Series: Colorado Water Law Basics #13 - Detention vs. Retention Stormwater Ponds

Detention vs. Retention Stormwater Ponds: The Water Rights Implications

There are two types of stormwater ponds: detention ponds and retention ponds. Detention ponds only detain water for a short period of time, then release the water to a natural watercourse, while retention ponds retain the water for an indefinite period of time. The way a detention pond works is there is typically an outlet at the lowest point in the pond that allows the water to slowly drain through the pond. Therefore, detention ponds typically are dry between storm events. Conversely, retention ponds typically have a weir or some type of overflow device which allows discharge of water but only when inflow exceeds the capacity of the pond and the water level rises above the outlet weir or pipe. As such, retention ponds are typically maintained at a relatively constant water level.

So both types of ponds aid in controlling stormwater flows and help improve water quality; but is there a difference in having a detention pond versus a retention pond when it comes to water rights issues? Yes, very much so.

Detention Pond

By statute, a detention pond is designed to only temporarily collect stormwater so flooding damage can be minimized by decreasing peak flows, and to act as a collector of debris and provide some water quality benefits by settling out potential contaminants from entering a natural watercourse. According to Senate Bill 15-212, water in a detention pond that results from a 5-year, or higher, frequency storm has to pass a minimum of 97 percent of the inflow within 72 hours, while 99 percent of the inflow from any storm frequency less than a 5-year storm, i.e., 10-year or 100-year storm, has to be passed in 120 hours. As long as stormwater is passed through a detention pond according to these parameters, no water rights are required for a detention pond.

One caveat regarding the release of water from a detention pond is that even though a water right is not required due to the presumption that releases within 72 or 120 hours will not cause material injury to any vested downstream water rights, a downstream water right owner can still allege injury if the water right owner believes the detention pond is causing injury to its water right. However, to prevail in an injury case against a detention pond owner it must be shown that (a) the operator is not releasing water according to statute or (b) peak downstream flows have been reduced below what would have occurred absent the urban development that caused increased peak flows at the time of the appropriation date of the water right. Obviously, this is a big hurdle to overcome.

Detention pond.

Retention pond outlet.

Retention Pond

Converse to detention ponds, based on their design retention ponds maintain a continuous pool. Therefore, not only is water prevented from contributing to downstream flows but there are also losses in the storage of water related to evaporation and seepage. By maintaining a permanent pool and only allowing water releases at the top of the pool allows sediment and potential contaminants to not be re-suspended and, therefore, they are contained in the retention pond. The outlet is typically at the far end of the inflow structure so settling occurs within the pond, i.e., the outflow is essentially decanted from the water surface.

Because there are water losses associated with a retention pond, the pond has to have a water right prior to operating. The water right could be a storage right and/or an augmentation plan. With a storage right, the pond would receive a priority date consistent with the date the application is filed in Water Court. While this is possible, the junior nature of a storage right would make it unlikely that the pond could maintain a permanent pool and, because of the passive nature of retention ponds where releases only occur when water levels reach the outlet elevation, it is difficult to drain a detention pond. Therefore, the most efficient way to operate a retention pond is with an augmentation plan. An augmentation plan allows the retention pond to maintain a permanent pool, with the associated depletions that could injuriously affect downstream rights. To prevent that injury, the augmentation plan requires that water from a reliable source is available to replace the depletions caused by the retention pond by delivering water into the stream upstream of the affected water right(s) so there is no net depletion to the downstream senior water rights.

If you have any water rights issues that you need assistance with, please give LWS a call (303-350-4090) or an email. LWS has extensive experience in numerous water rights issues!

Bruce Lytle bruce@lytlewater.com

Chris Fehn chris@lytlewater.com

Ben Bader ben@lytlewater.com

Anna Elgqvist anna@lytlewater.com

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