Loading...
(A) Manhole spacing. The maximum spacing between storm manholes shall be 500 feet.
(B) Minimum pipe size.
(1) The minimum diameter of any storm drain pipe shall be 12 inches, except that a maximum length of 60 lineal feet of eight-inch diameter pipe may be used between inlets and manholes.
(2) The minimum size of any storm drain culvert is 12 inches.
(C) Storm drain manhole ring and cover.
(1) The storm drain manhole ring and cover shall be East Jordan Iron Works Model 3 772Z 1 or an approved equal.
(2) The cover shall be marked “storm.”
(D) Curbs and gutters or swales.
(1) Streets and roads shall be designed to ensure proper drainage and curbs and gutters or swales shall be required in all subdivisions.
(2) Curbs and gutters of adjoining properties shall be extended to match any new curb and gutter. Gutter, ditch and swale flow line grades shall be greater than 0.5%.
(E) Culverts and drainage facilities. Culverts of adequate size shall be provided and installed by the developer where drainage channels intersect any street right-of-way or easement. All culverts shall extend at least the width of the base of the fill.
(1) The amount of backfill to be placed over the culvert and a culvert’s capacity shall be determined by a licensed professional engineer.
(2) This shall include arrangements for driveway culverts. Culverts larger than 24 inches shall have flared ends.
(F) Drain inlet frame and grate. The curb inlet frame and grate shall be Olympic SM 49, East Jordan Iron Works combination of product numbers 7222Ml or 7222M3 (Herringbone grate) or an approved equal.
(G) “Beehive” area drain frame and grate. The beehive area drain frame and grate shall be Neenah Foundry Model R-2560-D6 “beehive grate with frame,” East Jordan Iron Works 1205-2 or an approved equal.
(H) “Drive over” area drain frame and grate. The drive over area drain frame and grate shall be Neenah Foundry Model R-2556-A Type C frame with Type G grate, East Jordan Iron Works Model 1848-G or an approved equal.
(I) Sump requirements for catch basins and manholes. A 18-inch sump is required for catch basins and drain inlets. A 24-inch sump is required for storm drain manholes.
(Prior Code, § 11.05.040.050) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)
(A) This section outlines the requirements for sizing flow control facilities. The flow control standards are based on the Stormwater Management Manual for Eastern Washington and the Spokane County Stormwater Manual.
(1) Standard flow control facilities are detention, retention (natural depressions) and infiltration facility.
(2) Any other facility is considered a non-standard system and shall be evaluated individually by the City PWD.
(3) Flow control facilities are necessary to mitigate potential adverse impacts on down-gradient properties due to the increase in stormwater run-off caused by land development.
(B) (1) The peak rate of stormwater run-off from any proposed land development to any natural or constructed point of discharge downstream shall not exceed the pre-development peak rate of run-off.
(2) The post development volume of run-off can exceed the pre-development volume of run-off when the required down-gradient analysis demonstrates that there will be no adverse impacts on down gradient properties or existing natural and constructed conveyance systems.
(Prior Code, § 11.05.040.060) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)
(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)
Loading...