Posted by: Geoff Jones on Jun 4, 2009
Tagged in: Water
Many people look to Rainwater Harvesting primarily due to the benefits it provides in helping to meet irrigation demand and reduce dependency on municipal or well water supplies primarly during the summer months when outdoor water use increases. It also allows people to continue to irrigate there plants, shrubs, gardens, vegetables etc. during drought when watering bans are in place. These are all great benefits that we can all realize by using rainwater harvesting but there is another benefit that is often overlooked that I want to touch on today just to make you aware of it.
Rainwater Harvesting can help to reduce and manage stormwater flows.
What is Stormwater?
Stormwater is basically the water that flows from parking lots, roof tops, driveways, roads, lawns, fields etc. that is not absorbed into the ground during a rainfall event. This stormwater is typically redirected through storm sewers back out to lakes and rivers. Although some people might assume it is, stormwater is not treated before it is discharged into these surface water areas.
The Increasing Issue of Stormwater
As density of development increases the permeable (water can penetrate it and be absorbed by it) surface area in our neighbourhoods is decreasing. Houses are being built closer and closer together on smaller and smaller lots, and much of the green space is being developed for industrial use (factories, storage), commercial properties (i.e. malls, offices etc.) and institutions. With each new development permeable surfaces like grass fields and forests are replaced by non-permeable surfaces like sidewalks, driveways, parking lots, and buildings themselves. Now, when it rains there is less and less permeable surface area to absorb this storm water so more and more of it is redirected into the storm sewers. In many areas it is the case in some years that rainfall events are occurring further apart, but the rainfall event itself is actually more intense (more rainfall in a shorter period of time). When this occurs stormwater flows can increase significantly. Since water can only permeate the ground at a certain rate if it rains really hard in a very short period of time much of the water that could permeate into the ground during a light drizzle doesn’t. It runs off and ends up in the storm sewers because the ground cannot absorb it fast enough. The longer the period of time, as well as the more intense the heat between rainfall events, and subsequently the dryer the ground, the less effective the ground is at absorbing the rainfall.
If I look at my home for example; our backyard is graded such that the water flows toward our home and is directed into swales that channel the water between the houses and out onto the sidewalk. During a heavy rainfall there is a significant amount of water that is running between the houses and onto the road, into the storm sewers. This water running between the houses is a combination of water collected from the roof and then captured in your eaves trough, as well as water that is not absorbed that is running off the backyard and front yard lawn.
How Does Rainwater Harvesting Help Reduce Stormwater?
Well, it’s pretty simple actually. Let’s say you have a rainwater harvesting system at your home that collects the rainwater from your roof. If that system can store 1000L of water and you use it regularly, during the next rainfall event some portion, maybe all, of the water off your roof will be captured in your rainwater system (depending on how much rainfall and how much capacity is available in your storage tanks). The rainwater system essentially reduces the stormwater flows from your roof by the amount of available storage the system has at the time it rains. If you never use your rainwater during periods of drought and the tanks are always full, obviously during the next rainfall event the system will not actually reduce stormwater flows at all since it’s already full. However, in most cases we have the rainwater system to allow us to water between rainfall events at a more event rate so that it can be absorbed, and as such there should be storage capacity available at most rainfall events.
Why is Stormwater Management Important?
Managing stormwater is important for many reasons so let’s look a just a few very quickly. Excessive stormwater can cause an overload on the storm sewers which could result in flooding if there is insufficient drainage due to heavy rainfall events. I’m sure many of you can remember driving down the street or in a parking lot in 4-6 inches of stormwater over the past several years. Water that does go through the storm sewers is discharged into our lakes and rivers and can contribute to high water levels and flooding around these areas. Heavy stormwater flows can also cause excessive erosion and increased sediment which have a significant impact on our natural ecosystems. In addition, stormwater can also pick up all sorts of chemicals and contaminants from roads and driveways (oil, grease etc.) and take them into our lakes and rivers and affect the quality of the water in these environments. This is one of the primary reasons that pesticide bans are being put in place. It protects our ground and surface water supplies as well as the environment and ecosystems that are affected when these chemicals are washed off our lawns and ultimately into our water supplies.
There are many other technologies and approaches that can be used to help manage stormwater (i.e. green roof, naturescaping etc.) which we won’t go into right now. However, you might now be surprised to realize that your rainwater harvesting systems not only provide you with water for your gardens to help meet your irrigation needs, but they also help manage stormwater and thus help to protect our lakes, rivers, drinking water, ecosystems and wildlife that are impacted by stormwater.