Amid More Frequent Bonnet Carré Spillway Openings, Upriver Diversions Can be a Solution

06.23.2020 | In Coastal Restoration

The Army Corps of Engineers opens the Bonnet Carre Spillway during the 2016 flood.

The unprecedented  2019 Mississippi River flood was a stark reminder of the need to rethink the management of our rivers and how we confront a future with more water coming to Louisiana from the rest of the country.

In the last five years, the Army Corps has opened the Bonnet Carré Spillway (BCS) five times to relieve pressure on Mississippi River levees that protect one million people in Southeast Louisiana. 

To identify actionable solutions to confront this new normal, we set out to understand how operating diversions planned upriver of New Orleans to restore vital wetlands could also impact total water volume flowing through the spillway to manage river floods. Fortunately, a promising solution was hidden in plain sight. 

Louisiana’s 2017 Coastal Master Plan includes a number of freshwater and sediment diversions that will use the Mississippi River to build land, nourish wetlands and combat saltwater intrusion.

Aerial image from Google Earth

Some of those — Union, Ama and River Reintroduction to the Maurepas Swamp — would be constructed near the Bonnet Carré Spillway in locations that are critical for river water management in addition to sustaining surrounding wetlands. 

We hypothesized that operating these three diversions during river floods could reduce the total volume of water passing through the Bonnet Carré Spillway. To test this theory, we engaged Dr. Ehab Meselhe from Tulane’s Bywater Institute. 

The findings of this work suggest that operating these diversions could have reduced the volume of water flowing through the spillway in 2019 by almost 60% and allowed it to operate for fewer days.

Our approach and key findings

Dr. Meselhe’s team designed a series of experiments to model how opening these diversions would  influence the water levels and discharge at the spillway during floods.

To understand whether the diversions’ operation could help manage the river, they ran models analyzing the channel’s flow with simulated openings of the upper-river diversions. They studied the individual and combined impacts of each diversion. 

Results show a drastic reduction to the magnitude of the modeled Bonnet Carré discharge, especially with the joint operation of Ama North and Union diversions. According to the study, the flow volume released through the Bonnet Carré Spillway in 2019 was reduced by 57% to 61% and the length of the spillway’s opening was shortened by a third. 

Moving forward 

This study reinforces the idea that diversions that are intended for ecosystem restoration could also provide a vital new tool for flood water management. Luckily, Union and Ama have been included in the Section 7001 of the Water Resources Development Act (WRDA) Report to Congress and could receive federal funding for feasibility studies in the near future.

The Union Diversion also received some state funds that could be used to supplement initial studies by CPRA. While River Reintroduction to the Maurepas Swamp didn’t have a sizable impact on the pulse reaching BCS in this study, the project remains crucial to deliver much-needed freshwater and sediment to the forested swamps. Earlier this year, it received $130 million in RESTORE funds toward construction.

The next phase of this study has already begun. The Tulane team is running models to further test whether the diversions could distribute floodwaters in needed areas during high-water events to improve water quality in the Gulf, provide positive ecological benefits to degraded swamps while protecting populated areas from river flooding events. 

An aerial view of the Bonnet Carre Spillway taken during a flight with Southwings.

What’s at stake

If the last few years have been any indication, these high river flood events are becoming more and more frequent. Louisiana officials must work with the federal government and states upriver to ensure those states are doing their part to improve river management before the water gets to our state. However, we must also look for opportunities here to confront the new normal of higher, more frequent river floods. 

While opening the spillway is critical to protect lives and property in Southeast Louisiana, more frequent openings have caused a range of issues to wildlife and economies that extend beyond Louisiana.

For years, Restore the Mississippi River Delta partners have called on the Army Corps to update its century-old system and improve management of the Mississippi River. We hope this latest research will provide a big step forward toward realizing that vision of a more resilient future on the river.