EXTENDED DETENTION BASIN EXAMPLE

Transcription

1 EXTENDED DETENTION BASIN EXAMPLE July 2015 REFERENCE SECTION 1117 Given: TotalTributaryArea=7.5 o TributaryAreawithinExistingR/W=7.2 o TributaryArea,Impervious,OutsideofR/W=0.3 o TributaryArea,Pervious,OutsideofR/W=0.0 o Tributary Area, Pavement and Paved Shoulders = 1.5 o TributaryArea,BermsandSlopes4:1orFlatter=6.0 Rainfall Area B TimeofConcentration,t C =25min(calculationshowninthisexample) CalculatethewaterqualityvolumeWQ V: WQ V =(P*A*Cq)/12 P=0.75in A=7.5 Cq=0.858i i i+0.04 o i = frtion impervious o TheareawithinexistingODOTright of wayisconsideredimperviousareaforthe purpose of post construction BMP design considerations.(l&d Vol 2, Sec ) o i= =1.0 Cq=0.858* * *1+0.04=0.892 Per L&D Vol. 2, Section , consider all area within existing right of way to be impervious witharunoffcoefficientof0.90. Therefore,Cq=0.9 WQ V=(0.75in*7.5*0.9)/12 WQ V=0.422 ft Calculate the minimum detention basin volume, forebay volume, and micropool volume: Minimumbasinvolume=WQ V*1.20(dueto20%increase) =WQ V *1.2=0.422 ft*1.2=0.506 ft 10%WQ Vforforebayvolume,10%*0.422 ft=0.042 ft 10%WQ V formicropoolvolume,10%*0.422 ft=0.042 ft Layout a detention basin configuration that meets the following requirements: Forebay volume below the lowest outlet elevation at upstream end of the basin Micropool volume below the lowest water quality outlet invert elevation at the downstream endofthebasin Maximum 4:1 side slopes Include provisions for vehicle cess 1of13

2 EXTENDED DETENTION BASIN EXAMPLE(C0NTINUED) July 2015 REFERENCE SECTION 1117 Detention Basin Plan View: Detention Basin Profile View: Forebay: Theforebayvolumeisthevolumestoredupstreamofthelowflowchannel. Thevolumeinthe forebayisheldinapermanentpool,andisunabletoflowdownstreamtowardstheoutlet. The purposeoftheforebayistoallowrunofftoslowenoughforcoarsesedimenttosettleout. This improves performance and reduces the maintenance burden by concentrating sediment buildup in one location designed for maintenance cess. Forebayvolumemustbegreaterthanorequalto0.042 ft. Forebaydepth= =2ft Forebaybottomarea=25ft*18ft=450ft 2 Forebaytoparea=41ft*34ft=1,394ft 2 Forebayvolume=(450ft 2 +1,394ft 2 )/2*2ft/43,560=0.042 ft ft isequaltothe ftrequirement: Acceptable 2of13

3 EXTENDED DETENTION BASIN EXAMPLE(C0NTINUED) July 2015 REFERENCE SECTION 1117 Micropool: Themicropoolvolumeisthevolumethatisstoredbelowthelowestinvertelevationofthe lowest water quality outlet. The purpose of the micropool is to slow runoff draining towards the outlet structure, promote sediment settling below the outlet structure, and allow use of a non clogging outlet. This improves performance and reduces clogging and maintenance. Micropoolvolumemustbegreaterthanorequalto0.042 ft. Micropooldepth= =2ft Micropoolbottomarea=25ft*20ft=500ft 2 Micropooltoparea=41ft*36ft=1,476ft 2 Micropoolvolume=(500ft 2 +1,476ft 2 )/2*2ft/43,560=0.045 ft ft is greater than ft requirement: Acceptable WaterQualityVolume(WQ V )Storage: TheWQ V mustbefullystoredabovethelowestwaterqualityoutletelevation. Thelowestwaterqualityoutletisatanelevationof795.5ft. TheWQ V (0.422 ft)mustbestoredbetween795.5ftandcatchbasinoverflow. 3of13

4 EXTENDED DETENTION BASIN EXAMPLE(C0NTINUED) July 2015 REFERENCE SECTION 1117 Stage(Elevation) vs. Volume Curve: Thegraphshowsthatthe fullwq Visstoredbetween799.00and795.50ftinthedetention basin. The forebay and micropool have been excluded from this stage vs. volume graph since the volume associated with the forebay and micropool is constantly standing water. 4of13

5 EXTENDED DETENTION BASIN EXAMPLE(C0NTINUED) July 2015 REFERENCE SECTION 1117 Design the Detention Basin Water Quality Outlet: TheminimumdischargetimeoftheWQ Vis48hourswithnomorethan50%oftheWQ Vbeing releasedfromthedetentionbasininthefirstone thirdofthe48hourdraintime. WQ V =0.422 ft;musttake48hoursorlongertodrain 50%orlessoftheWQ V(i.e ft)mustbedrainedin16hours. Choose eight 0.5 inch diameter circular orifices at ft and eight 0.5 inch diameter circular orifices at ft. Calculate the drawdown curve. o Thiscalculationcanbedonebyhandbycreatingastagevs.dischargetableand interpolating between values, but it is generally easier to use a model to simulate runoff through a detention basin such as PondPk or HydroCAD. Do not route a design storm hydrograph through a detention basin to determine the drawdown curve. StartthesimulationwiththewatersurfeatalevelequivalenttotheWQ Vstorage(forthis example,atanelevationof799.00). ThenallowthepooledwaterfillingtheWQ V todrainbygravity out of the water quality outlet structure. Include all detention basin outlets that would affect this drawdown curve. Include any downstream constraints such as tailwater or limiting conveyance downstream. Forthisexample,thereisnotailwaterandthereisafreedischargefromthe detention basin. 5of13

6 EXTENDED DETENTION BASIN EXAMPLE(C0NTINUED) July 2015 REFERENCE SECTION 1117 Modeled Drawdown Curve using Pond Pk: Thegraphshowsthatittakeslongerthan48hourstodraintheWQ V(0.422 ft);therefore,itis ceptable. Thegraphshowsthatittakes16hourstodrainonehalfoftheWQV(0.211 ft);therefore,itis ceptable. Size the Primary Detention Basin Outlet: There are three main parts of a typical extended detention basin discharge structure: o Water quality outlet(s) o Primary outlet o Overflow weir 6of13

7 EXTENDED DETENTION BASIN EXAMPLE(C0NTINUED) July 2015 REFERENCE SECTION 1117 The primary detention basin outlet normally consists of a catch basin grate and the conduit that conveys discharges from the detention basin during all but the least frequent precipitation events. Theprimaryoutletshouldbesizedtoconveythe10 yeardesignstorm. ODOT Water Quality Catch Basin Detail(WQ 1.1): Determine the 10 year peak flow rate: Forthepurposesofpost constructionbmpcalculations,allexistingright of wayistobe considered impervious. For the purpose of general conveyance sizing, runoff coefficients should becalculatedusingtable1101 2inODOTsL&DVol.2. Q=CiA 7of13

8 EXTENDED DETENTION BASIN EXAMPLE(C0NTINUED) July 2015 REFERENCE SECTION 1117 Calculate the weighted C value: 1.5resoftributaryareaarepavementandpavedshoulders: C= resoftributaryareaarebermsandslopes4:1orflatter: C=0.5 C weighted = =0.58 Determine the precipitation intensity: Rainfall Area B t C=25min (Thetimeofconcentrationisgiveninthisexampleas25minutesbecausethereis significant overland flow over grassed area. The time of concentration should be calculatedforehsitebasedonthesite specificflowpath. 25minuteswouldlikelybe toohighofavalueifthedetentionbasinwerereceivingflowfromapipedsystem. See the time of concentration calculations below.) L&DVol.2,Figure1101 2: AreaB,10 yearfrequency,25mint C : i=3.4in/hr Q=0.58*3.4in/hr*7.5 =14.79cfs Time of Concentration (t C ) Calculations: o t C=Timeofoverlandflow(t O)+Timeofshallowconcentratedflow(t S)+Timeofchannel flow(t C) o OverlandFlow(t O ) t O = C = Runoff Coefficient(0.58 for this example) L=Distancetomostremotelocationindrainageinfeet(max.300ft.)(200ft.inthis example) s = Overland slope(percent)(0.33% in this example) t O = = minutes o ShallowConcentratedFlow(t S) V S =Velocityofshallowconcentratedflow(ft/sec)= 3.281ks 0.5 k=interceptcoefficient(l&dtable1101 1)=(0.457inthisexample) s = Overland slope(percent)(0.33% in this example) V S =3.281*0.457* =0.86ft/sec Length of shallow concentrated flow = 200 ft. t S=200ft./0.86ft/sec=233sec= 3.88 minutes 8of13

9 EXTENDED DETENTION BASIN EXAMPLE(C0NTINUED) July 2015 REFERENCE SECTION 1117 o ChannelFlow(t C) Mannings Equation: V = V=velocityinthechannel(ft/sec) r=hydraulicradius(0.69ft.inthisexample) s=channelslope(0.01ft/ftinthisexample) n=manningsroughnesscoefficient(l&dvol.2,table1102 3)(0.03inthis example) V= =3.88ft/sec Channel length = 500 ft.(for this example) tc=500ft./3.88ft/sec=129sec.=2.15 minutes o t C=t O+t S+t C= =25.19minutes. Uset C=25 minutes Size the primary detention basin discharge conduit: Thedischargeconduitmustbelargeenoughtoconveythe10 yeardesignstorm,keepingthe maximum hydraulic grade line within the crown of the pipe. This example has the following conduit charteristics: o Conduitslope=0.005ft/ft o No Tailwater; free discharge o Pipe Roughness Coefficient = 0.015(L&D Vol. 2, Section ) Theminimumconduitsizethatconveysthe10 yearpeakflow(14.79cfs)withthegiven charteristics is a 24 inch diameter pipe. Set the catch basin grate elevation: TheWQ Vfillsthedetentionbasintoanelevationof799.0ft. Atwatersurfeelevationsof ft and below, all discharge should pass through the water quality outlet. (In this example, thewaterqualityoutletisthetworingsof0.5inchorificesat795.5ftand796.5ft.) The ODOT standard water quality catch basin detail(scd WQ 1.1) calls for either Catch Basin No.2 3or2 4dependingontheoutletpipesize. Bothcatchbasinshavea6 inchhighsideinlet thatiseither3or4feetwidedependingonthecatchbasin. Seethedetail: 9of13

10 EXTENDED DETENTION BASIN EXAMPLE(C0NTINUED) July 2015 REFERENCE SECTION 1117 ODOTCatchBasinNo.2 3andNo.2 4: However,SCDWQ 1.1callsforatchbasinusedforadetentionbasintonothavethesideinlet windows. Therefore, side inlets should not be included in a detention basin outlet structure. TheelevationofthetopofthegrateshouldbesetattheWQ V elevation. Therefore,anyvolume abovethewq Vmaydischargeintothecatchbasinthroughthegrate. o Catchbasingrateelevation=799.0ft Set the Overflow Weir Invert Elevation: The 10 year peak flow rate(14.79 cfs) should pass fully through the primary discharge. Therefore,noflowshoulddischargefromtheoverflowweiruntilthe10 yearflowratehasbeen exceeded. Set the overflow weir invert elevation just high enough above the catch basin grate such that the full 10 year peak flow rate is conveyed through the primary discharge. 10of13

11 EXTENDED DETENTION BASIN EXAMPLE(C0NTINUED) July 2015 REFERENCE SECTION 1117 For this example, the primary discharge pipe is sized at 24 inches in diameter; therefore, Catch Basin No. 2 3 is appropriate. Catch Basin No. 2 3 has the same grate as Catch Basin No. 2 2B. The water quality catch basin has one openings to allow runoff inside; Grate No. 2 2 B. Use L&D Vol. 2, Figure to determine the necessary head above a No. 2 2 B grate to pass the 10 year peak flow rate (14.79 cfs). According to Figure , 1.25 ft of head is required to pass a flow rate of cfs. Add the necessary head to the grate elevation (799.0 ft) o ft ft = ft Set the overflow weir elevation at ft. Size the Emergency Overflow Weir and Set the Top of Basin Elevation: L&D Vol. 2, Section states that the hydraulic grade line should be checked for the 25 year storm. The 25 year peak flow rate should pass fully through the overflow weir. Calculate the peak flow rate: o 25 year t C = 25 min and Rainfall Area B: 3.8 in/hr o 25 year storm peak flow rate: Q = CiA = 0.58 * 3.8 in/hr * 7.5 = cfs Calculate the required weir length for eh design storm flow: o Overflow weir elevation = ft o Assume a top of detention basin elevation of ft. o Maximum height at overflow weir = ft ft = 0.5 ft o Weir equation: Q = C*L*H 1.5 C = 3 H = 0.5 L =? o Length of a weir: L = o 25 year: L = = 15.6 ft Provide a 16 ft wide overflow weir. The top of basin elevation is ft. The overflow weir length could be reduced by increasing the top of basin elevation. Or the top of basin elevation can be lowered by increasing the overflow weir length. The 25 year peak flow rate must fully pass through the overflow weir without overtopping the detention basin. Flow rates greater than the 25 year peak flow rate may overtop the detention basin uncontrolled. Provide erosion protection at the overflow weir, to the bottom of the berm, and continuing downstream if there is erosion potential. 11of13

12 EXTENDED DETENTION BASIN EXAMPLE (C0NTINUED) July 2015 REFERENCE SECTION 1117 Design Anti Seep Collars: Anti seep collars reduce the conveyance of flow along pipe bedding, outside of a conduit and increase the flow path for the seepage of water. This helps protect the berm above the discharge conduit from a detention basin from internal erosion. ODOT Standard Drawing WQ 1.2 Calculate the saturated zone length along the conduit (Ls) o Ls = Y(Z+4)[1+S/(0.25 S)] o Y = depth of water during the 10 year storm o Z = slope of embankment o S = slope of conduit Maximum elevation at 10 year storm = ft Conduit elevation = ft Y = ft ft = 5.25 ft Z = 4 S = Ls = 5.25(4+4)[ /( )] = ft Ls = 0.15*Ls = 0.15 * ft = 6.4 ft 12 of 13

13 EXTENDED DETENTION BASIN EXAMPLE (C0NTINUED) July 2015 REFERENCE SECTION 1117 Total Projection: P = W D o W = 2ft = 2 ft diameter = 2 ft = 6 ft o P = 6 ft 2 ft = 4 ft Number of collars = Ls / P = 6.4 ft / 4 ft = 1.6 o Minimum of 2 collars per outlet conduit o Use 2 anti seep collars Ple both anti seep collars in the saturation zone (within ft of front edge of berm). Sping between collars: between 10 and 25 feet Additional Considerations: Vegetate the sides of the detention basin with Item 670 Slope Erosion Protection per L&D Vol. 2, Section For all open water carriers at eh inlet and discharge from the detention basin, check the shear stress and ensure appropriate lining per L&D Vol. 2, Section For all culverts that discharge into or out of a detention basin, ensure that appropriate rock channel protection is included per L&D Vol. 2, Section Include calculated detention basin ponding elevations in the calculation of the hydraulic grade line for the upstream conveyance system per L&D Vol. 2, Section Attempt to locate structures outside of designated flood plains. If a detention basin encrohes on a flood plain, follow the flood assessment requirements in L&D Vol. 2 Section Ensure that safety criteria are met in the clear zone per L&D Vol. 1, Section Ensure that no more than one foot of permanent standing water is located within the clear zone without barrier protection, per L&D Vol. 1, Section Engage local project stakeholders in potential public safety considerations associated with detention basins. Develop a plan for how regular maintenance will be performed. o Vehicle cess o Mowing o Removal of woody vegetation o Regular unclogging of the water quality outlet 13 of 13

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21 Post-Construction BMP Review Checklists Project Wide Review Checklists for ODOT-Let Projects: Figures Project Limits Limits of Disturbed Area BMP Locations on a Site Plan Drainage Areas BMP Tributary Area (within and outside R/W) Waters of the State Floodplains and Wetlands (if applicable) Design Drawings Plan Sheets with BMP Locations BMP Plan and Profiles BMP Outlet Details Storm Sewer Plan and Profiles Figure Provided? Figure Provided? Values, Calculations, or Descriptions Value Units Project Earth Disturbed Area (EDA) Existing Impervious Area (Aix) New Impervious Area in New Permanent R/W (Ain) Treatment Percent % Treatment Requirements (Area) Quantity Treatment Required? Treatment Provided by all BMPs (Area)(within R/W) Treatment Provided by BMP #1(Area)(within R/W) Treatment Provided by BMP #2(Area)(within R/W) Treatment Provided by BMP #3(Area)(within R/W) Treatment Provided by BMP #4(Area)(within R/W) Description of any Deviation from Treatment Requirements

22 Manuftured System Review Checklist: Manuftured System Review Value Units BMP Tributary Area Area within R/W Draining to BMP Percent Impervious (for BMP Trib. Area) % Coefficient of Runoff (C) NA Water Quality Flow (WQF) cfs Manuftured System Type 1,2,3, or 4 Unit Footprint (length, width, depth) ft Description of Maintenance Access

23 Vegetated Filter Strip Review Checklist: Vegetated Filter Strip Review Value Units BMP Tributary Area Area within R/W Draining to BMP Width of Impervious Area Draining to Filter Strip ft Width of Vegetated Filter Strip ft Slope of Vegetated Filter Strip (Horizontal: Vertical) ft:ft Limits of Item 670, Slope Erosion Protection, Shown on the Plans? Limits of Item 659, Topsoil, Shown on the Plans?

24 Vegetated Biofilter Review Checklist: Vegetated Biofilter (Grass Swale) Value Units Review BMP Tributary Area Area within R/W Draining to BMP Percent Impervious (for BMP Trib. Area) % Coefficient of Runoff (C) NA Water Quality Flow (WQF) cfs Channel Bottom Width ft Channel Fore Slope (Horizontal: Vertical) (Z:1) ft:ft Channel Bk Slope (Horizontal: Vertical) (Z:1) ft:ft Longitudinal Channel Slope ft:ft Normal Depth of Flow during WQF in Velocity of Flow for during WQF fps Length of Vegetated Biofilter ft Design Storm Flow Rate cfs Scour Analysis and Scour Protection (if necessary) Limits of Item 670, Slope Erosion Protection, Shown on the Plans? Limits of Item 659, Topsoil, Shown on the Plans?

25 Extended Detention Basin Review Checklist: Extended Detention Basin Review Value Units BMP Tributary Area Area within R/W Draining to BMP Percent Impervious (for BMP Trib. Area) % Volumetric Runoff Coefficient (Cq) NA Water Quality Volume (WQV) -ft Extended Detention Volume (EDV) -ft Detention Basin Stage/Storage Table or Graph Description of All Detention Outlets Calculations of EDV Drawdown time Time to drain EDV hrs Time to drain 50% of EDV hrs EDV Drawdown Hydrograph Summary Input and Output from any Software Used Forebay Volume -ft Micropool Volume -ft Inflow and Discharge Design Velocities fps Description of any Inflow or Discharge Scour Protection Overflow Structure Sizing Calculations Description of Tailwater Assumptions Detention Basin located outside the Floodplain? Limits of Item 670, Slope Erosion Protection, Shown on the Plans? Anti-Seep Collars shown on the Plans? Maintenance (Vehicle) Access to Outlet Structure? Have Safety Concerns been Considered / Addressed?

26 Bioretention Cell Review Checklist: Source: ODNR Rainwater and Land Development Manual Bioretention Cell Review BMP Tributary Area Area within R/W Draining to BMP Impervious area Tributary to BMP Bioretention Cell Area Surfe Ponding Depth Side Slopes Depth of Soil Media Layer Depth of Filter Sand Layer Depth of Filter Pea Gravel Layer Depth of Gravel Drainage Layer Underdrain shown in Drawing Description of Pretreatment Provided Inflow and Discharge Design Velocities Description of any Inflow or Discharge Scour Protection Overflow Structure Sizing Calculations Depth: Bottom of Cell to Seasonal High Water Table Depth: Bottom of Cell to Bedrock Maintenance (Vehicle) Access to BMP? Value Units ft ft:ft in in in in fps ft ft

27 Post-Construction BMP Review Checklists Project Wide Review Checklists for Local-Let Projects: Review Questions Is a Post-Construction BMP Required? Is the Project EDA 1 or more res Is the project a routine maintenance project Is Water Quantity Treatment Required? Is more than 1 re of new impervious area in new permanent right-of-way being added? Is the project discharging directly to a 4 th order stream or a stream that has a drainage area greater than 100 square miles? Is an Appropriate Post-Construction BMP Provided? Input

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29 Post-Construction Stormwater BMP Design Training 2016 Review Worksheet 1. The following project information is given: Project EDA Paved Area within Existing Right-of-Way within Project EDA Grassed Area within Existing Right-of-Way, within Project EDA Paved Area in New Permanent Right-of-Way, within Project EDA Grassed Area in New Permanent Right-of-Way, within Project EDA 15 res 8 res 2 res 2 res 3 res What is the Treatment Percent (T%)? How much area must be treated to meet treatment requirements? Is quantity treatment required if the site does not drain to a large stream? 2. How much new impervious area in new permanent right-of-way must be added on a project to potentially trigger quantity treatment requirements for post construction? 3. Grade control is only an applicable BMP for quantity control for this type of site? 4. TRUE or FALSE (Circle One) Vegetated biofilters provide quality and quantity treatment credit. 5. For ODOT-let projects, who would approve alternative BMPs, outside of what is specified in the L&D? (Circle One) a. ODOT Office of Hydraulic Engineering b. Ohio EPA c. ODOT Office of Hydraulic Engineering and Ohio EPA 6. Generally, what is the cheapest and easiest post construction BMP, assuming adequate spe? Page 1

30 7. Circle all that apply: Which soil groups are appropriate for infiltration BMPs. a. HSG A b. HSG B c. HSG C d. HSG D 8. TRUE or FALSE (Circle Once) Treatment credit is given to area outside of the right-of-way draining to a post construction BMP. 9. TRUE or FALSE (Circle Once) All area within the right-of-way shall be considered as 100% impervious with a runoff coefficient of 0.9 for culvert design. 10. The 0.75 inches used in the WQ V calculation and the 0.65 in/hr used in the WQ F calculation represent approximately what percentage of the average annual runoff in Ohio? 11. TRUE or FALSE (Circle Once) Rational Method can be used to estimate runoff volume. 12. What are the minimum and maximum widths for vegetated filter strips? Min: Max: 13. Which of the following charteristics most affect treatment performance of vegetated filter strips? a. Type of plants used as vegetated cover b. Establishment of thick grass cover (at least 80% coverage) c. Slope of the filter strip d. Width of pavement draining to the filter strip 14. What is the maximum flow velocity allowed in a vegetated biofilter for the WQ F? 15. What is the maximum flow depth allowed in a vegetated biofilter for the WQ F? 16. What is the maximum flow rate allowed to a manuftured system for the WQ F? 17. How often must a manuftured system be maintained after installation? 18. How large must the forebay for a detention basin be compared to the calculated WQ V? Page 2

31 19. How much of the WQ V may discharge in the first 1/3 rd of the drain time for drawdown of a detention basin? 20. What design storm frequency should be used to design the grate and primary discharge for a detention basin? 21. TRUE or FALSE (Circle Once) A detention basin should be included in the Hydraulic Grade Line (HGL) calculations for pipes in and out of the basin. 22. How large must a bioretention cell be sized? a. 1% of the impervious tributary area b. 2% of the impervious tributary area c. 5% of the impervious tributary area d. 10% of the impervious tributary area 23. TRUE or FALSE (Circle Once) Pretreatment is not required for bioretention cells. 24. During which design stage submittal are final post construction stormwater BMP design calculations and documentation of any implementation issues due? a. Stage 1 submittal b. Stage 2 submittal c. Stage 3 submittal d. Final Trings 25. TRUE or FALSE (Circle Once) The BMP design submittal should include a figure showing the tributary area to eh BMP. Page 3