HomeMy WebLinkAbout04 - Water Quality AssessmentCity of Sebastian Stormwater Master Plan Update
4 Water Quality Assessment
4.1 Effects of Urban Stormwater Runoff
Stormwater runoff from urbanized areas often has negative effects on downstream waterways and receiving
waters. The extent and type of such effects vary, but they are often significant relative to other sources of pollution
and environmental degradation. Urban stormwater runoff has both physical and chemical effects that impact
water quality, water quantity, habitat and biological resources, public health, and the aesthetic appearance of
urban waterways.
Adverse effects on receiving waters associated with stormwater discharges fall into one of the three following
general classes:
• Short-term water quality effects during and after storm events. Such changes include temporary
increases in the concentration of one or more pollutants, toxics or bacteria levels.
• Long-term water quality effects. These are cumulative effects associated with repeated stormwater
discharges from many sources.
• Physical effects on the receiving waterbody. Examples include erosion, scour, deposition, plant and
aquatic species changes, and other effects associated with increased frequency and volume of runoff that
alters aquatic habitat.
A further discussion of these effects follows.
4.1.1 Water Quality Effects
Water quality effects pertain to changes in water chemistry and can significantly reduce the capacity of a
waterbody to support life or to maintain its existing ecosystem diversity. The introduction of pollutants into a given
waterbody is the most common means by which water quality is degraded, and the extent of degradation depends
on the type(s) and concentrations of the pollutants.
Pollutants associated with urban runoff potentially harmful to receiving waters fall into the categories listed below:
• Solids
• Oxygen -demanding substances
• Nitrogen and phosphorus
• Pathogens
• Petroleum hydrocarbons (oils and greases)
• Metals
• Synthetic organics (pesticides, herbicides, etc.)
These pollutants degrade water quality in receiving waters near urban areas, and often contribute to the impairment
of use and exceedances of criteria included in State water quality standards. The quantity of these pollutants per unit
area delivered to receiving waters tends to increase with the degree of development in urban areas. As shown in
Table 4-1 below, contaminants enter stormwater from a variety of sources in the urban landscape.
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Table 4-1. Contaminant Sources
Sediment and Floatables
Pesticides and Herbicides
Roof systems, streets, lawns, driveways, roads, construction activities,
atmospheric deposition, drainage channel erosion
Residential lawns and gardens, roadsides, utility right-of-ways, commercial and
industrial landscaped areas, soil wash -off, etc.
Roads, residential lawns and gardens, commercial landscaping, animal wastes
Automobiles, bridges, atmospheric deposition, industrial areas, soil erosion,
Organic Materials
Metals
corroding metal surfaces, combustion processes
Oil and Grease/Hydrocarbons
Roads, driveways, parking lots, vehicle maintenance areas, gas stations, illicit
dumping to storm drains
Bacteria and Viruses
Lawns, roads, leaky sanitary sewer lines, sanitary sewer cross -connections,
animal waste, septic systems
Nitrogen and Phosphorus
Lawn fertilizers, atmospheric deposition, automobile exhaust, soil erosion, animal
waste, detergents
The concentrations of pollutants found in urban runoff are directly related to the level of development within the
watershed. This relationship is shown in Table 4-2, a compilation of typical pollutant loadings from different urban
land uses13
Table 4-2. Typical Pollutant Loadings from Urban Runoff by Land Use (lbs./acre year)
Commercial
1000
1.5
6.7
Parking Lot
400
0.7
5.1
High Density Residential
420
1.0
4.2
Medium Density Residential
190
0.5
2.5
Low Density Residential
10
0.04
0.03
Freeway
880
0.9
7.9
Industrial
860
1.3 I
3.8
Park
3
0.03
1.5
Construction
6000
80
NA
Notes:
NA: Not Available - insufficient data to characterize
loadings.
1.9
3.1
2.9
2.0
1.4
0.1
62
47
27
13
NA
420
270
170
72
NA
2.7
0.8
0.8
0.2
0.01
2.1
0.8
0.7
0.2
0.04
0.40
0.04
0.03
0.14
0.01
2.0
0.8
0.5
0.02
1.5
4.2
NA
NA
4.5
2.1
0.37
0.2
1.3
NA
NA
2.4
7.3
0.5
NA
0.3
NA
2
0
NA
NA
NA
NA
NA
NA
NA
NA
NA
Other water quality parameters are not necessarily related to pollutants but are also very important to the overall
health of aquatic ecosystems. These include such parameters as pH, temperature, dissolved oxygen, and salinity.
13 Homer et al., 1994 in Protocol for Developing Nutrient TMDLs (USEPA 1999)
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4.1.1.1 Short -Term Water Quality Effects
The short-term effects of pollutants on receiving waters vary widely, ranging from minor changes in temperature
or salinity to complete loss of species or habitat, in the case of a major oil spill or other serious contamination
event. In most cases, these changes usually only last for a few days or weeks following a storm event. However,
repeated introduction of pollutants can lead to long-term effects, which are much more difficult to address.
4.1.1.2 Long -Term Water Quality Effects
Long-term water quality effects are those resulting from either large-scale contamination events or repeated
introductions of smaller pollutant loadings over time. In either case, significant amounts of pollutants are
introduced into a receiving water. Depending on the type of pollutant, they may bioaccumulate in aquatic plant
and animal species, be deposited into sediments, reduce dissolved oxygen levels, or provide food for aquatic
weed species. These effects can last for many years, can seriously impair the health and use of a waterbody,
and are difficult to correct.
4.1.2 Physical Effects
While water quality effects are not usually observed by the general public, the physical effects of stormwater
runoff are more visible. Stream channel and channel bank erosion provide direct evidence of water velocity
impacts caused by urban stormwater. The volume of urban stormwater runoff increases directly with the amount
of impervious area and the level of development within a watershed. As development continues, urban streams
are often forced to accommodate larger volumes of stormwater runoff that recur on a more frequent basis. This
leads to overloading the stream capacity and results in channel instability. The change in watershed hydrology
associated with urban development also causes channel widening and scour, and the introduction of larger
amounts of sediment and pollutants to urban streams.
Visible impacts include eroded and exposed stream banks, channel slope failures, fallen trees, sedimentation,
and recognizably turbid or murky conditions. The increased frequency of flooding in urban areas also poses a
threat to public safety and property. Both water quality and water quantity impacts associated with urban
stormwater negatively affect aquatic and riparian (shore edges or similar) habitat in urban streams. Rapid salinity
changes, higher levels of pollutants, increased flow velocities and erosion, alteration of riparian corridors, and
sedimentation associated with stormwater runoff can all negatively impact the integrity of aquatic ecosystems.
These impacts include the degradation and loss of aquatic habitat, and reduction in the numbers and diversity of
wildlife species.
Public health effects are for the most part related to bacteria and disease -causing organisms carried by urban
stormwater runoff into waters used for water supplies, fishing and recreation. Water supplies can potentially be
contaminated by urban runoff, posing a public health threat. People coming in contact with contaminated water at
beaches and other recreational sites can become seriously ill. Beach closures caused by urban runoff have a
negative impact on the quality of life and can impede economic development as well. Similarly, the bacterial
contamination of shellfish beds poses a public health threat to consumers, and shellfish bed closures negatively
impact the fishing industry and local economies.
Debris and litter floating in urban waterways and deposited on stream banks and beaches are aesthetic impacts
that are particularly noticeable to the general public. Stormwater is a major source of floatables that include paper
and plastic bags and packaging materials, bottles, cans, and wood. The presence of floatables and other debris in
receiving waters during and following storm events reduces visual attractiveness of the waters and detracts from
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their recreational value. Nuisance algal conditions including surface scum and odor problems can also be
attributed to urban stormwater in many instances.
Stormwater runoff from urban areas can contain significant concentrations of harmful pollutants that can
contribute to adverse water quality impacts in receiving streams. Effects can include such things as beach
closures, shellfish bed closures, limits on fishing and limits on recreational contact in waters that receive
stormwater discharges.
4.2 Best Management Practices
BMPs are techniques, approaches, or designs that promote sound use and protection of natural resources to
meet program goals and levels of service (LOS). BMPs are also required for permitting of both new development
and retrofit of existing systems. The BMPs discussed in this section offer the most potential for application in the
City. Since this SWMP deals primarily with existing systems in urbanized areas, emphasis was placed on BMPs
that are better suited for that situation. Where possible, BMP descriptions and design information were taken from
the latest version of the SJRWMD Permit Information Manual, dated June 1, 2018.
The use of a specific BMP depends on the site conditions and objectives such as water quality protection, flood
control, or volume control. In many cases, there are multiple goals or needs for a given project. Therefore,
multiple BMPs can be used in sequence to develop a "treatment train". The treatment train concept is intended to
maximize the use of available site conditions from the point of runoff generation to the receiving water discharge
in order to maximize water quantity (flood control), water quality (pollutant load reduction), and wetlands benefits.
4.2.1 Best Management Practices (BMPs) Considerations
This section presents descriptions of various BMPs that are either currently in use or should be considered for
use in the City SWMS. There are many BMPs available beyond those presented here, but the list was limited to
only those BMPs that the City is already using or those for which the City can receive significant pollutant
reduction credit under the CIRL BMAP if implemented. The BMPs are grouped as non-structural (regulation or
ordinances) and structural (constructed facilities).
4.2.1.1 Non-structural Source Controls
• Land use planning
• Public information programs
• Stormwater management ordinance requirements
• Fertilizer application controls
• Pesticide and herbicide use controls
• Solid waste management
• Directly Connected Impervious Area (DCIA) minimization
• Erosion and sediment control on construction sites
4.2.1.1.1 Land Use Planning
Land use planning and management presents an important opportunity to reduce/minimize pollutants in stormwater
runoff and control flooding by using a comprehensive planning process to integrate City goals into the development
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and redevelopment process. Management measures may include modification or restrictions of certain land use
activities and modification of existing City land development codes, perhaps even more restrictive than SJRWMD
requirements. Land development codes to include larger storage volumes, stormwater harvesting, or other
requirements for new developments or redevelopments can, in time, have significant positive effects on the City's
SWMS. Greater restrictions may be warranted where development can affect impaired, threatened, or significant
water bodies such as the IRL. Because increased pollutant loadings and flooding correspond to increase in
impervious cover, land use planning and local land development codes can become effective control measures.
4.2.1.1.2 Public Information Program
A public information participation plan provides the City with a strategy for informing its employees, the public, and
businesses about the importance of protecting stormwater from improperly used, stored, and disposed pollutants.
Many people do not realize that yard debris or trash thrown into ditches today will worsen tomorrow's flooding and
pollute surface waters. Municipal employees must be trained, especially those that work in departments not
directly related to stormwater but whose actions affect stormwater. Residents must become aware that a variety
of hazardous products are used in the home and that their improper use and disposal can pollute stormwater.
Likewise, improper disposal of oils, antifreeze, paints, and solvents can end up in streams and lakes, poisoning
fish and wildlife. If care is taken by individuals to properly dispose of yard debris, trash and hazardous materials,
many problems can be reduced in magnitude or avoided. Increased public awareness also facilitates public
scrutiny of industrial and municipal activities and will likely increase public reporting of incidents. Businesses,
particularly smaller ones that may not be regulated by Federal, State, or local regulations, must be informed of
ways to reduce their potential to pollute Stormwater. A key element of this program is public awareness of the
benefits of roadside swales. These BMPs cost- effectively provide both water quantity and water quality benefits.
The perception by many citizens is that shallow ponding (four to six inches) for one or two days after storms
during the wet season is a problem. In reality, this shallow ponding and infiltration is the onsite storage that saves
money by reducing pipe sizes and cost-effectively providing water quality treatment.
4.2.1.1.3 Fertilizer Application Control
Fertilizer application control is a voluntary control mechanism by citizens who use fertilizer as part of their
landscaping activities. Fertilizer application controls are implemented through a public information program by
making the public aware of the principals of environmental landscape maintenance and the problems associated
with overuse of fertilizers. Overuse of fertilizers will cause excessive runoff of nutrients to surface waters thereby
wasting money for the homeowner and potentially degrading the receiving water body. This is especially true
during heavy rainfall periods that produce yard and neighborhood flooding. Information programs should also be
extended to professional fertilizer users.
The City has implemented fertilizer application controls through its Integrated Pest Management (IPM) program,
as well as its fertilizer ordinance. See Section 1.4.2.6 for more information. However, further education and
enforcement of these items are necessary.
4.2.1.1.4 Pesticide Use Control
Pesticide use control is also a voluntary control by citizens who use pesticides as a part of their housekeeping
and lawn maintenance activities. Some pesticides are priority pollutants (e.g., Endrin, Lindane, and Silvex), which
can be toxic. Overuse of these chemicals can cause excessive runoff to surface waters and entry into the food
chain. Many professional applicators of pesticides are using approved pesticides in a safe and proper manner. An
information program on pesticide use will help to reduce the amount of pesticides entering the stormwater system.
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The City has implemented pesticide use controls through its Integrated Pest Management (IPM) program, as well
as its fertilizer ordinance. See Section 1.4.2.6 for more information.
4.2.1.1.5 Solid Waste Management
In some instances, problems can arise from trash and other debris flowing into and obstructing open channels,
culverts, and storm sewers. It is recommended that the public be informed of the adverse impacts of littering and
poor solid waste management. This can also include pet droppings and illegal dumping into storm drains, wooded
areas, and ditches. Pet droppings can be a source of coliform bacteria and pathogens.
4.2.1.1.6 DCIA Minimization
Another non-structural BMP option would be for the City to minimize the amount of Directly Connected Impervious
Area (DCIA) on a site and promote the use of green buffer zones around paved areas for infiltration. For example,
roof runoff from structures can be directed to green buffer zones or shallow swales around houses. Requiring
retrofitting of roof drainage systems for all existing and new construction would likely be difficult, but changing City
building codes to require roof discharge to green areas for all new construction or significant repairs/alterations
could be easier to accomplish. In addition, parking lots and driveways can be graded to landscaped/grassed
areas or swales, reducing direct runoff to the storm drainage system.
4.2.1.1.7 Erosion and Sediment Control on Construction Sites
Erosion and sediment control on construction sites provides for the protection of receiving waters from sediment
loads. Proper control during construction can be accomplished with gravel filter weirs, sediment fences, and
temporary berms or swales. The City has implemented erosion and sediment controls through City Ordinance No.
54-3-11.2 an ordinance requiring erosion and sediment control on construction sites.
4.2.1.2 Structural Stormwater Controls
• Retention systems
• Dry detention systems
• Underdrain systems
• Wet detention systems
• Wetland Systems
• Exfiltration trenches
• Grassed swales and channels
• Water quality inlets and baffle boxes
• Skimmers
• Dams
• Aeration Systems
4.2.1.2.1 Retention Systems
The SJRWMD defines a retention system as a storage area designed to store a defined quantity of runoff,
allowing it to percolate through permeable soils into the shallow ground water aquifer. Soil permeability and water
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table conditions must be such that the retention system can percolate the desired runoff volume within a specified
time following a storm event. After drawdown has been completed, the basin does not hold any water, thus the
system is normally "dry." Unlike detention basins, the treatment volume for retention systems is not discharged to
surface waters. Examples of retention systems include:
• Man-made or natural depressional areas
where the floor is graded as flat as
possible and turf is established to promote
infiltration and stabilize the basin slopes
(see Figure 4-1)
• Shallow landscaped areas designed to
store stormwater
• Vegetated swales with swale blocks or
raised inlets
• Pervious concrete with continuous curb
weir Creel Ele�uon
J
Slerog. [If r�quired�
51. . (IF q
inalmenl
Velum
54aroge
irye}meni V011ime
W
1
A.cnrery by
by
I
lnnn.
111
S60"a l high
g—nd—f.,
luhla .1-11—
Figure 4-1. Typical Retention Pond Design (from SJRWMD)
Stormwater retention works best where soils are highly permeable and the seasonal high water table is situated
well below the soil surface (at least 2 to 3 feet below pond bottom). The geology, soils, and groundwater
conditions in the City of Sebastian are generally not well suited for the use of retention systems. However,
retention systems may be suitable for use at individual urban redevelopment or retrofit sites within the watershed.
The application of retention systems should be considered on a case -by -case basis within the study area where
soils and water table conditions are suitable. The City currently uses several retention systems in the SWMS.
Potential Benefits of a Retention System
• Mimics the natural water balance of a site by promoting groundwater recharge close to the point of runoff
generation.
• Can provide offline or on-line treatment for environmentally sensitive waters (e.g., Class II)
• Reduces peak rate and volume of flood discharge by retaining water onsite.
• Can be used as sediment traps during the construction phase of a project.
• Are reasonably cost-effective in comparison with other BMPs for both construction and maintenance
costs (where soils are favorable).
• Effectively reduce pollutant loadings to receiving waters.
Potential Limitations of a Retention Basin
• Require well -drained soils to function properly.
• Unsuitable soils limit drawdown capacity, thereby reducing pollutant removal and flood control capacity.
• Soluble pollutants can be conveyed into groundwater.
• Possible nuisances such as odors, mosquitoes, and nuisance vegetation can occur if not designed,
constructed, or maintained properly.
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4.2.1.2.2 Dry Detention Systems
Dry detention systems are normally dry storage areas which are designed to store a defined quantity of runoff and
slowly release the collected runoff through an outlet structure to adjacent surface waters. After drawdown of the
stored runoff is completed, the storage basin does not hold any water, thus the system is normally "dry." A
schematic of a typical dry detention system is presented in Figure 4-2.
Dry detention basins are similar to
retention systems in that the basins
are normally dry. However, the main
difference between the two systems is
that retention systems are designed to
percolate the stored runoff into the
ground while dry detention systems
are designed to discharge the runoff
through an outlet structure to adjacent
surface waters.
Sedimentation is the primary pollutant
p 710.111
WNr Cn.f Flerellon I �
� Peak oilenuaHon y Weir
�Siena' (if M Ir1 LEJ
fraafmrnl Gonirol $lrw;lure
IIo.r /� pui[4ow Plpa
L Orff Ira
Rote! ants-ctoligtng d—t- a Seaaonai high
net ehern on control grenndwoier
�tructurr [see sis. 10.5] fobla �Lrplivn
Figure 4-2. Typical Dry Detention Basin Design (from SJRWMD)
removal process which occurs in dry detention systems. Unfortunately, only pollutants which are primarily in
particulate form are removed by sedimentation. Therefore, the pollutant removal efficiency of dry detention
systems is not as great as systems such as retention and wet detention which remove both dissolved and
particulate pollutants. Because of the limited pollutant removal efficiency of dry detention, this BMP must only be
utilized where no other BMP is technically feasible. For example, use of dry detention must be restricted to the
following situations:
(a) Where high ground water table or soil conditions limit the feasibility of other BMPs such as retention, and
(b) Small drainage basins (less than 5 acres). For larger projects (greater than 5 acres) other BMPs like wet
detention shall be utilized instead of dry detention.
There are several design and performance criteria which must be met in order for a dry detention system to meet
the District's requirements, including off-line detention volume, recovery time, outlet structure design, basin
bottom and control elevations, length -to -width ratio, and maintenance requirements. Note that SJRWMD currently
only permits off-line dry detention basins, which limits their application in some cases.
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 or sold (pond sediment removal).
• Low frequency of failure as compared with filtration systems.
Potential Limitations of a Dry Detention Pond
• Does not remove dissolved pollutants (nutrients) unless a permanent pool is included.
• Potential safety hazards if not designed and constructed properly.
• No permanent pool to store sediment inflow.
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• Occasional nuisance problems such as debris and mosquitoes.
• Regular maintenance is required to prevent nuisance plant species from emerging and to remove
accumulated sediments.
• Must be off-line.
4.2.1.2.3 Underdrain Systems
Stormwater underdrain systems consist of a dry basin underlain with perforated drainage pipe which collects and
conveys stormwater following percolation from the basin through suitable soil. Underdrain systems are generally
used where high water table conditions dictate that recovery of the stormwater treatment volume cannot be
achieved by natural percolation (i.e., retention systems) and suitable outfall conditions exist to convey flows from
the underdrain system to receiving waters.
A cross section of a typical underdrain
system is shown in Figure 4-2. Underdrain
systems are intended to control both the
water table elevation over the entire area
of the treatment basin and provide for the
drawdown of the treatment volume.
Underdrains are utilized where the soil
permeability is adequate to recover the
treatment volume since the on -site soils overlay
the perforated drainage pipes.
undrrdraln Pipe
Natural Craund 0"allon
F _ _ �
CFv�Mroler
Takla
fleuallon
Flller T.brk
Figure 4-3. Typical Underdrain System Design (from SJRWMD)
There are several design and performance criteria
which must be met in order for an underdrain system to meet SJRWMD requirements, including the following:
The system should be designed to provide for the drawdown of the appropriate treatment volume within
72 hours following a storm event. The treatment volume is recovered by percolation through the soil with
subsequent transport through the underdrain pipes.
The system should only contain standing water within 72 hours of a storm event. The pipe system
configuration (e.g., pipe size, depth, pipe spacing, and pipe inflow capacity) of the underdrain system
must be designed to achieve the recovery time requirement.
• The underdrain system must be designed with a safety factor of at least two unless the applicant
affirmatively demonstrates based on plans, test results, calculations or other information that a lower
safety factor is appropriate for the specific site conditions.
4.2.1.2.4 Wet Detention Systems
Wet detention systems are permanently wet ponds which are designed to slowly release collected stormwater
runoff through an outlet structure. A schematic of a typical wet detention system is shown in Figure 4-4.
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There are several components in a wet
detention system which must be properly
designed to achieve the required level of
stormwater treatment. A description of each
design feature and its importance to the
treatment process is presented below.
A wet detention system includes a permanent
pool of water, a shallow littoral zone with
aquatic plants, and the capacity to provide
detention for an extended time necessary for
LHIO G1 icn. c .b.. O,rlfnH 'I—N. Snd
T.o.a. {e r.ywr.d]
�
LP.ek eH.n�ellnn (If r.pdryd) W.In fi_
r— Orllke
Icbl. I.rollon b ^— ir.e}m.n! re/um� "��.'enlrol glweHen
P.rmen.nl peel
Figure 4-4. Typical Wet Detention System Design (from SJRWMD)
the treatment of a required volume of runoff.
In wet detention ponds, pollutant removal
occurs primarily within the permanent pool during the period of time between storm events. They are typically
sized to provide at least a two -week hydraulic residence time during the wet season. The primary mechanism for
the removal of a particulate forms of pollutants in wet detention ponds is sedimentation. Wet detention systems
can also achieve substantial reductions in soluble nutrients due to biological and physical/chemical processes
within the permanent pool such as uptake of nutrients by algae and rooted aquatic plants, adsorption of nutrients
and heavy metals onto bottom sediments, and biological oxidation of organic materials.
Wet detention systems are usually more visually appealing than dry ponds, particularly if there is desirable
wetland vegetation around the perimeter of the permanent pool. When properly designed and constructed, wet
detention ponds are actually considered as property value amenities in many areas. Also, wet detention ponds
offer the advantage that sediment and debris accumulate within the permanent pool. Since this accumulation is
out -of -sight and well below the pond outlet, wet detention ponds tend to require less frequent clean outs to
maintain an attractive appearance and prevent clogging. Sediment forebay areas (or sumps) are recommended
whenever possible.
The City has several wet detention systems in use, but due to the land area needed for these systems they may
not be suitable for future systems as undeveloped area is very limited within the City's boundaries.
Potential Benefits of a Wet Detention System
• Reduction of downstream flooding problems by attenuating the peak rate of flow.
• Reduction in pollutant loadings to receiving waters for dissolved and suspended pollutants.
• Reduction in cost for downstream conveyance facilities.
• Creation of local wildlife habitat.
• More aesthetically pleasing than dry detention/retention systems.
• Low frequency of failure.
• Can be used in areas with high water tables and less permeable soils.
• Pollutant removal can be optimized with pretreatment such as retention swales.
Potential Limitations of a Wet Detention Pond
• Land area needed to meet dimensional requirements.
• Potential safety hazards if not designed and constructed properly (gradual slide slopes are desirable).
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• Occasional nuisance problems such as odors, algae, debris, and mosquitoes.
• Regular maintenance of the littoral zone is required to control nuisance plant species.
• Recurring need for sediment removal from the permanent pool or sediment forebay.
4.2.1.2.5 Wetland Systems
Wetland systems incorporate either a natural or man-made wetlands area as part of a comprehensive stormwater
management system in combination with other best management practices to provide treatment of runoff. The
City's Stormwater Park was designed with a wetland system to improve nutrient removal and effluent quality. For
these systems, the SJRWMD must ensure that a proposed wetlands stormwater management system is compatible
with the existing ecological characteristics of the wetlands proposed to be utilized for stormwater treatment.
The only wetlands which may be considered for use to provide stormwater treatment are those which are:
(a) Isolated wetlands; and
(b) Those which would be isolated wetlands, but for a hydrologic connection to other wetlands or surface waters
via another watercourse that was excavated through uplands.
The District must also ensure that water quality standards will not be violated by discharges from wetlands
stormwater management system. To achieve these goals, specific performance criteria are set by the SJRWMD
for systems which incorporate wetlands for stormwater treatment, including the required treatment volume,
recovery time, maintenance of wetland hydroperiod, and others.
The design features of the system should maximize residence time of the stormwater within the wetland to
enhance the opportunity for the stormwater to come into contact with the wetland sediment, vegetation, and
micro-organisms (Livingston 1989).
Potential Benefits of a Wetland System:
• Reduction in pollutant loadings to receiving waters for dissolved and suspended pollutants.
• High -quality wildlife habitat
• Aesthetically pleasing
Potential Limitations of a Wetland System:
• Higher maintenance effort and cost — control of invasive species, periodic sediment removal, pump
systems and control structure maintenance
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4.2.1.2.6 Exfiltration Trenches
An exfiltration trench is a subsurface system consisting of a conduit such as perforated pipe surrounded by natural
or artificial aggregate which temporarily stores and infiltrates stormwater runoff (Figure 4-5). Stormwater passes
through the perforated pipe and infiltrates through the trench walls and bottom into the shallow groundwater aquifer.
The perforated pipe increases the storage available in the trench and helps promote infiltration by making delivery of
the runoff more effective and evenly distributed over the length of the system. Generally, exfiltration trench systems
are utilized where space is limited and/or land costs are high
Pavemen}
(i.e., downtown urban areas).
........................ ........
Soil permeability and water table conditions must be such that
the trench system can percolate the required stormwater
runoff treatment volume within a specified time following a
storm event. The trench system is returned to a normally "dry"
condition when drawdown of the treatment volume is
completed. Like retention basins, the treatment volume in
exfiltration trench systems is not discharged to surface waters.
Thus, exfiltration is considered a type of retention system.
Select Fill
O°�O 0 0 e0
nd'-D
o + -Coarse Aggregate
va 'oa
�8a Perforated Pipe
0
o�c en
-Fitter Cloth
O nOO�e DoeOoD
4
Seasonal High Groundwater Table
The operational life of an exfiltration trench is believed to be
short (possibly 5 to 10 years) for most exfiltration systems. Figure 4-5. Typical Exfiltration Trench Design
Sediment accumulation and clogging by fines can reduce the (from SJRWMD)
life of an exfiltration trench. Total replacement of the trench
may be the only possible means of restoring the treatment capacity and recovery of the system. Periodic
replacement of the trench should be considered routine operational maintenance when selecting this
management practice.
The City has a few exfiltration trench systems within its SWMS, but only on the east side of the Coastal Ridge,
where the groundwater conditions are suitable. Due to the high groundwater conditions throughout most of the
City and short operational life of these BMPs, they do not appear to be cost-effective and the City should consider
other options.
Potential Benefits of an Exfiltration Trench
• They mimic the natural groundwater recharge capabilities of the site.
• Can fit into space -constrained areas of a development site, including under 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
Potential Limitations of an Exfiltration Trench
• Require highly permeable soils to function properly.
• Highly susceptible to sediment and fines loading.
• Have relative short life spans before replacement or extensive restoration/maintenance of systems is required.
• Often more costly than other treatment alternatives, especially when operation and maintenance costs
are considered.
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City of Sebastian Stormwater Master Plan Update
4.2.1.2.7 Grassed Swales and Channels
According to the SJRWMD, swales are a man-made or natural system shaped or graded to required dimensions
and designed for the conveyance and rapid infiltration of stormwater runoff. Swales are designed to infiltrate a
defined quantity of runoff through the permeable soils of the swale floor and side slopes into the shallow ground
water aquifer (Figure 4-6). w:v z a:+
Turf is established to promote infiltration
and stabilize the side slopes. Soil
permeability and water table conditions
must be such that the swale can percolate
the desired runoff volume from the 3-year,
1-hour storm event. The swale holds water
only during and immediately after a storm
event, thus the system is normally "dry." Unlike
retention basins, swales are "open" conveyance
systems. This means there are no physical
w
ir�o}manl � —
rrF VoFume 4 Sdd� elopes Z S(Hi:1(V)
y S�orop�
Tr.almrnl Yalum� � !'. Y'
R.cowry by f — —� i
Inlillrcrlon
S0n20nnl NO
CI levallon
Figure 4-6. Typical Swale Design (from SJRWMD)
barriers such as berms or check -dams to impound the runoff in the swale prior to discharge to the receiving water.
Swales are normally used for conveyance systems to transport runoff offsite or to a stormwater facility. They are
best suited for sites with soils of moderate -to -high infiltration capacity (usually Hydrologic Groups A or B).
Grassed swales and ditches are widely used throughout the City of Sebastian's SWMS, but only really serve as a
conveyance structure in most areas because their infiltration capacity is greatly limited due to the high
groundwater conditions.
Potential 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 (3:1 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 be used in space -constrained areas such as along lot lines, rear of lots, and along roads.
• Can be used as water quality treatment or pretreatment with other BMPs in a treatment train.
Potential limitations of Shallow Grassed Swales
• Due to the long growing season in Florida, grassed swales are maintenance intensive and incur
significant recurring costs for mowing and sediment removal.
• Effective only as a conveyance system in unsuitable soils.
• Possible nuisances such as odors, mosquitoes, or nuisance plant species can occur if not designed,
constructed, or maintained properly.
• Aesthetically unpleasing if improperly designed and constructed (deep with steep side slopes -looks like a ditch).
• May not be suitable or may require geotextile matting in areas that serve as vehicle parking areas.
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City of Sebastian Stormwater Master Plan Update
4.2.1.2.8 Inlet Filters
Inlet filters are simple BMPs that are
designed to fit under the grate in a
storm inlet structure and capture and
separate solids (i.e., soil, road grit,
grass clippings, etc.) and certain
pollutants, such as hydrocarbons,
from stormwater runoff as it flows into
a storm inlet structure. They help
reduce the deposition of solids in
conveyance pipes and improve water
quality within the SWMS. Inlet filters
can fit a variety of inlet structures
and are readily available. One
example of an inlet filter is shown in
Figure 4-7 at right and is presented
in the section below.
GRATE
"ULTIMATE" BYPASS
FEATURES
GASKET
STAINLESS STEEL
SUPPORT BASKET
Fosslf Rot* —
ABSORBENT POUCHES
LINER
SUPPORT
BASKET
CATCH BASIN
FLAT GRATE STYLE)
The FlogardTM Inlet Filter DETAIL
manufactured by Oldcastle E%PLODEDMEW
Infrastructure is a specialized inlet
filter used specifically for grated Figure 4-7. FlogardTM Inlet Filter and Exploded View
catch basins. The unit is made of _
stainless steel for long service life
and durability. During a storm event, all incoming stormwater and solid material fall into the liner, which retains
solid particles and causes pollutants to come in contact with absorbent material. Treated stormwater passes into
the catch basin system, and the filter dries after each storm event. Collected debris is suspended and stored in a
dry state above static water level until removed during service. According to the manufacturer, servicing only
takes a few minutes and can be completed by simply lifting out the filter and dumping it into a container. No
vactor truck is required, but recurring labor time and costs should be considered. The purchase price of a Flogard
unit for a standard FDOT Type C inlet is approximately $1,500.00.
Benefits:
• Will not impede inlet water flow
• Captures debris and sediment
• Bypass openings prevent clogging
• Minimal space requirements
• Quick service times at an average of 15 minutes or less.
Potential Limitations:
• High initial cost due to number of inlets (approximately 320 in City's SWMS) but can be acquired a few at
a time to reduce the capital costs.
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City of Sebastian Stormwater Master Plan Update
• Require consistent, regular maintenance cleaning. Units should be cleaned approximately every four
weeks but maintenance scheduled should be adjusted depending on storm frequency and observed
loading rates.
4.2.1.2.9 Baffle Boxes
Baffle boxes are typically installed near or at the storm sewer outfall. They typically consist of a rectangular
concrete box divided into three chambers where floatables, sediment, grit, and oil are separated from stormwater
runoff as it passes through the chambers before exiting through an outlet to the storm drain system. The first
chamber is designed for sediment trapping, and the second chamber is designed for oil separation. Each
chamber contains a permanent pool and is accessible through manhole covers. The City currently has eight
baffle boxes in its SWMS.
A baffle box is a good choice for a water quality control device in areas where the other more traditional measures
discussed previously may not be applicable due to various constraints. The design of a baffle box is identical to a
primary clarifier with the addition of a skimmer for floatables. Target pollutant sizes are fine sands and larger size
particles. There are limited percent pollutant removal data on these devices, but the sediment and debris removed
can be quantified when the boxed is cleaned.
Maintenance requirements vary by device and application, but generally require a minimum of cleaning the
chambers at least twice a year to remove pollutants. The City currently cleans its baffle boxes on a quarterly
basis. Frequent maintenance is essential for the effective removal of pollutants using these systems. The cleaning
process from these devices includes pumping out the contents of each chamber into a tank truck. If the entire
contents are pumped out as a slurry, they are then transferred to a sewage treatment system. If the runoff is
separated from the sediments by onsite siphoning, the sediments can be trucked to a landfill for final disposal.
These maintenance operations can be costly.
4.2.1.2.10 Aeration Systems
The canals and ponds within the SWMS are shallow and warm, with very little water movement. Stormwater
runoff during storm events washes pollutants like nitrogen and phosphorus into the water, and their
concentrations increase, resulting in algal blooms, increasing BOD, reducing dissolved oxygen content, and many
other issues. The City has a few surface aerators in the canals, but they have a low oxygen transfer efficiency
and do not have much effect on overall water quality. Diffused aeration systems are much more effective at
introducing oxygen to water, which increases oxidation reactions and can significantly reduce the concentrations
of phosphorus, ammonia, BOD and COD, and promotes beneficial bacteria. Properly designed and constructed
diffused aeration systems could significantly improve water quality and are economical to operate and maintain.
However, aeration systems are not currently eligible for BMAP nutrient reduction credit and should only be
considered by the City if implementation is done in coordination with regulatory agencies and BMAP revisions that
will result in City nutrient reduction credits. A pilot study using an aeration system in a canal segment might be a
potential segway to regulatory acceptance for these systems, which have had demonstrated success improving
water quality in canals and ponds in Florida.
4.3 State Water Quality Standards
State water quality standards are established by FDEP and are set forth in chapters 62-4, 62-302, 62-520 and 62-
550, F.A.C. Surface and ground water discharges from stormwater management systems, works, and other
projects may not cause or contribute to a violation of state water quality standards.
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City of Sebastian Stormwater Master Plan Update
4.3.1 Surface Water Quality Standards
State water quality standards for surface waters are contained in chapters 62-4 and 62-302, F.A.C. The standards
apply at the point of mixing of discharge from the system with waters of the State. All surface waters of the State
have been classified according to designated uses as shown in Table 4-3. Water quality classifications are
arranged in order of the degree of protection required, with Class I water having generally the most stringent
water quality criteria and Class V the least. However, Class I, II, and III surface waters share water quality criteria
established to protect fish consumption, recreation and the propagation and maintenance of a healthy, well-
balanced population of fish and wildlife.
Table 4-3. Florida Surface Water Classifications
ANOWJ
CLASS I
Potable Water Supplies
CLASS I -Treated
Treated Potable Water Supplies
CLASS 11
Shellfish Propagation or Harvesting
CLASS III
Fish Consumption; Recreation, Propagation and Maintenance of a Healthy, Well -
Balanced Population of Fish and Wildlife
CLASS III -Limited
Fish Consumption; Recreation or Limited Recreation; and/or Propagation and
Maintenance of a Limited Population of Fish and Wildlife
CLASS IV
Agricultural Water Supplies
CLASS V
Navigation, Utility and Industrial Use
The IRL and the South Prong of the St. Sebastian River are classified as Class II surface waters. Chapter 62-
302.300(13), F.A.C. states that "...excessive nutrients (total nitrogen and total phosphorus) constitute one of the
most severe water quality problems facing the State. It shall be the Department's policy to limit the introduction of
man -induced nutrients into waters of the State. Particular consideration shall be given to the protection from
further nutrient enrichment of waters which are presently high in nutrient concentrations or sensitive to further
nutrient concentrations and sensitive to further nutrient loadings..." In addition to nutrient loadings of total nitrogen
and total phosphorus, dissolved oxygen levels play a critical role in the overall health of surface waters. A brief
discussion of the importance of these parameters on water quality is in the sections that follow.
4.3.2 Nutrient Loadings (Nitrogen and Phosphorus)
According to the EPA, nutrient pollution is one of America's most widespread, costly and challenging
environmental problems, and is caused by excess nitrogen and phosphorus in the air and water. Nitrogen and
phosphorus are nutrients that are natural parts of aquatic ecosystems. Nitrogen is also the most abundant
element in the atmosphere. Nitrogen and phosphorus support the growth of algae and aquatic plants, which
provide food and habitat for fish, shellfish and smaller organisms that live in water.
However, when too much nitrogen and phosphorus enter the environment, the air and water can become polluted.
The primary sources of nitrogen and phosphorus from human activities include urban and agricultural runoff,
wastewater treatment facilities, and septic systems. Septic systems are a particular concern in the City of Sebastian,
as more than 90% of the residences are equipped with them. According to the EPA, when a septic system is
improperly managed, elevated nitrogen and phosphorus levels can be released into local water bodies or ground
water. An estimated 10 to 20 percent of septic systems fail at some point in their operational lifetimes. The sheer
number of septic systems within the City and their typical failure rates make them a significant potential source of
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City of Sebastian Stormwater Master Plan Update
nitrogen and phosphorus loading on the City's SWMS. However, this issue cannot be quickly or easily solved,
because the City does not have sufficient wastewater treatment facility capacity or the sewer collection infrastructure
necessary for a centralized sanitary sewer system. The City has a septic -to -sewer conversion program, but to date
only a few of the existing septic systems have been eliminated.
Excessive nitrogen and phosphorus in the water causes algae to grow faster than ecosystems can handle.
Significant increases in algae harm water quality, food resources and habitats, and decrease the oxygen that fish
and other aquatic life need to survive. Large growths of algae are called algal blooms and they can severely reduce
or eliminate oxygen in the water, leading to illnesses in fish and the death of large numbers of fish. Some algal
blooms are harmful to humans because they produce elevated toxins and bacterial growth that can make people
sick if they come into contact with polluted water, consume tainted fish or shellfish, or drink contaminated water.
Federal regulations related to nutrient pollution are expected to get more stringent, which will drive State
regulations. In a memo dated April 5, 2022, EPA's Office of Water indicated plans to accelerate progress in
controlling nutrient pollution in the nation's waters by scaling up existing, foundational approaches and more
broadly deploying new data assessments, tools, financing approaches, and implementation strategies.
4.3.3 Dissolved Oxygen
The amount of dissolved oxygen (DO) an aquatic organism needs depends upon species, water temperature, and
other factors such as the life stage of an organism. Oxygen demand is a measure of the oxygen used by
microorganisms to decompose organic matter, and is expressed either as biochemical oxygen demand (BOD),
chemical oxygen demand (COD), or nitrogenous oxygen demand (NOD).
Biochemical oxygen demand is the amount of dissolved oxygen needed by aerobic biological organisms
to break down organic material present in a given water sample at certain temperature over a specific
time period. The BOD value is most commonly expressed in milligrams of oxygen consumed per liter of
sample during 5 days of incubation at 20°C.
• Chemical oxygen demand (COD) is a measure of the amount of oxygen that can be consumed by
reactions in a measured solution. It is commonly expressed in mass of oxygen consumed over volume of
solution and expressed in milligrams per liter (mg/L).
• Nitrogenous oxygen demand (NOD) is a quantitative measure of the amount of dissolved oxygen required
for the biological oxidation of nitrogenous material, for example, nitrogen in ammonia, and organic
nitrogen in wastewater.
Waters that have adequate DO under natural conditions but that are impaired for DO typically have one or more
anthropogenic sources of organic matter that create an oxygen demand. For urban stormwater, examples of
organic matter sources include leaves and other yard wastes, pet and animal wastes, fertilizer, sediment
containing organic matter (e.g. topsoil), and hydrocarbons (e.g. oil). A median value for BOD in stormwater runoff
in urban settings is about 8 mg/L, while a median value for COD is about 22 mg/L.
According to the FDEP's TMDL report, multiple environmental factors control DO concentrations in the IRL.
Theoretically, the DO concentration in a given waterbody can be influenced by temperature, salinity, flow, water
depth, photosynthesis, respiration, sediment oxygen demand (SOD), the oxidation of organic carbon or inorganic
reductants, and low DO ground water input. Typically, low DO concentrations were observed in the lagoon during
the summer months (May to September). Occasional DO concentrations lower than 5.0 mg/L were also observed
in other months, but with a much lower frequency. While temperature is an important factor responsible for
changes in DO, saturation DO concentrations under the typical summer water temperature (280 C) and salinity
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City of Sebastian Stormwater Master Plan Update
(above 3 percent) should still be higher than 6.0 mg/L, as long as no other chemical and biochemical processes
are involved (Clescerl et al. 1999). Therefore, DO concentrations lower than 4.0 mg/L in the IRL most likely result
from factors other than temperature and salinity.
Florida's Surface Water Quality Standards require that the DO concentration for Class II and III marine waters
"shall not average less than 5.0 mg/L in a 24-hour period and shall never be less than 4.0 mg/L. Normal daily and
seasonal fluctuations above these levels shall be maintained'.
4.4 Total Maximum Daily Load (TMDL) Requirements
As mentioned previously, the FDEP defines a TMDL as a "scientific determination of the maximum amount of a
given pollutant that a surface water can absorb and still meet the water quality standards that protect human
health and aquatic life." The Indian River Lagoon (IRL) TMDLs are targeted towards seagrass regrowth at water
depths where seagrass historically grew in the lagoon. The seagrass coverage in the IRL has decreased over the
years because of the degradation of water quality conditions.
Chapter 62-304.520, FAC contains the IRL TMDLs, which are divided into several areas of the IRL. The TMDLs
for the Central and southern South Indian River are 278,273 Ibs/year of TN and 53,599 Ibs/year of TP, which
represent a 56% reduction of TN and a 48% reduction of TP based on the year 2000 land use.
As mentioned previously, the Central Indian River Lagoon (CIRL) BMAP addresses adopted TMDLs for certain
tributaries to the CIRL, including the South Prong of the St. Sebastian River. Management actions provided by
stakeholders, including projects, programs, and activities that may reduce nutrient loads to the CIRL, are included
in the BMAP and have to meet several criteria to be considered eligible for credit.
The projects and activities in the CIRL BMAP are critical to the goal of recovering seagrass in the IRL, and their level
of completion are tracked to show stakeholder efforts and progress towards the total required milestone reductions.
FDEP conducts an assessment of progress towards the BMAP milestones every five years, and plan revisions are
made as appropriate. FDEP has established milestones for the years 2025, 2030, and 2035 as follows:
• 5-year milestone in 2025: 35 % or 320,614 Ibs/yr of TN and 77,290 Ibs/yr of TP.
• 10-year milestone in 2030: 70 % or 641,228 Ibs/yr of TN and 154,580 Ibs/yr of TP.
• 15-year milestone in 2035: 100 % or 916,040 Ibs/yr of TN and 220,828 Ibs/yr of TP.
4.5 Water Quality Model Evaluation
4.5.1 Water Quality Model — SWIL
The original BMAP nutrient loads were estimated using the Pollutant Load Screening Model (PLSM) and
represented year 2000 loading in most of the IRL Watershed. SJRWMD developed the seagrass depth limits
based on historical aerial photographs of the IRL from 1943 through 2001. FDEP used the PLSM and the
seagrass depth limits to develop the IRL TMDLs that were adopted by rule.
After the TMDLs were adopted, several MS4 permittees within the IRL Watershed expressed interest in creating a
new watershed model to improve upon the PLSM, which resulted in the development of the Spatial Watershed
Iterative Loading (SWIL) Model. The SWIL model was proposed to FDEP as an alternative to the PLSM to
calculate allocations for the IRL BMAPs and was selected for use. The sections below provide a brief summary of
the SWIL modeling and the processes used to update the TMDLs. For more detailed information, please refer to
the Indian River Lagoon Basin Central Indian River Lagoon BMAP (FDEP, 2021).
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City of Sebastian Stormwater Master Plan Update
As mentioned previously, the TMDLs for the IRL (in Rule 62.304.520, F.A.C.) include a 56% reduction of TN and a
48% reduction of TP from their starting loads. The TMDL allocations were calculated in two steps, first by using the
SWIL Model to establish the total TN and TP load, by project zone. To accomplish this, rainfall inputs from a
representative period were used to simulate a wide range of rainfall patterns and amounts over a multi -year period.
The SWIL outputs were used to generate a GIS-based Load Estimation Tool (LET) that included annual average
loads from the watershed. The LET was then used to calculate updated TN and TP baseloads from all existing
project treatment areas in the BMAP. To account for stakeholder credits for the various project types, the August
2020 FDEP BMP Efficiencies Guidance document was used to determine the appropriate credit calculations.
The second step in determining the TMDL allocations was to apply the respective TMDL percent reduction for TN
or TP to the starting load, which resulted in the TN or TP reduction value. The TN and TP allocations were then
calculated by subtracting the TN or TP reduction value from the TN or TP starting load. Additional adjustments
were made to account for TN and TP loads from natural land areas.
Once the total required reductions for each project zone were determined, the percentage of each stakeholder's
anthropogenic load relative to the total anthropogenic load for the project zone was calculated to determine their loading
contribution, and the percentage of the project zone's required reduction was applied to stakeholders accordingly.
4.5.2 Water Quality Conclusions/Recommendations
The Indian River Lagoon TMDLs were determined by the SWIL model and are included in Chapter 62-304.520,
FAC and divided into several areas of the IRL. The CIRL BMAP was developed to specifically address how the
total required nutrient reductions would be achieved by the stakeholders in the four CIRL project zones by 2035.
To date, the overall progress for TN and TP reductions over all four CIRL BMAP project zones relative to the
established milestones appears to be on track. Figures 4-8 and 4-9 show the latest results for Project Zone SEB,
which includes the City of Sebastian, where the FDEP reported a 23% reduction of TN load and a reduction of
51 % of TP load allocated to Project Zone SEB from stakeholder projects completed through July 31, 2020.
Under the CIRL BMAP, the City of
Sebastian's allowable loadings were
established as shown in Table 4-4. To
accomplish this, the City proposed several
management actions (projects) that were
included in the BMAP. To date, 16
projects have been completed by the City
or are currently underway, resulting in load
reduction credits of 5,223 lbs./year of TN
(15.7% of required reduction) and 620
lbs./year of TP (10.3% of required
reduction).
While the City's performance appears to
be lower than the overall progress
reported for CIRL Project Zone SEB, it
should be noted that the City has not yet
Central Indian River Lagoon, Project Zone SEB 2020 TV Project
Reductions
21,.009
TOW Rcnmb cn Rceucilen
262,949
s
2nm9
;; lti ^9
67,000
r
2013 2014 2015 2016 ton 2018 2019 2020 1021 2022 202.1 1024 20L5 N26 2627 2028 2029 20M 2091 2032 20:\i 2034 20J.5
V..e
Figure 4-8. CIRL Project Zone SEB - TN Reductions Progress (2020)
received reduction credits for seven completed projects, including the Stormwater Park. That single project may
account for a significant part of the City's required reduction goals.
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City of Sebastian Stormwater Master Plan Update
The FDEP conducts an annual review
of BMAP implementation efforts, and
during that review project -specific
information may be revised and
updated, resulting in changes to the
estimated reductions for those projects.
The revisions may increase or
decrease estimated reductions, and
FDEP will work with stakeholders to
address revisions as they are
identified. In addition, there are several
CIP projects eligible for reduction
credits included in this master plan that
may be proposed by the City as
management actions in future updates
of the CIRL BMAP.
Cenlral Indian River I.ngnnn, Pmjerl Jnne SER 21120-1-111'mprl
Redurlinns
i9.OM I l,i
-------------------------------------------------
a6.i9c
fl.^W
9 .4,Pp1
MOW
a
19j1I
9.WU
t., 1�1 .
W13 2014 =u1+ n116 Ali? 2Yn1 low luo M21 _-m 101' 21124 lllJIJ 2L2 Miff l m M_-Y MX 2031 Jon M&I n1M mm
Y
Figure 4-9. CIRL Project Zone SEB - TP Reductions Progress (2020)
Table 4-4. City of Sebastian TN and TP Required Reductions and Allowable Loadings
Total Nitrogen (TN)
Total Phosphorus (TP) 8.901 3.84 6,015 67.6 2,886
The City of Sebastian appears to be meeting the allowable loads for TN and TP established in the CIRL BMAP.
However, as additional development occurs, additional projects will be needed to ensure ongoing compliance with
BMAP requirements.
Some possible projects that could be considered for the BMAP include the following:
• Additional dry detention areas
• Dredging of sediment from existing retention/detention ponds
• Reduction of canal water surface elevation to reduce peak discharge
• Blossom Ditch Drainage Improvements (CIP Project 24) — addition of a Baffle Box to the existing outfall to
improve water quality.
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