Area Method: Slope & Examples

Instructor: Tiffany Price

Tiffany has many years of classroom teaching experience and has a masters degree in Educational Leadership.

This lesson discusses the Slope Area Method to calculate the flow of a waterway at a particular point. There is more than one way to calculate this information, but the Slope Area Method has its advantages, especially for man-made waterways.

The Slope Area Method uses the slope of the water and the area of a cross-section to calculate the discharge, or amount of water that moves though a particular point of the waterway. This information is key for hydrologist and hydraulic engineers, who help design and build canals and other bodies of water.

Below is the equation, called the Manning Formula, to calculate the discharge using the Slope Area Method.

Q = (1.49/n)A(Rh^2/3)S^1/2

This equation looks a bit complicated but it's not so bad once we break it down.

Q = the discharge

A = is the area of the cross-section

S = the slope of the waterway

n = Mannings Roughness Coefficient

Rh = the hydraulic radius of the cross-section

Now, let's look at each of these components a bit closer, to help us understand how to use this formula to get the discharge.

A = the area of the cross section

First off, let's talk about what a cross-section is. A cross-section is the surface that would be exposed if one were to make a straight cut though something. Here is an example of a cross-section of a man-made canal and the area of that cross-section must be calculated. For simplicity, we will use the semicircle shape for the examples in this lesson, but other shaped channels are common as well, such as the trapezoidal, rectangular, and triangular.

Area of a semicircle = pi x R² ÷ 2

pi = 3.14

cross section

The next two parts, the slope and the Mannings Coefficient, both illustrate why this method of calculating the discharge of a body of water works best with man-made bodies of water, not natural rivers, streams, channels, lakes, etc. Let me explain what I mean:

In order for the formula to work, certain assumptions must be made about the characteristics of the body of water that are not realistic for natural channels, such as the water flow is assumed to be even throughout and the lining at the bottom of the water is assumed to be uniform throughout. In natural bodies of water, many factors effect the flow of water and the bottom of he channel, such as plant life, fish, natural obstructions, etc.

S = the slope of the water

Those setting out to calculate the discharge of a particular channel of water, must find a uniform reach of the channel, where the water flow is constant in every way, meaning the volume of water flow is constant and free of any obstructions. It would be quite difficult to calculate the slope (rise over run) if these conditions are not met.


n = the Manning Roughness Co-efficient

This number is a constant based on the material(s) used as the lining or bottom of the waterway, such as concrete, bricks, or lead. Below is a chart that you'd use to find the value of n. This is a not a comprehensive list of all materials but a shortened table for simplicity's sake. An extensive table can be found in hydraulics textooks and government websites.


Rh = the hydraulic radius of the cross-section

Here is the simplest formula for the hydraulic radius of a semicircle.

Rh = Diameter/4

As a summary, let's do a couple of examples together.

Example 1:

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