HomeMy WebLinkAbout2013 Stormwater Master PlanColored copy on file in vault of City Clerk's office
City of Sebastian
Stormwater Management
Master Plan Update
December 6, 2013
Prepared by:
III_ NEEL— SCHAFFER
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Frank Watanabe Date
PE FL 66735
NEEL— SCHAFFER
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Table of Contents
December 6, 2013
1. Introduction ........................................................ ............................... 3
i. Background
ii. Purpose
iii. Modeling Approach
2. Data Collection and Methodology ...................... ............................... 9
i. Meetings with City of Sebastian and Project Identification
ii. Field Review Drainage Improvements from 2006 to 2012
iii. GIS Data Collection
iv. Hydrologic Model
v. Hydrologic Parameters
a) Topographic Data
b) Hydrologic Unit Areas
c) Time of Concentration
d) Curve Numbers
e) Boundary Conditions of South Prong Sebastian River
f) Soils Data
vi. Hydraulic Parameters
a) Existing Structure Inventory
b) New Structure Inventory since 2004
c) Modification of Existing ICPR Model
3. Engineering Analysis and Stormwater Deficiencies .......................... 17
i. Stormwater Model Analysis
ii. Level of Service
iii. Deficiency Areas
iv. Best Management Practices
4. Storm Water Quarter Round Program ................ ............................... 44
i. Testing and Analysis of Quarter Rounds
5. Conclusion and Findings .................................... ............................... 49
i. Storm water Improvement map
Appendix
Quarter Round Testing Calculations
• ICPR3 Model Run in separate Technical Binder
Sebastian Stormwater Master Plan Update Page 2
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1. Introduction
December 6, 2013
i. Background
The City of Sebastian started as a river front fishing haven for only a few dozen pioneers in
the late 1800's, expanding over the years due to its. ideal fishing location and treasure —
laden coastline. The current population is approximately 22,000. The City is located in the
northern section of Indian River County adjacent to the St. Sebastian Rivers and the County
limit. Sebastian is 95 miles Southeast of Orlando and 12 miles north of the City Vero
Beach or approximately midway through the east coast of the Florida Peninsula between
Melbourne and Vero Beach in an area known as the Treasure Coast. This location known
as "The Treasure Coast," is street accessible by US 1, 1 -95, the Florida Turnpike.
In February 2011 the City of Sebastian, Florida, contracted with Neel- Schaffer Inc. (NSI) to
update the north and southwest area of an existing stormwater model originally developed
by Camp Dresser & McKee Inc. (CDM) in 2004 - -- Master Stormwater Management Plan
(MSWMP). The City of Sebastian, by way of the South Prong of the St. Sebastian River,
drains to the Indian River Lagoon. The existing stormwater model parameters will be used
as a base to create a new 2012 drainage model using the Interconnected Channel and
Pond Routing version 3 (ICPR3) to verify the previous study and provide an update to storm
water improvement projects.
The ICPR3 model is an engineering software tool to solve problems of flood routing through
complex networks of interconnected and hydraulically interdependent stormwater ponds
using basins, nodes and links. In addition to the ICRP, ArcView (software application that
provides extensive mapping, data use, and analysis, along with simple editing and Geo-
processing capabilities) and ArcGIS (comprehensive name for the current suite of GIS
products used to create, import, edit, query, map, analyze, and publish geographic
information) will be used to update the existing storm water model mapping.
As noted, the original drainage model developed in 2004 had a base model developed in
1996 by Craven Thompson & Associates. The consulting firm CDM was retained to
develop the original model to identify Citywide drainage improvements. In 2010, Neel -
Schaffer, Inc. was retained to update the model based on recent drainage improvements
and changes in FEMA topographic data files. The extent of the new model is shown in
ICPR Model Boundary (page 5).
As part of the drainage update, the City's current implementation of the "Quarter Round"
program was incorporated into this report. Quarter Rounds are plastic pipes cut into
quarters. A quarter of the pipe is installed along the existing residential drainage swales to
assist in managing storm water runoff and filtering of pollutants. This program was initiated
in 2006 as an experimental project by St John's River Water Management District
(SJRWMD).
ii. Purpose
Florida receives on average 40 to 60 inches of rain each year, with less than one inch of
rainfall each time it rains; however the state also experiences torrential downpours and
hurricane rains. These cause runoff carrying sediment, fertilizers, pesticides, oil, heavy
metals, bacteria, and other contaminants to enter surface waters, causing adverse effects
from increased pollution and sedimentation.
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According to the Florida Department of Environmental Protection Agency (FDEP), the
Florida's stormwater regulatory program requires the use of Best Management Practices
(BMPs) during and after construction to minimize erosion and sedimentation and to properly
manage runoff for both stormwater quantity and quality. These BMPs are control practices
that are used for a given set of conditions to achieve satisfactory water quality and quantity
enhancement at a minimal cost. Each type of BMP has specific application, installation,
and maintenance requirements that should be followed to control erosion design of these
control measures, such as those established by the Florida Department of Environmental
Protection (FDEP), Florida Department of Transportation (FDOT) or other recognized
organizations.
Stormwater management is a critical component for the control of runoff and pollution. The
existing stormwater model data base was used to develop the new drainage parameters for
the modeling. These parameters include sub -basin boundaries, input conditions, new
information using FEMA elevations, soil characteristics and input from new City stormwater
projects. These parameters were used to analyze the previous drainage model and to
create the stormwater updates.
Therefore, the purpose of the MSWMP is to inventory and characterize the Previous
Stormwater Management System (PSWMS), update select hydrologic parameters of the
basin, update the existing stormwater model, identify areas that have indicated flooding and
develop alternatives to alleviate both flooding and water quality problems.
The study objectives addressed the following:
• Updating the existing stormwater model to represent the current hydrologic and
hydraulic conditions within the basin. This included incorporating several previous
models into a single model, incorporating approximately 80 culverts and 5 bridges
not represented in the original model, modifying the system storage represented in
the model, incorporating the St. Johns River Water Management District's
(SJRWMD) design of the regional stormwater park into the stormwater model,
verifying and modifying select channel cross sections, calculation of existing, and
future land use curve numbers and modification of hydrologic unit boundaries.
• Evaluating the existing capacity and the future demand of the PSWMS by
establishing the proper level of service, and determining the system's deficiencies
based upon local criteria.
• Developing alternative improvements (structural and non - structural) and providing
recommendations for reducing system deficiencies.
• Developing a master plan that prioritizes the recommended alternatives with
individual preliminary engineering cost estimates.
• Prioritizing areas for water quality retrofit and consider these areas in the design of
stormwater improvements for flooding.
Geographic Information Systems (GIS) data was gathered from the required county
government agency to create new maps with the most up to date information. Meetings
with the City of Sebastian, field reviews of the existing drainage network, and drainage
analysis using ICPR3 model were conducted to better understand the existing drainage
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December 6, 2013
system. As a result NSI provided a summary of stormwater deficiencies and provide a list
of future stormwater improvements.
It is noted that in order to obtain accurate data, the whole City was analyzed using GIS.
The time of concentration, stage /storage relationships, and sub - basins were calculated for
the entire City.
iii. Drainage Patterns
The City of Sebastian is located
between the South Prong of the
Sebastian River and the Indian River
Lagoon in Indian River County, Florida.
The City is approximately 13.4 square
miles or 8,600 acres. The modeling
extent or area of study is approximately
9.3 square miles. Low - density
residential land use consists of 6.2
square miles. There is a coastal ridge
along the eastern edge of the City and
slopes down to the Indian River Lagoon.
The sand is permeable allowing rainfall
to percolate rapidly through the soil.
However, majority of the City is located
to the west of the coastal ridge and is
flat. The average slope is less than 0.1
percent and the average elevation is
approximately 20 feet above Mean Sea
Level (MSL) [reference point used as a
standard for determining terrestrial and
atmospheric elevation or ocean depths
and is calculated as the average of
hourly tide levels measured by
mechanical tide gauges over extended
periods of time]. Soil conditions to the
west of the coastal ridge are virtually
impermeable and rainfall does not easily
percolate through the ground but
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UPDATED DRAINAGE MAP
remains as standing water until it evaporates.
Stormwater runoff generated in much of the City discharges to the South Prong of the
Sebastian River (South Prong). The City is drained by a series of major canals which
outfall in seven discrete locations to the south Prong. The secondary and tertiary (system
of rocks) systems tributary to these major canals tend to be back lot line ditches
(secondary) and side yard swales or pipes (tertiary). Upstream of the City, the South Prong
is controlled by the Sebastian River Water Control District (SRWCD). A radial gate dam is
located just south of the Scuthwest corner of the City and controls the surface discharge of
approximately 35,000 acres. Downstream of the City, the South Prong is more influenced
by tidal and storm surges that emanate for the Indian River Lagoon. Fifty -two percent
(52 %) of the Predominant :and uses in the City itself are low density, while 14 percent are
open and 11 percent are wetlands.
Sebastian Stormwater Master Plan Update Page 5
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Sebastian 2013 Drainage Map
December 6, 2013
Sebastian Stormwater Master Plan Update Page 6
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iv. Modeling Approach
December 6, 2013
The modeling approach consisted of updating a single regional model of the City of
Sebastian. The modeling approach utilized a combination of programs to calculate the
best data and update the report. Shape files were gathered from government. websites
such as Saint Johns River Water Management District, USDA, and FEMA. The shape files
were input into ArcView and using hydrologic data such as: area, elevations, and stages
were gathered. Using the ArcView information, the time of concentration was calculated.
[Hydrology is the science that encompasses the occurrence, distribution, movement and
properties of the waters of the earth and their relationship with the environment within each
phase of the hydrologic cycle. The hydrologic cycle is a continuous process by which water
is purified by evaporation and transported from the earth's surface (including the oceans) to
the atmosphere and back to the land and oceans.] The report created by CDM in 2004
used an advanced Interconnected Pond Routing model (ICPR) version 3.0 developed by
Streamline Technologies Inc.
In addition, City storm water projects from 2004 to the present were provided by the City of
Sebastian for the northern and southern portion of the City and were incorporated into the
new ICPR drainage modeling. After putting together information such as stage, storage
and time of concentration, the modeling was updated using Interconnected Channel and
Pond Routing 3 (ICPR3).
The Interconnected Channel and Pond Routing Model (ICPR) is a modeling tool that has
been used for over 25 years and successfully solves problems of flood routing through
complex networks of interconnected and hydraulically interdependent stormwater ponds. It
is listed with the Federal Emergency Management Agency (FEMA) as a Nationally
Accepted Hydraulic Model and is applicable to almost any type of terrain. The model now
includes hydrodynamic modeling of channel and pipe systems and has a fully integrated
hydrology component. The three primary building blocks in ICPR are Basins, Nodes and
Links. Stormwater runoff hydrographs are generated for basins and then assigned to
nodes in the drainage network. Nodes are used to represent ponds and specific locations
in the drainage network such as along channels, streams, rivers, and junctions in pipe
systems. Stages are calculated at each of the nodes. Links such as pipes, channel
segments, weirs (a small dam in a river or stream), and bridges are used to connect nodes
together. Flow rates are calculated for links based on stages at nodes.
The City in coordination with DEP and SJRWMD implemented a City's Stormwater Park
within the center of the City on a 166 -acre area known as Adam's Parcel. This facility is
intended to provide water quality treatment to surface water in the City's Stormwater
Management System that was previously untreated. Additionally, the City is also preparing
a stormwater master plan for the Sebastian Municipal Airport. A copy of that master plan is
available at the City Public Works Office.
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2. Data Collection and Methodology
December 6, 2013
i. Meetings with City of Sebastian and New Project Identification
To initially understand the drainage system within these areas, a meeting was held with
the City of Sebastian and field reviews were conducted to identify new drainage
sections since the original model was developed in 2004. The following is a summary
of the meeting held with the City of Sebastian:
Meeting on March 10, 2011
• Initial meeting with City staff regarding mapping and files.
• The City provided mapping and location of drainage projects constructed since
the previous drainage modeling in 2004.
• Create new IRCP model since the previous CDM model files were not available.
• Revise the model per 2004 projects and generate new storm water projects.
Data collection and research was also performed to gather all the necessary data files
to create the files using ArcGIS. GIS Shape files (shp) were collected in March 2011
from several different sources: Saint Johns River Water Management District
(SJRWMD), Indian River County Property Appraiser, Federal Emergency Management
Association (FEMA), Natural Recourses Conservation Service (NRCS), and the United
States Geological Services (USGS). With these files in place new curve numbers for
both existing and future land use conditions can be determined to update the new
model. Using information and the existing sub basin maps from the original CDM
report, a model run was performed. ICPR version 3 was used to create the model.
Field Review Existing
Drainage Improvements I
Field reviews were conducted to
identify the drainage improvements
in the City since 2004. Per the
City's direction there were six
stormwater project sites. Each site
was visited to verify installations
and take note on drainage
improvement for the modeling. The
following is a summary of the
stormwater projects and what they
consist of:
1. Twin Ditches:
Project location: Ditches
located between Main Street
and Airport Drive /Brush Foot
Drive from Fig Street to
Wimbrow Drive.
Converting the existing
double ditch system to a Best
Management Practice wet
detention treatment system.
" 7WIn Ditches Stormwater Retrofit
�i 40- 081 - 98504 -1
V M +Ao. 2004 Digital Ortho Quadrangle
4."25, NDS
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2. Davis Street Baffle Box 2006:
• Project Location: Street drainage improvement for Davis Street Area which
included the outflow from US Highway 1 and the Indian River Lagoon.
• Retrofit project involved the installation of a street drainage pipes and a Nutrient
Separating Baffle box and associated piping from an area of approximately 96
acres of light commercial and residential land. In addition, the project eliminated a
direct discharge of untreated stormwater into the Indian River Lagoon.
3. Collier Canal 2008
• Project Location: Collier Canal dredging
and seawall improvements from the
Barber Street Park /Hardy Dam near
Main Street and CR 512.
• Project consisted of retrofitting the
existing canal seawalls with 3:1 back
slopes. Project also consisted of
dredging the canal bottom to lower the
bottom elevation approximately 6 feet.
4. Periwinkle Stormwater Basin 2006
• Project Location: Residential area
adjacent to Periwinkle Street and other
streets.
• Project involved the installation of a 3:1
wet detention pond, drainage
improvements, passive recreation park
and a monitoring plan for the drainage
treatment.
• The project was well accepted by the
nearby residents as a positive
improvement to the community.
6. George Street Drainage 2010
• Project Location: George Street within
the residential neighborhood
• Consisted of retrofitting the existing
open drainage ditch with a new
drainage pipe and street drainage
crossing upgrade at George Street.
6. Potomac Drainage Improvement 2011
• Project Location: Potomac Avenue at
Roseland Road in City /County.
• Consisted of retrofitting the existing
drainage ditch with new drainage pipe,
street drainage crossing and new
nutrient baffle Box.
Sebastian Stormwater Master Plan Update Page 10
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iii. GIS Data Collection
GIS data collection was obtained from the SJRWMD website on March of 2011.
SJRWMD maintains an up to date data collection from governmental websites. The
district periodically updates the GIS Download Library as new data becomes available.
This data included: FEMA, topo (topographic) data, United States Geological Survey
(USGS) data; population characteristics data among others. Once files were
downloaded they could be imported into ArcView and data analysis could begin.
ArcView GIS is a desktop geographic information system (GIS) from Environmental
Systems Research Institute, Inc. (ESRI). A GIS is a database that links information to
location, allowing you to see and analyze data in new and useful ways.
iv. Hydrologic Model
NSI used the ICPR version 3.0 stormwater model. The model has three methods for
generating stormwater runoff: the Soil conservation Service (SCS) unit hydrograph
method, the Santa Barbara method, and the Overland Flow method. The SCS unit
hydrograph method was selected by the City. The ICPR model has two components to
determine the volume and rate of stormwater runoff. The first component is based upon
the amount of Directly Connected Impervious Area (DCIA) to the stormwater system
represented by a percentage of the contributing area. The resulting runoff from rainfall
over the DCIA does not pass over any pervious area and thus does not infiltrate into the
soil. The second component consists of the impervious areas and pervious areas that
are not directly connected to the PSMS and thus are subject to infiltration. The SCS
unit hydrograph method uses a Curve Number (CN) and a time of concentration (tc) to
determine the runoff volume and timing from this second component. The CN method
relates rainfall to direct runoff as a function of soil type and land use cover. The curve
number and time of concentration methodologies are fully documented in the National
Resource conservation Service's (NRCS) Technical Release 55 (TR55).
v. Hydrologic Parameters
a) Topographic Data
The study area consisted of detailed 2 -foot topographic contour data; therefore, the
hydrologic boundaries from the 2004 Study were modified significantly. Please refer to
the attached topographic map showing the 2 -foot contours (Figure: WOOLPERT 2 -foot
Aerial Topographic Data). Generally, the extent of the hydrologic boundaries consisted
of combining multiple basins loading to a single node into a single basin. However,
there was an exception, the basin named "BRIAR ". Due to new data obtained, the
basin was brand new and the data obtained was inconclusive as to whether it drained
into our study area or node water should flow to. It is noted that the basin "BRIAR" is
part of the southern area of the City and it is not modeled in the ICPR3 report. An
additional survey is recommended for this basin. It was determined that much of the
area that was previously assumed in the 1996 Study to load to the City's Primary
Stormwater Management system (PSWMS), in fact discharges directly to the South
Prong of the Sebastian River.
b) Hydrologic Unit Areas
The model extent of the City was divided into sub basins that have been further
subdivided into smaller hydrologic units based upon existing drainage patterns (see
Figure: Hydrologic Unit Location Map). The previous report by CDM, had a total of 204
hydrologic units. For our modeling purposes, the study area was subdivided into 216
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hydrologic units for which areas and time of concentration were compiled. The
hydrologic units averaged approximately 45 acres in size with a minimum of 3.8 acres
and a maximum of 155 acres.
c) Time of Concentration
The time of concentration (tc) is the time stormwater runoff takes to travel from the
hydraulically (operated by, moved by, or employing water or other liquids in motion)
most distant point of the watershed to the point of outflow from an area taking into
account the length of time required for the following:
Sheet flow - one in which the horizontal dimensions are much larger than the
vertical extent; The maximum sheet flow length should be no greater than 125-
150 feet;
Shallow concentrated flow - after a maximum of 300 feet shallow flow usually
becomes shallow concentrated flow. This 300 foot value has since been revised
down to a maximum of 150 feet on very uniform surfaces;
Open channel and /or pipe flow — This occurs within swales, channel streams,
ditches and piped storm drainage systems.
Complete documentation of this
methodology is available in
various publications such as
NRCS' TR -55 publication and
SJRWMD's Technical
Publication (TP) 85 -5.
During model development
several tc's appeared to be
inconsistent with the hydrological
unit size and apparent flow
length specified in the previous
report. New tes were calculated
based on the new 2 -foot topo
data and compared with the
original. Overall approximately
95 percent of the tc values of the
hydrologic units were updated in
ICPR3. The original and
updated tc values are shown in
Table 2 -1: Hydrologic Units and
Time of Concentration.
%/111
d) Curve Numbers
The curve numbers, which are used -to determine how much of the rainfall will be
converted to runoff, were calculated based on both the land use and hydrologic soil
group distribution in each hydrologic unit. For the purpose of this study, the same
guidelines as the previous drainage report were used to determine curve numbers and
were not modified.
Sebastian Stormwater Master Plan Update Page 12
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NEEL— SCHAFFER December 6, 2013
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e) Hydraulic Boundary Conditions in the South Prong of the
Sebastian River
To determine the boundary conditions to be used in the modeling effort, the conditions
specified in the CDM 2004 report were utilized. There are four nodes in the model that
represent various locations in the South Prong. The modeled system outfalls to these
nodes:
■ N -Stone — this is the boundary condition for the south end of the Stonecrop
portion (i.e., the southwest corner of the City that discharges to the South
Prong) of the model. This location is consistent with River Station 270 in the
FIS.
• Bridge — this is the boundary condition for the southern outlet from Unit 5.
This location is consistent with River Station 235 in the FIS.
• BC -210 — this is the boundary condition for the southern outlet from Unit 5.
This location is consistent with River Station 235 in the FIS.
• BC -210 — this is the boundary condition for the northern outlet from Unit 5.
This location is consistent with River Station 210 in the FIS.
• Ncollier — this is the boundary condition for Collier Canal. This location is
consistent with River Station 195 in the FIS.
f) Soils Data
Soil data are used to evaluate stormwater runoff, infiltration, and recharge potential for
pervious areas. Information on soil types was obtained from the U.E. Department of
Agriculture (USDA) NRCS (formerly the Soils Conservation service (SCS) Soil Survey
of Indian River County, Florida (NRCS, 1990) and in digital format from the SJRWMD.
V. Hydraulic Parameters
a) Existing Structure Inventory
Existing structures were collected from the CDM 2004 study. Revisions were made to
certain structures based on the new 2' topographic data and information obtained from
Google Earth Street View for cross -drain sections. The Google Street View NSI
identified crossings that were not part of the original model and have been updated in
the ICPR model.
b) New Drainage Inventory since 2004
As previously mentioned, meetings were held with the City of Sebastian to establish the
recent projects since the development of the previous model. These projects are the
following:
• Twin Ditches Project
• City Storm Water Park
• Collier Canal Dredging and seawall
• Davis Street Drainage and Baffle Box
• George Street Drainage
• Potomac Street Drainage and Baffle Box
• New Quarter Round Installation
• Replacement of Damaged Drainage Pipes
• Maintenance of existing ditches and swales
Sebastian Stormwater Master Plan Update Page 15
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c) Modification of Existing ICPR Model
The previous Stormwater Master Plan study by CDM which was developed in 1996 and
then updated in 2004 needed to be updated again due to storm water improvement
within the last nine years. The existing stormwater model originally developed as part of
the 1996 Study and later updated by CDM in February 2004 as a stormwater master
plan study. Since that time, there appeared to be inconsistency in the drainage master
plan in relation to the existing drainage system.
Based on the previous model analysis which included the original ICPR Model runs,
the study performed a limited verification of topographic data and channel cross -
sectional information. The previous master plan study noted the limited verification of
topographic data. As part of this updated study, there was verification of drainage data
which included comparison of the top widths of the modeled cross- sections to the top
widths measured on the aerial photograph. There was reasonable validation of the
cross - sections by using this method. A review of the UDS quad map indicated that the
vast majority of the study area was at elevation 20 foot -NGVD.
The inverts of the channels were then adjusted based on the assumption that the top of
bank (TOB) of the channels were also at 20 foot -NGVD. The cross - sectional
information was reviewed for each channel segment and the depth determined. The
invert was determined for each segment as the difference between the adjusted TOB
and the depth of the cross section. There was limited survey information available for
channels, typically associated with construction plans. This data was used to verify the
adjusted TOBs which appeared to be reasonable.
The original model developed by CDM was reviewed and then compared with the new
model run analysis using new topographic data. New FEMA topographic data was
used to determine new basins, time of concentration and re- routing of basins to certain
nodes. The stage /storage areas of the nodes and the sub -basin determination which
includes basin area and time of concentration values are shown in the technical
binders.
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3. Engineering Analysis
December 6, 2013
i. Stormwater Model Analysis
The original citywide stormwater model developed by the previous consultant CDM
performed simulation analysis for the mean annual 25 year and 100 year /24 hour design
storm event for both the existing and future land use conditions. The modeling analysis for
the new update reviewed both previous simulation analyses. New model simulations were
also conducted representing changes in the hydraulic conditions based on new stormwater
improvements.
The hydraulics analysis for both the existing and future land use conditions was analyzed
with new ICPR model simulation data. The new model runs were used to identify locations
of any hydraulic segments that appeared to be deficient base on top of road elevations and
finished floor elevations. The new model analyses were compared to previous model runs
for any discrepancies. It should be noted that the previous model findings assumed
topographic data for open channels and drainage systems which were not verified in the
field. Based on this assumption of non - verified drainage data, the new model simulation
runs were field verified for flood conditions, ponding, channels and drainage capacity
overflow conditions. Flooding is typically caused by undersized drainage systems or over
capacity conveyance of the system due to heavy storm period events. It was field verified
that the majority of the deficient storm water areas identified by the model analysis were
typically lack of conveyance for storm water runoff. In many of the field locations with
drainage flooding, improvements to the drainage system where flooding occurred would
resolve the situation. Alternatives were then developed to alleviate any deficiencies.
In addition to identifying the deficient drainage areas within the City, the City's stormwater
system needed to be updated base on recent drainage improvement by the City and new
topographic data provided by FEMA on the flood plan mapping. The modeling analysis
was based on the data calculated from the new FEMA GIS database and then re- analyzed
using the ICPR3 stormwater model to update the topographic data file and hydrologic unit
maps. The existing citywide group model was compressed into three sections to identify
and analyze the system as it should be modeled.
The City has a natural center line that separates the City into two main drainage groups.
These two main Stormwater groups are drainage north of CR 512 and drainage south of
CR 512
The model updated and reanalyzed the previous subgroupings of drainage areas
establishing two major groups to better analyze and quantify the deficiencies. The City is is
split with County Road (CR) 512 as the dividing line for the north and south City's drainage
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December 6, 2013
system. In addition to these two drainage groups, the City has a third area east of the
existing railroad tracks. This eastern section of the City of Sebastian is not included in the
two groups and was never modeled in previous drainage studies. The area of this third
group is defined by the railroad tracks to the west and the Indian River Lagoon to the east.
This section of Sebastian is known as the "River Front" and part of the City's
Redevelopment District.
ii. Levels of Service
As part of the stormwater management update there is a need to address the level of
service. There are essential components to any stormwater master plan and they are the
proper levels of service decisions.
The City is challenged financially to maintain the drainage system and to provide for the
proper level of service needed to maintain the existing drainage systems which include:
detention ponds, channels, side ditches, swales and the City's storm water park.
During the field review, there are several locations where the open channel system or side
ditches were overgrown with vegetation. In the drainage modeling analysis, these open
channels or ditches were assigned a coefficient of resistance for flow. This factor is called
a "Manning's Coefficient n" which represents the roughness coefficient of an excavated
channel with minor vegetation. The City ditches and open channels are all filled with weeds
and thick brush and vegetation which should have a friction factor worse than the channel
with no vegetation.
of
• Top of channel bank elevations were exceeded for the 25- year /24 -hour storm
event
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iii. Drainage Deficiencies
As part of the stormwater master plan update, the model runs identified some of the similar
drainage concerns from the previous study.
The regrouping of the systems into two regional groups helped to clarify where actual
drainage deficiencies exist or if the model is estimating potential drainage overflow due to
detailed topographic data of the City's drainage system. Most of the previous drainage
deficiencies have been addressed in the past few years with minor and major drainage
improvement projects within the City. These improvements sufficiently address most of the
drainage issues identified by the model. Based on local knowledge of the existing City
system, there are at present a few areas of drainage deficiencies that the model has
identified as potential overflow during high peak storm periods. These deficient drainage
areas are shown below in photographs below and identified in the 2013 Drainage Map
page 20).
The City has identified these drainage improvements in the five to ten year capital
improvement program. The goal of the City is to prioritize and apply for storm water grants
to help leverage the cost for the various improvement projects. The following projects are
illustrated on the 2013 Drainage Map (page 20).
Capital Stormwater Projects — Presented to City Council in February 2013
Stormwater Projects
Construction Cost
•
A - Indian River Drainage Improvement
$2,000,000
•
B - Collier Canal Hardy Dam
$ 100,000
•
C - Potomac Ave Lateral Pipe
$ 200,000
•
D — BlossomNVentworth Ditch Piping
$1,000,000
•
E - Stonecrop Pipe /culverts (Bevan & Laconia)
$1,000,000
•
1 — Future South Area Basin
$ 300,000
•
J — Southeast Dredge Basin
$ 500,000
•
K — Tulip Pipe Replacement (culvert 42 "x72 ")
$ 100,000
•
L — EastNNest Lateral Drainage Pipe
$ 500,000
•
M — Tulip Detention Basin
$ 250,000
•
N — George St. Canal Dredging
$2,500,000
•
O — Rosebush Terrace Pipe — Twin 48"
$ 100,000
•
P — Landdowne Dr. Pipe — Twin 36"
$ 100,000
Total Stormwater Improvements
$8,650,000
Annual Maintenance
F — Replace CMP — Maintenance $ 200,000
G — Quarter Rounds Installation $ 250,000
H — Backyard Ditches Maintenance $1,000.000
Total $1,450,000
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iv. Best Management Practices
December 6, 2013
A. Potential BMPs
This section presents various best management practices (BMPs) that may be considered
for use in the City for retrofit treatment. There are many other BMPs used throughout the
Country.
According to the Environment Protection Agency, "...dilution is the solution to pollution. If
people got rid of it quickly enough and far enough away, it would no longer be a problem.
In cities and towns, we focused on sewers to ferry and treat human or commercial waste.
They used storm sewers for rainwater and snow. In all cases, they have designed
convenient ways to ferry water into the nearest water body. Unfortunately, we have
discovered that dilution is not the solution for stormwater and its pollutants."
Best Management Practices (BMPs) is a term used to describe a type of water pollution
control. Stormwater BMPs are techniques, measures or structural controls used to manage
the quantity and improve the quality of stormwater runoff. The goal is to reduce or eliminate
the contaminants collected by stormwater as it moves into streams and rivers. Once
pollutants are present in a water body altering its physical makeup and habitat, it is much
more difficult and expensive to restore it. Therefore, the use of BMPs that prevent damage
to receiving waters is our target. Stormwater pollution has two main components:
• The increased volume and rate of runoff from water resistant surfaces, such as
roads and parking lots, and
• The amount of pollutants in the runoff.
Both components are directly related to urban development. They can cause changes in
water quality. This results in a variety of problems:
• Environment modification and loss,
• Increased flooding,
• Decreased native wildlife, and
• Increased sedimentation and erosion.
In turn, effective management of stormwater runoff offers a multitude of benefits:
• Protection of wetlands and ecosystems,
• Improved water quality of streams, rivers and other water bodies,
• Protection of water resources,
• Protection of public health, and
• Flood control.
There are two groups of BMPs: structural (constructed facilities) and non - structural
(regulatory or ordinances). The BMPs discussed appear to be the most applicable to the
City.
1. Structural Stormwater Controls
• Dry detention ponds
• Wet detention ponds
• Exfiltration trenches
• Shallow grassed swales
• Water quality inlets and baffle boxes
• Removal of septic tank systems
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2. Non - Structural Source Controls
• Public information programs
• Fertilizer application controls
• Pesticide and herbicide use controls
• Operation and maintenance
December 6, 2013
An explanation of each of the above BMPs follows and is a comparison for the treatment
and management of stormwater runoff. The use of a specific BMP depends on the site
conditions and objectives such as water quality protection, flood control, aquifer recharge,
or volume control. There might be many goals or needs for a project. BMPs can be used
with other BMPs to develop a "treatment train." The Stormwater Treatment Train (STT)
represents an ecological approach to stormwater management and has proven effective
and versatile in its various applications. The STT was designed with sequential components
that contribute to the treatment of stormwater before it leaves the site.
The components of the Stormwater Treatment Train system were designed to treat
stormwater runoff for water quality benefits and to reduce stormwater runoff peaks and
volumes. Based on hydrologic modeling and published information on BMP effectiveness,
the STT approach can be expected to reduce surface runoff volumes by 65 percent and
reduce solids, nutrients, and heavy metals loads by 85 percent to 100 percent. Source
controls (upstream from the initial swale component) minimize the impacts of the
development even further.
This alternative approach to stormwater management not only has the potential to reduce
infrastructure costs, but it also reduces maintenance costs. As described above, native
plants are adapted to the environment, and do not need extensive watering, chemical
treatment, mowing, and replanting that non - native species demand. In addition, there is
also a substantial benefit to downstream neighbors. By treating stormwater where it falls on
the land, responsible landowners are reducing their contribution to downstream flooding
and sedimentation. The STT incorporates a number of BMPs with varying effectiveness for
removing particulates and pollutants while also reducing runoff volume.
The advantages of an STT are as follows:
• Provides effective stormwater flood control by slowing down runoff and storing
water, including water infiltration into the soil.
• Improves water quality by filtering pollutants from stormwater (oils, greases,
metals, and sediments that can be picked up from paved surfaces).
• Reduces erosion.
• Flexible to incorporate existing natural features and /or introduced stormwater
control features.
• Provides open space that can be used for recreation and aesthetic value.
• Preserves natural /native vegetation and provides habitat for wildlife.
• Protects adjacent properties.
• Improves property values.
There are two disadvantages:
• May require more space than is available.
• Requires planning and stakeholder acceptance.
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There are implementation considerations and they are:
• Public outreach and acceptance for existing developments or communities.
• Effect on long -term stormwater management infrastructure.
• Demonstration of improved property values and cost of development with
implementation of the Stormwater Treatment Train.
• Planning and engineering of effective treatment train appropriate for each area.
• Determine the necessary space and length to achieve stormwater management
goals and water quality.
The cost of an STT will vary depending on best management practices and extent of the
treatment system. Overall cost is less, however, than stormwater collection and
conveyance systems for a similar area.
B. Structural BMPs
Structural BMPs involve building an engineered "facility" for controlling quantity and quality
of urban runoff. These structures treat runoff at either the point of generation or the point of
discharge to either the storm sewer system or receiving waters. Most require some level of
routine maintenance. Structural BMPs can be categorized as retention systems, detention
systems or other systems (Marshall, 2002).
Although the basic principles of management stormwater remain the same, they should be
uniquely adapted to the special requirements of each project. It should be understood that
no one BMP can be the "cure all" for a particular project, but if several are used together in
a linked fashion like cars in a train (a "BMP treatment train), adverse effects of urban
stormwater runoff can be reduced or alleviated (Marshall, 2002).
A careful assessment of stormwater management conditions should be made before
choosing a system of comprehensive BMPs. First, potential pollutant sources and high risk
areas of pollution must be identified. Then, the magnitude of the problem must be
evaluated by monitoring and analyzing runoff to determine the amount and type of
pollutants in terms of concentration or load. Understanding the source, amount, and
characteristics of pollutants in stormwater runoff is essential in applying a screening
process for selecting appropriate BMPs (Marshall, 2002).
General Information for Detention Practices
Detention refers to the temporary storage of excess runoff onsite prior to gradual release
after the peak of the storm inflow has passed. Runoff is held for a period of time and is
slowly released to a natural or manmade water course, usually at a rate no greater than the
pre - development peak discharge rate. For water quantity, detention facilities will not
reduce the total volume of runoff, but will redistribute the rate of runoff over a longer period
of time by providing temporary storage for the stormwater. Storage of stormwater runoff
within a stormwater management system is essential to providing the extended detention of
flows for water quality treatment and downstream channel protection, as well as for peak
flow attenuation (the process by which a virus, bacterium, etc., changes under laboratory
conditions to become harmless or less virulent) of larger flows for overbank and extreme
flood protection.
Dry Detention Ponds
Dry detention ponds (a.k.a. dry ponds, extended detention basins, detention ponds, and
extended detention ponds) are basins whose outlets have been designed to detain
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stormwater runoff for some minimum time (e.g., 24 hours) to allow particles and associated
pollutants to settle. Unlike wet ponds, these facilities do not have a large permanent pool of
water. However, they are often designed with small pools at the inlet and outlet of the basin.
They can also be used to provide flood control by including additional flood detention
storage.
Dry detention ponds have traditionally been one of the most widely used stormwater best
management practices. In some instances, these ponds may be the most appropriate best
management practice. However, they should not be used as a one size fits all solution. If
pollutant removal efficiency is an important consideration then dry detention ponds may not
be the most appropriate choice. Dry detention ponds require large amounts of space to
build them.
Potential Benefits of a Dry Detention Pond
• Reduction of downstream flooding problems by attenuating the peak rate of flow.
• Some removal of pollutant loadings to receiving bodies of water for suspended
pollutants.
• Reduction in cost for downstream conveyance facilities.
• Creation of fill that may be used on site for sold (pond sediment removal).
• Low frequency of failure as compared with filtration systems.
Limitations
Although dry detention ponds are widely applicable, they have some limitations that might
make other stormwater management options preferable:
• Dry detention ponds have only moderate pollutant removal when compared to
other structural stormwater practices, and they are ineffective at removing
soluble pollutants (See Effectiveness).
• Dry extended detention ponds may become a nuisance due to mosquito
breeding if improperly maintained or if shallow pools of water form for more than
7 days.
• Although wet ponds can increase property values, dry ponds can actually
detract from the value of a home (see Cost Considerations).
• Dry detention ponds on their own only provide peak flow reduction and do little
to control overall runoff volume, which could result in adverse downstream
impacts
Extended Dry Detention
Extended detention refers to a basin designed to extend detention beyond that required for
stormwater peak rate control to provide some water quality affect.. Extended dry detention
(ED) is used to drain a runoff volume over a specified period of time, typically 24 hours, and
is used to meet channel protection criteria (CPv). Some structural control designs (wet ED
pond and micro -pool ED pond) also include extended detention storage of a portion of the
water quality volume. Extended detention basins are viable and effective treatment
facilities. When properly designed, significant reductions are possible in the total
suspended sediment load and of constituents associated with these sediments. Typically
these basins are less effective in removing soluble solids. The amount of reduction
depends on a wide variety of factors, including:
• Surface area of the basin,
• Peak outflow rate,
• Size distribution of the particles,
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• Specific gravity of particles,
• Fraction of the sediment that is active clay,
• Type of associated pollutant concentrations,
• Fraction of influent solids are colloidal, dissolved, and non - settleable.
Extended detention basins will sometimes have a small permanent pool below the invert of
the low flow outlet. This is normally so small that it does not materially impact trapping of
sediment and chemicals, and is typically included for aesthetics or to cover deposited
sediments.
Wet Detention Ponds
Wet detention systems (a.k.a. stormwater ponds, wet retention ponds, retention basins, wet
extended detention ponds) are the most recognizable stormwater systems. They are
constructed basins that have a permanent pool of water into which stormwater runoff is
directed. Runoff from each rain event is detained and treated in the pond until it is
displaced by runoff from the next storm. They are designed to remove pollutants from
stormwater.
Ponds treat incoming stormwater
runoff by allowing particles to settle
and algae to take up nutrients.
The primary removal mechanism is
settling as stormwater runoff
resides in this pool. The pond's
natural physical, biological, and
chemical processes then work to
remove the pollutants.
Sedimentation processes remove
particulates, organic matter, and
metals, while dissolved metals and
nutrients are removed through
biological uptake. In general a
higher level of nutrient removal
and better stormwater quantity
control can be achieved in wet
detention ponds than can be achieved
trenches, or sand filters.
with other BMPs, such as dry ponds, infiltration
Wet detention ponds can be used as a stormwater retrofit. A stormwater retrofit is a
stormwater management practice (usually structural) put into place after development has
occurred, to improve water quality, protect downstream channels, reduce flooding, or meet
other specific objectives. Wet ponds are very useful stormwater retrofits and have two
primary applications as a retrofit design. In many communities, detention ponds have been
designed for flood control in the past. It is possible to modify these facilities to develop a
permanent wet pool to provide water quality control, and modify the outlet structure to
provide channel protection.
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Example of Stormwater Park in Sebastian, Florida
December 6, 2013
Design Considerations of Wet Detention Ponds
Specific designs may vary considerably, depending on site constraints or preferences of the
designer or community. There are some features, however, that should be incorporated
into most wet pond designs. These design features can be divided into five basic
categories: pretreatment, treatment, conveyance, maintenance reduction, and landscaping.
Pretreatment incorporates design features that help to settle out coarse sediment particles.
By removing these particles from runoff before they reach the large permanent pool, the
maintenance burden of the pond is reduced. In ponds, pretreatment is achieved with a
sediment forebay. A sediment forebay is a small pool (typically about 10 percent of the
volume of the permanent pool). Coarse particles remain trapped in the forebay, and
maintenance is performed on this smaller pool, eliminating the need to dredge the entire
pond.
Treatment design features help enhance ability of a stormwater management practice to
remove pollutants. The purpose of most of these features is to increase the amount of time
that stormwater remains in the pond. Stormwater should be conveyed to and from all
stormwater management practices safely and to minimize erosion potential. The out fall of
pond systems should always be stabilized to prevent scour. In addition, an emergency
spillway should be provided to safely convey large flood events. To help mitigate warming
at the outlet channel, designers should provide shade around the channel at the pond
outlet. In addition to regular maintenance activities needed to maintain the function of
stormwater practices, some design features can be incorporated to ease the maintenance
burden of each practice. In wet ponds, maintenance reduction features include techniques
to reduce the amount of maintenance needed, as well as techniques to make regular
maintenance activities easier.
The potential maintenance concern in wet ponds is clogging of the outlet. Ponds should be
designed with a non - clogging outlet such as a reverse -slope pipe, or a weir outlet with a
trash rack. A reverse -slope pipe draws from below the permanent pool extending in a
reverse angle up to the riser and established the water elevation of the permanent pool.
Because these outlets draw water from below the level of the permanent pool, they are less
likely to be clogged by floating debris. Landscaping of wet ponds can make them an asset
to a community and can also enhance the pollutant removal of the practice. A vegetated
buffer should be preserved around the pond to protect the banks from erosion and provide
some pollutant removal before runoff enters the pond by overflow. In addition, ponds
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should incorporate an aquatic bench (i.e., a shallow shelf with wetland plants) around the
edge of the pond. This feature may provide some pollutant uptake, and it also helps to
stabilize the soil at the edge of the pond and enhance habitat and aesthetic value
Wet Extended Detention Pond
The wet. extended detention pond combines the treatment concepts of the dry extended
detention pond and the wet pond. In this design, the water quality volume is split between
the permanent pool and detention storage provided above the permanent pool. During
storm events, water is detained above the permanent pool and released over 12 to 48
hours. This design has similar pollutant removal to a traditional wet pond and consumes
less space. Wet extended detention ponds should be designed to maintain at least half the
treatment volume of the permanent pool. In addition, designers need to carefully select
vegetation can withstand both wet and dry.
Wet Extended Detention Pond
The wet extended detention pond combines the treatment concepts of the dry extended
detention pond and the wet pond. In this design, the water quality volume is split between
the permanent pool and detention storage provided above the permanent pool. During
storm events, water is detained above the permanent pool and released over 12 to 48
hours. This design has similar pollutant removal to a traditional wet pond and consumes
less space. Wet extended detention ponds should be designed to maintain at least half the
treatment volume of the permanent pool. In addition, designers need to carefully select
vegetation can withstand both wet and dry.
Water Reuse Pond
Wet reuse ponds can act as a water source
for irrigation. In this case, the water balance
should account for the water that will be
taken from the pond. One study conducted
in Florida estimated that a water reuse pond
could provide irrigation for a 100 -acres golf
course at about One - seventhe the cost of the
market rate of the equivalent amount of
water at $40,000 versus $300,000.
Effectiveness of Wet Detention Ponds
Structural stormwater management practices can be used to achieve four broad resource
protection goals. These include flood control, channel protection, ground water recharge,
and pollutant removal. Wet ponds can provide flood control, channel protection, and
pollutant removal. One objective of stormwater management practices can be to reduce
the flood hazard associated with large storm events by reducing the peak flow associated
with these storms. Wet ponds can easily be designed for flood control by providing flood
storage above the level of the permanent pool.
When used for channel protection, wet ponds have traditionally controlled the 2 -year storm.
It appears that this control has been relatively ineffective and research suggests that control
of a smaller storm may be more appropriate. Wet ponds, cannot provide ground water
recharge. Infiltration is impeded by the accumulation of debris on the bottom of the pond.
Wet ponds are among the most effective stormwater management practices at removing
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stormwater pollutants. A wide range of research is available to estimate the effectiveness
of wet ponds.
Limitations of Wet Detention Ponds
Limitations to wet detention ponds include:
• If improperly located, wet pond construction may cause loss of wetlands or
forest.
• Wet ponds are often inappropriate in dense urban areas because each pond is
generally quite large.
• Wet detention ponds must be able to maintain a permanent pool of water.
Therefore, ponds cannot be constructed in areas where there is insufficient
precipitation to maintain the pool or in soils that are highly permeable.
• In cold water streams, wet ponds are not a feasible option due to the potential
for stream warming.
• Wet ponds may pose safety hazards.
• Without proper maintenance, the performance of the pond will drop off sharply.
Regular cleaning of the fore bays is particularly important. Maintaining the
permanent pool is also important in preventing the re- suspension of trapped
sediments. The accumulation of sediments in the pond will reduce the pond's
storage capacity and cause a decline in its performance. Therefore, the bottom
sediments in the permanent pool should be removed about every 2 to 5 years.
Exfiltration Trenches
An exfiltration trench is an underground drainage system consisting of a perforated pipe
surrounded by natural or artificial aggregate such as sand, which stores and infiltrates
runoff. They are similar to infiltration trenches with the exception they can be placed below
paved surfaces such as parking lots and streets. The exfiltration trench performs well at
removal of fine sediment and pollutants. They are sometimes referred to as subsurface
detention or retention, percolation tanks, soak - always or underground infiltration basins.
While infiltration trenches are usually rock filled ditches into which stormwater enters from
the top, exfiltration trenches often involve a pipe in the middle of the trench through which
stormwater enters. The stormwater runoff is collected by catch basins located at the end of
each exfiltration trench segment; the perforated pipe delivers the stormwater into the
surrounding aggregate through the pipe perforations. The stormwater ultimately exfiltration
into the ground water aquifer through the trench walls and bottom. As the treatment volume
is not discharged into surface waters, exfiltration trench systems are considered a type of
retention treatment.
The objectives of these structures are to capture and discharge stormwater at a controlled
rate. They function in concert with pervious surfaces by enhancing the infiltration and
storage capacity of on -site soils and treating runoff before it recharges the ground water.
Exfiltration systems act as small, distributed, underground stormwater retention ponds.
Exfiltration tanks and trenches can be used to convey and distribute captured runoff across
a lot or subdivision. These exfiltration structures provide a storage area for rapid runoff
during a storm, then allow it to infiltrate gradually through the soil into the ground water.
Runoff water enters the underground chamber at the inlet and a physical filtration process
removes pollutants as some pollutants can remain in the exfiltration water, so additional
source control is needed where ground water contamination is a concern.
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Example of an Exfiltration Trench
December 6, 2013
arc,
The permeability of the soils at the ,• :; ,�
exfiltration trench location and the
--�.
anticipated water table elevation
determine the applicability and d I;
performance of the exfiltration trench ffi Er:_
system, which has to be able to --
infiltrate the required stormwater
treatment volume and drawdown the m �a •�
treatment volume to return to its normal condition within a specific time after the design
storm event. When the trench Bottom is located at or above the average wet season water
table, the exfiltration trench is considered a dry system.
Exfiltration trenches, like other types of retention systems, are able to efficiently remove the
storm water pollutants. Additionally, exfiltration trenches contribute to recharge of the
ground water aquifer thus assist in combating saltwater intrusion in coastal areas.
Exfiltration tanks and trenches can vary considerably in size. Large underground
exfiltration designs generally utilize concrete and large pipe systems. Modular products are
available that are usually constructed of lightweight but durable plastic wrapped in a geo-
textile.
Water Quality
The exfiltration trenches to provide water quality treatment to a watershed can be installed
off -line or online in the drainage system. The off -line treatment method diverts runoff into
the exfiltration trench designed provide the required treatment volume; subsequent runoff in
excess of the treatment capacity bypass the off -line exfiltration trench towards the outfall. A
diversion drainage structure is usually required for off -lien systems. The on -line exfiltration
trench provides the required water treatment but the treatment volume is mixed with the
total runoff volume. As such, runoff volume in excess of the treatment capacity carries a
portion of the pollutant load to the receiving water body.
Water Protection Benefits
• Water conservation implications — Exfiltration systems do not benefit potable
water supplies directly, but do assist in groundwater recharge and reducing some
demand by on site vegetation.
• Stormwater implications — Sub - surface infiltration systems such as exfiltration
tanks reduce peak veloCity and volume of stormwater runoff. When significant storage
volumes and mitigation of peak runoff veloCity are attained, zero stormwater discharge
from the lot may be achieved. This in turn can reduce the size of the centralized
stormwater retention ponds.
Operations and Maintenance (O and M)
Successful operation depends on maintaining the percolation rate of the trench's sides and
bottom. The keys to long -term performance are accurate estimation of percolation rate,
proper construction, pretreatment, offline design, and maintenance accessibility. Exfiltration
trenches can become clogged, so it is important to prevent sediments and materials from
entering the system as much as possible and periodically remove those that accumulate.
Frequency of clogging is dependent on effectiveness of pretreatment, such as vegetative
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buffer strips and street sweeping, at removing sediments. Accumulated sediments need to
be removed from the pipe to allow percolation into filter media. If filter media becomes
clogged, it can be expensive to remove pipe and replace media to allow for proper
percolation.
Access for maintenance should be considered in the design, potentially including an
observation well of PVC pipe leading to the bottom of the trench to allow for monitoring of
the drawdown rate. Some systems incorporate an underdrain below the filtering system,
which can be used as an overflow should clogging occur.
Applications of Exfiltration Trenches
• Residential lots
• Commercial development
• Parking lots
• Green spaces
• Golf courses
Benefits of Exfiltration Trenches
• They mimic the natural groundwater recharge capabilities of the site.
• Are relatively easy to fit into the margins, perimeters, and other space -
constrained areas of a development site, including underground pavement.
• Can provide offline treatment for environmentally sensitive waters (e.g., Class I,
Class II, or OFW).
• Can be used to retrofit already developed sites where space is limited.
• Detention
• Infiltration
• Stormwater reuse
• Groundwater recharge
• Runoff attenuation
• Reduction in peak veloCity
• Reduction in stormwater runoff volume
• Possible reduction in size of central stormwater retention ponds
Potential Limitations of an Exfiltration Trench
• Require highly permeable soils to function properly.
• Difficulties in keeping sediment out of the structure during site construction.
• Not recommended for clayey or highly erodible soils.
• Have relative short life spans before replacement or extensive
restoration /maintenance of system is required.
• Often more costly than other treatment alternatives, especially when operation
and maintenance costs are considered.
Shallow Grassed Swales
In the context of BMPs to improve water quality, the term swale (a.k.a. grassed channel, dry
swale, wet swale, bio filter, or bios wale) refers to a vegetated, open channel management
practices designed specifically to treat and attenuate stormwater runoff for a specified water
quality volume. As stormwater runoff flows along these channels, it is treated through
vegetation slowing the water to allow sedimentation, filtering through a subsoil matrix,
and /or infiltration into the underlying soils. Variations of the grassed swale include the
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grassed channel, dry swale, and wet swale. The specific design features and methods of
treatment differ in each of these designs, but all are improvements on the traditional
drainage ditch. These designs incorporate modified geometry and other features for use of
the swale to treat and convey stormwater runoff.
Grassed swales can be applied in most situations with some restrictions. Swales are well
suited for treating highway or residential road runoff because they are linear practices.
They can also be used to provide a low -cost drainage option for farms, industrial, and
commercial areas. Swales are also useful as one of a series of stormwater BMPs or as
part of a treatment train, for instance, conveying water to a detention pond and receiving
water from filter strips. Furthermore, swales are highly recommended by proponents of
design approaches such as Low Impact Development (LID) and other green designs.
Grassed swales can be used as a retrofit. A stormwater retrofit is a stormwater
management practice (usually structural) put into place after development has occurred to
improve water quality, protect downstream channels, reduce flooding, or meet other specific
objectives such as reducing loadings to comply with a TMDL waste load allocation. One
retrofit opportunity using grassed swales modifies existing drainage ditches. Ditches have
traditionally been designed only to convey stormwater. In some cases, it may be possible
to incorporate features to enhance pollutant removal or infiltration such as check dames
(i.e., small dams along the ditch that trap sediment, slow runoff, and reduce the effective
longitudinal slope). Since grassed swales cannot treat a large area, using this practice to
retrofit entire water shed would be expensive because of the number of practices needed to
manage runoff from a significant amount of the watershed's land area. Designers need to
consider site conditions. In addition, they need to incorporate design features to improve
the longevity and performance of the practice while minimizing the maintenance burden.
Drainage Area
Grassed swales should generally treat runoff from small drainage areas (less than 5 acres).
If used to treat larger areas, the flows through the swale become too large to produce
designs to treat stormwater runoff in addition to conveyance.
Slope
Grassed swales should be used on sites with relatively flat slopes of less than 4 percent
slope; 1 to 2 percent slope is recommended. When site conditions require installing the
swales in areas with larger slopes, check dams can be used to reduce the influence of the
slope. Runoff velocities within the channel become too high on steeper slopes. This can
cause erosion and does not allow for infiltration or filtering in the swale. Grassed swales
can be used on most soils, with some restrictions on the most impermeable soils. In the dry
swale a fabricated soil bed replaces on -site soils in order to ensure that runoff is filtered as
it travels through the soils of the swale.
The required depth to ground water depends on the type of swale used. In the dry swale
and grassed channel options, the bottom of the swale should be constructed at least 2 feet
above the ground water table to prevent a moist swale bottom or contamination of the
ground water. In the wet swale option, treatment is provided by creating a standing or slow
flowing wet pool, which is maintained by intersecting the ground water. According to
SJRWMD, a swale is defined as a manmade trench that:
• Has a top width -to -depth ratio of the cross - section equal to or greater than 6:1 or
side slopes equal to or greater than 3 feet horizontal to 1 foot vertical.
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• Should contain contiguous areas of standing or flowing water only following a
rainfall event, thus the system is normally "dry."
• Is designed to percolate 80 percent of the 3- year /1 -hour storm (approximately
2.3 inches in 1 hour) within 72 hours.
• Is planted with or has stabilized vegetation suitable for soil stabilization,
stormwater treatment, and nutrient uptake.
• Is designed to take into account the soil errodability, soil percolation, slope
length, and drainage area to prevent erosion and reduce the pollutant
concentration of any discharge.
Design Considerations
Although there are different design variations of the grassed swale, there are some design
considerations common to all designs. An overriding similarity is the cross - sectional
geometry. Swales often have a trapezoidal or parabolic cross section with relatively flat
side slopes (flatter than 3:1), though rectangular and triangular channels can also be used.
Designing the channel with flat side slopes increases the wetted perimeter. The wetted
perimeter is the length along the edge of the swale cross section where runoff flowing
through the swale contacts the vegetated sides and bottom. Increasing the wetted
perimeter slows runoff velocities and provides more contact with vegetation to encourage
sorption, filtering, and infiltration. Another advantage to flat side slopes is that runoff
entering the grassed swale from the side receives some pretreatment along the side slope.
Design Variations to the Grassed Swale
There are variations to the Grassed Swale and they include the grassed channel, dry
swales, and wet swales. Of the three grassed swale designs, grassed channels are the
most similar to a conventional drainage ditch, with the major differences being flatter side
slopes and longitudinal slopes, and a slower design veloCity for water quality treatment of
small storm events. Of all the options, grassed channels are the least expensive but also
provide the least reliable pollutant removal. The grassed channel is a flow- rate -based
design. Based on the peak flow from the water quality storm (this varies regionally, but a
typical value is the 1- inch /24 hour storm), the channel should be designed so that runoff
takes, on average, 10 minutes to flow from the top to the bottom of the channel.
Dry Swales
Dry swales are similar in design to bio retention areas. These incorporate a fabricated soil
bed into their design. The native soil is replaced with sand /soil mix that meets minimum
permeability requirements. An underdrain system is installed at the bottom of the soil bed.
This underdrain is a gravel layer that encases a perforated pipe. Stormwater treated in the
soil bed flows into the under drain, which routes this treated stormwater to the storm drain
system or receiving waters. Dry swales are a relatively new design, but studies of swales
with a native soil similar to the man -made soil bed of dry swales suggest high pollutant
removal.
Wet Swales
Wet swales intersect the ground water and behave similarly to a linear wetland cell. This
incorporates a shallow permanent pool and wetland vegetation to provide stormwater
treatment. The wet swale also has potentially high pollutant removal. Wet swales are not
commonly used in residential or commercial settings because the shallow standing water
may be a potential mosquito breeding area
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Maintenance Considerations of Grass Swales
Maintenance of grassed swales mostly involves litter control and maintaining the grass or
wetland plant cover. Typical maintenance activities are as follows:
• Inspect pea gravel diaphragm for clogging and correct the problem
• Inspect grass.alongside slopes for erosion and formation of rills or gullies and
correct.
• Remove trash and debris accumulated in the inflow forebay.
• Inspect and correct erosion problems in the sand /soil bed of dry swales.
• Based on inspection, plant an alternative grass species if the original grass
cover has not been successfully established.
• Replant wetland species (for wet swale) if not sufficiently established.
• Rototill or cultivate the surface of the sand /soil bed of dry swales if the swale
does not draw down within 48 hours.
• Remove sediment build -up within the bottom of the swale once it has
accumulated to 25 percent of the original design volume.
• Mow grass to maintain a height of 3 -4 inches.
Effectiveness of Grass Swales
Structural stormwater management practices can be used to achieve four broad resource
protection goals. These include flood control, channel protection, ground water recharge,
and pollutant removal. Grassed swales can be used to meet ground water recharge and
pollutant removal goals. Grassed channels and dry swales can provide some groundwater
(subsurface water contained in the interconnected pores below the water -table of an
aquifer) recharge (process by which aquifers are replenished with water from the surface)
as infiltration is achieved within the practice. Wet swales, however, generally make little, if
any, contributions to ground water recharge. Infiltration is impeded by the accumulation of
debris on the bottom of the swale. A number of factors influence the rate of recharge
including the soil type, plant cover, slope, rainfall intensity, and the presence and depth of
confining layers and aquifers. Most of Florida's groundwater recharge occurs in the
summer months when precipitation is highest. Recharge also occurs with locally heavy
rainstorms during the rest of the year. Groundwater typically discharges into a lake or river,
maintaining its level or flow in dry seasons.
Benefits of Shallow Grassed Swales
• Usually less expensive than installing curb and gutters, and usually less
expensive than other water quality treatment controls.
• Hardly noticeable if shallow swales (0.5 to 1.0 ft. maximum depth) are designed
and constructed with gradual slopes (4:1 to 6:1).
• Can provide off -line treatment for environmentally sensitive waters (e.g. Class I,
Class II., or OFW).
• Can reduce peak rates of discharge by storing, detaining, or attenuating flows.
• Can reduce the volume of runoff discharged from a site by infiltrating runoff with
a raised inlet or check dam.
• Maintenance can be performed by the adjacent landowner.
• Can be used in space- constrained areas such as along lot lines, rear of lots, and
along roadside.
• Can be used as water quality treatment or pretreatment with other BMPs in a
treatment train.
• Recovers storage and treatment volumes quickly wher4e soils are permeable.
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Can be used as recessed landscape areas (part of green space requirement),
and runoff collection becomes the source for irrigation and some nutrients
(saving money) provided the use does not impact long -term maintenance or
impact existing trees.
Limitations of Grass Swales
Grassed swales have some limitations, including the following:
• Grassed swales cannot treat a very large drainage area.
• Wet swales may become a nuisance due to mosquito breeding.
• If designed improperly (e.g., if proper slope is not achieved), grassed channels
will have very little pollutant removal.
• Effective only as a conveyance system in unsuitable soils.
• Possible nuisances such as odors, mosquitoes, or nuisance plant species can
occur if not designed, constructed or maintained.
• Aesthetically unpleasing if improperly designed and constructed (deep with
steep side slopes — looks like a ditch).
• Improperly designed swales may also negatively impact the frequency of
maintenance by the responsible entity.
• May not be suitable or may require geotextile matting in areas that serve as
vehicle parking areas.
Water Quality Inlets and Baffle Boxes
Water quality inlets (WQls), also commonly called trapping catch basins, oil /grit separators,
consist of one or more chambers that promote sedimentation of course materials and
separation of free oil (as opposed to emulsified or dissolved oil) from storm water. The first
provides effective removal of coarse particles and helps prevent premature clogging of the
filter media. A second chamber contains a sand filter to provide additional removal of finer
suspended solids by filtration.
Water quality inlets rely on settling to remove pollutants before discharging water to the
storm sewer or other collection system. They are also designed to trap floating trash and
debris. When inlets are coupled with oil /grit separators and /or hydrocarbon absorbents,
hydrocarbon loadings from high traffic/parking areas may be reduced. However, experience
has shown that pollutant - removal effectiveness is limited, and the devices should be used
only when coupled with extensive clean -out methods (Schuler et al., 1992). Maintenance
must include proper disposal of trapped coarse - grained sediments and hydrocarbons.
Clean -out and disposal costs may be significant. Catch basins are water quality inlets in
their simplest form. They are single chambered inlets with a lowered bottom to provide 2 to
4 feet of additional space between the outlet pipes for collection of sediment at the bottom
of the structure.
Selection Criteria
Applicable too many sites, including high density areas with poorly drained soils
and extensive impermeable areas.
Small Drainage area.
Flexibility to retrofit existing drainage areas with minimal or no additional land
requirement.
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Limitations
• Pollutant removal effectiveness is limited, and the devices should be used only
when coupled with extensive clean -out methods.
• Not effective for water quality control during intensive storms.
Design and Sizing Considerations
• Retrofitting devices can be installed in any shape or size of grate or cub inlet.
Accurate measurement of inlets must be taken to ensure proper fit.
• Should not obstruct flow or cause excessive hydraulic head losses.
• Need removable grates or manholes to install and clean devices.
Inspection /Maintenance Considerations
High sediment loads can interfere with the ability of the WQI to effectively separate oil and
grease from the runoff. During periods of high flow, sediment can be suspended and
released from the WQI into surface waters. Maintenance of WQls can be easily neglected
because they are underground. Establishment of a maintenance schedule is helpful for
ensuring proper maintenance occurs. The required maintenance effort will be site - specific
due to variations in sediment and hydrocarbon loading. Since WQI residuals contain
hydrocarbon by- products, they may require disposal as hazardous waste. Many WQI
owners coordinate with waste haulers to collect and dispose of these residuals.
Separation Devices
Separation devices include sumps, baffle boxes, oil /grit separators, and sediment basins to
capture trash, sediments, and floating debris. They are efficient only within specific ranges
of volume and discharge rates. Control units usually have a forebay to pretreat discharges
by separating heavy grit and floating debris before it enters the separator. Separation
processes use gravity, vortex flow, centrifugal force, and even direct filtration. Further
treatment may be accomplished by adding chemicals such as alum. After separation, the
sediment is collected and transported or pumped to a waste treatment facility. These
devices may have a high initial investment cost.
Nutrient Baffle Boxes
Nutrient baffle boxes are concrete or fiberglass structures containing a series of sediment
settling chambers separated by baffles. The primary function of baffle boxes is to remove
sediment, suspended particles, and associated pollutants from stormwater. Baffle boxes
may also contain trash screens or skimmers to capture larger materials, trash, and
floatables. Baffle boxes are located either in -line or at the end of storm pipes. The use of
baffle boxes for pollutant removal is based on the concept of slowing the flow veloCity
through the box, thereby allowing solids and associated pollutants to settle to the bottom of
the box. Stormwater enters the box and begins to fill the first chamber. As water
encounters the baffles, flow veloCity decreases, allowing particles with a settling veloCity to
settle to the bottom of the box. In addition to decreasing flow velocities, the baffles impede
particle movement. As suspended solids strike the baffles, they begin to settle. Larger
particles usually settle out first and accumulate in the first chambers while smaller particles
usually settle out in subsequent chambers.
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Nutrient baffle boxes have proven effective in removing sediment from stormwater runoff.
They are mainly utilized in areas where sediment control is a primary concern, while other
stormwater BMPs may be more effective in areas where additional stormwater pollutants,
such as dissolved nutrients, oil and grease, or metals, are prevalent
Nutrient baffle boxes are ideally suited for retrofitting into existing storm pipes. Baffle boxes
for pipes up to 48 inches in diameter can be precast, making installation quick and cost -
efficient. Baffle boxes can be used for pipes up to 60 inches in diameter, but these boxes
must be cast in place, making them more expensive and time - consuming to install. Baffle
boxes are principally designed for sediment removal, but trash racks, screens, or skimmers
can be installed to trap floatables and oil and grease as well.
Design Criteria
The design concept of a sediment
(baffle) box is similar to the design of
a three - chamber water quality inlet
(also known as an oil /grit separator).
Typical baffle boxes are 3 to 5
meters (10 to 15 feet) long, 0.6
meters (2 feet) wider than the pipe,
and 2 to 2.7 meters (6 to 8 feet) high.
Weir height is usually 1 meter (3
feet).
Weirs are usually set at the same
level as the pipe invert to minimize
hydraulic losses. Manholes are set
over each chamber to allow easy
access for cleaning and
maintenance. Manholes should be located
access by vacuum trucks for box maintenance.
within 15 feet of a paved surface to allow
The design of the baffle box can be modified to promote easy cleaning and to prevent
nutrient leaching from accumulated biota. Some fiberglass baffle boxes have been
designed to include sliding gates on both ends. These gates are closed during cleaning to
block flow, allowing removal of accumulated sediments and trash without vacuuming up
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incoming or residual flows. These baffle boxes also have rounded bottoms that cause
accumulated sediment to collect in the middle of the box, making it easier to vacuum it out.
Baffle boxes can also be designed with aluminum screens installed below the inflow pipe
but above the baffles. In this design, incoming flow drops through the screen, trapping
trash, yard waste, and other debris away from the accumulating water below. Leaching is
reduced because this debris is kept out of standing water. Therefore, there is less chance
of introducing nutrients into the outflow. Trash deflectors are set at the outflow end of the
box, reducing the chance of carrying garbage out with excess flow. Preliminary modeling
by the Florida Institute of Technology indicates that these screens do not become clogged
even under heavy loads of debris (unpublished data reported by Sun tree Technologies,
Inc. 2000).
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As flow accumulates in the first chamber, it is forced over a baffle into the next chamber.
Flow deflectors at the top of the baffle reduce the possibility of sediment being carried from
one chamber to the next. Flow exits through the outlet pipe. Possible modifications to a
standard baffle box design to accommodate site - specific conditions include: A two- chamber
box for small pipes and small drainage areas; A three - chamber box for larger pipes; and
Two multi- chambered boxes in a series.
These design modifications have not been fully studied. However, the Florida Institute of
Technology used hydraulic scale - modeling to evaluate box size and shape, along with
baffle size and placement, on pollutant removal efficiency. Using three, four; and five -
chambered baffle boxes, this study evaluated the sediment removal efficiencies of fine and
coarse - grained sediments under several typical flow rates and sediment concentrations.
The researchers also evaluated the effect of changing the depth of the box and raising the
height of the baffles. The results showed that, in general, adding more chambers to the box
did not increase sediment removal because each chamber became shorter, and thus
sediment did not settle out as efficiently. Re- suspension of sediments in the box was a
consistent problem because incoming flow disturbed sediments that had already settled,
causing them to be re- suspended and carried out of the settlement chamber. The study
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suggested that reducing re- suspension in the box would increase its overall efficiency, but
this has not been investigated.
Performance
Baffle boxes are an effective BMP to remove sediments from stormwater. Baffle boxes
have been shown to remove from 225 to 22,500 kilograms (500 to 50,000 pounds) of
sediment per month, depending on the sediment load feeding into the baffle box. However,
pollutant removal efficiencies (e.g., the percentage of pollutants removed by the BMP)
depend on factors such as land use, drainage basin area, soil types, stormwater velocities
through the box, and the frequency and thoroughness of box cleaning. Limited data exists
on the pollutant removal efficiencies of baffle boxes. Only one laboratory and one field
evaluation are complete, while several more field tests are scheduled for the future.
Benefits of the Nutrient Separating Baffle Boxes
• Fits Within Existing Easements
• Retrofits Existing Systems
• Easy & Quick To Install and Maintain
• Captures Foliage, Litter, Sediment & Hydrocarbons
• Separates Foliage and Litter From Water & Sediment
• Will Not Go Septic Between Storm Events
• Captures thousands of pounds of sediment, debris and gross pollutants
• Excellent treatment structure for Recharge wells
Stormwater Quality Improvement Project — Nutrient Separating Baffle Box
Skimmers
Oil and grease simmers are a cost - effective method of prohibiting oil and grease from
following onto receiving water bodies. Oil and grease skimmers are easily installed and
maintained. Skimmers should also be considered in the design phase of all
storage /treatment facilities such as the wet detention ponds. The SJRWMD requires the
use of skimmers or baffles at BMP outlets where oil and grease are expected (e.g.,
gasoline station) and where the upstream tributary has more than 50 percent of impervious
surfaces. The skimmers are designed to retain the oils and greases at the surface of the
retention /detention system.
C. Non - Structural BMPs
Non - structural BMPs are practices that improve water quality by reducing the accumulation
and generation of potential pollutants at or near their source. They do not require
construction of a facility, but instead provide for the development of pollution control
programs that include prevention, education and regulation. These can be classified as
follows:
• Planning and regulatory tools
• Conservation, recycling and source controls
• Maintenance and operational procedures
• Educational and outreach programs
The Sebastian City Council recently had an education presentation on the City's erosion
control illicit discharge relating to the National Pollutant Discharge Elimination System
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( NPDES) which the City now has an ordinance relating to the City's NPDES and the
National Clean Water Act.
Other samples of educational campaigns include; posters, brochures and webpages.
NIN
Government Agencies and Regulatory Programs
All government agencies play an important role in establishing programs to address
stormwater pollution. Federal agencies are tasked with establishing nationwide programs.
The State of Florida has established regulations by adopting the appropriate Code of
Federal Regulations title into the Florida Statutes and the Florida Administrative Code.
Water management districts such as St John's Water Management District, function under
these codes and require agency permits for the construction and operation of storm water
management systems, storm water usage, or storm water quality monitoring plan.
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Local governments play a role in establishing regulatory programs that provide
opportunities to meet specific objectives. These measures must comply with state and
federal mandates and should address such issues as hazardous materials, codes, zoning,
land development and land use regulations, water shortage and conservation policies, and
controls on types of flow allowed to drain into sanitary municipal storm sewer systems. For
a local program to be successful, the following elements should be considered:
• Community /business composition
• Land use patterns
• Local practices
• Community concerns
• Institutional characteristics.
Ordinances are rules or laws issued by a local government under legal authority granted by
statutes. They include findings of fact, objectives or purposes, definitions, permitting
requirements, variances, performance /design standards, and enforcement policies.
Low Impact Development (LID)
LID is an approach to land development (or re- development) that works with nature to
manage stormwater as close to its source as possible. LID employs principles such as
preserving and recreating natural landscape features, minimizing effective imperviousness
to create functional and appealing site drainage that treat stormwater as a resource rather
than a waste product. There are many practices that have been used to adhere to these
principles such as bio retention facilities egetated rooftops, rain barrels, and permeable
pavements. By implementing LID principles and practices, water can be managed in a way
that reduces the impact of built areas and promotes the natural movement of water within
an ecosystem or water shed. Applied on a broad scale, LID can maintain or restore a
watershed's hydrologic and ecological functions. Hydrologic functions such as filtration,
frequency, and volume of discharges, and ground water recharge can be maintained by
reducing impervious surfaces, functional grading, open channel sections, reuse of runoff
and using multifunctional landscape features such as rain gardens, swales, mulch, and
conservation areas. LID has been characterized as a sustainable stormwater practice by
the Water Environment foundation and others (EPA, 2013).
Educational and Outreach Programs
Public education and outreach programs can be implemented to meet any individual or
community needs. The public is often unaware that the combined effects of their actions
can cause significant non -point source pollution problems. Outreach programs should be
part of a community's overall plan for stormwater management to educate employees, the
public, and businesses about the importance of protecting stormwater from improperly
used, stored, and disposed pollutants. Proper education on day -to -day activities such as
recycling of used automotive fluids, household chemical and fertilizer use, animal waste
control and other activities can significantly reduce non -point source pollutant loadings to
urban streams. A public education plan should consist of several kinds of activities that may
include the following:
Public surveys to assess use of toxic materials, disposal practices, and overall
environmental awareness.
Frequent and consistent campaign messages using a mission statement, logo,
and tag line.
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• Campaign products such as door hangers, pamphlets, guidebooks, signs, press
releases, or classroom /library displays.
• Public outreach activities such as having a field day where a local water quality
expert comes to a community to demonstrate ways of reducing pollution.
• Neighborhood programs such as the following:
• Identifying storm drains with stenciling.to discourage dumping.
• Distributing toxics checklist for meeting household hazardous waste
regulations.
• Producing displays and exhibits for school programs.
• Distributing free seedlings for erosion control.
• Creating volunteer opportunities such as water quality monitoring.
• Informing residents about picking up pet waste or installing pet waste bags and
containers
• Demonstrating to residents how to compost lawn debris.
• Distributing brochures about recycling of oil and antifreeze.
• Distributing brochures about pesticides and fertilizers.
See the City of Sebastian, Florida's website — http: / /sebastiannrb.com
Operation and Maintenance (O & M)
Maintenance programs are necessary in order to reduce the pollutant contribution from the
urban landscape and to ensure that stormwater collection and treatment systems are
operating as designed. Non - structural maintenance and operational procedures can be
used to prevent or reduce the need for more costly structural treatment controls. The
Florida Department of Environmental Protection has reported that nearly 70 percent of
existing treatment facilities in Florida are not properly maintained and therefore do not
provide the intended pollutant removal effectiveness. Because of this, one of the most
effective non - structural BMPs is routine maintenance of existing treatment facilities. The
following are a few of the non - structural maintenance operations:
• Turf and landscape management,
• Street cleaning,
• Catch basin cleaning,
• Road Maintenance,
• Canal /ditch maintenance, and
• Modification of structural operations.
Conservation Plan
All users (domestic, utility, commercial, agricultural, and recreational) of water have a
responsibility and an opportunity to conserve water, to reduce or eliminate the amount of
water potentially requiring stormwater runoff treatment. Conservation practices should be
promoted in all communities. A good conservation water plan should include a framework
for the following components (Marshall, 2002):
• Appropriate lawn irrigation.
• Adoption of landscape ordinances.
• Installation of ultra -low volume plumbing fixtures in new construction.
• Adoption of conservation- oriented rate structures by utilities.
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• Implementation of leak detection programs by utilities with unaccounted for
water loss greater than 10 percent.
• Institution of public education programs for water conservation.
Using Reclaimed Water
Recycling water involves disinfecting and treating wastewater and using the reclaimed
water for new, beneficial uses such as the following (Marshall, 2002):
• Landscape irrigation for parks, golf courses, highway medians, and residential
lawns.
• Agricultural irrigation for crops, pasture lands, and nursery operations.
• Ground water recharging either directly or through rapid infiltration basins
• Industrial cooling or in- manufacturing processes.
• Creating or restoring wetlands.
• Fire protection.
• Separate toilet piping systems in industrial or commercial buildings.
• Aesthetic enhancements for ponds, fountains, and landscape features.
• Dust control for construction sites or unpaved road communities.
Pesticide /Herbicide Use
Pesticides and herbicides can be a significant source of water quality impairment in urban
streams due to their high aquatic toxiCity. According to a 2011 report from the EPA, the
United States pesticide usage was 1.1 billion pounds in 2007 or 22 percent of the world
estimate of 5.2 billion pounds of pesticide use. The total pounds of U.S. pesticide used
decreased by approximately 8 percent from 1.2 to 1.1 billion pounds from 2000 to 2007.
The use of conventional pesticides decreased about 3 percent from 2002 to 2007 and 11
percent from 1997 to 2007 (EPA, 2011).
Herbicides remained the most widely used type of pesticide in the in the agricultural market
sector and were also the most widely used type of pesticide I the home and garden and
industrial, commercial, and governmental market sectors (EPA, 2011).
A significant portion of these applications find their way into stormwater runoff and
ultimately into receiving streams through spray drift, transport by soils, solubilization by
runoff, and by spillage, dumping and improper disposal of containers and residuals.
Education on the proper methods of application, application rates and alternatives to
pesticides can help to reduce the amount of pesticides that are carried by urban runoff.
Alternatives to pesticides, such as in integrated pest management program and pesticide
alternatives such as insecticidal soap or natural bacteria, can also reduce the need for
pesticides.
Fertilizer Use
A significant amount of nutrients in urban runoff results from misapplication of fertilizer to
the urban landscape. Residential lawn and garden maintenance and maintenance of
landscape and turf grass at golf courses, schools and commercial areas uses significant
amounts of fertilizers containing nitrogen and phosphorous. Since most fertilizers are water
soluble, over — application or application before rainfall events can allow significant quantities
to be carried away by stormwater runoff. Education on proper application of fertilizers can
help to reduce the quantities of nutrients reaching receiving waters.
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The City of Sebastian, Florida has an Ordinance (Ordinance No. 0- 12 -06) regarding the
use of fertilizer. See the City of Sebastian Website (www.Cityofsebastian.org) for further
details.
Automotive Product Disposal
Discharge of automotive fluids such as antifreeze and motor oil to storm drains or land can
cause significant water quality problems. "Do -it- yourself' automobile mechanics often
incorrectly assume that materials that are dumped into storm drains will receive treatment
at wastewater treatment plant prior to discharge. Education on appropriate recycling and
disposal techniques for these materials can help to reduce pollutant loadings to streams.
Education programs should identify the location of community automotive products
recycling centers. In addition to impacts associated with dumping used oil and antifreeze,
potential runoff pollutant sources from home automobile maintenance activities include dirt,
cleaners, oils and solvents from car washing, leaking fluids such as brake and transmission
fluid and gasoline spills.
City of Sebastian Website
upeaning BusNs
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4. Stormwater Quarter Rounds
December 6, 2013
L Quarter Rounds System
The Quarter Rounds are standard 12 -inch plastic pipe which is cut into four quarters with
holes that allow water to percolate into the grass swales to evenly drain away more
efficiently. The quarter section of pipe is connected together and pinned into the existing
grass swale. The existing swale is graded to provide a gradual slope. The pipe installation
is combined with culvert replacement as needed and driveway replacement by the City
crews.
The City started the pilot program for the quarter round system as an innovative measure to
assist in the managing storm water runoff within the residential streets of the City. The pilot
program was initiated with a field review meeting with consultants and members from the St
John's River Water Management agency to ensure that the project was permitted by the
management agency. At that field review meeting, it was noted by the SJRWMD staff that
a typical grass yard swale has the ability to infiltrate about 80 percent of the 3- year /1 hour
storm or approximately 2.3 inches within 72 hours. This treatment of grass swales and
percolation is questionable and it has been challenged that the treatment provided by the
grass swale is marginal as best for infiltration. The City currently is continuing to install
quarter rounds within the residential neighbors as a storm water management program.
In February, 2011, the City of Sebastian had
approximately 40 miles of quarter round installed.
There is more than 300 miles of stormwater quarter
round swales in the City of Sebastian. The use of
the quarter rounds is based on the severity of
drainage problems. The quarter rounds have
improved the stormwater system overall. Quarter
Rounds in Progress — 2012 show that the City has
installed over 3,400 linear feet of quarter rounds.
The City staff has indicated that many residents do
not take care or maintain the plastic quarter round within the yard swale frontage. In many
cases, these quarter round swales have become over grown with grass or the quarter
round plastic is buried in siltation of soil from the runoff. There were many locations where
the installed quarter rounds within the last year were covered with grass for silt. The City
has a contract grass cutting crew which cut and maintains the drainage swales.
ii. Testing of Quarter Rounds
As part of the stormwater update, the City's implementation of the quarter round system
was field tested to determine the infiltration and conveyance of stormwater runoff. A test
site was identified by City staff for an actual field condition testing of the quarter rounds.
The site scheduled was installed with quarter rounds within a two week period. The field
test required a pre- condition test and then a post- condition test with the new quarter round
installed. The City provided a tanker truck which carried 3,000 gallons of water to be
discharged into the test site for the pre and post condition tests. The site was measured
and timed for the drainage runoff of water from the starting point to where the drainage flow
ended along the existing grass swale.
Sebastian Stormwater Master Plan Update Page 44
NEEL- SCHAFFER
Solut ions you can build upon
December 6, 2013
Pre - condition Grass Swale:
March 27, 2013 was the initial test date for the grass swale. The nature of the grass swale
slope was very flat, which resulted in more ponding than movement. The water movement
only occurred when there was sufficient head for the water to move. The water pump truck
stopped discharging water at 1:53 pm, resulting in about 53 minutes of total pump time.
On March 28, 2013 another test was conducted and the ground was moist and partially
saturated from the 3/27 test. Water hit the 3/27 test 10 min water point at 7minutes and 25
seconds. The water pump stopped discharging water at 10:15 am, resulting in 15 minutes
of pump time. The water hit the 3/27 test's final water movement point (after 45min) after
only 20 minutes.
The vegetation and dry soil passed the 3/27 test's 45 min point resulted in higher
absorption and slower movement. The water stopped progressing at the 25 min mark. A
little water moved passed that, but was absorbed into the ground almost immediately.
Before Quarter.Round
After Installation
Post Condition Test with Quarter Round
The initial grass swale ground was very dry. Once the ground became saturated, the water
entered the quarter rounds. This took approximately 2 minutes. The slope was very small,
which resulted in more ponding than movement because the ground was very dry.
Movement in the quarter round only occurred when there was sufficient water head
(energy) to move the water, but once the water had more head (energy), it flowed freely on
the plastic quarter round.
The post testing was conducted twice since the original ground with new quarter round was
significantly dryer than normal condition. A second test was conduct to reflect a more
normal quarter round runoff and percolation conditions.
Sebastian Stormwater Master Plan Update Page 45
NEEL- SCHAFFER
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December 6, 2013
Dry Test Condition
The water pump stopped discharging water at 1:20 pm, resulting in about 14 minutes of
total pump time.
Wet Test Condition
The discharge of water began at 9:58 am on Tuesday, April 16, 2013 and ended at 10:15
am, resulting in 17 minutes of pump time.
The ground was moist and saturated from the rain that occurred on April 14, 2013.
(Amount of rain unknown.) Because of the saturation of the ground, the water entered the
Quarter Round at 10:00 am or 2 minutes after the water was discharged.
• At the 5 minute mark, the water had traversed 82 feet which was 44 feet (38 feet
mark) farther than the unsaturated test on April 11tH
• The water hit the 4/11 test's final water movement point after 13 minutes.
• Because the ground had been saturated from the rain on April 14, 2013, the water
traveled quickly down the Quarter Round. It traveled 294 feet in 25 minutes before
it came to a complete stop. The water was being absorbed the slower the water
moved.
The data collected from the four test sampling are tabulated on the chart below for the pre
and post condition tests with the velocity, flow and distance traveled by the flow. It is noted
that in both pre and post test samples, that the wet condition allowed for the flow of water to
travel farther along the swale and quarter rounds. (See appendix for calculations)
Pre'= Coridit'on 1`est. Post Condition Test
Test Factois' -
Collectd
Ma1cl12Z, 2013 March128;,�Ld1, Apr, l T1-c',2 Q',%3, ,April 17;, 2013
VeloCity 2.87 ft/min 4.48 ft/min 11 ft/min 11.8 ft/min
Flow 1 12.3 ft/sec 1 20.08 ft/sec 1 82.15 ft/sec 1 115 ft/sec
Distance 1 129 ft — 48 min 1 134 ft — 30 min 1 222 ft — 20 min 1 294 ft — 25 min
Based on the field testing, the storm water within the quarter round system traveled farther
than water traveling on grass swales. This travel distance is based on the roughness
coefficient of grass which is greater than the smooth surface of the plastic quarter round
material. In addition, water is conveyed a distance farther than water on the grass swale
with or without percolation. Therefore, the percolation rate for the quarter rounds is not
equal, but considerably less than the percolation rate for grass swales. Base on the data
collected, the water travels almost twice the distance in the quarter round system than on
the grass swale from 134 feet (pre) to 294 feet (Post). Therefore, the quarter round system
is conveying storm water with double the travel flow distance than grass swale. This means
the storm water within the quarter round system is approximately twice as fast in flowing the
runoff during peak storm events.
Sebastian Stormwater Master Plan Update Page 46
NEEL- SCHAFFER
Solutions you can build upon
5. Conclusion and Findings
December 6, 2013
10. Updated two systems
• Northern 1,769 acres
• Southern 4,583 acres
11. Third area not modeled - East of US 1
Capital Stormwater Projects — Presented to City Council in February 2013
Stormwater Projects
• A - Indian River Drainage Improvement
• B - Collier Canal Hardy Dam
• C - Potomac Ave Lateral Pipe
• D — Blossom/Wentworth Ditch Piping
• E - Stonecrop Pipe /culverts (Bevan & Laconia)
• I — Future South Area Basin
• J — Southeast Dredge Basin
• K — Tulip Pipe Replacement (culvert 42 "x72 ")
• L — East/West Lateral Drainage Pipe
• M — Tulip Detention Basin
• N — George St. Canal Dredging
• O — Rosebush Terrace Pipe — Twin 48"
• P — Landdowne Drive Pipe — Twin 36"
Total Stormwater Improvements
Annual Maintenance
F — Replace CMP — Maintenance
G — Quarter Rounds Installation
H — Backyard Ditches Maintenance
Total Stormwater Improvement Costs (per 2013)
Construction Cost
$2,000,000
$ 100,000
$ 200,000
$1,000,000
$1,000,000
$ 300,000
$ 500,000
$ 100,000
$ 500,000
$ 250,000
$2,500,000
$ 100,000
$ 100,000
$8,650,000
$ 200,000
$ 250,000
$1,000,000
$1,450,000
Sebastian Stormwater Master Plan Update Page 49
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14L NEEL- SCHAFFER
Solutions you can build upon
Quarter Round Base "Wet" Tests and
After Quarter Round Installation
Results and Notes
Prepared by:
Neel - Shaffer
March 28, 2013
April 11, 2013
2925B 20" Street • Vero Beach, FL 32960 • phone 772 -770 -4707 • fax 772 -770 -4640 • www.neel- schaffer.com
March 27th, 2013
First Quarter round base "wet" test:
Total: 4,500 gallons of water sprayed at the corner of Warren Street and Spire Avenue, in front
of 701 Spire Avenue.
Total distance of water movement:
1St discharge (3,000 gallons): 129' 0"
2 "d discharge (1,500 gallons): 84' 6"
Water Movement (from first discharge site):
Time (min)
Distance (feet)
10
49' 11"
15
691399
20
98' 1"
25
112' 6"
30
115' 3"
END OF 3,000 GAL. MOVEMENT
35 116' 4"
40 121' 1"
45 129' 0"
END OF ADDITIONAL 1,500 GAL. MOVEMENT
Percolation Rate = Amount of water (gal) / Percolation time (min):
4,500 gallons / 45min =100 gal/min
Velocity = Distance (ft) / Time (min):
129 ft / 45 min= 2.87 ft /min
Area (sf) = Width x Length
2' X 129' = 258 sf (square feet)
Flow Rate = Area (sf) x Velocity (ft/min):
258 sf x 2.87 ft/min = 740.46 cf (cubic feet) /min
NOTES:
Slope was very small, which resulted in more ponding than movement. Movement only occurred
when there was more head to the water.
Water pump stopped discharging water at 1:53 pm, resulting in about 53 minutes of total pump
time.
2 � NEEL— SCHAFFER
soi..�io... I...... bu71d ..no.,
March 28th, 2013
Second Quarter round base "wet" test:
Total: 3,000 gallons of water sprayed at the corner of Warren Street and Spire Avenue, in front
of 701 Spire Avenue.
Total distance of water movement:
From discharge point to end movement point: 134' 6 ".
Water Movement (from discharge site):
Time (min) Distance (feet)
10 62' 9"
15 98' 3"
20 129' 4"
25 134' 0" "Water movement stopped here
30 134'6"
END OF 3,000 GAL. MOVEMENT
Percolation Rate = Amount of water (gal) / Percolation time (min):
3,000 gallons / 30 min =100 gal /min
Velocity = Distance (ft) / Time (min):
134.5 ft / 30 min= 4.48 ft /min
Area (sf)= W x L
2 X 134.5 = 269 sf
Flow Rate = Area (sf) x Velocity ( ft/min):
269 sf x 4.48 ft/min = 1205.1 cf/min
NOTES:
The ground will be moist and partially saturated from the 3/27 test.
Water hit the 3/27 test 10 min water point at 7minutes and 25 seconds.
Water pump stopped discharging water at 10:15 am, resulting in 15 minutes of
pump time
The water hit the 3/27 test's final water movement point (after 45min) after only
20 minutes.
The vegetation and dry soil passed the 3/27 test's 45 min point resulted in higher
absorption and slower movement.
The water stopped progressing at the 25 min mark. A little water moved passed
that, but was absorbed into the ground almost immediately.
RL NEEL— SCHAFFER
3 _ 5.1 Non.... can bu11d upon
April 111h, 2013
First Quarter round base test with quarter round installed:
Total: 3,000 gallons of water sprayed at the corner of Warren Street and Spire Avenue, in front
of 701 Spire Avenue.
Total distance of water movement:
Discharge (3,000 gallons): 222'
Water Movement:
Time (min)
Distance (feet)
5
38'
10
111'
15
174'
20
222'
Percolation Rate = Amount of water (gal) / Percolation time (min):
3,000 gallons / 30 min =100 gal/min
Velocity = Distance (ft) / Time (min):
222 ft / 20 min= 11.1 ft /min
Area (sf) = W x L
Area = 222' x 2' = 444 sf
Flow Rate = Area x Velocity (ft/min):
444 sf x 11.1 ft/min = 4928.4 cf/min
NOTES:
Quarter rounds were installed. (A quarter round is a piece of plastic piping with wholes for
percolation cut into quarters and installed in a swale.)
The ground was very dry. Once the ground became saturated, the water entered the quarter
rounds. This took approximately 2 minutes. The slope was very small, which resulted in more
ponding than movement because the ground was very dry. Movement in the quarter round only
occurred when there was more head to the water but once the water had more head, it flowed
freely on the plastic quarter round. The water pump stopped discharging water at 1:20 pm,
resulting in about 14 minutes of total pump time.
4 RL NEEI— SCHAFFER
ms.4,.1.... r..... 4.116 .P.n
Page 1 of 2
Jeanette Williams
From: Sally Maio
Sent: Tuesday, January 28, 2014 8:44 AM
To: Andrea Coy; Bob McPartlan; Jerome Adams; Jim Hill; Richard Gillmor
Cc: Jeanette Williams
Subject: FW: Stormwater Master Plan and Addendum
Council - FYI in regard to Stormwater Master Plan approved in December.
Sally
From: Sally Maio [mailto:smaiol @comcast.net]
Sent: Monday, January 27, 2014 6:52 PM
To: Frank Watanabe
Cc: Sally Maio; Joseph Griffin; Jerry Converse; Tim Walker
Subject: Re: Stormwater Master Plan and Addendum
Thanks Frank. That's we'll do then.
Sally
On Jan 27, 2014, at 4:53 PM, Frank Watanabe <frank.watanabekneel- schaffer.com> wrote:
Sally,
I would scan and record the Council approved final Stormwater master plan as of 12/13/13.
Any comments and revisions by SJRWMD can be filed as an updated final with comment
revisions by SJRWMD.
Frank Watanabe
Neel- Schaffer, Inc.
(772) 770 -4707
From: Sally Maio rmailto :smaio @cityofsebastian.orgl
Sent: Monday, January 27, 2014 4:48 PM
To: Frank Watanabe
Cc: Joseph Griffin; Jerry Converse; Tim Walker
Subject: Stormwater Master Plan and Addendum
Frank - Has the approved Stormwater Master Plan and Addendum been signed off on by
SJRWMD yet? We need to know if the plan approved by Council on 12 -11 -13 can be scanned
as final into LaserFche.
Sally
Sally Maio, MMC
City Clerk
Sebastian, FL 32958
(772) 388 -8214
smaio cityofsebastian.org