HomeMy WebLinkAbout03 - Data and MethodologyCity of Sebastian Stormwater Master Plan Update
3 Data and Methodology
3.1 Meetings with the City and Available Data
A kickoff meeting was conducted with the City to introduce the Arcadis project team and review the City's
drainage system, historical problems, recent projects, management issues, and identify data available for the
master planning effort. The City had extensive hard copy files of previous master plans and previous project
documentation, but there were no previous model files of the stormwater system. In addition, the only
topographic information available was the Florida Division of Emergency Management (FDEM) Coastal Program
2007 Light Detection and Ranging (LiDAR) elevation data of Indian River County in vertical datum NAVD88.
After reviewing available documentation and several discussions with City staff, it was determined that a new GIS
map of the major components of the City's stormwater system would be needed for the hydraulic and hydrologic
modeling effort and to satisfy MS4 permit requirements. In order to make the GIS map as accurate as possible,
Arcadis reviewed and digitized more than 280 as -built drawings, permits, sketches, and other documentation of
the drainage features within the City's stormwater system. A field crew was mobilized and spent more than three
weeks in the City locating, identifying, measuring, and surveying elevations for more than 4,400 stormwater
drainage components to populate the GIS map.
3.2 Stormwater System GIS Map Development
The stormwater system GIS Map was developed to provide a geospatial database of the City of Sebastian
stormwater system incorporating the best available information. The City provided several stormwater-related GIS
layers and record drawings as noted in section 3.1. The database incorporated available City of Sebastian
information such as CAD drawings, GIS layers including the ditches feature class and Weirs Dams feature class
and georeferenced PDFs including "2015 — Ditch Location Map.pdf' and Plan view figures from the 2018 City of
Sebastian Stormwater Master Plan. The GIS database and subsequent H&H modeling was completed in vertical
Datum NAVD 88. Information collected from record drawings, PDF's, etc. in vertical datum NGVD 29 were
converted to NAVD 88 by subtracting 1.463ft based on the NOAA National Geodetic Survey Coordinate
Conversion and Transformation Tool (NCAT), see Appendix F for conversion documentation.
The information provided by the City of Sebastian was utilized as a starting point. The team utilized aerial
imagery, surface topography, and Google Street View to identify stormwater assets not provided in the City of
Sebastian data. Through desktop analysis, approximately 2,381 nodes and 1,875 pipes, culverts and open
channels were identified. 75% of stormwater assets were missing critical parameters such as pipe size, invert
elevation and rim elevation. Therefore, an ArcGIS online map was created for the survey crew to take
measurements via Collector or Field Maps application while in the field.
Two shapefiles were created to represent the stormwater system: a point layer for stormwater structures and a
polyline layer for closed pipes and open channels. Additional spot elevations were collected in areas with
documented historic flooding. Private driveway culverts and cross pipes were also included in the geodatabase
where drawings were available; however, these assets were not included in the modeling work, which focused on
infrastructure owned by the City of Sebastian.
Due to survey time limitations, a priority ranking from 1 to 6 was assigned to all nodes and conduits. The highest
priority for survey, Rank 1, was given if there was missing information necessary for the H&H modeling work such
as conduit type, pipe material, pipe size, invert elevations, rim elevations and blockage conditions. Rank 2 was
assigned to ditches that are adjacent to high priority Rank 1 nodes and conduits, and were missing cross-section
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City of Sebastian Stormwater Master Plan Update
dimensions, invert elevations and vegetation conditions. Rank 3 was assigned to nodes and conduits which had
at least one critical attribute identified from information provided by the City of Sebastian. Ranks 4-6 were low
priority nodes and conduits that were not in critical locations for the H&H model development but were identified
as part of the stormwater system.
Of the 2,381 nodes and 1,875 conduits identified for the field survey activities 2,170 nodes and 1,539 conduits
were inspected. Of the 2,381 Redes and 1,875 coRduit6 identified f9F the field 6uFvey aGtovitles 2,3690- ne-d-es and
Arcadis developed a stormwater system asset geospatial database in ArcGIS with the best available information
from the desktop analysis, field inspections and survey and the H&H stormwater model development. The
database is provided in Appendix J Stormwater Asset GIS Database.
3.3 Infrastructure Asset Survey and Assessment
3.3.1 Asset Assessment Goals, Criteria and Methodology
Arcadis reviewed available stormwater system information as mentioned in Section 1, 3.1, and 3.2. A GIS based
inventory of stormwater assets was first developed using desktop analysis followed by a site visit stormwater
inspection and field survey.
The infrastructure asset assessment included inspections of over 4,500 stormwater assets including:
• 756 bulkheads,
• 2,170 nodes which include culvert inlets/outlets, outfalls, baffle boxes, curb inlets, yard inlets, and
manholes, and
• 1,539 conduits which include pipes, canals, and ditches.
In addition, 111 point elevations were taken at low points that were in flood hazard areas. The complete GIS
database also includes inlets and conduits that were provided by the City or located in as -built drawings. The
total asset inventory developed for field inspections and survey is comprised of 756 bulkheads, 2,381
inlets/nodes, 1,875 conduits (canals, ditches, etc.), and 27 outfalls for a total of 5,039 features.
3.3.2 Inspection Methodology
The inspections were performed using a Trimble R2 GNSS receiver and a mobile based application that allowed the
inspectors to update the inventory of the stormwater conveyance system by taking elevations, dimension
measurements, taking note of the material/size, uploading pictures of the asset, and writing down any comments
specific to the asset to include condition assessments. The Trimble units were tested with Keynet VRS correction
and the ESRI Field Maps App to verify the vertical and horizontal accuracy. The vertical accuracy was documented
as +/- 0.2ft and the horizontal accuracy was documented +/-0.1ft. Previously unknown or undocumented assets
which were discovered during the inspections were recorded and located with GPS coordinates.
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City of Sebastian Stormwater Master Plan Update
3.3.3 SWMS Components
3.3.3.1 Canals
The City's Stormwater Management System (SWMS) includes
approximately eight miles of canals, which have bulkheads on both
sides for most of their length. The canals themselves showed
varying amounts of vegetation and shoaling, and generally
appeared to be in need of vegetation removal and maintenance
dredging to reset their flow areas to their original design
configurations. The bulkheads along the canal banks are
constructed of a variety of materials, primarily concrete or steel
sheet piling. The canals themselves are within the City's drainage
easement and are maintained by the City, but the majority of the
bulkheads are privately owned and maintained.
As indicated above, a total of 756 bulkheads were inspected and
about 85% were generally found to be structurally sound, but many of
those showed signs of deterioration (i.e., cracks along the bulkhead
face, concrete cap, etc.) and need maintenance. At the time of the
inspections, 108 bulkheads had structurally failed and need to be
replaced immediately, especially in the Schumann Lake area. An
example of the observed bulkhead failures is shown in Figure 3-1, a
bulkhead toe failure that occurred over multiple properties.
The bulkheads that were constructed of metal were consistently in
Figure 3-1. Bulkhead Toe Failure
better condition with fewer cases of failure, but that is likely because they are newer than the concrete bulkheads.
Many of the properties with bulkheads also have moderate to high amounts of vegetation growing around and on
top of the structures, which blocked the view for inspections and, if left unchecked, could cause future failures due
to erosion and uplift from tree roots. For these reasons, no trees, shrubs or other vegetation with substantial root
systems should be present within 20 feet of a bulkhead structure.
3.3.3.2 Green Infrastructure
The City's SWMS relies primarily on Green Infrastructure, a network of interconnected roadside swales, side yard
swales, and backyard ditches, for conveyance of stormwater to canals and outfall points. Due to the sheer
number of swales and ditches, our inspections were limited to the major flow paths for use in H&H model
development. The thousands of roadside swale sections, though not specifically included in our inspections, were
observed throughout the City during the field work and generally appeared to be in good condition, with some
exceptions. However, the large number of roadside swale sections and their connecting driveway culverts in
series create a collection system with a high probability for operational difficulties. A detailed inspection and
hydraulic analysis of the roadside swale systems throughout the City should be performed in the future.
The GIS infrastructure database developed to facilitate the field inspections and survey activities described in
Section 3.3.1 included 473 line features representing roadside swales, side yard swales, and backyard ditches.
For each asset, the inspection data and a photo documenting its condition at the time of inspection was entered
into the GIS database. Per City code, the private property owners are the responsible party for maintenance of
swales and ditches. The City completes limited ditch regrading in association with road repaving projects.
Approximately 60% of these swales and ditches were either in good condition or had evidence of being recently
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City of Sebastian Stormwater Master Plan Update
3.3.3.3 Conveyance Structures
The remainder of the stormwater inspections consisted of assessing the physical and operation condition of the
conveyance structures, such as culvert inlets/outlets, outfalls, baffle boxes, curb inlets, yard inlets, manholes, and
pipes. Except as noted, all accessible structures identified during the desktop analysis discussed in Section 3.2
were inspected and surveyed. Elevations were taken at the rim and invert(s) of these stormwater structures,
when accessible. For pipes, additional field measurements were taken to document the pipe diameter and length,
and the pipe material was noted. For each asset, the inspection data and a photo documenting its condition at
the time of inspection was entered into the GIS database.
Note that the thousands of driveway culverts in the SWMS were not included in these inspections, as they are
part of the residential roadside swale network and not specifically included in the hydraulic model due to a lack of
survey and physical data. Note that the thousands of driveway culverts in the SWMS were not included in these
inspections, as they are part of the residential roadside swale network and not specifically included in the
hydraulic model due to a lack of survey and physical data. However, as mentioned in the previous section, the
large number of roadside swale sections and their connecting driveway culverts in series create a collection
system with a high probability for operational difficulties. A detailed inspection and hydraulic analysis of the
roadside swale systems throughout the City should be performed in the future.
In all, 2,165 nodes were included in the GIS map for these conveyance structures. In all, 2,381 node features and
1,440 line features were included in the GIS map for these conveyance structures. Their condition was
determined based on the amount of sediment inside the asset, whether the asset was blocked due to vegetation,
or if the asset was damaged or collapsed. In most cases these assets were in good condition, with little or no
sediment blocking the flow. However, there were a few locations in which sediment and vegetation blocked
enough of the structure to prevent water from flowing at their intended levels.
The GIS infrastructure database developed to facilitate the field inspections and survey activities described in
Section 3.3.1 included 473 line features representing roadside swales, side yard swales, and backyard ditches.
3.4 Hydrologic and Hydraulic Modeling
3.4.1 H&H Model Goals and Limitations
A dynamic 1 D Hydrologic and Hydraulic (H&H) computer model of the City of Sebastian stormwater network was
developed to evaluate the system performance in response to various conditions. The H&H model also supported
capital improvement project development, permitting, grant funding applications and stormwater system
performance evaluations.
3.4.2 Software Selection
Arcadis worked with the City of Sebastian to select a modeling software for the stormwater analysis that met the
needs of the City. Arcadis completed a technical review of the existing City of Sebastian modeling approach and
the capabilities of standard software programs and commonly applied modeling approaches. The existing City of
Sebastian stormwater model files created in modeling software Interconnected Pond Routing model (ICPR)
version 3 developed by Streamline Technologies Inc. were not available for review and therefore Arcadis
reviewed the model documentation provided in the Master Stormwater Management Plan Report, 2004' and the
7 CDM Smith. Master Stormwater Management Plan. City of Sebastian, Florida. February 2004.
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City of Sebastian Stormwater Master Plan Update
Stormwater Management Plan Update, 20181. Based on discussions with the City of Sebastian the top priorities
when selecting the modeling software were low cost, wide software usage, model accessibility, and suitability for
use on stormwater planning projects. The Team reviewed stormwater modeling software ICPR version 4,
PCSWMM provided by Computational Hydraulics Int., and InfoWorks ICM by Innovyze. The software selected
with input from the City of Sebastian for the hydrologic and hydraulic evaluation was PCSWMM which uses the
EPA's public domain Stormwater Management Model (SWMM) engine. A summary of the PCSWMM software
attributes is provided below:
• Relatively low cost
• High accessibility — PCSWMM model files can be accessed using the publicly available EPA SWMM5 software.
• Flexibility across projects - PCSWMM model network can be exported to GIS for easy data management
and review in support of ancillary City of Sebastian objectives.
• Support and Software Usage - PCSWMM software and EPA SWMM5 is used throughout the US and has
an extensive user group and online community.
• Suitability — PCSWMM is a dynamic hydrologic and hydraulic model software developed to support
stormwater management.
The City of Sebastian is interested in 2D stormwater modeling capabilities. 2D modeling is helpful when
evaluating surface flooding extents and complex overland flow paths associated with various design storm events.
Arcadis recommended InfoWorks ICM as the software of choice for 2D modeling. PCSWMM software has a
recommended maximum 2D area of 500 acres which would not support a system -wide 2D model of the City of
Sebastian (7,243 ac). The City of Sebastian selected PCSWMM over InfoWorks ICM based on software costs
and accessibility which are higher priorities to the City compared to 2D modeling capabilities. Utilizing the
PCSWMM software the City of Sebastian will have the ability to create individual 2D models for areas of concern
within the stormwater network with a total 2D surface areas less than 500 acres as necessary.
3.4.3 Model Extents and Level of Detail
The model extents were developed based on discussions with the City of Sebastian and are provided in Figure 3-3.
The modeled network is focused on the pipes, culverts, and open channels owned by the City of Sebastian. In some
locations roadside swales were included in the model extents in order to maintain system connectivity between City
owned infrastructure. The model includes 2,210 conduits (pipes, culverts, open channels, swales, canals, etc.), 2,150
junctions (culvert inlets/outlets, catch basins, manholes, open channel confluence points, etc.), and 46 storage nodes
(detention basins, ponds, GI features, etc.). There are an additional 57 storage nodes in the model that track and store
runoff from Planned Unit Development (PUD) areas where stormwater runoff is managed onsite and disconnected
from areas without stormwater infrastructure that are not tributary to the City of Sebastian stormwater network.
8 CWT Engineering, LLC. Stormwater Management Master Plan update. City of Sebastian, Florida. August 2018.
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City of Sebastian Stormwater Master Plan Update
There are a total of 27 outfall locations in the City of Sebastian, the locations are described in Table 3-1. Fourteen
of the outfalls drain to the Indian River Lagoon, the outfalls are managed by the FDOT and the City of Sebastian
as described in Table 3-1. Thirteen of the outfalls drain to the South Prong of the St. Sebastian River, the outfalls
are managed by the City of Sebastian, Indian River County, and a private entity as described in Table 3-1. Two of
the outfalls drain to Collier Creek which is tributary to the South Prong of the St. Sebastian River, the outfalls are
managed by the City of Sebastian. There are 23 outfall locations included in the H&H model, as shown in Figure
3-3. The modeled outfalls are not a direct match to the outfalls documented in Table 3-1. The model includes a
single outfall along Collier Creek labeled Collier Creek Outfall located 375ft downstream of the Hardee Dam and
receives flow from the Twin Ditches Outfall and Hardee Outfall. Also, the model includes a single outfall along the
Elkcam Waterway labeled Elkcam Waterway Outfall located 1,500ft downstream of the Elkcam Dam and includes
flow from the Elkcam outfall, Laconia outfall and Lanfair outfall. Finally, the Sunport outfall is not included in the
H&H model due to a lack of available information.
Table 3-1. Outfaii Locations
Davis Outfall
City of Sebastian
Indian River Lagoon
DT12110-Jefferson Outfall
FDOT
Indian River Lagoon
DT12115-Davis South Outfall
FDOT
Indian River Lagoon
DT12124-Harrison South Outfall
FDOT
Indian River Lagoon
DT12294-Washington Outfall
FDOT
Indian River Lagoon
DT12295-Coolidge Outfall
City of Sebastian
Indian River Lagoon
DT12342-Oyster Point North Outfall
FDOT
Indian River Lagoon
Fisherman's Landing Outfall
City of Sebastian
Indian River Lagoon
Harrison Outfall
City of Sebastian
Indian River Lagoon
Main North Outfall
City of Sebastian
Indian River Lagoon
Main South Outfall
City of Sebastian
I Indian River Lagoon
Main St Outfall
City of Sebastian
Indian River Lagoon
Martin Outfall
City of Sebastian
I Indian River Lagoon
Truman Outfall
City of Sebastian
Indian River Lagoon
Hardee Dam Outfall
City of Sebastian
I Collier Creek
Twin Ditches Outfall
City of Sebastian
Collier Creek
Blossom Outfall
City of Sebastian
South Prong of the St. Sebastian River
CR512 Outfall
Indian River County
South Prong of the St. Sebastian River
Elkcam Outfall
City of Sebastian
South Prong of the St. Sebastian River
Gladiola Outfall
City of Sebastian
South Prong of the St. Sebastian River
Laconia Outfall
City of Sebastian
South Prong of the St. Sebastian River
Lanfair Outfall
City of Sebastian
South Prong of the St. Sebastian River
Potomac Outfall
Indian River County /
South Prong of the St. Sebastian River
City of Sebastian
San Sebastian Springs Outfall
Private
South Prong of the St. Sebastian River
Stonecrop Basin Outfall
City of Sebastian
South Prong of the St. Sebastian River
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City of Sebastian Stormwater Master Plan Update
Sunport Outfall
Watering Hole Outfall
Table 3-1. Outfall Locations
City of Sebastian South Prong of the St. Sebastian River
City of Sebastian South Prong of the St. Sebastian River
Note 1: The Potomac Outfall is owned by Indian River County. The City of Sebastian has historically maintained the outfall
and placed a Baffle Box at this location in coordination with Indian River County.
3.4.4 Hydraulic Parameters
The 1 D hydraulic stormwater network was developed from limited GIS data provided by the City of Sebastian,
digitized record drawings, field investigations, survey data and photos. The network includes closed pipe and
open channel stormwater conveyance and is represented in the model with a collection of nodes and links. As
discussed in Section 3.2 Stormwater System GIS Map Development a desktop analysis was completed to create
the City of Sebastian existing stormwater network geospatial database which included a review of available GIS
data and record drawings, aerial imagery and Google Street View to identify open channels and pipes not
included in the GIS data and record drawings. 75% of the stormwater assets identified during the desktop
analysis were missing hydraulic attributes (pipe diameter, open channel geometry, invert elevations, rim
elevations, pipe material, and open channel vegetation). Pipe and open channel attributes were collected by
survey crews in 2022, as documented in Section 3.2. Open channel cross-section geometry and channel slope
was based on surveyed cross -sections and channel elevations supplemented with surface topography. The
Florida Division of Emergency Management (FDEM) Coastal Program 2007 Light Detection and Ranging (LiDAR)
elevation data of Indian River County with a vertical accuracy of +/- 4 inches was used for the analysis and is in
vertical datum NAVD88. An average open channel geometry is assigned to the length of the modeled open
channel sections.
As part of the hydraulic data quality review the surveyed invert elevations were compared against the LiDAR
topography and adjacent invert elevations documented in record drawings. The hand-held GPS unit has a
documented vertical accuracy of +/-0.2ft. A limited number of surveyed invert elevations were found to have a
larger range of accuracy than expected, approximately +/-1.5ft. The hand-held GPS unit used for survey data
collection can result in less accurate data if satellite connectivity issues occur. Locations with inaccuracies in the
surveyed elevations were determined based on the following criteria: negatively sloped pipes and open channels,
invert elevations that exceeded the ground surface elevations derived from LiDAR or survey data. Locations with
the invert elevation inaccuracies or missing invert elevations were either estimated based on the LiDAR
topography or interpolated based upstream and downstream invert elevations while assuming constant slope.
The invert elevation adjustments were focused on the larger pipes and open channels that were significant to
stormwater conveyance. Due to limited existing system information and the flat nature of the City of Sebastian
topography flow directions of the pipes and open channels were determined by reviewing invert elevations and
pipe sizes under the assumption that smaller pipes flow to larger pipes.
The team reviewed the stormwater network following the surveying effort and identified gaps in the system
connectivity; pipes, roadside ditches, and open channels not included in the hydraulic network developed through
desktop analyses from available information. The roadside swales are not maintained by the City of Sebastian but
were included as necessary to connect the City of Sebastian culverts, open channels, etc. in the stormwater
model. For the assets not included or inaccessible during the surveying effort, hydraulic attributes were assumed
based on topography, Google Street View, and upstream and downstream stormwater infrastructure. The open
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City of Sebastian Stormwater Master Plan Update
channels with missing cross-section geometry were grouped into 6 categories according to vegetation condition
and size of the channel according to 2007 LiDAR data and recent aerial imagery provided by Indian River County
dated June 2021. Roadside ditches were grouped as one category assuming they have similar cross-section
geometry. The average cross-section geometry was calculated for each category and was applied to each open
channel and roadside ditch lacking cross-section geometry. Cross-section geometry for the large open channels
was developed individually based on the 2007 LiDAR elevation data.
The Manning's, n, roughness coefficients for pipes and culverts were adjusted according to material type as noted
in the record drawings and field survey. Open channel Manning's, n, roughness coefficients used in PCSWMM
were based on existing conditions documented in field surveyed vegetation, site photographs, aerial imagery, and
Manning's, n, roughness coefficients for channels'. Table 3-2 summarizes the Manning's, n, roughness
coefficients used in the PCSWMM modeled pipes and open channels. In locations where detailed information
regarding pipe material or open channel conditions was unavailable a default Manning's, n, roughness coefficient
of 0.015 and 0.035 was assigned for pipes and open channels respectively.
Table 3-2. Manning's, n, Roughness Coefficients
Reinforced Concrete Pipe - Default
Polyvinyl Chloride and Cured In Place Pipe Lining
Concrete
Corrugated Metal/Aluminum Pipe
HDPE
Excavated Channel - Earth, straight, and uniform with short grass, few weeds
Clean, straight, full stage, no rifts, or deep pools, with some stones and weeds
(Default or Clear)
Channels not maintained, weeds and brush uncut, clean bottom, brush on sides
(Some vegetation)
Channels not maintained, weeds and brush uncut, dense weeds, high as flow depth
0.015
0.01
0.015
0.02
0.012
0.027
0.035
0.05
0.08
(High vegetation)
Pipe blockages documented in the field survey efforts, Section 3.2 and 3.3, were not included in the existing
conditions model. The H&H model assumes that pipe blockages have been addressed through regular O&M
activities. This condition allows for identification of undersized infrastructure and supports the identification of CIP
projects. System maintenance to remove sedimentation and pipe blockages is critical to the operation of the
stormwater system as sedimentation and blockages negatively impact the conveyance capacity of the existing
stormwater infrastructure and can increase the severity and frequency of surface flooding within the City.
The stormwater storage structures including detention basins and ponds were represented in the model as storage
nodes. The storage nodes are assigned a stage -storage curve based on available record drawings or LiDAR
topography. Specialty hydraulic features including weirs and orifices were included in the model to represent outlet
control structures at the canals, outfalls, detention basins, etc. The Stormwater Park pump station is included in the
9 Chow, V.T., 1959, Open -channel hydraulics: New York, McGraw-Hill.
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City of Sebastian Stormwater Master Plan Update
PCSWMM model but based on discussions with the City of Sebastian staff the pump is not operated during wet -
weather events and is therefore the pump station is turned off in the design storm simulations.
3.4.4.1 Outfall Boundary Conditions
There are 23 outfall locations included in the H&H model, as shown in Figure 3-3. Fourteen of the outfalls drain to
the Indian River Lagoon and the remaining nine outfalls drain to the South Prong of the St. Sebastian River. The
boundary conditions of the outfalls in the Indian River Lagoon are based on the mean sea level elevation data
collected at the Trident Pier Station at Port Canaveral (Station No. 8721604) which is the closest National
Oceanic and Atmospheric Administration (NOAA) Tide Station to the City of Sebastian. According to Figure 5-1 in
Section 5 - Sea Level Rise Assessment, data collected at this station from 1994 to 2022 show the maximum of
mean sea level elevation is 2.15ft NAVD 88 (1.2 MSL ft). The fixed stage water surface elevation of 2.15 ft was
assigned to the 14 Indian River Lagoon outfalls for the 10-year, 25-year, and 100-year design storm events. The
Indian River Lagoon boundary condition set to the maximum mean sea level allows for review of the capacity of
the stormwater conveyance system. There are additional risks to stormwater infrastructure draining to the Indian
River Lagoon which are present during periods of elevated water levels such as King Tides, Hurricanes, etc. The
outfalls tributary to the South Prong of the St. Sebastian River were assigned a fixed stage elevation based on the
FEMA Flood Insurance Study (FIS) for Indian River County, Florida" and incorporated areas revised on
December 4, 2012. The FIS provides still water elevations by transect for 10-, 50-, 100-, 500-year recurrence
intervals for use in floodplain management. The 25-year storm event at each transect was interpolated from the
provided recurrence interval floodplain data. The location of each City of Sebastian outfall to the South Prong of
the St. Sebastian River in relation to the FIS transects along the stream centerline was used to interpolate the
water surface elevations for the 10-, 25-, and 100-year recurrence interval events. The boundary conditions at
each outfall location are summarized in Table 3-2.
Table 3-3. Outfaii Boundary Conditions
Blossom Outfall
6.27
7.37
Collier Creek OutfaII2
12.30
13.12
CR512 Outfal13
6.34
7.43
Davis Outfall
2.15
2.15
DT12110-Jefferson Outfall
2.15
2.15
DT12115-Davis South Outfall
2.15
2.15
DT12124-Harrison South Outfall
2.15
2.15
DT12294-Washington Outfall
2.15
2.15
DT12295-Coolidge Outfall
2.15
2.15
DT12342-Oyster Point North Outfall
2.15
2.15
Elkcam Waterway Outfall5
7.97
8.91
Fisherman's Landing Outfall
2.15
2.15
Gladiola Outfall
6.90
7.94
,r
ation,
100-year Water
Surface
Elevation, ft
Invert Elevation, ft
0.46
0,
•.
10 Federal Emergency Management Agency, Flood Insurance Study Indian River County, Florida and Incorporated Areas, 4 December 2012.
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City of Sebastian Stormwater Master Plan Update
Harrison Ou
Main North
Main South
Main St Outf
Martin Outfall
Potomac Ou
San Sebasti
Stonecrop B
Truman Outf,
Watering Hol
Table 3-3. Outtaii Boundary Conditions
a
00-year Water
Surface Surface Elevation, Surface Invert Elevation, ft
fall 2.15 2.15 2.15 -1
Outfall 2.15 2.15 2.15 -1.46
Outfall 2.15 2.15 2.15 2.54
all 2.15 2.15 2.15 3.94
2.15 2.15 2.15 -2
tfall 1.98 3.13 4.85 7.56
n Springs Outfall 5.26 6.38 8.00 6.15
asin Outfall 6.60 7.67 9.32 0.54
all 2.15 2.15 2.15 0
e Outfall 6.72 7.77 9.46 8.1
Note 1: Elevations are provided in vertical datum NAVD88
Note 2: The modeled Collier Creek outfall is located 375ft downstream of the Hardee Dam and includes flow from the Twin Ditches outfall
and Hardee outfall
Note 3: The CoR512 outfall included in the H&H model is not owned by the City but receives flow from City property along CoR512. The
outfall is located South of CoR512 and drains to the South Prong of the St. Sebastian River.
Note 4: The Sunport outfall is not included in the H&H model due to a lack of available information.
Note 5: The modeled Elkham Waterway outfall is located 1,500ft downstream of the Elkham Dam and includes flow from the Elkham
outfall. Laconia outfall and Lanfair outfall
3.4.5 Hydrologic Parameters
Hydrologic parameters for the area tributary to the stormwater collection system were developed to define the
PCSWMM calculations which convert rainfall into stormwater runoff and infiltration.
3.4.5.1 Subcatchment Delineation
Site topography, storm sewer inlet locations, site survey and visual inspections were used to delineate drainage
areas throughout the system. The total modeled drainage area is 7,243 acres and was split into 1,898
subcatchments (see Figure 3-3). The City provided a map of locations and boundaries of Planned Unit
Development (PUD) areas where stormwater runoff is managed onsite and these areas were delineated as
individual subcatchments. Additionally, areas not identified as PUD's without stormwater infrastructure that were
not tributary to the City of Sebastian stormwater network were delineated as individual subcatchments and are
noted as a "Disconnected area" in Figure 3-3. There are 28 PUD subcatchments and 29 disconnected area
subcatchments with a total acreage of 1,087 ac and 734 ac respectively.
The calculated subcatchment hydrologic parameters developed for the stormwater model are listed below.
• Subcatchment Parameters
- Area
- Average Flow Path Length
- Average Subcatchment Slope
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Note 1: Elevations are provided in vertical datum NAVD88
Note 2: The modeled Collier Creek outfall is located 375ft downstream of the Hardee Dam and includes flow from the Twin Ditches outfall
and Hardee outfall
Note 3: The CoR512 outfall included in the H&H model is not owned by the City but receives flow from City property along CoR512. The
outfall is located South of CoR512 and drains to the South Prong of the St. Sebastian River.
Note 4: The Sunport outfall is not included in the H&H model due to a lack of available information.
Note 5: The modeled Elkham Waterway outfall is located 1,500ft downstream of the Elkham Dam and includes flow from the Elkham
outfall. Laconia outfall and Lanfair outfall
3.4.5 Hydrologic Parameters
Hydrologic parameters for the area tributary to the stormwater collection system were developed to define the
PCSWMM calculations which convert rainfall into stormwater runoff and infiltration.
3.4.5.1 Subcatchment Delineation
Site topography, storm sewer inlet locations, site survey and visual inspections were used to delineate drainage
areas throughout the system. The total modeled drainage area is 7,243 acres and was split into 1,898
subcatchments (see Figure 3-3). The City provided a map of locations and boundaries of Planned Unit
Development (PUD) areas where stormwater runoff is managed onsite and these areas were delineated as
individual subcatchments. Additionally, areas not identified as PUD's without stormwater infrastructure that were
not tributary to the City of Sebastian stormwater network were delineated as individual subcatchments and are
noted as a "Disconnected area" in Figure 3-3. There are 28 PUD subcatchments and 29 disconnected area
subcatchments with a total acreage of 1,087 ac and 734 ac respectively.
The calculated subcatchment hydrologic parameters developed for the stormwater model are listed below.
• Subcatchment Parameters
- Area
- Average Flow Path Length
- Average Subcatchment Slope
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City of Sebastian Stormwater Master Plan Update
- Impervious Area
- Infiltration Parameters
Subcatchment parameters affect stormwater runoff volume and peak flow rates. Site topography is based on
Florida Division of Emergency Management (FDEM) Coastal Program 2007 Light Detection and Ranging (LiDAR)
elevation data of Indian River County with a vertical accuracy of +/- 4 inches. The LiDAR data along with the
stormwater inlet locations identified through the review of record drawings, survey activities, and publicly available
aerial/street-view imagery were used to calculate the subcatchment area in Arc Geographic Information System
(ArcGIS). The subcatchment flow path length is defined by the distance runoff travels within a subcatchment to
reach the outlet. The flow paths were developed in ArcGIS for each subcatchment based on the existing
topography. The slope for each subcatchment is determined based on the average slope of the subcatchment
and was calculated based on the existing topography.
The subcatchment impervious area was calculated in ArcGIS based on geospatial information regarding building
footprints and roadway extents. The building footprint information was developed from the Indian River County
building footprint shp file and was reviewed and updated. The IRC Building Footprints poly file was compared
against recent aerial imagery provided by Indian River County dated June 2021 and buildings identified as
missing in the building footprint file were added and buildings that were not present in the aerial imagery were
removed. The updated IRC Building Footprints poly file is provided for reference in Appendix J Stormwater
Asset GIS Database. When calculating the percentage of impervious area within the subcatchments the building
impervious areas were increased by 25% to account for driveways. The roadway impervious area shp file was
developed using a buffer of the street centerline file provided by the City of Sebastian. The existing system
percent impervious is approximately 20%.
The modified Green-Ampt Infiltration method was selected for modeling the infiltration of rainfall into the pervious
surfaces within the drainage area. Infiltration parameters are based on physical soil attributes and include the
initial moisture deficit of the soil, soil hydraulic conductivity (infiltration rate), and the suction head at the wetting
front. The stormwater infiltration within the City of Sebastian functions differently East versus West of the coastal
divide. Detailed City of Sebastian soils information is provided in Section 2.1 - Geology and Soils. West of the
coastal divide the groundwater is noted as within 1ft of the ground surface and there is a documented hardpan
layer, both of which limit the infiltration potential. East of the coastal divide infiltration functions based on the
physical characteristics of the soils. Two different approaches were used to develop infiltration parameters for the
varied hydrogeology East and West of the coastal divide in the City of Sebastian.
East of the coastal divide the subcatchment Green-Ampt infiltration parameters were developed from a composite
of the soils documented in the NRCS Web Soil Survey. The documented soil types were assigned Green-Ampt
parameters based on the relative soil texture, as referenced from Rawls (1983)", the NRCS provided soil
hydraulic conductivity ranges and the most hydraulically restrictive layer within each soil type. The Green-Ampt
parameters for the various soil types are provided in Table 3-3.
" Rawls, W.J. et al. 1983. J. Hyd. Engr., 109:1316.
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City of Sebastian Stormwater Master Plan Update
Table 3-4. Green Ampt Parameters assigned City of Sebastian USDA/NRCS So# Types
Chobee loamy fine sand, frequently ponded, 0 to 1 percent slopes
I C/D
0.06
8.66
0.262
EauGallie fine sand
A/D
0.06
8.66
0.262
Immokalee fine sand
B/D
2.49
6.49
0.378
Myakka-Myakka, wet, fine sands, 0 to 2 percent slopes
A/D
2.49
6.49
0.378
Oldsmar fine sand
A/D
0.06
8.66
0.262
Pepper sand
D
0.43
4.33
0.368
Riviera fine sand, 0 to 2 percent slopes
A/D
2.49
6.49
0.378
St. Lucie sand, 0 to 8 percent slopes
A
4.74
1.93
0.413
Archbold sand, 0 to 5 percent slopes
A
4.74
1.93
0.413
Wabasso-Wabasso, wet, fine sand, 0 to 2 percent slopes
2.49
6.49
0.378
B/D
Pineda-Pineda, wet, fine sand, 0 to 2 percent slopes
2.49
6.49
0.378
A/D
Quartzipsamments, 0 to 5 percent slopes
A
4.74
1.93
0.413
Pomello sand, 0 to 5 percent slopes
A
4.74
1.93
0.413
Arents, 0 to 5 percent slopes
A
4.74
1.93
0.413
Floridana sand, frequently ponded, 0 to 2 percent slopes
C/D
0.06
8.66
0.262
Immokalee-Urban land complex
A/D
4.74
1.93
0.413
Astatula sand, 0 to 5 percent slopes
A
4.74
1.93
0.413
Satellite fine sand, 0 to 2 percent slopes
A
4.74
1.93
0.413
Malabar fine sand
A/D
0.43
4.33
0.368
Myakka fine sand, frequently ponded, 0 to 1 percent slopes
A/D
2.49
6.49
0.378
Holopaw fine sand, 0 to 2 percent slopes
2.49
6.49
0.378
A/D
Electra sand, 0 to 5 percent slopes
0.43
4.33
0.368
A
Pompano fine sand, 0 to 2 percent slopes
4.74
1.93
0.413
A/D
Riviera fine sand, frequently ponded, 0 to 1 percent slopes
A/D
0.43
4.33
0.368
Lokosee fine sand
A/D
0.06
8.66
0.262
West of the coastal divide the infiltration is affected by additional factors that limit the potential for infiltration. The
high water table and hardpan layer are reflected in the hydrologic soil groups (HSG) assigned by NRCS, 78% of
the soils within the drainage area are hydrologic soil group D or two HSG groups (i.e., A/D, C/D) which indicate
that a confining layer or high water table when present affects the infiltration rate that a given soil group would
normally exhibit and the soil acts like HSG D. Additionally, the East Central Florida Transient Expanded Model
(ECFTX) groundwater model recharge rates are less than 0.005in/hr in the City of Sebastian which confirms the
limited infiltration potential. Therefore, the PCSWMM stormwater model Green-Ampt parameters West of the
coastal divide were set to the most restrictive soil type documented in the City, a Sandy Clay Loam, which are
provided in Table 3-4. For reference the hydraulic conductivity assigned to the area west of the coastal divide is
reflective of the infiltration rate that would be achieved using the Curve Number method in this area.
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City of Sebastian Stormwater Master Plan Update
Table 3-5. Green Ampt Parameters assigned to Subcatchments West of the Coastal Divide
1 sanay play Loam u.ub b.bb I U.ZbZ
Once the variable parameters were generated for each subcatchment, general hydrology parameters for all
subcatchments were applied and are listed in Table 3-5.
Table 3-6. Subcatchment Hydrology Parameters
Overland Roughness (Manning's N) - Impervious 0.03
Overland Roughness (Manning's N) - Pervious
0.25
Depression Storage - Impervious
Depression Storage - Pervious
Percent Impervious with Zero Depression Storage
0.075 in
0.75 in
25%
Subarea Routing
Pervious
Percent Routed
75%
3.4.5.2 Rainfall
The existing City of Sebastian stormwater network performance was evaluated for the 10yr-24hr, 25yr-24hr, and
the 100yr-24hr design storm events. The NOAA Atlas 14 Precipitation Frequency Data12 was used to develop
design rainfall depths for these design events and is included in Appendix G. The design event rainfall depths are
provided in Table 3-6. The design rainfall depths were distributed over the 24-hour duration using the NRCS Type
II FL Modified Distribution with a 30-minute interval. The NRCS Type II FL Modified Distribution is one of the
recommended design storm distributions in the St. Johns River Water Management District Permit Information
Manual and was selected because it was derived to more accurately simulate hydrographs for the State of Florida
compared to the standard NRCS Type II rainfall distribution.
Table 3-7. Design Storm Rainfall Depths and Distributions
0:00
0.000 0.000
0.000 0.000
0:30
0.006
0.043
0.055
0.074
1:00
0.012
0.087
0.110
0.149
12 Point Precipitation Frequency Estimates, National Oceanic and Atmospheric Administration Atlas 14, Volume 9, Version 2. Accessed 28
February 2022.
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City of Sebastian Stormwater Master Plan Update
Table 3-7. Design Storm Rainfall Depths and Distributions
1:30
0.019
0.138
0.174
0.236
2:00
0.025
0.181
0.229
0.310
2:30
0.032
0.232
0.293
0.397
3:00
0.039
0.282
0.358
0.484
3:30
0.047
0.340
0.431
0.583
4:00
0.054
0.391
0.495
0.670
4:30
0.062
0.449
0.569
0.769
5:00
0.071
0.514
0.651
0.880
5:30
0.080
0.579
0.734
0.992
6:00
0.089
0.644
0.816
1.104
6:30
0.099
0.717
0.908
1.228
7:00
0.110
0.796
1.009
1.364
7:30
0.122
0.883
1.119
1.513
8:00
0.134
0.970
1.229
1.662
8:30
0.148
1.072
1.357
1.835
9:00
0.164
1.187
1.504
2.034
9:30
0.181
1.310
1.660
2.244
10:00
0.201
1.455
1.843
2.492
10:30
0.226
1.636
2.072
2.802
11:00
0.258
1.868
2.366
3.199
11:30
0.308
2.230
2.824
3.819
12:00
0.607
4.395
5.566
7.527
12:30
0.719
5.206
6.593
8.916
13:00
0.757
5.481
6.942
9.387
13:30
0.785
5.683
7.198
9.734
14:00
0.807
5.843
7.400
10.007
14:30
0.826
5.980
7.574
10.242
15:00
0.842
6.096
7.721
10.441
15:30
0.857
6.205
7.859
10.627
16:00
0.870
6.299
7.978
10.788
16:30
0.882
6.386
8.088
10.937
17:00
0.893
6.465
8.189
11.073
17:30
0.904
6.545
8.290
11.210
18:00
0.913
6.610
8.372
11.321
18:30
0.923
6.683
8.464
11.445
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City of Sebastian Stormwater Master Plan Update
Table 3-7. Design Storm Rainfall Depths and Distributions
3.4.6 H&H Model Validation
The H&H model was validated to historical flooding locations documented by the City of Sebastian. Locations of
historic flooding and the relative flooding depth were provided geospatially and were focused on roadway areas.
Hydrologic and hydraulic parameters were adjusted in the model to match observed flooding. Additionally
modeled peak flows throughout the stormwater network were compared to the flows provided in the 2004 Master
Management Plan developed by CDM Smith. The PCSWMM model simulated higher flows on average compared
to the 2004 ICPR model. This is likely due to the additional detail in the PCSWMM model and the fact the 2004
ICPR model was a skeleton model which only included the main conveyance network, approximately 80 culvert
crossings and 204 subcatchments. The PCSWMM model is a planning level tool and provides a dynamic
representation of the stormwater network performance based on the best available hydrologic and hydraulic input
data. The model focuses on the main conveyance network and does not include all pipes and open channels
within the model extents. A potential future improvement to the H&H model is the addition of stormwater overflow
from the PUD areas into the City of Sebastian Stormwater network. The PUD areas are disconnected from the
City of Sebastian stormwater network but can overflow into the network during extreme wet weather events. The
addition of the PUD flows can improve the accuracy of the modeled system conditions.
3.4.7 H&H Model Limitations
The model is a planning level representation of the City of Sebastian stormwater network and is based on the
information available at the time of development. The model focuses on the main conveyance network and does
not include all pipes and open channels within the model extents. The H&H model is a living model and as
improvements are made to the system and additional information is collected, including new survey data and field
investigations, the model will require updates to reflect the current conditions. Application of the model for future
evaluations and design projects should include review of the project area to assess any model improvement
needed to reflect the condition under analysis, comparison of modeled flows and/or heads with the most recent
monitoring data available, and review of available survey data of the stormwater conveyance network.
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City of Sebastian Stormwater Master Plan Update
3.5 Existing Conditions Design Storm Analysis Results
The validated 1 D Hydrologic and Hydraulic stormwater model was used to identify locations and volumes of
simulated surface flooding and review hydraulic gradelines for design wet weather events. According to the
simulated existing conditions H&H model 1073, 1424, and 1738 locations experience flooding for more than two
hours for the 10yr-24hr, 25yr-24hr, and 100yr-24hr design storm events respectively. The conveyance capacity of
the existing stormwater infrastructure as calculated with Manning's Equation was used to identify pipes and open
channel that are capacity limited for the design storms The surface flooding locations, relative flooding volumes,
and capacity limited pipes/open channels simulated in the existing conditions model for the 10yr-24hr, 25yr-24hr,
and 100yr-24hr design storm events are shown in Figures 3-4, 3-5, and 3-6. Modeled pipes and open channels
identified as capacity limited for the design storm events in the plan view figures are provided in the Appendix K
Stormwater Network Summary Tables. The current City of Sebastian Stormwater conveyance network
experiences surface flooding during frequent wet -weather events which is represented in the model. The H&H
model is a 1 D model and the surface flooding is modeled with an assumed surface flooding area of 500 sf at each
node; manhole, catch basin, culvert inlet/outlet, etc. The H&H stormwater modeling results identify areas with
limited conveyance capacity and supported the development of capital improvement projects which reduce
surface flooding within network. 2D modeling of the stormwater network including the surface topography in
relation to the stormwater network will affect the simulated flooding locations and flooding extents compared to
the 1 D modeling results.
The existing conditions design storm results assume that drainage through Collier Creek is consistent with the
conditions included in the 2012 FEMA Flood Insurance Study and that the Hardee Dam is able to discharge into
the Collier Creek. The stormwater drainage through the Hardy Dam is a critical outfall in the City of Sebastian
stormwater network. The loss of drainage and flood control capabilities through Collier Creek would have serious
impacts on the flooding frequency and extent within the City of Sebastian.
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