§ 15.28.040. Design requirements.

All drainage plans shall conform to the following design requirements:

A.

Engineered Plans. All elements of a drainage plan shall be prepared by a registered civil engineer and certified to have been so prepared;

B.

Stormwater Management Manual Requirements. All drainage plans shall be prepared in accordance with the drainage requirements of the Snohomish County Engineering Design and Development Standards (EDDS), 2003 edition;

C.

Peak Discharge Rate. The rate of surface water leaving the property from the design storm after construction shall not exceed the rate before construction. This requirement may be met by retaining water for on-site disposal, by detaining water on-site for release off-site at the allowable rate, or by a combination of the two methods;

D.

Critical Drainage Areas. In critical drainage areas, the town supervisor may require that the volume of water leaving the site over a period of time may not exceed the preconstruction volume for the same period of time, or may require other special measures related to the critical drainage problems. Critical drainage areas are areas with special drainage problems because of existing flood potential, soil instability or high erodibility of the soil. Critical drainage areas shall be identified by the engineer employed or used by the town;

E.

Receiving Facilities. There shall be no change in location or capacity of existing facilities or natural features that receive off-site surface water entering the site;

F.

Discharge Facilities. Where possible, surface water discharge off-site shall be released at the same points that handled the runoff before construction, with no significant change in the proportion of water handled at each point. Where significant changes are necessary, downstream drainage facilities shall be improved to allow them to accommodate the increased drainage without damage to the drainage facilities, nearby property or water quality;

G.

Design Basis. All drainage calculations shall be based upon a design rainstorm with a ten-year average return period. A twenty-five-year design rainstorm shall be used if the drainage area is more than fifty acres or if the drainage area produces runoff of more than twenty cubic feet per second during a twenty-five-year design rainstorm;

H.

Setbacks. All structures on the site shall be set back at least ten feet from any closed drainage facility, and at least fifteen feet from the edge of any open drainage channel or pond;

I.

Deviations. The town supervisor may approve minor deviations from the requirement of this section. The decision shall be written, shall state the reasons for the decision, and shall be based upon the following factors:

1.

Capacity and condition of downstream facilities;

2.

Effect on receiving bodies of water;

3.

Significant damage from on-site disposal of water;

4.

Existence of regional retention or detention facilities;

5.

Major difficulties in maintaining the drainage system.

J.

Water Quality. To the extent technically feasible, as determined by the engineer employed or used by the town, the quality of water released offsite or disposed of on-site shall be at least as high as the preconstruction water quality.

(Ord. No. 605, Att. A, 11-10-2004; Ord. 419 § 4, 1988)