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Calcimator

Drainage Swale Calculator

Design a drainage swale using Manning's equation to determine channel dimensions for a target flow rate.

Inputs

cfs
%
:1

Results

Bottom Width

2 ft

≈ 7 credit cards

Swale Depth

0.31 ft

Flow Velocity2.26 ft/s
Cross-Section Area0.91 sq ft
Top Width3.9 ft
How to Use This Calculator
  1. Enter the drainage area in acres that drains to the swale.
  2. Set the design storm rainfall intensity in inches per hour and runoff coefficient for your surface type.
  3. Input the desired swale slope in percent and Manning's n for the channel material.
  4. Review the peak flow (CFS), required channel cross-section, and recommended dimensions.
  5. Verify velocity stays within non-erosive limits (< 3 ft/s for bare soil, < 6 ft/s for grass-lined).

What each input means

Design Flow Rate
Required flow rate in cubic feet per second from stormwater calculations.
Channel Slope
Longitudinal slope of the swale in percent.
Side Slope (H:V)
Horizontal to vertical ratio of side slopes (3:1 recommended for mowing, 4:1 for safety).
Manning's n
Manning's roughness coefficient (grass ~0.035, bare soil ~0.02, rip-rap ~0.04).

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Design Flow Rate = 2, Channel Slope = 2, Side Slope (H:V) = 3, Manning's n = 0.035 = 4 input(s) provided
  2. Calculate Bottom Width
    Bottom Width
    2 = 2
  3. Calculate Swale Depth
    0.31 = 0.31
  4. Calculate Flow Velocity
    Flow Velocity
    2.26 = 2.26
  5. Calculate Cross-Section Area
    0.91 = 0.91

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