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HomeMy WebLinkAbout09 - Design Report - Villas at West Winds - Storm Preliminary Stormwater Investigation For The Villas at West Winds Prepared By: DOWL HKM 920 Technology Blvd. Suite A Bozeman, MT 59718 tDOWL HKM March, 2009 045067.138 Preliminary Stormwater Investigation For The Villas at West Winds Prepared For: Cascade Homes,Inc. 1627 West Main St. Bozeman, MT 59715 Prepared By: DOWL HKM 920 Technology Blvd., Ste. A Bozeman, MT 59718 March, 2009 045067.138 The Villas at West Winds: Stormwater Investigation Report September 2008 Purpose This engineer's report is intended to address the stormwater management system for the proposed Villas at West Winds development, and will supplement the Stormwater Investigation Reports submitted for Phase 4 and 5 Subdivisions of the West Winds Planned Community. This report will confirm the previously recommended sizing of the stormwater detention basins and conveyance facilities within the Phase 4 and 5 subdivisions. This report was prepared in accordance with the design criteria set forth in the City of Bozeman's Design Standards and Specifications Policy. Proiect Overview The Villas at West Winds is a multi-family/senior living development containing 21 single- family units, 6 duplex units, 64 fourplex units and a clubhouse. The proposed development is located within Phases 4 and 5 of the West Winds Planned Community(see Figure 1), which is located in the northeast %4 of the northwest %4, Section 2, Township 2 South, Range 5 East. The Villas at West Winds project is bordered by Oak Street on the south, Windward Avenue to the east, Winter Park Street to the north and Davis Lane to the west. Preliminary Conditions Summary The pre-development condition of the proposed 17.49-acre development is agricultural with no residential structures on-site. The property is currently being used for hay production and topsoil stockpiling. The site is not currently irrigated, but the property has water rights through the Farmer's Canal Ditch Company alk/a Harmon Ditch. The current topography of the site can be generalized as flat with a consistent slope of approximately 2% from south to north. The MRCS Soil Survey has identified three soil types on the property: Amsterdam silt loam (53B); Meadowcreek loam(510B); and Fairway silt loam(51 IA). These soil types correlate to the SCS Hydrologic Soil Group `B". Soil pits were excavated and groundwater monitors were installed in April 2003 and monitored through September 2003. From these pits and subsequent pits excavated in April of this year, it was determined that the soil horizon is typical of the Bozeman area with approximately 12"to 18" of topsoil underlain with a layer of silty-clay, followed by sandy gravel. The groundwater depth varied dependant on elevation and proximity to irrigation channels with a typical minimum depth to water being in the range of 4' to 8' below ground surface within proposed housing areas. ry ✓ ® TRADE WIND LANE I � ; FLt1RRY LANE 1 I I =;; � � • TSCHACFE LANE 1 �.,1 1 TURBIA.ENCE LANE �lkl �' ` "a "`s,z a r �S` c z3x � Ye WMER PAW STREET� a `7 WLLAS 1 � F I i AT WEST ' Y f1� T WINDS i ,��. raLrTorrE PL l�,r lY 1 1 a r 3 � 4 9 E ffEEZEtlCLE TEMPE6T COURT @§� TT O B 1 ekaxaH¢¢¢e¢sfem.vv.¢.fffiaiv.9.CY Cmwa¢¢mmeC H.m �1.Y 836 :�;tgg.;.Ye,4;Yr;pYt= to6 E LEGEND WEST WINDS COMMUNI T Y PROPOSED PHASE SUNES YSTEM PROPOSED TRAH.SYSTEM NW 114 SECT/ON 2, T.2.S, R.5.E., P.M.M. o ——— PROPOSED LOT UNE ARIKLA GALLA TIN COUNTY, MONTANA Ific-)0 W L H K M MMM=mFUTURE PHASES —N— 42O TED 111T.A Y BLVD. MUASPREMOUS PHASES VILLAS A T WEST WINDS ®MITE A �`y'^"�"'1 yRyAS AT tt1=ST WNDS SERVICE AREA 9DZEMAN,MT 5971 E7 ���•�•J o 406-5H6-30IF --30 PROPOSEDSTORMWASIN MAIN STORM WA TER PLAN 4D6.5B6-t 730 tFA%1 �O PROPOSED CATT?18A9N/NANHOLE PRE TEWP CONVEYANCE SWALE® ST)MWATER DETENTION AREA FIGURE I �--pp TTUP.STORMWAIER DE71NYM k 200 0 200 ul\lY RETENnON AREAS iiiiiia FUTURE STORMWATLR DETENTION AREA e-de feet DATE: MARCH,2009 PROJECT NO.04SO67.138 Copyright 0 2009 DOWI.HKM,All Rights Reserved. The Villas at West Winds: Stormwater Investigation Report September 2008 Basis of Calculations The calculations and recommendations within this report are based on the regulations set forth in the City of Bozeman Design Standards and Specifications Policy. The Rational Method was used to determine the pre-developed release rates and, in turn, the post-developed minimum required storage volumes. All calculations associated with the release rates and required storage volumes were based on a 10-year, 2-hour storm event. The conveyance facilities, described further in this report, are based on a 25-year, 2-hour storm event. Overflow conveyance facilities from the master detention pond are based on a 100-year, 2-hour storm event. Calculations are provided in the appendices of this report. Stormwater Detention The Villas at West Winds development area was originally comprised of six drainage basins identified as 4A through 41), 5A and 5B in the Stormwater Investigation for West Winds planned Community Phase S report. The proposed site layout and preliminary grading have resulted in revisions to the original drainage basins. There are a total of 11 drainage basins within the revised Villas at West Winds development boundary; Basins 1 through 11 (see Figure 2). The proposed drainage basins are only preliminary at this time due to potential revisions to the site layout and grading as a result of Preliminary Site Plan review comments which may affect the basin boundaries. The runoff from drainage Basins 1, 2, 3 and 5 throughl 1 will be conveyed north to a detention pond located within the West Winds Development, adjacent to Baxter Lane. This detention pond is one of two master detention ponds to be constructed for the West Winds western development area as discussed in the P.U.D. and Phase 5 report. Runoff from Basin 4 will be retained in the proposed pond located within the basin. Preliminary calculations for the development indicate that the composite c-value for the proposed development is 0.62 (see Appendix A). The original calculations used for determining the volume storage requirements and sizing the detention facilities for the Phase 4 and 5 reports assumed a c-value of 0.61 for the area. The small increase in c-value will not be an issue from a storage aspect since the runoff from almost 2 acres of area within Basin 4 will no longer contribute to the overall storage requirements of the master pond. Preliminary retention volume storage calculations for Basin 4 are included in Appendix A and show that the proposed pond will increase in depth by approximately 0.2' in order to store the runoff from the basin. The pond is designed with approximately four feet of free board above that which is required to store the stormwater runoff. Further calculations will be performed once preliminary Site Plan review has been completed. DAVIS LANE (NORTHBOUND) a uj Ttl M BASIN 8 ( ! AREA=0.43 AC I , BASIN 6 I II I BASIN 3 AREA=3.02 AC r E , I J 1 AREA=0.91 AC ` BASIN 1 `....AREA=2.30 AC 4r' " 1 o-CB —�—.�?�_, ca --- --- _ l ' I CB __-- CB BASIN 9 I AREA=1.78 AC !I I.I �B �—� FUTURE PHASE 5 I f f-.< __ „z8 -- _--- DEVELOPMNT AREA 1 � _ TT / N BASIN 4 %�)� 1IVI ( I ---- ,� �` AREA 1.97 EXISTING 21°0 PVC STORM STUB-OUT G+{B , � I • 1 I BASIN 2 1 { ; _, AREA=2.43 ACC' '6 EXISTING 15"0 PVC i I STORM STUB-OUT k�CB - BASIN 10CB I � EA 0.65 A BASIN 5 I -- _ _ �� BASIN 7 i I AREA=1.74I AC 1i. ,; AREA=1.65 AC ti 1 f p 1 t ' r : j i f ►TI 1 I � BASIN 11 :_. ___ , .ice.•- Z _v I t : VIINWARD AVE w _ ZAt PROPOSED DRAINAGE BASIN BOUNDARY THE VILLAS AT WEST WINDS FIGURE 2 100 0 100 CB PROPOSED CATCH BASIN a STORMWATER DRAINAGE BASINS (SURFACE) /-- FLOW PATTERNS BOZEMAN, MONTANA oowL HKM scale feet (PIPE) 045067.138 I MARCH, 2009 Copyright 0 2009 DOWt_ HKM, All Rights Reserved. The Villas at West Winds: Stormwater Investigation Report September 2008 Conveyance Facilities Stormwater runoff from this site, with the exception of Basin 4, will be conveyed to the stormwater detention pond at the north end of the Planned Community, adjacent to Baxter Lane as shown in Figure 1. Stormwater will be conveyed by means of curb and gutter, catch basins, storm mains and conveyance swales within the Villas project boundary. All conveyance facilities in Phases 4 and 5 leading to the temporary conveyance swales were previously sized to handle the 25-year, 2- hour storm event from the proposed site as required in the City of Bozeman Design Standards and Specifications Policy. The proposed storm sewer mains within the Villas development are shown graphically on Figures I and 2 and will discharge to the storm mains within Winter Park Street. Further calculations will be preformed in order to confirm previously determined storm sewer main sizes as well as main sizes within the proposed development once Preliminary Site Plan review has been completed. Standard 24"x 36"square storm drain inlets and 48"combination manhole and curb inlets (C.O.B Standard Drawing No. 02720-1 and 02720-113, respectively) will be used to capture the stormwater runoff at the points specified on Figure 2. These catch basins are in conformance with City of Bozeman design standards and include a 9"sump for ease of maintenance. Maintenance The storm drainage system within the Villas at West Winds is defined as a private and public system. The mains and catch basins that lie within the publicly dedicated right-of-ways are defined as public systems. The public systems shall be maintained by the City of Bozeman. The private system, those facilities that do not lie within the publicly dedicated right-of-ways, will initially be maintained by the developer and then the Homeowners Association. Due to sediment in the storm runoff and other variables, regular maintenance will be required to maintain proper performance of the storage and drainage network. The following steps are minimum requirements for the maintenance of the storm facilities. A) Inspection Program—On an annual basis, the following elements of the storm water facilities should be inspected for excessive sediment deposits: 1) Catch Basins 2) Curb Cut Openings 3) Storm Mains 4) Detention Ponds B) Maintenance Program—The following maintenance measures should be completed based on the inspection program: The Villas at West Winds: Stormwater Investigation Report September 2008 1) Catch Basin, &Curb Cut Openings— excessive sediments shall be removed either manually(with a shovel) or with a vacuum truck and flushed. 2) Storm Mains—Mains shall be flushed if necessary. 3) Detention Pond—A stake will be set six (6) inches above the original bottom of the basin. If sediment is over the stake, it must be removed and the basin must be revegetated according to the original landscape plan. The Homeowners Association is responsible for conducting the inspection program and maintaining the storm facilities outside of the public right-of-way—this typically includes the retention and detention ponds. During the annual stormwater inspection, if any facility within the street right-of-way needs attention, the Homeowners Association should contact the City of Bozeman Streets Department and inform them of the problem. Conclusion This report establishes the preliminary projected stormwater flows, detention requirements and conveyance recommendations for the Villas at West Winds development. The basis for this report is the approved West Winds P.U.D. application of September 9, 2005 and subsequent Stormwater Investigations for West Winds Phases 4 & 5. THE VILLAS AT WEST WINDS STORMWATER POND VOLUME CALCULATION Basins 1,2,3 and 5 through 11 Building y Area Driveway/Parking Sidewalk/Patio Total impervious Pervious/LS Area Roadway Area Area Area Area Total Area (sf) 184,846 64,381 76,827 60,143 386,197 290,034 676,231 (acres) 4.24 1.48 1.76 1.38 8.87 6.66 15.52 %of Total: 57.1 42.9 Composite C= 0.9*(%Impervious)+0.25*(%Pervious) Composite C= MEMEMEM Basin 4 Building Sidewalk/Patio Pond/Water Total impervious Pervious/LS Area Total Area Area Area Surface Area Area (sf) 19,584 6,075 18,537 44,196 41,498 85,694 (acres) 0.45 0.14 0.43 1.01 0.95 1.97 %of Total: 51.6 48.4 Composite C= E Storage Volume Requirements -Basin 4 Pond (Contributing Basin: 4) The following calculations were used to determine the minimum required on-site storage volume for the storm water runoff from Basin 4. The volume was calculated using the Rational Method, and the retention volume was determined based on a 10-year 2-hour storm event. Area = 1.97 Acre C = 0.59 Weighted C I = 0.41 in./hr. Determine post-developed runoff rate: Q = C x I x A 0.48 cfs Determine minimum required storage volume: V= 7200xQ 342 : cf -Proposed Pond 4722.0 18,448 0 0 normal water el7in4722.1 18,678 1,856 1,856 4722.2 18,908 1,879 376 WSE w/