What is C in rational method?
The Rational method runoff coefficient (c) is a function of the soil type and drainage basin slope. The duration is usually equivalent to the time of concentration of the drainage area. The storm frequency is typically stated by local authorities depending on the impact of the development.
How do you calculate weighted C?
The weighted C for a drainage area is computed by dividing the sum of all subarea CAs by the total area, where CA is the subarea C value multiplied by the area of the subarea.
What is C in Q CiA?
Q = CiA. where. Q= design peak runoff rate, cfs. C = runoff coefficient, dimensionless – defined as the ratio of the peak runoff rate to the rainfall intensity.
What is the rational method formula?
The Rational method predicts the peak runoff according to the formula: Q=CiA, where C is a runoff coefficient, i is the rainfall intensity, and A is the subcatchment area. The peak runoff is sustained until the event duration has elapsed, and then decreases linearly to zero.
What is SCS Curve Number?
The SCS curve number method is a simiple, widely used and efficient method for determining the approxient amount of runoff from a rainfall even in a particular area. The curve number is based on the area’s hydrologic soil group, land use , treatment and hydrologic condition.
When can you use the rational method?
One of the most commonly used procedures for calculating peak flows from small drainages less than 200 acres is the Rational Method. This method is most accurate for runoff estimates from small drainages with large amounts of impervious area. Examples are housing developments, industrial areas, parking lots, etc.
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What is time of concentration in rational method?
For a given watershed, the time of concentration is the time required for rainfall landing on the farthest point of the watershed to reach the watershed outlet. The time of concentration should be used as the duration of the design storm for calculating peak storm water runoff rate with the Rational Method equation.
What is the purpose of rational method?
The Rational Method expresses a relationship between rainfall intensity and catchment area as independent variables and the peak flood discharge resulting from the rainfall as the dependent variable. It has been used for over 150 years, and known as the Rational Method for nearly 100 years.
What is rational method used for?
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.
What does SCS method stand for?
The SCS Runoff Curve Number method is developed by the United States Department of Agriculture (USDA) Soil Conservation Service (SCS) and is a method of estimating rainfall excess from rainfall (Hjelmfelt, 1991).
What is SCS model?
Basic Concepts and Equations. The Soil Conservation Service (SCS) Curve Number (CN) model estimates precipitation excess as a function of cumulative precipitation, soil cover, land use, and antecedent moisture, using the following equation: Pe=(P−Ia)2(P−Ia)+S.
What is the rational method (rational formula)?
The Rational Method (or Rational Formula) is: Q = C f C i A (Equation 1) Where: Q = Peak flow in cubic feet per second (cfs) C f= Runoff coefficient adjustment factor to account for reduction of infiltration and other losses during high intensity storms
When is the rational method appropriate for estimating peak discharge?
The rational method is appropriate for estimating peak discharges for small drainage areas of up to about 200 acres (80 hectares) with no significant flood storage.
What is the rational method of estimation of rainfall?
The Rational Method (or Rational Formula) is: Q = C f C i A (Equation 1) Where: Q = Peak flow in cubic feet per second (cfs) C f = Runoff coefficient adjustment factor to account for reduction of infiltration and other losses during high intensity storms . C = Runoff coefficient to reflect the ratio of rainfall to surface runoff
What are the assumptions and limitations of the rational method?
Use of the rational method includes the following assumptions and limitations: The method is applicable if t c for the drainage area is less than the duration of peak rainfall intensity. The calculated runoff is directly proportional to the rainfall intensity. Rainfall intensity is uniform throughout the duration of the storm.