Stormwater Runoff Management

Today with the increase in cities and urban growth, there are fewer places for rainwater to soak into the ground, causing more runoff in streams and rivers. This extra water that comes during storms or melting of snow and ice, can cause erosion as well as transporting the pollution of oil and trash to the rivers and eventually, the ocean. Without stormwater management, higher flood stages and lower water levels between rainfalls are becoming more common.

Historically, this has been a consideration in agriculture for thousands of years, while the majority of the urban runoff has been considered and handled as waste. Even though stormwater runoff has been recognized as a major factor contributing to poorer water quality in rivers, it has only had significant regulation in the US within the last 40 years. This leaves much of the existing infrastructure behind and many times they are in conflict with the new regulations.

Why we need stormwater runoff management

The old system mostly dealt with floodwaters and not stormwater runoff management and retention. Today the goal is to prevent as much runoff as possible and to give the water a place to soak into the ground right where it lands. This is done with many methods, the simplest being retention ponds that are designed into the property layout before it is developed.

The more complicated systems can be engineered vaults that are installed under parking lots, to make more efficient use of the real estate. These drain systems include areas to catch and store sediments and oils that are in the runoff water. These are periodically serviced by pumping out the sediment and replacing the oil catch material.

stormwater runoff management sediment and storage tanks

These accept and hold most rainfall, and only in the biggest downpours, will they spill over and release some of the water into nearby streams or storm drains.

Many areas around the world have shown improvement of groundwater levels and healthier soil and plant life with the water being directed back into the ground. The goal is to slow the water down like the trees and shrubs did when they were covering the landscape. This method has also has been shown to be effective in barren areas that seem to have died, when the landscape is bermed to slow the rainwater down, within 3-6 years, there is a marked increase in plant growth, bringing birds, bugs, and small animals back into these areas.

Similarly, these urban systems and ponds are not only used to retain the water but also provide habitats for plants, animals, and insects that may have been displaced by the adjacent construction.

What is required in stormwater runoff management

Commonly if one acre or more is disturbed with new construction, there needs to be appropriate runoff retention put in place. The amount of stormwater retained requirement varies state to state in the US, from only catching the first 1/2 inch of rainfall in Montana, to capturing 90% of the annual storms in Vermont.

Some areas also require that the water that leaves the property must be treated by screening, separation, and filtration. No two projects are the exact same, so the methods will also vary according to application and local regulations.

Screening is the first step to remove trash and large debris and hold it to be cleaned up later. Separation happens next and removes solids, grit, sand, and metals that might be present. Filtration is usually the last stage, and this is to capture oils and other pollutants.

stormwater filter

Screening is theoretically simple, but removing the trash must be done without causing a restriction and causing ponding or backing up of the system. Many have provisions to screen most of the normal rainfalls and only if there is an unusual amount of rainfall, there are bypasses to allow more water to flow past the filter and speed up the water evacuation to prevent flooding

Separation can be done by slowing the water down and allowing the solids to settle out or speeding the water up and using a vortex to get the debris to drop out, as the cleaner water is directed up and out. The filtration can be done using manmade filter material or natural sand and soil can be used depending on the pollutants that are in the incoming stormwater.

Interestingly, human hair has been found to be one of the best natural materials to soak up oil and has been used for over 40 years in oil spills in the ocean and across Texas, to prevent oil from entering the storm drains.

Using Human Hair to Soak Up Oil Spills

This video shows the underground storage and filtration of runoff water in a parking lot.

Maintenance of stormwater runoff systems

All of these systems must be periodically maintained and cleaned to operate correctly, and this responsibility falls on the landowner who may be federal or state government, commercial property, or a Homeowners association. Lack of care can cause the systems to be less or completely ineffective, resulting in flooding, erosion, and trash entering the waterways.

Retention ponds are designed to hold the water and let the sediments collect at the bottom. This can also include organic material like leaves, grass, and the remains of animals and birds that live in the area. This must all be cleaned out periodically, usually about every 5 – 20 years, to regain the original capacity and effectiveness of the pond.

The underground boxes and vortex collection points need much more frequent servicing to maintain their effectiveness, this includes removing the large garbage, the sediment, and replacing oil-soaked filters. In the more natural retention ponds, there is the maintenance of mowing and cleaning up trash, and making sure the outflow pipes and spillways are not obstructed by anything foreign. A very thorough article about the maintenance methods and standards is available here.

Stormwater Wet Pond and Wetland Management Guidebook, February 2009

Slowing the water down

Prevention of erosion is important and there is a myriad of solutions, the simplest being vegetation. The roots grow into a mat that holds the earth in place, and also cover the soil from the heat of the sun and the wind that could take the soil away.

It also can be accomplished by implementing swales or ditches that are lined with plants and rocks to slow the water flow down and filter out sediments and give the water more time to soak into the ground before it runs into streams and rivers. These can be beautiful additions to the landscaping and provide opportunities to include water features like meandering waterfalls or fountains.

stormwater runoff management swale

Other methods are used tires with the sidewalls cut off, laid and fastened together, and filled with rocks and gravel to form a strong base for roads and driveways. There are also many plastic mats and grids that will accomplish the same goal while requiring less fill gravel to be used. Water is heavy and powerful, so the goal is to constantly slow it down, and not let it gain momentum to erode the land.

Maintaining the health of standing water

By circulating the water in a reservoir with a fountain or a pump-driven waterfall, more oxygen is introduced into the water, organisms can break down the organic matter and the fertilizer runoff. This helps to keep the water clear and cool, because if it becomes stagnant and algae bloom starts, as they absorb the heat of the sun at the surface of the water, the temperature tends to rise, promoting more growth of the algae.

stormwater runoff management pond with fountain

One example is in 2011 in Lake Erie, where record torrential spring rains washed fertilizer into the lake, promoting the growth of microcystin-producing cyanobacteria blooms. More info on Lake Erie is available here. Record-setting algal bloom in Lake Erie.

While these blooms usually don’t last long, when they die off they leave significant amounts of organic material in the water. This material, when it decomposes, uses up the oxygen in the water, may times depleting it significantly and killing off other plants and fish that depend on the oxygenated water. This can be the cause of death for much of a system in a short amount of time.

Although stormwater entering retention systems is supposed to percolate into the ground within 72 hours, retention areas often remain wet for longer periods. Floodwater mosquitoes are normally the first to appear in retention areas. Overall, abundant populations of pests and disease-vectoring mosquitoes are much more frequently associated with retention systems than they are with detention systems.

In certain situations, mosquitoes must be controlled with larvicides. For a larvicide operation to be effective, it must be supported with a quality inspection program. More info on mosquitos and retention ponds is available here. Mosquitoes Associated with Stormwater Detention/Retention Areas

Changing roofing and drains to reduce runoff

Another method that is being adopted for reducing the runoff water, is collecting the water from roofs and using it for consumption, irrigation, or just flushing toilets.

“Rainwater is naturally soft and, in most areas, pH neutral to slightly acidic,” Mike Ruck explains. He is the vice president of the American Rainwater Catchment Systems Association (ARCSA) and chief water officer for Certified B Corp RainWater Solutions in Raleigh, NC. He went on to say, “The end use will determine the complexity of the system and how much filtration is required. For irrigation and other non-potable uses, filtration is generally very basic. For water to be consumed, it will need to be cleaned and filtered to a higher standard.”

What does Certified B Corp mean?

While this can cost more to install initially, many systems have been shown to quickly save money, and with more building projects striving to get LEED-certified, the practice is becoming more common. There are also systems that have been tested with good results, of misting roofs of homes and businesses, and working just like sweat on your body, as the water evaporates, it takes some of the heat with it. It has been proven in areas of low humidity, to drop roof temps up to 40 degrees, thereby taking some load off the air conditioner units and actually extending the life of the roof due to less temperature change.

stormwater runoff management for roofs

Taking the different roofs a step further are the rooftop gardens that are also gaining popularity. The buildings must be built to handle this extra weight, and the roofing and drain system built to shed excess water efficiently. This does the job of keeping the building cooler and provides a beautiful green area in the city.

Parking lot material

Parking lots, driveways, and roads cover a lot of ground and generally, the water has to run off the side to soak in. But there are ways of paving with materials that allow water to soak through. Concrete and asphalt can be made without adding sand and while it does affect the strength of the material, it makes it porous so the water can collect in the deep layer of stones under the asphalt, where it can slowly seep into the ground. Asphalt shingles can and are recycled to be reused in the production of roadways.

Why Recycle Asphalt Shingles?

While much of the porous concrete and asphalt is still in the early stages, pavers have been proven to work well too. Sometimes called grass pavers, they are sometimes X-shaped and allow the grass to grow up in each space, slowing the water from sheeting off during rain. also allowing water to soak into the ground. These solutions will not work everywhere as the power of water is destructive when it turns to ice and expands. Ice can easily break concrete and with freeze and thaw, it will open cracks up, bugger and bigger each time.

Water retention projects

One very interesting method of stormwater runoff management has taken hold in the northern part of India, where the increasingly dry springtime is making it much harder for the farmers to get their plants to grow. Later in the summer, they have water that melts from the glaciers high in the mountains that provides water almost the whole winter.

Using this available water, and the freezing conditions, one local has started to grow his own glaciers, that slowly melt in the springtime, providing water during the dry season. In 2013, using just gravity, a pipe with a sprinkler head, and dead tree branches, he made the first one 20 feet tall, and it held about 40,000 gallons of water in the form of ice. This ice, shaped like a cone, melted slowly in the spring providing water during the dry season.

Since then, many more of the ice cones, or Stupas, as they are called, have been constructed, 12 in 2019, 25 more in 2020, and more are planned.

Dams can also be used in the stormwater runoff management plan, but there must be enough room for all the water in a given storm season, or the extra will have run off the overflow. There are dams that are operated to adjust the water level according to the season and the given rainfall for the year. While the main goal for dams is electric production, they also help more water soak into the ground, and provide more consistent flow downstream.


While we have just touched the surface of the stormwater runoff management, there is a lot more to learn and we will cover more soon, but I hope you were intrigued to look up and learn more about it. Water affects all of our lives and we need to be conscious of the next generation that will live here and need the water too.

If you found this interesting, share it with a friend, and stay curious!

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