How large of a storm drain do you need? The simple answer is one large enough to take care of the runoff. But, how are you going to figure out how big that is? Your area may need small or large stormwater drains depending on a variety of factors.
Normal and Above Normal Rainfall
The first factor is to determine how much rain your area really gets. What is the average rainfall during a month? What is the largest reported rainfall in a day in your area? You can find out answers to these questions from the National Weather Service. If they do not have the statistics, they will point you to the right department.
You should be able to look at rainfall reports which give you daily breakdowns, record rainfalls, and other information for your general area.
Knowing how much rainfall the area gets will get you started on determining the size of storm drain you need for your area, but it is not the entire story.
The Lay Of The Land
While it is important to know about rainfall, it is even more important to know about the elevation in comparison to the surrounding area. Are you installing a storm drain at the bottom of a hill? If so, you are probably needing large stormwater drains, not small ones.
Look around the area for signs of runoff. Is the area going to receive runoff from surrounding fields and yards in addition to runoff from the road? The more runoff from the surrounding area, the larger the drain and underlying drainage system need to be.
What appears to be relatively flat areas should not lull you to sleep. Slight changes in elevation on flat areas can cause one stormwater drain to receive the bulk of the runoff, while other drains receive almost nothing. Pay close attention to changes in elevation.
Speed Of Rainwater Runoff
Another factor you need to pay close attention to is how fast will the runoff happen. Water moves slower through grassy yards and fields than it does on concrete and paved parking lots and roads. You need to plan the size of your drain to account for how fast the water will reach the drain.
A torrential downpour on a paved road can overwhelm a stormwater drain due to the speed the water runs to the drain.
You can estimate the size of your drain by looking at the amount of expected rainfall, the area that will drain into your stormwater drain, and its position relative to other drains. As a simple example, if you receive 2 inches of rain in an hour on a square 200 feet on each side that will drain into your storm drain, you will need a drain that can handle nearly 50,000 gallons of water per hour.
You can find calculators to help you determine the volume of water you can expect. Remember, it is better to have an oversized stormwater drain than one that cannot take care of the drainage and leads to street flooding.