Understanding the subsurface makes New Orleans more robust

Published: 25 July 2018

New Orleans subsides by an average of 6-8 mm annually and a large part of the city is below sea level. Hurricane Katrina had such an enormous impact because water flooded the low-lying areas and then had to be pumped out. To improve the resilience of the city, the U.S. Department of Housing and Urban Development granted The City of New Orleans $141.3 million in National Disaster Resilience Competition (NDRC) funds for its Gentilly Resilience District plan to create a replicable model of urban resilience and water management.

The City’s Office of Resilience and Sustainability is partnering with Deltares and Deltares USA Inc., to assist with development of the plans, as well as implementation of many of the plan’s scientific and technical components.

Resilient city

New Orleans’ NDRC proposal, titled “Reshaping the Urban Delta,” describes how New Orleans can become a resilient city that can cope better with water and land-subsidence challenges, by investing in spatial planning. It is expected that those challenges will become more intense in the decades to come because of sea-level rise, intense rainfall and ongoing land subsidence. Robust city structures and water infrastructure are crucial for the city’s survival and economic prosperity. The plan will initially be implemented in the Gentilly area before being scaled up to cover all of New Orleans.

Focus on the subsurface

In collaboration with The Water Institute of the Gulf (the Institute) and with the help of other local partners such as Tulane University and Deltares USA Inc., Deltares will focus on the New Orleans subsurface over the next 18 months. The aim is to ensure that rainwater, surface water, groundwater and land subsidence in the urban area can be managed in coordinated ways.

Analysis and phases

An analysis of the current situation is the first step: What is the status of the subsurface and groundwater flows at present and what are the expectations with regard to climate change, sea-level rise and further land subsidence? The analysis will be made on the basis of 63 soil drilling operations around the city. In addition, specifications for measuring instruments will be drawn up and a database will be established for measurement data. Finally, work is taking place on a numerical groundwater/soil subsidence model which will be followed by the design and installation of an integrated water monitoring network. The result with be New Orleans’ first-ever groundwater monitoring network; providing decisionmakers with information necessary to set data-based priorities to maximize impacts.

Roelof Stuurman (technical project leader/groundwater expert at Deltares): “You can’t manage what you don’t know. You can’t see the subsurface. If you lack knowledge about the system — and that knowledge has been sparse until now — you can’t know how groundwater and surface water respond to interventions or to climate change. I’m proud of the focus on the subsurface in this project. The subsurface is associated with threats such as land subsidence but it also offers opportunities for storing water, for example in rain gardens.”

“This new work builds on the resilience strategy New Orleans launched in 2015 and an understanding of the best available science on future challenges,” said Ehab Meselhe, Ph.D., Vice President for Engineering at the Water Institute of the Gulf. “Knowing how water flows through and under the city is vital in the overall plan of how New Orleans learns to live with, and adapt to, water.”

Measures

Once the subsurface has been charted, it will be possible to study the impact of possible solutions such as reduced drainage and increased groundwater recharge. Current green solutions such as water-permeable streets and gardens that can store water will also be evaluated: How effective are they really and what is needed in terms of maintenance? Finally, a study will be conducted of the feasibility and opportunities of a “real-time” automated pumping system using weather forecasts.

Roelof Stuurman (Deltares) teaches subsurface by drilling in the school garden of KIPP Central City Primary (New Orleans)

The project, which involves the stakeholders (including local residents), will contribute to a greater awareness of the importance of the subsurface for low-lying cities. The results will also show what can be done at different scales; from private land to the district levels, to reduce land subsidence and floods, and to support the policy of the city and the Sewage and Water Board of New Orleans.