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HomeMy WebLinkAboutStormwater Management Report_06-03-2020 STORMWATER MANAGEMENT DESIGN REPORT FOR: COLLINS APARTMENTS LOTS 2 & 3, BLOCK 1 CATTAIL CREEK SUBDIVISION, PH. 3 BOZEMAN, MT Prepared By: MADISON ENGINEERING Madison Engineering 895 Technology Drive, Suite 203 Bozeman, MT 59718 (406) 586-0262 FEBRUARY 2020 REVISED DUNE, 2020 STORMWATER MANAGEMENT DESIGN REPORT FOR: COLLINS APARTMENTS LOTS 2 & 3, BLOCK 1 CATTAIL CREEK SUBDIVISION, PH. 3 BOZEMAN, MT f i .]. � 6 F+may e pltnESKI . r • � !��ONA�����• Madison Engineering 895 Technology Blvd Ste 203 Bozeman, MT 59718 (406) 586-0262 FEBRUARY 2020 REVISED JUNE 2020 COLLINS APARTMENTS SITE PLAN LOT 2 & 3, BLOCK 1, CATTAIL CREEK SUBDIVISION, PHASE 3 STORMWATER DESIGN REPORT A. Introduction The proposed development for the Collins Apartments will be constructed on Lots 2 and 3, Block 1, of Cattail Creek Subdivision Phase 3. The site will have four buildings, each with four apartments, for a total of 16 dwelling units. The site will contain stormwater onsite and allow infiltration/retention through underground infiltration chambers consisting of 18-inch diameter perforated corrugated metal pipe (CMP) that capture runoff via stormwater inlets in the parking lot and drive lanes. The following references were used in the preparation of this report: a. COB Design Standards and Specifications Policy, 2004. Addendum #6 b. COB Modifications to Montana Public Works Standard Specifications (MPWSS) B. Stormwater Management The infrastructure for Lots 2 and 3 will be constructed in one phase although the construction of the buildings will be phased. Stormwater from Lot 2 flows in a northerly direction from the drive aisle and driveways into two storm inlets at the center of the lot. These flows are directed into an underground infiltration facility on the west side of the site on Lot 2. The flows from Lot 3 flow in a northerly direction into an inlet at the northwest corner of the drive aisle, which directs flows into an underground infiltration facility along the west side of the site. All four buildings will tie their roof downspouts into an 8-inch ADS pipe that connects to the storm system and ultimately drains into the infiltration chambers. Storm water run-off is calculated based on the Rational Formula and infiltration/retention facilities are sized per the City of Bozeman's design standards for a storm intensity of 10-year frequency and a 2-hour duration. Table 1 below shows the required and proposed storage volumes for the two (Lot 2: southern; Lot 3: northern) infiltration basins. The calculations that produce these volumes can be found in Appendix A. Table 1:Facility Storage L'olurnes Facility ID Required Storage Volume (cu ft) Proposed Storage Volume (cu ft) Northern 1,172 1,178 Southern 1,115 1,178 The subsurface stormwater facilities have been redesigned to provide storage for the 10-year event with no infiltration reduction in volume and the bottom of the basin to be above the seasonally high groundwater levels. The basin size has been revised to 2-18" side by side pipes and 2.5' deep from bottom of topsoil to bottom of basin stone. In addition, the proposed finish Collins Apartments Stonn mdter Design Report Page 1 of 2 grade at the basin locations is approximately 6" above existing ground. Based on the revisions it is anticipated the bottom of the basin will be approximately 2.5' above high ground water levels. C. Curb inlet adequacy check The three curb inlets that received surface flow from the site were checked for capacity. All standard 24-inch by 36-inch curb inlets are adequate to handle the runoff from a 25-year storm event. See Appendix B for more information. D. Groundwater Monitoring No groundwater monitoring has been performed on this site. However, in early May a ground water monitoring well was installed on a lot to the north, near the proposed Collins development, and has been monitored weekly since. As shown in Appendix E - Groundwater Monitoring Summary, the ground water levels are approximately 5.5'±below existing ground. We will continue monitoring the groundwater through the peak. Appendices A. Stormwater calculations B. Curb Inlet adequacy check C. Stormwater Maintenance Plan D. Drainage Exhibit E. Groundwater Monitoring Summary Collins Apartments Stormwater Design Report Page 2 of 2 Appendices Appendix A — Stormwater Calculations Collins Apartments 6/3/2020 South Basin Stormwater Run-off and Infiltration Calculation Calculation of Required Volume for Storm Water Retention & Infitration System Design Rainfall Freq. 10 year-2 Hour event OF coefficient a 0.64 OF coefficient b OF coefficient n 0.65 Post-development Calculations: C Areas(ft): Impervious 16.965 0.90 Landscaped 5,951 0.20 Total: 22,916 0.72 Retention Basin Calculation: Q=CIA C= 0.72 (post-development) 1 = 0.41 in/hr(10-yr,2-hr storm) A= 0.53 acres Qpsst= 0.15 cfs Required retention storage(ft3)= 1,115 ft3 (10-yr,2-hr storm) Contech Corrugated CMP Pipe Storage Calculations: Pipe Area A=3.14xR2 Diameter Radius 0.75 3.14 = 1.77 fit` Pine Volume A=3.14xR2 Diameter Radius 1.5 0.75 3.14 = 1.77 ft2 Length(2-rows @ 80'each) = 160 ft Volume = 283 W Stone Volume Stone Width = 4.5 ft Stone Depth = 3.5 ft Area = 15.75 ft2 Less Pipe Area = 1.77 ft2 Stone Area = 13.98 ft2 Pipe Length = 160 ft Storage Volume in Stone(40%voids) = 895 ft3 Total Storage Volume = 1,178 ft3 Infiltration Calc-Corrugated CMP-South REV 1.xls Collins Apartments 6/3/2020 North Basin Stormwater Run-off and Infiltration Calculation Calculation of Required Volume for Storm Water Retention& Infitration System Design Rainfall Freq. 10 year-2 Hour event OF coefficient a 0.64 OF coefficient b OF coefficient n 0.65 Post-development Calculations: C Areas(ft): Impervious 17,455 0.90 Landscaped 7,942 0.20 Total: 25,397 0.68 Retention Basin Calculation: Q=CIA C= 0.68 (post-development) I = 0.41 in/hr(10-yr,2-hr storm) A= 0.58 acres QPos= 0.16 cfs Required retention storage(ft3)= 1,172 ft3 (10-yr,2-hr storm) Contech Corrugated CMP Pipe Storage Calculations: Pipe Area A=3.14xR2 Diameter Radius 0.75 3.14 = 1.77 ft` Pipe Volume A=3.14xR2 Diameter Radius 1.5 0.75 3.14 = 1.77 ft2 Length (2-rows @ 80'each) = 160 ft Volume = 283 fe Stone Volume Stone Width = 4.5 ft Stone Depth = 3.5 ft Area = 15.75 ft2 Less Pipe Area = 1.77 ft2 Stone Area = 13.98 ft2 Pipe Length = 160 ft Storage Volume in Stone(40%voids) = 895 W Total Storage Volume = 1,178 Ife Infiltration Calc-Corrugated CMP-North REV 1.xls Appendix B — Curb Inlet adequacy check Collins Apartments Peak flow calculations Curb Inlet Drainage Basins-Post Development Peak Flow Summary(cfs) Area Drainage Area No. Area(Ac.) Weighted C 100 Yr 25 Yr 10 Yr S.drive aisle DA 1 0.420 0.72 1.62 1.22 1.06 E.drive aisle DA 2 0.110 0.72 0.81 0.61 0.53 N.drive aisle DA 3 0.580 0.68 2.00 1.59 1.41 OF Equations from Bozeman Stormwater Master Plan for City of Bozeman,March 1982 General Equation_ i=a/(b+D)"where 0 is duration in hours,i=intensity in inches/hour Storm Information Design Rainfall Freq. 100 25 10 OF coefficient a 1.01 0.78 0.64 OF coefficient b 0 0 0 OF coefficient n 0.67 0.64 0.65 Adjustment Factor Cf. 1.25 1.1 1 Peak Q Values Drainage Area 1 Area 0.420 acres Weighted C 0.72 Average slope 0.75 percent Travel Distance 146 feet Design Rainfell Freq. 100 25 10 years C`Cf 0.90 0.79 0.72 (Shall not exceed 1.00) Total tc: 4.96 7.63 9.42 minutes intensity at t� 5.37 4.02 3.49 in/hr peak runoff: 1.62 1.22 1.06 Os Drainage Area 2 Area 0.110 acres Weighted C 0.72 Average slope 2.72 percent Travel Distance 5o feet Design Rainfall Freq. 100 25 10 years C`Cf 0.90 0.79 0.72 (Shall not exceed 1.00) Total tc: 1.89 2.92 3.60 minutes intensity at tc 10.23 7.66 6.65 in/hr peak runoff: 0.81 0.61 O.S3 cfs Drainage Area 3 Area 0.580 acres Weighted C 0.68 Average slope 0.75 percent Travel Distance 130 feet Design Rainfall Freq. 100 25 10 years C'Cf 0.85 0.75 0.68 (Shall not exceed 1.00) Total tc: 5.87 8.26 9.86 minutes intensity at t, 4.80 3.81 3.39 in/hr peak runoff: 2.00 1.59 1.41 Os Page 1 of 1 Collins Apartments Gutter Capacity Calculations Allowable Pavement Encroachment Given: T= 9 feet(max per city) T W= 1.5 feet W is Ts= 7.5 feet Sw= 0.0625 ft/ft Sx= 0.03 ft/ft a a= 0.59 inches °` s' d= 3.24 inches e n= 0.015 s. a Sw/Sx= TM= Capacity for Gutter equations: Q=Qw +Qs Where: Qw =E.Q Qs.= Discharge within the Roadway above the depressed section(cfs) Qs Qw= Discharge within the depressed Q—]—Eo (gutter)section(cfs) Cf= 0.56 for English units Qs =Cr Sx3TS3S��1 Sx= Pavement cross slope(ft/ft) n Ts- Width of flow in the roadway above depressed section So= Gutter longitudinal slope(ft/ft) S W/SX Sw= Gutter depression cross slope(ft/ft) ]+ a T= Spread(ft) ]+ Sw/SX - I(T/W)—1 W= Width of gutter depression(ft) , _Capacity solution_ Gutter Capacity-Drainage Area 1 Gutter Capacity-Drainage Area 2 So= 0.0075 So= 0.0272 Qs= cfs Qs= cfs Eo= cfs Eo= 0.42 cfs Q= cfs G- 6.67 cfs Gutter Capacity-Drainage Area 3 So= 0.0075 Qs= 2.02 cfs Eo= 0.42 cfs �Q= 5 C Cis DA AREA 25 Yr Design Flow(cis) 1 S.drive aisle 1.22 2 E.drive aisle 0.61 3 N.drive aisle 1.59 Summary The gutter capacity is adequate for all three proposed curb inlets. Page 1 of 1 Collins Apartments Inlet Capacity Calculations Gutter Section Given T= 9.0 feet T W= 1.50 feet Ts= 7.50 feet "' TB SW= 0.06 ft/ft Sx= 0.03 ft/ft _ a= 0.59 inches 0. d= 3.24 inches o. s. n= 0.015 d So= 0.016 f Where: S. a Qs= Discharge within the Roadway above the depressed section(cfs) Qw= Discharge within the depressed (gutter)section(cfs) Capacity for Inlets on Grade Cf= 0.56 for English units (Standard 24x36 Curb inlet) Sx= Pavement cross slope (ft/ft) Ts= Width of flow in the roadway above Drainage Area 1 depressed section So= Gutter longitudinal slope(ft/ft) Qw= 1.48 cfs Sw= Gutter depression cross slope(ft/ft) Qs= 2.02 cfs T= Spread (ft) Cross-sectional area of flow W= Width of gutter depression(ft) A= 1.22 ft2 Gutter Velocity V= 2.88 ft/sec Fraction of side flow intercepted Rs= 0.27 Total flow capacity intercepted by the inlet Qint= 2.07 1 cfs Qbypass= 1.43 cfs Design Q for inlet#1 025= 1.22 cfs Single Inlet Sufficient Drainage Area 2 Qw= 2.83 cfs Qs= 3.84 cfs Cross-sectional area of flow A= 1.22 ft2 Gutter Velocity V= 5.49 ft/sec Fraction of side flow intercepted Rs= 0.10 Total flow capacity intercepted by the inlet Page 1 of 2 Qint = 3.32 cfs Qbypass = 3.34 cfs Design Q for inlet#2 Q26= 0.61 cfs Single Inlet Sufficient Drainage Area 3 Qw= 1.48 cfs Qs= 2.02 cfs Cross-sectional area of flow A= 1.22 ftz Gutter Velocity V= 2.88 ft/sec Fraction of side flow intercepted Rs= 0.27 Total flow capacity interce ted b the inlet Qint= 2.07 cfs Qbypass= 1.43 cfs Design Q for inlet#3 Q25= 1.59 cfs Single Inlet Sufficient Page 2 of 2 Appendix C — Stormwater Maintenance Plan STORMWATER MAINTENANCE PLAN Collins Apartments Owner's responsibility for routine inspection and maintenance 1. Keep the inlets of the facilities free of leaves,rocks, and other debris. 2. Re-sod damaged or maintained areas immediately, or use grass plugs from the adj acent up-sl ope area. 3 See that litter and other debris are removed from inlets, swales, and vegetated and paved areas. 4. Maintenance of the underground systems are as follows: • Minimum required maintenance includes a quarterly inspection during the first year of operation and a yearly inspection thereafter. Utilize inspection ports for inspections. • The inspection ports can be used to pump water into the system and re- suspend accumulated sediment so that is may be pumped out. Flush and pump as inspections deem necessary. • Utilize the adjacent upstream storm sewer manhole or cleanout for removal (vacuuming) of debris from the end of the storm drain at the connection point with the underground chamber. 5. Owner to maintain and fund Operation and Maintenance of stormwater facilities. Shane Collins, Owner's Representative G.\NIADISON CNGINEERINGTROJBC'M2019\1 9-116 Springer-Collins\Stonn`.STORht\%'ATr.R MAINTENANCE PLAN.doe Appendix D — Drainage Exhibit r' n i i i _ 1 i /4061 § 1 ------ - - —`66&0 NORTH S 1 - ' 1 " •t� Up �i,5 � I w 1 DAM 1 — 6R ws I ---� o UP UP UP UP p P o a C S I o L SAM 0 467 s 67Q3 1670. -21�11 "M i I I � I � 0 N 0 U MADISON ENGINEERING EXHIBIT A COLLINS APARTMENTS eoa.rECENC&OGYM nSU== DRAWN BY:CGB DRAINAGE BASIN F-NIMIT BDZML%N.MT MM 02/04/20 PROJECT#19-116 Appendix E Groundwater Monitoring Summary n Qo rn 3 o N � 3 N � � � O a Q � 3 "' cD N r'� N � r N `D N 7 � M CN W G R o N � N r3 r N � N W a M o 10 N � 0 3 w o6 L R N v Orq M 0 3 N � N v�i O O � cuC LU `� b a � E % (7 o Ita o o w °° U o o ° c� L rL U 3 aa .0 o0 O 0