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HomeMy WebLinkAbout15 - Design Report - Highland Apartments (Norton Ph 1 Lot 1 Blk 3) - Stormwater STORMWATER DESIGN REPORT for HIGHLAND APARTMENTS LOT 1, BLOCK 3, NORTON EAST RANCH SUBDIVISION, PHASE 1 BOZEMAN, MONTANA Prepared for: Norton Properties, LLC 63026 NE Lower Meadow Dr. Suite 200 Bend, OR 97701-5877 Prepared by: t C & H Engineering and Surveying, Inc. 1091 Stoneridge Drive Bozeman, MT 59718 (406) 587-1115 Project No.: 15128 DECEMBER 2015 INTRODUCTION Lot 1,Block 3,Norton East Ranch Subdivision,Phase 1 is located east of Water Lily Drive,north of Fallon Street, and south of May Fly Street in Bozeman, Montana. This project will include the construction of a 20-unit apartment building,parking lot,21 storage units(8ft by 8ft each),domestic water, fire, and sewer services. The parking lot will be graded using curb and gutter to direct stormwater runoff to flow off site onto May Fly Street at which point it travels to the regional detention pond for Norton East Ranch, Phase 1. The project is called Highland Apartments. NORTON EAST RANCH, PHASE 1 —STORMWATER BACKGROUND INFORMATION Norton East Ranch, Phase 1 included the construction of a regional detention pond that handles the entire Phase 1 runoff by curb and gutter and underground stormwater infrastructure designed by Morrison-Maierle,Inc.Lot 1,Block 3 was assigned a Rational Method Runoff Coefficient of 0.5 per Bozeman Design Standards and Specifications Policy (DSSP) in that original design. A Runoff Coefficient of 0.5 is an inadequate estimation for the proposed development of this lot. The actual Runoff Coefficient for the proposed Highland Apartments on Lot 1, Block 3 is 0.69 (See Appendix B for calculations).This original underestimation of the future development of this lot means that the regional detention pond for Phase 1 is inadequate to handle the entire runoff contribution of Lot 1, Block 3 as proposed. LOT 1,BLOCK 3 STORMWATER DESIGN At first it seemed that a retention pond would need to be placed on Lot 1, Block 3 to account for the original under designed detention pond as previously discussed. However, a recently approved project called the Snowcrest Apartments (October 13, 2015 -File no. Z15046)located immediately to the north, on Lot 2, Block 2 was designed to handle most of its stormwater runoff in a 1,719 ft3 onsite retention pond.The Snowcrest site was split into two drainage areas,with the larger Drainage Area 1 flowing into the 1,719 ft3 onsite retention pond and the remaining Drainage Area 2 flowing offsite and contributing to the regional detention pond. This volume of stormwater that runs off of the Snowcrest Apartments project from Drainage Area 2 is less than the original Morrison-Maierle design allocated. It is proposed with this project to utilize that leftover volume of stormwater from the Snowcrest Apartments project as a "credit" to account for the additional runoff from the Highlands Apartments project because both lots stormwater runoff flows to the same regional detention pond for Norton East Ranch, Phase 1. See supporting calculations in Appendix B. `—` Design Report-Page 2 of 2 APPENDIX A DRAINAGE AREA MAPS FOR HIGHLAND APARTMENTS AND SNOWCREST APARTMENTS EXhWff DRAINAGE AREA MAP HIGHLAND 20-PLEX APARTMENTS i --MA-Y-FLY STREE-T- i m C- LOT 1 LOT 2 LOT 3 � i ••yyam� --� LOT 4 = ( 20'cc 0 >L LOT 5 r OD J �� OPEN SPACE I I� > d 4 LOT s J d •1 •� I� III � 7 I� LOT 8 d � Seale In Feet 50 0 50 , I 1 LOT 9 �_� 1Z J �_u�'�. 15 0 15 0 - 8 �811 61 ` !�— `� Seale In Meters I � '� r-- r — Contour Intervals: 1 Foot FALL©N STREET ngineering and Surveying Inc. 1091 Stoneridge Drive•Bozeman,MT 59718 Phone(406]587-1115•Fax(406)587-9768 www,chengineers.com•info@chengineem.com Sheet 1 of 1 #15045 I I i EXHIBIT A DRAINAGE AREA MAP I SNOWCREST 16—PLEX APARTMENTS a i 138.03 090'25'5U' 1,719 C.F. RETENTION (� � I POND RIP RAP cbi' -F: 0 I 1110 Co T 2 tpl ORY 7,404 Sq. Ft._ ._ .I 1co 16fPLEX 2 Acresi �Lo I - - --- f N J 138.03 27925'50" Scale In Feet , - MAY FLY- STREET._._ ._ 50 0 50 - 0 ' Engineering and Surveying Inc. 15 0 15 ``1091 Stoneridge Drive•Bozeman,MT 59718 Scale In Meters hone'4061 587-1115•Fax W6)587-9768 v.chengineets.com•info@chengineers.com Sheet I of 1 Contour Intervals: 1 Foot #15045 APPENDIX STORMWATER CALCULATIONS l Pc_ .levelopment Runoff Coefficient Calculatioi. Weighted Runoff Coefficient for original design, Norton Ranch, Phase 1. Lot 1 , Block 3 (estimation from original design) Morrison-Maierle designed the regional detention pond for Phase 1 using a Runoff Coefficient of 0.5 for future development classification of"Dense Residential" per Bozeman DSSP Table 1-1. However, the proposed development is more dense than originally estimated. Original Weighted C 0 5pI Determine Weighted Runoff Coefficient for the entire lot as proposed. Lot 1 , Block 3 (as proposed) Site Area(Sq. Ft.) Area(acre) C FACTOR Pavement 23266 0.5341 0.95 20-plex roof 11880 0.2727 0.95 Landscape 14545 0.3339 0.05 Total 49691 1.1407 Weighted C 0 6,9 Stormwater runoff volume calculations for 10 year,2 hour storm Calculate stormwater runoff volume for original Morrision-Maierle design. Lot 1 , Block 3, Original design (C=0.5) Q=CiA Retention Pond Summary-10 yr.2 hr storm Total Area= 1.14 acres Weighted C= 0.50 *Rainfall intensity= 0.41 in./hr. Runoff,Q= 0.23 cfs Volume,V= 1684 ft *rainfall intensity value obtained from Bozeman DSSP Figure 1-2 for 10 yr,2 hr storm Calculate stormwater runoff volume for proposed design. Lot 1 , Block 3, as proposed (C=0.69) Q=CiA Retention Pond Summary-10 yr,2 hr storm Total Area= 1.14 acres Weighted C= 0.69 *Rainfall intensity= 0.41 in./hr. Runoff,Q= 0.32 cfs Volume,V= 2312 ft *rainfall intensity value obtained from Bozeman DSSP Figure 1-2 for 10 yr,2 hr storm Determine the difference in stormwater runoff between original Morrison-Maierle design and proposed Lot 1,Block 3 design. Lot 1,Block 3(original) 1684 ft3 Lot 1,Block 3(as proposed) 2312 ft3 Difference 628 ft3 Therefore,628 ft3 of stormwater must be accounted for either through onsite retention or other means.We elect to account for that 628 ft3 through other means.As discussed in the Stormwater Design Report we will use a stormwater"credit'from the neighboring Snowcrest Apartment project to account for the additional 628 ft3 of runoff from this project. The following information is taken from the Snowcrest Apartments(Lot 2,Block 2)stormwater design report. It shows Drainage Area 2 runoff calculation.DA#2 is the west half of the 16-plex roof and west portion of the landscaped lot that does not flow into the 1,719 cf retention pond located in the northeast corner of the site. *Drainage Area 2 Site Area(Sq.Ft.) Area(acre) C FACTOR Impervious 7445 0.1709 0.95 Landscape 5207 0.1195 0.05 D.A.#1 34752 0.7978 0.00 Total 47404 1.0882 *For the purpose of calculating the weighted Runoff Coeffiecient value of Drainage Area#2,the Runoff Coeffient of Drainage Area#1 is considered to have a value of 0.00,because all of Drainage Area#1 is being retained onsite by the retention pond in the northeast corner.This yields a Runoff Coefficient of 0.15 for Drainage Area 92,which is the only area that contributes to the Norton East Ranch,Phase 1 regional detention pond.This proves that the runoff from the developed site that contributes to the regional detention pond is far less than it was originally designed to accommodate. Calculate runoff volume from Drainage Area#2 (on Lot 2,Block 2). Lot 2, Block 2, DA#2 (C=0.15) Q=CiA Retention Pond Summary-10 yr,2 hr storm Total Area= 1.09 acres Weighted C= 0.15 Rainfall intensity= 0.41 in./hr. Runoff,Q= 0.07 cfs Volume,V= 497 ft Calculate runoff volume from the entire Lot 2,Block 2 based on the original Morrison-Maierle design. Lot 2, Block 2, Original design (C=0.5) Q=CiA Retention Pond Summary-10 yr.2 hr storm Total Area= 1.09 acres Weighted C= 0.50 *Rainfall intensity= 0.41 in./hr. Runoff,Q= 0.22 cfs Volume,V= 1606 ft Determine the difference in stormwater runoff between original Morrison-Maierle design and proposed Lot 2,Block 2 design. Lot 2,Block 2(original) 1606 ft3 Lot 2,Block 2(DA#2) 497 ft3 Difference 1109 ft3 1,606 ft3 of stormwater was eligible to be released from the Snowcrest Apartment project,but only 497 ft3 was released from Drainage Area#2.Therefore,1,109 ft3 of stormwater was not release from the Snowcrest Apartment project and can be applied toward the the Highland Apartment project as stormwater"credit".The Highland Apartments only exceed their allocated stormwater runoff volume by 628 ft3,so the 1,109 ft3 credit is sufficient to cover the Highland Apartment project;therefore,no _ insite retention is required. APPENDIX C TABLE 3-1 -RECOMMENDED RUNOFF COEFFICIENTS (ASCE) F, Table 3-1. Recommended Runoff Coefficients. Description of Area Runoff Coefficient: Business Downtown 0.70 to 0.95 Neighborhood 0.50 to 0.70 Residential Single-family 0.30 to 0.50 20,000 sq ft 0.49 10,000 sq ft 0.52 8,500 sq ft 0.57 Multi-units,detached 0.40 to 0.60 Multi-units,attached 0.60 to 0.75 Residential(suburban) 0.25 to 0.40 Apartment 0.50 to-0.70 Industrial Light 0.50 to 0.80 Heavy 0.60 to 0.90 Parks,cemeteries 0.10 to 0.25 Playgrounds 0.20 to 0.35 Railroad yard 0.20 to 0.35 Unimproved 0.10 to 0.30 Character of Surface Pavement Asphalt and Concrete 0.95 Brick 0.85 Roofs 0.95 w.... Lawns,Sandy soil Flat,2% 0.05 to 0.10 t Average,2 to 7% 0.10 to 0.15 Steep,7%+ 0.15 to 0.20 Lawns,Heavy soil Flat,2% 0.13 to 0.17 Average,2 to 7% 0.18 to 0.22 Steep,7%+ 0.25 to 0.35 Source: Design and Construction of Sanitary and Storm Sewers, ASCB and the Water Pollution Control Federation, 1969.