How to Do a Stormwater Management Plan?
As the climate change problem becomes one of the biggest problems of the era, more responsibilities pile on engineers and geoscientists.
As we all know, engineers are responsible for both a primary and a secondary goal. The primary goal for every single engineering design in the world is the public's well-being and safety. The second goal is to make sure that the design provides a high level of serviceability and durability. Just like in every single design, stormwater management also required implementing the two goals of providing a safe and sustainable design that resulted from careful strategic thinking.
What is Stormwater Management?
According to the United States Environmental Protection Agency (EPA), “Stormwater management refers to the practice of controlling and directing the flow of stormwater runoff to minimize its negative impacts on the environment and infrastructure. It involves the design, implementation, and maintenance of systems and strategies to manage stormwater, including its collection, storage, treatment, and release.”
It‘s worth mentioning that stormwater management does not include the drainage of water alone. It requires providing economical, efficient, and environmentally friendly solutions to anticipated future problems.
What Are the Main Design Criteria for the Stormwater Management Plan?
The ultimate goal of stormwater management is to make sure that new development will not affect the environment by releasing additional runoff that exceeds the capabilities of existing natural and municipal drainage systems. Overwhelming such systems may cause flooding in areas that directly present a high risk to the public's health and well being.
In order to fulfill this criterion, we need to make sure that we provide suitable stormwater devices to control the flow rate so that the generated peak flow rate after development is less than or equal to the existing generated flow rate.
For example, if we consider an existing small lot that is found to generate 100 L/S of runoff before development. After the development, as a result of increasing impermeable surfaces. Runoff will increase to 150 L/S. Therefore, engineers required to find solution to reduce new generated runoff by 50 L/S so that post development rate is less or equals to 100 L/S.
The solution to the aforementioned design problem can vary depending on many factors, such as the type of soil, geographic location, rainfall intensity, and volume generated due to development.
According to opentextbc.ca ,
“There are four basic ways to handle a risk.
Avoid: The best thing you can do with a risk is avoid it. If you can prevent it from happening, it definitely won’t hurt your project. The easiest way to avoid this risk is to walk away from the cliff, but that may not be an option on this project.
Mitigate: If you can’t avoid the risk, you can mitigate it. This means taking some sort of action that will cause it to do as little damage to your project as possible.
Transfer: One effective way to deal with a risk is to pay someone else to accept it for you. The most common way to do this is to buy insurance.
Accept: When you can’t avoid, mitigate, or transfer a risk, then you have to accept it. But even when you accept a risk, at least you’ve looked at the alternatives and you know what will happen if it occurs. If you can’t avoid the risk, and there’s nothing you can do to reduce its impact, then accepting it is your only choice.”
However, in stormwater management, we mostly handle risk by two ways (Avoidance and Mitigation)
Avoidance Solution
This includes providing a green and sustainable design that reduces runoff. In this way, we may not have to do anything on the site to mitigate the problem if we can simply avoid it!
- Permeable Paver
Instead of using typical impermeable asphalt pavement, we can use permeable pavers, which are designed in a way that allows water to infiltrate. This will contribute to reducing runoff and, therefore, decreasing the post-development runoff flowrate.
2. Green Roofs
As we are looking to be more sustainable in our designs, we can use green roofs, which significantly reduce runoff, reduce carbon, and are better looking. However, there are always downsides to innovations; here, for example, having a green roof may not always be a wise solution to go with. Especially since we have more than one factor to consider during the design phase, such as the maintenance cost and the durability of the design.
Mitigation Solution
This is the most commonly used solution to mitigate the design problem. This includes using stormwater infrastructure that detains or retains water to provide an acceptable flow rate.
- Retention Pond
As a result of limiting the runoff released to municipal systems, water will accumulate in retention areas, so-called ponds. Engineers are required to design retention systems to hold water during that time. It could be a dry temporary retention pond or a wet permanent retention pond. Both have cons and pros and choosing the right one depends on various factors.
2. Underground Chambers
This is the cheapest and most reliable solution. However, the small holding capacity makes it unsuitable for big projects. Clear stone chambers hold a maximum of 40% of their overall volume. This comes from the fact that the maximum void ratio of the clear stone is 40%.
3. Infiltration Devices
This includes using innovative ways to enhance infiltration so runoff infiltrates through the soil and, therefore, the retained volume decreases.
There are also more innovative ways that can be used to reduce the impact of new development on the environment. I mentioned some of the most commonly used methods to give a big picture of storm design and the problems that engineers face during design.
This was published based on scientific resources and mostly on my experience as a civil engineering designer.
