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Rational Method Formula:
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The Rational Method is a widely used empirical formula for estimating peak discharge from small drainage areas. It relates rainfall intensity, catchment area, and runoff characteristics to predict maximum flow rates for hydraulic design.
The calculator uses the Rational Method formula:
Where:
Explanation: The method assumes that peak discharge occurs when the entire catchment is contributing to runoff and rainfall intensity remains constant during the time of concentration.
Details: Accurate peak discharge estimation is essential for designing drainage systems, culverts, bridges, and flood control structures to prevent overflow and flooding.
Tips: Enter runoff coefficient (0-1), rainfall intensity in mm/h, and catchment area in hectares. All values must be positive numbers within reasonable ranges.
Q1: What is the typical range for runoff coefficient C?
A: Runoff coefficients range from 0.05-0.95 depending on surface type: paved areas (0.7-0.95), lawns (0.05-0.35), forests (0.05-0.25).
Q2: How is rainfall intensity determined?
A: Rainfall intensity is typically obtained from intensity-duration-frequency (IDF) curves based on local rainfall data and return period requirements.
Q3: What are the limitations of the Rational Method?
A: Best suited for small catchments (< 200 ha), assumes uniform rainfall distribution, and doesn't account for storage effects or complex hydrology.
Q4: How is catchment area measured?
A: Catchment area is determined through topographic mapping, identifying all land that drains to the point of interest through surface flow paths.
Q5: What are common applications of this method?
A: Urban drainage design, small bridge and culvert sizing, stormwater management planning, and preliminary flood assessment studies.