Detention Basins: Design, Calculation, and Components, Slides of Water and Wastewater Engineering

Download Detention Basins: Design, Calculation, and Components and more Slides Water and Wastewater Engineering in PDF only on Docsity! Detention Basics 1 http://emengineers.com/images/Wh2o1lo.jpg www.b-e-c.com/PromoPages/GatewayBrownfields4.jpg Docsity.com Objectives • Know what a detention basin is • Know how to develop an inflow hydrograph • Know how to determine a stage-storage curve • Know how to determine an outflow curve 2 Docsity.com * On-Site ¢ Regional Detention Basins 5 Docsity.com Detention Basins • Inflow (ditch or pipe) • Storage • Outflow (single/multiple stage – Orifice – Weir • Emergency spillway 6 Docsity.com Routing • Method used to model the outflow hydrograph • Based on continuity equation – Water in varies – Water out varies 7 Docsity.com TR-55 Hydrograph (NRCS Method) TR-55 Hydrograph 0 200 400 600 800 1000 0 5 10 15 20 Time (hrs) Fl ow (c fs ) Undeveloped Developed Flow (cfs) Flow (cfs) Time (hrs) Undeveloped Developed 12.7 246 631 12.8 284 670 13 366 739 13.2 433 820 13.4 503 861 13.6 575 872 13.8 636 861 14 686 833 14.3 720 755 14.6 701 679 15 631 568 15.5 529 412 Peak flow is higher after development Peak flow occurs earlier after development Docsity.com Rational Method: Simple Symmetrical Triangle Triangular Runoff Hydrograph Tc=2 hours; Qp=200 cfs 0 50 100 150 200 250 0 1 2 3 4 Time (hrs) Q (c fs ) Docsity.com Rational Method: Time base of 2.67 tc Base of 2.67*Tc Tc=2 hours; Qp=200 cfs 0 50 100 150 200 250 0 1 2 3 4 5 6 Time (hr) Q (c fs ) Area under hydrograph? Docsity.com Elevation-Area Method: Ex 14-1 Elev (ft) Area (ft2) Incr. Vol (ft3) Cum. Vol (ft3) 230 0 0 0 231 250 (250/2*1)=125 125 232 840 ((250+840)/2*1)= 545 670 233 1350 1095 1765 234 2280 1815 3580 235 3680 2980 6560 236 5040 4360 10,920 Docsity.com Stage vs Volume 0 2000 4000 6000 8000 10000 12000 230 231 232 233 234 235 236 Elev (ft) Vo lu m e (c ub ic ft ) Docsity.com Computing Storage Volumes • Average end-area (pipes) – Find u/s area at elevation increments – Find d/s area at elevation increments – Average the areas & multiply by length – This gives you total volumes (not incremental volumes) – See Example 14-2 (page 343) 17 Docsity.com Discharge Rating • Calculate outflows based on water elevation in the detention pond • Orifice and weir equations are used – Single stage (see pg 345) – Two stage (see page 348) • If more than one stage, calculate each outlet separately and add to get stage-discharge curves 20 Docsity.com 21 Orifices • Hole in a wall through which water flows – Square edge – Beveled edge Docsity.com 22 Orifice • When water flows through an orifice the water contracts with a smaller area than the original orifice opening (vena contracta) www.spiraxsarco.com www.diracdelta.co.uk Docsity.com 25 Orifice-Free Discharge • Given: Dia=6”, WSE=220.0 ft; Elev of orifice centerline=200.0 ft • Q=ca(2gh).5 • Q=0.62*0.196*(2*32.2*20).5 • Q=4.4 cfs Docsity.com 26 Weir • Horizontal surface over which water is allowed to flow • Used to regulate and measure flows http://www.flow3d.com/appl/weir.htm Docsity.com 27 Rectangular, Sharp-Crested Weir • Q=cLH3/2 – Q-flow (cfs) – c-adjusted discharge coefficient (careful) • c=3.27+0.4(H/P) where P is ht of weir above channel bottom – L-effective crest length, ft • L=L’-0.1nH – L’=actual measured crest length and n=# of contractions – H-head above crest, ft Docsity.com 30 Other Weir Types • Cipoletti (trapezoidal) • Ogee (dam spillway) www.lmnoeng.com youngiil.co.kr Docsity.com 31 Detention Outlet Structures • Single Stage (culvert or orifice) • Multi-Staged to handle different flows • Combination of orifices &/or weirs Docsity.com 32 Single Stage Outlet Example (Ex14-3) • An outlet consisting of a 12” pipe is proposed for a detention basin. The invert of the pipe is 320.0 feet and the top of berm is 325.0 ft. Compute the discharge rating for the outlet. • Area=0.785 sq ft • Assume c=0.62 • Use orifice equation: Q=ca(2gh).5 Docsity.com 35 Multi-Stage Outlet Example 14-4 (pg 349) • 4” Orifice and 2 weirs L=1.5’ and L=12.5’ Docsity.com Multistage Outlet Example 14-4 Multi Stage Outlet (4" orifice @ 560'; 1.5' weir @ 562.37, 12.5'weir @ 563.37; weir wall thicknesses=0.5') Total Q (cfs) WSE (ft) h (to cl-ft) Q (cfs) h (to cl-ft) c (FigA-5) Q (cfs) h (to cl-ft) c (FigA-5) Q (cfs) cfs 560 0 0 0 0 0 0 0 561 0.83 0.40 0 0 0 0.00 0.40 562 1.83 0.59 0 0 0 0.00 0.59 562.67 2.50 0.69 0 0 0 0.00 0.69 563 2.83 0.73 0.33 3.00 0.85 0 0.00 1.58 563.67 3.50 0.81 1.00 3.32 4.98 0.00 0.00 5.79 564 3.83 0.85 1.33 3.32 7.64 0.33 2.86 6.78 15.27 565 4.83 0.95 2.33 3.32 17.71 1.33 3.32 63.65 82.32 4" ORIFICE 1.5' WEIR 1.5' WEIR 36 Equations: c*a*(2gh)^.5 orifice cLH^1.5 weir Docsity.com 37 0 10 20 30 40 50 60 70 80 90 559 560 561 562 563 564 565 566 Di sc ha rg e (c fs ) WSE (ft) Stage-Discharge Curve Docsity.com