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The Peak Discharge from Drainage Basin Runoff calculator computes discharge rate of a drainage basis based on the runoff coefficient, average rainfall intensity and the size of the drainage area.
INSTRUCTIONS: Choose units and enter the following:
- (C) Runoff Coefficient (see table below)
- (I) Average Rainfall Intensity
- (A) Drainage Area
Peak Discharge from Drainage Basin Runoff (PD): The discharge rate is returned in cubic meters per second and cubic feet per second. However, these can be automatically converted to compatible units via the pull-down menu.
The Math / Science
The Peak Discharge from Drainage Basin Runoff is found using the Rational Method:
`Q=C*I*A`
where:
- Q = Peak Discharge Rate
- C = Runoff Coefficient
- I = Average Rainfall Intensity
- A = Drainage Area
| Land Cover Type | Runoff Coefficient (C) |
|---|---|
| Pavement / Concrete | 0.70 - 0.95 |
| Urban Areas (high density) | 0.5 - 0.70 |
| Lawns (sandy soil, flat) | 0.05 - 0.15 |
| Lawns (clay soil, steep) | 0.25 - 0.35 |
| Agricultural Land | 0.10 - 0.50 |
| Bare Soil | 0.30 - 0.60 |
| Gravel Areas | 0.30 - 0.70 |
Use the Rainfall Intensity Calculator if you need `I`.
The Rational Method estimates peak discharges for drainage areas of up to about 200 acres (80 hectares) and no significant flood storage. Although the Rational Method estimates the peak discharge, it cannot estimate a flow volume or time series of flow.
Assumptions under the Rational Method: (If any one of these conditions is not true, the user should try an alternative method.)
- tc for the drainage area must be less than the duration of peak rainfall intensity
- Runoff is directly proportional to rainfall intensity
- Rainfall intensity is uniform throughout the storm
- Rainfall is distributed evenly over the drainage area
- Rational Method does not account for storage
- Available storage is assumed to be filled
- Frequency of occurrence for the peak discharge is equal to the frequency of the rainfall producing that event
- Minimum duration of rainfall intensity is 10 minutes
- If the time of concentration (tc) computed is less than 10 minutes, then 10 minutes should be adopted for rainfall intensity computations
| Type of snow or ice | (kg/m3) |
| Fresh New snow | 50-70 |
| Damp new snow | 100-200 |
| Settled snow | 200-300 |
| Depth hoar | 100-300 |
| Wind packed snow | 350-400 |
| Firn (granular) | 400-830 |
| Very wet | 700-800 |
| Glacier ice | 830-917 |
| Rain/Melt | 997 |
Hydrology (Water, Rain and Snow) Calculators:
- Snow Pack to Liquid Water Calculator
- Volume of water in snow or rain
- Weight/Mass of Snow on an Area.
- Weight of Snow on a Rectangular Area.
- Weight of Snow on a Roof.
- Weight of Snow on a Polygon Shaped Area.
- Volume of Water in a Rainfall
- Flow Rate of Broad Crested Weir
- Water Capillary Rise.
- Volume of Flood Water.
- Number of sandbags needed to build a wall or dike.
- Water Saturation Ratio
- Rain Fall Harvesting Calculator
- Peak Discharge from Drainage Basin Runoff
- Gauckler-Manning Equation
- Cipolletti Weir Flow Rate
References
Online Manuals (http://onlinemanuals.txdot.gov/txdotmanuals/hyd/rational_method.htm)
Geography AS Notes (https://geographyas.info/rivers/discharge-and-hydrographs/)
