Retention ponds why do we need them? My sub-division has two. Can’t the water just get drained to the river?
In: Engineering
Houses and streets don’t absorb as much water as trees, grass, bare dirt, whatever. So when it rains more water washes downhill from a subdivision than a natural area.
That higher flow causes erosion. To stop it, the water is sent into a detention pond and is allowed to drain at a rate similar to what it would have drained before construction.
To replenish ground water and prevent flooding during heavy rains. Modern urban infrastructure is not very permeable, so we use artificial structures like retention ponds to emulate natural landscapes where marshes and other wetlands served a similar purpose.
I’m a geologist, so this is technically not my area of expertise, but I had a conversation with a hydrogeologist once where this came up.
Retention ponds drain slowly. If a large amount of rain falls in an area, how quickly it ends up in creeks/bayous/rivers is a big factor in how bad an area floods.
If you just have a ton of impermeable concrete channels and pipes that instantly direct all the rain into the waterways the flood level will peak quickly and catastrophically. However, if you have retention ponds that allow large amounts of rain to absorb into the ground or otherwise drain slowly it reduces the strain on the creeks and rivers.
This was how it was explained to me. Perhaps someone with more knowledge can weigh in.
I’m a civil engineer and design stormwater retention and detention facilities. They serve a few functions, the main ones being treatment of runoff for pollutants, and to control the flow rate leaving a site to match the pre-developed conditions. Most municipalities have some requirements to match the runoff flow rate that the site produced prior to development. To do this you need to hold on to the water and releases it slowly. This usually takes a lot of storage volume, surface ponds are the most cost effective way to do this, but sometimes if a project doesn’t have the real estate for surface ponds, underground vaults, or gravel filled trenches are used.
TLDR: retention/detention ponds help prevent worsening of flooding as a watershed is urbanized and developed.
Okay, does this pond ALWAYS have water in it? Or does it only fill up during a storm?
If it always has water in it, then yea, it is a retention pond, but it cannot simply be drained because that is the normal groundwater. Retention ponds can lower the ground water of a subdivision slightly, but that isn’t just water stuck there, that is the naturally occurring groundwater in that area. So it can’t totally be drained.
Unlike a DEtention pond, which is dry/mostly dry besides during and after rainstorms, where it fills up and holds onto the water for a while before slowly draining into the storm water system, to help from keep the storm water system from overflowing.
When you change the water properties of a piece of land, the water that was flowing off the land needs to be the same as before. You cannot increase the amount of water that leaves the site.
When you develop land, you and to make it less water absorbent so you need retention ponds to retain the water that would have run off.
What if all the water gets drained into the river at once uncontrolled? What if the river gets full? We call that a flood.
The retention pond allows the water to sit and drain slowly into the environment. Our houses, roads, and other structures don’t absorb water like the ground does, so when we build things, we increase the risk of flooding. The retention ponds are there to hold the water that would otherwise be absorbed by the ground we built on, reducing the risk of flooding.
Lots of people getting the first half of the answer right: when you develop an area, you increase the rate of runoff. To avoid flooding streams or stormwater pipes downstream, you set up a retention pond to slow the rate at which water drains off your site. It rains, and the water gets stored in the retention pond and released slowly into the stream or stormwater pipe to leave the site.
The answer to your second question is the answer to all things civil engineering: you can do it, but it costs more. Sometimes developers use underground retention. Basically, instead of an open pond, you have an underground structure that holds the water while it is let out slowly. You might use a really big pipe, an underground concrete vault, or this weird looking modular product that looks like milk crates. That way you can put a park, a road, or a parking lot over top of it. The only downside is it costs more money.
I’m just gonna tell you that our well-engineered retention ponds and drainage system handled four days of +12” rain (Hurricane Harvey) where I live with no flooding.
I’m a civil engineer and work in land development design and the ELI5 is basically grassed areas allow water to get back into the ground and hard surfaces do not.
When you take a piece of land that was all grass and replace it with roads and houses and other hard surfaces a lot less water end up back into the ground which means more water needs to go to the river. Because the river isn’t used to this extra water we need to keep it on site and slowly release it to avoid flooding downstream areas.
Being able to hold this water longer also allows us to remove some of the pollutants that it may have picked up on the streets and driveways on its way to the storage pond and prevent those bad things from entering into the river.
Basically we have to trick the river into not knowing there is a subdivision there so it thinks everything’s still all grassland or forest or whatever – we can’t give it more water then was there before construction and we can’t give it dirtier water than it got before construction or it will harm the wildlife.
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