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Home My WebLink About07 - Design Report - Annie Street Cottages - Stormwater Storm Water Design Report Revised 9/25/07 ANNIE STREET COTTAGES STREET AND UTILITY IMPROVEMENTS STORMWATER DESIGN REPORT A. Introduction This design report and management plan is for storm water runoff for the proposed Annie Street Cottages project located at Annie and Hanson Lane, east of Meagher Avenue. The zoning for the proposed 5 acre, 31 unit condominium development is R2. Construction for the first phase of the development is expected to begin in fall of 2007. The Methodology and analysis procedures utilized in the report are in conformance with the design standards for in the City of Bozeman Design Standards and Specifications Policy,Dated March 2004. B. Drainage Storm water runoff from the proposed development will be discharged into a detention basin which will be located in the low spot of the existing site, situated in the NW confer of the tract. The storm water will be conveyed to the detention basin via piping,overland sheet flow and a swale.The discharge from the detention basin will be into the historic channel from the site. The detention basin will be sized to detain the difference between the flow from the developed site and the pre-developed site,for a 10-year storm event. C. Design Methodology The area analyzed in the study is 5 acres. It is proposed to utilize gutters, inlets,piping, and swales to collect storm water runoff for the proposed project. The runoff will be routed to a detention basin located in the low spot of the existing site,situated in the NW conger of the tract. The detention basin is sized using the 10-year,2-hour design storm event as required by the City of Bozeman Design Standards and Specifications.All conveyance systems including gutters, inlets and storm piping have been sized to accept the 25-year design storm event as required by the City of Bozeman Design Standards and Specifications. The Ration Method was used in calculating the storm runoff for the proposed development and all contributing areas. D. Basin Information The proposed storm water runoff analysis has been divided into 4 drainage basins,Basin A through D. A basin map and calculations are provided in the appendix of this report. Basin A Basin A consists of the runoff collected from Hanson Drive and the south half of Annie Street. Storm water from basin A will be collected in curb inlets located along Annie Street. Basin A consists of Open Spaces: 0.40 acres C=0.2 Impervious Area: 0.98 acres C=0.9 Total 1.38 acres C=0.70 composite Annie Street Cottages—Storm Water Design Report Page l of Revised 9/25/07 Basin B Basin B consists of the runoff collected from the north half of Annie Street. Storm water from Basin B will be collected in curb inlets located along Annie Street.Basin B consists of- Open Spaces: 0.05 acres C=0.2 Impervious Area: 0.26 acres C=0.9 Total 0.31 acres C=0.79 composite Basin C Basin C consists of the runoff collected from the developed area south of Annie and west of Hanson Lane. Storm water from Basin C will be collected in a curb inlet located on the north end of the private drive. Basin C consists of- Open Spaces: 1.51 acres C=0.2 Impervious Area: 0.98 acres C=0.9 Total 2.49 acres C=0.48 composite Basin D Basin D consists of the runoff collected from the developed area north of Annie Street. Storm water from Basin D will be routed to the detention basin via a curb cut and grass lined swale. Basin D consists of: Open Spaces: 0.93 acres C=0.2 Impervious Area: 0.40 acres C=0.9 Total 1.33 acres C=0.41 composite E. Storm Water Run Off Calculations Storm water runoff calculations were preformed using the modified ration method for both the pre and post-development conditions.The following are the results of the calculations, detailed calculation can be found in the appendix. Pre-Development Conditions 10-Year Storm Event Open Spaces: 4.81 acres C=0.2 Impervious Area(dirt Road): 0.19 acres C=0.8 Total 5.00 acres C= 0.22 composite Average Slope= 1.6% Time of Concentration=33 minutes Rainfall Intensity: 0.95 in/hr Q= .26 x 0.95 in/hr x 5.0= 1.04 cfs Annie Street Cottages—Storm Water Design Report Page 2 of 4 _. Revised 9/25/07 Table I Post-Development Conditions 10-Year Storm Event Area Basin (acres) C Value A 1.38 0.70 B 0.31 0.79 C 2.49 0.48 D 1.33 0.41 Total 5.31 0.55 (composite) F. Detention Basin Sizing Summary Pond A Pre-development flow rate: 10-year event= 1.23cfs Post-development flow rates: 10-year event=2.76 cfs Required detention storage: 10-year event=3,399 ft3 Detention provided: 3,594 ft3 Outlet Pipe Size: 15-inch @ 0.28%min(capacity 4.44 cfs) Weir Width: 2.0 inches Detail detention basin calculations can be found in the Appendix of the report. G. Roadway and Gutter,and Inlet Capacity Calculations The impact of storm water runoff on the roadways is an important design consideration. The city of Bozeman Design Standards and Specification 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 criterion,the available gutter capacity was calculated for Annie and Hanson Lane.The following are the results for the 25 storm event: Calculations are provided in the Appendix of this report. Gutter Capacity: Annie Street(Southside)=3.7 cfs Annie Street(Northside)=3.7 cfs 25-year Calculated Runoff Rate Annie Street(Southside)(Basin A)= 1.55 cfs Annie Street(Northside)(Basin B)=0.85 cfs 100-year Calculated Runoff Rate Annie Street(Southside)(Basin A)=2.68 cfs Annie Street(Northside)(Basin B)= 1.88 cfs Inlet Capacity: Inlet#2 Basin A=4.03 cfs Annie Street Cottages—Storm Water Design Report Page 3 of 4 Revised 9/25/07 Inlet#3 Basin B=4.03 cfs H. Pipe Capacity Calculations The pipes and culverts are illustrated in Figure 4. The following is summary of the calculations. See Appendix A for details calculations. TABLE 3 Pipe Capacity Calculations 25-Year Storm Event Pipe ID Calculated Size Used for Pipes Peak runoff(cfs) Design(in) Ca acity(cfs) PI 2.01 12 2.45 P2 3.49 15 4.44 P3 4.34 15 4.44 Minimum size required by the City of Bozeman for storm sewers inlets is 12-inches Minimum size required by the City of Bozeman for storm sewer mains is 15-inches. I. Maintenance Considerations Maintenance of the detention basin is both essential and costly. General objectives of maintenance are to prevent clogging, standing water, and the growth of weeds and wetland plants. This requires frequent removal of debris from the outlet and mowing. Cleaning out sediment will be necessary in 10 to 20 years. Culverts and inlets should be inspected at least once a year.Any necessary repair of maintenance should be prioritized, and scheduled through the spring, summer,and fall.These items may include removing blockages, cleaning and flushing the length of pipes,trimming and removing brush at culverts ends, moving grass and weeds, cutting and removing trees and limbs that threaten to fall and block upstream swales, establishing vegetation on bare slopes at culvert ends, and mark the culvert ends in the winter for snowplow operators. J. Storm Water Summary The included analyses and calculations show that the proposed storm water management system for Annie Sreet and Hanson Lane will adequately store the 10-year,2 hour storm event and adequately convey the 25-year storm event.Available gutter capacity and the location of inlets limit the encroachment of runoff on the pavement surfaces to acceptable levels. The recommended pipe sizes would effectively convey storm runoff to the designated detention area. Therefore,the proposed storm water management system meet or exceeds the City of Bozeman requirements and will satisfy the property owner. Annie Street Cottages—Storm Water Design Report Page 4 of 4 Revised 9/25/07 Appendix A Basin Calculations Storm Water Design Report SCALE I linah- 100 ., I I = BASIN D --'MATCH LINE --,, BASIN B � I I BASIN A "I •I ' I ; 'I I I BASIN C o •jil � � �� II li I J4 C MATCHI LINE ®' it w ® 1 11 I U I., iII rn 11 a I u -------___-- j \ I I N 0 U EXHIBIT A ANNIE STREET COTTAGES MADISON ENGINEERING DRAWN BY: CGB DRAINAGE ]BASIN EXHIBIT 924 S'I''0NERIDGE DR. S T E 1,BOZEMAN,MT 59718 PHONE(406) 585-7355 FAX(405)585-7357 DATE: 8/20/07 REVISED: 9/25 Annie Street and Hanson Lane Basin Runoff Calculations 10-year Storm Event Storm Information Design Rainfall Freq. 10 IDF coefficient a 0.64 IDF coefficient b 0.00 IDF coefficient n 0.65 Adjustment Factor Cf: 1 Rational Method Calculations Basin A Open Space 0.40 acres C: 0.2 Pavement 0.98 acres C: 0.9 Total Area: 1.38 acres Weighted C: 0.70 Average slope: 1.70 Percent Travel Distance 1250.00 feet Total t.: 22.32 minutes intensity at t� 1.22 in/hr peak runoff: 1.17 cfs Basin B Open Space 0.05 acres C: 0.2 Pavement 0.26 acres C: 0.9 Total Area: 0.31 acres Weighted C: 0.79 Average slope: 1.70 Percent Travel Distance 252.00 feet Total t.: 7.78 minutes intensity at tc 2.41 in/hr peak runoff: 0.59 cfs Basin C Open Space 1.51 acres C: 0.2 Pavement 0.98 acres C: 0.9 Total Area: 2.49 acres Weighted C: 0.48 Average slope: 1.60 Percent Travel Distance 372.00 feet Total1.: 19.12 minutes intensity at t. 1.35 in/hr peak runoff: 1.61 cfs Basin D Open Space 0.93 acres C: 0.2 Pavement 0.40 acres C: 0.9 Total Area: 1.33 acres Weighted C: 0.41 Average slope: 1.60 Percent Travel Distance 273.00 feet Total tc: 18.23 minutes intensity at t. 1,39 in/hr peak runoff: 0.76 cfs PAGE 1 OF 3 Revised 9125/07 Annie Street and Hanson Lane Basin Runoff Calculations 25-year Storm Event Storm Information Design Rainfall Freq. 25 IDF coefficient a 0.78 IDF coefficient b 0.00 IDF coefficient n 0.64 Adjustment Factor Cf: 1.1 Rational Method Calculations Basin A Open Space 0.40 acres C: 0.2 Pavement 0.98 acres C: 0.9 Total Area: 1.38 acres Weighted C: 0.70 Average slope: 1.50 Percent Travel Distance 1250.00 feet Total t�: 19.24 minutes intensity at t. 1.61 in/hr peak runoff: 1.55 cfs Basin B Open Space 0.05 acres C: 0.2 Pavement 0.26 acres C: 0.9 Total Area: 0.31 acres Weighted C: 0.79 Average slope: 1.70 Percent Travel Distance 252.00 feet Total tc: 5.83 minutes intensity at t, 3.47 in/hr peak runoff: 0.85 cfs Basin C Open Space 1.51 acres C: 0.2 Pavement 0.98 acres C: 0.9 Total Area: 2.49 acres Weighted C: 0.48 Average slope: 1.60 Percent Travel Distance 372.00 feet Total tc: 17.79 minutes intensity at t. 1.70 in/hr peak runoff: 2.01 cfs Basin D Open Space 0.93 acres C: 0.2 Pavement 0.40 acres C: 0.9 Total Area: 1.33 acres Weighted C: 0.41 Average slope: 1.60 Percent Travel Distance 273.00 feet Total t�: 17.13 minutes intensity at t� 1.74 in/hr peak runoff: 0.95 cfs Page 2 OF 3 Revised 9/25/07 Annie Street and Hanson Lane Basin Runoff Calculations 100-year Storm Event Storm Information Design Rainfall Freq. 100 IDF coefficient a 1.01 IDF coefficient b 0.00 IDF coefficient n 0.67 Adjustment Factor Cf. 1.25 Rational Method Calculations Basin A Open Space 0.40 acres C: 0.2 Pavement 0.98 acres C: 0.9 Total Area: 1.38 acres Weighted C: 0.70 Average slope: 1.50 Percent Travel Distance 1250.00 feet Total t,: 13.20 minutes intensity at t, 2.78 in/hr peak runoff: 2.68 cfs Basin B Open Space 0.05 acres C: 0.2 Pavement 0.26 acres C: 0.9 Total Area: 0.31 acres Weighted C: 0.79 Average slope: 1.70 Percent Travel Distance 252.00 feet Total t,: 2.89 minutes intensity at t. 7.71 in/hr peak runoff: 1.88 cis Basin C Open Space 1.51 acres C: 0.2 Pavement 0.98 acres C: 0.9 Total Area: 2.49 acres Weighted C: 0.48 Average slope: 1.60 Percent Travel Distance 372.00 feet Total t�: 15.59 minutes intensity at t. 2.49 in/hr peak runoff: 2.95 cfs Basin D Open Space 0.93 acres C: 0.2 Pavement 0.40 acres C: 0.9 Total Area: 1.33 acres Weighted C: 0.41 Average slope: 1.60 Percent Travel Distance 273.00 feet Total tc: 15.50 minutes intensity at tc 2.50 in/hr peak runoff: 1.37 cfs Page 3 OF 3 Revised 9/25/07 Appendix B Gutter & Inlet Calculations Storm Water Design Report Annie Street and Hanson Lane Gutter Capacity Calculations Allowable Pavement Encroachment Given: T= 9 feet(max per city) T W= 1.5 feet W--Ts Ts= 7.5 feet SW= 0.08 ft/ft Sx= 0.03 ft/ft Q. a= 0.96 inches S. d= 3.24 inches d n= 0.012 So= 0.005 Annie Sta. 12+90 (Southside) a S. So= 0.005 Annie Sta. 12+90(Northside) Sw/Sx= 2.67 T/W= 6 Capacity for Gutter equations: Q=Qw+Qs Where: Q= Capacity of roadway Qw =E,,Q Qs= Discharge within the Roadway above the depressed section (cfs) _ QS Qw= Discharge within the depressed Q 1—EQ (gutter)section (cfs) Cf= 0.56 for English units Cf s a t Sx= Pavement cross slope(ft/ft) QS =-rJx3T53s42 n Ts= Width of flow in the roadway above depressed section So= Gutter longitudinal slope(ft/ft) SOX Sw= Gutter depression cross slope(ft/ft) Eo = 1+ S /S T= Spread (ft/ft) 1+ w " —1 W= Width of gutter depression (ft/ft) (T/W)-1)8,3 Capacity solution Annie Sta. 12+90(Southside) Annie Sta. 12+90 (Northside) Basin A Basin B So= 0.005 ft/ft So= 0.005 ft/ft Qs= 2.06 cfs Qs= 2.06 cfs Eo= 0.44 cfs Eo= 0.44 cfs Q= 3.70 cfs Q= 3.70 cfs Design flow Design flow Q10= 1.17 cfs QJo= 0.69 cfs Q25= 1.55 cfs Q25= 0.85 cfs Q100= 2.68 cfs Q100= 1.88 cfs Summary The gutter capacity was calculated at two different locations where peak flows where also calculated. The gutter capacity at both locations is well above the calculated peak flows.Therefore, the pavement encroachment will be less than the allowable(9-feet)by the City of Bozeman. _ Page 1 of 2 Revised 9/25/07 Annie Street and Hanson Lane Inlet Capacity Calculations Gutter Section Given: T= 9.0 feet T W= 1.50 feet w Ts Ts= 7.50 feet Sw= 0.08 ft/ft Sx= 0.03 ft/ft Q. a= 0.96 inches d= 3.24 inches d n= 0.012 So= 0.005 Curb Inlet#1 s. So= 0.005 Curb Inlet#2 From Gutter Capacity Inlet#1 Qw= 1.64 cfs South Side Qs= 2.06 cfs Inlet#2 Qw= 1.64 cfs North Side Qs= 2.06 cfs Capacity for Inlets on Sag-Annie Street(Southside) Inlet#2 Based on Neenah inlet R-3067-L Weir Equation Q=3.3P(hj 5 Where: P= perimeter(ft) h = Head (ft) P: 5.90 feet h: 0.35 feet(max per city) Qwier= 4.03 cfs Design Q for inlet#2 (Basin A) Q10= 1.17 cfs Q25= 1.55 cfs Q100= 2.68 cfs Inlet Design Ok Capacity for Inlets on Sag-Annie Street(Northside) Inlet#3 Based on Neenah inlet R-3067-L Weir Equation Q=3.3P(hj-5 Where: P= perimeter(ft) h= Head (ft) P: 5.90 feet h: 0.35 feet(max per city) Qwler= 4.03 cfs Design Q for inlet#3 (Baisn B) Qio= 0.59 cfs Q25= 0.85 cfs Qjoo= 1.88 cfs Inlet Design Ok Page 2 of 2 Revised 9/25/07 Appendix C Pipe Capacity Calculations Storm Water Design Report Annie Street and Hanson Lane Pipe Capacity Calculations 25-year Storrs Event Required Capacity for Pipes Pipe 1 From Curb Inlet#1 to Combo Manhole/Inlet#2 Accepts runoff from Basin C peak runoff: 2.01 cfs (See 25-year storm event basin run off calculations) Required Pipe Size Diameter(flowing full) n: 0.013 Slope: 0.28 % Length: 43 feet Size used for design 12 inch Capacity 2.45 cfs OK Velocity 3.12 fps Pipe 2 From Combo Manhole/Inlet#2 to Combo Manhole/Inlet#3 Accepts runoff from Basin A&Pipe 1 peak runoff from Basin A: 1.55 cfs (See 25-year storm event basin run off calculations) Peak runoff from Basin C: 2.01 cfs Total peak runoff: 3.56 cfs Required Pipe Size Diameter(flowing full) n: 0.013 Slope: 0.28 % Length: 78 feet Size used for design 15 inch Capacity 4.44 cfs OK Velocity 3.62 fps Pipe 3 From Combo Manhole/Inlet#3 to Outlet Accepts runoff from Basin B&Pipes 1 &2 Peak runoff from area A: 1.66 cfs Peak runoff from area B: 0.86 cfs (See 25-year storm event basin run off calculations) Peak runoff from area C: 2.01 cfs Total peak runoff: 4.41 cfs Required Pipe Size Diameter(flowing full) n: 0.013 Slope: 0.28 % Length: 164 feet Size used for design 15 inch Capacity 4.44 cfs OK Velocity 3.56 fps Page 1 of 1 Revised 9/25/07 1 -inch Storm Sewer Capacity Project Description Friction Method Manning Formula Solve For Full Flow Capacity Input Data, Roughness Coefficient 0.010 Channel Slope 0.28 Normal Depth 12.00 in Diameter 12.00 in Discharge 2.45 fts/s Results Discharge 2.45 ft'/s Normal Depth 12.00 in Flow Area 0.79 ttz Wetted Perimeter 3.14 ft Top Width 0.00 ft Critical Depth 0.67 ft Percent Full 100.0 % Critical Slope 0.00448 ft/ft Velocity 3.12 ft/s Velocity Head 0.15 ft Specific Energy 1.15 ft Froude Number 0.00 Maximum Discharge 2.64 ft'/s Discharge Full 2.45 ft3/s Slope Full 0.00280 ft/ft Flow Type SubCritical GVF Input Data Downstream Depth 0.00 in Length 0.00 ft Number Of Steps 0 GVF Output Data Upstream Depth 0.00 in Profile Description Profile Headloss 0.00 ft Average End Depth Over Rise 0.00 Normal Depth Over Rise 100.00 % Bentley Systems,Inc. Haestad Methods Solution Center FlowMaster [08.01.058.00] 1011/2007 11:43:06 AM 27 Siemons Company Drive Suite 200 W Watertown,CT 06795 USA +1-203-755-1666 Page 1 of 2 1 -inch Storm Sewer Capacity GVF Output Data Downstream Velocity Infinity ft/s Upstream Velocity Infinity ft/s Normal Depth 12.00 in Critical Depth 0.67 ft Channel Slope 0.28 % Critical Slope 0.00448 ft/ft Bentley Systems,Inc. Haestad Methods Solution Center FlowMaster [08.01.058.00] 10/112007 11:43:06 AM 27 Siemons Company Drive Suite 200 W Watertown,CT 06795 USA +1-203-755-1666 Page 2 of 2 1 -inch Storm Sewer Capacity Project Description Friction Method Manning Formula Solve For Full Flow Capacity Input Data Roughness Coefficient 0.010 Channel Slope 0.28 Normal Depth 15.00 in Diameter 15.00 in Discharge 4.44 ft'/s Results Discharge 4.44 ft'/s Normal Depth 15.00 in Flow Area 1.23 ft2 Wetted Perimeter 3.93 ft Top Width 0.00 ft Critical Depth 0.85 ft Percent Full 100.0 % Critical Slope 0.00426 ft/ft Velocity 3.62 ft/s Velocity Head 0.20 ft Specific Energy 1.45 ft Froude Number 0.00 Maximum Discharge 4.78 ft'/s Discharge Full 4.44 ft'/s Slope Full 0.00280 ft/ft Flow Type SubCritical GVF Input Data Downstream Depth 0.00 in Length 0.00 ft Number Of Steps 0 GVF Output Data Upstream Depth 0.00 in Profile Description Profile Headloss 0.00 ft Average End Depth Over Rise 0.00 Normal Depth Over Rise 100.00 Bentley Systems,Inc. Haestad Methods Solution Center FlowMaster [08.01.068.00] 10/1/2007 11:42:46 AM 27 Siemons Company Drive Suite 200 W Watertown,CT 06796 USA +1-203.755-1666 Page 1 of 2 15-inch StormSewer Capacity GVF Output Data Downstream Velocity Infinity ftts Upstream Velocity Infinity ftls Normal Depth 15.00 in Critical Depth 0.85 ft Channel Slope 0.28 % Critical Slope 0.00426 ft/ft Bentley Systems,Inc. Haestad Methods Solution Center FlowMaster (08.01.058.001 10111200711:42:46 AM 27 Siemons Company Drive Suite 200 W Watertown,CT 06795 USA +1-203-755-1666 Page 2 of 2 Appendix D Detention Basin Calculations Storm Water Design Report Annie Street Cottages Storm Water Detention Pond Calculations Calculation of Required Volume for Storm Detention Pond Pond ID A (Reference: Bozeman Stormwater Master Plan-1982) Design Rainfall Freq.e 10 year(see page III-5 of master plan) IDF coefficient a 0.64 IDF coefficient b IDF coefficient n 0.65 Pre-development Calculations Post-development Calculations C C Areas(ft): open space >�217,800 ' 0.26 Areas(ft�): open space 0.20 med. res. 0.35 med.res. 0.35 dense res. 0.50 dense res. 0.50 comm.neigh 0.60 comm.neigh. 231,303 0.55 comm.down 0.80 comm.down. 0.80 industrial 0.80 industrial 0.80 Total: 217,806 Total: 231,303 total area: 5.00 acres total area: 5.31 acres composite C: 0.26 composite C: 0.55 Overland t� Overland t. average slope 1 6 °`"percent average slope: >1 7 percent travel distance , _ 600 ;, feet travel distance 1250_, :_�feet t�: 33 minutes t�: 33 minutes Channel t� Channel tc channel t, minutes channel t.: minutes Total t.: 33 minutes Total tc: 33 minutes intensity at t,(fig 23): 0.95 in/hr intensity at tc(fig 23): 0.94 in/hr pre-devel peak runoff: 1.04 cfs post-devel peak runoff: 2.76 cfs Storm Duration Intensity Future Runoff Runoff Release Required (minutes) (in/hr) Rate(cfs) Volume(cf) Volume(co Storage(cf) 33 0.94 2.76 5458 2059 3399 35 0.91 2.65 5572 2184 3388 37 0.88 2.56 5681 2309 3373 39 0.85 2.47 5787 2434 3353 41 0.82 2.39 5889 2558 3331 43 0.79 2.32 5988 2683 3305 45 0.77 2.25 6084 2808 3276 47 0.75 2.19 6178 2933 3245 49 0.73 2.13 6268 3058 3211 51 0.71 2.08 6357 3182 3174 53 0.69 2.03 6443 3307 3136 55 0.68 1.98 6527 3432 3095 57 0.66 1.93 6609 3557 3052 59 0.65 1.89 6689 3682 3008 61 0.63 1.85 6768 3806 2961 63 0.62 1.81 6645 3931 2914 65 0.61 1.77 6920 4056 2864 required detention storage(ft)=r 3,399 Pagel of 3 _ Annie Street Cottages Revised 9/25/07 Detention Pond Calculations: design depth of pond 1.50. feet max side slope 4.00 horizontal to 1.00 vertical length/width ratio 3.00 min. particle removed =40 " microns(1 micron=1 x 10-6 meters) settling velocity of partic 0.0069 feet/second min. pond to settle parti 400 square feet pond dimentions assuming vertical side slopes(actual pond footprint will be larger) width 27 length 82 Volume held between contours: Cumulative Contour Area(ft) Delta V(ft) Volume(ft3} ;'.4645.75 50 4646 0 r. 1 836; 236 236 4647 0 3 202",1 2,519 2,755 4647 25 3,51 °,' 839 3,594 Design storage at 1.5'depth(ft)= 3,594 Paoe 2 of 3 Annie Street Cottages Revised 9/25/07 Annie Street Cottages Storm Water Detention Pond Calculations Detention Pond Outlet Calculations (Reference: City of Bozeman, Design Standards and Specifications Policy, March 2004, II.D.2, page 24) Note: see Figure A-2 in above reference. Rectangular weir-Q=3.33LH311 Determine Outlet Slot Width needed: Pre-development flow rate= 1.04 cfs Vertical Slot Height=' 18 ' inches Req'd Outlet Slot Width= 0.17 feet or 2 1/32 of an inch Determine Outlet Flow: Outlet Slot Width= < ' 2.0q inches stage ft Q cfs O(gpm) 0.25 . 0.069 31 0.50 0.196 88 0.75 0.360 162 1.00 0.555 249 1.25 0.776 348 1.50 1.020 458 Flow through outlet Stage vs. Discharge 1.20 1.00 y ,{ 0.80 0.60 ca 0.40 U 0 0.20 0.00 0 0.5 1 1.5 Stage (ft) Page 3 of 3 Annie Street Cottages Revised 9/25/07