20xxpereviewhydrologyandhydraulics

20xxpereviewhydrologyandhydraulics内容摘要：

Number method CN Method, continued Time of Concentration, tc CN values Runoff Volume determination Runoff Example In a previous problem, a design rain event in NE Illinois was determined to be ”. Assuming the watershed in question was a pleted 300 ac residential area with an average lot size of 189。 ac, all on Hydrologic Group C soils, what is the needed pond volume, if all runoff is to be retained? A: runoffinches B: 53 acreinches C: 630 acreft D: 53 acreft Runoff Example, continued Runoff Volume determination Answer to Runoff Example The answer is D, 53 acreft. From the table, the CN for Hyd group C soil with 189。 ac lot is 80. Using the graph with a ” rainfall, runoff depth is ”. Volume is then 300ac* = 630 acin, divided by 12 is 53 acft. Additional example You discover that the subdivision is actually 100 acres of 189。 ac lots on C soils, 100 acres of 189。 ac lots on D soils, 50 acres of 188。 ac lots on B soils and 50 acres of townhouses on A soils. What CN value would you use? A: 79 B: 85 C: 80 D: 75 Add’l Runoff Example, cont. Answer The correct answer is C, 80. Use an areaweighted average, similar to Theissen method. The respective CN values for 189。 ac on C, 189。 ac on D, 188。 ac on B and townhouses on A are 80, 85, 75 amp。 77. The areaweighted CN is then (80*100+85*100+75*50+77*50)/300 = , which is more appropriately 80. Peak Discharge The CN method also provides for Peak Discharge estimation, using graphs or tables. Required information includes average watershed slope, watershed flow path length, CN, and rainfall depth. The graphical method from the EFM is: Peak Runoff Discharge Peak Discharge Example Same residential watershed that produced ” of runoff from a ” rainfall. Flow length is 2500’, slope is 2%. CN is 80, so S is ”. Ia = *S = ”. Ia/P = Tc = 2500^*(1000/809)^^ = Peak Runoff Discharge Example solution From graph, with Ia/P of and Tc of , unit peak discharge is cfs/ac/in or qp = *300* = 360 cfs Rational Method The Rational Equation is: Qp = CiA where: C is a coefficient i is rainfall intensity of duration tc A is area in acres C is approximately , A is 300ac, i is 2” in 30min, so 4iph, peak rate is then *300*4 = 480 cfs Questions on runoff? Hydraulics of Structures Flow through structures is important given that such structures control the rate of flow. Sizing of such structures is then important to allow flow to pass while protecting downstream areas from the effects of too high a flow rate. Str。