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(A) All required stormwater retention/detention facilities must be constructed and in operation prior to paving and building construction, unless otherwise approved by the Department of Public Works.
(B) The capacity of the downstream drainage course is required to be evaluated for a minimum distance of one-fourth mile from the point of discharge of the development.
(1) The downstream conveyance system must be adequately sized to handle off-site peak flows conveyed through the development plus the on-site developed peak flow discharged to the drainage course from the controlling ten-year storm.
(2) This system should be shown on the drainage layout.
(3) The size of the controlled outlet shall be calculated for the total drainage basin when the total run-offs from both off-site and on-site areas are combined.
(C) The limits of the FEMA mapped 100-year floodplain shall be required to be delineated on the final plat, as well as the engineering drawings for plat construction.
(D) Arrows indicating drainage direction in all public and private property shall be shown on the construction drawings for all hydraulic conveyance systems.
(E) (1) A topographical map sufficient in area to show all areas draining onto the proposed land development must be submitted and show enough of abutting downstream properties to indicate natural or human-made drainage course into which the proposed land development is to drain.
(2) The direction of flow, acreage of areas contributing drainage to the development, the outline of the development and location of intermittent streams or drainage courses which are within the site boundaries shall be indicated on this map.
(F) Surface water entering the subject property shall be received at the naturally occurring location, and surface water exiting the subject property shall be discharged at the natural location, with adequate energy dissipaters within the subject property to eliminate the potential for detriment to the existing downstream infrastructure.
(Prior Code, § 11.05.040.040) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)
(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)
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