Retention ponds why do we need them? My sub-division has two. Can’t the water just get drained to the river?
In: Engineering
No, the subdivision has a lot of hard surfaces that weren’t there before. More runoff is generated in the developed area than the undeveloped one. In order to reduce the potential for flooding downstream retention and detention features are developed to release the water slowly to prevent overwhelming local waterways.
In addition if the hold water most of the year it can help firefighting efforts to have accessible surface water in the case of wildfires and earthquakes.
Another point to add on to the other correct answers:
Why does your subdivision have *two* retention ponds?
It’s probably newer.
When cities expand outwards (generally in suburban sprawl) it usually goes further away from the river. You build the stormwater drain for your new subdivision and drain it where? *To the previous subdivision*
This means that as you keep expanding outwards, the existing stormwater pipes keep getting more and more drainage area added to them, and these usually end at the oldest parts of the system which may not have been designed to handle so much water.
So as subdivisions get further and further out, they need to start dedicating more and more land for retention, since it’s constantly adding water to an increasingly overloaded system. Usually the retention is cheaper than replacing the entire downstream drainage pipe with a bigger one.
Retention ponds help with pollution as well as cooling the water.
All of the storm sewers in your development lead to your retention ponds. When it rains (or snow melts), small rocks, road salt/ash, oil from the asphalt, oil and antifreeze that may have leaked from vehicles, trash are all washed down into the storm sewer the retention ponds collects most of this pollution and prevents it from entering nearby creeks and streams.
Have you ever noticed after a summer rainstorm steam rising from the road and driveways. This is because the 70 degree rainwater is hitting the hot asphalt and some of it evaporates. What doesn’t evaporate is heated and flows into the storm sewer. The retention ponds allows the hot water coming off the roads to cool down so the hot water doesn’t go directly to the creeks and streams and kill fish and temperature sensitive organisms.
Rivers work best when they’re surrounded by lots of plants. Rain falls and plants and soil soak up most of the rain and only after a large amount is absorbed does excess rain flow directly into the river.
People like to live in homes and drive and that means making lots of waterproof things (roads, roofs, etc). When rain falls on these very little is soaked up and most of the water flows into a gutter and then stream or/then a river.
That means if a river only started getting water from rainstorms that dropped more than 3 cm or an inch of water, now it might get half the rain that falls on a large city plus any rain that falls over that amount. That’s a lot more water for the river to carry which means it floods far more than it did previously. This tends to make the people downstream upset.
To cut down on how much water they discharge, more areas have begun to require retention ponds to act more like the undisturbed area did (absorbing some of the water that the buildings and paved surface are shedding).
Retention ponds keep the water in the area, so plants can drink and the soil stays wet and strong.
If all the water flows away immediately, all the plants would dry out if it didnt rain for a week or two
Two things:
Pollution – there’s often plants or other filters to catch pollutants from getting into the main watercourses kind of like a coffee filter.
Flood Prevention – They drain slowly, preventing water infrastructure from being overwhelmed by sudden water run-off.
Can also be for fire prevention. If an area has retension ponds it does not need fire hydrants because they can use the water in the pond.
Aquatics Tech here. Retention and detention ponds main function is to slow down water entering the drainage system. This is to prevent flooding and erosion. A pond should have a functioning forebay at every inlet. This is like a tiny pond for the main pond. The forebays are meant to trap debris, heavy metals, and sediment. This way one small spot can be dredged instead of the whole pond.
There is also a “detention” pond.
At a super high level, a “retention” pond should always have some level of water in it.
A “detention” pond is meant to eventually dry up between the rains.
These two types filter and dispense with the water in different ways.
[https://www.budatx.gov/ArchiveCenter/ViewFile/Item/858](https://www.budatx.gov/ArchiveCenter/ViewFile/Item/858)
During a storm, the peak water levels can last as little as 10 minutes. If you can reduce that peak even a little, then flooding downstream can be prevented.
Where I live, they encourage everyone to have a small 100L water butt attached to domestic roofs, just to help a little bit. If enough people do this, neighbours could be saved from flooding.
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