Example. Have precipitation and streamflow data, need to estimate losses

Transcription

1 Excess Rinfll

2 Excess rinfll Rinfll tht is neither retined on the lnd surfce nor infiltrted into the soil Grph of excess rinfll versus time is clled excess rinfll hyetogrph Direct runoff = observed stremflow - bseflow Excess rinfll = observed rinfll - bstrctions Abstrctions/losses difference between totl rinfll hyetogrph nd excess rinfll hyetogrph

3 f-index f-index: Constnt rte of bstrction yielding excess rinfll hyetogrph with depth equl to depth of direct runoff Used to compute excess rinfll hyetogrph when observed rinfll nd stremflow dt re vilble

4 f-index method Gol: pick t, nd djust vlue of M to stisfy the eqution Steps 1. Estimte bseflow 2. DRH = stremflow hydrogrph bseflow 3. Compute r d, r d = V d /wtershed re 4. Adjust M until you get stisfctory vlue of f 5. ERH = R m - ft r r d d R f M m t Rm ft M m1 depth of observedrinfll Phiindex #intervlsof contributingto driectrunoff time intervl direct runoff rinfll

5 Stremflow (cfs) Exmple Time Observed Rin Flow in cfs 8: : : : : : : : : : : : : : : : : Hve precipittion nd stremflow dt, need to estimte losses :30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM Time No direct runoff until fter 9:30 And little precip fter 11:00 Bsin re A = 7.03 mi2

6 Stremflow (cfs) Exmple (Cont.) Estimte bseflow (stright line method) Constnt = 400 cfs :30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM Time bseflow

7 Exmple (Cont.) Clculte Direct Runoff Hydrogrph Subtrct 400 cfs Time Observed Direct Runoff Rin Flow in cfs cfs 8: : : : : : : : : : : : : : : : : Totl = 43,550 cfs

8 Exmple (Cont.) Compute volume of direct runoff V d Qnt t n1 n1 3600s/hr*0.5hr*43,550 ft 7.839*10 7 ft 3 Compute depth of direct runoff r d V d A Q n 3 /s 7.839* mi* ft 4.80in 7 ft 2 3 ft 2

9 Exmple (Cont.) Neglect ll precipittion intervls tht occur before the onset of direct runoff (before 9:30) Select R m s the precipittion vlues in the 1.5 hour period from 10:00 11:30 r d 4.80 M m1 R m ft ( f *3*0.5) r d 4.80in f 0.54in ft 0.27in

10 Stremflow (cfs) Exmple (Cont.) Direct Runoff Excess Rinfll Time Observed Rin Flow in cfs cfs in 8: : : : : : : : : : : : : : : : : ft= :30 PM 9:00 PM 10:30 PM 12:00 AM 1:30 AM 3:00 AM 4:30 AM 6:00 AM Time

11 Precipittion SCS method Soil conservtion service (SCS) method is n experimentlly derived method to determine rinfll excess using informtion bout soils, vegettive cover, hydrologic condition nd ntecedent moisture conditions The method is bsed on the simple reltionship tht P e = P - F I P e is runoff volume, P is precipittion volume, F is continuing bstrction, nd I is the sum of initil losses (depression storge, interception, ET) I P P e F P e I F t p Time

12 Precipittion Abstrctions SCS Method In generl P e P After runoff begins F S Potentil runoff P I SCS Assumption F S Combining SCS ssumption with P=P e +I +F 2 P I P e P e P I P I S I t p P P e F P e I P Totl Rinfll P I e F RinfllExcess F InitilAbstrction ContinuingAbstrction Time S PotentilMximumStorge

13 Cumultive Direct Runoff, Pe, in SCS Method (Cont.) Experiments showed So I 0. 2S P 0.2S P e P 0. 8S 1000 S 10 CN (AmericnUnits;0 CN 100) S 254CN CN (SIUnits;30 CN 100) Surfce Impervious: CN = 100 Nturl: CN < Cumultive Rinfll, P, in

14 SCS Method (Cont.) S nd CN depend on ntecedent rinfll conditions Norml conditions, AMC(II) Dry conditions, AMC(I) 4.2CN( II) CN( I) CN( II) Wet conditions, AMC(III) 23CN( II) CN( III) CN( II)

15 SCS Method (Cont.) SCS Curve Numbers depend on soil conditions Group Minimum Infiltrtion Rte (in/hr) Soil type A High infiltrtion rtes. Deep, well drined snds nd grvels B Moderte infiltrtion rtes. Modertely deep, modertely well drined soils with modertely corse textures (silt, silt lom) C Slow infiltrtion rtes. Soils with lyers, or soils with modertely fine textures (cly loms) D Very slow infiltrtion rtes. Clyey soils, high wter tble, or shllow impervious lyer

16 Exmple - SCS Method - 1 Rinfll: 5 in. Are: 1000-c Soils: Clss B: 50% Clss C: 50% Antecedent moisture: AMC(II) Lnd use Residentil 40% with 30% impervious cover 12% with 65% impervious cover Pved rods: 18% with curbs nd storm sewers Open lnd: 16% 50% fir grss cover 50% good grss cover Prking lots, etc.: 14%

17 Exmple (SCS Method 1, Cont.) Hydrologic Soil Group B C Lnd use % CN Product % CN Product Residentil (30% imp cover) Residentil (65% imp cover) Rods Open lnd: good cover Open lnd: Fir cover Prking lots, etc Totl CN vlues come from Tble CN

18 Exmple (SCS Method 1 Cont.) Averge AMC Wet AMC CN 83.8 S P e in 2 P 0.2S 5 0.2*1.93 P 0.8S 5 0.8* in 23CN( II) 23*83.8 CN( III) CN( II) * S in 92.3 P e 2 P 0.2S 5 0.2*0.83 P 0.8S S CN * in

19 Exmple (SCS Method 2) Given P, CN = 80, AMC(II) Find: Cumultive bstrctions nd excess rinfll hyetogrph Time (hr) Cumultiv e Rinfll (in) Cumultive Abstrctions (in) Cumultive Excess Rinfll (in) Excess Rinfll Hyetogrph (in) P I F Pe

20 Exmple (SCS Method 2) Clculte storge Clculte initil bstrction Initil bstrction removes 0.2 in. in 1 st period (ll the precip) 0.3 in. in the 2 nd period (only prt of the precip) Clculte continuing bstrction F P P S e P I P e I F F F S I S( P I ) ( P I S) CN S 0.2* in 2.5( P 0.5) ( P 2.0) 2.5( ) (2hr) 0.34in ( ) Time (hr) 2.50in Cumultive Rinfll (in) P

21 Exmple (SCS method 2) Cumultive bstrctions cn now be clculted Time (hr) Cumulti ve Rinfll (in) Cumultive Abstrctions (in) F 2.5( P 0.5) ( P 2.0) P I F

22 Exmple (SCS method 2) Cumultive excess rinfll cn now be clculted P e P I F Excess Rinfll Hyetogrph cn be clculted Time (hr) Cumultive Rinfll (in) Cumultive Abstrctions (in) Cumultive Excess Rinfll (in) Excess Rinfll Hyetogrph (in) P I F Pe

23 Exmple (SCS method 2) Cumultive excess rinfll cn now be clculted Excess Rinfll Hyetogrph cn be clculted P e P I F Time (hr) Cumultive Rinfll (in) R i n f l l ( i n ) Cumultive Abstrctions (in) Cumultive Excess Rinfll (in) P I F Pe T i me ( h o u r ) R i n f l l H y e t o g r p h s 4 5 Excess Rinfll Hyetogrph (in) R i n f l l E x c e s s R i n f l l

24 Time of Concentrtion Different res of wtershed contribute to runoff t different times fter precipittion begins Time of concentrtion Time t which ll prts of the wtershed begin contributing to the runoff from the bsin Time of flow from the frthest point in the wtershed Isochrones: boundries of contributing res with equl time of flow to the wtershed outlet

25 Quntittive wy of studying strems. Developed by Horton nd then modified by Strhler. Ech hedwter strem is designted s first order strem When two first order strem combine, they produce second order strem Only when two strems of the sme order combine, the strem order increses by one When lower order strem combines with higher order strem, the higher order is retined in the combined strem Strem ordering