ARTICLE IN PRESS Ocean Engineering 37 (2010) 37–47 Contents lists available at ScienceDirect Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng Potential impact of sea level rise on coastal surges in southeast Louisiana Jane McKee Smith , Mary A. Cialone, Ty V. Wamsley, Tate O. McAlpin US Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199, USA a r t i c l e in fo abstract Article history: Received 23 October 2008 Accepted 19 July 2009 Available online 28 July 2009 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. Published by Elsevier Ltd. Keywords: Hurricane Katrina Sea level rise Southeast Louisiana Storm surge Waves ADCIRC STWAVE IPET 1. 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  Corresponding author. Tel.: +1 601 634 2079; fax: +1 601 634 4314. E-mail address: Jane.M.Smith@usace.army.mil (J.M. Smith). 0029-8018/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.oceaneng.2009.07.008 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,b) 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 70.124 cm/yr (1939–1974) for Eugene Island and 0.924 cm/yr with a 95% confidence interval of 70.059 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 ARTICLE IN PRESS 38 J.M. Smith et al. / Ocean Engineering 37 (2010) 37–47 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 0.1870.05 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 0.3170.07 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 1C) 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, 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. Gonza lez 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, Sections 3 and 4 describe the surge and wave results, respectively, and conclusions are given in Section 5. Table 1 Hypothetical hurricane parameters. CP, mb Rmax, km Vf, m/s Storm 9 Storm 15 Storm 17 Storm 24 Storm 53 Storm 126 900 40.4 4.9 930 47.8 4.9 900 27.6 4.9 930 47.8 4.9 900 34.1 4.9 900 32.8 4.9 2.1. Hypothetical hurricanes The six hypothetical storms selected for this analysis are summarized in Table 1. The storms have central pressure (Cp) of 900 or 930 mb, radius of maximum winds (Rmax) of 28–48 km (15–26 nm), and forward speeds (Vf) of 4.9 m/sec (11 mph). The tracks for these storms are shown in Fig. 1. These storms were selected because they produce water levels with approximately a 1% probability of occurrence in a given year in southeast Louisiana (east Orleans Parish, St. Bernard Parish, Plaquemines Parish, and southern Jefferson Parish) based on model simulations of hypothetical hurricanes using the joint probability method—optimal sampling developed through a joint Interagency Performance Evaluation Task Force (IPET), US Army Corps of Engineers, Federal Emergency Management Agency, and National Oceanic and Atmospheric Administration effort (IPET, 2007b,c). Wind fields for these hypothetical storms were generated using a planetary boundary layer (PBL) model (Thompson and Cardone, 1996). 2.2. Vegetative roughness Spatially variable Manning n coefficients were assigned to the circulation and wave model grids using land cover definitions from the United States Geological Survey (USGS) Gap Analysis Program (GAP) LA-GAP in Louisiana, USGS MS-GAP in Mississippi, and the USGS National Land Cover Data (NLCD) in Texas and Alabama (National Wetlands Research Center, 2004; Bunya et al., 2009). In Louisiana, the n values for saline to fresh marsh range from 0.035 to 0.055, for wetland to upland scrub/shrub range form 0.06 to 0.09, and for wetland to upland forest the range is 0.14–0.18. As relative sea level increases in wetlands, the vegetation type and n value will change. Wetlands evolve from freshwater marsh to brackish marsh to open water with increasing water level. Little guidance exists on how this transition takes place, so simple, but realistic rules were used to evolve the Manning n values for the increased water level simulations: 2. Methodology  If the bottom elevation is above mean tide level (MTL), The potential impact of RSLR on surge and waves due to hurricanes is evaluated using numerical surge and wave models for southeast Louisiana. A base case was run using post-Katrina bathymetry and levee heights expected to be in place in 2010. Then 0.5 and 1.0 m of additional water were added to represent future RSLR scenarios and the runs were repeated. For this evaluation, six hypothetical hurricanes were simulated. These hurricanes generated approximately 100 yr water levels in areas in southeast Louisiana (IPET, 2007c). For these simulations, the bathymetry and topography were not modified to represent coastal erosion or wetland loss (no modeling of erosion or sedimentation), but bottom roughness values were updated to reflect vegetation appropriate for the increased water levels. This section describes the six storms, the methodology used to modify the vegetative roughness, application of the surge model, the application of the wave model, and modeling uncertainty. Additional information on the modeling methodology is given by Bunya et al. (2009) and Dietrich et al. (2009). including the RSLR, the n value is not changed.  If the bottom elevation is below 0.3 m MTL (tide range in  southeast Louisiana is the order of 0.3–0.5 m): 1 If the initial n value is greater than 0.1, the value is halved and wind sheltering due to canopy is turned off. 1 If the initial n value is less than 0.1, it is set to 0.02 for open water. If the bottom elevation is between MHW and MLW: 1 If the initial n value is greater than 0.1, the value is halved and wind sheltering due to canopy is turned off. 1 If the n value is less than 0.1, it is set to 0.035 for saline marsh. Fig. 2a–c shows the Manning n values applied for the base case, 0.5 m RSLR, and 1.0 m RSLR. The roughness values are reduced in all wetland areas with RSLR. Terrebonne Bay, Barataria Bay, the West Bank, Caernarvon Marsh, and Biloxi Marsh change largely to open water for the 1 m RSLR, and the wetlands around Lake Maurepas are also significantly degraded. ARTICLE IN PRESS J.M. Smith et al. / Ocean Engineering 37 (2010) 37–47 39 Fig. 1. Hypothetical hurricane tracks for Storms 9, 15, 17, 24, 53, and 126 in southeast Louisiana. Fig. 2. Manning n values assigned for (a) base case, (b) 0.5 m, and (c) 1.0 m of relative sea level rise. 2.3. Surge model The ADCIRC unstructured coastal ocean circulation model was applied to compute water surface elevation and currents. The ADCIRC model solves the depth-integrated barotropic shallowwater equations in spherical coordinates using a finite-element solution, resulting in second-order accurate elevation and velocity solutions (Luettich and Westerink, 2004; Westerink et al., 2008). The solution is accurate and robust when applied to the wide range of scales of motion. The unstructured grid allows for high grid resolution where solution gradients are large and low grid resolution where solution gradients are small, minimizing both local and global errors for a given computational cost (Blain et al., 1998). The ADCIRC simulations were approximately 6 days in ARTICLE IN PRESS 40 J.M. Smith et al. / Ocean Engineering 37 (2010) 37–47 duration, with 2 days of river spin up (ramped up from zero to their full value), 2 days of wind and pressure forcing only, and 2 days (bracketing hurricane land fall) of wind, pressure, and wave forcing. Because the waves and surge interact, the surge was first calculated with no wave forcing, these surges were passed to the wave model to simulate wave transformation and generation, and then ADCIRC was rerun including the wave forcing. Simulations were run for six storms with three imposed mean water levels (0, 0.5, and 1.0 m of RSLR) for a total of eighteen runs. 2.4. Wave modeling Gulf of Mexico-scale wave generation and propagation was calculated with the deepwater spectral wave model WAM (Komen et al., 1994). WAM solves the time-dependent wave action balance equation, including the source-sink terms of atmospheric input, nonlinear wave–wave interactions, white capping, bottom friction, and depth-limited wave breaking. WAM was applied on a 0.05 deg. latitude–longitude grid covering the entire Gulf of Mexico with 28 frequencies and 24 direction bins. WAM was forced with the PBL wind fields. Near-coast wave spectra from WAM were then used to drive a nested nearshore spectral wave generation and transformation model STWAVE (Smith et al., 2001; Smith, 2007). STWAVE solves the steady-state conservation of spectral action balance along backward-traced wave rays. The source terms include wind input, nonlinear wave–wave interactions, white capping, bottom friction, and surf-zone breaking. The assumptions made in STWAVE include a mild bottom slope; negligible wave reflection; steady waves, currents, and winds; linear refraction and shoaling; and a depth-uniform current. STWAVE was run over four nearshore grids (including the Alabama, Mississippi and eastern Louisiana coasts and Lake Pontchartrain) with a spatial resolution of 200 m and the same 28 frequencies from WAM, but increased directional resolution (5 deg. interpolated from the 15-deg.-resolution WAM spectra). STWAVE bathymetry and friction coefficients were interpolated from the ADCIRC mesh. STWAVE was applied in half-plane mode, where only waves propagating toward the coast are represented, along the open coast and in full-plane mode, allowing generation and propagation in all directions, in Lake Pontchartrain. Wave breaking in the surf zone limits the maximum wave height based on the local water depth and wave steepness. STWAVE was run at 30 min intervals for 2 days around storm landfall using spatially variable winds and water levels interpolated from ADCIRC. Additional information on the modeling methodology and validation of the methodology for Hurricanes Katrina and Rita is given by Bunya et al. (2009) and Dietrich et al. (2009). Additionally, validation of STWAVE in Biloxi Marsh for Hurricane Gustav data is presently in progress. All wave heights reported are significant wave heights. 2.5. Modeling uncertainty Uncertainty in the RSLR modeling results from errors or uncertainty in the models themselves and in the input conditions, including winds, bathymetry, friction coefficients, and boundary conditions. ADCIRC errors based on high-water marks for Hurricanes Katrina and Rita were 0.3 and 0.2 m, respectively. STWAVE errors for the only nearshore gauges along the coast for Katrina and Rita were 0.4 and 0.1 m, respectively. The sensitivity of ADCIRC and STWAVE to model inputs for Hurricane Katrina was investigated by IPET (2007b). Increasing and decreasing the wind speeds by 5% led to a maximum of 0.3–0.6 m changes in the surge and 0.2–0.3 m changes in the nearshore waves. Erosion of the Chandeleur islands as occurred during Hurricane Katrina (representing uncertainty in bathymetry) increased the nearshore surge in areas by 0.1–0.2 m, but did not significantly change the nearshore waves. Modification of the bottom friction coefficients from pre-Katrina to post-Katrina conditions (to represent wetland losses) increased nearshore surges by up to 0.3–0.5 m and nearshore waves up to 0.2 m. Katrina was a more severe storm for this area than the simulations run for this analysis, so the error and sensitivity analyses provide an upper bound for the expected uncertainty. Also, the focus of this paper is not the absolute surge and wave magnitudes, but the relative differences between the base and the RSLR scenarios. 3. Surge results The surge results for the six storms show consistent trends in terms of the impact of RSLR on water levels in the very complex region of southeast Louisiana (complex in terms of bathymetry, landscape, and levees). Fig. 3 shows the shallow bathymetry/ topography of southeast Louisiana with the areas of interest labeled. The brown lines in the figure are levees and elevated road beds, which are represented as weirs in ADCIRC. To illustrate the surge response to RSLR, the results from Storm 17 are shown in detail, and summary plots and discussion of all the storms are presented. 3.1. Storm 17 The track of Storm 17 is through the middle of the region of interest and it is one of the more intense, smaller storms simulated (Cp ¼ 900 mb and Rmax ¼ 27.6 km (14.9 nm)). Fig. 4a–c shows the contours of maximum surge over the duration of the storm for 0, 0.5 and 1.0 m of RSLR, respectively. In the base case (0 m RSLR), maximum surge elevations are over 6.5 m in Caernarvon Marsh, 5.5 m in Lake Borgne, 5 m between Lake Pontchartrain and Lake Maurepas, 4.5 m at the Louisiana– Mississippi state line (north of Lake Borgne) and 4 m at Grand Isle. The addition of 0.5 and 1.0 m of RSLR (Fig. 4b and c) shows relatively linear increases in the peak surges in these peak-surge areas. For example, in Caernarvon Marsh, the peak surge increases from 6.5 to 7 m and 7.5 m for 0.5 and 1.0 m of RSLR, respectively. In the area between Lakes Pontchartrain and Maurepas, the peak surge response is slightly less than linear. In other areas, such as the West Bank, Lake Pontchartrain, and Lake Maurepas, where the base surge levels were moderate (2–3 m), the 0.5 m RSLR increases the peak surge to 3.5–5 m and 1.0 m RSLR increases the peak surge to 4.5–6 m. The values are as much as double and triple the RSLR over broad areas and as much a five times the RSLR in isolated areas. Fig. 5a shows the differences in maximum surge between the 0.5 m RSLR and the base case (0 RSLR) minus the 0.5 m RSLR (thus a zero value indicates that the increase in total water level is equal to the RSLR). The yellows and reds in these figures represent surge increases greater than the RSLR and the blues represent surge increases less than the RSLR. Fig. 5b shows the differences in maximum surge between the 1.0 m RSLR and the base case (0 RSLR) minus the 1.0 m RSLR. The patterns of surge increase and decrease between the 0.5 and 1.0 m RSLR are very consistent, although the ratios of differences normalized by the RSLR are slightly greater for the 0.5 m RSLR. What the areas of increased surge have in common is that they are on the right-hand side of the storm track (strong side, winds blowing generally onshore) and they are in very shallow bathymetric/topographic areas where the RSLR is a significant portion of the total water depth. Some of these areas are also in ‘‘pockets’’ that are surrounded on two or three sides by levees or topographic features. Many of the ARTICLE IN PRESS J.M. Smith et al. / Ocean Engineering 37 (2010) 37–47 41 Lake Maurepas Lake Pontchartrain Lake Borgne Biloxi Marsh West Bank West Bank Caenarvon Marsh Barataria Bay Terrebonne Bay Lower Plaquemines Parish Grand Isle Grand Isle Fig. 3. Bathymetry of southeast Louisiana. Fig. 4. (a) Maximum surge for Storm 017 base case (0 m RSLR). (b) Maximum surge for Storm 017 for 0.5 m RSLR. (c) Maximum surge for Storm 017 for 1.0 m RSLR. ARTICLE IN PRESS 42 J.M. Smith et al. / Ocean Engineering 37 (2010) 37–47 Fig. 5. (a) Increase in maximum surge for Storm 017 for 0.5 m RSLR (0.5 m RSLR surge minus base case 0.0 m RSLR surge minus 0.5 m). (b) Increase in maximum surge for Storm 017 for 1 m RSLR (1.0 m RSLR surge minus base case 0.0 m RSLR surge minus 1 m). (c) Increase in maximum surge for Storm 009 for 1 m RSLR (1.0 m RSLR surge minus base case 0.0 m RSLR surge minus 1 m). (d) Increase in maximum surge for Storm 053 for 1 m RSLR (1.0 m RSLR surge minus base case 0.0 m RSLR surge minus 1 m). (e) Increase in maximum surge for Storm 126 for 1 m RSLR (1.0 m RSLR surge minus base case 0.0 m RSLR surge minus 1 m). (f) Increase in maximum surge for Storm 024 for 1 m RSLR (1.0 m RSLR surge minus base case 0.0 m RSLR surge minus 1 m). enhanced surge areas coincide with wetlands, which are areas where the Manning n coefficients were reduced due to RSLR (Fig. 2). These locations are also generally further inland, where the shallow depths/slow surge propagation speeds or constrictions (e.g., the passes between Lake Borgne to Lake Pontchartrain) limit the volume of water inundating these locations, so RSLR and changes in marsh roughness increase the speed and inland propagation of the surge. 3.2. Storm 15 Storm 15 has the same track as Storm 17, but the storm is much larger (Rmax ¼ 25.8 nm) and slightly less intense (Cp ¼ 930 mb). The surge response is very similar to Storm 17, with slightly lower surges near the track line due to the lower intensity, but similar values away from the eye of storm. The largest increases in maximum surge are 0.5–1.0 m less than those computed for Storm ARTICLE IN PRESS J.M. Smith et al. / Ocean Engineering 37 (2010) 37–47 17. The maximum amplifications occur on the West Bank (values of 1.5 times the RSLR, compared to 2 for Storm 17) and around Lake Maurepas (values of 1.5 times the RSLR, compared to 2.5 for Storm 17). 43 South STWAVE grids (Fig. 6). The storm selection was based on illustrating the greatest RSLR impact, although the trends were generally consistent for all storms. 4.1. Lake Pontchartrain grid, Storm 17 3.3. Storms 9, 53, and 126 Storms 9, 53, and 126 have very similar surge parameters. The tracks of Storms 53 and 126 are parallel with Storm 126 making landfall further to the west. Storm 9 makes landfall west of Storm 126 and has a north–northwest track instead of the northwest tracks of Storms 53 and 126. The peak storm surge patterns for these storms are similar to each other with the highest surges in Caernarvon Marsh, Lake Borgne, and Terrebonne Bay. Storm 9 has the most westerly landfall location and thus has the largest surge in Terrebonne Bay (5 m), while Storm 53 has a more easterly landfall location and has the largest surge in Caernarvon (7.5 m). Fig. 5b shows the differences in maximum surge between the 1.0 m RSLR and the base case with the RSLR subtracted for Storm 9. This is representative of Storms 53 and 126 (Fig. 5c and d, respectively), as well. As with Storm 17, in the regions of maximum surge the surge increases over RSLR are relatively linear (although they increase by as much as 0.5 m over the RSLR). The surge increases 1–3 m (above the 1 m RSLR) in Lake Maurepas and the West Bank, areas of moderate 2–2.5 m surge in the base cases, again similar to Storm 17. 3.4. Storm 24 Storm 24 differs somewhat from the other storms in that it makes landfall further to the east along the eastern edge of Lake Pontchartrain. This means that the West Bank and Lake Maurepas are west of the storm track and the responses in these areas are much smaller for this storm, compared to the other five storm. Storm 24 has the same parameters as Storm15 (these storms are larger, but weaker than the other storms). The maximum storm surge for the base case is 5 m in Caernarvon and 5.5 m along the Mississippi River levees in lower Plaquemines Parish, east of the river. Fig. 5e shows the differences in maximum surge between the 1.0 m RSLR and the base case with the RSLR subtracted for Storm 24. The surge response to the 1.0 m RSLR shows less amplification than in the other storms, although the position of the maximum surge moves from the east river levee in lower Plaquemines Parish northward into the Caernarvon Marsh and surge height in this area increases by 1 m above the RSLR. There is also an increase of 0.5–1.0 m above the RSLR in lower Plaquemines Parish on the west side of the Mississippi River (2–4 m base surge level), 0.5 m in Lake Pontchartrain, and 1–1.5 m in Lake Maurepas. 4. Wave results Waves are impacted not only by the RSLR itself, but by the spatially variable changes in surge described in the previous section (in some cases up to 3 m). The main impacts of the RSLR and increased surge on waves are increased wave growth, reduced wave breaking, reduced refraction and shoaling, and reduced frictional dissipation. In areas of depth-limited wave breaking, wave height reduction due to breaking can be quite strong and approximately linear with the change in depth. The other effects are generally far less than linear, except in very shallow depths (on the order of meters). Refraction, shoaling, and dissipation are reduced as depth increases (and Manning n values decrease) and wave growth increases. In the interest of space, only the results of one storm are shown on each of the Pontchartrain, Southeast, and Storm 17 tracks just west of Lake Pontchartrain and produces strong westerly winds over the western part of the lake. Fig. 7a and b shows the maximum wave height for each grid cell over the duration of the storm and the associated mean wave direction for the base case and 1 m RSLR (including the increased surge), respectively. Waves are generated locally in Lake Pontchartrain and grow to maximum heights of 3.4 m for the base case and 3.6 m for the 1 m RSLR case, due to the differences in depth. For all the storms, RSLR has a minimal effect on wave growth in the lake. A larger effect is the nearshore breaking wave heights along the shoreline. As the shoreline shifts inland with RSLR, the breaking location also shifts shoreward. The differences in the peak wave heights between the base and 1 m RSLR cases are shown in Fig. 7c (positive values indicate increased wave height for the RSLR case). The most significant differences in wave height are seen in the wetlands to the northwest of Lake Pontchartrain. The wave heights in this area are as much as 1.6 m higher for the 1 m RSLR case. The higher waves are due to less wave breaking on the lake shore, less frictional dissipation (deeper depth and smaller Manning n coefficients), and possibly more wind input due to the reduced tree canopy. 4.2. Southeast grid, Storm 53 Storm 53 tracks parallel to the Mississippi River and west of the river. Fig. 8a and b shows the maximum wave height for each grid cell on the southeast grid over the duration of the storm and the associated mean wave direction for the base case and 1 m of RSLR, respectively. The southeast grid results are relevant east of the Mississippi River. For the base case, the wave height decreases gradually across the shelf from 12 to 6 m offshore of the Chandeleur Islands due to depth- and steepness-limited wave breaking. The islands are flooded at the peak of the storm, but the shallow depth induces additional, concentrated breaking and wave heights are less than 1 m in the lee of the island for the base case. In the sound landward of the Chandeleurs, the waves grow slightly and then dissipate across Biloxi and Caernarvon Marshes due to both depth-limited breaking and frictional dissipation. Wave heights in Lake Borgne are 0.6–0.8 m. With 1 m RSLR, the general patterns in wave height are the same as the base case, but the wave heights are 0.5–1.3 m higher, largely due to less dissipation across the Chandeleurs, less dissipation in the marshes, and slightly more wave growth. Wave heights are also as much as 1 m higher along the southeast portion of the River Delta. The differences in the peak wave heights between the base and 1 m RSLR cases are shown in Fig. 8c (positive values indicate increased wave height for the RSLR case). Note that the surge is not greatly enhanced due to RSLR in this area for Storm 53 (see Fig. 5c). 4.3. South grid, Storm 53 Fig. 9a and b shows the maximum wave height for each grid cell on the south grid for Storm 53 over the duration of the storm and the associated mean wave direction for the base case and 1 m of RSLR, respectively. The south grid results are relevant west of the Mississippi River. Wave height decreases gradually across the shelf from 14 to 4 m offshore of the barrier islands due to depthand steepness-limited wave breaking (similar to the southeast ARTICLE IN PRESS 44 J.M. Smith et al. / Ocean Engineering 37 (2010) 37–47 Depth (m MTL) 30.0 25.0 20.0 15.0 10.0 5.0 0.0 Pontchartrain Grid MS-AL Grid Southeast Grid South Grid Fig. 6. STWAVE grid locations overlaid on bathymetry. Fig. 7. (a) Maximum wave heights Storm 17 for base case, Pontchartrain grid. (b) Maximum wave heights Storm 17 for 1 m RSLR, Pontchartrain grid. (c) Maximum wave heights for 1 m RSLR minus base case for Storm 17, Pontchartrain grid. ARTICLE IN PRESS J.M. Smith et al. / Ocean Engineering 37 (2010) 37–47 45 Fig. 8. (a) Maximum wave heights Storm 53 for base case, southeast grid. (b) Maximum wave heights Storm 53 for 1 m RSLR, southeast grid. (c) Maximum wave heights for 1 m RSLR minus base case for Storm 53, southeast grid. grid). The islands additionally dissipate energy through breaking, and transmitted wave heights are on the order of 1 m for the base case and 2 m for the 1 m RSLR case. In Barataria and Terrebonne Bays, the wave heights are generally decreasing towards the shoreline. In the base case, there is a small increase in wave height in the center of Barataria Bay (peak period of 4 sec). In the 1 m RSLR case, this same area is dominated by 8 s peak periods that penetrate through and over the islands and suppress the local growth. The differences in the peak wave heights between the base and 1 m RSLR cases are shown in Fig. 9c (positive values indicate increased wave height for the RSLR case). In the wetlands fringing the bays and the West Bank, wave heights are 0.5–1.3 m higher for the 1 m RSLR case (the surge in these areas was 0.5–2.0 m higher for the RSLR case). The wave height differences are due to reduced breaking and frictional dissipation. Wave directions generally were consistent between the base and RSLR cases as shown in the figures. Although, differences in refraction in very shallow depths are expected. Wave periods were also generally within 1 s, except in areas were significantly more swell propagation over and around barriers or structures. 5. Conclusions This study investigates the potential impact of RSLR on hurricane storm surge from hypothetic hurricanes that produce approximately 100 yr water levels in southeastern Louisiana. RSLR in the region in the next 50–100 yr is expected to be in the range of 0.5–1 m (although these estimates are still open to a great deal of debate). RSLR will strongly impact wetland vegetation in this micro-tidal environment. The Manning n roughness values were modified to reflect the evolution of marsh type with RSLR, as wetlands evolve from freshwater marsh to brackish marsh to open water with increasing water level. The bathymetry was not modified to reflect possible erosion or accretion of sediments, but erosion is more likely in this sediment starved region. Numerical surge and wave models were used to simulate the hurricanes under the base case (no RSLR) and 0.5 and 1 m of RSLR. The trends shown in the simulations were similar:  In the peak-surge areas, the maximum surge generated under the RSLR scenarios increased relatively linearly with RSLR. Although, the surge values increased by as much as 0.5–1.0 m in some simulations. The reason for the relatively linear response is likely due to the large surge values in these areas (5–7.5 m), so the RSLR was a smaller portion of the total surge in these areas. Thus, the surge propagation and the interaction with the bottom were not so significantly changed by RSLR. But, certainly the additional of 0.5–1.0 m, in addition to RSLR, is not insignificant in the design and construction of sea walls and levees. In some areas the maximum surge is also limited by the surrounding topography and levee heights. ARTICLE IN PRESS 46 J.M. Smith et al. / Ocean Engineering 37 (2010) 37–47 Fig. 9. (a) Maximum wave heights Storm 53 for base case, south grid. (b) Maximum wave heights Storm 53 for 1 m RSLR, south grid. (c) Maximum wave heights for 1 m RSLR minus base case for Storm 53, south grid.  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 (above the RSLR) for the RSLR simulation. The water level increases are as much as double and triple the RSLR over broad areas and as much as five times the RSLR in isolated areas. The areas most impacted are the West Bank and Lake Maurepas. The deeper water depths (due to RSLR) and the degredation of the wetlands (reduced Manning n values) appear to increase the surge propagation speed and allow greater inundation. Similar amplification of surge occurred in other wetland areas, e.g., Lake Borgne and Lower Plaquemines Parish, but the amplification factors were not as high. The loss of the cypress canopy in the Lake Maurepas area also contributed to large local variations in surge associated with RSLR. Surge levels in Lake Pontchartrain also showed a consistent increase over the RSLR (0.5 for 1 m RSLR). This is due to gradients across the passes between Lake Borgne and Lake Pontchartrain pushing more water into Pontchartrain. This also contributes to higher water levels in Lake Maurepas. The surge amplification patterns were very similar for 0.5 and 1 m of RSLR, but the relative impact of RSLR (surge increase divided by RSLR) was greater for the 0.5 m RSLR. Waves increase significantly in shallow areas due to the combined increases in water depth due to RSLR and surge increases. Maximum increases for the storm suite were 1–1.5 m. The main mechanism for increased wave energy is reduction of depth-limited wave breaking, but reduced frictional dissipation and increased wave growth also contribute. In areas of depth-limited wave breaking, increased wave height is approximately linear with the change in total depth (RSLR plus surge). The major conclusion is that surge does not increase linearly with RSLR. Linear addition of RSLR to design water levels is not appropriate in this region. Although it appears that the peak surge may increase modestly (0.5–1 m above the 0.5–1 m of RSLR), regions of more modest surge (2–3 m) may see very significant increases in surge (1–3 m above the 0.5–1 m RSLR). Thus, in a statistical analysis of water levels, the long return period water levels would increase modestly above the RSLR, but shorter return period water levels could increase significantly. Surge propagation over broad, shallow, wetland areas is highly sensitive to RSLR. Waves 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. Smith et al. (2008), in a similar study where Manning n values were not changed with RSLR, found similar surge amplification results, indicating that the sensitivity to RSLR ARTICLE IN PRESS J.M. Smith et al. / Ocean Engineering 37 (2010) 37–47 is due primarily to the shallow water depths in the wetlands and not the change in Manning n roughness values. As wetlands deteriorate, it is likely that water depths will increase further and RSLR will impact surge levels to an even greater extent, compared to the base case estimates. Acknowledgments Permission to publish this paper was granted by the Office, Chief of Engineers, US Army Corps of Engineers. This research was conducted under the Wave Computations for Ecosystem Modeling under the System-Wide Water Resources Research Program of the Coastal and Hydraulics Laboratory, US Army Engineer Research and Development Center. References Blain, C.A., Westerink, J.J., Luettich, R.A., 1998. 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