Do Retention Ponds Actually Work?

0:01 This is the Historic Fourth Ward Park in Atlanta, Georgia.

0:04 It’s got all the stuff you could want a park to have:

0:08 landscaped walkways, benches, grassy fields, a playground,

0:11 and even a splashpad and amphitheater.

0:14 The focal point is the 5-acre or half-a-hectare pond running through the middle.

0:19 But this pond isn’t just for looks.

0:21 In fact, this park would never have been built at all

0:24 except for the fact that it solves a serious flooding problem.

0:28 For years, the Fourth Ward neighborhood

0:30 struggled with drainage and flooding issues.

0:32 In the 90s, the city came up with a plan:

0:36 a massive underground tunnel to carry runoff away.

0:38 Don’t get me wrong.

0:39 I love flood tunnels.

0:41 I have a whole video about them.

0:43 But they’re not always the right call.

0:45 One engineer in Atlanta had a better idea- a solution

0:48 that would address the flooding issue for a lower cost,

0:51 and significantly beautify the area,

0:53 a rare opportunity to improve form and function.

0:57 You’ve almost certainly seen a stormwater pond,

1:00 whether you realized that’s what it was or not.

1:02 They kind of blend into the urban landscape

1:04 to the point where they’re basically hiding in plain sight.

1:08 Some have been turned into amenities in places

1:10 like parks where the primary purpose is disguised.

1:13 But I think it’s fair to say

1:15 that no other single solution has been installed more

1:19 extensively in modern cities or delivered greater cumulative

1:22 protection against runoff than the humble stormwater pond.

1:27 Let me show you how they work with a model I built in the garage

1:30 and some of the ways these ponds are evolving in the 21st century.

1:34 I’m Grady, and this is Practical Engineering.

1:47 The problem that stormwater ponds solve is pretty easy to understand.

1:52 Storms bring water, and that stormwater has to go somewhere.

1:56 Spray a garden hose on some grass and some concrete,

1:59 and just watch what happens.

2:01 How much of that water soaks in, and how much runs off the surface?

2:05 Depending on the type of soil below

2:07 the grass (and the duration of the experiment),

2:10 the answers are pretty different for the two situations.

2:13 Let’s do a little development to make this clearer.

2:16 Say we buy up this piece of land on the edge of the city.

2:20 Add roads and sidewalks; some commercial parcels with parking lots;

2:24 a park with a gazebo, tennis and basketball courts;

2:28 apartments and homes with roofs, driveways, patios, and sheds.

2:32 Before our project, this entire area was natural ground-

2:36 soil that could absorb at least some amount of precipitation,

2:39 allowing it to infiltrate into the earth, recharging aquifers.

2:43 Now, it’s covered in all kinds of impervious surfaces.

2:47 Let’s see what happens when it rains.

2:50 Essentially, two things can happen to rainfall when it hits the ground.

2:53 It either soaks in or it runs off.

2:56 How much of each happens depends on quite a few factors.

3:00 For soil, it matters what kind.

3:02 Sandy soils with large particles and interstitial spaces can absorb a lot.

3:07 Clays, with microscopic particles and almost no voids, very little.

3:11 It also matters how much water is already in the soil.

3:15 If it’s wet before the storm, there’s less room for more water to flow in.

3:20 And as soil absorbs water throughout a storm,

3:23 its ability to infiltrate more decreases.

3:26 Any water that can’t infiltrate the soil

3:29 will run off into creeks or rivers nearby.

3:31 But, for impervious surfaces like asphalt, concrete,

3:35 and roofs, there aren’t really any variables.

3:38 Essentially all the water that falls on them runs off.

3:41 When it rains in our new development,

3:43 all the runoff still flows to the same place:

3:46 maybe into a channel that runs to a creek

3:48 that eventually connects to a larger river.

3:51 It’s just that now, there’s a lot more of it.

3:54 As I mentioned, depending on the type of soil and the size of the storm,

3:58 the difference between pre- and post-development

4:01 conditions can be pretty significant.

4:03 But a single development usually only represents a small

4:06 portion of the watershed for a creek or river.

4:09 So, even with all these new impervious surfaces,

4:13 the marginal increase in water levels during

4:15 a storm downstream may be fairly insignificant.

4:18 But zoom out to the scale of an entire city, and the problem becomes obvious.

4:24 It’s basically all impervious.

4:26 Development left unchecked can dramatically increase the frequency

4:30 and severity of flooding because when it rains,

4:33 a much greater proportion of that rain runs off

4:36 into creeks and rivers instead of soaking into the ground.

4:40 So, most cities don’t let development go unchecked,

4:43 at least from a flooding standpoint.

4:46 Rules vary a lot among cities and across the world,

4:49 but the most basic requirement you’ll see in most places is pretty simple:

4:53 To get a building permit to develop a piece of property,

4:56 you’re going to have to limit the peak

4:59 runoff from the property to pre-development levels.

5:01 That means that for a given storm, on a given site,

5:04 you can’t have a higher flow rate

5:07 after development than it would have been beforehand.

5:10 Most development is going to involve adding impervious surfaces,

5:14 whether they’re roads, buildings, sidewalks, or parking lots.

5:17 And that means more runoff.

5:19 You can’t just get rid of the water (in most cases), so somehow,

5:23 you’re going to have to store it and release

5:25 it gradually to keep the peak flow below pre-development levels.

5:29 And the simplest way to do it is a pond.

5:32 This is my garage-built stormwater pond.

5:37 It’s just an acrylic flume I use for some of my demos.

5:40 But I’ve built this outlet structure that should slow down the water,

5:44 backing it up into the pond.

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6:16 I’m measuring flow with a meter on the inflow pipe.

6:19 I also have a level sensor measuring the volume

6:22 of discharge over time in this tank below.

6:25 These are both feeding into an Arduino so we can look at the data.

6:29 I’m going to simulate a storm event using this valve.

6:32 So, this is a hand-crafted, artisan inflow event.

6:35 A typical storm has kind of a bell-shaped runoff curve.

6:39 Starts slow, builds to a peak as more and more of the watershed contributes,

6:43 and then tapers off as the storm moves away.

6:46 And you can see that my stormwater pond captured some of that peak.

6:49 Because of the outlet structure (that just has

6:52 a small hole at the bottom right now),

6:54 the discharge from the pond is much lower than the inflow.

6:57 And, after a little post-processing, I can show you the data.

7:01 This is a plot of flow versus time.

7:04 Inflow is the solid line.

7:06 Outflow is the dashed line.

7:07 The units are arbitrary since this is just for comparison,

7:11 but I did calibrate the sensors so they match as closely as possible.

7:14 You can see that the area under both curves is the same.

7:17 Just as much water came out of the pond as into it.

7:21 But the peak outflow rate was a lot lower.

7:23 And that’s a big deal.

7:25 The peak of the flood is everything.

7:27 That’s what determines how high the water rises downstream.

7:30 It correlates closely with the total amount of damage that occurs.

7:34 So most drainage rules in cities don’t really focus on total volume;

7:38 they focus on the peak flow rate leaving the site.

7:41 And you can see that the peak coming out

7:44 of my pond is significantly lower than the peak going into it.

7:47 So great, the pond did its job.

7:50 Problem solved right?

7:51 But you know this wouldn’t be an engineering

7:54 challenge if there wasn’t something to balance.

7:56 You can imagine a pond with no outlet at all that just fills up with runoff.

8:01 In that case, the peak discharge is zero.

8:03 We’ve maximized the performance, right?

8:06 Obviously not, since that storage is expensive,

8:10 not only in the construction cost to build it but also

8:13 in the valuable real estate it takes up on the site.

8:16 So really, the optimal solution is the one that uses the least volume necessary,

8:21 while still keeping the peak discharge below what

8:23 it would have been without any development at all.

8:26 The problem is storms vary in intensity and duration.

8:29 So most of the time, you’re going to have to show that your design

8:34 works for several different storm events of varying magnitude.

8:36 A little hole at the bottom of the outlet structure might work for a small one.

8:41 However, for a larger storm,

8:42 you can see that my pond fills up pretty quickly and eventually overflows.

8:46 Sure, you could make the pond bigger to hold more volume,

8:50 but we’re just trying to trim the peak

8:53 off the flow rate to match pre-development conditions.

8:56 We can release more water from the pond;

8:58 we just have to be careful about how much.

9:01 When I remove this conspicuous piece of tape from my outlet,

9:04 you can see that I’ve already built this in.

9:07 I have a larger hole higher up on the structure,

9:10 so it can release more during more intense storms.

9:13 Let me simulate that now.

9:19 You can see as the water reaches that level,

9:21 the flows from the two holes combine,

9:23 and we get more water released from the pond, so it doesn’t overflow.

9:26 Here’s the graph of the small storm again.

9:29 And here’s the graph for the big one.

9:31 You can see that in both cases, we’re not completely eliminating the flow.

9:35 The pond and outlet structure are just shaving off

9:38 the peaks to reduce the impact of the impervious surfaces.

9:42 But that can be a tricky thing to do when

9:45 you have a lot of different storm magnitudes to consider.

9:48 Take a look at a stormwater ponds in the wild

9:51 and you’ll start to notice the wide variety of outlet designs.

9:54 Placing the various orifices or weirs is kind

9:56 of an art as much as it is a science,

10:00 because every site is different and every city has different rules.

10:04 An engineer has to tune the structure to balance the amount

10:07 of storage with the additional runoff from all the impervious surfaces.

10:11 I added a third hole on top of my structure so it can handle a really big storm.

10:17 The flow through all three holes in the outlet

10:19 combines to create more flow out of the pond.

10:28 Here’s the graph of that run.

10:30 You can see the discharge is much higher, but it’s still below the peak inflow.

10:35 But, this gets quite a bit more complicated,

10:38 because stormwater runoff doesn’t just create flooding.

10:41 It also carries pollution.

10:43 We think of rain as cleansing,

10:45 but the stuff rain washes off the landscape has to go somewhere.

10:50 That means everything from trash, oil, dog poop, sediment, road salt,

10:55 and a whole lot more ends up in creeks and waterways.

10:59 A lot of the contaminants in stormwater are

11:02 either attached to sediment (or are sediments themselves).

11:05 So stormwater ponds can serve double duty,

11:08 reducing flooding and downstream contamination.

11:11 You’re not going to get the water really clean like at a wastewater plant,

11:15 but the treatment for suspended solids can be as simple as letting water sit

11:20 still for a day or two so bits of stuff can settle to the bottom.

11:25 You may have heard the terms detention pond or retention pond.

11:29 We’ve been talking about detention ponds that simply slow down runoff,

11:33 but they eventually empty out.

11:35 Retention ponds are related,

11:36 but they keep some of that water stored permanently,

11:39 and it makes a big difference when it comes to treatment.

11:42 Let me show you in the model.

11:45 I added a bunch more mica powder to the water so

11:48 you can easily see how the water flows through the pond.

11:51 Contamination is worst during the beginning of a storm,

11:54 sometimes called the “first flush,” when streets and surfaces are dirtiest.

11:58 You can see in my model, before the pond starts filling,

12:02 everything suspended in the water is making it through to the outlet.

12:06 The water is moving pretty quickly, and it’s relatively turbulent,

12:09 there’s just not enough time for anything to settle out.

12:13 But I can put a plug in the bottom outlet of the structure

12:16 and prefill the pond so it acts like a retention facility.

12:18 Now when I turn on the pump to the same flow rate, you can see a big difference.

12:24 There’s a lot of turbulence where the water flows

12:27 in, but things slow way down toward the outlet.

12:30 It’s still just a scale model,

12:31 so most of the mica powder is still suspended at the end,

12:34 but you can imagine if we scaled this up so

12:37 the water took several hours or more before reaching the outlet,

12:40 most of the solids in the flow would have enough time to settle out.

12:45 And retention ponds have other benefits too.

12:47 They help with groundwater recharge by giving water more

12:50 time to soak in, and they often look nicer, since water features are an amenity,

12:55 and these are often landscaped like any other

12:58 pond you might intentionally install on a site.

13:00 But, obviously, there’s a tradeoff here.

13:02 You get cleaner water out, but you need a bigger pond,

13:06 since some of the volume is already taken up before a storm arrives.

13:11 However, there is a way to have your pond-cake and eat it, too.

13:15 Outlet structures don’t have to be passive like my demo here.

13:19 Imagine if you could actively control how much water flows

13:23 out of the pond based on sensors and weather forecasts.

13:26 You could hold water in the pond for longer

13:28 periods of time when there isn’t too much rain,

13:31 improving the quality of the treatment,

13:33 and then pre-drain the pond ahead of a storm,

13:36 freeing up that space for the next runoff event.

13:39 This is known as Continuous Monitoring

13:41 and Adaptive Control- it’s basically “smart” stormwater management.

13:45 It’s a pretty cool idea that’s only just starting to catch on in cities,

13:51 but it has disadvantages too.

13:53 One is disease vector control.

13:55 Because there’s no stable pool,

13:57 you can’t reliably stock fish to eat mosquito larvae,

14:00 so there are limits on how long you can

14:02 hold water before you have to drain the pond.

14:05 It’s also quite a bit more technically sophisticated,

14:08 so there’s a tradeoff there too.

14:10 Usually, these types of systems are

14:13 operated by specialized companies that install, manage, and maintain them.

14:17 Some even sell the capacity on an open marketplace,

14:21 allowing developers to buy credits in lieu of on-site ponds.

14:25 This stuff gets pretty creative- addressing

14:27 the lot-level needs of individual developments with larger,

14:31 watershed-scale outcomes.

14:32 And in fact, they’re often part of a larger idea called regional detention.

14:38 Even though on-site detention or retention is great in theory,

14:42 it can be messy in practice: small lots don’t have room for meaningful storage,

14:47 building dozens of tiny basins inevitably leads to uneven maintenance,

14:51 the small pipes and outlets of minor ponds are more susceptible to clogging,

14:56 and in some cases, they can actually make flooding worse.

15:00 You could see on my graphs that detention lowers the peak at each site,

15:04 but it also delays it.

15:06 If many basins are designed with similar outlet controls,

15:09 their attenuated peaks can arrive all at once at a confluence downstream,

15:14 spiking the creek level worse than if there were no detention at all.

15:19 Water quality benefits are hit-or-miss, too,

15:22 because performance depends on how each little system is built and maintained.

15:26 So, there are cases where developers get together or a city

15:30 or drainage district solves the problem at a regional scale,

15:34 building a single, larger facility that can

15:37 handle the runoff from multiple sites.

15:39 By routing excess runoff to a shared basin (or a network of them),

15:44 you gain real storage volume,

15:46 coordinated release rates that match downstream capacity,

15:50 and professional, centralized upkeep.

15:52 It also lets you optimize water-quality treatment and pipe

15:56 sizes across the area instead of overbuilding each parcel.

15:59 Keep the small storms where they fall for infiltration and local benefits;

16:04 send the larger pulses to regional detention so the watershed sees a calm,

16:09 controlled hydrograph instead of a patchwork

16:11 of ponds releasing a chorus of overlapping peaks.

16:15 I should make clear that detention and retention

16:18 are far from the only stormwater management tools.

16:21 Regional geology and hydrology often drive the design.

16:24 I live near Austin, which has strict

16:27 environmental rules because of the Edwards Aquifer.

16:30 Where the limestone reaches the surface,

16:32 contaminated runoff can easily enter the groundwater.

16:35 So many sites in Austin require filtration ponds that actually

16:39 pass water through a layer of sand before it’s discharged downstream,

16:45 removing pollutants before they can reach the groundwater.

16:48 I’ve talked about permeable pavement in a previous video,

16:51 and there are a lot more solutions out there.

16:53 Many civil engineers spend their entire

16:56 careers solving urban stormwater puzzles,

16:58 trying to balance the important watershed functions

17:01 with the challenge of flooding and pollution.

17:04 Detention and retention ponds are just one piece of it.

17:08 Part park, part plumbing, mostly hiding in plain sight,

17:11 they are often carefully tuned pieces of infrastructure

17:14 that help keep the city’s head above water.