(a) Minimum Control Requirements.
(1) All stormwater management practices will be designed so that the specific storm frequency storage volumes (e.g., recharge, water quality, channel protection, 10 year, 100 year) as identified in the current stormwater design manual are met, unless the City of Dunbar grants the applicant a waiver or the applicant is exempt from such requirements.
(2) In addition, if hydrologic or topographic conditions warrant greater control than that provided by the minimum control requirements, the City of Dunbar reserves the right to impose any and all additional requirements deemed necessary to control the volume, timing, and rate of runoff.
(b) Site Design Feasibility.
(1) Stormwater management practices for a site shall be chosen based on the physical conditions of the site. Among the factors that should be considered:
Topography
Maximum Drainage Area
Depth to Water Table
Soils
Slopes
Terrain
Head
(2) Location in relation to environmentally sensitive features or ultra-urban areas. Applicants shall consult the Stormwater Design Manual for guidance on the factors that determine site design feasibility when selecting a stormwater management practice.
(c) Conveyance Issues. All stormwater management practices shall be designed to convey stormwater to allow for the maximum removal of pollutants and reduction in flow velocities. This shall include, but not be limited to:
(1) Maximizing of flowpaths from inflow points to outflow points.
(2) Protection of inlet and outfall structures.
(3) Elimination of erosive flow velocities.
(4) Providing of underdrain systems, where applicable.
(5) The Stormwater Design Manual shall provide detailed guidance on the requirements for conveyance for each of the approved storm water management practices.
(d) Pretreatment Requirements. Every stormwater treatment practice shall have an acceptable form of water quality pretreatment, in accordance with the pretreatment requirements found in the current stormwater design manual. Certain stormwater treatment practices, as specified in the Stormwater Design Manual, are prohibited even with pretreatment in the following circumstances:
(1) Stormwater is generated from highly contaminated source areas known as "hotspots".
(2) Stormwater is carried in a conveyance system that also carries contaminated, nonstormwater discharges.
(3) Storm water is being managed in a designated groundwater recharge area.
(4) Certain geologic conditions exist (e.g., karst) that prohibit the proper pretreatment of storm water.
(e) Treatment/Geometry Conditions. All stormwater management practices shall be designed to capture and treat stormwater runoff according to the specifications outlined in the Storm Water Design Manual. These specifications will designate the water quantity and quality treatment criteria that apply to an approved stormwater management practice.
(f) Landscaping Plans Required. All stormwater management practices must have a landscaping plan detailing both the vegetation to be in the practice and how and who will manage and maintain this vegetation. This plan must be prepared by a registered landscape architect or soil conservation district.
(g) Maintenance Agreements. All storm water treatment practices shall have an enforceable operation and maintenance agreement to ensure the system functions as designed. This agreement will include any and all maintenance easements required to access and inspect the stormwater treatment practices, and to perform routine maintenance as necessary to ensure proper functioning of the storm water treatment practice. In addition, a legally binding covenant specifying the parties responsible for the proper maintenance of all stormwater treatment practices shall be secured prior to issuance of any permits for land disturbance activities.
(h) Non-Structural Stormwater Practices. The use of non-structural storm water treatment practices is encouraged in order to minimize the reliance on structural practices. Credit in the form of reductions in the amount of storm water that must be managed can be earned through the use of nonstructural practices that reduce the generation of stormwater from the site. These nonstructural practices are explained in detail in the current design manual and applicants wishing to obtain credit for use of non-structural practices must ensure that these practices are documented and remain unaltered by subsequent property owners.
(Ord. 654. Passed 7-6-10.)