HomeMy WebLinkAbout2018 - Updated
City of Sebastian
Stormwater Management
Master Plan Update
Updated
August 30, 2018
Prepared by:
CWT Engineering, LLC.
___________________
Frank Watanabe
Florida PE 66735
CWT Engineering, LLC
August 30, 2018
Table of Contents
1. Introduction ....................................................................................... 3
i. Background
ii. Purpose
iii. Modeling Approach
2. Data Collection and Methodology ..................................................... 8
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) Updated drainage projects from 2006 to 2018
d) Update 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 ............................................... 42
i. Testing and Analysis of Quarter Rounds
5. Conclusion ........................................................................................ 45
i. 2018 Storm water Map
6. Appendix 2015 Monitoring and Testing Award ............................... 47
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1. Introduction
i. Background
The City of Sebastian is located in the northern section of Indian River County adjacent to
the St. Sebastian Rivers and the North County limit. The current population is approximately
22,000. 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
US 1, I-95, the Florida Turnpike.
The original stormwater drainage model developed in 2004 had a base model created in
1996 by Craven Thompson & Associates. The consulting firm CDM, Inc. was retained in
2004 to develop the ICPR model to identify citywide drainage improvements. In 2010, Neel-
Schaffer, Inc. was retained to update the model using the latest drainage program and
verifying drainage improvements with changes in FEMA topographic data files. The 2013
model used new topographic maps to help improvement terrain modeling. In 2017, the City
retained CWT Engineering, LLC, to update the improvement projects and to submit to
SJRWMD for permit determination. SJRWMD has reviewed and determined this type of
comprehensive master plan report does not meet the requirements for Environmental
Resource Permit or any other permits, so this application has been closed with SJRWMD.
In performing drainage studies, a modeling program known as Interconnected Pond Routing
(ICPR) is typically used to forecast storm events and for the 2013 update, the latest version
of ICPR3 was used. The ICPR3 program is an engineering software tool to analyze the flood
routing through complex networks of interconnected and hydraulically interdependent
drainage ponds and lakes 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 drainage model network.
into this report. Quarter Rounds program uses plastic pipes cut into quarters and a quarter
of the pipe is installed along the existing residential drainage swales to convey storm water
runoff and help to filter pollutants. This program was initiated in 2006 as an experimental
cancelled by the City Council in 2017.
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. According to the Florida Department of
Environmental Protection Agency (FDEP), the ry 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
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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 drainage 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 from any new drainage
improvement projects. These parameters were used to analyze the previous drainage model
and used to create the 2013 drainage model.
The purpose of the 2018 master plan update is to review the 2013 updated report which
inventoried and characterized the previous 2004 Stormwater Management System (SWMS).
The 2013 model created revised hydrologic parameters of the basins and then updated the
2004 model with revised basin zones and to identify areas that have indicated flooding and
develop alternatives to alleviate both flooding and water quality problems.
The 2013 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
design of the regional Sebastian Stormwater Park into the drainage 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 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
drainage 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 system. The
time of concentration, stage/storage relationships, and sub-basins were calculated for the
entire City per the 2013 analysis. In the 2013 modeling analysis, the results provided a
summary of drainage deficiencies and a list of future stormwater improvements. The 2013
project list was reviewed and updated with 2018 drainage projects.
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iv. Modeling Approach
The 2013 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, storm water projects from 2004 to 2013 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
Version 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 CityStormwater Park
within the center of the City on a 166-
intended to provide water quality treatment to surface water in the City Stormwater
Management System that was previously untreated.
Additionally, the City has also prepared a separate stormwater master plan for the Sebastian
Municipal Airport which includes the municipal golf course. A copy of the Airport Stormwater
Master Plan is available at the Sebastian Municipal Airport offices. The Sebastian Airport
and the Riverfront Areas were not included in the original or updated 2013 storm water master
plan.
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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.
iii. Hydrologic Model
The ICPR Version 3 stormwater model was used in the analysis. 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
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
ue 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.
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 CityPrimary Stormwater Management system
(PSWMS), in fact discharges directly to the South Prong of the Sebastian River.
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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
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.
Documentation of methodology for the modeling analysis is available in various
-55 publnical
Publication (TP) 85-5. During the model development stage, s appeared to
be inconsistent with the hydrological unit size and apparent flow length specified in the
previous 2004 report. New 2013 calculated based on the new 2-foot
topographic data and compared with the original model data. The 2-foot topographic map
of Sebastian is shown on Page 11.
Overall approximately 95% of the 2013 TC new ICPR3 were rerun data created
from the new model network. The original and updated TC valves were checked to
reflect the Hydrologic Units zones and Time of Concentration.
The revised 2011 Hydrologic Units maps is shown on Page 12.
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.
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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 which used 5 foot contour
for topographic data. Revisions were made to certain structures based on the new 2 foot
topographic data and information obtained from Google Earth Street View for cross-drain
sections. The revised topo data from Google Street View identified crossings that were
not part of the original model and have been updated in the new ICPR3 model.
b) New Drainage Projects and Programs 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 and
programs include the following:
Twin Ditches Project
City Storm Water Park
Collier Canal Dredging and seawall
Davis Street Drainage and Baffle Box
New Quarter Round Installation
Replacement of Damaged Drainage Pipes
Maintenance of existing ditches and swales
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c) Sebastian Drainage Projects from 2006 to 2017
The following is a list of recent City drainage project from 2006 to 2017 and the ones with
SJRWMD permits are shown below.
Projects SJRWMD Permit No.
1. 2006 Davis Street Drainage/Baffle Box 98724-1
2. 2008 Periwinkle Detention Pond 103638-1 & DEP permit
3. 2009 George Street Drainage 18714-4
4. 2009 Collier Canal Dredging/Seawall 104663-2 & DEP permit
(Section 319 Grant Funds)
5. 2009 Potomac Street Baffle Box 119623-1 & DEP permit
6. 2009 Schumann Park Imp./Drainage 40952-2
7. 2010 Barber Sport Complex Drainage 40775-4
8. 2012 City Storm Water Park Update No permit
9. 2013 Quarter Round Review No permit
10. 2013 Stormwater Master Plan Update No permit
11. 2014 Coolidge Street Baffle Box 135385-1
12. 2014 Presidential Street Drainage 130339-1
(DEP TMDL Grant Funds)
13. 2014-2017 Water Monitoring/Testing No permit
14. 2014-2016 Seawall Investigations No permit
15. 2015 Northern Area Ditch Cleaning No permit
16. 2016 DEP MS4 NPDES update MS4 permit
17. 2016 Tulip Detention Pond 134274-1 & ACOE permit
(DEP TMDL/Section 319 Funds)
18. 2016 Working Waterfront Baffle Box 145977-1 & DEP permit
(SJRWMD grant and TMDL grant)
19. 2016 Oyster Bag Pilot Project 142124-1 & ACOE permit
20. 2016 CavCorp Parking Drainage 142058-2
21. 2017 Jefferson Street Drainage Repair FDOT US 1 RW Permit
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c) Update 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 2004 model analysis which included the original ICPR Model runs,
the 2013 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. Therefore, due to limited topographic data, 2004 model runs had some
limitations on accuracy.
As part of the 2013 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.
As noted, the original model developed by CDM was reviewed and then compared and
validated 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. These IRCP output reports provides for the data of the
stage/storage areas of the nodes and the sub-basin determination which includes basin
area and time of concentration values.
This 2018 report which is mainly an update to the 2013 master plan provides revisions to
the capital drainage projects with updated 2018 construction costs. The report was
modified for any technical change, program updates and any completion of drainage
projects. The 2013 maps was also updated with new 2018 Stormwater Map.
The 2018 Stormwater Master Plan was submitted to SJRWMD for permit determination.
Based on technical review by SJRWMD engineers, this type of comprehensive study
does not meet their requirements for permitting, so no permit was issued at this time. As
individual projects identified in the study is ready for construction, then each project will
need to be submitted for Environmental Resource Permit.
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3. Engineering Analysis
i. Stormwater Model Analysis
The original citywide stormwater model developed by the previous consultant CDM and
updated in 2004, 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 2013
modeling analysis reviewed and updated the previous simulation analyses. In addition, the
2013 model simulations conducted changes in the hydraulic conditions based on stormwater
improvements from 2006 to 2010 and additional topographic data.
The hydraulics analysis for both the existing and future land use conditions was analyzed
with new ICPR3 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 2013 modeling was 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 2013 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. The deficient storm water areas identified by the
model analysis were field verified and typically most were lack of conveyance for storm water
runoff. In many of the flooding area, the identified improvements to the drainage system
would resolve the situation.
In addition to identifying the deficient drainage areas within the City, the stormwater system
needed to be updated base on recent drainage improvements 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 is divided by County Road (CR) 512 into two drainage groups for the northern and
southern drainage systems. The 2013 model reanalyzed the previous subgroupings of
drainage areas establishing the two major groups to better analyze and quantify the
deficiencies. In addition to these two drainage groups, the City has a third area east of the
existing railroad tracks. This eastern section (Riverfront Area) of the City of Sebastian was
not included in the analysis of the two groups and was never modeled in any previous
drainage studies. The area of this third group is defined by the area east of the railroad tracks
to the Indian River Lagoon. This section of Seront Area
part of the City.
ii. Levels of Service
As part of the drainage management update, there is a need to address the drainage 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
water park.
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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
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.
Keeping the citizens of the City safe from flooding is the primary focus of the study. In addition
the City needs to maintain emergency and evacuation route access. Level of Service
requirements include retrofits to address known flooding problems. The decisions made
directly affect the size and cost of any recommended alternative and have been formulated
to establish improvement goals. The City does not have a defined level of service for
stormwater management in the City
following criteria were used to define flooding when analyzing the results of the stormwater
model:
Top of road elevations were exceeded for the 25-year/24-hour or 100-year/24-
hour storm event; or
Top of channel bank elevations were exceeded for the 25-year/24-hour or 100-
year/24-hour storm event
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
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 three photographs which are
identified in the 2013 Drainage Map.
The modeling analysis identified the drainage basins which need improvements to meet the
required 25 year/24 storm period to ensure conveyance and minimize flooding. Based on
reviewing the past model outputs and identified areas of improvements, the previous
improvement projects were reviewed and validated as still needed. The 2013 list of
improvement projects were also checked for ones which have been completed and new ones
which recently become issues in the field.
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The following is an updated list of 2018 capital projects identified in the 2013 modeling and
by actual field conditions as needed improvements. These projects will be included in the
capital improvements 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.
Projects Costs
1. Elkcam, George, Schumann Seawall Repair/Replace $16,000,000
2. Ocean Cove Drainage Repair $278,500
3. Day Drive Retention $241,000
4. Schumann Pipe Lining $200,000
5. Pelican Island Place Drainage $475,000
6. Stonecrop Drainage Ditch $1,000,000
7. Blossom Ditch Piping $1,000,000
8. Empress Canal Pipe $250,000
9. Lansdowne Box Culvert $178,000
10. Rosebush Box Culvert $242,000
11. Tulip Box Culvert $232,000
12. Albatross Box Culvert $178,000
13. Bayfront Box Culvert $178,000
14. Benedictine Ditch Piping $1,515,000
15. Bryant/Friar Court Retention Pond $315,000
16. Oyster Point Drainage Box $100,000
17. Hardy Dam Retrofit Gates $150,000
18. Potomac Ditch Piping $393,000
19. Potomac Baffle Retrofit for filter $60,000
20. Davis Street Baffle Retrofit for filter $60,000
21. Rosebush Retention Basin $315,000
22. Indian River Drive Drainage Improvements $1,000,000
Realign Mulligan pipe, baffle box at Southern outfall
Baffle box at Jackson outfall/clear outfall area
Total Drainage Cost $24,360,500
The above listed of drainage improvement projects are illustrated on the update 2018
Stormwater Master Plan map shown on the following page 20.
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Best Management Practices
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
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 drainage runoff. The goal is to reduce or eliminate
the contaminants collected by stormater as it moves into channels, 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
An explanation of each of the above BMPs follows and is a comparison for the treatment and
management of drainage 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. The Stormwater
Treatment Train (STT) represents an ecological approach to stormwater management and
has proven effective and versatile in 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 is to treat drainage runoff
for water quality benefits and to reduce drainage 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.
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.
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.
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B. Structural BMPs
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.
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
erse effects of urban drainage
runoff can be reduced or alleviated. 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.
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 drainage 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 drainage
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.
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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 is
used to drain a runoff volume over a specified period of time, typically 24 hours, and is used
to meet channel protection criteria (CP v). Some structural control designs wet extended pond
and micro-pool extended 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,
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. drainage ponds, wet retention ponds, dry 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 drainage runoff is directed.
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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
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 with other BMPs, such as dry ponds, infiltration
trenches, or sand filters.
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. Example of the Stormwater Park in
Sebastian, Florida.
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.
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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 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
0ne-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
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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
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 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 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 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 pcapacity 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 drainage 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 drainage 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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The permeability of the soils at the exfiltration trench location and the anticipated water table
elevation determine the applicability and performance of the exfiltration trench system, which
has to be able to infiltrate the required stormwater treatment volume and drawdown the
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 drainage 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 o
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
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.
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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 drainage 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 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 drainage 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
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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
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 drainage 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.
Should contain contiguous areas of standing or flowing water only following a
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.
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Is designed to take into account the soil erosion, 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
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.
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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
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 where soils are permeable.
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.
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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 (WQIs), 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.
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
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Preliminary modeling by the Florida Institute of Technology indicates that these screens do
not become clogged even under heavy loads of debris. The Florida Institute of Technology
used hydraulic scale-modeling to evaluate baffle box size and shape, along with baffle size
and placement, on pollutant removal efficiency. 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.
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
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
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principles such as bio retention facilities vegetated 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
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
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 which 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.
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:
o Identifying storm drains with stenciling to discourage dumping.
o Distributing toxics checklist for meeting household hazardous waste
regulations.
o Producing displays and exhibits for school programs.
o Distributing free seedlings for erosion control.
o Create volunteer opportunities such as water quality monitoring.
Inform residents of picking up pet waste or install 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.
Distributing brochures about the NPDES on illicit Discharge
See the City http://sebastiannrb.com
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the City received the Environmental Stewardship Award from the Florida League of Cities for
creating this innovative program. See appendix for award letter and news article.
Operation and Maintenance
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 non-structural maintenance operations:
Turf and landscape management,
Street sweeping & cleaning,
Catch basin cleaning
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:
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.
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:
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.
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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, solubility 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,
overapplication 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. The City of Sebastian, Florida
has an Ordinance (Ordinance No. O-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
-it-
incorrectly assume that materials that are dumped into storm drains will receive treatment at
wastewater treatment plant prior to discharge. Education on recycling and disposal
techniques for these materials can help to reduce pollutant loadings into streams. Education
programs should identify the location of community automotive 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.
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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
th
mark) farther than the unsaturated test on April 11.
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.
Pre-Condition Test Post-Condition Test
Test Factors
Collected
March 27, March 28, 2013 April 11, 2013 April 17, 2013
2013
Velocity 2.87 ft./min 4.48 ft./min 11 ft./min 11.8 ft./min
Flow 12.3 ft./sec 20.08 ft./sec 82.15 ft./sec 115 ft./sec
Distance 129 ft. 48 min 134 ft. 30 min 222 ft. 20 min 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-condition) to 294 feet (Post-condition).
Therefore, the quarter round plastic pipes provide for conveyance of storm water by doubling
the travel flow distance when compared to typical grass swale. This means the storm water
within the quarter round system is approximately twice as fast in flowing drainage runoff
during peak storm events.
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5. Conclusion
The 2018 Master Plan Stormwater Update provides the City with a drainage analysis for the mean
annual 25 to 100 year/24 hour storm event for both the existing and future land use conditions. Based
on an updated 2013 modeling analysis and new topographical GIS mapping and using Interconnected
Channel and Pond Routing (ICPR3) model to perform simulations runs of the stormwater system and
then to identify areas of drainage deficiencies. The following is a summary of the report:
Review existing 2004 CDM drainage model
Field verified existing 2013 model network systems
Created new GIS maps for the network system
Developed Northern and Southern drainage groups
Created new hydrologic parameters using 2 foot aerial topographic data
Analyzed system using Interconnected Channel and Pond Routing (ICPR3) model
Modeled two systems: Northern 1,769 acres, Southern 4,583 acres
Adjust the model per stage flow and capacity
Verified Stormwater deficiencies
Identified drainage improvement projects and construction costs
Created new 2018 drainage master plan map
List Structural and non-
Reviewed past previous quarter round program
for Stormwater Monitoring and Testing Program
The following is a listing of storm water improvement projects with estimates 2018 construction costs.
Projects Cost__
1. George, Schumann Seawall Repair/Replace $16,000,000
2. Ocean Cove Drainage Repair $278,500
3. Day Drive Retention $241,000
4. Schumann Pipe Lining $200,
5. Pelican Island Place Drainage $475,000
6. Stonecrop Drainage Ditch $1,000,000
7. Blossom Ditch Piping $1,000,000
8. Empress Canal Pipe $250,000
9. Landsdowne Box Culvert $178,000
10. Rosebush Box Culvert $242,000
11. Tulip Box Culvert $232,000
12. Albatross Box Culvert $178,000
13. Bayfront Box Culvert $178,000
14. Benedictine Ditch Piping $1,515,000
15. Bryant/Friar Court Retention Pond $315,000
16. Oyster Point Drainage Box $100,000
17. Hardy Dam Retrofit Gates $150,000
18. Potomac Ditch Piping $393,000
19. Potomac Baffle Retrofit for filter $60,000
20. Davis Street Baffle Retrofit for filter $60,000
21. Rosebush Retention Basin $315,000
22. Indian River Drive Drainage Improvements $1,000,000
Total Drainage Cost $24,360,500
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