The URL can be used to link to this page

Home My WebLink AboutSWP272943(1) ' P ETERSON 1 CONSULTING ' 4030 Lake Washington Blvd. N.E., Suite 200, Kirkland, WA 98033 JOHNSTON SHORT PLAT Drainage Report City of Renton August 2, 2001 Prepared for: Mr. Jack Willing Lyle Homes Inc. 1601 - 114th Ave. SE r Suite 100 Bellevue, WA 98004 of w Prepared By: �� 20704 0 �4 Santhosh J. Moolayil o� �GISTE�� 0\ 1 GNAL ' Reviewed By: EXPRES 11/10/02 ' Jon W. Nelson, P.E. PCE Job No. LYLE-0001 r r TABLE OF CONTENTS 1 ' PROJECT OVERVIEW VICINITY MAP REQUEST FOR EXEMPTION FROM FLOW CONTROL AND WATER QUALITY CONVEYANCE CALCULATIONS 1 1 1 r 1 1 r r r i 1 r r ' PROJECT OVERVIEW The proposed project Johnston Short Plat is located in Section 5, Township 23 North, and Range 5 East, Willamette Meridian, City of Renton. Please refer to the vicinity map attached later in this report. The project contains approximately 0.45 acres and is located on the east side of Jones Avenue NE. The site is generally flat with a gentle slope towards the northwest to Jones Avenue NE. Per Soils Survey of King County area by the US Soils Conservation ' Services, the project contains Indianola loamy fine sand (InC), classified as outwash soils by Table 3.2.2.B in the 1998 King County Surface Water Design Manual (KCSWDM). This report will follow the stormwater requirements per the 1998 King County Surface Water Design Manual (K- CSWDM). The project proposes to subdivide 0.45 acres into a four lot short plat. The access driveway will be from Jones Avenue NE. Please refer to the construction plans for more details. Under the 1998 King County Surface Water Design Manual (KCSWDM)this project is exempt from both flow control and water quality enhancement. Please refer to the letter and the supporting calculations addressed to City of Renton dated July 18U', 2001, included later in this report for more details. 1 r � t _ MA Y CREEK PARK ' N NE 28th ST ' KENNYDALE Ll NS PARK SITE J24th ST: f COULON BEACH NE 20th ST VICINITY MAP (NOT TO SCALE) 1 LETERSON ONSULTING ' July 18, 2001 Drainage Review ' City of Renton 1055 South Grady Way Renton, WA 98055 ' RE: Johnston Short Plat Exemption From Flow Control and Water Quality PCE Job No. —Lyle-0001 Under the 1998 King County Surface Water Design Manual (KCSWDM) this project is exempt ' from both flow control and water quality enhancement. Page 1-28 and 1-50 of the 1998 KCSWDM outline the peak flow exemption for urban redevelopment' projects and the water quality surface area exemption, respectively. Flow Control Exemption From Page 1-28 KCSWDM (response to each of the exemption requirements are provided below): 4. "Peak Flow Exemption for Urban Redevelopment Projects Any natural discharge area of a redevelopment project located within the Urban Growth Area is exempt if the project improvements within the natural discharge area generate less than a 0.1 cfs increase in the existing site conditions 100-year peak flow, AND all of the following criteria are met: a. The application of this exemption to natural discharge areas within a proposed project must not result in more than a 0.4 cfs increase in the existing site conditions 100-year peak flow rate for any threshold discharge area of the project, AND" , ' "A project that proposes to add,replace,and/or alter impervious surface(for purposes other than routine maintenance, resurfacing,regarding,or repair)on a site that is already substantially developed(i.e.,has 35%or more of existing impervious coverage)" (from page 1-4 KCSWDNq 4030 LAKE WASHINGTON BLVD. NE, SUITE 200 • KIRKLAND, WASHINGTON 98033 • (425) 827-5874 (425) 822-7216 FAx Drainage Review City of Renton July 18,2001 Page 2 iThe difference between the existing and developed 100-year storm event is 0.044 cfs (see attached calculations). Therefore, this criteria is met. b. "Flow control BMP's must be applied to runoff from the new impervious surfaces as specified in section 5.2.1, AND" Page 5-14 of the KCSWDM states, "redevelopment projects utilizing the [redevelopment exemption] must infiltrate the runoff from the new roof areas if feasible.....If downspout infiltration is not feasible then runoffftonz the roofs shall be dispersed using dispersion trenches......if this dispersion requirement can not be met, then a perforated stub-out connection may be used......." ' Downspout infiltration is not feasible due to space limitations. Soil characteristics on site would require 75 lineal feet of trench per 1,000 square feet (sf) of roof area per Page 5-6 of the KCSWDM. This would equate to approximately 750 lineal feet (10 total or 190 if per lot. Infiltration trenches require a 5 foot setback from both buildings and property lines and a minimum trench spacing of 6 feet on center. Given these restrictions, the maximum length of trench which could be constructed on each lot is 130 If Therefore infiltration is not a viable option. Dispersion trenches are designed using criteria similar to that cited above for infiltration trenches. Therefore, the dispersion trench is also not a valid option for this site. The last option, and the preferred method for this site, is perforated stub-out connections from the downspouts to the local storm system. The stub-outs would collect runoff from the roofs and discharge it to the existing storm system within Jones Avenue NE. Runoff from the roads would be collected in a closed conveyance system which would also discharge to the existing storm system in Jones Avenue NE. c. "The project improvements within the natural discharge area must not be located with in a Landslide Hazard Drainage Area and must not significantly impact a "severe erosion problem" or"severe flooding problem" (see page 1-21), AND" The site is not located within or near a landslide hazard area, so there are no severe erosion or flooding potentials. d. "The manner in which runoff is discharged from the project site must not create a significant adverse impact per Core Requirement 91." Runoff from the site will be discharged into the existing storm sewer system in Jones Avenue NE. The 12-inch concrete storm system runs northerly toward May Creek, which is approximately 0.4 miles from the site. A 12-inch pipe at minimum slope (0.5 %) can convey 2.73 cfs. The difference in flow that will be added due to development of this site ' is 0.044 cfs, less than 2 percent of the minimum capacity of the existing pipe system. In our professional opinion, this will have no impact on the existing downstream system. i ' Drainage Review o- City of Renton ' July 18, 2001 Page 3 Water Quality Exemption From Page 1-50 KCSWDM (response to each of the exemption requirements are provided below): "❑ EXEMPTIONS FROM CORE REQUIREMENT #8 [WATER QUALITY] 1. Surface Area Exemption ' A project or any threshold discharge area within a project is exempt if it meets all proposed of the following criteria: 5 000 square feet of new PGIS2 will be added AND" a. Less than q , This project has 2,600 square feet of PGIS, which includes the driveways and frontage improvements, thus it meets this criteria. b. "Less than 5,000 square feet of contiguous PGIS3 will be created from any combination of new and/or replaced impervious surface as a part of a redevelopment project." This project has 2,600 square feet of contiguous PGIS, thus it meets this criteria. c. "Less than 1 acre of contiguous PGPS'will be will be added and/or modified', OR there is ' a formal agreement with King County to implement a landscape management plan for the PGPS areas on the site(or a farm management plan in the case of an agricultural land use)." The project site is 0.45 acres total, thus it meets this criteria. 2 "PGIS(Pollution-generating impervious surface)are those surfaces considered to be a significant source of pollutants in stormwater runoff. Such surfaces include those which are subject to vehicular use or storage of erodable and leachable materials,wastes, or chemicals,and which receive direct rainfall or the run-on or blow-in rainfall. Metal roofs are also considered to be PGIS unless they are treated to prevent leaching" [from page 1-4 KCSWDM]. 3 "Contiguous PGIS means a discrete patch of PGIS that is all together as opposed to being sparated in different locations on the project site" [from page 1-50 KCSWDM] 4 "Contiguous PGPS(pollution-generating pervious surface,a non-impervious surface with vegetative ground cover subject to use of pesticides and fertilizers,i.e. lawn)means a patch of PGPS that is all together as opposed to being 1 separated in different locations on the project site" [from page 1-50 KCSWDM]. 5 "Modified PGPS means any existing PGPS that is re-graded or re-contoured by the proposed project" [from page 1-50 KCSWDM]. 6"Landscape Management Plan means a King County approved plan for defining the layout and long-term maintenance of landscaping features to minimize the use of pesticides and fertilizers" [form page 1-50 KCSWDM]. Drainage Review City of Renton July 18, 2001 Page 4 ' Based on the information provided, this project meets all of the requirements for both the peak flow exemption for urban redevelopment projects and the surface area exemption for water quality. If you have any questions or comments regarding these exemptions or calculations please contact me at (425) 827-5874. Sincerely, PETERSON CONSULTING ENGINEERS, INC. Robert W. Stewa Engineer Enclosure cc. Mr. Jack Willing w/enclosure 3 36 4 SSIGNAL 1yG F7CPIREs- 9/09/cq- 1 FLOW RATE CALCULATIONS ' The soils on site are Indianola loamy fine sand (InQ, classified as outwash soils by Table 3.2.2.13 in the 1998 KCSWDM. Existing Peak Flow Calculations There is one existing house on site, along with a garage and shed. The remainder of the site contains ground cover that consists of grass and lawn. Existing Conditions: Site Area = 0.448 acres Outwash Lawn = 0.274 acres Impervious = 0.174 Flow Frequency Analysis Time Series File:predev.tsf Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- FlowRate Rank Time of Peak - Peaks - Rank Return Prob (CFS) (CFS) Period ' 0.051 5 2/09/01 2:00 0.102 1 100.00 0.990 0.037 8 1/05/02 16:00 0.063 2 25.00 0.960 0.063 2 2/27/03 7:00 0.063 3 10.00 0. 900 0.043 7 8/26/04 2:00 0.051 4 5.00 0. 800 0.051 - 4 10/28/04 16:00 0.051 5 3.00 0.667 0.045 6 1/18/06 16:00 0.045 6 2.00 0.500 0.063 3 10/26/06 0:00 0.043 7 1.30 0.231 ' 0.102 1 1/09/08 6:00 0.037 8 1.10 0.091 Computed Peaks 0.089 50.00 0.980 Developed Peak Flow Calculations ' The development of this site will consist of 4 lots and approximately 2600 square feet of proposed P . road/driveway. Peterson ConsultingEngineers Page I Developed Conditions: Site Area = 0.448 acres ' Impervious Surface: Roads = 0.06 acres ' 4 lots = 0.224 acres Total Impervious = 0.284 acres Outwash Grass = 0.164 acres Flow Frequency Analysis Time Series File:dev.tsf ' Project Location:Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis------- FlowRate Rank Time of Peak - - Peaks - - Rank Return Prob (CFS) (CFS) Period 0.074 5 2/09/01 2:00 0.146 1 100.00 0.990 0.060 8 1/05/02 16:00 0.103 2 25.00 0.960 0.089 3 2/27/03 7:00 0.089 3 10.00 0.900 0.071 7 8/26/04 2:00 0.084 4 5.00 0.800 0.084 4 10/28/04 16:00 0.074 5 3.00 0.667 0.074 6 1/18/06 16:00 0.074 6 2.00 0.500 0.103 2 10/26/06 0:00 0.071 7 1.30 0.231 0.146 1 1/09/08 6:00 0.060 8 .1.10 0.091 Computed Peaks 0.132 50.00 0.980 Conclusion Based on the information provided above, there is only 0.044 cfs difference between the 100-year developed flow, and the 100-year existing flow. Peterson Consulting Engineers Page 2 r 8/2/01 1: 14 : 24 pm Peterson Engineering Consulting page 1 JOHNSTON PROPERTY 100YR/24HR BACKWATER REACH SUMMARY Hydraulic Gradeline for Reach HG-100 -FROM- <--TO--> <FLOW> floss Ent-HGL EntLoss ExtLoss Outlet Inlet AppHead BndHead JunHead HeadWtr -cb/mh- -------- -------- cfs -loss- -Elev-- --loss- --loss- --Elev- --Elev- --loss- --loss- --loss- --Elev- --Rim-- SXCB 247.21 CBI EXCB 0.23 1.40 tw & do <dc: Inlet Ctrl-Assumed 248.58 0.00 0.00 0.00 248.58 250.50 CB2 CBI 0.23 0.24 249.74 0.00 0.00 249.74 0.00 0.00 0.00 0.00 249.74 251.70 C83. CB2 0.23 0.00 249.74 0.00 0:00 249.74 249.53 0.00 0.00 0.00 249.74 253.31 8/2/01 1 : 14 : 8 pm Peterson Engineering Consulting page 1 JOHNSTON PROPERTY 100YR/24HR CONVEYANCE --------------------------------------------------------------------- --------------------------------------------------------------------- REACH SUMMARY ' Network Reach NET REACH <-AREA> <-DIA> LENGTH SLOPE < n > DSGN Q 5 PIPE Ndepth $Depth Vact Vfull C Area ID (Ac) (ft) (ft) ft/ft ------ (cfs) ------ (ft) ------ (fps) (fps) P3 1.00 38.50 0.0182 0.0120 0.23 4.10 0.14 14.30 3.34 7.34 DA3 P2 1.00 47.92 0.0050 0.0120 0.23 7.81 0.20 19.59 2.12 3.85 P1 1.00 27.87 0.0502 0.0120 0.23 2.46 0.11 11.21 4.76 12.19 ' Dal Flow Frequency Analysis Time Series File:dal .tsf Project Location: Sea-Tac ---Annual Peak Flow Rates--- -----Flow Frequency Analysis-- -Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob _ (CFS) (CFS) Period 0 . 083 6 8/27/01 18 : 00 10 .227 1 100. 00 0. 990 0 . 058 8 9/17/02 17 : 45 0 . 161 2 25. 00 0. 960 0. 161 2 12/08/02 17 : 15 0 . 113 3 10 . 00 ' 0. 900 0. 067 7 8/23/04 14 : 30 0 . 093 4 5. 00 0. 800 ' 0 . 088 5 10/28/04 16 : 00 0 . 088 5 3 . 00 0. 667 0. 093 4 10/27/05 10 : 45 0 . 083 6 2 . 00 0. 500 0 . 113 3 10/25/06 22 : 45 0 . 067 7 1 . 30 0 .231 0.227 1 1/09/08 6 : 30 0 . 058 8 1 . 10 0 . 091 Computed Peaks 0 . 205 50 . 00 0. 980 ' FLOW C- H t-\T k,4_�T-�� Page 1