Harvesting Rainwater for Use During California Droughts: What is the Difference Between Detention and Retention Ponds?

Between a growing agricultural sector and an ever-expanding population, California’s need for water is getting bigger all the time. Droughts are common in this region—and they exacerbate the problem. Rainwater harvesting is one part of the solution for water shortages.

The thing is, when speaking about rainwater harvesting systems, the terms “detention pond” and “retention pond” are often used interchangeably. However, these two types of ponds are actually very different things. Understanding the difference will help you grasp how rainwater harvesting works, and how to create your own rainwater harvesting system.

The Key Difference Between Detention and Retention Ponds

The biggest thing to understand is that detention ponds hold water temporarily, whereas retention ponds contain water permanently.

That’s the biggest difference, but these two types of ponds share some things in common. First, is the flow orifice. This is the part of the pond that controls the water level within the pond. A detention pond, sometimes also called a “dry pond,” will feature a flow orifice at the bottom of the pond because it is designed to be emptied completely. It may fill up when a storm sweeps through, but in the days and weeks after the storm, the pond empties until it’s dry again.

Retention ponds have flow orifices above the pond’s bottom. Operators place the flow orifice at the desired depth level when flooding is not occurring. Similarly, to how detention ponds work, when water levels in a retention pond are higher than the flow orifice, water flows out until the pond reaches its normal level. Thus, they never run dry even though they can flood during stormy weather.

Why Use Detention Ponds?

Detention ponds and retention ponds are similar enough that you may be wondering why one might use a detention pond instead of a retention pond. The answer deals with flood control. Detention ponds are meant to prevent flooding in the moment. They capture runoff water and divert it away from valuable areas like homes or crop land. Because of this, they’re made to be able to stand up to fast flowing storm water—and slow that water down as it is channeled into more permanent holding areas.

Detention ponds are not good for things like pollution control or water treatment since they are only a temporary measure to collect and divert rapid flows.

Retention ponds are where water can be treated for use on crops or as potable water. Here, sediments can be allowed to settle out, and the water can be channeled to crops or wherever else it needs to go.

How do Detention Ponds Work?

Detention ponds are best used in areas where more than ten acres need to be protected against stormwater. For plots of land smaller than this, other options, like storm drains or storm ditches, may be a better alternative. These ponds work best with a small slope that helps divert water. Inlets shouldn’t be more than 15% higher than outlets to ensure that water flows at the correct volume through the system.

Most detention ponds allow for a large area in which to collect stormwater. They capture that water and direct it out slowly through the outlet into retention ponds, rivers, lakes, or other holding facilities where vast volumes of water can be safely handled. Often, near the outlet, baffles in the form of concrete blocks or other structures are installed to help slow down the rate of flow even more.

How Do Retention Ponds Work?

At their heart, retention ponds are a long-term storage option for collected stormwater. They’re typically deep enough to hold large amounts of water, and they will have outlets high on their walls to drain them when they are too full. These ponds are ideal for storing water so that it can be directed into irrigation systems or other areas where water supplies are low.

Another advantage to these ponds is that natural processes allow for the treatment of the water. Stormwater runoff is often highly polluted with sediments and other compounds picked up as floodwater runs over lawns, crop fields, roads, and so on. Water in a retention pond has a chance to settle, with silt and sediment falling to the bottom. In a good stormwater management system, these ponds will also have plenty of vegetation planted around them to serve as a natural filter to remove various contaminants. Water processed in this way can then be used for crops or a variety of other purposes.

Both retention and detention ponds are invaluable parts of stormwater management—and in areas like California, stormwater management can also double as a rainwater harvesting system that helps shore up water supplies during periods of drought.

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