Chapter 13.15 STORMWATER DRAINAGE SYSTEMS

Chapter 13.15
STORMWATER DRAINAGE SYSTEMS

Sections:

13.15.010 Stormwater requirements.

Each development shall be provided with a drainage system for the collection, control, and/or disposal of the surface water runoff consistent with the following requirements:

A. A stormwater drainage plan, prepared by an engineer licensed and registered in the state of Washington, shall be required for any new development that creates more than 5,000 square feet of impervious surfaces;

B. Design calculations for peak flow and peak volume storage requirements shall be based on a design storm frequency of 10 years. Design calculations for treatment shall be based on 64 percent of the two-year recurrence interval, 24-hour storm runoff event;

C. The plan shall provide for the on-site treatment and detention/retention of all increased runoff based on the design storm over the predevelopment conditions;

D. Treatment standards shall be based on best management practices identified in the Washington State Department of Ecology’s Stormwater Management Manual for the Puget Sound Basin or an equivalent manual deemed acceptable by the city;

E. The overflow of runoff in excess of the design storm quantities must be situated or directed to locations to where it would have overflowed prior to development. Appropriate methods must be used to attenuate flows so that erosion or damage to downstream properties does not occur;

F. The drainage plan shall include all calculations for the determination of the required size of the system. Said calculations shall be based on the required criteria and upon an analysis of estimated runoff from areas contributing to the facilities. Peak flow analysis shall be done using the rational method. Storage volume quantities shall be calculated by the rational-stored rate method. The assumption for the outflow rate used in the stored rate method will need to be verified by the developer by actual field testing in the case of infiltration systems. Collection systems shall be gravity pipe systems;

G. In calculating the amount of impervious surfaces, the area of roofs shall not be included if the roof drains to downspout or other systems that discharge directly to the ground and not onto paved or other impervious surfaces;

H. Treatment of runoff from sidewalks and detached bikeways is not required if the sidewalk or bikeway drains away from roadways so the stormwater does not mix with runoff from the roadway;

I. Stormwater facilities should be incorporated into on-site open spaces and should be given a natural appearance where practicable. [Ord. 938 § 1, 2003.]

13.15.020 Costs reimbursement.

Any and all costs incurred by the city of Roslyn to review a planned action under this chapter or any other cost incurred by the city under this chapter shall be reimbursed by the proponent of the proposed development. [Ord. 938 § 1, 2003.]