(A) Purposes and intent. The developer shall provide such facilities as may be needed to drain the subdivision or land development to positive outlets that can be legally maintained in permanent use, or into a public system of adequate capacity which discharges into such positive outlets, including all rights-of-way, easements and necessary construction at no expense to the town. Side ditches along public roads may not necessarily be considered as such public drainage systems or positive outlets. Wherever technically feasible within current design standard storm sewers and treatment facilities shall be provided to control storm water quality by providing for on-site percolation and/or detention or any other appropriate treatment facilities for storm water.
(B) General drainage specifications. Drainage piping of all kinds shall be fitted with headwalls, endwalls, inlets or other appropriate terminating or intermediate structures; the design of headwalls, endwalls, inlets, catch basins, manholes, ditch paving, box culverts and minor drainage and related structures in general shall broadly follow current State Department of Transportation practices. Piping used within a storm sewer shall be 15 inches in diameter or larger. Distances between terminating or intermediate structures shall not exceed 400 feet without the construction of a maintenance inlet or manhole.
(C) Requirements for a complete drainage system. A complete system shall be provided for draining roads, streets, alleys and other areas in the subdivision or land development, and for handling drainage run-off that enters or crosses the subdivision or land development. The system shall be designed according to accepted engineering principles for rainstorms of the maximum intensity predicted for the county area at three-year intervals according to current State Department of Transportation charts and data. The run-off coefficients used shall be those which will be applicable to the areas involved after complete development has occurred. The drainage systems shall be designed for long life, low maintenance cost, and ease of maintenance by normal maintenance methods.
(D) Swales, curbs and gutters. When there is sufficient width for road rights-of-way, the development shall be designed so as to provide roadside swales or curbs and gutters or both. Such drainage improvements shall be located within the right-of-way and shall have slopes that parallel the centerline road grade. Swales and small ditches of similar size and capacity shall have slopes not flatter than 0.0025 foot per foot in the direction of the flow. Ditch pavement or other adequate permanent protection against scour or erosion shall be provided where necessary. Run-off in excess of the capacity of swales and gutters shall be diverted therefrom and carried away in storm sewers or in outfall ditches or by other means separate from the roads and streets. Road grades shall be shown on the development plans by the direction and percentage of fall and with the centerline lineal distance between the control points. Minimum road grades for swaled and guttered sections shall be 0.0025 per foot.
(E) Drainage outfalls. Outfall ditches and other open channels shall be designed so they will not overflow their banks; and if practicable, they shall be designed for flow velocities that will not cause scour or erosion. Where higher velocities must be used, ditch pavement or other adequate permanent protection against scour or erosion shall be provided.
(F) Hydraulic gradient. The hydraulic gradient for storm sewer systems shall be so designed that the down-stream end of the gradient is at the high-water elevation for the design storm. Storm sewers are to be designed flowing full with the hydraulic gradient at the grate of the critical inlet.
(G) Drainage design report. The design data of the drainage system shall be submitted along with the construction plans in a report form prepared by the developer’s engineer indicating the method of control of storm water and groundwater, including the method of drainage, existing water elevations, recurring high water elevations, proposed design water elevations, drainage structures, canals, ditches and any other pertinent information pertaining to the system. The drainage system shall be designed using acceptable engineering principles with consideration being given to the protection of all future buildings from a one-in-100-year storm. In addition, the systems shall provide for the necessary maintenance of groundwater levels to prevent over drainage for the intended land use.
(H) Accommodation of contributory drainage. The system shall provide for the drainage of lots, streets, roads and other public areas including surface waters which drain into or through the property. The design for drainage of the subdivision or land development must be adequate to provide for surface water drainage of adjacent contributory areas. Where additional ditches and canals are required to accommodate contributory surface waters, right-of-way shall be provided for future needs; however, the developer may be permitted to excavate or open sufficient capacity to provide for existing drainage needs whenever the developed or undeveloped status of adjacent areas so warrants, as determined by the engineer retained by the town for such purpose.
(I) Drain pipe requirements. Pipe shall be sloped and structures channeled to develop sufficient scouring to minimize sediment. The pipe used in the system shall be reinforced concrete or metal meeting ASTM, ASSHTO and current State Department of Transportation specifications. Concrete pipe shall have gasket joints, meeting the requirements of ASSHTO. Metal pipe will not be used beneath the pavement or parallel within the right-of-way. Drainage pipe shall be fitted with headwalls, endwalls, inlets and other appropriate terminating and intermediate structures. structure design shall meet or exceed town and county standards.
(J) Rainfall run-off management. Rainfall run-off, surface waters and groundwaters shall be managed in subdivisions and land developments to minimize degradation of water quality, nutrients, turbidity, debris and other harmful substances, and maximize percolation and detention to promote the reuse of this resource. Storm water treatment facilities shall be designed, sized and performance evaluated to accommodate as a minimum requirement a three-year storm. Run-off from roads, parking lots, roofs and other impervious surfaces should be directed over areas where percolation into the soil can be accomplished prior to introduction into any storm sewer or other receiving facilities. Pervious areas shall be covered with vegetation.
(K) Run-off flow distance. The maximum recommended run-off flow distance over impervious surfaces before being diverted to percolation areas should be 50 feet, excluding building roofs, recreational areas, roadway gutters and storm sewers.
(L) Percolation areas for run-off. Run-off which must be carried directly into the closed storm sewer system without previously crossing percolation areas should be discharged to percolation areas prior to conveyance to on-site receiving waters in order to promote detention, disposition of silt and other particulates and the removal of nutrients or other undesirable constituents in the water prior to discharge from the subdivision or land development. Water storage and detention capabilities of on-site bodies of water shall be governed by the discharge limitations of the area. Temporary ponding is allowable in areas where specifically designed.
(M) Minimum drainage accommodation. The minimum capacity of the town’s storm drainage facilities shall be designed and maintained so as to accommodate the run off caused by one inch of rainfall in a period of one hour.
(N) Drainage easements. In order to accommodate adequate drainage facilities, canals or ditches of sufficient width and capacity, according to the engineer’s certified design standards, shall be acceptable to the town. For purposes of maintenance and control, such canals or ditches shall be placed under the authority of the drainage district having jurisdiction, or a property owners’ association or corporation formed for such purpose, or the town. Drainage easements shall be provided to facilitate surface waters from contributory areas. When a subdivision or land development is traversed by or develops canals, watercourses, lakes, streams, drainage ways or channels, there shall be provided a drainage easement of right-of-way conforming substantially with the lines of such watercourse and of such further width or construction or both, as will be adequate for the purpose.
(P) Alternate drainage treatment. Alternate treatment methods or facilities which in the opinion of the engineer retained by the town for such purpose, are equal or superior to the above requirements may be approved. Applications for such approvals shall be accompanied by written data, calculations and analyses which show, by accepted engineering principles, that the alternate treatment methods or facilities are equal or superior to those specified.
(Q) Variance request. In the event that the engineer retained by the town for the purpose of designing a storm water management system or facility concludes that the drainage criteria and design requirements of the South Florida Water Management District, or those provided within this subchapter, can not be met by reason of the location of presently existing improvements, vested property rights, insufficiencies in the dimensions of area, length, width or elevation of the land area under consideration for drainage improvements, or for other reasons based upon sound engineering principles, the town may submit an application to the South Florida Water Management District for a variance to the aforesaid requirements for drainage, or apply for a project permit approval for an alternate designed system, which, if issued, shall govern and control the town’s proposed drainage improvement project.
(Ord. 1-90, passed 1-31-1991)