§ 801. Stormwater Management.
801.1. Purpose and Intent.
It is the intent of this Section to impose reasonable standards to control stormwater runoff, to conserve water resources, to pursue a non-structural approach to stormwater management and to minimize the construction of drainage facilities designed to increase conveyance. It is the further intent of this Section to:
A.
Protect the quantity and quality of ground and surface waters;
B.
Promote recharge into the groundwater system;
C.
Prevent and reduce salt water intrusion;
D.
Minimize the production of disease carrying mosquitoes;
E.
Prevent erosion and loss of valuable topsoils and subsequent sedimentation of surface water bodies;
F.
Prevent adverse impact on adjacent property caused by diverting or blocking surface water flow; and
G.
Prevent flood hazards, prevent loss of life and property damage, and reduce the capital expenditures associated with floodproofing and the installation and maintenance of storm drainage systems.
801.2. Stormwater Management Plan Required.
A.
Generally. A stormwater management plan shall be prepared and submitted for all new construction, redevelopment, filling, excavating, earthmoving or where other topographic changes occur as part of a development. The plan, which shall be submitted in conjunction with construction drawings for subdivision plats or site plans, as applicable, shall be reviewed for compliance with this Section. The regulations of this Section shall be administered by the Public Works Department. Any improvement specified in an adopted master drainage plan for the proposed development shall be installed by the developer.
B.
Submittal. Stormwater Management Plans, which shall be prepared by a professional engineer unless exempted by F.S. ch. 471, shall include requirements found in the Manatee County Public Works Standards.
801.3. Stormwater Management Standards.
The following requirements and the requirements of F.S. ch. 373, and the Manatee County Public Works Standards Manual shall apply to all development.
A.
Preservation of Watercourses. In accordance with Title 44, Code of Federal Regulations (C.F.R.), and the Comprehensive Plan, the alteration of watercourses and floodways shall be prohibited unless part of an overall drainage basin plan.
This prohibition may be waived if a finding of overriding public interest is made by the Board. The prohibition shall not apply to irrigation and drainage ditches constructed in the uplands which are not more than thirty-five (35) square feet in total cross section area and which normally have a water depth of no more than three (3) feet, provided they are not in and do not directly connect to Outstanding Florida Waters, Class I Waters or Class II Waters.
Any alteration shall be in accordance with this Section; Section 802, Floodplain Management; Section 706, Wetland Protection; other applicable sections of this Code; and F.S. ch. 373. The flood carrying capacity within the altered or relocated portion of the watercourses shall be maintained or improved.
B.
Stormwater Management Permits within the Watershed Protection Overlay WPE and WPM Districts. All proposed stormwater management permits required under Chapters 62-4, 62-25, 40D-4, 40D-40 or 40D-400 of the Florida Administrative Code for development within the WPE and WPM shall be required to comply with all requirements contained in Section 403.10 of this Code.
C.
Development within the WPE, and WPM Watersheds. All developments located within the WPE and WPM watersheds shall meet Florida Department of Environmental Protection Stormwater System Design Standards for discharge into Outstanding Florida Waters, (OFW) as provided by F.S. ch. 403, and Chapters 62-4, 62-25, 62-40 and 62-302, and other applicable chapters of the Florida Administrative Code. At minimum, development within the WPE, or WPM Overlay Districts shall provide one hundred fifty (150) percent water quality treatment and shall be designed and constructed to detain, and permit the filtration of, the runoff from the first one and one-half (1.5) inches of rainfall. The Board may allow system designs different from the FDEP Stormwater System Standards if the applicant demonstrates that the development will utilize a stormwater management system which provides for equivalent, or greater, levels of stormwater treatment, providing that overall existing ground water hydrology characteristics are not altered, impacted or changed from the existing predevelopment condition. Equivalent stormwater treatment shall utilize a combination of treatment practices so as to ensure that the construction, alteration, or operation of the system will not discharge, emit, or cause pollution in contravention of applicable state Outstanding Florida Water quality standards of the Florida Administrative Code. To demonstrate compliance with the OFW Standards, the water quality in the receiving water body will be monitored in the receiving water body in an area adjacent to the point of discharge. Acceptable treatment practices may include but are not limited to:
1.
Retreatment/pretreatment detention systems shall be designed to provide detention times that exceed detention times normally required, and providing repetitive water quality treatment. These systems include those providing pretreatment/retreatment using natural or artificial wetland systems, without adverse impact on the biocharacter of the wetlands. Use of the natural wetland systems for retreatment can be considered when no adverse impacts to the wetlands, fish, and wildlife and other resources can be demonstrated.
2.
Off-line treatment systems using retention/percolation shall be designed to retain at least the first three-quarter (¾) inch of stormwater runoff, without release of pollutants out of the retention area.
3.
Stormwater system designs that significantly increase the time of concentration (i.e., time taken for stormwater runoff to reach the detention/retention areas), or increase the percentage of post-treatment discharge to greenspace suited for additional water quality treatment (i.e., swales or buffer areas). Increased times of concentration may, for example, be achieved through the use of swales in major drainage ways to convey stormwater to detention/retention areas. Times of concentration shall be determined to be the point of ultimate outfall to a receiving watercourse
4.
Mechanical treatment systems shall be designed to reduce the amount of directly connected impervious surface (e.g., limits on downspout locations, trench drains). Mechanical systems that irrigate, function as exfiltration systems or provide groundwater recharge or stormwater reuse.
5.
Other systems which can be demonstrated to provide additional stormwater treatment equivalent, in effect, to treatment criteria associated with discharge into Outstanding Florida Waters.
D.
Development Outside the WPE and WPM Watersheds. All development outside the WPE or WPM Overlay Districts shall be designed and constructed to detain and permit the filtration of the runoff from the first one (1) inch of rainfall, unless required to detain additional volume pursuant to State regulations. When the watershed drainage areas are less than one hundred (100) acres, the alternative design criteria allowed by Chapter 62-25, 40D-4, or 40D-40, Florida Administrative Code, is allowed. This alternative allows the treatment of the first one-half (½) inch of runoff. Biological filtration and sand filtration shall be in accordance with SWFWMD design criteria and Manatee County Public Works Standards.
E.
Stormwater Discharge from New Development. The rate of stormwater discharge from new development shall be equal to, or less than, the rate of discharge that existed prior to development, based on a twenty-five-year frequency twenty-four-hour duration storm event, or as prescribed in the Manatee County Public Works Standards. The post-development point(s) of discharge shall be the same as the pre-development point(s) of discharge for the project site. An analysis using accepted computer modeling software shall be performed to determine pre- and post-development flows. In certain watersheds in Manatee County which are known to flood, the rate of runoff shall be reduced by fifty (50) percent, or as determined by Public Works staff.
F.
Storm Sewers and Drainage Channels. Trunk storm sewers and major drainage channels shall accommodate the stormwater runoff resulting from a design storm of twenty-five-year frequency twenty-four-hour duration, consistent with the requirements of Chapters 62-4, 62-25, and 62-40, Florida Administrative Code.
G.
Internal and On-site Drainage Facilities. All internal and on-site drainage facilities on any project shall accommodate the stormwater runoff resulting from a design storm of ten-year frequency twenty-four-hour duration based on the project site's time of concentration, consistent with the requirements of Chapters 62-4, 62-25, and 62-40, Florida Administrative Code and this Code, unless located within the 100-year floodplain or floodway.
H.
Diverting Stormwater to Adjacent Property. A stormwater management system shall be designed for all areas to be developed or redeveloped such that any filling or other topography changes within a site will not divert stormwater to any adjacent property or lot. Existing and new swales along property lines shall be maintained by each property owner, Homeowners Association or other maintenance entity to insure positive flow to a detention basin, retention basin or to a primary drainage facility.
I.
Increasing the Impact of Stormwater Discharge. All development that increases the impact of stormwater discharge from the site, either in terms of volume of water or any pollutant, shall provide for on-site detention and filtration of stormwater runoff to remove oils, floatables, silt, sediment, nutrients, and heavy metals at levels required by applicable federal, state, and regional regulations. Design criteria for developing a Stormwater Management Plan are contained in the Manatee County Public Works Standards Manual.
J.
Design Storm. Design storm requirements for drainage systems are contained in the Manatee County Public Works Standards.
K.
Detention/Retention Basins Slopes. All detention or retention basins shall be designed utilizing the Manatee County Public Works Standards.
L.
Floodplain Requirements within 25- and 100-year Floodplain. All fill within the twenty-five-year or one hundred-year floodplains shall be compensated by creation of storage of an equal or greater volume, credited above the seasonal high water table, with such compensatory storage also located within the twenty-five-year and one hundred-year floodplains. Stormwater retention/detention volumes below the seasonal high water table shall not be used to compensate for the placement of fill within the one hundred-year floodplain.
M.
Floodplain Compensation. Developments within a project area greater than one hundred (100) acres shall provide sole use floodplain compensation. This shall include projects that may be less than one hundred (100) acres but are part of a larger project area (i.e. DRIs, multi-phased projects, etc.).
N.
Excavation. Within any twenty-five-year floodplain, it shall be a condition of any permit to provide equal excavation on the same lot to compensate for any filling. The prohibition of habitable structure applies only to areas where the mapping of the twenty-five-year floodplain has been completed, or where existing water surface profiles can permit the identification of the twenty-five-year floodplain.
O.
Substantiate Engineering Data. It shall be the responsibility of the applicant to provide engineering data to substantiate compliance with this Section. The Department Director shall administer and approve development plans only after verifying compliance with this Section for each development. Areas within the one hundred-year floodplain along or adjacent to a tidal influenced water body shall not be subject to this level of service performance standard.
P.
Limitation of Habitable Structures within twenty-five-Year Floodplain. For parcels within the twenty-five-year floodplain, new habitable structures shall be located outside of the twenty-five-year floodplain, if reasonably possible, and all residential lots shall be located outside of the post development twenty-five-year floodplain. If the property owner cannot locate the structure outside of the twenty-five-year floodplain, then the property owner shall obtain an administrative permit. An administrative permit for such uses shall be granted upon a finding that the proposed structure is located and designed to minimize possible property damage in the event of a twenty-five-year storm event.
This limitation shall not preclude the development of water dependent uses, stormwater management facilities, or non-habitable structure, where appropriate. This limitation of habitable structures applies only to areas where the mapping of the twenty-five-year floodplain has been completed and adopted as part of the Code. It shall be the responsibility of the applicant to provide engineering data to substantiate compliance with this Section. The Department Director shall administer and approve development plans only after verifying compliance with this Section for each development.
Q.
Wetlands Protection. Stormwater management systems shall be designed to protect against harmful siltation and adverse impacts of wetlands that are to be left undisturbed and that could result in the reduction of their natural flood control and water quality control capacity, based upon the twenty-five-year frequency storm for flood control and the one-year storm for water quality control. Development shall not impede or divert the naturally occurring flow in wetlands. The new drainage facilities shall maintain groundwater levels sufficient to protect and maintain pre-existing wetland vegetation, habitat and hydroperiod proposed to be preserved, through the use of weirs, filtration berms, or other means. Wetlands may be incorporated into a stormwater management system subject to the requirements of the State, SWFWMD and the Manatee County Public Works Standards and this Code. The developer shall show that the requirements set out in this Section can be met without significantly altering the existing natural wetlands. See also Section 706, Wetland Protection.
R.
Swale Design. Swales shall allow the positive flow of water without any obstruction. Refer to Section 701 for landscaping specifications. Swales, and similar open drainage facilities shall be designed in accordance with the Manatee County Public Works Standards to minimize the creation of mosquito breeding habitat.
S.
Design Principles and Practices. The design of stormwater management systems shall be based upon current and reliable best management practices. All analyses shall be performed using the best available data and such other information (i.e. the adopted watershed studies. See the Manatee County Public Works Standards) as may be required to assure compliance with the requirements of this Section and Manatee County Public Works Standards. Except for single family detached, single family attached dwellings, and duplex dwellings the driveway entrances shall not be utilized to discharge stormwater from on-site to off-site. Swales shall not be constructed across roadways to convey stormwater.
Any stormwater management system shall address the existing total hydrology and water budget for the proposed development. Continuance of existing overall hydrology in the proposed development is required in order to prevent or minimize impacts to the natural components of surface and groundwater hydrology. Stormwater reuse, groundwater recharge, pollutant removal rates, volume and discharge attenuation are some of the required elements that shall be considered in review and approval of any stormwater management plan as appropriate based on the size and nature of the project and included in the proposed stormwater management plan. The extent to which the hydrology design specifically outlined in this paragraph is considered for any development shall be determined by the County Engineer; provided the design is in compliance with the Southwest Florida Water Management District (SWFWMD), the Manatee County Public Works Standards and the other provisions of this Section.
T.
Finished Floor Elevations. The finished floor elevation for each habitable structure shall comply with this section and Section 802 (Floodplain Management).
U.
Roadway Drainage. Where a developer constructs or widens a public roadway immediately adjacent to the development, the associated drainage systems shall comply with the Manatee County Public Works Standards.
V.
Drainage Easements. When a public drainage easement is required, the easement shall be dedicated to Manatee County and shown on the final subdivision plans and the final plat for the project. In addition, all public drainage easements shall be accessible from public right-of-way or dedicated ingress/egress easement. All drainage easements and ingress/egress easements shall be on clear and level ground, free of obstructions, including landscaping. Manatee County is responsible for maintaining only the free flow of drainage through these easements.
W.
Erosion Control. The development shall include sedimentation facilities and other erosion control measures to protect against erosion and sediment discharges during clearing and construction and to protect against erosion and sedimentation of drainage facilities during the life of the development. No grading, or clearing, except brush removal for surveying, or filling shall be commenced until an inspection is conducted by the Department Director to insure that all erosion and sedimentation measures have been applied to all the disturbed areas and specifically around any water bodies, watercourses, or wetlands.
Sediment and erosion control measures will be required to control and minimize damage to downstream and adjacent property, the conveyance system, and to preserve water quality. The method for sediment and erosion control shall be indicated and shown on all plans being submitted to the Building and Development Services Department for approval, and the control measures installed prior to any construction. All swales shall be either sodded or have an equivalent stand of grass prior to approval.
The Building and Development Services Department shall be responsible for the following:
1.
The review of topographic data submitted for construction approval for sinkhole activity and limiting, where necessary, development to achieve compliance with Comprehensive Plan policies and other regulation[s];
2.
The review, at time of construction drawing submittal on any project, that the proposed use or alteration of any slope that is equal to or greater than twenty-six and one-half (26.5) degrees, complies with the Comprehensive Plan policies and other regulations; and
3.
Conditioning all building permits that compliance with soil stabilization, soil erosion and transport policies and regulations is required.
801.4. Ownership and Maintenance.
The ownership and maintenance of stormwater management systems shall be clearly indicated on subdivision plans and comply with the Manatee County Public Works Standards.