What is Flood Discharge?

Flood discharge, often referred to as peak runoff, is the volume of water that passes through a specific point in a drainage system per unit of time during a storm event. Calculating this value is essential for civil engineers, hydrologists, and urban planners to design safe infrastructure such as culverts, bridges, and storm sewers.

How to Use the Flood Discharge Calculator

This tool utilizes the Rational Method, one of the most widely accepted formulas for estimating peak discharge for small watersheds (typically under 200 acres or 80 hectares). To get an accurate result, follow these steps:

1. Select your unit system: Choose between Imperial or Metric units.
2. Drainage Area (A): Enter the total surface area that contributes water to the point of interest.
3. Rainfall Intensity (i): Enter the average rainfall rate for a duration equal to the time of concentration.
4. Runoff Coefficient (C): This is a dimensionless value representing the fraction of rainfall that becomes runoff. For example, asphalt has a high C value (0.90), while a grassy field has a lower value (0.20).

Understanding the Rational Method Formula

The core formula used by this calculator is Q = CiA. In the Imperial system, where A is in acres and i is in inches per hour, the result Q is conveniently expressed in cubic feet per second (cfs) because 1 acre-inch per hour is approximately equal to 1.008 cfs. In the Metric system, a conversion factor of 0.00278 is applied to ensure the output is in cubic meters per second (m³/s) when area is in hectares and intensity is in mm/hr.

Frequently Asked Questions (FAQs)

Why is the Runoff Coefficient important?

The Runoff Coefficient (C) accounts for the permeability of the surface. Heavily urbanized areas with concrete and rooftops prevent water from soaking into the ground, leading to higher discharge values and increased flood risk.

When should I not use the Rational Method?

The Rational Method is best for small, relatively uniform watersheds. For very large basins or areas with significant storage (like lakes or wetlands), more complex hydrologic models like TR-55 or HEC-HMS are recommended.