(A) Facility volume.
(1) The CN method requires the selection of, or the input of, a rainfall distribution and the precipitation associated with a design storm.
(2) The NRCS Type I 24-hour ten- and 100-year storm events are the design storms to be used for all flow control facilities that use a surface discharge.
(3) Detention ponds shall be sized using one of the following methods:
(a) Sized to contain the difference between the post development and the pre-development 24-hour storm events; or
(b) Post development level pool routing method with a draw down time of 72 hours.
(B) Setbacks. When a detention facility is proposed upslope of developed property or at the top of a slope inclined 15 % or greater, the minimum setback from the slope must be greater than or equal to 50 feet or as determined by a licensed engineer with geotechnical experience.
(C) Release point.
(1) Generally.
(a) Stormwater run-off from a developed site shall leave the site in the same manner and location as it did in the pre-developed condition.
(b) The distance between the outlet structure and the inlet into the detention facility shall be maximized.
(2) Outflow control structures.
(a) Control structures are manhole or catch basins with a restrictor device used for controlling outflow from a facility to meet a desired standard.
1. Outflow structures are required for all stormwater detention facilities.
2. The restrictor device is usually multiple orifices, consisting of two or more orifices and/or a weir section sized to meet performance requirements.
(b) 1. Run-off shall enter the detention facility through a conveyance system separate from the control and outflow conveyance system.
2. The distance between the inlet and outlet shall be maximized to reduce sediment from accumulating in the outflow structure. The types of outflow control structures are listed below.
a. Multiple orifice restrictors.
i. In most cases, control structure need only two orifices: one at the bottom and one near the tip of the riser;
ii. Minimum orifice diameter is three inches; and
iii. Orifice shall be constructed on a tee section or on a baffle.
b. Risers and weir restrictor.
i. Properly designed weirs maybe used as flow restrictors. However, they must be designed to provide for primary overflow of the developed 100-year peak flow discharging to the detention facility; and
ii. The combined orifice and riser (or weir) overflow may be used to meet flow requirements. However, the design must still provide for primary overflow of the developed 100-year peak assuming all orifices are plugged.
c. Outflow control structure minimum requirements. The following minimum access requirements shall be met. Additional requirements are specified in Appendix A.
i. An access road to the control structure is required for inspection and maintenance; and
ii. Manhole and catch basin lids for control structure shall be locking and rim elevation shall match proposed finish grade.
(Prior Code, § 11.05.040.070) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)