## methods of computing runoff from a catchment area

ADVERTISEMENTS: Read this article to learn about the following three methods of calculating average depth of precipitation upon the area of the basin, i.e., (1) Arithmetic Mean, (2) Theissen Polygon Method, and (3) Iso-Hyetal Method. The equation is based on Newton’s second law of motion and is written as: Where, y is the flow depth, V is the mean velocity, g is the gravitational acceleration, So is the bed slope, and Sf is the friction slope. SWMM, the Storm Water Management Model, program configuration. There are several empirical formulae in use. The lognormal distribution has the advantages over the normal distribution that it is bounded (X > 0) and that the log transformation tends to reduce the positive skewness commonly found in hydrologic data, since taking logarithms reduces large number proportionately more than it does for small numbers. Centimetres of water over a catchment area. Chow(1954) reasoned that this distribution is applicable to hydrologic variables formed as the product of other variables since if X = X1  X2  X3 X4 ....Xn, then Y , which tends to the normal distribution for large n provided that the Xi are independent and identically distributed. The data are then arranged in descending order of magnitude and the probability P of each event being equal to or exceeding (plotting position) is calculated by the plotting position formula. (19.6) is written for field application as (19.7) (Source: http://rpitt.eng.ua.edu/Class/Computerapplications/Module9/Module9.htm). If the maximum intensity of rainfall is 40 mm/hour, calculate the quantity of storm water which will reach sewer lines. Uploader Agreement, Measurement of Run-Off | Functions | Precipitation | Geography, Factors Affecting Runoff | Functions | Precipitation | Geography, Runoff: Process and Sources | Functions | Precipitation | Geography, Catchment Area: Slope and Its Classification | Geography, Essay on Heat Waves in Europe—A Natural Calamity | Environment, International Tourism in India: Introduction, History, Trends, Opportunities and Future, Forestry: Definition, Branches, Costs, Programme and Conclusion | Geography, Contribution of Russia to World Geography (In Hindi), French Scholars and their Contribution to Geography in Hindi. Many hydrologic methods are available for estimating peak flows (runoff) from a catchment area, and no single method is applicable to all catchments. UK Hydrology: New UK, Wallingford, ReFH, FEH, FSR … the width of the confidence band for the regional curve at the selected return period. Runoff estimation becomes necessary, as the numbers of gauged watersheds are generally small. Geography, Earth, Precipitation, Functions, Runoff, Estimation of Runoff. Its main parameter is the topographic index derived from a digital elevation model. The routing portion of SWMM transports this runoff through a conveyance sys­tem of pipes, channels, storage/treatment devices, pumps, and regulators. Fig. Subsurface routing is calculated in the runoff block. where K = Constant and its value for various types of catchment areas are shown in Table 7.2. In practices it is the value of X for a given P that is required, y, Since the value of the variate for a given return period, x, For a confidence probability c, the confidence interval of the variate, x, If the random variable Y = log X is normally distributed, then X is said to be log-normally distributed. The equation is based on the principle of the conservation of mass and is written as: Where, Q is the discharge (m3 s-1), A is the cross-sectional area (m2), q is the lateral inflow per unit length (m3 m-1), x is the space coordinates (m), and t is the time (seconds). 4. The curve fitting methods based on frequency factor for hydrologic frequency analysis: Where, x = flood magnitude of given return period T,  = mean of recorded floods, s = standard deviation of recorded floods, K = frequency factor, Gumbel's distribution is one of the widely used probability distribution functions for extreme values in hydrologic and meteorologic studies for prediction of flood peaks, maximum rainfall, maximum wind speed etc. (Source: http://research.guilan.ac.ir/cjes/.papers/958.pdf), Fig. The momentum equation applies to the full dynamic wave. If the catchment area is symmetrically distributed around the drainage network, an approximate value for the overland flow length can be found by dividing the area by twice the length of the drainage channel. The Rational Method is widely used to estimate the peak surface runoff rate for design of a variety of drainage structures, such as a length of storm sewer, a storm water inlet, or a storm water detention pond. The first is the development of the basic dimensionless frequency curve representing the ratio of the flood of any frequency to the mean annual flood. computing the runoff from ungauged catchments, a set of regional parameters have been used, parameters which, as an average, give an acceptable accuracy within the region. computing terms with the ability to perform long simulations. The amount of rainfall which is flowing across the catchment surface can become a runoff under the assumption and experimental procedures. The Modified Rational Method (as shown in Fig. These are based on the Rational Method which simply multiplies the catchment area by the rainfall intensity, modified by the runoff coefficient.Each method estimates the rainfall intensity and the runoff coefficient differently. Rainfall data is uncertain and Runoff is one of the important hydrologic variables used in the water resources management and planning. The regional flood frequency analysis aims at utilizing available records of stream in the topographically similar region on either side of the stream in question so as to reduce sampling errors. The graph will normally be a straight line having the following equation (Fig. This technique of frequency analysis to develop a frequency curve at a gauging station on a stream has been dealt with at length in the preceding section. As the catchment area for most gauging stations is above A set of flood ratios (ratio of flood to mean annual flood) is computed for each of the stations satisfying the homogeneity test for different return periods with the help of station frequency curves. Tables are prepared showing the relation between precipitation and the resulting runoff, taking into consideration the catchments. Deducting Abstractions from Precipitation: The runoff can also be estimated by deducting all abstractions from the precipitation. The second part is the development of relation between topographic characteristics of the drainage area and mean annual flood to enable the mean annual flood to be predicted at any point within the region. Since the value of the variate for a given return period, xT determined by Gumbel's method can have errors due to the limited sample data used; an estimate of the confidence limits of the estimate is desirable. Or; The total water in cubic-metre or hectare-metre for given catchment. The inter-connection between neurons is accomplished by using known inputs and outputs, and presenting these to the ANN in some ordered manner; this process is called training. The standard AR method assumes that the runoff per unit area in the donor catchment is equal to that in the target catchment. These ratios are averaged to obtain the mean 10-year ratio for the year. farm ponds) or for safe disposal (e.g. All stations in the region with 10 or more year’s record are selected. Centimetres of water over a catchment area. 5. A method of dealing with the runoff directly is called the flood frequency method. 2.5. The method is typically used to determine the size of storm sewers, channels, and other drainage structures. 6. The precipitation range as well as the type of catchment is covered in this approach. Gumbel defined a flood as the largest of the 365 daily flows and the annual series of flood flow that constitutes a series of largest value of flows. For a confidence probability c, the confidence interval of the variate, xT is bounded by values x1 and x2 given by. The catchment area is computed from the contour map and the flood discharge is estimated from the “Run-off ” formula. SWAT model divides the . If the probability of an event occurring is P, the probability of the event not occurring in a given year is q = (1-P). However, it gives a rough, estimate. Rainfall depths from actual storms are used to make the estimates. ... T o calculate aerial precipitation. The resulting ratio is taken as the error in the regional error estimate, i.e. A homogeneity test on the 10-year flood is performed as: (a) The ratio of the 10-year flood to the mean annual flood is determined from the frequency curve of each station. 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