Calculate the Cross-Sectional Area (A) of the Stream Channel

• 1). Calculate the cross-sectional area (A) of a straight stretch of stream channel by multiplying the length (in feet) by the width (in feet) to determine the area in square feet. For natural channels with irregular cross-sections, use geometric equations to calculate the area in square feet.
  
  
• 2). Repeat the calculation of cross-sectional area along the river if necessary to determine the bankfull discharge at multiple locations.
  
  
• 3). Enter the cross-sectional areas into a spreadsheet to make subsequent calculations easier.
  
  

Calculate the Velocity in the Stream Channel

• 1). Calculate the hydraulic radius (R) by dividing the cross-sectional area (A) by the wetted perimeter. The wetted perimeter is the distance from the waterline at one bank, down the side of the channel, across the stream bottom and up the other bank to the waterline.
  
  
• 2). Measure and calculate the slope (S) or gradient of the bottom of the channel by dividing the change in elevation by the horizontal length along the channel.
  
  
• 3). Calculate the channel-bottom roughness coefficient (n). This number can vary from as low as 0.011 for plane alluvial channels with no vegetation to 0.070 for channels that are weedy and winding with deep pools. Clean winding channels with a few pools and ripples and mountain streams with beds of gravel or cobbles have roughness values of 0.040.
  
  
• 4). Calculate the velocity (V) of the water flow in feet per second by using the Manning equation:
  
  V = [1.00] / [ (n) * (R) to the 2/3 power * (S) to the 1/2 power].
  
  
• 5). Repeat calculations of velocity as necessary at different locations along the stream reach, if you need more than one bankfull discharge calculation.
  
  

Calculate the Bankfull Discharge (Q)

• 1). Calculate the bankfull discharge by multiplying the cross-sectional area (A) by the velocity (V):
  
  Q = A * V
  
  
• 2). Convert units if necessary to ensure the flow is in cubic feet per second:
  
  Q = (A in square feet) * (V in feet/second)
  
  Q = cubic feet per second, or cfs
  
  
• 3). Report the bankfull discharge for the location along the stream where you measured the cross-sectional area. Bankfull discharge depends on the cross-sectional area; for large planning projects, you must calculate bankfull discharge along multiple sections of stream channel.