HomeMy WebLinkAboutEx 07_Drainage Assessment.pdf Interlaken Engineering and Design, PLLC
Seattle, WA | (206) 470 – 9572
Written Drainage Assessment for
New Single-Family Residence
5xxx NE 10th Street
Permit #
Narrative Description of Project
The project is located in Renton on a 0.26-acre lot zoned R-4 (Parcel No. 102305-9360) on the south side
of NE 10th St between 148th Ave SE and Kitsap Ave NE. The lot is presently undeveloped. The lot slopes
from northeast to southwest at an average magnitude of approximately 24% and a total relief of 20’
between property lines. A new single-family residence is proposed for the northeast portion of the lot.
Access will be provided by a driveway extending south into the lot from NE 10th Street. The total
proposed roof area for the new residence is 1,962 sf. Total proposed hard surface area for the driveway
area is 320 sf. The project also proposes 1,060 sf of new offsite hard surface area due to half street
improvements. The total proposed onsite hard surface area for the project is 2,282 sf, with a total
disturbed onsite area of approximately 4,665 sf.
A Soils Report was prepared by GEO Group Northwest, Inc. dated December 7th, 2022. The site was
found to be underlaid by glacial till soils. The full report is included in the appendix.
Because the subject lot is smaller than 22,000 square feet, it is subject to the Small Lot BMP
Requirements in Appendix C of the 2022 City of Renton Surface Water Design Manual. Because the
project is a single-family residential project that results in ≥2,000 sf of new plus replaced impervious
surface, results in less ¾ acres of new pervious surface, and as a project on predominately till soils with <
7,947 sf of target impervious surfaces and proposed pervious area is < 20,242-2.96 x (total target
impervious surfaces) [proposed pervious area = 2,711 sf < 13,487 sf], this project is subject to Simplified
Drainage Review.
On-Site BMP Analysis for Feasibility
To address the requirements for mitigation of target impervious surface, the applicability and feasibility
of the on-site BMPs were evaluated based on the order described per Section C.1.3. The following
summarizes the feasibility analysis:
1) Full Dispersion: Full dispersion is infeasible because the site’s steep slope areas of over 15% do
not allow adequate area for required flowpath segments.
2) Full Infiltration: Full infiltration is infeasible as the site is underlaid by glacial tills, inadequate for
infiltration per the Geotech report prepared by Geo Group Northwest, Inc.
3) Limited Infiltration: Limited infiltration is infeasible as the site is underlaid by glacial tills,
inadequate for infiltration per the Geotech report prepared by Geo Group Northwest, Inc.
4) Rain Gardens: Rain gardens are in feasible because the site’s slope areas of over 15% do not
allow adequate area for rain garden implementation.
5) Bioretention: Bioretention is infeasible because the site contains slopes in excess of 15%.
6) Permeable Pavement: Permeable Pavement is infeasible as the site is underlaid by glacial tills,
inadequate for infiltration per the Geotech report prepared by Geo Group Northwest, Inc.
7) Basic Dispersion: Basic dispersion is feasible and proposed to the maximum extent practical. The
onsite driveway of 320 sf will be mitigated using sheet flow basic dispersion. According to
CORSWDM Section C.2.4.5 a 2 foot wide, 4-6 inch-deep strip of crushed rock or extended base
course will be provided at the edge of the impervious strip. Additionally, a flowpath of 10’ must
be provided for impervious lengths of 20’ or less. The driveway is 16’ in width, therefore a 10’
trench is sufficient.
Interlaken Engineering and Design, PLLC
Seattle, WA | (206) 470 – 9572
According to CORSWDM Section C.2.4.4 a 2 foot wide, 18-inch deep trench filled with ¾ to
1½” washed rock mitigates 700’ of impervious surface. As such, the onsite roof area of 1,920 sf
will be mitigated using a 28’ dispersion trench. A flowpath length of 25’ will be provided and the
trench is situated parallel to the ground contours to the maximum extend possible.
Proposed ESC and SWPPS Measures
In order to prevent erosion and trap sediments within the project site, the following BMPs will be used
approximately as shown in the ESC details on the CSWPP Plan:
- The limits of disturbance will be delineated with Hi-Viz type snow fence and silt fence in
accordance with CORSWDM C.3.6.
- Before any other work proceeds a stabilized construction entrance will be installed in accordance
with CORSWDM C.3.1.
- Runoff will not be allowed to concentrate and no water will be allowed to point discharge onto
slopes.
- All exposed soils will be stabilized through mulching when not being worked per CORSWDM
C.3.2.
- It will be the responsibility of the applicant and contractor to maintain all proposed ESC and
SWPPS measures.
Supporting Documentation
Geotechnical Evaluation prepared by GEO Group Northwest, Inc. dated December 7, 2022 has been
appended to this report.
ESC and BMP details have been included on the TESC and Site Drainage Plans.
Special Studies or Soils Reports
Soils information is included in the Geotechnical Evaluation.