20xxpereviewhydrologyandhydraulics-免費(fèi)閱讀
【正文】 ac on B and townhouses on A are 80, 85, 75 amp。 Hayes (1994) Example ? Answer is C. From map, 10yr24hr rainfall in NE Illinois is just over 4”, use ” to be conservative. Questions on precipitation? Abstractions from Rainfall ? Abstractions from rainfall are “l(fā)osses” from rainfall that do not show up as storm water runoff: – Interception – Evapotranspiration – Storage ? In bank ? On surface – Infiltration Runoff by other names… ? “Effective” rainfall ? Rainfall “excess” Runoff If rainfall rate exceeds the soil infiltration capacity, ponding begins, and any soil surface roughness creates storage on the surface. After at least some of those depressions are filled with water, runoff begins. Additional rain continues to fill depressional storage and runoff rate increases as more of the hill slope and subsequently the watershed contributes runoff. Rainfall/Runoff process Time of Concentration, tc The time from the beginning of runoff to the time at which the entire watershed is contributing runoff that reaches the watershed outlet is called the Time of Concentration. It is also described as the “travel time from the hydraulically most remote point in a watershed to the outlet”. Curve 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。 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
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