Presentation on theme: "Watershed Management Runoff models"— Presentation transcript:
1 Watershed Management Runoff models Hydrology and Water Resources, RG744Institute of Space TechnologyNovember 13, 2013
2 Runoff models Peak runoff models Continuous runoff models Provide only the estimates of peak discharge from the watershedContinuous runoff modelsThis class of runoff models provides storm hydrographs for a given rainfall hyetographProvide an estimate of runoff vs. time series
4 Rational Method To calculate peak runoff from small watersheds Provides peak runoff rate from a catchment given:the runoff coefficient C,the time of concentration Tc,the area of the catchment, andthe information to calculate the input or design storm or rainfall eventAlso called Lloyd Davies Method, 1850Source: The McGraw-Hill civil engineering PE exam depth guide: water resources By Emmanuel U. Nzewi
5 Rational Method: Assumptions Catchment is small (less than 200 acres)Catchment is concentratedRainfall intensity is uniform over the area of studyThe runoff coefficient is catch-all coefficient that incorporates all losses of the catchmentSource: The McGraw-Hill civil engineering PE exam depth guide: water resources By Emmanuel U. Nzewi.Catchment is concentrated: that is, rainfall duration is longer than or at least equal to the time of concentration of the watershed.
6 Rational method: formula Qp = CIiAQp = Peak discharge (cfs)C = runoff coefficient (function of soil type and drainage basin slope)Ii = average rainfall intensity (in/hr) for a storm duration equal to the time of concentration, TcA = area (acre)For runoff coefficient refer Bedient Table 6-5 page 381Obtain i from IDF curve with tr (duration) and T defined (assume tr = Tc)C = runoff coefficient, ratio of the total runoff rate to the precipitation, dimensionless
7 Determining Tc Take Tc = 5min when A (acres) < 4.6 S (slope %) Or use Kinematic Wave Theory (iterative process)L = length of overland flow plane (feet)S = slope (ft/ft)n = Manning roughnessIi= Rainfall intensity (in/hr)C = rational runoff coefficientSource:
8 Rational Method: example 6-6 Bedient Drainage design to be accomplished for a 4 acre asphalt parking lot in Tallahassee for a 5 yr return period. The dimensions are such that the overland flow length is 1000 ft down a 1% slope. What will be the peak runoff rate?Refer Table 6-5, 4-2 & Figure 6-5Assume tr value, for that value read rainfall intensity from IDF curve. Calculate Tc using Kinematic wave theory. (iterate till tr = Tc)Page 383
9 Runoff coefficient for nonhomogeneous area Weighted runoff coefficient based on area of each land useCw = ∑j=1 n Cj Aj/ ∑j=1 n AjExample McCuen page 381C reflects the effect of landuse, soil and slope on runoff potential assuming watershed is homogeneous. If drainage basin has different runoff potentials then it’s subdivided.
11 NRCS Runoff Curve Number Methods By the USA Soil Conservation Service (now called the Natural Resources Conservation Service), division of the USDA (USA Department of Agriculture)To predict peak discharge due to a 24-hr storm eventEmpirically derived relationships that use precipitation, land cover and physical characteristics of watershed to calculate runoff amount, peak discharges and hydrographsMore sophisticated approach than Rational MethodSource:National Resources Conservation Service (NRCS) (formally SCS)The RCN (Runoff Curve Number) method was originally established by the SCS in 1954.Initially developed as a design tool to estimate runoff from rainfall events on Agricultural fields. Now used for computing peak runoff rates and volumes for Urban Hydrology.Note: TR (Technical Release) -55 a simplified NRCS tool essentially joins the NRCS runoff equation with unit hydrograph theory for the computation of these runoff rates.
12 NRCS Curve numberCurve number is a coefficient that reduces the total precipitation to runoff potential, after “losses”EvaporationAbsorptionTranspirationSurface StorageHigher the CN value - higher the runoff potential will beIt is essential to use the CN value that best mimics the Ground Cover Type and Hydrologic Condition
13 NRCS Rainfall-Runoff Equation Following equation presents relationship between accumulated rainfall and accumulated runoffWhere:Q = accumulated direct runoff (in. or mm)P = accumulated rainfall (potential maximum runoff) (in. or mm) (24-Hour Rainfall Depth versus Frequency Values)Ia = initial abstraction including surface storage, interception, evaporation and infiltration prior to any runoff occurring (in. or mm)S = potential maximum soil moisture retention after runoff begins (in. or mm)Note: for P ≤ Ia, Q = 0Equation 1Eq-1 Derived by NRCS from experimental plots for numerous soils and vegetative cover conditions.The simplified NRCS method can be used for drainage areas up to 2,000 acres.Ia is highly variable, but generally is correlated with soil and landcover parameters. Through studies of many small agricultural watersheds, Ia was found to be approximated by the following empirical equation: Ia = 0.2S.S is related to the soil and cover conditions of the watershed through the curve number, CN (0-100 range).If P- Ia is negative then Q=0
14 potential maximum retention (S) potential maximum retention (S) can be calculated using Equation 2Where:z=10 for English measurement units, or 254 for metricCN = Runoff Curve NumberGenerally, Ia may be estimated asIa = 0.2 S Equation 3Substituting Ia value in Equation 1Equation 2Equation 2 was developed mainly for small watersheds from recorded storm data that included total rainfall amount in a calendar day but not its distribution with respect to time. Therefore, this method is appropriate for estimating direct runoff from 24-hour or one-day storm rainfallEquation 4
19 Curve Number CNThe principal physical watershed characteristics affecting the relationship between rainfall and runoff areland use,land treatment,soil types, andland slope.NRCS method uses a combination of soil conditions and land uses (ground cover) to assign a runoff factor to an area (CN)CN indicates the runoff potential of an areaHigher the CN, the higher the runoff potentialSoil properties also influence the relationship between runoff and rainfall since soils have differing rates of infiltrationBased on infiltration rates, the NRCS has divided soils into four hydrologic soil groups
20 Composite Curve Number When a drainage area has more than one land useWhen Composite CN is usedAnalysis does not take into account the location of the specific land usesdrainage area is considered as a uniform land use represented by the composite curve numbercan be calculated by using the weighted method
21 Hydrologic Soil groups Hydrologic Group is a grouping of soils that have similar runoff potential under similar storm and cover conditionsGroup A Soils: High infiltration (low runoff). Sand, loamy sand, or sandy loam. Infiltration rate > 0.3 inch/hr when wet.Group B Soils: Moderate infiltration (moderate runoff). Silt loam or loam. Infiltration rate 0.15 to 0.3 inch/hr when wet.Group C Soils: Low infiltration (moderate to high runoff). Sandy clay loam. Infiltration rate 0.05 to 0.15 inch/hr when wet.Group D Soils: Very low infiltration (high runoff). Clay loam, silty clay loam, sandy clay, silty clay, or clay. Infiltration rate 0 to 0.05 inch/hr when wet.Effects of Urbanization: Consider the effects of urbanization on the natural hydrologic soil group. If heavy equipment can be expected to compact the soil during construction or if grading will mix the surface and subsurface soils, you should make appropriate changes in the soil group selected.Antecedent soil moisture conditions: AMC I, II and IIIAverage antecedent soil moisture conditions (AMC II) are recommended for most hydrologic analysis
22 Antecedent soil moisture conditions-AMC AMC is the preceding relative moisture of the pervious surfaces prior to the rainfall eventLow: when there has been little preceding rainfallHigh: when there has been considerable preceding rainfall prior to the modeled rainfall eventACM I (dry), ACM II (average) and ACM III (wet)For modeling purposes, we consider watersheds to be AMC II, which is essentially an average moisture conditionalso referred to as Antecedent Runoff Condition (ARC).
23 CNsA CN of 100 is to be used for permanent water surfaces such as lakes and ponds
29 Continuous Runoff Models Time area methodUnit Hydrograph TechniquesSource: The McGraw-Hill civil engineering PE exam guide: breadth and depth By Roger Dodge Woodson, Kimberly Griffin.
30 Time Area MethodDevelop to address non-uniform rainfall in large areasConvert rainfall excess into hydrographConcept of time-area histogram is usedThis method assumes that outflow hydrograph results from pure translation of direct runoff to the outlet at uniform velocity, ignoring any storage effects in the watershedWatershed divided into subareas with distinct runoff translation times to the outletSubareas are delineated with isochrones of equal translation time (numbered upstream from the outlet)Source: Bedient page 111
31 Time Area MethodIf a rainfall of uniform intensity is distributed over the watershed area, water first flows from are as immediately adjacent to the outletPercentage of the total area contributing increases progressively in timeExample : Surface runoff from area A1 will reach the outlet first, followed by contributions from A2, A3, …..and so onQn = RiA1+Ri-1A2+…….R1AjQn = Hydraulic ordinate at time n (cfs)Ri= excess rainfall ordinate at time i (ft/s)Aj= time-area histogram ordinate at time j (ft2)
32 Isochrones: a line on a map or diagram connecting places from which it takes the same time to travel to a certain point. Isochrones of equal translation time numbered upstream from the outletSource:
33 Example 2-2 from Bedient page 112 Time area histogram method. The following is an example of the inflow for hour 5 using the 5 hour rainfall and 4 sub-basins. Assume constant rainfall intensity of 0.5in/hr
35 Unit Hydrograph (UHG) Theory Unit Hydrograph of a watershed is defined as the direct runoff hydrograph resulting from a unit depth of rainfall excess (1 in or 1 cm) distributed uniformly over a drainage area at a constant rate for an effective durationThe effective rainfall is considered uniformly distributed within its duration and throughout the whole area of the basinUniquely represents storm-flow response (hydrograph shape) for a given watershedTime area method is a special case of the unit hydrograph method.Introduced by Sherman in 19321 cm in SI unitsTo be applied only to direct runoff.
36 Unit Hydrograph Theory: Assumptions The effective rainfall is uniformly distributed within its durationThe effective rainfall is uniformly distributed throughout the whole area of the basinThe base period of the direct runoff hydrograph produced by effective rainfall of same duration (intensities may be different) are also sameThe ordinates of the direct runoff hydrographs of a common base period are directly proportional to the total volume of direct runoff represented by the respective hydrographsFor a given drainage basin the hydrograph of runoff due to a given period of rainfall reflects the unchanging characteristics of the basinSource: A Textbook of Hydrology By P. Jaya Rami ReddyIf there are N number of rain gages spread uniformly over the basin, then all gages should record almost same amount of rainfall during the specified period.1 & 2 incorporated in definition of UHG, 4 is linearity principle, 3 and 5 imply that shape of runoff hydrograph remains same irrespective of time it occurs as long as the duration of rainfall producing it is same.
37 Unit Hydrograph Theory: Assumptions 2 Basic Assumptions:Time InvarianceDirect runoff response to a given effective rainfall in a catchment is time invariant, i.e. direct-runoff hydrograph (DRH) for a given excess rainfall in a catchment is always the same irrespective of when it occursLinear ResponseDirect runoff response to the rainfall excess is assumed to be linear. Means if inputs x1(t), x2(t) cause outputs y1(t) and y2(t) respectively then an input x1(t) + x2(t) will cause an output y1(t) + y2(t). Also if x1(t) = r x2(t) then y1(t) = r y2(t).Source: A Textbook of Hydrology By P. Jaya Rami Reddy????
38 Linear ResponseSource: usf (tp= time to peak from start of rainfallUH is 1/2 the size but TB (time base) and TP (time to peak) are the same
39 Typical Unit Hydrograph Source: Engineering Hydrology By K. SubramanyaRunoff volume = meter cubeDirect runoff = 1 cmRunoff volume= Catchment area x direct runoffCatchment area = Runoff volume/direct runoff = 1292 square km
40 Properties of unit hydrograph Volume under unit hydrograph is equal to 1 unit rainfall excess (1 in or cm)If duration of 2 rainfall excess events is equal without regard to their respective rainfall intensities, they must result in the same hydrograph time baseResults in a linear system whereby the direct runoff for storm of specified duration is directly proportional to the rainfall excess amount or volumeRainfall distribution for all equal duration storms is identical in time and spaceSource: The McGraw-Hill civil engineering PE exam guide: breadth and depth By Roger Dodge Woodson, Kimberly Griffin.
41 Development of UHGExamine records of watershed for single peaked, isolated stream flow hydrographs resulting from short duration rainfall hyetograph of relatively uniform intensityDetermine depth of storm precipitation spread over the watershed equivalent to the volume of water divided by areaVolume is equal to area under hydrographOrdinates of UHG can be calculated by dividing the ordinates of the DRH by the storm depthCheck: recalculate area under UHG and divide it by watershed area. That should give unit storm depth.Source: Brooks pg 444
42 Example:Determine the UHG ordinates for the hydrograph shown in figure. The area of watershed is 16.2 square kmSource: Intro To Env Engg (Sie), 4E By Davis
48 Drg ordinates from uhg with Variable Rainfall excess values (M pulses of excess rainfall Discrete Convolution Equation is used to compute direct runoff hydrograph ordinatesQn = direct runoff hydrograph ordinates, Pm= Rainfall excess, Un-m+1 = unit hydrograph ordinates,n= direct runoff hydrograph time interval (1, 2, …N), m= precipitation time intervalM = pulses of excess of rainfallN = pulses of direct runoffN-M+1 = L, Number of UH ordinatesReverse process is called ‘deconvolution’ to derive unit hydrograph given Pm and QnSource: Water Resources Engineering By Larry W. Mays
55 Instantaneous Unit Hydrograph Limiting the duration of UHG to zero an Instantaneous Unit Hydrograph is obtainedInstantaneous Unit Hydrograph is the hydrograph resulting from an instantaneous rainfall of one unit uniformly over the basinUHG depends on duration of excess rainfall (disadvantage).
56 Synthetic UHGWhen observed rainfall/runoff data for a catchment is not available to derive UHGConstruct synthetic UHG based on empirical functions (basin’s physical characteristics)Developing UHG for other locations on the stream in the same watershed or other watersheds that are of similar character with known dataSnyder’s Unit HydrographSCS Dimensionless Unit HydrographClark (time area method)
57 Snyder UHGRelates the time from the centroid of the rainfall to the peak of the UHG to geometrical characteristics of the watershedImportant factors to be considered for a UGH are;peak flow and time of peak flowCoefficient derived from gaged watershed in the area Cp and CtCp = peak flow factor, and Ct = lag factor.Basic Assumption of Synder’s Method:that basins which have similar physiographic characteristics are located in the same area will have similar values of Cp and CtTherefore, for ungagged basins, it is preferred that the basin be near or similar to gaged basins for which these coefficients can be determinedWater Resources Engineering By Larry W. MaysSource:Ct = represents variation in watershed slopes and storage characteristics.Cp = represents the effect of retention and storage.
58 Snyder’s UHG: Computing Ct tp = basin lag time (hrs)Lc= distance from outlet to a point on the stream nearest the centroid of the watershed area -in km (miles)L = length of main stream from outlet to the upstream divide - in km (miles)C1 = 0.75 ( 1.0 for English units)𝐶𝑡= 𝑡𝑝 𝐶1 𝐿.𝐿𝑐 0.3L Lc = measure of watershed shape. L and Lc are determined from gaged watershed and tp from derived unit hydrograph for the gaged basin.
59 Snyder’s UHG: Computing Cp Qp = peak direct runoff rate - in m3/s (cfs)A = watershed area - in km2 (mi2),qp= peak discharge/unit watershed areaC2 = 2.75 (640 for English units)𝑄𝑝= 𝐴 𝐶2𝐶𝑝 𝑡𝑝Or for a unit discharge (discharge per unit area)𝑞𝑝= 𝐶2𝐶𝑝 𝑡𝑝Or𝐶𝑝= 𝑞𝑝𝑡𝑝 𝐶2English units (ft)
60 Characteristics of a Standard UHG tp = 5.5 trtp = C1Ct (LLc)𝑄𝑝= 𝐴 𝐶2𝐶𝑝 𝑡𝑝tp = basin lag – time from the centroid of excess rainfall hyetograph to the peak runoff (hrs)Qp = peak direct runoff rate
62 Snyder: Development of a Required UHG assuming that Ct, Cp, L, and Lc are knownRequired: a unit hydrograph whose associated effective rainfall pulse duration is tR for an ungagged watershedUse equation 2 to determine the lag-time, tpIf tR meets the criterion for a standard UHG (tp = 5.5 tR) then the required unit hydrograph is a standard UHGequations 2 and 3 can be used directly to estimate the peak discharge and the time to peak of the required unit hydrographIf tR does not meet the criterion for a standard UHGIn this case, the lag-time of the required unit hydrograph, tpR, istpR = tp – (tr – tR)/where tp is obtained from equation 2, tr is obtained from equation 1 and tR is given.Source: Snyder’s UHG
63 Conti… The peak discharge of the required UHG, QpR, is, QpR = Qp tp/tpRwhere Qp is obtained from equation 3Assuming a triangular shape for the UHG, and given that the UHG represents a direct runoff volume of 1 cm (1 in), the base time of the required UHG may be estimated bytb = C3A/QpRwhere C3 is 5.56 (1290 for the English system)
64 Drawing Snyder’s UHGRelationships for the widths of the UHG at values of 50% (W50) and 75% (W75) of QpR developed by U.S. Army Corps of Engineers;W% = Cw(QpR/A)-1.08where the constant Cw is 1.22 (440 for English units) for the 75% width and equals to 2.14 (770 for English units) for the 50% width
66 Example: Snyder’s Method A watershed has a drainage area of 5.42 mi2; length of the main stream is 4.45 mi, and the main channel length from the watershed outlet to the point opposite the center of gravity of the watershed is 2.0 mi, Using Ct = 2.0 and Cp = 0.625, determine the standard synthetic UHG for this basin. What is the standard duration? Use Snyder’s method to determine 30 min unit hydrograph parameters.Solution:Given: Ct = 2.0, Cp = 0.625, Lc = 2.0 mi, L= 4.45 mi, tR= 30 mi (0.5 hr), C1 = 1, C2 = 640, C3 = 1290Equation tp = C1 Ct (LLc)0.3 =1 x2 (4.45 x 2) 0.3 = 3.85 hrsEquation Standard rainfall duration, tr = tp/5.5 = 3.85/5.5 = 0.7 hrs, tr ≠ tR.Equation tpR = tp – (tr – tR)/4 = 3.85 – (0.7 – 0.5)/4 = 3.8 hrsEquation 𝑄𝑝= 𝐴 𝐶2𝐶𝑝 𝑡𝑝 = 5.42 x 640 x 0.625/3.85 = 563 cfsEquation QpR = Qp tp/tpR = 563 x 3.85/3.8 = cfsEquation tb = C3A/QpR = 1290 x 5.42 /570.5 = hrsExample pg#295, Water Resources Engineering By Larry W. Mays
67 SCS unit hydrographSlide source:484 = 2/2.67 *(5280 ft/mi x 5280 ft/mi) x 1/12 (ft/in) x 1/3600 (hr/sec) = cfs
68 Clark’s IUH Time-Area Method (concept of isochrones) Slide source:Isochrones are lines of equal travel time. Any point on a given isochrone takes the same time to reach the basin outlet.
69 HYDROGRAPHS Floods 2013 in Sindh Map layout prepared by M. Arslan student MS-5 batch at RS and GISc, IST Karachi Campus.HYDROGRAPHSFloods 2013 in Sindh