Changing Stormwater System Dynamics in Coastal Transition Zones

We will present an overview of how evolving watershed system dynamics and large-scale watershed improvements, combined with increasing climate and environmental risks, are substantially changing the dynamics of urban stormwater systems in the Acadiana Region of Louisiana. While practitioners typically focus stormwater design on localized drainage system requirements, large-scale watershed improvements have been made over the years that have created unintended consequences impacting local/regional stormwater and flood risk management systems. In 1925, George H. Maxwell, a California lawyer who helped pass the National Reclamation Act in 1902, proposed a system of spillways, floodways, and storage basins on the Mississippi River that he called 'The Atchafalaya National Safety Valve'. He challenged the illusion of safety the existing Lower Mississippi River (LMR) flood control levees provided and warned that additional protections were needed. His proposal along with many others were considered by the Mississippi River Commission (MRC) but no action was taken, and the status quo remained until the devastating 1927 flood. This catastrophic flood ended the MRC's reliance on levees for flood control. The need for the Atchafalaya Floodway diversion and LMR gated diversions was no longer debatable. In 2025, a century later, we find that these large-scale improvements are evolving and changing drainage characteristics within adjoining watersheds and consequentially impacting local stormwater management systems. The design challenges and debates are increasing with regards to providing stormwater management and flood resiliency for an increasingly at-risk population and rapidly changing environment, particularly in the Acadiana region located approximately 200 miles east of Houston. HDR developed the Louisiana Watershed Initiative (LWI) regional-scaled watershed models for Louisiana DOTD in Region 5 (Atchafalaya, Teche, Vermilion, Mermentau Headwaters, and Mermentau HUC8 watersheds). HDR engaged in this complex modeling effort with extensive coastal transition zones utilizing coupled coastal and riverine models for this critical drainage area of over 10,000 square miles. The Region 5 calibrated/validated models have been completed for south-central Louisiana at a highly refined scale, with AEP frequency design storms and consequence models scheduled for completion before June. We will present a brief overview of the Region 5 modeling effort and changing stormwater system requirements, along with highlights of the stakeholder engagement and modeling efforts that led to 2023's newly created 'Acadiana Watershed District', one of the first regional watershed management districts in Louisiana. This initiative intends to promote understanding and encourage collective action among public and private sector partners to improve resilience and collaboration for local/regional stormwater and flood risk management systems. The benefit of this presentation is to show how fully integrated regional-scaled models can positively impact and inform practitioners in the design of local stormwater systems, such that the impacts are assessed more holistically, and proposed improvements/benefits are better understood. This is especially needed to address climate resilience and adaptation in evolving high-risk coastal transition areas.