How to Calculate Catchment Area for Drainage

Do you want to know the method of how calculating the catchment area for drainage? If so, you’re in the right place. In this tutorial article, L’ll actually a real-world problem from the field of water resources to help you get a better understanding of catchment area and the method of its calculation specifically for drainage.

In a watershed area, the net precipitation after deducting infiltration was found to be 3.5 inches. The main outlet, located at the furthest end of the watershed area, had 51,560 cubic feet of water passing through it during a storm. The task is to find the total acreage of the drainage.

Following are the steps through which you can calculate the catchment area for drainage:

  1. To solve this problem, we’ll be using the Civil Engineering Reference Manual. Specifically, we’ll be using Chapter 20, Equation 20.21 for the unit hydrograph. This equation states that the runoff (V) equals the drainage area times the net precipitation.
equation for the Catchment Area for Drainage
The equation for the Catchment Area for Drainage
  1. We’re given the runoff (V) as 51,560 cubic feet, the net precipitation (P) as 3.5 inches, and we need to find the area (A). We’ll start by converting the net precipitation from inches to feet. Using the equation, we find that the area of the watershed is 176,077.7 square feet. Plugging in the Numbers and Converting to Acres.
  1. The question is asking for the total acreage of the watershed, so we need to convert from square feet to acres. There are 43,560 square feet in one acre, and when we divide the total square feet by 43,560, we get 4.06 acres.
  2. Here is the complete solution in picture format:
How to Calculate Catchment Area for Drainage
Practical Example solution on How to Calculate Catchment Area for Drainage

Final Thoughts

This was a great example of a hydrology problem that you might encounter on the different civil engineering exams like the Professional Engineer (PE) exam. At Cool Civil Engineering, we have a wealth of resources to help you prepare for the exam, including more hydrology problems and tips for success. Hit us up for more information and let’s get you one step closer to your PE license.

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Also, Learn: How to Find the Catchment Area of a River and How To Calculate the Catchment Area Easily

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