Potential impact of sea level rise on coastal surges in southeast Louisiana

Abstract

Potential impacts of 0.5 and 1.0 m of relative sea level rise (RSLR) on hurricane surge and waves in southeast Louisiana are investigated using the numerical storm surge model ADCIRC and the nearshore spectral wave model STWAVE. The models were applied for six hypothetic hurricanes that produce approximately 100 yr water levels in southeastern Louisiana. In areas of maximum surge, the impact of RSLR on surge was generally linear (equal to the RSLR). In wetland or wetland-fronted areas of moderate peak surges (2–3 m), the surge levels were increased by as much as 1–3 m (in addition to the RSLR). The surge increase is as much as double and triple the RSLR over broad areas and as much as five times the RSLR in isolated areas. Waves increase significantly in shallow areas due to the combined increases in water depth due to RSLR and surge increases. Maximum increases in wave height for the modeled storms were 1–1.5 m. Surge propagation over broad, shallow, wetland areas is highly sensitive to RSLR. Wave heights also generally increased for all RSLR cases. These increases were significant (0.5–1.5 m for 1 m RSLR), but less dramatic than the surge increases.

Introduction

Sea level rise (SLR) and subsidence are significant issues in the design of flood protection for southeast Louisiana. Flood walls, in particular, cannot be easily raised after construction, so future SLR must be considered in their initial design. Global or regional SLR is an increase in water level due to climate change (primarily due to thermal expansion of ocean waters and melting of glaciers and ice caps). Locally, sea level may also rise relative to the land level due to subsidence (e.g., due to compaction of subsurface sediments, extraction of subsurface hydrocarbons or water, collision of tectonic plates, isostatic response, sediment loading, and subsurface faulting). The relative sea level and its rise over time alter the extent and degree that surge and wave heights generated by hurricanes impact coastal areas. In the past, relative sea level rise (RSLR) was included in coastal protection design by raising design water levels an amount equivalent to the RSLR. But, surge generation and propagation are nonlinear processes and linear addition of RSLR to design water levels underestimates the impact in many areas. In addition to the surge elevation, wave heights also increase with water level in coastal areas where the wave height is limited by water depth. RSLR impacts not only the storm response, but also landscape type in southeast Louisiana. Higher water levels in wetlands impact vegetation type, where a wetland may change from freshwater marsh to brackish marsh to open water with increasing water level. RSLR may also lead to wetland loss, shoreline erosion, erosion of protective barrier islands (through overwash and breaching), and an overall change in the local morphology (islands transforming to submerged shoals and wetlands becoming open lakes or bays). Wamsley et al., 2009a, Wamsley et al., 2009b discuss the potential impact of wetland loss on hurricane surges.

RSLR has been estimated using a number of techniques for southeast Louisiana, resulting in a large range of RSLR projections. Penland and Ramsey (1990) use National Ocean Survey (NOS) tidal records from Eugene Island, Louisiana, (1934–1974) and Grand Isle, Louisiana, (1947–1987) to estimate RSLR rates of 1.19 and 1.04 cm/yr, respectively. Estimates given by NOS (http://tidesandcurrents.noaa.gov/sltrends) of RSLR rates are 0.965 cm/yr with a 95% confidence interval of ±0.124 cm/yr (1939–1974) for Eugene Island and 0.924 cm/yr with a 95% confidence interval of ±0.059 cm/yr (1947–2006) for Grand Isle. Based on NOS tide gauge records, these rates in the Mississippi River Delta region (southeast Louisisana) are the highest rates of RSLR in the Gulf of Mexico. The NOS estimated rates on the Gulf of Mexico coast of Florida are approximately 0.08–0.24 cm/yr, the rate for Dauphin Island, Alabama, is 0.30 cm/yr, and the rates for Texas are 0.19–0.68 cm/yr (with the Texas rates generally highest in the east and decreasing to the west). Penland and Ramsey (1990) also provide estimates of approximately 1 cm/yr of RSLR in southeast Louisiana based on US Army Corps of Engineers tide stations in Terrebonne, Jefferson, Plaquemines, and Orleans Parishes with records of 37–57 yrs. Evaluating RSLR over a shorter and more recent time period (1983–2006), IPET (2007a) estimated RSLR of 0.58 cm/yr for Grand Isle and 0.89 cm/yr in the Inner Harbor Navigation Canal at Florida Avenue in New Orleans.

The Intergovernmental Panel on Climate Change (IPCC) (Solomon et al., 2007) estimates a global SLR of 0.18±0.05 cm/yr over a similar time period (1961–2003) based on tide gauges, and for a more recent period (1993–2003) they report global SLR of 0.31±0.07 cm/yr based on satellite altimeter data. They attribute 90% of the later SLR estimate to climate change (thermal expansion and glacier and ice sheet melting). The IPCC projects future global SLR over the next 100 yr based on modeling of six scenarios (with global surface warming of 2–4 °C) with a median range of 0.2–0.6 m. Pfeffer et al. (2008) investigate kinematic constraints on glacier contributions to SLR and estimate the range of SLR as 0.8–2.0 m by 2010 based on glaciological conditions.

Yet another view is given by Törnqvist et al., 2004, Törnqvist et al., 2006 who use basal peat records to trace RSLR over thousands of years. For the time period 600–1600 AD, they report RSLR of 0.055 cm/yr for the Mississippi River Delta, based on locations that are about 10 km from the coast. González and Törnqvist (2006) suggest that the peat record analysis results represent the glacio-isostatic contribution to subsidence in coastal Louisiana.

For the Mississippi River Delta, the RSLR includes contributions from global SLR, regional glacio-isostatic subsidence, and local factors, such as compaction of Holocene sediments and anthropogenic contributions (e.g., hydrocarbon and water extraction). There is significant uncertainty in future projections of each of these components, but it is clear that the global SLR component is accelerating. The RSLR estimates given above are based on differing time periods and spatial areas, but indicate the rapidly changing conditions along the coast of southeast Louisiana. For the purposes of the analysis given here, we evaluate RSLR of 0.5 and 1.0 m, which would likely occur in the next 50–100 yr.

The purpose of this paper is to estimate the potential impact of RSLR on hurricane surge and waves in southeast Louisiana. This is accomplished through numerical surge and wave modeling of six hypothetical hurricanes that produce approximately 100 yr water levels in the area (water levels with approximately 1% chance of occurrence in a given year based on historical hurricane frequency), comparing a base case (present day conditions) with 0.5 and 1.0 m of RSLR. Section 2 describes the modeling methodology applied, 3 Surge results, 4 Wave results describe the surge and wave results, respectively, and conclusions are given in Section 5.