(A) A storm drainage plan shall be submitted for development or redevelopment that includes the creation of more than 5,000 square feet of new impervious surface area.
(B) (1) A detailed stormwater report shall be prepared by a registered professional engineer.
(2) The stormwater report shall contain the stormwater calculations and a discussion of the design rationale. The plan shall include the following:
(a) A map showing building site(s), open areas, drainage ways, ditches, culverts, storm sewers, inlets, storage ponds, roads, streets and any other infrastructure improvements which may affect drainage characteristics. The map shall also include identification of the various ground surfaces (i.e., vegetation, gravel and pavement) and their respective coverage (square feet);
(b) Topographic contours (one-foot intervals) and sufficient spot elevation data;
(c) Description of the ultimate destination of stormwater run-off from the project and an evaluation of its impact on down slope drainage facilities;
(d) Design calculations determining run-off quantities, storage requirements and peak rates;
(e) Description of water quality design method and calculations;
(f) A storm drainage facilities operation and maintenance plan. The plan shall:
1. Identify ownership of all facilities;
2. Establish a schedule for maintenance activities necessary to keep the system operationally effective; and
3. Identify the responsible party in charge of the specific maintenance duties.
(g) Details and specifications (including invert and other pertinent elevation information) for all storm drainage improvements, such as storm sewer, manholes, inlets, discharge structures, and retention/detention pond dimensions and volume, side slope, and top, bottom and maximum water surface elevations.
(C) Pre-development and post-development run-off calculations for sizing drainage facilities shall include:
(1) Depiction of the drainage area on a topographical map, with acreage indicated (the off-site contributing area must be included).
(a) Provide a pre-development topography and drainage basin map and a post-development topography and drainage basin map.
(b) The change in peak discharge and volume of surface water entering and leaving the subject property.
(2) The capabilities of all downstream drainage systems within one-fourth mile of the project site downstream boundary to handle the change in peak discharge and volume of surface water caused by the development.
(3) In areas where ground water may be a factor, the analysis shall include the effects of the ground water on the total storm drainage system proposed for the development.
(Prior Code, § 11.05.040.020) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)