(A) Methods of computing runoff volume and peak rate discharge. The design method used to establish runoff volume and peak rates of discharge shall be by current techniques approved by the St. Johns River Water Management District and/or the Florida Department of Transportation. In order to provide for a reasonable measure of consistency, the following methods of computation are encouraged:
(1) For basins or sub-basins 0 to 5 acres in area, use the Rational Method (Q = CIA).
(2) For basins or sub-basins 5 to 300 acres in area, use the SCS Method or the Santa Barbara Urban Hydrograph Method.
(3) For basins over 300 acres in area, use advanced stormwater modeling software such as TR-20 by the U.S.D.A, Soil Conservation Service; HEC-1, by the U.S. Army Corps of Engineers; Advanced I.C.P.R. by Streamline Technologies, Inc.; and the like.
(B) Design storms for various structures:
(1) Minimum design storms:
Facility | Design Storm |
Bridges | 50-year, critical duration* |
Canals, ditches, or culverts for drainage external to the development | 25-year, critical duration |
Cross drains, storm sewers | 10-year, critical duration |
Roadside swales for drainage | 10-year, critical duration internal to the development |
Detention basins | 25-year, 24-hour |
Retention basins (no positive outfall) | 25-year, 24-hour or 100-year, 24-hour |
*Note: Critical duration is that storm event which generates the peak discharge rate for the post-development conditions. Use the Florida Department of Transportation methods to determine this event.
(2) The design frequency for major drainage systems may be increased if deemed necessary by the City Engineer to protect upstream or downstream properties or to comply with other regulations.
(C) Rainfall intensity, storm duration, rainfall distributions, and rainfall amounts.
(1) The following guidelines are for use in the design of the stormwater management system. For the Rational Method, time of concentration will dictate the intensity. Rainfall intensities for the Rational Method are to be obtained from the State Department of Transportation Rainfall Curves for Zone 7.
(2) For other methods, such as the SCS method or the Santa Barbara Urban Hydrograph method, use the durations given above in (B) Design storms for various structures.
(3) For the 24-hour durations, use the Soil Conservation Service's Type II Florida modified rainfall distribution. Tabular rainfall ratios (accumulated total/24-hour total) for this distribution are given in section 13.1.2 of the Applicant's Handbook for the Management and Storage of Surface Waters of the St. Johns River Water Management District. For the 25-year, 96-hour storm, use the distribution given in section 13.2.3 of the Applicant's Handbook for the Management and Storage of Surface Waters of the St. Johns River Water Management District. For all other distributions, use the distributions developed by the Florida Department of Transportation for this district.
(4) The rainfall amounts to be used with the above storm events are to be amounts recommended by any of the three following agencies: the U.S.D.A. Soil Conservation Service, the Florida Department of Transportation, or the St. Johns River Water Management District for the Lake Mary area.
(D) Retention/detention design criteria. As a minimum, all facilities shall meet the St. Johns River Water Management District's criteria but the city reserves the right to demand stricter standards if the City Engineer deems it necessary in order to protect property from flood damage, protect the water quality of the lakes in the city, or to protect special wetland areas, such as Soldiers Creek.
(1) Dry-bottom ponds.
(a) Retention with infiltration/recharge facilities.
1. A soils report by a geotechnical engineer registered in the state documenting the soils types and the seepage rates at each pond location is required. Infiltration may be used in the stormwater routing analysis to reduce the peak rate of discharge, but the rates to be used must be determined by the geotechnical engineer.
2. The bottom of the pond must be a minimum of three feet above the seasonal high water table, except for ponds that are to service the drainage system of the city’s downtown redevelopment area as depicted in the downtown master plan, which may have a one foot separation.
3. Water evacuation period. Ponds shall be dry within 72 hours of the cessation of rainfall.
4. Ponds not designed for the 25-year, 24-hour post-development volume or the 25-year, 96-hour or the 100-year, 24-hour storm events are required to have an outfall structure with an oil skimmer. These outfall structures must discharge to appropriate drainage facilities.
(b) Detention with filtration.
1. A soils report by a geotechnical engineer registered in the state documenting the soils types and the seepage rates at each pond location is required. Infiltration may be used in the stormwater routing analysis to reduce the peak rate of discharge, but the rates to be used must be determined by the geotechnical engineer.
2. The bottom of the pond must be a minimum of three feet above the seasonal high water table, except for ponds that are to service the drainage system of the city’s downtown redevelopment area as depicted in the downtown master plan, which may have a one foot separation. Where this is not possible due to a high ground water table, the city will accept underdrains. The underdrains will be installed with a minimum invert elevation of one foot below the pond bottom and one-half a foot above the seasonal high water table. There must be at least two feet of filtration, except for ponds that are to service the drainage system of the city’s downtown redevelopment area as depicted in the downtown master plan, which may have one foot of filtration.
3. Water evacuation period. Ponds shall be dry within 72 hours of the cessation of rainfall.
4. Ponds not designed for the 25-year, 24-hour post-development volume or the 25-year, 96-hour or the 100-year, 24-hour storm events are required to have an outfall structure with an oil skimmer. These outfall structures must discharge to appropriate drainage facilities.
5. All underdrains and filtration systems shall be designed to meet the St. Johns River Water Management District's design criteria or the Florida Department of Transportation's design criteria.
(2) Wet retention/detention facilities.
(a) These facilities must be designed to meet the St. Johns River Water Management District's criteria as a minimum.
(b) A minimum of six feet of depth below the design low water must be maintained.
(c) The side slopes must have a horizontal/vertical ratio of 6:1 or greater from three feet below normal pool elevation to design high water elevation. Above design high water elevation slopes can be 4:1.
(E) Open drainage ways or retention/detention ponds.
(1) Right-of-way and easements.
(a) Outfall ditches and canals shall have sufficient right-of-way for the facility plus an unobstructed maintenance berm on one or both sides.
(b) Ponds shall have a sufficient easement to allow for installation plus an unobstructed maintenance berm all around the perimeter of the pond.
(2) The minimum requirement for maintenance berms is as follows:
Ditch or Canal Width | Minimum Maintenance Berm Required |
Ditch or Canal Width | Minimum Maintenance Berm Required |
Less than 16 feet
|
20 feet one side
|
16 feet to 32 feet
|
20 feet both sides
|
32 feet to 55 feet
|
20 feet one side and 30 feet on the other side
|
Over 55 feet
|
30 feet both sides
|
Ponds
|
Minimum Maintenance Berm Required
|
With fencing
|
20 feet all around perimeter
|
Without fencing
|
5 feet
|
(3) Areas adjacent to open drainage ways and ponds shall be graded to preclude the entrance of stormwater except at planned locations. Where retention/detention areas are located on the project periphery, the developer may be required to provide additional landscaping or screening to adequately protect abutting properties
(4) Maximum side slopes.
(a) Open dry bottom drainage ways shall have a horizontal/vertical side slopes ratio of 3:1 or greater. Open wet bottom drainage ways shall have a horizontal/vertical side slopes ratio of 5:1 or greater.
(b) Dry bottom ponds shall have a horizontal/vertical side slopes ratio of 4:1 or greater. Wet bottom ponds shall have a horizontal/vertical side slopes ratio of 6:1 or greater to at least three feet of normal water depth.
(c) The use of retaining walls shall be allowed on Commercial/Office site plans for up to one-third of the circumference of dry bottom retention/detention ponds when the retaining wall fits into the architectural scheme of the building. The retaining wall must be on the side of the pond adjacent to the proposed buildings.
1. Refer to § 154.21 for requirements on constructing retaining walls. All items apply except no landscaping will be required in the retention pond area.
2. Keystone retaining wall systems or similar products are acceptable.
(d) Retaining walls, fences and slopes greater than 3 to 1 may be approved based on the developer demonstrating a significant savings of trees and native vegetation (see § 154.21).
(5) Minimum bottom width. The minimum bottom width for ponds and open drainage ways shall be four feet.
(6) Erosion protection:
Open Drainage Ways, Grade | Protection Required |
Open Drainage Ways, Grade | Protection Required |
Less than 1%
|
Grassing and mulch
|
1% to 2%
|
Sodding
|
Greater than 2%
|
Paving
|
Side Slopes
|
Sod
|
Ponds
|
Protection Required
|
Side slopes and berms
|
Sod
|
Bottom
|
Grass and mulch
|
A dense stand of grass is required to be established within all dedicated rights-of-way and easements.
(7) Fencing, ponds.
(a) If side slopes have a horizontal/vertical ratio of 4:1 or greater, fences will not be required.
(b) If side slopes have a horizontal/vertical ratio steeper than 4:1 or if retaining walls are used, the city reserves the right to require fencing if the staff (City Engineer) deems it necessary for safety. Fencing shall not consist of wood, wire, or chainlink.
(8) Freeboard, open drainage ways and ponds: one-foot minimum above design storm elevation.
(F) Porous concrete. The city will allow the use of porous concrete parking areas in stormwater management systems as a means of reducing retention pond size, creating more usable land, and protecting trees and natural vegetation. The following conditions must be met:
(1) The mean annual high groundwater table must be a minimum of one foot below the compacted subgrade.
(2) The city will require the owner and all future owners to maintain the porous concrete drainage system. If at any time the system does not drain within 72 hours, the owner will be required to rebuild the system at his expense. This includes replacing the clogged subgrade materials with clean sands and placing new porous concrete.
(a) If the porous concrete drains to a dry bottom retention pond, which holds 50% or more of the required volume, only the pond bottom would have to be replaced.
(3) All porous pavement designs must meet the specifications and guidelines set forth in the "Pervious Pavement Manual" published by the Florida Concrete and Products Association, Inc.
(G) Exfiltration systems. The city will allow the use of exfiltration systems in stormwater management systems as a means of reducing retention pond size, creating more usable land and protecting trees and natural vegetation. The following conditions must be met:
(1) The mean annual high groundwater table must be a minimum of one foot below the exfiltration pipe invert elevation.
(2) The city will require the owner and all future owners to maintain the exfiltration system. If at any time the system does not drain within 72 hours, the owner will be required to rebuild the system at his expense. This includes replacing the pipe, the filter fabric and any sands or granular materials within five feet (horizontally) from the circumference of the pipe and two feet below the invert of the pipe.
(3) All exfiltration systems shall be designed to meet the St. Johns River Water Management District's or the Florida Department of Transportation's specifications for exfiltration systems.
(Ord. 241, passed 11-27-85; Am. Ord. 457, passed 12-21-89; Am. Ord. 482, passed 1-4-90; Am. Ord. 527, passed 9-20-90; Ord. 632, passed 2-4-93; Am. Ord. 1162, passed 3-17-05) Penalty, see § 154.04