A Paper presenaon on EXPERIMENTAL INVESTIGATION OF RAINFALL RUNOFF PROCESS by Ank Cakravar M.K.Jan Kapl Rola Deparmen of Hydrology, Indan Insue of Tecnology, Roorkee-247667
Inroducon Ranfall-runoff processes plays an mporan role n ydrologcal cycle. Te movemen of waer as manly arsen from e need o evaluae e amoun of avalable runoff waer a a parcular locaon o mee local demand as well as rsk of floodng due o excess waer. A number of models suc as pyscally based and concepual models ave been used o smulae e ranfall runoff process. However, due o s complexy and spao-emporal varaon, few models can accuraely smulae s gly non-lnear process.
Runoff s generaed by ransorms and s occurrence and quany are dependen on e caracerscs of e ranfall even,.e. nensy, duraon and dsrbuon. Te expermenal nvesgaons could be performed o undersand surface, sub-surface or ground waer runoff generaon mecansm. Te mecansm of surface runoff generaon s owever e focus of e presen nvesgaon. Te man objecve of s laboraory sudy s o quanfy e ranfall runoff process.
Descrpon of e laboraory se-up Te ranfall smulaor s desgned o smulae e ranfall even over e cacmen. Basc componens of e smulaor are- Tlng flume for slope adjusmen(recangular cross-secon avng.25 m wde and 5 m long), Spray Nozzles, Sorage Tank, Pressure Gauges, Elecrc Moor Slope Adjusmen Devce (0-5%) Runoff Recordng Sysem.
Fg. Advanced Hydrologc sysem S-2 MKII-50 (Ranfall Smulaor)
Meodology Te expermens s carred ou on a sand layer placed over an mpermeable plane surface (smoo meal see), Sedmen parcle dameer of 0.5-.0 mm s requred o fll over e flume. Smlar arrangemen for ranfall nensy from 30 mm/r o 90 mm/r. W e elp e spray nozzle arfcal ranfall s spread over e cacmen area n e flume flled w sand. Durng ranfall even waer flows over surface of sand reaces o e oule and e runoff waer flows roug collecng ank. Te Dep sensor s aaced below e collecng ank o gve e eg over wer.
Maemacal Modelng KW formulaon as been wdely used for modelng overland and sream flow. KW formulaon uses pysograpc parameers suc Overland rougness, slope, dranage area and leng, and sol caracerscs for compuaon of overland flow. Te KW approxmaon of San-Venan's equaon can be wren as:- A ( va ) x = q l.() m Q = αa.(2)
Were; A s e area of waersed (m 2 ), v s e flow velocy (m/s); Q s flow rae (m 3 /s), s e me (s), x s e dsance measured n e drecon of flow (m) α and m are parameers of e knemac wave model wc are closely relaed o e caracerscs of e flow. A α ma m A x = q l (3) α o m o m o x = r f (4) q o m = α o (5) o
Te governng dfferenal equaon were solved numercally usng explc numercal meod. From equaon () and (2) usng mplc fne dfference meod, = q x q ( r f ) (6) ( ) m q = α ( ) m q = α.(7). (8) ( ) m q = α. (9) Were r = ranfall nensy n mm/r, f = nflraon rae n mm/r, = 5 / 3, α o = s / 2 o n o m o
By solvng e above equaon usng Newon-Rapson erave meod, ( ) ( ) ( ) ( ) k k k k f f / = ( ) ( ) / = m m x f α Were Afer smplfyng e ese equaon, ( ) ( ) ( ) ( ) = m m f r x x f α α. (0).. (). (2)
Nas Suclffe Creron Were, NES(%) = n n ( Y ( Y o o Y Y c m ) ) 2 2 00 Y o Y c Y m = Observed flow (Expermenal ranfall flow) value a me = Compued flow (knemac flow) value a me = Mean of observed values. Error n Runoff Volume Compuaon Volumerc error, n % Y c = volume compued Y o = volume observed Y c 00 Y = o
Analyss of e daa Expermenal daa obaned from Advanced Hydrologc sysem (ranfall smulaor) s analyzed usng one dmensonal KW overland flow smulaon model usng FORTRAN programmng. Te smulaon was done usng me sep of 5 sec and spaal grd sze s aken as cm. Expermenal daa were smulaed usng developed model n order o sudy e overland flow rougness due o varaon of slope and ranfall nensy on e cacmen. Daa was observed for cacmen slope beween % o 4 % and ranfall nensy vares beween 30 o 90 mm/r. Developed KWM was calbraed for mannng s rougness coeffcen n o f no daa observed from expermen for 30 mm/r ranfall nensy for dfferen slopes.
Table Pernen caracerscs of observed and compued ydrograp for 30 mm/r nensy of ranfall. Volume Tme o Peak Slope NSE S.No Observed Compued Error Observed Compued Error (%) (l) (l) (%) (mn) (mn) (%) (%) 9.3 7.9 4.3 2.7 2.4. 98 2 2 6.3 5.9 6.3 2.3 2. 8.6 97.2 3 3 8.3 7.9 4.8 2. 2.0 4.7 9 4 4 7.6 7.3 3.9 2.0.8 5 93.2
Table 2 Pernen caracerscs of observed and compued ydrograp for 60 mm/r nensy of ranfall Volume Tme o Peak S.No Slope Observed Compued Error Observed Compued Error NSE (%) (l) (l) (%) (mn) (mn) (%) (%) 2.5 2.2 2.4 3.9 3.5 0.2 97.8 2 2 0.4 0. 2.8 3.2 2.9 9.3 96.6 3 3 6.2 5.8 2.4 2.8 2.6 7. 97.0 4 4 2.5 2. 3.2 2.5 2.4 4 92.6
Table 3 Pernen caracerscs of observed and compued ydrograp for 90 mm/r nensy of ranfall Volume Tme o Peak Slope NSE S.No Observed Compued Error Observed Compued Error (%) (l) (l) (%) (mn) (mn) (%) (%) 6. 5.7 2.4 3.6 3.3 8.3 96 2 2 8.3 7.9 2. 3. 2.9 6.4 97 3 3 5.4 4.9 3.2 3.2 3 6.2 96. 4 4 5. 4.7 2.6 2.6 2.5 3.8 96.7
Fg. 2 Comparson of observed and compued ydrograp for ran fall nensy 30 mm/r a % slope. Fg. 3 Comparson of observed and compued ydrograp for ran fall nensy 30 mm/r a 2 % slope.
Fg 4 Comparson of observed and compued ydrograp for ranfall nensy 60mm/r a 2% slope of e plane. Fg 5 Comparson of observed and compued ydrograp for ranfall nensy 60mm/r a 3% slope of e plane.
Fg 6 Varaon of Mannng s rougness w overland flow plane slope for ranfall nensy 30 mm/r.
Fg. 7 Varaon of Mannng s rougness w ranfall nensy for % slope of e plane.
Dscusson of resuls Te ploe beween observed and model compuaed ydrograp are sown n Fg. 2 o 5 for slope of %, 2%, and 3% respecvely I can be seen a, afer e sar of e ranfall on e cacmen, e waer flow rae a e oule ncreases rapdly w me, s poron of ydrograp s known as e rsng lmb. However a a ceran me e waer flow rae a e oule equals e ranfall nensy, s me s known as me o peak. Wen e ranfall nensy s sopped e waer flow rae a e cacmen oule sar reducng a a very slow rae, s s known as e fallng lmb of e ydrograp.
Te KWM could reproduce resonably well rsng lmb of e observed ydrograp as well as seady sae dscarge. In case of fallng lmb of e ydrograp, wo segmens are clearly vsble. Were only surface runoff domnaes wc s reproduced well by KWM. Te release of waer from sand bed, wc e KWM as no smulaed because e mecansm of release of waer from sand bed was no ncorporaed n e KWM. Form Fg 6 e value of n decreases lnearly w ncrease n overland flow plane slope. Ts may be due o rapd dranng of e waer due o ncreased surface slope.
Concluson Te overland runoff ydrograps presened n s sudy was conduced n e laboraory w equpmen conssng of a ranfall smulaor (nozzle spray), an mpermeable overland flow plane. Te resuls ndcae a ere s less dfference n e amoun of runoff volumes, peaks a dfferen ranfall nenses and slope of e cacmens. Bu consderable dfference n fallng lmb of ydrograp sapes. Laboraory and feld expermens wll es relaonsps for a wder range of condons a wll nclude e use of oer ranfall nensy, dfferen slopes and nflrang surfaces. Fuure work sould also nclude e comparson of e expermenal resuls w numercal resuls.e. knemac wave eory.
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