Well Monitoring: Can riverine wetlands reduce downstream flooding?

Well Monitoring: Can Riverine Wetlands Reduce Downstream Flooding?

Written on: March 7th, 2018 in Wetland Assessments

By Kenny Smith, DNREC, Wetland Monitoring and Assessment Program

Wetlands provide many vital benefits to the State of Delaware, like habitat for all kinds of plants and animals, improved water quality, and erosion control. Another benefit that wetlands provide is flood control. Wetlands have the ability to collect and store storm waters and lessen flooding down river, and we are working on a project to study just that.

Riverine Wetlands

No matter where you are in Delaware, there is at least one wetland located within a mile of you. Wetlands do not always have water at the surface. For this reason, we determined a wetland to be an area where water is held in the top 12 inches of the soil or higher for two or more weeks. This particular project focused on riverine or floodplain wetlands. Riverine wetlands are found along non-tidal creeks or rivers surrounding the banks of the creek. One of their main benefits is that they absorb and hold onto any excess water in the system. Creeks and rivers without wetlands along them can have significant damage or erosion to nearby properties from storm events.

Monitoring Water Levels in Riverine Wetlands

Our program, WMAP, installed three wells in an area along the Blackbird Creek in the Blackbird Forest in 2015. We placed the wells throughout the riverine wetland to figure out what the water in the soil is doing at the site to then determine how much flood protection that particular wetland can provide. The wells collect data on how much and often the wetland is soaked with water. Every four months we go back to the site to download the data and do any maintenance.

Does the Blackbird Forest Wetland Offer Flood Protection? And How Long was the Wetland Wet?

The 2016 data showed that this particular wetland had water in the top 12 inches of the soil or higher for for about 179 days throughout the year (if you combine the days together, that is almost six months)! The water level was even above the ground surface on four different days. Looking at what months were the wettest, the data also showed that the majority of the water was held for 162 days from December 15, 2015 to May 27, 2016. This information supports the fact that wetlands hold more water in the winter and early spring months.

Can the Wetland Soak Up Water?

To figure out how much flood protection the Blackbird Forest riverine wetland provided, we compared the water depth of our wells with a downstream water level gauge in the creek. The thought process is that as precipitation falls the creek water level will rise. Once it has rained, snowed, or sleeted enough, the water level will reach the creek banks and overflow into the wetland. Then, the wells should show the increase of water in this wetland.

We have had a few large precipitation events since we installed these wells and can visualize how much water these wetlands can hold in these events. One example of this was around December 25, 2015. The site received about two inches of precipitation and in response the creek water gauge increased by 27 inches and our well water levels increased by 19 inches. Another example was on July 6, 2017. The site received approximately seven and a half inches of precipitation, the creek water gauge rose to almost 33 inches above the normal level and our wells reported an increase in 45 inches.

*This graph shows how much water this wetland is holding during and after a rain event. Notice the sharp incline (uptake of water), and the slow and gradual decline (the slow release of water).*

Can the Wetland Slowly Release the Water?

The well data also showed that this wetland surrounding the creek was able to quickly soak up the overbank flooding and slowly release it. The water level in the wells usually increased exponentially faster than decreased after the precipitation events. If the wetland hadn’t been there, it is likely the water would have quickly rushed down creek causing flooding or erosion damage in both of these rain events.

Future Monitoring Efforts

We will continue to monitor our Blackbird Forest site to further explain the function of riverine wetlands to the landscape. In addition, we also have three wells in a flat wetland in the Ted Harvey Wildlife area in Dover, DE where we have been collecting data. Here we will look at the relationship between flat wetlands and precipitation.

Want more information about this project? Contact kenneth.e.smith@delaware.gov or 302-739-9939.