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HomeMy WebLinkAboutStormwater Drainage Analysis - PLN General - 4/17/1995 EC V ED MAY 0 2 lg95 A4,kS; N CO. PLA14016 kz?T. STORMWATER DRAINAGE ANALYSIS CHURCH OF THE NAZARENE E. 1331 Shelton Springs Road Shelton, Washington April 17, 1995 ENGINEERING MANAGEMENT SERVICES OLYMPIA, WASHINGTON :Value Engineering •Pre-Construction •Training EC7�i"Xs •Value Analysis ENGINEERING MANAGEMENT SERVICES *Value Management April 17, 1995 Mason County Department of Public Works Mr. Alan Tahja P.E. Hydraulics Engineer 411 N. Fifth St. Shelton, Wa. 98584 Re: Shelton Nazarene Church E. 1331 Shelton Springs Road Stormwater Drainage Design Dear Mr. Tahja: Enclosed is a stormwater drainage analysis of the proposed improvements for the Shelton Nazarene Church, located at E. 1331 Shelton Springs Road. The actual improvements planned are fairly minimal, involving only construction of a 3200 sq. ft. sanctuary, and a gravel parking lot. The existing site is relatively flat and there are no apparent drainage problems in the area. All surface drainage appears to be sheetflow and is apparently infiltrated into the granular soil without significant surface accumulation or runoff. The stormwater analysis was evaluated using the"Stormwater Management Manual for the Y g Puget Sound Basin" prepared by the Washington State Department of Ecology. Best Management Practices include: control of runoff to prevent pollution of groundwater or surface water runoff; optimization of infiltration techniques; and on site retention and sedimentation facilities. The proposed design will safely control and contain the surface water runoff(if any)and will provide a natural,low maintenance facility. The construction will involve grading of about five hundred cubic yards of material to establish the collection and retention ditch and basin. This ditch and basin area will be revegetated with natural grasses to enhance the filtration characteristics of the facility. We request your review and comments regarding the proposed design. If you have any questions, or need additional information,please advise. Thank you for your consideration. Sincerely, Clyde L. Slemmer P.E., CVS Attachment Clyde L. Slemmer, P.E.,CVS, Principal/Owner, 416 Shadow Lane NE., Olympia, WA 98506 Business: (2%)456-4887 Fax: (2%)456-4887*51 Home: (24W� 491-5893 SHELTON NAZARENE CHURCH STORMWATER ANALYSIS REPORT ra Gene 1 The proposed project is located at E. 1331 Shelton Springs Road, in Shelton, Wa., on Lots 2 and 3 as identified in the Short Plat for John Oakes. The proposed improvement will include a new church sanctuary building of approximately 3200 sq. ft. and a graded gravel parking lot of 16,310 sq. ft.(.45 acre). The total area of Lots 2 and 3 is 4.25 acres. The new improvements will be located adjacent to Lot No. 1 which has been developed as a residential site and is now being used by the church. 1 toward the southwest. There ' relatively flat with a slight slope e o The topography of the land is e y gh p are no apparent surface drainage channels or wetlands and no evidence of poor drainage or ponding. The general soils in the area are of glacial origin, and consist of compacted sands and gravel with layers of alluvial outwash granular materials primarily on the surface. These surface materials are generally more free draining and provide a good medium for infiltration of stormwater. Three percolation test holes have been placed on the site, and they indicate relatively clean sand and gravel to a depth of approximately four feet with denser materials below four feet. Goals and Objectives Following are the goals and objectives for this stormwater analysis: * Investigate the existing site conditions including terrain, topography, vegetative cover, soil conditions, and drainage patterns and performance. * Evaluate the proposed project improvements, and determine the magnitude of the impacts, and the land use description for each distinct element. * Determine the appropriate Soil Group Classification, and determine the applicable Soil Conservation Service (SCS) Runoff Curve Number for the land use and soil group. * Select the appropriate precipitation designation from the regional Isopluvial Maps, calculate the potential maximum detention for the areas and the volume of stormwater runoff for each design storm event. * Design a stormwater drainage system which will provide capacity to allow infiltration,provide for sedimentation to occur,and prevent excessive discharge onto adjacent property. - Reduce the preconstruction runoff quantity by fifty percent,for the two year design storm. - Provide capacity to retain runoff quantities exceeding the preconstruction conditions for the ten, and one hundred year design storms. Small Parcel Determination The proposed improvement for the church will add approximately 4890 sq.ft.of impervious area including the roof area and sidewalks. The total area for construction(land disturbance) is estimated to be about 22,100 sq. ft. (less than one acre)including the building and gravel parking area. In addition,revegetation and landscaping will be accomplished to enhance the infiltration and transpiration capability of the soil and ground cover. The construction access route is an existing,mature driveway access directly to the Shelton Springs Road. Due to the land terrain, slope gradient and existing vegetation buffers, the adjacent properties will not be affected by the construction activities or stormwater runoff (if any). Considering this minimal construction activity and land disturbance the site appears to meet the criteria for the"Small Parcel'requirements as outlined in the Puget Sound Water Quality Management Plan. Hydrologic Soil Group Classification The general appearance of the site indicates moderately good drainage characteristics. There is no evidence of surface drainage channels, significant ponding, or any stormwater runoff flowing from the property. A significant portion of the property along the east and south boundary lines is vegetated with mature second growth timber and heavy brush, and constitutes about sixty percent of the total site. These areas will not be impacted by the proposed improvements. The remaining major portion of the site is open space which will be vegetated with grass and landscaping. This area is approximately thirty percent of the total 4.25 acre site. The proposed construction will affect only about ten percent of the property. Based on the visual observations of the soil type and description and the existing drainage characteristics and performance, the classification meets the requirements of the Soil Conservation Service (SCS) Soil Group "B". "Moderate low runoff potential". Soils having moderate infiltration rates when thoroughly wetted, and consisting chiefly of moderately fine to moderately course textures. These soils have a moderate rate of water transmission". Land Use Description and Runoff Curve Numbers The site can be divided into four distinct land use descriptions as follows: 1. Impervious surfaces, pavement, roofs, sidewalks. This classification covers about 6140 sq. ft., or .141 acres. 2. Gravel roads and parking lot. This classification covers about 15750 sq. ft., or.362 acres. 3. Open space, good condition, with grass cover. This classification covers about 54910 sq. ft., or 1.26 acres. 4. Wood or forest land, mature second growth and brush. This classification covers about 108375 sq. ft., or 2.49 acres. Total property area(Lots 2 and 3) =4.25 acres. SUMMARY Area Designation Description Area(acresl SCS Curve No. 1 Impervious .141 98 2 Gravel areas .362 85 3 Open Space, Grass 1.26 80 4 Forest and Woods 2.49 72 Isopluvial Map - Pred-pitation Designation For this analysis,precipitation values for the two,ten and one hundred year,24 hour design storms were taken from Isopluvial maps prepared by NOAA Atlas 2 for the Puget Sound Basin. Storm Frequency Isopluvial Number Precipitation 2 year 40 (.10) 4" 10 year 65 (.10) 6.5" 100 year 80 (.10) 8" Potential Maximum Detention By Area S = (1000/CN) - 10 Area Designation "S" Detentionl 1 1000/98 - 10 = .204 2 1000/85 - 10 = 1.76 3 1000/80 - 10= 2.5 4 1000/72 - 10 = 3.89 Following are calculations to determine the precipitation depth (Qd), and the total runoff volume for the design storm events. '7 L51, L S -9�yo rt-)d �Uurnn/It 7U-L OOL bS Q W,2.:�qi 7IV101 -7 Z ,N ��-qZ�g- 4s`� j 7 S'O� p wrvly -b2 SiSti 7 /Vt! 1 rPn-3 u��QLs' V44;�jS IYRL. v l o -pt"R Ej R44OW7011 -1, a`o- •NO L�/��=� (7VON/74)f YWO-)olj 72c4.L ivnv4 L )8 ] / ZL` / a jro Oufb Z.= � �/ �0'Z O�'I)c = n C .X • +LL �OZ • ��a'�£ � �t�oZ�R' t� � ) Z" - fr PC7 1 z Xb%,, vm/ & 4 YSls (Z4 wouY� .�rr��errI �v .Vr �iv�3 ,O- i z V = 36.30 (-7,'76y*,i4 i4Qeh z C8 -,'L(I,?(, 7.z / 8 + (/_7�)� = e,zZ V = 34,3 o C.t S�, 36Z) -- Y/ -7.3 C",F L(2,5 1 [e f3(2.S> = S,4-3 5630 (q.6.3y/,Z4) C, f cl ��� ��d -. L(5,-S )7Z [./ 8 t, e,�3_�39�� ✓- Z30 (4.G9 2,4`t) = g23Y t C, F j?>�L I�c�GUrrlE �4ND�f I! Tor"RL l�o�u�� �uNa,afr I�, 1704Z0 C.f CIMA16 E V ct--cipr 6- Storm Drainage Design The drainage design proposes to construct the gravel parking lot at the elevations shown: Location Elevation Southwest Corner 400.00 Northwest Corner 399.50 Southeast Corner 398.50 Northeast Corner 398.50 The buffer area to the north between the parking lot and the Shelton Springs Road will be graded away from the parking area gently toward the road. The drainage gradient will be parallel to the road sloping gradually to the east to the drainage ditch along the easterly property boundary. A twelve inch C.M.P. culvert will be placed under the driveway access (if necessary) to assure continuity of flow to the ditch on the easterly boundary. The collection and retention ditch will be located along the easterly boundary of the property terminating in a retention basin located at the southeast corner. The east boundary ditch elevation will slope from elevation 396.5 on the north to elevation 394 at the southeast corner. The ditch will be trapezoidal in section with a three foot wide bottom and 4H:1V side slopes. At the southeast corner the retention basin with a 27 foot diameter bottom and 5H:1 V or flatter side slopes will be constructed with a top elevation of 398. A twelve inch diameter C.M.P. overflow pipe will be installed with an invert elevation of 396.5. The discharge will be to the southeast following the natural slope of the terrain. On Site Retention Capacity With the proposed design all surface water flowing from the parking area will flow across the grass infiltration area between the parking lot and the road and the excess water,(if any), will be contained in the easterly ditch and retention basin. The risk of hazardous waste contamination is very low as the parking area will be used primarily for passenger vehicles. All sediments will be contained in the bio-swale ditch. The buffer zone between the parking area and the Shelton Springs Road will provide approximately 9200 sq.ft.of bio-filtration area before the water enters the easterly retention ditch. The easterly retention ditch will provide storage capacity for 9340 C.F. The retention basin at the southeast corner will provide capacity for an additional 5300 C.F. of storage. The total storage capacity will be 14,640 C.F. This will reduce the pre-development runoff volume by about fifty three percent. By placing the top of the ditch berm at elevation 398, an additional volume of storage capacity is created in excess of 5000 C.F. The twelve inch overflow pipe located with invert elevation at 396.5 will control the 10 year and 100 year storm volumes and is assumed to be operating at forty percent efficiency. Summary and Conclusions With the observed infiltration characteristics of the site, it is improbable that the retention capacity will be reached with the design storm events. The design,however,provides a level of safety to prevent runoff discharge to adjacent property. The bio-swale ditches and retention basin will provide for containing sediments and assure high quality for any overflow runoff which may occur. As mentioned in the introduction, there is no evidence of surface runoff on the site. Considering that the physical construction changes will effect only about ten percent of the area, and with the additional retention capacity planned in the drainage design, the post construction drainage conditions will provide an even greater factor of safety for off sits drainage and improved water quality. The construction staking for the location of the collection and retention ditch and basin will take advantage of saving large trees and significant brush clusters to maximize the existing vegetation. In other words, the actual construction may meander slightly to optimize the terrain and topographic features, but the essential elevations and capacities will be maintained. r- AUE/U IFN__•! �"'r X15d'1 R.S� •t '� • - • • ��S � Paz � . , - . . :' ' �E�/CAT/ON f.•. ` 29• , a FZ.l^2. '�` 1 K� S7- /z -,pear//V ra.4vT 4W •� ;Ems, o Q,�. SHOR7�" PLAT To ono/Cw`.4NOA.�O.c WAY .o�/Rf-bS• \� TO MA3o v CoUNTy LY/n � �~ NORTXIBRLY OF 7y� .3t7U: M.4,Qa/N OA< S.*�►4/NG �• �/� ROAD 'Its -7"o wev �IQ �� JO H Ill OAK�.S syELTO.,V .TP RO. > �63 SHELTON, Wii., %9 ¢ S, o T !% _ NORT,vEA ST CORNER vw%4 i @ss SECT/ON 12, TOWNSH/P 20 N7 407 RANGE vtc 1' Z I 74 Z Z i m -:•1 c4 Q d()�03 7 2 189. 40 — _ 4 73 IInn 1 c V1 Q 'S.VI/. COR. S�7`4 3 7Co3. �7(.:31 3'.58�_'• t . SEC. /2 S E. COR. 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SGtLE 1 N= So A;� A__A• S C$ d D. 7z -- I . cv / yw� Vlt \ O �S � N ��✓, 398 , s h k'Y sLvpE Z cam. �v SI.bPE 4*1 P, �Ld /f'✓�cEss 6L�V. a 57 �S� ALE d• '',4�5 •o P Rk 47 169rlW j s� �e7' I I Sl- �-1 25 *;/ SLOPE SEGTiC�1.l a-8 SCOh LE t r I I f ' l SukF/�cE sN�,ErftoW fOLLOk/l�vL� ' a 3qg 6be-✓, 398 AI,47aR9l. gRouvO 7ER941A/ DRAWAyE PArfC-,eA/, n- R AgEO /3 w !� S� Np Rd-TEN7-104 BRA/Af a,7' — O�/ERFC.Ot1/ f pipe, i 1 1646-V#n0N O6 86-0-M seeno,y C-C o M _ 398 - SCAt.E , 1 /0 r=t-&Vn n om OF RET=N-51Oki Btv5 N 394 a�E�FIoW PlP� C /2" C.M.P. —_ _-_ _ SATE D.QA IAfAqE: PLA/V - MASON COUNTY PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER Shelton, Washington 98584 April 21, 1995 INTER-DEPARTMENTAL COMMUNICATIQNS DATE Mason County Planning and Building Departments TO: —ALAN A. -T - lir--ngineer FROM: --SMItu"azarene Church Stormwater Drainage Plan SUBJ: Mr. Clyde L. Slemmer, P.E. , of ENGINEERING MANAGEMENT SERVICES of Olympia, submitted a stormwater drainage proposal for a planned Nazarene Church to be located on the Spring Road in Mason County Washington. The plans are dated April 17, 1995, and Public Works review of the proposal finds that they meet current standards required by Public Works for commercial sites. Mr. Slemmer employed a static design method and attempted to achieve complete infiltration of design storm events through a 100 year, 24 hour event. Computer simulation of the site with the proposed improvements indicates that the facilities may not infiltrate all stormwater runoff as Mr. Slemmer intended, but that during storm events larger than a 2 year 24 hour event, some runoff may occur at the same location as current runoff escapes the property. The rates of the runoff from the site during larger events is well within PSWQA downstream erosion control discharge standards, and is accepted as an alternative standard when complete infiltration of surface water runoff is not practicable or achievable. When Mr. Slemmer delivered the plans to Public Works, he only left one set, and was reluctant to produce more until he was reasonably sure that they would not be wasted by revision. At that time, I provided Mr. Slemmer with a blank copy of the Operation & Maintenance Covenant required of all privately owned and maintained stormwater control facilities. I explained to him that the completed document should be notarized and recorded with the County Auditor's Office, and a copy of the recorded document should be furnished to the County's Public Works Department. In the body of the drainage report, Mr. Slemmer indicates that the site grading will require approximately 500 yards of earthwork and affect approximately 10 percent of the property. I feel Mr. Slemmers estimate of 500 cubic yards is a high estimate. If additional copies of the stormwater plan are needed by your departments, please contact Mr. Slemmer at 456-4887 (Office) or 491-5893 (Home) . ALAA A. TAHJA : __ File: H:\WP\STRMWTR\NAZ-CHRH.IDC