Stormwater Infrastructure for Water Quality

Stormwater Infrastructure helps to convey rainwater, prevent flooding of homes, roadways, all while protecting water quality. Many factors can impact water quality. In this article we will look at water temperature and how stormwater infrastructure can help maintain healthy streams and lakes.
As areas become more developed there is an increase in impervious surface area. On a sunny day paved surfaces can transfer heat to the air above them and to stormwater as it flows over. The surface temperature of a parking lot can exceed 140°F on a clear summer day. This is almost hot enough to fry an egg. During a rainstorm, pavement also allows water to flow unimpeded into the storm drains. Water flowing off hot pavement becomes warmer and can cause thermal stress to organisms living in the receiving stream.

Fish and macroinvertebrates, tiny organisms which make up the base of the aquatic food chain, have a temperature range that they are well adapted to. Think of trout living in a cool mountain stream. Fish have adapted over many thousands of years to the environments in which they live. Increasing the temperature of a stream over time or subjecting the stream to sudden fluctuations in temperature will stress the fish populations in that stream. Some heat tolerant species will survive while less tolerant species will see a drop in numbers. There are many reasons for this, one of which is dissolved oxygen. Colder water can hold more oxygen which fish breath through their gills. Warmer water holds less oxygen. While our native fish populations do worse with increased stream temperatures, certain algae species can do well in warmer waters. These algae will bloom, creating additional stress on the fish since they can deplete oxygen in the water. To further complicate things, urbanization typically results in less tree canopy around streams resulting in less shade and higher water temperatures.

​Stormwater controls can help to reduce thermal stress on fish and other aquatic organisms. Grassed swales can slow down water and help it to infiltrate. Establishing clearing limits around streams helps to maintain a filter buffer or better yet, a tree canopy around the streams.
How can stormwater infrastructure help to maintain good water quality. One piece of infrastructure that can make a big difference is the basin.

Figure 1 General layout of a stormwater retention basin, post construction

In our example of hot water flowing off a parking lot, the basin slows the flow down giving the water time to cool to safer levels. Once in the basin heat transfer can occur between the water and the basin. Some transfer also takes place between the water and the air. The rate of heat loss is determined by the “specific heat” of water. The specific heat it is the amount of energy needed to change the temperature of water by 1 degree. A significant amount of energy needs removed to reduce the temperature of water. We can’t change the air temperature or how much rainfall is received.  What can be influenced is how long it takes water to flow through the basin. This is done by oversizing the basin and by installing the water quality orifice when converting the outlet structure over to post construction design. A larger basin will have more storage, increasing the water quality storage volume. The orifice plate or restriction plate is made of stainless steel and restricts the volume of water that can leave the basin in a given time. The person installing the plate should consult the Storm Water Pollution Prevention Plan (SWPPP) for which size orifice plate to install. Properly designed and maintained stormwater infrastructure helps make our rivers and streams a little cooler.

For more information, contact our office at 513.695.1337