How To Calculate The Wetted Perimeter

Wetted perimeter is one measurement used to analyze river and streambeds. It is the total linear distance in a cross section of the river or stream that is in contact with the water. Measuring wetted perimeter is easy if the bed is flat and smooth, like a concrete drainage channel, but river and streambeds rarely are. The complex contours require a series of depth measurements that are then used to calculate the wetted perimeter. The more measurements taken for a given cross section, the more accurate the estimate will be.

    Attach a string to the top of the lower bank of the stream. Stretch the string across the stream perpendicularly and attach it to the other bank. The string must be horizontal, so use the level to check and correct. If you are taking measurements in a deep river or lake, you will probably be using a boat and a depth finder rather than a string and a ruler.

    Take depth measurements at equal intervals along the string. It will be easier if you have a partner recording the data as you take the measurements. For deep water, you will need a precision navigation system to guide you as you take measurements with the depth finder.

    Calculate the wetted perimeter of each interval. If the depth at one side of the interval (D1) was 2 feet, the depth on the other side (D2) was 4 feet and the interval (W) was 6 inches, the calculation would be: P = SQRT((D2-D1)^2 + W^2) P = SQRT((4 - 2)^2 + 0.5^2) Note: 6 inches = 0.5 feet P = SQRT(4.25) = 2.06 feet

    Add the wetted perimeters for all the intervals to get the total wetted perimeter for that cross section.


    • If the bed is rough and bumpy, you will need more measurements at closer intervals to get an accurate result.


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