Model Development to Assess Groundwater Flooding and Levee Underseepage in New Orleans, Louisiana

Abstract

Flooding is a major threat to New Orleans due to its geographic location and geologic condition. However, potential groundwater flooding is seldom studied and poorly understood even though groundwater level is expected high in the city. High groundwater level might result in groundwater flooding in low-lying areas. High uplift pore water pressures may cause strong underseepage and risk levee safety. The objective of this study is to assess the impacts of hydrogeology on groundwater flooding and evaluate potential underseepage-induced hazards along levees in New Orleans. In this study, a groundwater flow model development which involves stratigraphy modeling, groundwater flow model design, sensitivity analysis, model calibration, and uncertainty analysis was conducted in this study. Groundwater flooding was indicated by seepage rate. Underseepage along levees was evaluated by a factor of safety analysis. Parameter uncertainty was incorporated in the assessments of groundwater flooding and levee underseepage. The stratigraphy model showed that 74% of sediments in shallow New Orleans are fine grains. The Pine Island Beach sand along the shore of Lake Pontchartrain is the most pronounced shallow sand in the region. By simulating groundwater flow in 2018, the study found that 40% of New Orleans metropolitan area may have groundwater level 1.0 m above ground surface all year round. The results indicated that relatively severe groundwater flooding comes from well recharged surficial coarse sediments. Residential areas around the Inner Harbor Navigation Canal in the vicinity of Lake Pontchartrain are the most concerned about groundwater flooding, where seepage rate was estimated 4 to 5 mm per day. These areas also exhibit high chance of strong underseepage and potential sand boils. The modeling assessment results agreed well to the IPET’s levee assessment for the Hurricane Katrina disaster.