Methods of runoff estimation and its measurement

1 Methods and estimation of excess rainfall ( runoff )- Theory and PracticalDr. Ranu Rani SethiPrincipal ScientistICAR-Indian Institute of Water ManagementBhubaneswar, OdishaOutline runoff and factors affecting runoff estimation of runoff Rational formula Curve Number method Use of remote sensing and GIS runoff measuring devicesDirect discharge Methods Weirs Orifices FlumesVelocity - Area Methods Float method Current meterRunoffPortion of precipitation that makes its way towardsstreams, lakes or oceans assurfaceorsubsurface flowFactors affecting runoff Climatic factors Physiographic factorsRunoff measurement methodsRational MethodWhere, Q=Peak rate of runoff , m3/sec.

2 I=Intensity of rainfall, mm/hour (time of concentration)C= runoff coefficientA=Area of the catchment, haRunoff coefficient (C)Sl typeLand useCultivation Pasture , ,noclay pans, loams andsimilar soil3 Soilwithbelowaverageinfiltrationrates, heavy clay soilsor soils with a claypan near the surface,shallow soils aboveimpervious of rainfall (I)Icalculatedfor theperiodequal tothetimeof concentrationof (Tc)isdefinedasTheTimeofConcentration(Tc )isdefinedasthetimerequiredfor water flowfromthemost remotepoint of thecatchment totheoutletTime of Concentration (Tc) This is the longest time it takesfor a part of the catchment tocontribute water to the outlet.

3 It is the time it takes for all thepartsofthewatershedtobepartsofthewate rshedtobecontributing water to the outlet. Thedivideorwatersheddivides the flow of water alongdifferent slopes. All runoffs flow fromthe wholecatchment to the ,tc=Time of concentration, min;L =Length of channel reach, m;S =average slope of the channel reach, m/mLimitations It assumes that rainfall intensity is uniformoverthe entire watershed during the duration of thestorm, which is very rare Theinitiallossesduetodepressionstoragean d Theinitiallossesduetodepressionstoragean dinitial infiltration are not considered. runoff coefficient changes with respect toseason as well as rainfall characteristics, whichis not considered in rational formulaCurve number methodWhere, F=Actual retention, mm; F=Actual retention, mm; S =Potential maximum retention, mm;Q =accumulated runoff depth, mm;P =accumulated rainfall depth, mmIa=Initial abstraction, mmFactors determining curve number values Indicate runoff response characteristics of thedrainage basin.

4 Parameterslikelanduse,landtreatment, Parameterslikelanduse,landtreatment,hydr ological condition, hydrological soil groupand antecedent soil moisture condition in thedrainage basin are related to curve of runoff by using remote sensing and GIS PreparationofLanduse/Land cover map byusing the remote sensingand GIS. Preparation of hydrologicsoilgroupmapformakingsoilgroup mapformakingappropriate hydrological soilclassification A, B, C & D Decide the curve number(CN) values based on fieldcondition. Calculate S value and thenestimate runoff QEstimation of runoff depth -ExamplePreparation of Various Thematic Maps: 1.

5 Contour map2. drainage map3. DEM4. Slope map5. Land land use/ land cover map. Kanhaiya nala watershed In Tons River catchment Elevation - 480 to 620 m above MSL Satna District ( ) Soil and Land use land coverGenerating CN and runoff depth map runoff measuring devicesDirect discharge Methods Weirs Orifices FlumesVelocity - Area Methods Float method Current meter Tracer methodWeirQ = C L HmWhere,Q = discharge ;C = coefficient dependent onthe nature of weir crest andapproachconditions;Rectangular weir with no end contractionapproachconditions;L = length of crest;H = head on the crestm = exponent depending onweir should be calibrated todeterminetheseparameters before weir with no end contractionRectangular weir with two end contractionOrificeWhere,Q = discharge rate, l/sec;C = coefficient of discharge ( );( );a = area of the orifice opening,sq cm.

6 G = acceleration due to gravity,cm/s2h = head of water causing theflow, cmFlumesParshal FlumesUsed under free flow and submergedflow ,primarypointofmeasurement,denotedasHa,w hichdeterminetheflowratewhichdetermineth eflowratethrough the short throated flume, theHaisupstream of the throat at a specificlocation - 2/3 of the sidewall lengthas measurement back from wherethe converging section meets thethroatFor large Parshall flumes the point ofmeasurement is closer to the throatStandard design dimensions(from USDA-SCS 1965) Throat width W (feet)A (feet, inches)BCD13-04-4 7/82-02-9 1/41 3-24-7 7/82-63-4 3/823-44-10 7/83-03-11 23-44-10 7/83-03-11 33-85-4 3/44-05-1 7/844-05-10 5/85-06-4 1/454-46-4 6-07-6 5/864-86-10 3/87-08-975-07-4 8-09-11 3/885-47-10 1/89-011-1 3/4 Dimension A = 2/3 (w/2 + 4)Cut Throat Flume Improved version of Parshalflume.

7 It is having flat bottom, For free flowcritical depth ismeasured at throat. Free flowdepends on flumesize and submergence flow monitoring Methods The volume of water thatmoves through the channelisthencalculatedbydividing the channel intosmaller units of known orapproximated areas (width depth) Step1:Selectingthechannel location Step 2:Developing a cross-section of the site andestablishing a reference orstaffgauge depth) Measuring the flow withineacharea(velocity-distance over time). By multiplying area andvelocity, flow is calculatedfromthe locationstaffgauge Step 3:Measuring intervaland depthMeasurement of stream flow dischargeMid section methodVelocity-area methodFloat Method A floating object is put inwater and time is recorded tocover a known distance.

8 Assuming the float travels D meters in t seconds Velocity of water at surface =(D/t) m/s, Average velocity of flow = (D/t),Flowrate,Q=Crosssectional area velocity offlowCurrent meter Currentmeterisasmallinstrumentcontaining arevolving wheel or vane that isturned by the movement ofwater. Current meter is inserted to thedepth= (d=depthofflow).depth= (d=depthofflow). Number of rotations (rpm) wasrecorded and then velocity offlowiscalculatedfromcalibrated chart. A current meter is used tomeasure velocity at and or at only level recorder Non recording streamgage Staff Wire(String) Crest staff Recording streamgage Float type Digital gageSl.

9 NoDistance from mouth,Av. Count/minuteMeasured Velocity, Top width,Bottom width,Water depth, Area, (A) discharge ,Qmm of discharge in Sunity Creek availability in creeks and water qualityCreekDischarge at creek cross section( m3/sec)Volume of Water available(November -January),m3 Creek (Creeks) Creek Creek Creek Creek (Sub creeks) Estimating discharge and stream flows a guide for sand and graveloperators (July 2005 Ecology Publication Number 05-10-070) Murty, and Jha, Land and Water ManagementEngineering. Ritzema, , drainage Principles and application. ILRI Publication 16, Raghunath, ,1985.

10 Hydrology Principles, Analysis and DesignTHANK YOU