USING GIS TECHNIQUES TO DETERMINE RUSLE’S ‘R’ AND ‘LS’ FACTORS FOR KAPINGAZI RIVER CATCHMENT MBUGUA W. ESTHER EN 382-0069/2008 Department Geomatic Engineering and Geospatial … Calculating Custom CN Values from percent impervious cover values ... then needs to substitute the corresponding CN of the land use of interest in place of 98 in the above formula. Three formulas provided a similar fit to the data with the simple relationship with catchment area ultimately adopted (equation 1). In the case that such factors do not change, the runoff coefficient is expected to be more stable in time and space than other climatic or hydrological factors [14, 18]. Pervious and Impervious Area C Factor Inputs: Establish the Pervious and Impervious areas, as well as the C factor calculation method and value. R = geographic rainfall factor. More the area, more will be the runoff. It is calculated by measuring the percentage of a catchment area which is made up of impervious surfaces such as roads, roofs and other paved surfaces. This can usually be taken as anything between 30% and 100%, though there may be some underestimation at the lower end of this scale. The Effect of Sponge City Construction for Reducing Directly Connected Impervious Areas on Hydrological Responses at the Urban Catchment Scale April 2020 Water 12(4):1163 With HydroCAD 7.1 and earlier, a single composite Curve Number is calculated for each subcatchment. If different ARIs are required, and for situations in which there are a range in values in the table, or where the proposed land use is different to that prescribed, the fraction impervious must be estimated and taken into consideration. The table and the graph on the right show the CN values for percent impervious cover on the four hydrologic soil groups with the pervious cover being grass in fair condition. The allowance for GWI assumes that good quality materials and workmanship have been used for sewer system construction and that ongoing condition assessment, inspection and maintenance is performed. If using the Curve Number method for infiltration, the impervious parts of the catchment can be accounted for by modifying the curve number (CN) that is used for the infiltration accordingly. The impervious area is included in a simple percentage of impervious area factor (PIMP) which assumes 100% runoff from manmade areas such as roofs and pavements. This clearly suggests that the formula is not suitable to be used for an urban catchment as the formula does not account for the roughness coefficient. Catchment Area formula: This involves the catchment area only. So also, the shape will have a definite effect on the runoff. Since impacts are assessed through a hydrologic or hydraulic analysis of the highway drainage works, every proposed land development should provide … Total Area (Ha) Users can type in a total catchment area, pick a polyline from the drawing or draw a polyline for the catchment. The technique requires a modest amount of information, including the subwatershed drainage area and impervious cover, stormwater runoff pollutant concentrations, and annual precipitation. For three of the four species (all but speckled madtom), occurrence probability was predicted to approach zero at approximately 2-4% effective impervious cover. One of the important implications of the probabilistic rational method approach is that the time of concentration used for design must be calculated using the same formula that was used in the derivation of the runoff coefficients (equation 3). The runoff coefficient value is an attempt to take account of the volume of runoff which is absorbed into the surface. In addition, different catchment descriptions are presented in order to investigate how the hydrological reduction factor depends on the level of detail regarding the catchment description. 2008. Equivalent Impervious Area (EIA): is the area of a catchment that would produce a design flood of the same size as that estimated for the catchment if that area had a runoff coefficient of 1, in other words, if all of the rainfall falling on the EIA runs off. Introduction . Catchment Area of Triangular Surface (standard math formula) multiply the triangle’s base times its height then divide by 2 = catchment area where the base can be any side, and the height is measured perpendicularly from the base to the opposite vertex. Determine the time of concentration, t. c, for the catchment. ExAMpLE: You want to know the area of a triangular section of patio. The catchment is assumed to be represented by two idealized, rectangular inclined planes - one for the pervious surface and the second for the impervious fraction. The Rational Method is a commonly used procedure for estimating the peak surface runoff rate from an urban watershed for use in design of storm water management facilities, such as storm sewers, storm water inlets, and storm water detention ponds. They are derived for specific are and cannot be used university. Chow, 1988 and Hall, 1984). From Wenger SJ et al. Applicability: The Rational Method establishes an empirical formula that is commonly used in urban areas for computing peak rates of runoff for designing drainage structures. Q = 11.4 A0.75 (cfs) for A in Sq-km. SOIL. Define the flow path from the upper-most portion of the catchment to the design point. P = uncorrelated value of runoff, i.e., obtained from the runoff-curve. The accuracy of the Bransby Williams formula was also questioned in the UK (Beran, 1979). Size and Shape of the Catchment Area: The size of catchment has a definite effect on the runoff. Pervious/Impervious Runoff Calculations Overview. Impervious nature of the catchment generate variable EMCs. The use of rational formula for estimating the storm water (or rain water) flow or runoff for the design of sewers is usually limited to small catchment areas or drainage areas, say up to about 400 hectares. Equation gives the formula for calculating RMSE. F = recurrence interval factor. Other work suggested that the Bransby Williams formula wasn’t satisfactory as a measure of the characteristic response time of a catchment because it gave inconsistent values particularly when used with the equal area slope (McDermott and Pilgrim, 1982, p 33; Pilgrim and McDermott, 1982). 2. It has also been identified as one of the key factors in the occurrence of flash floods. 2.4. This flow path should be divided into reaches of similar flow type (e.g., overland flow, shallow swale flow, gutter flow, etc.). FACTORS FOR KAPINGAZI RIVER CATCHMENT Master of Science Research Project Report MBUGUA W. ESTHER Supervisors Dr. Thomas Ngigi Dr. Mwangi Gathenya Mr. Thomas Yatich December 2009 . impervious surface). 3. This value is required for the rational method which simply multiplies the catchment area by the rainfall intensity. S = shape factor … Use of the rational equation should be limited to drainage areas less than 20 acres (Amer. Pick Area: Click to select a closed 2D polyline from the drawing. It is useful in estimating runoff on relatively small areas such as roof tops and parking lots. The various formulae that have resulted from the individual studies are only applicable to the areas where they have been derived and it is not advisable to use them for the areas with different climates and topography (Shaw, 1983). Frequency Factors for Rational Method Runoff Coefficients (After AR&R, 1987) Direct Input Runoff Coefficient A constant runoff coefficient may be directly entered. Impervious Runoff Volume= (Impervious Runoff/1000(mm-m))* Impervious Area • Total Runoff Volume (m3) = Pervious Runoff Vol + Impervious Runoff Vol (m3) Figure 1: Total predicted runoff in Coogee catchment 2030-2050 INTEGRATION OF RAINWATER TANKS IN THE CATCHMENT It is desired to know the effect the Rainwater tanks can have on the predicted total runoff calculations for the catchment. The allowance for GWI should ta Impervious Cover Data Limitations of the Simple Method References. The two planes are commonly assumed to be inclined at the same gradient, but MIDUSS lets you define this and all other characteristics to be different. Instead its value is highly variable and depends on the above described catchment-specific factors and on the rainstorm characteristics. This is done by using the following formula: Q = PRFS …..(2.14) Where, Q = peak runoff rate for a specified geographic location and desired recurrence interval. Stream fish occurrence in response to impervious cover, historic land use, and hydrogeomorphic factors. Some of the formulas are: Dicken formula: Q = 825 A0.75 for A in sq-miles. When this option is selected, a global runoff coefficient for all impervious surfaces is entered in Job Control with the pervious runoff coefficient entered at each sub-catchment. Q = CAn Where C and n are constants. Users can also check the total infiltration volume in Table R5 of the output file. In rural catchments where no or only small parts of the area are impervious, the coefficient K, which describes the percentage of runoff resulting from a rainstorm, is however not a constant factor. sandstones should be classified as high groundwater table portions of the catchment, or as otherwise advised by the responsible local agency or technical specialist. This is so because for large areas the selection of suitable values of runoff coefficient and intensity of rainfall requires extreme care and judgement. The Simple Method estimates stormwater runoff pollutant loads for urban areas. Considering that a catchment is composed of impervious and pervious areas, the rainfall volume for the given event is estimated as: 5 VR PA (16) Where P=precipitation depth in [L per watershed]. Factor # 2. Impervious surface coverage can be limited by restricting land use density ... is an important factor when considering drainage of water. In a study of a partially urbanised catchment, ... significant relationships between stream condition and both the total impervious area and the mean patch size of impervious areas. reduce the catchment response time (e.g. We recommend a CN value of 98 for the impervious part of the catchment. As a matter of fact, the roughness coefficient itself represents the type of the catchment and the land use characteristics (e.g. In case of a fan-shaped catchment area, the base period of the resulting hydrograph will be less and thus more peak flow may be expected. This is the Urban Catchment Wetness Index value. Selecting Computational Methods. Area of catchment. Morgan’s formula: Q= 3000A0.5 (cfs) A in sq-miles. This paper examines the impact of different methods for estimating impervious surface cover on the outcome of a distributed rainfall-runoff model for the upper catchment of the Woluwe River in the southeastern part of Brussels. UCWI. 8 R M S E = 1 n ∑ i = 1 n M S E i = 1 n ∑ i = 1 n (E M C i − E M C i p r e d) 2 Where, E M C i and E M C i p r e d are the i th observation and the i th prediction from the LOOCV and n is the number of observations. This value needs to be modified to take account of the characteristics of the surfacing. Where a proposed land development will be discharging stormwater runoff to a receiving drainage system, any potential impacts to the receiving drainage system or the highway drainage system must be examined. This value is a weighted average of all the CN values in the subcatchment, including all pervious and impervious surfaces. The length and slope of each reach should be measured. This is the total sub-catchment area draining to a … The effective impervious fraction was related to total impervious area and the directly connected impervious fraction estimated from maps. coefficient, and catchment area. This is the WRAP soil index value. 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