# Storm Water Drainage System Model

Question: How would you use PIPE-FLO to model the rainfall runoff for a storm water drainage system?

**Answer:**

PIPE-FLO can be used to model certain portions of a storm water drainage system. One of the key assumptions used in PIPE-FLO is that the pipeline being modeled is completely full of water or "fully charged." This is not always the case in portions of a typical storm water drainage system, depending on the amount of rainfall the system is designed to handle.

Rainfall intensity, duration, and distribution are all factors in sizing storm water systems. When it rains, the storm water can go into the ground, infiltration, or become runoff. Assuming the pipes are 100% full, the runoff can be modeled in PIPE-FLO Professional.

The first step is converting the rainfall into a volumetric flow rate in gallons per minute. This is done by multiplying the area of the ground (or collection surface) by the rate of the rainfall.

For example:

Area: 100 ft x 100 ft = 10,000 ft^{2}

Rate: 0.1 {\frac{in}{min}} (0.0083 {\frac{ft}{min}})

Next: Multiply the area times the rate to get the volumetric flow rate in cubic feet per minute

10,000 ft^{2} x .0083 {\frac{ft}{min}}= 83.3 {\frac{ft^3}{min}}

Then convert to gpm:

83.3 {\frac{ft^3}{min}} x 7.48 {\frac{gallons}{ft^3}} = 623.3 gpm

So, 0.1 inch per minute of rain over a 10,000 square foot area, equals 623.3 gallons per minute of flow. This assumes either the soil is saturated or the area is nonporous (none of the water is absorbed) and all the rainfall is captured by the storm water drainage system.

Once the rainfall is converted into a flow rate, PIPE-FLO Professional can be used to model the flow as a demand entering a system. A demand can be added at the beginning of a pipe. Shown below is the demand dialog box with 50 US gallons per minute flow entering the system.

An example PIPE-FLO model might look like the following:

This model shows the rainfall being simulated as three demands going into three pipes. The pipes drain into one sump tank and the sump tank is being emptied by a sump pump.