Why Stormwater Management
Stormwater management is necessary to prevent the loss of life and property due to flooding, to reduce expenses related to flood control and maintenance of storm drain systems, and to minimize the adverse impact of development and related construction activities. The rights of other landowners must be respected with regards to the volume and rate of discharge from a project site. This includes upstream and downstream impacts in collection, storage and conveyance systems, and impacts to other hydraulically connected areas. Special consideration must also be given to sediment generated during construction and impacts to "volume sensitive" areas which may have limited or no drainage outlet.
A stormwater permit is required from the St. Johns River Water Management District for almost any work that affects natural or man-made drainage systems or changes the use of a property or the structures on it. The following is a summary of the requirements from the St. Johns County Land Development Code. This is presented as informational only, and persons planning a project should refer to the Code for details or contact one of the county technical review engineers. There are levels of stormwater design requirements depending on the type, size and impact of a project. There are exempt projects, minor projects and those requiring full design and code compliance.
Exempt projects are those associated with agricultural activities, maintenance of drainage canals and existing stormwater systems, pavement or buildings. Isolated single and two family dwelling units are also exempt. However, they must meet finished floor elevations and lot grading requirements.
Minor Project Requirements
Minor projects are not required to limit post-development peak rate of discharge to the pre-development rate for the standardized 5 year and 25 storm event, but must meet all other requirements. They must however, provide retention of the first one inch of rainfall resulting from the developed area, or where conditions require, detention of the same volume which must be released between 24 and 72 hours after the event.
These type projects include:
a. single triplexes and quadraplexes if lot coverage <35% of total lot
b. existing project expansions or modifications, if all the following are met.
1. site is currently served by a stormwater system
2. total impervious area < 70% of total site
3. total expansion of building, sidewalks and new parking is less than 2800 square feet
4. the existing system can be enlarged to accommodate the additions
5. there will be no unreasonable impact to adjacent properties and all other Land Development Regulations have been met
c. new project with no more than 35% impervious coverage (up to 9000 square ft.) if all the following are met.
1. no more than 15,000 square feet of site is altered
2. retention or detention of runoff can be provided
3. there will be no unreasonable impact to adjacent properties and all other Land Development Regulations have been met
All other projects must meet the following requirements at a minimum: (See Land Development Code for specifics)
1. Drawings and documents related to stormwater management must be prepared, signed and sealed by a Florida Registered Engineer. Design must be certified as being in compliance with the St. Johns County Land Development Code.
2. All drainage calculations must be submitted
3. A maintenance entity must be specified
4. An As-Built survey must be submitted
5. A legal positive outfall must be identified
6. system elements designed to be "dry" must have a geotechnical report indicating this condition. If seasonal high water table is < 4’ from bottom of proposed retention area, a mounding analysis is required to substantiate design and recovery.
7. pre and post development hydrographs are required for the 5 and 25 year storm events (The preferred method is the SCS runoff curve number method based on TR 55 values)
8. The flow must be routed through the storage system to size for storage and discharge control to limit post development rate and volume to pre-development levels.
9. There must be recovery of total volume within 7 to 14 days after event
10. Temporary roadway flooding is permissible if roadway will be recovered at end of storm event and if no more than ½ of travel lane will be covered for a 25 year storm event, and not exceed a depth of 1 ft. or finished floor elevations for the 100 year storm event.
11. Normal roadway design shall be for a 5 year storm event.
12. Temporary ponding in parking lots is allowable if of shallow depth and if there will be full recovery at end of storm event.
13. Tailwater conditions must be addressed
14. Erosion and sediment control best management practices shall be used during construction to retain sediment on site.
15. If more than 2 feet of water is designed to be in a pond there must be a 4:1 slope out to a depth 2 feet below the surface control elevation or the pond may be fenced.
16. Finished floor elevations shall be at a minimum 1 foot above the FEMA 100 year elevations in A-flood zones or as specified in county approved subdivision lot grading plans.
17. All buildings except those on one acre or more of upland area will require a Lot Grading Plan unless fill is to be placed within 10 feet of the property line.