Collaborative Development of a Forecast Capability for Water Levels in the Lower Pearl River Basin, LA/MS

The Pearl River drains approximately 8,760 sq. mi. extending from central Mississippi to southwestern Mississippi and southeastern Louisiana. Starting in Marion County, MS, 760 sq. mi. of drainage is labeled the Lower Pearl River Basin. Within the Lower Pearl River Basin, the main stem of the river splits into two distinct channels west of Picayune, MS to form the East and West branches of the Pearl, which flow into Lake Borgne and the Rigolets, respectively eventually emptying into the Mississippi Sound and the Gulf of Mexico. The East Pearl river serves as the political boundary between Louisiana and Mississippi. While the lowest reaches of the East Pearl are much wider than the corresponding region of the West Pearl, the West Pearl is historically the branch with larger flows. However, recent imagery, in-situ depth measurements, and other anecdotal evidence suggest that flow on the West Pearl is diminishing below Interstate-10. With a tidal range of as much as 1 m on spring tide and 0.3 m on neap at the ocean mouth, flows in both the West, and especially the East, Pearl River show a strong tidal signal with tidal fluxes generally much greater than river outflow during low flow conditions. Both channels regularly flood and drain the intertidal marshes near the river mouth.

Improved knowledge of the Pearl River basin is needed for diverse interests such as flood warning and control, pollution abatement, water supply, and recreation. With approximately 12,700 structures located in the Lower Pearl River Basin in LA and MS, the National Weather Service (NWS) and local officials in LA and MS are responsible for warning emergency managers and residents of potential flood events. Both the LA Department of Wildlife & Fisheries and the US Fish & Wildlife Service desire enhanced understanding of water level and habitat impacts throughout the basin. Furthermore, local planners desire to quantify the impact of proposed storm surge mitigation scenarios on the river and its surrounding flood plain. To address such diverse interests, a collaborative group has formed consisting of federal and local government partners from St. Tammany Parish, LA and the NWS's Lower Mississippi River Forecast Center (LMRFC), Slidell, LA, together with research partners at the Naval Research Laboratory, Stennis Space Center, MS, Mississippi State University and Oklahoma University. The objective of the collaboration is development of a modeling system that simulates existing conditions in the Lower Pearl River Basin for water levels and currents. Once developed, this model can serve as a tool for forecasting and analyses that address the broad array of concerns enumerated above.

To be discussed is the formulation of a model for such a complex, braided river system as the Lower Pearl River Basin, which draws on multiple sources of information and state-of-the-art modeling practices. Initial configuration of a channel model applies strategies developed for data poor environments, using imagery to define the river channels and algorithms to estimate river depths when no information is available. Addition of the surrounding flood plain to the channel model is cross-checked against imagery obtained by unmanned aerial sensors flown over the basin in 2014 that delineate wet from dry terrain at spatial scales of meters. In-situ measurements of water depth, particularly along the West Pearl River contribute to a more realistic characterization of flow in that channel. A network of new water level gages installed throughout the system also provides snapshots of reality.

The final model configuration consists of a hydrodynamic model of the Lower Pearl River Basin that receives upstream discharges at three locations, capturing flow contributions from the Bogue Chitto and E. and W. Hobolochitto rivers, as well as the northern stem of the Pearl at Bogalousa. NWS's LMRFC provides the forecast hydrographs used by the model at each of these locations. The downstream boundary of the modeling system extends into the open waters of the Mississippi Sound and well beyond where tidal forcing from validated tidal databases can be applied. The modeling system is designed to produce water levels and currents throughout the Lower Pearl River Basin. Initial validation efforts to be presented include consideration of recorded low-flow, average-flow, and high-flow scenarios. More comprehensive validation of the modeling system will then extend to reproducing events associated with the historical Hurricane Isaac that impacted the region in 2012. Additional discussions will highlight new understanding on flow patterns throughout the system based on numerical simulations, in-situ, and remotely sensed observations.