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HomeMy WebLinkAbout09 - Design Report - Norton East Ranch Ph 1 - Stormwater Revised S A N D E R S 0 N t!•� STEWART • • !: �����rrrrrrrr�rrrrrrrrr� 3 BR DO ' D 31 EK = REVISED STORMWATER MANAGEMENT DESIGN REPORT 0. 1 975PE •LUZ NA IL���``�� FO R 1nm110 ,� NORTON EAST RANCH SUBDIVISION, PHASEI ON-SITE IMPROVEMENTS BOZEMAN, MONTANA PREPARED FOR NORTON PROPERTIES, LLC 63020 LOWER MEADOW RD., SUITE 200 BEND, OR 97701 APRIL 2009 NO. PROJECT# BOZ-07004.04 r.. ` i � ` . - �i i � � � i i ;. � .�. it � .+ .L_ 5 _ '� � r r�� ��1��M�MJ rr v�-Rr� � I • I� -r. i n i i � ti _ iiirY i •'�� � - i 1 i•ti� .�. _ i i �1i i+l Norton East Ranch Subdivision, Phase 1 On-Site Improvements Bozeman, Montana Revised Stormwater Management Design Report Table of Contents I. INTRODUCTION.......................................................................................................................... 1 II. EXISTING SITE CONDITIONS.............................................................................................. 1 III. PROPOSED SITE CONDITIONS............................................................................................2 IV. HYDROLOGICAL METHODOLOGY................................................................................... 2 V. STORMWATER ANALYSIS AND DESIGN......................................................................... 3 VI. CULVERT DESIGN...................................................................................................................... .5 I VII. MAINTENANCE............................................................................................................................ 6 VIII. CONCLUSION.................................................................................................................................6 REFERENCES List of Tables Table 1 10year, 2 hour Post-DevelopedPeak Flow Calculations ............................................................ 4 Table 2 DetentionlBetention Pond Storage Volumes............................................................................... 5 Appendices AppendixA Vicinity Map Revised Drainage Basin Map Appendix B Detention Pond Calculations P:BOZ-07004.04 1 s J .• 1 � �i • 1 April2009 Project No. BOZ-07004.04 REVISED STORMWATER MANAGEMENT DESIGN REPORT FOR NORTON EAST RANCH SUBDIVISION, PHASE 1 ON-SITE IMPROVEMENTS BOZEMAN, MONTANA I. INTRODUCTION The Norton East Ranch Subdivision—Phase I Stormwater Management report for the original 40- acre residential development,prepared by Sanderson Stewart (formerly Engineering, Inc.) and dated April 30,2008,was approved by the City of Bozeman on May 13, 2008. The development is located in Section 9,Township 2 South, Range 5 East,Principal Meridian Montana, Gallatin County, Montana, as shown on the Vicinity Map in Appendix A. It is generally bounded by Babcock Street to the north, Fallon Street to the south and Laurel Parkway on the west. The development is planned for 314 residential dwelling units. The subject property is currently owned by: Norton Properties 63020 Lower Meadow Road, Suite 200 Bend, OR 97701 Over the past year, deteriorating real estate market conditions have led the developers to scale down the project into three, smaller phases. The proposed Phase 1 area, shown on the Vicinity Map,is approximately 16 acres in area,was reviewed and approved by the City of Bozeman Planning Department on April 10, 2009. The purpose of this report is to analyze the stormwater drainage characteristics for the revised site boundaries and determine the appropriate stormwater management facilities required for the development, as required by state and local regulations. This analysis is being completed for the revised road and utility design plan set submittal. The design standards governing this project are found in City of Bozeman Design Standards and Specifications Policy,March 2004, and any addenda thereto. II. EXISTING SITE CONDITIONS Site Features and Vegetation Currently, the property consists primarily of pasture land for grazing. The land is not being used for agricultural production at this time. The topography across the site slopes to the northwest at an P:BOZ-07004.04_STORM_REPORT_PHASEl_REVISION 1 approximate grade of 1-3%. There are approximately 4.02 acres of delineated wetlands situated on the northwest part of the property. Soils and Groundwater The Natural Resources Conservation Service (MRCS) Soil Survey had identified four soil types for the property:: Hyalite-Beaverton Complex (448A),Enbar Loam (509B),Meadowcreek Loam (510B), and Hyalite-Beaverton Complex (748A). These soils correlate to hydrologic group C (clay loam, shallow sandy loam, soils low in organic contents, soils usually high in clay). According to a Geotechnical Investigation,prepared by Rimrock Engineering, Inc. in April 2007, the following soil horizons were observed on the subject parcel. The top one foot was comprised of topsoil and vegetation. This horizon was underlain by a layer of lean clay and sandy lean clay ranging from 1.5 feet to 3 feet below the existing grades. Beneath the clay layer was gravel with sand and cobble that extended to the explored depth of 14.5 feet. Information obtained from Montana's Ground-Water Information Center (GWIC) website indicates that the static water level in the area of the proposed development ranges in depth from 50 feet to two feet below existing ground surfaces. The average groundwater depth was 11.24 feet. Six groundwater wells were installed on the project site in December 2006. The wells indicate a static- groundwater,level of 1.5'below existing ground surfaces in the northwest corner. III. PROPOSED SITE CONDITIONS As previously mentioned, the development consists of a total of 110 dwelling units — including single family and multi-family units located on 39 lots. Proposed site improvements include the construction of water and sewer mains, paved roads with curb and gutter, and storm drainage facilities,where necessary. All roads are proposed to be constructed to a crowned section with a three percent cross slope. Roadway drainage will be collected and conveyed via curb and gutter, curb inlets, and detention ponds. All local interior roads will be paved to a 33-foot wide section to back of curb. Laurel Parkway and Babcock will be paved to a 45-foot wide section to back of curb. Boulevard and sidewalk width vary between the two street sections. In general, the roads running north-south flow to the north at grades ranging from approximately 1% to 2%. The east-west roads vary from running parallel to the contours to a slight grade to the northwest. These roads have a minimum of 0.5% grade with high and low points to maintain minimum grades. Valley gutters are used to convey east-west stormwater across north-south running roads. In some areas, the east-west roads have been designed to flow to the north-south roads with valley gutters utilized to convey east-west stormwater to the north. IV. HYDROLOGICAL METHODOLOGY The calculations and recommendations within this report are based on the regulations set forth in the City of Bozeman Design Standards and Specifications Policy,March 2004, and any addenda thereto. Stormwater management will be addressed with the following conveyance facilities: surface flow, drainage swales, curb inlet,pipe conveyance and detention or retention ponds. Both open space within the development and offsite areas will be utilized for detention pond storage. All offsite detention ponds will be located on easements located outside of the public right-of-way. P:BOZ-07004.04_STORM_REPORT_PHASEI_REVISION 2 The Rational Method was used to determine the pre-developed release rate and,in turn, the developed minimum required storage volume. All calculations associated with the release rate 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. V. STORMWATER ANALYSIS AND DESIGN The Rational Method was used to analyze stormwater runoff under conditions which include the property in both a pre-developed and developed state. Runoff Coefficient (C) Runoff coefficients were used from Table I-1 of the City of Bozeman Design Standards and Specifications Policy. A runoff coefficient of 0.20 was used for open land conditions and 0.50 was used for dense residential for developed conditions. Intensity (i) Rainfall intensity values were determined by using the Rainfall Intensity-Duration Curves (IDF) (Figures 1-2 and I-3) from the City of Bozeman Design Standards and Specifications Policy. From the curves, specific intensities equal to the time of concentration were determined and used for peak flow calculations (see Appendix B for calculations). Time to Concentration (Tc) Time to concentration for overland flow for each drainage basin was calculated using the Figure I-1 for distances less than 1200 feet. For distances greater than 1200 feet, the TR-55 method for shallow concentrated flow was used (see below). These times were then summed for a total time to concentration for each basin. Shallow Concentrated Flow (Channel Flow) To calculate shallow concentrated flow the TR-55 method assu�_that sheet QoW becomes shallow concentrated o_w2f 4_a maximum of 300 feet. The average velocity is derived as a function of water course slope and land use. The relationship is expressed as: V = k(100s)°'S where: V = average velocity (ft/sec) k = land use parameter (see Table 3-12,McCuen,page 121) s = average land slope (ft/ft) The travel time for shallow concentrated flow is then calculated as: T, = L / (3600V) where:t, = time of travel for shallow concentrated flow (hours) L = flow length (ft) V = average velocity (ft/sec) Drainage Basins/Peak Flow Calculations Drainage areas which contribute runoff to the proposed development were delineated and analyzed for developed conditions to aid in the sizing of detention or retention ponds, culverts, and storm drainage conveyance facilities. P:BOZ-07004.04_STORM_REPORT_PHASEI_REVISION 3 �j Of the seven original onsite drainage basins that were delineated on the property, two of the basins will be utilized for the revised first phase. The two onsite basins are Basin 2 and Basin 4 (see ,r Drainage Basin Map in Appendix A of this report). Each basin contributes storm runoff to a detention pond down gradient of the drainage basin.'All of the infrastructure associated with Basin 4 will be installed with the first phase,including Detention Pond 4."Basin 2 will be scaled down, as shown on the Drainage Basin Map, and a temporary retention pond will be installed with the first �P\ 'G phase. The remaining infrastructure and the permanent Detention Pond 2 will be constructed with 1 4 the second phase of the development. The three offsite drainage basins (OS-1 - OS-3) were delineated for the extensions of West Babcock Street and Fallon Street, and will remain unchanged from the original stormwater investigation. Pre-development peak flow calculations were made for each basin for the 10 year, 2 hour storm event. Detailed calculations are provided in Appendix B and are summarized in Table 3 below. Table 1 — 10 Year, 2 Hour Pre-Developed Peak Flow Calculations Area Rainfall Peak 1 th+ ? Basin Description C Intensity Discharge 0�1 (acres) in/hour cfs 2 Open Land 4.216 0.20 ✓ 0.41 N/A -4y- 'y 4 Open Land 18.576 0.20 0.73 2.71 OS-1 Open Land 1.436 0.20 1.13 0.32 OS-2 Open Land 1.06 0.20 0.82 0.17 OS-3 Open Land 1.01 0.20 1.93 0.39 Detention Pond Sizing The City of Bozeman De ign Standards and Specifications Policy requires that detention basins be designed to accommodate the difference in peak runoff between the pre-development and post- development 10-year design storm while limiting the release rates to pre-development runoff rates. The required storage is determined by subtracting the total basin release volume from the runoff volume at different storm intervals. The pond locations are shown on the attached Drainage Basin Map. A minimum basin area of 145 square feet per 1 cfs release rate is required for sediment control. Each detention pond will have an outlet pipe with an orifice plate sized to convey the pre- development flow from the 10-year, 2-hour storm event. The volumes of the ponds are shown in Table 2 below. The pond depth totals 2.5 feet—allowing for 1.5 feet of storage, as acceptable by City design standards, and one foot of freeboard. Due to high static groundwater levels in the northwest corner of the subdivision,Ponds 3 and 4 were designed with an overall depth of 1.5 feet by eliminating the one foot of freeboard. The ponds were intentionally oversized, as shown in Table 2, to account for the lack of freeboard. The bottom of the outlet control structures on the shallow ponds will be sunk down below the pond bottom,with the inlet grate being located at the pond bottom elevation, to allow for the "T" fitting to be included. The sides of the ponds will be sloped to finished grade at 4H:1V slope. Detailed calculations are provided in Appendix B. P:BOZ-07004.04_STORM_REPORT_PHASEl_REVISION 4 Table 2—Detention/Retention Pond Volumes Pond Min' Required Pond Volume Storage Vol. (cf) (cf) 2* 6,223 ✓ 610:'t (a C/¢l C 3 1,291 2,985 ' 4 125058 16,288 OS-1 1,819 1,952 OS-2 1,597 1,736 OS-3 960 1,130 *Pond 2 will be a retention pond. Roadside Drainage and Gutter Capacity Calculations The impact of storm water runoff on roadways is also an important design consideration.The City of Bozeman De ign Standards and Specifications Policy provides that for city streets, the flow in the gutters shall not be greater than 0.15 feet below the top of curb. Using this criteria, the available gutter capacity was calculated using Manning's Formula for the roadways running north-south and east-west(see Appendix B for Calculat_ons). A 3%pavement crown slope was used as required by City design standards. L � C..,ZOS PA-9— got NLS4.Low 4j g�v_e� 6—, rr' 'fir" 'l�u W d 08 . Inlet Spacing and Capacity Calculations The proposed roadway design for the local streets includes 33-feet to back of curb,boulevard and sidewalk. West Babcock Street will be paved to a 45-foot wide section to back of curb with a boulevard and sidewalk on each side of the road. Storm water runoff will be captured from the local streets through the use of storm drain inlets and direct it to the proposed detention ponds draining on-site basins. The location of curb inlets and storm sewer mains are shown on the Storm Sewer exhibit located in Appendix A of this report. Similarly, storm drain inlets will be used to capture runoff from West Babcock Street and Fallon Street (basins OS-1 - OS-3). The runoff will then be directed to the offsite ponds. Inlet spacing calculations were performed to determine minimum spacing requirements throughout the development. The analysis included determining allowable capacity of both gutter and curb inlets. - c�-P� a.A- c d � w. r� lJ.t�,�r wo w��t✓t�fd ��/3�OB VI. CULVERT DESIGN Culverts will be installed in four locations along West Babcock Street and in one location along Fallon Street. Two 40" x 65" arch RCP culverts will ire installed across Fallon Street and West Babcock Streets to convey water from Baxter Creek. e culverts$ t4 a design capacity of 150 cfs which exceeds the 25-year storm flow of 148 cfs from Baxter Creek. A second 40" x 65" arch RCP culvert will be installed in both locations to provide 100% overflow protection. 9° In addition to the Baxter Creek culvert on West Babcock, a 12" RCP culvert was installed to provide connectivity to the wetlands near the northwest corner of Phase 1. The culvert has a design capacity of 3.57 cfs, which is more than adequate according to Barbara Vaughn, who prepared the wetland �a study for this project. There is natural swale, with no apparent beginning or end, that is currently passed under the existing two track road via an 18" CMP culvert. This culvert will be removed and P:BOZ-07004.04_STORM_REPORT_PHASEI_REVISION 5 replaced with an 18" RCP culvert with a design capacity of 10.03 cfs. Finally, a 36" RCP arch equivalent culvert will convey water from Baxter Ditch under West Babcock Street, just west of the intersection with Cottonwood Road. The culvert size matches the existing culvert immediately upstream from the crossing and a crossing into the soccer park just south of Durston Road. This culvert has a design capacity of 26.51 cfs. Detailed calculations are included in Appendix B of tJi�it xgpert-. 4 � d 11,LpJ-ck y(AfL1-(J 5/(3/08 , VII. MAINTENANCE CONSIDERATIONS The storm drainage system within the Norton East Ranch Subdivision—Phase 1 is defined as a private and public system. The storm drainage facilities 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 he within the publicly dedicated right-of- way,including the retention and detention ponds, will be maintained initially by the developer and then the Homeowner's Association, once established. Due to sediment in the storm runoff and other variables,regular maintenance will be required by the City to maintain proper performance of the conveyance network. The following steps are minimum requirements for the maintenance of the storm facilities. In pection Program—On an annual basis, the following elements of the stormwater facilities should be inspected for excess sedimentation: 1) Curb Cut Openings 2) Drainage Swales 40 A 3) Detention Ponds 4) Catch Basins Maintenance Program—The following maintenance measures should be completed based on the inspection program: 1) Curb Cut Openings—excess sedimentation should be removed manually. 2) Drainage Swales—swales should be mowed when necessary and any excess sedimentation removed. 3) Detention Ponds—a stakeg(should be set six inches above the original bottom of the basin. If sediment is over the stake,it should be removed and the basin should be re- vegetated according to the original landscape plan. 4) Catch Basins—excess sedimentation should be removed either manually or with a vacuum truck and flushed. VIII. CONCLUSION The included analyses and calculations show that the proposed storm water management system for the Norton East Ranch Subdivision,Phase 1 development will adequately handle the 10-year and 25-year storm events. Available inlet capacity will limit encroachment of runoff on pavement surfaces to acceptable levels. P:BOZ-07004.04_STORM_REPORT_PHASEI_REVISION 6 REFERENCES 1.City Engineering Division. (2004). Design Standards and Specifications Policy and any addenda thereto. Bozeman, MT:Author. 2.Lindeburg, Michael R., PE. (2003). Civil Engineering_Reference Manual for the PE Exam. Ninth Edition. Belmont, CA:Professional Publications,Inc. 3.McCuen,Richard H. (1998).Hydrologic Analysis and Design.Second Edition.Upper Saddle River,NJ:Prentice Hall. 4.Montana Department of Transportation. (1998).AASHTO Model Drainage Manual.Chapters 7,9-10 5.United States Department of Agriculture. Natural Resources Conservation Service. Conservation Engineering Division. (1986).Urban Hydrology for Small Watersheds:TR-55.Washington,DC:Author. P:BOZ-07004.04_STORM_REPORT_PHASEl_REVISION 7 t r, �: 1 ' � _ � �. �� !ji 1! Appendix A Figures r i NORTON RANCH SUBDIVISION GALLATIN COUNTY, MONTANA f -- Tm f- -- — - {���( P . 9 V TV•� 11 -.- 8/7 - -- A0� 9TB 7 �' .i �• — �— SITE 7 �� — __-j.Po�K/ i 1 . 4786AT I _ W BABCQCK S c " Ir i I'...1 - I. I FALLON I:•I I T aa2q''. HUFFINE LN. T. UFFIN — -- -- -� i7 � - s 9B=9T I T -- • VICINITY MAP 4891 -r - n. SANDERSON 5 11:\FYA k I 2000 1000 0 2000 www.sandersonstawart.com i NORTON VICINrY MAP.DWG [30Z 07004 01 06/07/07 RDH .. ,t, •; ,�� ;1 �'� �_ ,t Y GS 1 �' c � � � � � � � � � � � � � � �i NOFtTON RANCH SUBDIVISION - PHASE 1 REVISED BOZEMAN, MONTANA 1 II I II II II DETENTION II I II I - REVISED PHASE 1 I BOUNDARY — OFF SITE BASIN 3 ........... w«y�, tea+ BABCOCK ST OFF - + 1.01 ACRES„ `...`....... .......:... '"""" .. S06 ACRES 2 I (I DETENTION DETENTION POND OS-3 DETENTIQ POND OS-2 POND 3 �j { iSWAINSON STREET I BASIN14 1 < ' Q OPEN SPACE/ \ 1sseACREs X WETLANDS AREA BAXTERCREEK I 4.02 ACRES T 100-YR 0_ Z# I I, :3: LL r FLOODPLAIN I WETLAND Ix w BOUNDARY 3 � w wI E I a r a r I ^I I CALLI PESTREE m I TEMPORAR RETENTION POND 2ILL 1 I I I IZII I I I BASIN 2 m I I I 4 22 ACRES o I I i =I I I I I N IQ I I I U I I I I I DETENTION POND OS-1 13 O~ FF SITE BASIN 1 - - - - — - - - - - - -- - FAL�ONST — — - --- _ 144ACRES a a z v) Q m �I DRAINAGE BASIN MAP a LEGEND ZI — —4787— — EXISTING CONTOUR S A N D E R S O N S54a BASIN 2 1--.— 1BASIN 4 OFF STREET DRAINAGE ARROWS o o OFFSITE BASIN 1 STREET DRAINAGE ARROWS z OFFSITE BASIN 2 NOT TO SCALE www.sandersonstewart.com OFFSITE BASIN 3 z tau;Tut:_STOR1dDRNNAGE"-0/uINM.V'_D,2209.i]'NG BOZ-07004.09 On/2?/(l9 BDS ni a .r �: Appendix B Calculations RETENTION POND CALCULATIONS � _, � .. r: i a �� h S f Norton East Ranch Subdivision - Phase 1 Revised Temporary Pond Basin 2 Bozeman, MT The following calculations were used to determine the minimum required storage volume for storm water runoff. The volumes were calculated using the Rational Method, and the retention facilities were sized based on a 10-year 2-hour storm event. Area = 4.216 Acre I = 0.41 In/Hr. C = 0.5 ✓ Dense Residential Calculate Developed Peak Runoff Rate Q10 = ciA, using the above parameters Qto= 0.86 ✓ cfs Calculate Developed Minimum Required Volume Storage For 10-Year Event V= 7200 x Quo Dense Residential 6223 - cf � _. _ , _ _. ".�; �' �1' �\ ~�. �� y.'n 1 �1 :� ;� tmp#2.txt #units=E1evation,ft,Area,ac,volume,ft3 Volume,ft3 # Elev Area Cummil Avg Cumml Conic # ft ac ft3 ft3 4787.0000 0.1200 6502.8364 6488.0572 V*L. .4r A pEPril 4786.0000 0.0925 1874.0881 1872.3391 4785.5000 0.0796 0.0000 0.0000 �F (,sFbIL'L r MAY 0 4 2999 7 r�rrrr��rrrrr---..--«---- Page 1 Table 2—Detention/Retention Pond Volumes Pond Min. Required Pond Volume Storage Vol. (COc 2* 6,223 6,488 3 1,291 2,985 4 12,058 16,288 OS-1 1,819 1,952 OS-2 1,597 1,736 OS-3 960 1130 *Pond 2 will be a retention pond. Roadside Drainage and Gutter Capacity Calculations The impact of storm water. noff on roadways is also an important design consideration.The City of Bozeman Design Staxndairls and Sbeci 27catims Policy provides that for city streets, the flow in the gutters shall not be gteatef than 0.15 feet below the top of curb.Using this criteria, the available gutter capacity was calculated using Manning's Formula for the roadways running north-south and east-west(see Appendix B for Calculations). A 3%pavement crown slope was used as required by City design standards. Inlet Spacing and Capacity Calculations The proposed roadway design for the local streets includes 33-feet to back of curb,boulevard and sidewalk. West Babcock Street will be paved to a 45-foot wide section to back of curb with a boulevard and sidewalk on each side of the road. Storm water runoff will be captured from the local streets through the use of storm drain inlets and direct it to the proposed detention ponds draining on-site basins. The location of curb inlets and storm sewer mains are shown on the Storm Sewer exhibit located in Appendix A of this report. Similarly,storm drain inlets will be used to capture runoff from West Babcock Street and Fallon Street(basins OS-1 - OS-3). The runoff will then be directed to the offsite ponds. Inlet spacing calculations were performed to determine minimum spacing requirements throughout the development. The analysis included determining allowable capacity of both gutter and curb inlets. VI. CULVERT DESIGN Culverts will be installed in four locations along West Babcock Street and in one location along Fallon Street. Two 40" x 65" arch RCP culverts will be installed across Fallon Street and West Babcock Streets to convey water from Baxter Creek. The culverts have a design capacity of 150 cfs which exceeds the 25-year storm flow of 148 cfs from Baxter Creek. A second 40"x 65" arch RCP culvert will be installed in both locations to provide 100% overflow protection. In addition to the Baxter Creek culvert on West Babcock,a 12" RCP culvert was installed to provide connectivity to the wetlands near the northwest corner of Phase 1. The culvert has a design capacity of 3.57 cfs, which is more than adequate according to Barbara Vaughn, who prepared the wetland study for this project. There is natural Swale, with no apparent beguining or end, that is currently passed under the existing two track road via an 18" CMP culvert. This culvert will be removed and P:BOZ-07004.04_STORilS_REPORT PHASI ]_RT'NNgSTON 5 Retention Pond Sizing The following tables were used to determine the detention pond volumes. The volumes were calculated by using the prismoidal method and are based on the detention pond configuration shown on the drainage basin map. All elevations are assumed. TemDorary Retention Pond 2 ND� L'E E*L�i OL E 0U u c , y ' .' ft 97 9 0 0 97.5 2854 \ 503.88 503.88 bottom z 98 3413 1,564.67 2,068.55 0 98.5 4000 1,851.31 31919.86 Z,5 tr 3 Z 99 4613 2,151.43 6,071,29 WSE �n7� 6�2 2 99.5 5252 2,464.52 8,535.81 z 100 5918 ,790.84 11,326.65 tuN w t— w 0 i � • � ;► �� �.