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HomeMy WebLinkAboutSWP272340 • ! s COUGHLINPORTERLUNDEEN 0 a • Engineering Report v • w Drainage Analysis • St. Matthew's Lutheran Church Renton, Washington �196 • 0 i OCT 2$ 19 • Prepared For: St. Matthew's Lutheran Church • 1700 Edmonds Avenue NE Renton,Washington • • U VIV f � � —2?_S4 U Prepared Through: Baylis Brand Wagner Architects 10801 Main St. Bellevue,WA 98004 • r' ''. (206) 454-0566 E • • • • • • • Prepared By: Coughlin Porter Lundeen 217 Pine Street, Suite #520 • w4� °4 �j'��'�' Seattle,WA 98101 h (206) 343-0460 i Oho '��js54ti°��w4 October 28, 1996 • `�Sf�NALG` • EXPIRES 01/23/ • ti�w 9 p p y, Jam, "�1 ; ,Ay • 217 PINE STREET•SUITE 520•SEATTLE,VVA 98101 P:206/343-0460 F:206/343-5691 • DRAINAGE ANALYSIS • St. Matthew's Lutheran Church Coughlin Porter Lundeen Job No.95-0040-28 October 16,1996 General Project Description • The subject property is located at the northeast corner of the intersection between Edmonds Avenue N.E. and N.E. 16th Street in Renton, Washington (see Figure 1: Vicinity Map in Appendix 'B'). The St. Matthews Lutheran Church property encompasses approximately 3.7 acres. Presently, the western half of the site is occupied by the church building, associated a parking lot, and open landscaped areas. The remaining eastern portion of the property consists of a more recently constructed Washington State Department of Transportation (WSDOT) Park- and-Ride facility. A mutual agreement had been established to allow WSDOT to construct their commuter service facility on church property. i The proposed project area is located immediately south of the existing building and consists of approximately 0.7 acres. The project development involves the construction of a 9,400 square foot . single-story addition to the south side of the existing building. The addition will accommodate a new sanctuary for the church. As part of the proposed development, a small portion of the WSDOT parking lot will be • demolished to facilitate the project construction. Existing Site and Drainage Conditions Existing Site Conditions The church property is situated on a local topographic high point The existing site topography is relatively flat and declines toward the west and east at an average slope of 6%. The maximum topographic rise throughout the project area is on the order of 15 feet (High: 363' Low: 348'). • The site has no upstream tributary drainage area. • The site soils are classified as Soil Group'C'which has a moderate runoff potential. • The on-site vegetation consists of a light mixture of ornamental deciduous/coniferous trees with supporting underbrush and open lawns. • Existing Drainage System • Based on our field investigation and review of existing records, there is no runoff control or detention system for the existing church development. When the church was initially constructed, stormwater detention was not required. However, a conveyance system was s installed. Surface runoff from the project site presently sheetflows overland in a westerly direction and enters the parking lot drainage system. From this point, flows are conveyed to the 12-inch storm line in NE 16th Street. • Unlike the church site, the WSDOT Park-and-Ride facility was required to provide on-site stormwater detention at the time of it's development. As a result, the existing WSDOT Park-and- Ride facility has a separate drainage system from the church. Existing catch basins and conveyance pipes are connected to 142 lineal feet of 42-inch diameter concrete detention pipe. ! Peak runoff rates are controlled by a single orifice frop tee regulator. The detention system provides approximately 1,400 cubic feet of storage volume. Stormwater runoff released from the ! detention system is discharged into the 12-inch storm line in NE 16th Street. Proposed Site and Drainage Conditions Proposed Site Conditions The project development is primarily comprised of a 9,400 square-foot church addition along the south side of the existing building. The proposed addition will also include new building • entryways, a new 4" sanitary side sewer service, a new 2" domestic water service line servicing both the existing building and new addition (the existing 1-1/2" domestic service will be capped and abandoned -see Civil Engineering plans), and a fire sprinkler system for the entire building (i.e.both existing and proposed)using a 6"wet-tap connection from NE 16th Street. In addition to the new building, the project will also include the removal of approximately 6,000 square-feet of the existing WSDOT Park-and-Ride asphalt lot. The drainage and traffic patterns of the remaining portion of the parking lot will be revised to mitigate the demolition. Conversely, ! the existing church parking lot will remain undisturbed. The proposed site grading will accommodate the new building addition and also create a new play area near the site's south property line. A four foot berm will be constructed to separate the recreation area from NE 16th Street. ! Proposed Drainage System • Drainage calculations were performed using methods described in Chapter 3 (Hydrologic Analysis) of the King County Surface Water Drainage Manual. Peak runoff rates for the pre- developed and post-developed conditions were calculated using the Santa Barbara Urban Hydrograph Method. We used the Water Works hydrology program to develop and manipulate hydrographs. Our drainage analysis resulted in pre- and post-developed peak runoff flow rates of 2.61 cfs and 2.66 cfs, respectively, for the 100-yr/24-hr design storm. This represents an • increase of 0.05 cfs, which is well below the 0.50 cfs threshold for runoff control exemption as defined in the King County Surface Water Design Manual. The increase in the peak runoff rate for the 100-year design storm event is insignificant (refer to Core Requirement #3) and consequently, should not adversely impact downstream properties. Therefore, no stormwater ! detention is proposed for the site. Yard drains and catch basins will be included in the proposed drainage design to collect surface runoff from landscaped areas and also tight-lined roof downspouts. The existing WSDOT ` stormwater detention system is located near the southeast corner of the proposed project area. The new drainage system for the site will connect downstream of the detention system's control ! structure. Downstream Analysis (Level 1) The purpose of the downstream analysis is to determine the condition and configuration of the • existing downstream drainage system from the St. Matthew's Church project site. Existing record drawings (i.e. City of Renton Storm Drainage Maps and topographic maps of the vicinity) and our field investigation were used to describe the downstream system. • Field observations were completed on December 8, 1995. The weather was cold and clear. There had been heavy rains during the previous week. No surface runoff was observed; however, running water was observed in some of the catch basins downstream of the subject property. . Currently,runoff in the area of the proposed building addition sheetflows overland in a westerly direction into the existing drainage system in the church parking lot. The church drainage • system discharges into an existing 12-inch concrete storm line in NE 16th Street. At this point,the stormwater from the site is conveyed west for approximately 85 feet along the north side of NE 16th Street. The downstream system then conducts flow north for approximately 1,400 lineal feet in an 18-inch concrete conveyance line along the east side of Edmonds Avenue NE (end of 1/4 • mile downstream). The drainage system continues north along Edmonds Avenue for another 4,000 feet where it discharges into May Creek. The flows in May Creek are conveyed west for • another 1.75 miles where it eventually drains into Lake Washington. Based on our observations, there is no evidence of any drainage problems within the vicinity of the project area nor are any drainage problems predicted after development. At the time of our field investigation, there was no evidence of past flooding in the downstream drainage system. The Level I Downstream Analysis indicates that the downstream conveyance system is adequate to accommodate the runoff from the proposed development. Temporary Erosion and Sedimentation Control Since the site is relatively flat and the amount of disturbance is minimal, the risk of erosion or sediment laden runoff disturbance is very low. Therefore, we recommend installing silt fences located along the south and west sides of the construction area, and protection of existing catch basins with filter fabric to prevent sediment from entering the storm system. The site should be monitored and additional B.M.P.'s implemented, if necessary, as construction continues. The project construction drawings will include the City of Renton Standard Construction and Erosion Control Notes. These notes address the required provisions for T.E.S.C. during construction and additional T.E.S.C. B.M.P.'s that may be necessary. Summary • As discussed previously in the report, the net increase in the peak runoff rate during the 100-yr storm was determined to be insignificant. Based on this analysis, no stormwater detention is required by the City of Renton and/or King County. • Our field investigation verified that there is no evidence of any drainage problems within a 1/4 mile downstream of the subject property. The Level I downstream analysis showed no evidence • of any bank sloughing,erosion,flooding. • • • • • • • • • • APPENDIX A • (ENGINEERING CALCULATIONS) • • • • • • • • • • • • • • • • • a • • 12/13/95 9 : 28 : 5 am Coughlin, Porter, Lundeen Inc . page 1 St . Matthew' s Church • Stormwater Drainage Analysis 1990 King County Drainage Manual • BASIN SUMMARY=====_______________________ • BASIN ID: 100ydv NAME: 100 Year Storm Dev. Condition SBUH METHODOLOGY TOTAL AREA. . . . . . . : 3 . 70 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE. . . . : TYPElA PERV IMP . PRECIPITATION. . . . : 3 . 90 inches AREA. . : 1 . 41 Acres 2 . 29 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 00 TC. . . . . 5 . 00 min 5 . 00 min ABSTRACTION COEFF: 0 . 20 • PEAK RATE : 2 . 66 cfs VOL: 0 . 99 Ac-ft TIME: 480 min BASIN ID: 100yrex NAME : 100 Year Storm Exist Condition SBUH METHODOLOGY TOTAL AREA. . . . . . . : 3 . 70 Acres BASEFLOWS : 0 . 00 cfs RAINFALL TYPE . . . . : TYPEIA PERV IMP • PRECIPITATION. . . . : 3 . 90 inches AREA. . : 1 . 60 Acres 2 . 10 Acres TIME INTERVAL. . . . : 10 . 00 min CN. . . . : 86 . 00 98 . 00 TC. . . . : 5 . 00 min 5 . 00 min ABSTRACTION COEFF: 0 . 20 • PEAK RATE : 2 . 61 cfs VOL: 0 . 97 Ac-ft TIME : 480 min lsl f o u ' r, 2• �e � L�S 2•�6C-FS I Q " O.4�� c..'t'S -C� �• S L� �s���� • 0 LEGEND Basin id 100ydv • • 0 0 • M • w a r�� �� z Z. �� G� �a� Floes a • c • �0 0 • • 0 0 • • • 3 b 9 12 15 18 21 A 27 • Time in Hours • • • • • • • • • • • • • • • • • 0 • p LEGEND Basin id IONrex_ • • 0 0 • M cis � k -% "0 yN 0 • 0 0 • • • 3 6 9 12 15 A 21 A 27 Time in Hours • • • • • • • • • • • • • • • • • • • • • • • • • • APPENDIX B • (REFERENCE MATERIALS & SUPPORTING MAPS) • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Scale:1" = (POD Contour: kl.A Project Number: 95- ob r 3C TRANSMITTAL MEMO • at Wsef Project Name: S• • tA6 4 0AUyCJi ``'' M 1 7TGVG- Month: ��- Year 1 q 5 r r, c40FAM Co Page: of PHOW M Map Name: G.�J of ReKfDb Pra:�.,< Aajd iiw,� ZZ?-!v(9Z —T rAx 6 FAX 0 2 - z r SEE SHEET 7 A, �: , t ►�,' R y� 00w�4Ng" I OWIA6"114. fv OHO p " 1 '.� ► r' FE A A NE N N a A E _) �+ w ru . 1NDE A E !21 GT N y 0 b NE NG N s N , � i NE -44 �• JE FE SON i' E •�'. 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E 9711 ST sI d �I SF I leTN STNj�- I S „ 12<> 1 IS$< �'S NE _ xl o •'e z S( o ` _,z t„��Klfi! �T '' - ` I NN�}1R" 1 S„+r. 211� I T� H HIW l l +n.�5 -NE BTH�<S� �._� = ST 120TH_ ST a_ CRIVER ?Iq0 1)I PARR F erS S�\00 NE 7TH ST +� N1 I?IST 51 < .l S c 122ND S7 s ul_v sly. 3 yi t} 1' 300 cr �z Y S-� I2Lro STD 1x,NW 7TH ST1 Z 1TH $� 6. %' NFr > y GI 656 MAP - - - • Mi usillo . I Flo - � 1 � _. WWII _. F FA FAA • , ��,• • • KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL Infiltration facilities must be designed based on infiltration testing and a soils report prepared by a professional civil engineer with expertise in soil engineering. To maintain outflow rates of the infiltration tanks and ponds, all inflow must be pretreated for sediment removal (see Section 5.4). • An emergency overflow path must be identified for infiltration facilities and noted on the engineering plan. This overflow path must be analyzed to meet the requirements of Core Requirements ##1 (see Section 1.2.1) and #12 (see Section 1.2.2) for the 100-year, 24-hour duration design storm, except Downspout Infiltration Systems (see Section 4.5.1). Infiltration facilities may be especially useful in the following circumstances, provided the proper soil conditions are present and all requirements can be met. (1) The proposed project discharges to a closed depression. (2) The proposed project discharges to a severely undersized conveyance system that restricts the runoff volume that can be accommodated. • (3) The proposed project is in a Critical Drainage Area requiring runoff volume control. • Exemptions From On-Site Peak Rate Runoff Control On-site peak rate runoff control will not be required for a proposed project in the following • situations. Negligible Peak Runoff Rate Increase': (1) The proposed project site post-developed peak runoff rate for the 100-year, 24-hour duration design storm event is calculated for each discharge location' to be less • than 0.5 cfs more than the peak runoff rate for the existing site conditions; OR, (2) The project proposes to construct 5,000 square feet, or less, of new impervious surface. a Direct Discharge: The proposed project will discharge surface and stormwater runoff without on- site peak rate runoff control directly to: • A Regional Facility. Direct discharge of surface and stormwater runoff to a regional facility will be allowed if: o the facility has been demonstrated to adequately control the proposed project's increased peak rate of runoff by an adopted King County basin plan or by a detailed drainage analysis approved by the SWM Division; AND o the facility will be available by the time of construction of the project; AND • o the conveyance system to the regional facility can accommodate, with no significant adverse impact to the drainage systems, the design peak runoff for the • proposed project site and the equivalent area' developed to the full zoning potential. ' Proposed projects in adopted critical drainage areas, basin plans and community plans requiring peak runoff rate or runoff volume controls more strict than standard controls shall not quality for this exemption. ' A threshold discharge area is an on-site area which drains to a single natural discharge location or multiple natural discharge locations that combine within 1/4-mile downstream. s The equivalent area is the area tributary to the receiving water body equal to or less than the shortest, straight line distance from the discharge point from the receiving water body (or regional facility) to the furthermost point of the proposed development. • ��� 1.2.3-5 11/92 .. ,-------..�.._,�.-______ �.-.-i-- _.. :.,;, . .. � rM �:�:. e �.. a� .