(A) All drainage facilities, including on-site detention, drainageways, detention ponds, and drainage channels, shall be shown on the drainage plan. The developer is required to expand the drainage plan to include other properties within the drainage basin when the potential exists for impact beyond the development area, both upstream and downstream.
(B) The plan shall provide the following information:
(1) Existing and proposed contour lines and the surface water drainage system, including any major alteration of the existing drainage pattern. The contour interval shall be of such detail that the final drainage pattern is adequately illustrated;
(2) A natural or established watercourse requires a minimum of a 40-foot easement;
(3) The five-, 25-, and 100-year storm event for predevelopment conditions and post development conditions shall be shown. The post development flow rate from the subdivision shall not exceed pre-developed flow rates from any location as reviewed and approved by the county;
(4) (a) Detention ponds and outlet control structures shall be sized to mitigate increases in the flowrate for the five-, 25-, and 100-year storm occurrence.
(b) Detention shall be calculated using a method approved by the county and detention storage shall begin at an elevation equal to or above:
1. The discharge location ground elevation;
2. Floodplain;
3. Wetland elevation;
4. Top of bank; or
5. Other elevation as determined by the county.
(c) Driveway culverts shall be designed for the five-year storm occurrence. Any driveway culverts that exceed the minimum size of 18 inches shall be constructed by the developer along with the road improvements. Roadway culverts, drainageways, and road ditches should generally be designed for the 25-year storm occurrence;
(5) The boundaries of all drainage easements and detention ponds. A maintenance agreement for the upkeep of all drainage facilities, including on-site detention, drainageways, detention ponds, and drainage channels, and natural or established watercourses shall be filed with the plat;
(6) Individual lot drainage shall be coordinated with the general surface drainage pattern for the area. Drainage shall be designed so as to avoid a concentration of storm drainage water from each lot to adjacent lots. Concentrated flows from the subdivision shall only occur at locations where concentrated flows previously existed;
(7) It shall be the responsibility of each lot owner to install and maintain a culvert under the driveway, unless installed by the developer, when lot development commences; and
(8) (a) Discharge or concentrated flows onto adjacent properties shall be located generally at existing drainage conveyance locations and will be reviewed and approved with final development plans.
(b) The applicant shall provide written permission from adjacent landowner, drainage easement, or energy dissipation methods as reviewed and approved by the county.
(Ord. 1104-63, passed 4-26-2011; Ord. 2009-25, passed 9-1-2020)