HomeMy WebLinkAboutRS_Drainage_Exemption_Spec_191206_v11.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES
2017 City of Renton Surface Water Design Manual 12/12/2016
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DIRECT DISCHARGE EXEMPTION
Any onsite natural drainage area is exempt from the flow
control facility requirement if the area drains to one of the
major receiving waters listed in Table 1.2.3.B, AND meets
the following criteria for direct discharge 23 to that receiving
water:
1. The flowpath from the project site discharge point to the
edge of the 100-year floodplain of the major receiving
water will be no longer than a half mile, except for
discharges to Lake Washington, AND
2. The conveyance system between the project site and the
major receiving water will extend to the ordinary high
water mark, and will be comprised of manmade
conveyance elements (pipes, ditches, etc.) and will be
within public right-of-way or a public or private drainage easement, AND
3. The conveyance system will have adequate capacity 24 to convey the 25-year peak flow (per Core
Requirement #4, Conveyance System), for the entire contributing drainage area, assuming build-out
conditions to current zoning for the equivalent area portion (the area that is contained within an arc
formed by the shortest, straight line distance from the conveyance system discharge point to the
furthermost point of the proposed project) and existing conditions for the remaining area, AND
4. The conveyance system will be adequately stabilized to prevent erosion, assuming the same basin
conditions as assumed in Criteria (c) above, AND
5. The direct discharge proposal will not divert flows from or increase flows to an existing wetland or
stream sufficient to cause a significant adverse impact.
A. PEAK RATE FLOW CONTROL STANDARD AREAS
The Peak Rate Flow Control Standard is a peak-rate matching standard intended to prevent increases of
peak flows for specific events rather than match flow-durations over a range of flows. The standard is
appropriate for use in areas where the concern is flooding rather than stream bed erosion. Within the City
of Renton, this standard is allowed for those areas that are highly urbanized prior to 1985 and that drain to
pipes or non-fish bearing constructed conveyance systems leading to the lower Cedar River, Lake
Washington or the portion of the Green River Valley floor located in Renton.
Minimum Required Performance
Facilities in Peak Rate Flow Control Standard Areas must comply with the following flow control
performance standards and assumptions unless modified by offsite analysis per Core Requirement #2 (see
Table 1.2.3.A):
Peak Rate Flow Control Standard: Match the developed peak discharge rates to existing site
conditions peak discharge rates for 2-, 10-, and 100-year return periods.
Intent
The Peak Rate Flow Control Standard is intended to protect flow-carrying capacity and limit
increased erosion within the downstream conveyance system for runoff events less than or equal to the
10-year event. Matching the 2-, 10-, and 100-year peak flows is intended to prevent increases in
return-frequency peak flows less than or equal to the 100-year peak flow down to the 2-year peak
22 Footnote 22 is not used.
23 Direct discharge means undetained discharge from a proposed project to a major receiving water.
24 Note: The City does not charge a special use fee.
TABLE 1.2.3.B
MAJOR RECEIVING WATERS22
• Cedar River downstream of Taylor
Creek confluence
• Johns Creek downstream of
Interstate-405 (I-405) east right-of-
way
• Lake Washington
Note: The major receiving waters listed
above do not include side adjacent or
associated channels, spring- or
groundwater-fed streams, or wetlands.