To determine stormwater volume, we need to figure out how much water will flow during a rainstorm. Imagine you have a big bucket outside, and when it rains, water collects in the bucket. We want to know how much water fills up the bucket.

To calculate stormwater volume, we typically use the Rational Method equation: Q = CiA, where Q is the peak flow rate in cubic feet per second (cfs), C is the runoff coefficient, i is the rainfall intensity in inches per hour (in/hr), and A is the drainage area in acres. This equation estimates the stormwater volume based on the characteristics of the area and rainfall data.

First, we need to know how big the area is where the water collects. It could be your yard, a parking lot, or a field. We measure this area in acres or square feet. This number is “A” in our equation.

Next, we think about the rain. How hard is it raining? We measure this as rainfall intensity, which tells us how much rain falls in a certain time, like inches per hour. This number is “i” in our equation.

Now, every area is different. Some areas let more rain soak into the ground, while others don't absorb as much water. We call this the runoff coefficient. There’s are reference tables for coefficient rates. It tells us how much of the rain turns into runoff and flows away instead of soaking into the ground. To simplify, a parking lot would be .9 because most of the water is runoff; where a grassy area’s rate would be .3 because most of the water is absorbed. This number is “C” in our equation.

To calculate the stormwater volume, we multiply three things: the runoff coefficient, the rainfall intensity, and the size of the area. This will give us an estimate of how much water will flow or runoff during the rainstorm.

To calculate the stormwater volume for a 1-acre grassy field receiving 0.5 inches of rain in one hour, we'll need to consider the runoff coefficient for a grassy surface.

Let's assume that for a grassy field, the typical runoff coefficient is 0.3 .

First, we convert the acreage to square feet since the runoff coefficient is typically used with square feet:

1 acre = 43,560 square feet.

Next, we multiply the rainfall intensity (0.5 inches) by the area (43,560 square feet) and the runoff coefficient (0.3):

Stormwater volume = 0.5 inches * 43,560 square feet * 0.3 = 6,534 cubic feet.

Therefore, the estimated stormwater volume for a 1-acre grassy field receiving 0.5 inches of rain in one hour would be approximately 6,534 cubic feet, or 48,877.71 gallons. That’s a lot of water.

By figuring out the stormwater volume, we can understand how much water we need to manage and plan for things like drainage systems, pipes, and ponds. It helps us prevent flooding and take care of the environment by keeping the water clean.

Remember, the stormwater volume changes depending on the size of the area, how hard it's raining, and how much water runs off. By studying and calculating these things, we can be better prepared and take care of our surroundings when it rains.