Storm Surge Response Research Articles

Trend of landfalling tropical cyclones in the East China Sea and their storm surge response

In recent years, the influence of global tropical cyclones (TCs) has a tendency to expand towards the polar direction, and the proportion of high-intensity TCs has also increased greatly. The Northwest Pacific is a high incidence area of TCs, the northward shift of landfalling TCs in China and the increase of high-intensity TCs will lead to an increased risk of storm surge in the East China Sea (ECS). However, due to the lack of long-term observations in the ECS, previous studies have focused on the analysis of individual TC, and the response mechanism of storm surge to long-term trends of TCs is not clear. Based on the long-term observations of TCs from 1949 to 2022 and the numerical experiments of storm surge, the trend of landfalling TCs in China and the response characteristics of storm surge in the ECS were studied in this paper. The observations showed that the probability of TCs entering ECS became greater in the latter 37 years compared with the former 37 years. The average intensity of landfalling TCs (maximum wind speed) increased by 36% to 30 m/s, and the average speed of landfalling TCs increased by 100% to 8 m/s. The model results showed that the intensity of the storm surge is closely related to the intensity and translation speed of TCs. Under the condition of the same translation speed of TCs, the storm surge enhanced with the intensity of TCs. For the TCs with high intensity (strong typhoon and super typhoon), the intensity of the storm surge continues to increase as the translation speed of the TCs increases. However, for the TCs with low or medium intensity (severe tropical storms and typhoons), the intensity of the storm surge reaches a peak with the increase of the translation speed of TCs. The intensity of the storm surge is determined by both the wind speed and the duration of strong wind. The TCs with faster translation speed can produce a stronger wind field but shorter duration of strong wind. Therefore, the intensity of the storm surge decreases when the translation speed of TCs exceeds the threshold value.

Relative Sea Level Rise Impacts on Storm Surge Flooding of Transportation Infrastructure

Relative sea level rise increases the vulnerability of coastal infrastructure to storm surge flooding. In this study, we develop, validate, and apply a coupled hydrodynamic and wave model to simulate storm surge inundation under different local sea level projections to quantitatively assess some of these vulnerabilities. Our study area is located in southeast Virginia, where the rate of relative sea level rise is the highest in the US East Coast. The model is developed in two levels of nesting in which a region-scale model with relatively low resolution provides boundary conditions for a high-resolution city-scale model. The model is first validated with documented water levels in water and over land during Hurricane Irene (2011). It is then applied to investigate changes in vulnerability of transportation infrastructure to relative sea level rise by quantifying changes in flood intensity and duration over access roads to critical bridges that are identified based on elevation and traffic volume. Furthermore, the length of flooded roadways as well as the extent and volume of inundation is quantified under different combinations of relative sea level rise and storm return periods. With a 1.8-m sea level rise, an upper bound of sea level projections in the study area in 2070, and under 10- and 50-year storms, collective flood intensity and duration increase by factors of 23 and 51, respectively, and the length of flooded roadways increase by factors of 9 and 7 from their present values, respectively. Variations in flood intensity, duration, area, and volume under storm and sea level rise combinations indicate that storm surge response to relative sea level rise is generally nonlinear, and linear superposition of these factors mostly overestimates inundation intensity while consistently overestimating inundation volume. Variation of rate of change in total inundation volume is more complex and suggests that the linear summation overestimates flood volume for small to moderate rates of sea level rise while it underpredicts flood volume for high sea level projections. This suggests that nonlinear effects act to intensify flood vulnerability at an accelerated rate as sea level rises.

Development of a tropical storm surge prediction system for Australia

Abstract The Australian tropical storm surge forecasting system is described, including the development of a tropical cyclone atmospheric forcing model and the configuration of the ocean hydrodynamic model. The atmospheric model is developed as an asymmetric modified Rankine vortex and the resulting time dependent stress and pressure fields are applied to a shallow water hydrodynamic model. The system was benchmarked against seven contemporary tropical cyclones occurring within the northern Australian region between 2011and 2017. The model storm surge response to the synthetic forcing was compared against tide gauge observations. For the seven test cases, the root mean square error for maximum sea level was 0.30 m, the mean absolute error was 0.21 m and the mean bias error was 0.11 m. For peak timings, the root mean square error of the model was 62 min, the mean absolute error was 48 min and the mean bias error was 8 min. Surface forcing fields were compared against observations for TC Yasi and found to be in general agreement.

Influence of offshore shoals on spatial variability of storm surge

Idealized bathymetries were subjected to idealized cyclones in order to measure the storm surge response to a range of shoals, under various storm conditions, for the purpose of informing the development of bathymetry thinning algorithms. Nine bathymetries were considered, including eight large, offshore shoals and a featureless reference domain. Six storm realizations (two different sizes/intensities and three different landfall directions) were used as meteorological forcing. Shoal influence on surge was examined over the entire continental shelf throughout the surge event. Quantification of shoal impact on surge was performed prior to landfall, near the time of peak surge, and during surge recession. Three elevation difference contours (1.0, 0.5, and −0.5 m) were used to quantify the spatial variation in surge caused by the shoals at the three times examined. Positive contours (1.0 and 0.5 m) were used to show that a shoal increased surge locally and the negative contour (−0.5 m) to show that a shoal decreased surge locally. The influences of three shoal parameters (depth below mean sea level (MSL), cross-shore width, and distance from shore) were isolated and examined to see which characteristics caused the shoals to alter surge.In many cases, the shoals did not have much effect on the surge response. Only 13% of the samples used to quantify shoal impact on surge indicated any shoal influence whatsoever, and the shoals did not alter maximum elevation within the threshold considered (Δη≥|±0.5m|) anywhere in the domain. However, analysis of the elevation difference contours revealed multiple types of shoal influence. The shoals increased surge when they reduced setdown in storms with offshore winds prior to landfall and when they delayed surge recession after all storms. The shoals decreased surge when they impeded shoreward flow, delaying the arrival of surge. In cases when the shoals altered surge, 95% of the impact was caused by Shallow features (0.5 m below MSL). Also, there appears to be a maximum area over which a shoal can influence surge. All changes in surge greater than the threshold of Δη≥|±0.5m| occurred over the shoal or approximately between the shoal and the shore.The implications of our results for bathymetry thinning follow: In general, offshore shoals do not need to be resolved in bathymetry data sets used in surge modeling, especially if their impact on maximum elevation is the primary concern. Since a shoal's depth below MSL was an excellent indicator of whether it would impact storm surge, we recommend that feature depth below MSL be used as a bathymetry thinning criterion. Finally, a feature's maximum area of influence could be a useful property for determining whether it should be retained in a bathymetry dataset – if one's area of interest is outside of a feature's area of influence, the feature could be omitted regardless of its impact on surge.

An objective and efficient method for estimating probabilistic coastal inundation hazards

The joint probability method (JPM) is the traditional way to determine the base flood elevation due to storm surge, and it usually requires simulation of storm surge response from tens of thousands of synthetic storms. The simulated storm surge is combined with probabilistic storm rates to create flood maps with various return periods. However, the map production requires enormous computational cost if state-of-the-art hydrodynamic models with high-resolution numerical grids are used; hence, optimal sampling (JPM-OS) with a small number of (~ 100–200) optimal (representative) storms is preferred. This paper presents a significantly improved JPM-OS, where a small number of optimal storms are objectively selected, and simulated storm surge responses of tens of thousands of storms are accurately interpolated from those for the optimal storms using a highly efficient kriging surrogate model. This study focuses on Southwest Florida and considers ~ 150 optimal storms that are selected based on simulations using either the low fidelity (with low resolution and simple physics) SLOSH model or the high fidelity (with high resolution and comprehensive physics) CH3D model. Surge responses to the optimal storms are simulated using both SLOSH and CH3D, and the flood elevations are calculated using JPM-OS with highly efficient kriging interpolations. For verification, the probabilistic inundation maps are compared to those obtained by the traditional JPM and variations of JPM-OS that employ different interpolation schemes, and computed probabilistic water levels are compared to those calculated by historical storm methods. The inundation maps obtained with the JPM-OS differ less than 10% from those obtained with JPM for 20,625 storms, with only 4% of the computational time.

Assessing the Multiple Impacts of Extreme Hurricanes in Southern New England, USA

The southern New England coast of the United States is particularly vulnerable to land-falling hurricanes because of its east-west orientation. The impact of two major hurricanes on the city of Providence (Rhode Island, USA) during the middle decades of the 20th century spurred the construction of the Fox Point Hurricane Barrier (FPHB) to protect the city from storm surge flooding. Although the Rhode Island/Narragansett Bay area has not experienced a major hurricane for several decades, increased coastal development along with potentially increased hurricane activity associated with climate change motivates an assessment of the impacts of a major hurricane on the region. The ocean/estuary response to an extreme hurricane is simulated using a high-resolution implementation of the ADvanced CIRCulation (ADCIRC) model coupled to the Precipitation-Runoff Modeling System (PRMS). The storm surge response in ADCIRC is first verified with a simulation of a historical hurricane that made landfall in southern New England. The storm surge and the hydrological models are then forced with winds and rainfall from a hypothetical hurricane dubbed “Rhody”, which has many of the characteristics of historical storms that have impacted the region. Rhody makes landfall just west of Narragansett Bay, and after passing north of the Bay, executes a loop to the east and the south before making a second landfall. Results are presented for three versions of Rhody, varying in the maximum wind speed at landfall. The storm surge resulting from the strongest Rhody version (weak Saffir–Simpson category five) during the first landfall exceeds 7 m in height in Providence at the north end of the Bay. This exceeds the height of the FPHB, resulting in flooding in Providence. A simulation including river inflow computed from the runoff model indicates that if the Barrier remains closed and its pumps fail (for example, because of a power outage or equipment failure), severe flooding occurs north of the FPHB due to impoundment of the river inflow. These results show that northern Narragansett Bay could be particularly vulnerable to both storm surge and rainfall-driven flooding, especially if the FPHB suffers a power outage. They also demonstrate that, for wind-driven storm surge alone under present sea level conditions, the FPHB will protect Providence for hurricanes less intense than category five.

Surge dynamics across a complex bay coastline, Galveston Bay, TX

While tidal exchange across bay coastlines has been well studied, research to date has not provided a comprehensive analysis of the volume of surge that can flow across bay coastlines during tropical cyclone events. Such insight can particularly be useful for understanding coastal storm surge dynamics that influence regional inundation and can help guide surge mitigation strategies for semi-enclosed bay systems. In this study, a suite of 80 synthetic tropical cyclones were simulated to characterize the volume of surge that flows across Galveston Bay's complex 98 km coastline, which is made up of tidal inlets, intermittent barrier islands, and existing surge defenses along the Upper Texas Gulf Coast. Tropical cyclone characteristics analyzed in this study include the wide variety of tropical cyclone intensity (Vmax) and size (Rmax) combinations possible for the region, with the storm surge response of each tropical cyclone simulated using the ADCIRC + SWAN model. In-depth analysis of coastal and in-bay storm surge responses was performed for baseline or existing conditions as well as scenarios representing severe barrier island erosion and future conditions based on intermediate sea level projections for the year 2050 and 2 100. Findings demonstrate that while the majority of surge may flow across tidal inlets for smaller, weaker storms, a storm intensity and size threshold exists that results in greater flow across barrier islands, having important implications for surge mitigation strategies. Analysis demonstrates that while this threshold may occur for extreme tropical cyclones under baseline conditions, this transition can occur for relatively, significantly weaker and smaller storms under the erosion and sea level rise scenarios evaluated.

Development of a Real-time Storm-surge Response System for Decision-making Support on the Korean Coast

Lee, H.-Y.; Kim, D.-S.; Jeong, Y.-H., and Hong, S.-J., 2018. Development of a real-time storm-surge response system for decision-making support on the Korean coast. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 911–915. Coconut Creek (Florida), ISSN 0749-0208.A real-time storm surge prediction system based on typhoon advisories issued by the Korea Meteorological Administration was constructed to support decision-making regarding the mitigation of storm surge disasters. This approach was tested in real time by providing storm surge prediction data during the approach of Typhoon Chaba in 2016, to support National Disaster and Safety Status Control Center decision-making in response to that system's storm surge. A 140 core-based parallel cluster was used for storm surge prediction and the entire process (from storm surge height prediction to provision of information for decision-making) lasted three hours. The root mean square errors of the maximum water surface elevation and the maximum surge height provided were 0.09–0.11 m and 0.14–0.26 m, respectively, and the relative errors were 6–9% and 34–69%.

Operation of Real-time Storm Surge Response System for Decision-making Support

Operation of Real-time Storm Surge Response System for Decision-making Support

The mechanism of shallow submarine landslides triggered by storm surge

Submarine landslides triggered by storm surge have a major impact on human life by destabilizing offshore installations and generating tsunamis. Based on disaster-related marine geology, this paper analyzes the characteristics of storm surge load and response of seabed soil under the action of storm surge, and then examines mechanisms of submarine landslides triggered by storm surge. Important advances in this field are summarized and a scope for future studies is suggested. Current studies show that there are three load forms of storm surge: storm tide, storm wave, and storm current. Among these, the storm wave is the main factor in submarine slope instability. Under storm surge load, there is a series of changes in seabed soil, such as pressure consolidation, liquefied rheology, and mass transport. Therefore, two types of theory can be used to explain the mechanism of submarine landslide, liquefaction failure, and shear failure. Future studies in this field may focus on the mechanism of soil–water interaction under storm surge, establishing the critical condition of slope failure and failure modes of submarine landslides, and improving techniques of field monitoring and testing.

Quantifying the potential impact of land cover changes due to sea-level rise on storm surge on lower Texas coast bays

In this study we investigated the impacts of potential changes of land cover due to sea-level rise (SLR) on storm surge (i.e., the rise of water above normal sea level, namely mean-sea level and the astronomical tide, caused by hurricane winds and pressure) response inside bays on the lower Texas coast. We applied a hydrodynamic and wave model (ADCIRC+SWAN) forced by hurricane wind and pressure fields to quantify the importance of SLR-induced land cover changes, considering its impacts by changing bottom friction and the transfer of wind momentum to the water column, on the peak surge inside coastal bays. The SLR increments considered, 0.5m to 2.0m, significantly impacted the surge response inside the bays. The contribution of land cover changes due to SLR to the surge response, on average, ranged from a mean surge increase of 2% (SLR of 0.5m) to 15% (SLR of 2.0m), in addition to the SLR increments. The increase in surge response strongly depended on storm condition, with larger increases for more intense storms, and geographical location. Although land cover changes had little impact on the surge increase for SLR increments lower than 1.0m, intense storms resulted in surge increase of up to 10% even for SLR below 1.0m, but in most cases, the geometry changes were the major factor impacting the surge response due to SLR. We also found a strong relationship between changes in bottom friction and the surge response intensification; demonstrating the importance of considering land cover changes in coastal regions that are highly susceptible to SLR when planning for climate change.

Dynamics of sea level rise and coastal flooding on a changing landscape

AbstractStandard approaches to determining the impacts of sea level rise (SLR) on storm surge flooding employ numerical models reflecting present conditions with modified sea states for a given SLR scenario. In this study, we advance this paradigm by adjusting the model framework so that it reflects not only a change in sea state but also variations to the landscape (morphologic changes and urbanization of coastal cities). We utilize a numerical model of the Mississippi and Alabama coast to simulate the response of hurricane storm surge to changes in sea level, land use/land cover, and land surface elevation for past (1960), present (2005), and future (2050) conditions. The results show that the storm surge response to SLR is dynamic and sensitive to changes in the landscape. We introduce a new modeling framework that includes modification of the landscape when producing storm surge models for future conditions.

A Numerical Simulation of Extratropical Storm Surge and Hydrodynamic Response in the Bohai Sea

A hindcast of typical extratropical storm surge occurring in the Bohai Sea in October 2003 is performed using a three-dimensional (3D) Finite Volume Coastal Ocean Model (FVCOM). The storm surge model is forced by 10 m winds obtained from the Weather Research Forecasting (WRF) model simulation. It is shown that the simulated storm surge and tides agree well with the observations. The nonlinear interaction between the surge and astronomical tides, the spatial distribution of the maximum surge level, and the hydrodynamic response to the storm surge are studied. The storm surge is the interaction of the surge and the astronomical tides. The currents change rapidly during the storm surge and turn to be the unidirectional at some places where the tidal currents are usually rectilinear. The results show that the local surge current velocity in each depth, with a magnitude of the same order as the astronomic tidal currents, increases or decreases rapidly depending on the relationship between the winds and current directions. Furthermore, the current pattern gets more complicated under the influence of the direction of the winds, which might affect sand movement in the coastal water of the Bohai Sea.

Sea-Level Rise Effects on Storm Surge and Nearshore Waves on the Texas Coast: Influence of Landscape and Storm Characteristics

AbstractSea-level rise (SLR) estimates vary broadly, but most estimates predict significant increases in sea levels within the next century. Through the use of validated, coupled wave and hydrodynamic models, this study investigates SLR effects on storm surge and nearshore waves, including variation with coastal landscape type and storm characteristics. Both the wave and hydrodynamic models account for bottom friction based on land-use type, so the modeling procedure accounted for SLR-related changes to land cover, in addition to higher mean sea levels. Simulation of storms with similar meteorological characteristics and track but with landfall in the northeastern and southwestern coastal areas allowed examination of how the coastal features influenced the storm surge and wave response to increased sea levels. The simulation results analyzed here demonstrate that the relationship between storm surge and relative SLR (RSLR) varies between geographic region and storm scenario. The increase in an inundated a...

Effect of hurricane paths on storm surge response at Tianjin, China

A hurricane induced storm surge simulation system was developed for Tianjin coast, which consists of a hurricane model and a storm surge model. The peak storm surge result of the simulation agreed well with that of the observation. Three observed paths (Rita, Mimie and WINNIE) and a hypothetical path (Rita2) were chosen as the selective hurricane paths according to their positions relative to Tianjin. The sensitivity of Tianjin storm surge to the four paths was investigated using the validated storm surge simulation system. Three groups of experiments were done. In group one, the models were forced by the wind field and air pressure; in group two and three the models were forced by the wind only and the air pressure only respectively. In the experiments, the hurricane moved with a fixed speed and an intensity of 50 year return period. The simulation results show that path of the type Rita2 is the easiest to cause storm surge disaster in Tianjin, and the effect of air pressure forcing is most evident for path of the type Rita in Tianjin storm surge process. The above conclusions were analyzed through the evolution of the wind fields and the air pressure distributions. Comparing the experiment results of Group one, two and three, it can be seen that the storm surge is mainly induced by the wind forcing and the nonlinear interaction between the effect of wind forcing and air pressure forcing on the storm surge tends to weaken the storm surge.

A Retrospective Evaluation of the Storm Surge Produced by Hurricane Gustav (2008): Forecast and Hindcast Results

Abstract The evolution and convergence of modeled storm surge were examined using a high-resolution implementation of the Advanced Circulation Coastal Ocean and Storm Surge (ADCIRC) model for Hurricane Gustav (2008). The storm surge forecasts were forced using an asymmetric gradient wind model (AWM), directly coupled to ADCIRC at every time step and at every grid node. A total of 20 forecast advisories and best-track data from the National Hurricane Center (NHC) were used as input parameters into the wind model. Differences in maximum surge elevations were evaluated for ensembles comprised of the final 20, 15, 10, and 5 forecast advisories plus the best track. For this particular storm, the final 10–12 forecast advisories, encompassing the last 2.5–3 days of the storm’s lifetime, give a reasonable estimate of the final storm surge and inundation. The results provide a detailed perspective of the variability in the storm surge due to variability in the meteorological forecast and how this changes as the storm approaches landfall. This finding is closely tied to the consistency and accuracy of the NHC storm track forecasts and the predicted landfall location and, therefore, cannot be generalized to all storms in all locations. Nevertheless, this first attempt to translate variability in forecast meteorology into storm surge variability provides useful insights for guiding the potential use of storm surge models for forecast purposes. Model skill was also evaluated for Hurricane Gustav by comparing observed water levels with hindcast modeled water levels forced by river flow, tides, and several sources of wind data. The AWM (which ingested best-track information from NHC) generated winds that were slightly higher than those from NOAA’s Hurricane Research Division (HRD) H*Wind analyses and substantially greater than the North American Mesoscale (NAM) model. Surge obtained using the AWM more closely matched the observed water levels than that computed using H*Wind; however, this may be due to the neglect of the contribution of wave setup to the surge, especially in exposed areas. Several geographically distinct storm surge response regimes, some characterized by multisurge pulses, were identified and described.

A surge response function approach to coastal hazard assessment. Part 2: Quantification of spatial attributes of response functions

In response to the 2004 and 2005 hurricane seasons, surge risk assessment approaches have been re-evaluated to develop more rapid, reliable methods for predicting the risk associated with extreme hurricanes. Here, the development of dimensionless surge response functions relating surge to hurricane meteorological parameters is presented. Such response functions present an opportunity to maximize surge data usage and to improve statistical estimates of surge probability by providing a means for defining continuous probability density functions. A numerical modeling investigation was carried out for the Texas, USA coastline to develop physical scaling laws relating storm surge response with hurricane parameters including storm size, intensity, and track. It will be shown that these scaling laws successfully estimate the surge response at any arbitrary location for any arbitrary storm track within the study region. Such a prediction methodology has the potential to decrease numerical computation requirements by 75% for hurricane risk assessment studies.

Coupling of Hydrodynamic and Wave Models: Case Study for Hurricane Floyd (1999) Hindcast

This paper demonstrates a practical application of coupling a hydrodynamic model with a wave model for the calculation of storm tide elevations in the St. Johns River (Northeastern Florida). Hurricane Floyd (1999) is chosen as the storm of interest due to its track which paralleled the northeast coast of Florida without making a direct landfall on the St. Johns River. The advanced circulation (ADCIRC) numerical code is applied as the hydrodynamic model for the computation of two-dimensional circulation resulting from astronomic tides and meteorologically induced storm surge. The simulating waves nearshore (SWAN) numerical code is applied as the wave model for the computation of wind-induced waves. Two model implementations are considered in order to investigate the effect of short wave contributions on the overall storm tide water level: (1) a one-way coupling procedure that transfers gradient of wave radiation stresses from SWAN to ADCIRC; and (2) a two-way coupling procedure that builds on the former to include feedback of water levels and currents from ADCIRC to SWAN. Simulated storm tide elevations are compared to historical National Ocean Service data to result in two major conclusions: (1) wind-induced waves play a significant role in the storm tide (contributing 10–15% of the peak water level), namely with respect to the transfer of momentum from the dissipation of short waves to the long-wave motion of the storm surge; and (2) a local-scale hydrodynamic model requires the application of a hydrograph boundary condition in order to account for the remote effects of the storm surge response.

The influence of domain size on the response characteristics of a hurricane storm surge model

The influence of domain size on boundary condition specification and on computed storm surge response is investigated. Storm surge response along the Florida shelf in the Gulf of Mexico due to Hurricane Kate is examined over three domains using two different open ocean boundary forcing functions, a still water (or zero elevation) condition and an inverted barometer condition which accounts for the atmospheric pressure component of the meteorological forcing. The first domain is relatively small and is situated primarily on the continental shelf in the region of intense storm surge generation. A second domain includes the entire Gulf of Mexico basin. The final domain covers the Gulf of Mexico, contiguous basins, and extends out into the deep Atlantic Ocean. The computed storm surge response indicates that the small domain is inadequate, since cross‐shelf boundaries are in regions of significant storm surge generation where surge and therefore boundary conditions are not known a priori. Also, the behavior of resonant modes that are physically excited within the Gulf of Mexico due to the passage of the hurricane is unknown at the boundaries of this small domain. The domain that includes the entire Gulf of Mexico captures the primary storm surge well but may not correctly model resonant modes. In general, these resonant modes are difficult to accurately set up by boundary condition specification, since they may be dependent on interactions between the Gulf and contiguous basins. The primary storm surge response as well as resonant modes excited by the storm are best represented using a domain which encompasses the western North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. This domain with deep Atlantic Ocean boundaries facilitates simple boundary condition specification and minimizes the influence of boundary conditions on storm surge generation in coastal regions. Basin resonant modes and basin to basin interactions are also captured.

STORM SURGE OVER THE MISSISSIPPI RIVER DELTA ACCOMPANYING HURRICANE BETSY, 1965

Abstract Selected tide water gage readings taken during hurricane Betsy, 1965, along the Mississippi River Delta and adjacent areas are graphically presented. The various processes involved in the change in tide water level associated with the passage of a hurricane are discussed. The data are divided into three main categories in order to show the storm surge response along the coast (Gulf Coast gages), on inland waterways (New Orleans area gages), and on an inland lake (Lake Pontchartrain gages). A discussion of surge elevation along the Mississippi River from the mouth upriver to New Orleans is also included. The topography of the City of New Orleans is described and the extent and probable cause of the flooding in the City are discussed. The storm surge elevations along the coast predicted by currently used methods are found to compare favorably with the actual readings during Betsy.