2008-01-23_PERMIT FILE - M2007069 (2)

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CIVIL ENGIlYEERING REFERENCE MANUAL a particular project. The DBF is nasally determined from economic considerations, or is specified as peat of the contract document. Although the one percent flood is a commaoa choice for the design basis flood, shorter recurrence intervals are often used, particularly in low value areas such as crop- land. For example, 5-year storm curves are used in a+es- idential areas, 10-year curves for business sections, and 15-year frequencies for higlrvaiue districts where flood- ing will realilt in more e~ctensive damage. The ultimate choice of a recurrence interval, however, must be made am the basis of economic considerations and tradeoffs. The probability that a flooding eveait in aaly year wiIl equal or exceed the design basis flood based on a recur- sence interval or fregiaeacy, F, is p{F event in ~e year} _ ~ 6.9 The probability of an F event occurring in n pears is p{F event in n years} =1- Cl - F ~ ~ 6.10 Ezampk 6.2 A wastewater treatment plant has been designed to-be in use for 40 years. What is the probability that a one percent flood will occur within the usefill lifetime of the plant? F = 100 // ~ p{100 year flood is 40 years} = 1- (l - 1~} = 0.33 \\ () 5 SUBSURFACE WATER Subsurface water is a major source of all water used in the United States. In dry areas, it may be the only sonrc:e of water used for domestic and irrigation ns~. Snbsirrface zones ass divided into two parts by the water table. The txidoat zone exists above the water table, and pores in the vadase zone map be either empty or fall. Below the water table is the phraatic zone, whoee pores are always full. Soil moisture content is measured in pounds per cu- bic foot. Soil moisture i$ usually -determined by a~ven drying a sample of soil and measuring the weight Ions. The moisture content can also be determined with a tensiometer, which measures the vapor pressure of the moisture in the soil. Movement of crater thmugh an aquifer i$ given by equa- tion 6.11 in which Kp is known ss the ooe,(~icient of Pe*meabilit;l (ar hydraulic conduetivity)1 Q = (1.157EE-5)KrAG~ 6.11 Ky is the IIalited States is usnalijr given in Mei-iar:r units (gallams per day per square foot)- The aquihr Saw area, A, is given by equalise 6.13. The hydraulic gradient G~ = H/L, depends on the length, L, of the Table 6.?l Approximate Coefficients of Permeability, KP ft/day or material ft$/nay-ft~ gallaas--ft' darcya Clay 1.3 EF. 3 EE-2 E]<-3 Sand 1.3 EEZ EE3 EE2 Gravel 1.3 EE4 =ro~~EES F.E4 Gravel/Sand 1.3 E1~: i EE4 EE3 Sandstone 1.3 EEl EE2 F.El Dense shale k limestone 1.3 EE-1 1.0 EFf-1 Qaartaite do granite 1.3 EFr3 EE-2 EE 3 The tranamiaaivity of flow frame a saturated aquifer of thickness Y grad width B is given b3' equation 6.12. T = KpY 6.12 The crass sectional area in flow depends on the aquifer dimensnams? A = BY = BT 6.13 P Combining equation 611 with equation 6.13, Q = (1.I57E1r5)BTG~ 6.14 6 WELL DRAWDOWN IN AQUIII'ERS A virgin aquifer with a well is shown in £igare 6.2 (a). Once pumping of the well starts, the water table wdl be lowered in the vicinity of the well, and the resulting water table surface is kaurwn as the eons o/ dcpnaaion. The decrease in water level at the weIl ~ known as the drntudown, do. The drawdown at some distance r from the well is d,.. if the drawdoavn is sanall with aespect to the aquifer thiclQiess, Y, and the well completely penetrates the aquifer, then the equih'brium (steady state} well disc a This is ~ foam of llbtipb 1aa. The 1.157 EE-b is a coureisiou factor frean cubic feet per d;y to ds. ~ Sinoe wager can Duty flow between rock, not aIl of the atlnifet area oomributee to Saw. ft is impaetaat to know that the area, A, and the pesmeabality, Kg,sae comeistent. If the pesmeabiiitp value ie based as the clear Clow eta only, it will be see~aay to multiply the aquifer area by the pacaeity to reduce the area is Saw. PROFESSIONAL PUBLICATIONS INC. ~ P.O. Box 199. San Carlos, CA 94070