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To: Jill Ding, Senior Planner From: Brian Wolf, PE
City of Renton, Community and Economic Stantec
Development 11130 NE 33rd Place Suite 200
1055 S Grady Way, 6th Floor Bellevue WA 98004
Renton, WA 98057
File: 2002003611 Date; October 2, 2018
Reference: Drainage Review Waiver for the Thunder Hills Sanitary Sewer Replacement Project
Per the conditions of the construction permit issued by the City of Renton, the Thunder Hills Sanitary Sewer Replacement
Project is required to meet the drainage requirements as established by the 2009 King Count Surface Water Design
Manual (KCSWDM). Section 1.1.1 of the KCSWDM lists the criteria necessitating a drainage review, as follows:
Drainage review is required for any proposed project (except those proposing only maintenance)
that is subject to a King County development permit or approval, including but not limited to those
listed at right, AND that meets any one of the following conditions:
1. The project adds or will result in 2,000 square feet or more of new impervious surface,
replaced impervious surface, or new plus replaced impervious surface, OR
2. The project proposes 7,000 square feet or more of land disturbing activity, OR
3. The project proposes to construct or modify a drainage pipe/ditch that is 12 inches or
more in size/depth, or receives surface and storm water runoff from a drainage pipe/ditch
that is 12 inches or more in size/depth, OR
4. The project contains or is adjacent to a flood hazard area as defined in KCC 21A.06, OR
5. The project is located within a Critical Drainage Area, OR
6. The project is a redevelopment project proposing $100,000 or more of improvements to
an existing high- use site, OR
7. The project is a redevelopment project on a single- or multiple -parcel site in which the total
of new plus replaced impervious surface is 5,000 square feet or more and whose
valuation of proposed improvements (including interior improvements and excluding
required mitigation and frontage improvements) exceeds 50% of the assessed value of
the existing site improvements.
If drainage review is required for the proposed project, the type of drainage review must be
determined based on project and site characteristics as described in Section 1.1.2. The type of
drainage review defines the scope of drainage requirements that must be evaluated for compliance
with this manual.
The KCSWDM defines maintenance as "those usual activities taken to prevent a decline, lapse, or cessation in the use of
currently serviceable structures, facilities, equipment, or systems if there is no expansion of the structure, facilities,
equipment, or system and there are no significant hydrologic impacts. Maintenance includes the repair or replacement of
non-functional facilities and the replacement of existing structures with different types of structures, if the repair or
replacement is required to meet current engineering standards or is required by one or more environmental permits and
the functioning characteristics of the original facility or structure are not changed." (KCSWDM, Page 1-2).
The Thunder Hills Sanitary Sewer Replacement Project includes replacement of approximately 1,205 lineal feet of existing
sewer with 12 -inch HDPE sewer and associated manholes and structures. Approximately 845 lineal feet of existing sewer
will be lined. These improvements will be performed to address several maintenance and structural deficiencies in the
existing sewer pipe.
The ravine's hillside has slumped in several places and erosion created by the creek has compromised the stability of the
maintenance access road and has narrowed the existing maintenance access road in various locations. Past stream bank
Design with community in mind
Stantec
October 2, 2018
Jill Ding, Senior Planner
Page 2 of 2
Reference: Drainage Review Waiver for the Thunder
stabilization measures implemented by the City have included rock gabion walls and placing rock rip rap in the stream,
along the stream bank and where the hillside has slumped. Issues with erosion impacts continue to be a problem for the
stream bank and hillside. Due to these deficiencies, the existing maintenance access road will be replaced with a 7 -feet
wide maintenance access road, generally within the same location as the existing access road. A vehicle tumaround will
be added, and the existing entrance road will be reconstructed. This will allow maintenance access for the sewer facilities
while reducing or limiting future erosion impacts to the hillside and stream bank. A complete description of project
activities is included in the City of Renton Environmental Checklist, which is included with this submittal.
The nature of the improvements for sewer and the maintenance access road meet the definition for maintenance as noted
in the KCSWDM. Because it meets the criteria for maintenance, the project is not subject to a drainage review.
Please contact me if you have any further questions.
Best Regards,
Brian Wolf
Project Enginee
Phone: (425) 289-7328
brian.wolf@stantec.com
Attachment: None
c. File
Design with community in mind
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