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. 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. This flow path should be divided into reaches of similar flow type (e.g., overland flow, shallow swale flow, gutter flow, etc.). 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 … With HydroCAD 7.1 and earlier, a single composite Curve Number is calculated for each subcatchment. Pervious and Impervious Area C Factor Inputs: Establish the Pervious and Impervious areas, as well as the C factor calculation method and value. 3. 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. This is the Urban Catchment Wetness Index value. Impervious Cover Data Limitations of the Simple Method References. Determine the time of concentration, t. c, for the catchment. 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]. So also, the shape will have a definite effect on the runoff. More the area, more will be the runoff. 2.4. 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. Factor # 2. Define the flow path from the upper-most portion of the catchment to the design point. Use of the rational equation should be limited to drainage areas less than 20 acres (Amer. 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. The effective impervious fraction was related to total impervious area and the directly connected impervious fraction estimated from maps. 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. They are derived for specific are and cannot be used university. 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. Area of catchment. UCWI. impervious surface). From Wenger SJ et al. Frequency Factors for Rational Method Runoff Coefficients (After AR&R, 1987) Direct Input Runoff Coefficient A constant runoff coefficient may be directly entered. Morgan’s formula: Q= 3000A0.5 (cfs) A in sq-miles. ExAMpLE: You want to know the area of a triangular section of patio. S = shape factor … 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. Other factors which influence the effects of imperviousness . Q = CAn Where C and n are constants. 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. Instead its value is highly variable and depends on the above described catchment-specific factors and on the rainstorm characteristics. We recommend a CN value of 98 for the impervious part of the catchment. 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. 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. SOIL. 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. 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. 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. This can usually be taken as anything between 30% and 100%, though there may be some underestimation at the lower end of this scale. 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. P = uncorrelated value of runoff, i.e., obtained from the runoff-curve. 2. 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). This is so because for large areas the selection of suitable values of runoff coefficient and intensity of rainfall requires extreme care and judgement. This value is required for the rational method which simply multiplies the catchment area by the rainfall intensity. Equation gives the formula for calculating RMSE. The Simple Method estimates stormwater runoff pollutant loads for urban areas. Selecting Computational Methods. Catchment Area formula: This involves the catchment area only. This is the total sub-catchment area draining to a … This is the WRAP soil index value. Users can also check the total infiltration volume in Table R5 of the output file. Units in Rational Equation calculation: ft 3 =cubic foot, m 3 =cubic meter, mm=millimeter, s=second Rational Method Equation The Rational equation is the simplest method to determine peak discharge from drainage basin runoff. It is useful in estimating runoff on relatively small areas such as roof tops and parking lots. 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. 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]. 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.