How to Calculate Water Flow for Culverts

Culvert flow may vary throughout the culvert due to rocks and other obstructions.

Determining the water flow for a culvert enables a land-owner to determine the approximate amount of water pushed through a culvert at any time. Operators of hydraulic energy generators or pumps running off the culvert utilize this information to estimate their peak, average, and minimal energy generation. Calculating the water flow for culverts also facilitates wetland and wild-land restoration efforts by enabling conservationists to determine if culvert modifications are necessary.

Things You'll Need

• Stopwatch
• Small floating object
• Rope
• Tape measure

Instructions

1. Measure a length along the culvert, drop a floating object at one event of the culvert, and utilize the stop watch to time how long it takes to reach the other end of the span you have measured.

2. Divide the length chosen along the culvert by the time for the floating object to travel from one end of the culvert to the other end to determine the velocity of the culvert.

3. Tie a rope from one size of the culvert to the other and mark off each foot.

4. Utilize a tape measure to measure the depth of the culvert at each one foot interval and average all measures to estimate the average depth of the culvert.

5. Measure the width of the culvert.

6. Multiple the velocity by the average depth of the culvert by the width of the culvert to determine the water flow for the culvert.

Tips & Warnings

• Repeat the assessment of the velocity several times and average the results for increased accuracy.

• Utilize intervals of less than one foot for depth measurements to increase accuracy.

• Divide the culvert into smaller areas and calculate the velocity and volume for each area independently for more accurate flow calculations.

References

• Worsleys School: Calculating the Flow of Water in a Culvert
• Bradshaw's Pond: How to Calculate Water Volumes and Flow Rates in Different Units
• Oregon: Step by Step "Cookbook" for Culverts Placed Flat and for Embedded Sunken Culverts

Resources

• United States Geologic Survey: Investigation Report 97-4037