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

Coastal Engineering 138 (2018) 165–183

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Surge dynamics across a complex bay coastline, Galveston Bay, TX Benjamin Bass a, *, Jacob M. Torres b, John N. Irza c, Jennifer Proft d, Antonia Sebastian e, Clint Dawson f, Philip Bedient e a

Woodard & Curran, United States Lockwood, Andrews, and Newnam, Inc., United States c Walter P. Moore, United States d Department of Aerospace Engineering, University of Texas at Austin, United States e Department of Civil and Environmental Engineering, Rice University, United States f Department of Aerospace Engineering and Engineering Mechanics, University of Texas at Austin, United States b

A R T I C L E I N F O

A B S T R A C T

Keywords: Hurricane Storm surge ADCIRC Galveston bay Barrier islands Sea level rise

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 þ SWAN model. In-depth analysis of coastal and inbay 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.

1. Introduction The U.S. Gulf and North Atlantic Coasts are incised by numerous bay environments that commonly serve as metropolitan areas and industrial ports. While such geographic locations may provide exceptional social and economic opportunities, they are also vulnerable to tropical cyclone storm surge. Storm surge-driven disasters can dramatically impact the short and long-term well-being of the local inhabitants and infrastructure of these regions, and can potentially have larger, regional or national effects because of the economic importance of these communities. In recent years, a number of inhabited bays have experienced severe storm surge impacts, including Chesapeake Bay (Hurricane Isabel – 2003; Sheng et al., 2010), Galveston Bay (Hurricane Ike – 2008; Hope et al.,

2013; Sebastian et al., 2014), and New York Harbor (Hurricane Sandy – 2012; Brandon et al., 2014). During tropical cyclones, barrier islands provide a natural form of protection that can help prevent coastal surge from entering a bay, whereas tidal inlets serve as a direct connection through which coastal surge can enter a bay. Although tidal exchange that occurs across tidal inlets is generally well-characterized, exchange that takes place across bay coastlines during tropical cyclone events is less predictable. This is due to non-linear surge interactions across coastline-bay environments that depend on various factors, including the bathymetry of tidal inlets and topography of barrier islands, tropical cyclone characteristics and their respective influence on storm surge, and erosion and breaches that can alter barrier islands during tropical cyclone strikes. In addition, while

* Corresponding author. Woodard & Curran, 888 Figueroa St., Suite 1700, Los Angeles, CA, United States. E-mail addresses: [email protected] (B. Bass), [email protected] (J.M. Torres), [email protected] (J.N. Irza), [email protected] (J. Proft), [email protected] (A. Sebastian), [email protected] (C. Dawson), [email protected] (P. Bedient). https://doi.org/10.1016/j.coastaleng.2018.04.019 Received 12 July 2016; Received in revised form 26 April 2018; Accepted 27 April 2018 Available online 8 May 2018 0378-3839/© 2018 Elsevier B.V. All rights reserved.

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it is known that sea level rise will exacerbate coastal flooding, exactly how it will impact the exchange of storm surge across bay coastlines during TC events is not known. Such analysis is important to understand coastal storm surge dynamics that can help guide surge mitigation strategies for present-as well as future-conditions. Past studies have evaluated TC storm surge flows across different coastal sections (e.g. tidal inlets and barrier islands) of the coastlines of Louisiana (Grzegorzewski et al., 2011), Long Island, New York (Ca~ nizares and Irish, 2008), and Galveston Bay (Rego and Li, 2010; Sebastian et al., 2014). However, these studies only considered the impacts of a few historical or synthetic tropical cyclone events with randomly varying characteristics or only portions of a bay's coastline. While single event analyses can be useful for understanding what may have happened during a historical event, in order to comprehensively understand and characterize the non-linear variations and relative changes in flow that can occur across tidal inlets and barrier islands of a bay's coastline, a range of storms with standardized characteristics must be evaluated using physics-based numerical models. The objective of this paper is to characterize general trends in flow across Galveston Bay's complex coastline using the tightly coupled ADvanced CIRCulation (ADCIRC) and Simulating WAves Nearshore (SWAN) model (referred to as ADCIRC þ SWAN). This study uses the term “coastal surge” to refer to surge that flows across a bay's coastline and “local surge” to refer to surge solely generated within a bay. Coastal surge was quantified by determining the volume of surge or net flow entering Galveston Bay due to inundation overwash across the bay's coastline for a suite of 80 synthetic tropical cyclones and Hurricane Ike (2008) for reference. Galveston Bay is located along the Upper Texas (TX) Coast of the Gulf of Mexico, and was chosen as the study location because of vulnerability, societal importance and its complex coastline, which is made up of various tidal inlets, barrier islands, and an existing seawall. By using tropical cyclones with a wide variety of possible storm size (represented by radius to maximum winds or Rmax) and intensity (represented by maximum 1-min sustained wind speed or Vmax) combinations that could impact the study area, non-linear trends in the volume of surge that flows across Galveston Bay's coastline and its different tidal inlet and barrier island sections was evaluated. In addition to understanding general trends in coastal surge dynamics due to variations in the Rmax and Vmax of storms, sensitivity analysis was performed with respect to TC forward speed (Vf). Finally, variations in coastal surge dynamics relative to baseline or existing conditions were evaluated for projected sea level increases in the year 2050 and 2 100 and severe erosion scenarios for the bay's barrier islands based on conditions observed post-Ike (2008). Since, as discussed later, the volume of surge across the bay's coastline and hence its different coastline sections can serve as an indicator of the extent of inland inundation behind the bay's coastline, information from this study can be useful for understanding how regional surge impacts to the bay may vary for different TC characteristics. Furthermore, the location and height of surge mitigation designs for the region can be designed more effectively by understanding where and how much surge flows across Galveston Bay's tidal inlets and barrier islands for different tropical cyclone characteristics under current and future sea level conditions. Since analysis of coastal surge dynamics provides useful insight for surge mitigation strategies in the study region, proof-of-concept simulations involving the introduction of surge mitigation features (i.e. levees) were additionally performed to inform coastal engineers of some of the practical findings from this study. This analysis involved optimizing levee placement based on further understanding baseline coastal storm surge dynamics. In addition, local surge was evaluated to understand the residual surge that can still develop within Galveston Bay when fully protected by a coastal barrier that prevents coastal storm surge from flowing into the bay. This analysis highlights the levels of local surge that can develop due to wind-fetch that acts across the bay, which depends on various factors, including the volume of water in the bay, the local

bathymetric and topographic features of the bay, and tropical cyclone intensity and other meteorological characteristics. While results apply to Galveston Bay, the methods used to advance the understanding of surge dynamics and guide general surge mitigation concepts can be applied to other bay environments. Section 2 outlines the study area of interest and Section 3 the methodology used to evaluate the coastal and local surge response in the study area. In Section 4 results are provided, followed by a discussion of the study's findings and its implications in Section 5. Finally, a brief conclusion is provided in Section 6. 2. Study area Galveston Bay is a semi-enclosed, tidally influenced bay located on the Upper Texas Gulf Coast with a surface area of approximately 1 554 km2 and an average depth of roughly 3 m (Fig. 1a). As the seventh largest estuary in the United States, the bay serves as the primary connection between the Gulf of Mexico and inland communities of the greater Houston-Galveston region. Primary freshwater tributary dischargers to the bay include the Lower San Jacinto River (LSJR) and the Trinity River, while minor tributaries include smaller bayous such as Clear Creek (Fig. 1a). Along the perimeter of Galveston Bay, more than half a million (545,346) people live less than 7.6 m above mean sea level (U.S. Census Bureau, 2010). The bay is also home to critical facilities, including the second largest petrochemical complex in the world, and the number one port in the U.S. in terms of export tonnage (Port of Houston Authority, 2015). The Houston-Galveston region's historical vulnerability to extreme tropical cyclone events has demonstrated the need to protect it from storm surge. Since 1900, the bay has experienced 21 TC strikes within a 100 km radius from the center of the bay, 8 of which were Category 3 (49.39 m/s or 110 mph) or greater (NOAA, 2016a). While the 5.2 m Galveston Seawall and the 4.6–7.6 m Texas City Levee help protect localized areas (Fig. 1a), a significant number of people and infrastructure are still vulnerable to storm surge in the Houston-Galveston region. Storm surge can flow across various sections of Galveston Bay's coastline, including its tidal inlets, barrier islands, and the Galveston Seawall. To help understand, predict, and protect against surge impacts to the entire Houston-Galveston region, this study evaluates the volume of storm surge that can flow across the bay's coastline and its different sections. As discussed in the results section, the volume of surge that flows across the bay's coastline can provide a direct indication of regional impacts as compared to peak surge which can vary significantly throughout the bay. Fig. 1a and b outline Galveston bay's different coastline sections, which are referred to throughout the paper, from west to east, as Tidal Inlet 1 (T1) for San Luis Pass, Barrier Island 1 (B1) for west Galveston Island, Barrier 2 (B2) for the Galveston Seawall, Tidal Inlet 2 (T2) for Bolivar Roads, and Barrier Island 3 (B3) for Bolivar Peninsula. B1 and B3 partially protect the region against storm surge as natural sand barrier islands. While the topography along these barrier islands varies significantly in their respective alongshore and cross-sectional dimensions (Fig. 1b), B1 and B3 stand from 0.75 to 2 m above MSL. Peak dune elevations along the seaward facing side of these barrier islands range from 1.5 to 3 m above MSL for B1 and B3. The main tidal inlets along the bay's coastline include T1 and T2, which, respectively, contribute roughly 20% and 80% of the diurnal tidal exchange that occurs between the bay and the Gulf of Mexico (Lester and Gonzalez, 2011). T2 makes up the majority of tidal exchange because of its extensive width (3.75 km) and maximum depth of roughly 15 m, as compared to T1, which has a width of 1.1 km and a maximum depth of 4.25 m (Fig. 1b). Various local and regional surge mitigation strategies have been proposed for the Houston-Galveston region since Hurricane Ike (2008) made landfall directly at Bolivar Roads (T2) (Merrell et al., 2011; GCCPRD, 2015; SSPEED Center, 2015; Torres et al., 2017). Many of the strategies include coastal barriers or levees along Galveston Bay's barrier islands and/or flood gates across the bay's tidal inlets; however the extent 166

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Fig. 1. 1a) Plan-view of the Galveston Bay study region. 1b) Cross-section view of Galveston Bay's different coastline sections.

previous parameters were specified by FEMA at 6-hr intervals along the tropical cyclones track for the PBL model. The resulting output included wind and pressure fields at 15-min intervals with a 2-km resolution. To standardize landfall locations, intensities, and sizes of the storms and develop a storm set representative of nearly the full range of possible tropical cyclone sizes and intensities that could impact the study region, the FEMA wind- and pressure-fields were then shifted to different landfall locations and/or scaled. Landfall locations were standardized by shifting storm wind- and pressure-fields while preserving their original spatial and temporal characteristics. To subsequently allow for standardized variations in storm intensity at a given landfall location, wind- and pressure-fields were scaled or increased/decreased at each time-step along its path, similar to the methodology used in Sebastian et al. (2014) and Torres et al., 2017. To evaluate baseline (existing) coastal surge conditions, 20 storms were simulated at four landfall locations near Galveston Bay, for a total of 80 simulations. Characteristics of the storms are listed in Table 1 and represent unique combinations of maximum wind speeds (35.8–71.5 m/ s, 80 to 160 mph) and radius to maximum winds (15–62 km). All storms were simulated using the same forward speed (6.7 m/s), track, and angle of approach (41 relative to due north). Fig. 2a depicts the different landfall locations and tracks of the storm suite. Fig. 2b illustrates the different storm sizes and intensities evaluated, and how they compare to historically observed storms in the Gulf of Mexico. Typical landfall Holland B values for the region (0.9–1.2), which define the shape of a TC's wind and pressure profile, were utilized (FEMA, 2011). As demonstrated in Fig. 2b, the synthetic storm suite encompasses possible tropical cyclone sizes and intensities that could impact the study area based on historical data. Since previous studies (e.g., Sebastian et al., 2014) have indicated that the regional surge response is highly-sensitive to landfall location, the four different landfall locations were chosen to generate a large gradient of surge responses. The angle and forward speed of the synthetic storm suite were selected based on the most probable parameters for the study region (FEMA and USACE, 2011; Dorst, 2014). These values include an average forward speed of 6.7 m/s, which can range from 4.5 m/s to 8.9 m/s, and an average angle of approach of 41 from due north, which can range from 78 to þ10 relative to due north based on historically observed TC characteristics for the study region (FEMA and USACE, 2011). In addition to the synthetic storms, Hurricane Ike (2008) was simulated at its original landfall location (same as landfall C reference Fig. 2a for landfall and Table 1 for characteristics) to put results into context of a historical storm with reliable windfield data that

of this coastal barrier has not been determined. By characterizing the relative amount of surge that can flow across the bay's different tidal inlets and barrier islands, this study provides information that can be used to guide surge mitigation designs along the bay's coastline. In addition, this study also highlights local surge that can develop within the bay itself, demonstrating whether or not a “multiple lines of defense” approach may be required to protect the Houston-Galveston region. The following section outlines the storm suite and model set-up utilized to quantify the volume of surge across the bay's coastline for baseline conditions, for projected sea level increases in the year 2050 and 2100, and a severe erosion scenario in which barrier islands are represented with similar conditions to that of post-Ike (2008). In addition, a coastal barrier, introduced to evaluate the levels of local surge that can develop within the bay itself, is also discussed. 3. Methodology This study simulates possible tropical cyclone intensity (Vmax) and size (Rmax) combinations that could impact the study region using the physics-based, tightly coupled ADvanced CIRCulation (ADCIRC) and simulating WAves Nearshore (SWAN) numerical model (referred to as ADCIRC þ SWAN). As a tightly coupled model, ADCIRC þ SWAN simultaneously solve for water levels, currents, and wave action to represent the evolution of storm surge and waves from their initial oceanic development to their generation, propagation, and dissipation at the coast and within complicated nearshore systems including estuaries, tributaries and floodplains (Dietrich et al., 2011a). The coupled model has been shown to be effective in capturing the regional surge and wave response of various historical tropical cyclones such as Katrina and Rita (2005) and Gustav (2008) (Rego and Li, 2009; Dietrich et al., 2010, 2011b). Furthermore, ADCIRC þ SWAN has been validated for the Houston-Galveston study region with Hurricane Ike (2008) (Hope et al., 2013). The synthetic storm suite utilized in this study was developed by modifying storms created for FEMA's latest comprehensive flood insurance study for the U.S. Gulf Coast (FEMA and USACE, 2011). FEMA's synthetic storms were developed with the coupled hurricane vortex-planetary boundary layer (PBL) model (Holland, 1980; Thompson and Cardone, 1996; FEMA and USACE, 2011). The PBL model requires the following tropical cyclone characteristics as inputs: central pressure deficit, peakedness and wind field distribution as defined by the Rmax and Holland B parameter, storm forward speed, and storm position. The

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Table 1 Hurricane Ike (2008) and the 20 synthetic storms simulated at landfall locations A, B, C, and D to evaluate coastal surge under baseline conditions. Storm ID

Vmax (m/s)

Rmax (km)

Pmin (mb)

6-hr Vf (m/s)

Angle of Approach (degrees from due North)

Ike (2008): Only Simulated at Landfall C 1a 1b 1c 1d 1e 2a 2b 2c 2d 2e 3a 3b 3c 3d 3e 4a 4b 4c 4d 4e

48.7 35.8 44.7 53.6 62.6 71.5 35.8 44.7 53.6 62.6 71.5 35.8 44.7 53.6 62.6 71.5 35.8 44.7 53.6 62.6 71.5

56 15 15 15 15 15 28 28 28 28 28 41 41 41 41 41 62 62 62 62 62

952 985 975 961 932 903 982 962 942 912 900 982 962 942 912 900 982 962 942 912 900

5.0 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7 6.7

36 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41

Fig. 2. 2a) Synthetic landfall locations simulated in this study. Landfall A and B are 120 km and 60 km west from landfall C, which is considered a direct hit at the mouth of Galveston Bay. Landfall D is 30 km east of landfall C. The bottom left image demonstrates the entire track of the tropical cyclones simulated. 2b) Intensity (Vmax) and size (Rmax) of synthetic storms simulated as compared to historical storms in the Gulf of Mexico with available storm size and intensity data (data from NOAA, 2016b).

impacted the study region. The wind and pressure fields for Ike were derived from post-storm reconstruction analysis via NOAA's Hurricane Research Division Wind Analysis System (H*Wind) (Powell et al., 1998; Hope et al., 2013). TCs can take on a wide variety of parameters, including variations in landfall location, wind intensity, storm size, forward speed, and angle of approach. Studies to date have shown that storm landfall location, wind intensity, and size have the strongest influence on a storm's peak surge and volumetric surge response (Irish et al., 2008; Rego and Li. 2009; Bass et al. 2016). While forward speed and angle generally have a relatively minor influence on a storm's coastal peak surge response, the forward speed of a storm can impact the duration of a storm over a body of water and thus the time-variable surge dynamics analyzed in this study. Thus, while this study focuses on general trends in coastal storm surge dynamics by focusing on the main factors that influence storm surge

(landfall, wind speed, storm size), a brief sensitivity analysis is performed to evaluate how forward speed can impact the volume of surge that flows across Galveston Bay's coastline, and subsequent surge levels and inundation throughout the Houston-Galveston region. The analysis representing sensitivity to forward speed can be referenced in Appendix B. For each storm simulated, the net volume of water or storm surge that flowed across the entirety of the bay's coastline as well as its individual coastline sections (T1, T2, B1, B2, and B3 in Fig. 1) was calculated. Fig. 3 provides a schematic of the method used to determine the volume of surge across the bathymetry/topography of a given coastline section. First, as shown in Fig. 3a, flow at a given point in time was determined by splitting a given coastal section into discrete points, and multiplying the bathymetry, water surface elevation, and unit normal velocity of storm surge simulated using ADCIRC þ SWAN at each point using the trapezoid rule as shown in Equation (1):

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Fig. 3. Schematic demonstrating how volume of surge was calculated across a given coastal stretch. 3a) water levels produced by Hurricane Ike (2008) at a given point in time across a portion of West Galveston Island's (B1) topography and associated equation utilized to determine flow for a given time-step, and 3b) time-variable flow throughout the entire duration of Hurricane Ike (2008), where positive flows represent water entering Galveston Bay and negative flows represent water flowing back into the Gulf of Mexico. Positive flows were integrated to represent the volume of surge entering Galveston Bay.


Q

Length3 time

 ¼

 Δx h  ⇀ yo velocityn 0 2   i  ⇀ ⇀ þ 2y1 velocityn …2yn  velocityn 1

n

Table 1 was performed for baseline or existing conditions that did not take into account erosion or breaching of barrier islands. After understanding baseline conditions, a reduced set of storms were utilized to evaluate possible variations in storm surge dynamics due to erosion of barrier islands and projected increases in sea level for the year 2050 and 2 100. These different scenarios are listed in Table 2. Storms 1a, 3c, and 4e at landfall locations A, B, C, and D were simulated against the additional erosion and sea level scenarios. These storms include the smallest and weakest storm (1a), an intermediate storm (3c), and the largest and strongest storm (4e), respectively (Table 1). While this reduced set of storms precludes predictive analysis such as that discussed for baseline conditions in the results section, this representative set of storms suffice to provide a general description of variations in surge dynamics for the erosion scenario and sea level conditions evaluated. Erosion to barrier islands was only evaluated for current sea level conditions given the large uncertainty associated with morphodynamic changes to barrier islands under future conditions. Post Hurricane Ike (2008) conditions were utilized to guide the development of a severe erosion scenario that could occur due to a direct TC strike. Post Hurricane Ike Light Detection and Ranging (LiDAR) coverage obtained by the U.S. Army Corps of Engineers (USACE, 2009) and additional documentation (Doran et al., 2009; Rego and Li, 2010) was utilized to guide the modification of baseline barrier island conditions along Galveston Bay's coastline. After Ike (2008), elevations on B3 were reduced to 25% of their original height, except for a roughly 500 m wide strip behind peak dune elevations, where elevations increased by 0.3 m (Rego and Li, 2010). West Galveston Island (B1) experienced a similar reduction of its peak dune elevations to 25% of their original height during this event, with less extensive erosion across the entire barrier island since right-hand side winds fell to the right of B1 (Doran et al., 2009). To represent a severe, but possible erosion scenario based on post-Ike conditions, the elevation of both barrier islands was reduced to 25% of their original height and a 0.3 m increase was introduced for a 350 m wide strip behind the peak dune elevations of each respective barrier island (Fig. 4). For the scenarios representing sea level rise, relative sea level rise projected for the year 2050 and 2 100 was represented by increasing ADCIRC þ SWAN's initial water level by 0.62 m and 1.55 m, respectively. Relative sea level rise values were obtained from the USACE's sea-level change calculator (USACE, 2017) using NOAA's (2017) intermediate (50%) scenario for a tidal gauge (Pier 21) located within Galveston Bay (Fig. 5). In addition to evaluating flow across Galveston Bay's coastline, the ADCIRC þ SWAN mesh was reduced to isolate Galveston Bay from coastal surge influences and evaluate the amount of local surge that could develop within the bay itself. Given that a coastal barrier is being

(1)

where Q is the flow or flux across a given coastal stretch of interest, Δx is the distance between two points along a given coastal transect, 0 to n represent the points along the transect, y represents the depth of water which is equivalent to the water surface elevation minus the bathymetry, ⇀

and velocityn represents the unit normal velocity relative to the transect. This calculation was performed at each hour throughout a given TC simulation to obtain flow throughout an event (Fig. 3b). Then, the volume or net flow into the bay was determined by simply summing flows into Galveston Bay for each hour and multiplying by the duration of the event, which approximates the integral of positive flows: t X   Volume ¼ Net Flow into Bay m3 ¼ t* Qi ; if Qi > 0

(2)

i¼1

This analysis was performed to further the current understanding of storm surge dynamics across the bay's coastline, and to provide insight for surge mitigation designs that could most effectively reduce surge impacts to the Houston-Galveston region. Analysis for the 80 storms in Table 2 Different scenarios evaluated in this study. Scenario

Qualitative Description

Quantitative Description

Source

Baseline

Existing Conditions

Validated in Hope et al. (2013)

Erosion

Severe erosion of barrier islands based on post-Hurricane Ike (2008) conditions

2050

Intermediate relative sea level rise conditions represented for the year 2050 Intermediate relative sea level rise conditions represented for the year 2 100

B1 and B3 stand from 0.75 to 2 m above MSL, with dune elevations as high as 3m 2/3 reduction in B1 and B2 height. ~350 m wide strip behind peak dune elevations with increase of 0.3 m Initial water level increased by 0.62 m

2 100

Initial water level increased by 1.55 m

USACE (2012), Rego and Li (2010)

USACE, 2017; NOAA, 2017 Intermediate (50%) Projection for 2050 USACE, 2017; NOAA, 2017 Intermediate (50%) Projection for 2 100

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simulated at landfall B to characterize the range of local surge levels produced within the bay for varying wind intensities. Storm 3e was additionally evaluated at locations C and D in order to evaluate what levels of local surge could develop in different parts of Galveston Bay for an extreme TC. While the peak surge response within bays can be more sensitive to variations in angle and forward speed (Weisberg and Zheng 2006) as compared to coastal surge (Irish et al., 2008; Bass et al. 2016), this study intended to demonstrate the potential need for in-bay defenses even after preventing coastal surge from flowing from the Gulf of Mexico into Galveston Bay, rather than provide an extensive characterization of local surge levels under a hypothetical coastal barrier scenario. In the ADCIRC þ SWAN simulations, the primary surge forcing mechanisms, including direct wind stress, barotropic water level adjustments, and wave radiation stress were accounted for; however, oscillating astronomical tides were not included to avoid random variations in the surge response of the tropical cyclones. A semi-implicit numerical scheme with a 1 s time-step was used in the model setup. The sector based wind-drag coefficient relationship was used as in the Ike (2008) validation study by Hope et al. (2013) and described in Powell et al. (2003) and Dietrich et al. (2011b). Simulations were performed using the Stampede Linux cluster at the Texas Advanced Computing Center at the University of Texas using 1 200 cores with roughly a 1-h wall-clock time per simulation of baseline conditions. 4. Results The first section of the results discusses surge dynamics for baseline conditions. The first part of this section (4.1.1) discusses maximum or peak surge levels that are typically used to describe a TC's surge response, 4.1.2 discusses how the integrated volume or net flow across the bay's coastline can be used to characterize regional surge impacts to the bay, and 4.1.3 outlines detailed, non-linear trends in the volume of surge that flows across the bay's different coastline sections. The sensitivity of these surge dynamics are subsequently evaluated in Section 4.2 due to severe erosion of barrier islands typical of conditions after a direct TC strike and sea level conditions for the years 2050 and 2 100. Finally, Section 4.3 provides an evaluation of the local surge levels that can still develop within the bay even when protected by a coastal barrier that prevents any coastal storm surge from entering the bay. Fig. 6 shows Western (W), Northern (N), Eastern (E), Southern (S), Central (Ctr), and Coastal (Coast) reporting locations or points where peak surge was analyzed.

Fig. 4. Modifications to 4a) baseline bathymetry made to represent 4b) severe erosion along Bolivar Peninsula (B3).

considered as a comprehensive solution for surge mitigation throughout the Houston-Galveston region (Merrell et al., 2011), this analysis was performed simply to highlight remaining local surge that could develop within Galveston Bay even after the introduction of an idealized coastal barrier. The boundary of this reduced mesh is defined along the bay's coastline and then cuts inland at the ends of the bay's coastline (referred to as “Coastal Barrier” condition). By isolating the bay and defining the edge of the mesh as a no-flow boundary condition, this mesh configuration resulted in conditions analogous to that if the bay were protected by a solid coastal barrier of an infinite height (i.e. no influence from coastal surge). Defining the mesh in this manner, allowed for an evaluation of reductions, as well as remaining local surge after the introduction of an idealized coastal barrier. A reduced suite of 7 storms were simulated against this coastal barrier condition. Five of these storms (storms 3a to 3e in Table 1) were

4.1. Peak surge levels throughout the bay Fig. 7 shows the range of peak surge levels that occur at the W, N, E,

Fig. 5. Cross-section representation of original topography of the barrier islands (black) and severe erosion of the barrier islands (grey). Sea level increases for the year 2050 and 2 100 are additionally displayed, which were evaluated for baseline conditions (no barrier island erosion). 170

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Fig. 8. Example of time-variable flow across the entire coastline of Galveston Bay.

small, but intense Category 5 storm. Peak surge levels and their sensitivity to different TC characteristics provide useful insight; however, for a given storm, peak surge levels can vary by as much as 2 m for different point locations within the bay (Fig. 7). To provide a comprehensive representation of a storm's regional impacts, from a hydraulic perspective, the following section describes the volume of storm surge that enters Galveston Bay via its coastline, which can have a strong relationship to resulting surge levels within the Bay and inundation throughout the surrounding Houston-Galveston region.

Fig. 6. Representative reporting locations where peak surge was analyzed.

4.2. Volume of surge entering the bay Fig. 8 demonstrates the time-variable flow across Galveston Bay's coastline for storms 1a, 3c, and 4e making landfall at location B. These storms represent the smallest and weakest storm, a storm intermediate in intensity and size, and the largest and strongest storm, respectively, from the suite of storms evaluated (Table 1). As discussed in the methodology section, the volume of surge was determined by summing the total flow occurring across each coastal section and into the bay, where positive flows represent flow into the bay that was integrated throughout the entire duration of a storm, while negative flows represent flow back into the Gulf of Mexico via tidal inlets or back across barrier islands. Note that these results include flow across barrier islands due to overtopping; however, they are non-conservative since barrier island erosion that can occur during TC strikes was not included for the baseline analysis. Possible changes in the surge response due to barrier island erosion is discussed in Section 4.2. Fig. 9 shows the volume of water that flowed across the coastline and into Galveston Bay for the suite of 20 synthetic storms analyzed at each landfall location A, B, C, and D. While these results represent general trends in the volumetric surge response along the bay's coastline for a range of representative landfall locations, simulations at additional landfall locations may be required to fully represent sensitive variations in volumetric surge (i.e. between landfalls B and C). Like the highest peak surge levels, greater volumes of surge developed and flowed across the bay's coastline for storms making landfall west of the bay (landfall A and B), as compared to storms making landfall directly at the bay (C) and east of the bay (D). However, as compared to peak surge which can vary significantly throughout the bay for a given storm, the volume of surge entering the bay is useful owing to its ability to represent regional surge impacts from a hydraulic perspective. As shown in Fig. 10a, the volume of surge across the bay's coastline has a nearly direct relationship to the inland area inundated along and behind the bay's coastline (including the

Fig. 7. Range in peak surge levels due to the range of storm sizes (Rmax from 15 to 62 km) evaluated for the weakest (Vmax of 35.8 m/s) and most intense (Vmax of 71.5 m/s) storms. Range in peak surge is shown at W, N, E, and S point locations for storms making landfall at locations A, B, C, and D.

and S points due to variations in storm size for the weakest (35.8 m/s) and most intense (71.5 m/s) storms. As shown in this figure, the highest surge levels generally occur for landfall B, followed by landfall locations A, C, and then D. This was expected since storms that made landfall southwest of the Bay's coastline (landfall locations A and B) resulted in more intense winds directed towards the bay's coastline, as compared to storms making landfall directly at (C) or east (D) of the bay. The range of peak surge levels, due to variations in storm size, is shown to be much greater for storms with greater wind intensity (Fig. 7). In addition, the peak surge response was either more sensitive to the range of storm sizes or intensities depending on the landfall location and the reporting location within the bay. Specifically, for the range of storms evaluated, greater sensitivity to storm size (intensity) is demonstrated by floating bars in Fig. 7 that (do not) overlap for a given landfall and point location. Floating bars that overlap for a given landfall and point location indicate that a large, Category 1 storm can result in greater surge levels than a

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Fig. 9. Volume of surge across Galveston Bay's coastline for landfalls A, B, C, and D. Each node represents the result from a simulation. For context, the star at landfall C represents Hurricane Ike (2008).

Fig. 10. 10a) Nearly direct relationship between volume of surge across the bay's coastline and inland area inundated. 10b) Inundated area was evaluated for inland areas within the dotted lines. This includes the bay's barrier islands and inland area of the greater Houston-Galveston region behind the bay's coastline.

roughly 3- to 4.5-fold for all landfall locations except landfall D, where only a 2- to 3-fold increase was observed. At fixed wind intensities (Vmax), increases in Rmax from 14.8 to 64.4 km resulted in a 5.5- to 7.5-fold increase in the volume of surge across the bay's coastline for landfall A, a 4.5- to 5.5-fold increase for landfall B, a 3.5- to 4.5-fold increase for landfall C, and a 3- to 4-fold increase for landfall D. This sensitivity analysis provides useful insight regarding a given storms volumetric surge response and subsequent potential for regional surge impacts (i.e. inundated area) as compared to peak surge. Fig. 11 demonstrates the relationship between the volume of surge that enters the bay and peak surge levels at a central (Ctr) point location within the bay (reference Fig. 6). Since surge levels vary throughout the bay, this only provides a general indication of increases in water levels within the bay due to increases in the volume of water entering the bay; however, such analysis is useful for understanding the relative influence of coastal surge on peak surge levels. Under normal conditions, the bay contains roughly 4.5 cubic kilometers of water. A doubling of this volume of water results in a ~3 m increase in water levels, a 3x increase in volume results in a ~5 m increase, and a 4x increase in volume results in a ~6.5 m increase. Unlike the direct, linear relationship between volume of surge and inundated area shown in Fig. 10a, this demonstrates that increases in the volume of surge results in a quadratic increase in peak surge levels that eventually begins to level out for larger, more intense storms.

bay's barrier islands and the greater Houston-Galveston region within the bounded area of Fig. 10b). Based on the simulated results, dimensional analysis was performed to allow for the evaluation of the inundated area expected from any TC intensity and size of interest for the landfall locations considered in this study (included in Appendix A). Note, while sensitivity analysis discussed in Appendix B suggests that the volume of positive flow into the bay does not vary significantly for storms with varying forward speeds, the resulting inundated area throughout the Houston-Galveston region can increase significantly for slower storms due to increased residence time over the bay. As expected, increasing storm intensity and size resulted in greater surge volumes to flow across the bay's coastline (Fig. 9). However, contrary to peak surge, which was shown to fluctuate in its sensitivity to the range of storm intensities (Vmax) and sizes (Rmax) evaluated, storm size was shown to always have a greater influence on the volume of surge a tropical cyclone introduced into the bay. At fixed storm sizes (Rmax), increases in wind intensity from 35.76 m/s (80 mph) to 71.5 m/s (160 mph) resulted in the volume of surge across the bay's coastline to increase

4.3. Relative volume of surge across different coastline sections In this section, the relative volume of surge that flows across Galveston Bay's different coastline sections is evaluated. Aside from providing insight regarding general trends in surge dynamics across the bay's coastline, this analysis provides guidance regarding where surge mitigation along the bay's coastline could provide the greatest reductions in regional inundation.

Fig. 11. Relationship between volume of surge entering the bay and increases in peak surge at a Central (Ctr) location within the bay. 172

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Fig. 12. Relative volume of surge (%) that flowed across the different tidal inlets and barrier islands making up Galveston Bay's coastline for landfall locations A to D. For all plots x-axis is maximum wind speed (Vmax in m/s), y-axis is radius to maximum winds (Rmax in km), and z-axis is relative volume of surge (%). For context, Hurricane Ike is represented by a star at landfall C (its original landfall location).

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Fig. 12 demonstrates the relative volume of water (as a percent) that flowed across Galveston Bay's different coastline sections, where the cumulative flow (100%) across the bay's entire coastline is equivalent to the sum of the five individual sections for a given storm's landfall location. For example, Hurricane Ike (2008), which made landfall at location C and had a Vmax of 48.7 m/s and Rmax of 56 km, introduced 6.57 km3 of water into Galveston Bay, of which 4.6% flowed across T1, nearly 0% flowed across B1 and B2, 66.2% flowed across T2, and 29.2% flowed across B2. Since significant amounts of surge developed along the entirety of the bay's coastline for landfall locations A and B, trends in the relative amount of flow across all sections of the bay's coastline are readily apparent for these landfall locations. However, for landfall locations C and D, a combination of weaker northerly and southerly winds on the left hand side of the tropical cyclones act across T1, B1, and B2 to result in lesser amounts of flow and less discernible trends in the surge response for these sections. For landfall locations A and B, trends in the relative volume of surge across the tidal inlets and barrier islands were non-linear, and generally approached asymptotic limits (Fig. 12). For example, relative contributions across the barrier islands are initially flat for the smallest, weakest storms, then exponentially increase as storm size and intensity increased, and then begin to flatten out for the largest, most intense storms. The asymptotic behavior in the surge response for the small, weaker storms was expected since the storm surge was required to reach a minimum height to crest the barrier islands. Once crested, increases in storm size and intensity, resulted in non-linear, nearly exponential increases in the volume of surge across the barrier islands. Finally, the relative volume of surge across the barrier islands began to reach an asymptotic upper limit, which is likely due to a limit or upper bound on the amount of surge that can develop along the shallow continental shelf of the Gulf of Mexico for the larger storms (Irish and Resio, 2010). As illustrated in Fig. 13, a storm size and intensity threshold was reached for landfall locations A, B, and C that led to a transition in the majority of flow occurring across tidal inlets to barrier islands. As discussed in Section 4.2, these results are non-conservative, and this transition point could occur at a lower threshold due to erosion of barrier islands. These findings provide insight for understanding complex surge dynamics and their impacts to

Fig. 14. Minimum and maximum relative volume of surge that occurred across each coastline section (T1, B1, B2, T2, B3) for all of the storms evaluated at each landfall location (A, B, C, D). Grey dashes at landfall C represent Hurricane Ike.

Galveston Bay, where volume of surge across tidal inlets may dominate for small, weak storms, whereas the volume of surge across barrier islands can become relatively more important for larger, more intense storms. The relative volume of surge across components other than T2 and B3 were fairly small. Fig. 14 demonstrates the minimum and maximum relative volume of surge that occurred across each coastline section for all of the storms evaluated at each landfall location. For nearly all of the storms and landfall locations evaluated, the majority of surge, ranging from 40 to 92%, flowed across T2 because of its large cross-sectional area without obstructions (i.e. roadways). However, as storm size and intensity increased, the volume of surge across T2 became nearly equivalent to B3 for the most intense and largest storms making landfall at locations B and C. In contrast to T2, the relative volume of surge across T1 only reached a maximum of 8% likely due to this tidal inlets small width and shallow bathymetry. The relative volume of surge across B2 only reached a maximum of 5% owing to the small amounts of surge that crest the 5.2 m height of the seawall barrier. Relative volume of surge across B1 were generally small; however, for the larger and more intense storms

Fig. 13. Relative volume of surge across the tidal inlets (T1 and T2 – represented by solid lines) and the barrier islands (B1, B2, and B3 – represented by dashed lines) for the suite of storm intensities (increasing from cool to warm colors) and sizes (x-axis) evaluated at each landfall location. This figure illustrates a transition, for landfall locations A, B, and C, at which a given size and intensity of a TC results in the relative volume of surge across the barrier islands to exceed that of the tidal inlets. 174

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percent increases in volume, peak surge at a north point location (see Fig. 6), and inundation area of the Houston-Galveston region. Negligible differences in the volume of surge across Galveston Bay's coastline as well as peak surge (Fig. 16c) for the north point location was observed for the erosion scenario compared to baseline conditions. However, for this same erosion scenario, noticeable increases in inundation area (Fig. 16d) were observed ranging from 64% for the smallest, weakest storm (1a) to 31% for the intermediate and largest, strongest storm (4e). The primary reason for increases in inundation for the smallest, weakest storms is due to inundation of the barrier islands, which flood more easily for lower surge levels due to peak barrier island elevations of 0.25–0.75 m above MSL for the erosion scenario as compared to 1.5–3 m above MSL for baseline conditions. For the intermediate storm (3c) and the largest, strongest storm (4e), increases in inundation area may additionally be attributed to increases in the volume of surge across barrier islands as compared to tidal inlets which results in more distributed flow across the bay's entire coastline. As shown in Fig. 17, under the erosion scenario, the volume of surge across the main tidal inlet (T2) decreases, while the volume of surge across both barrier islands (B1 and B3) increases. This change in coastal surge dynamics suggests that a shift in preferential flow paths occur for the erosion scenario due to the lower-lying barrier islands. This shift in coastal storm surge dynamics can lead to increases in inland inundation. Generally, all surge responses, including volume of surge across the bay's coastline, peak surge levels, and inundated area, increase due to increases in sea level. For the year 2050, minor increases ranging from 25% to 3.5% are observed for the volume of surge across Galveston Bay's coastline, while significant increases ranging from 75% to 15% are observed for the year 2100, respectively, for the smallest, weakest to the larger, strongest storm (Fig. 16b). Increases in peak surge levels were found to generally scale linearly with increases in sea level rise. As a result, the same increase for a lower surge level (i.e. caused by a smaller, weaker storm) results in a greater percent increase as compared to a larger, stronger storm (Fig. 16c). This same reasoning holds for the percent increases observed in inundation area due to sea level rise for the smallest, weakest storm (1a) to the largest, strongest storm (4e) (Fig. 16d). Fig. 18 demonstrates variations in the relative volume of surge that flows across the different barrier islands and tidal inlets that make up Galveston Bay's coastline for the different scenarios evaluated. As expected, the erosion scenario and sea level rise result in increases in the relative volume of surge that flows across barrier islands as compared to tidal inlets. In addition, the range of the relative volume of flow significantly narrows for 2 100 conditions, due to the increasing (decreasing) amount of flow across barrier islands (tidal inlets) for smaller, weaker

at landfall A, the volume of surge across this section became significant, with a maximum relative contribution of 22.5%. Lessons learned from this analysis can help guide surge mitigation strategies that have a reduced structural footprint, and thus less environmental impact Specifically, for Galveston Bay, minor volumes of surge were found to enter the bay via T1, suggesting that a flood gate across this tidal inlet, which allows for fresh/salt-water exchange necessary for the health of the bay, is likely unnecessary. On the other hand, the majority of flow was found to occur across T2 and B3, suggesting that surge defenses across these sections could achieve nearly the same surge reductions as a uniform coastal barrier across the entirety of the bay's coastline. As a proof-of-concept, storm 3c was simulated at landfall B against baseline (existing) conditions, a scenario with barriers introduced along T2 and B3, and a scenario with a fully enclosing barrier along the bay's coastline (Fig. 15). Storm 3c has a moderate Rmax of 41.3 km and a Vmax of 53.6 m/s, which, when compared to FEMAs preliminary flood insurance maps, results in approximately 250-yr stillwater levels throughout the Houston-Galveston region (FEMA, 2011). Levees along T2 and B3 and the fully enclosing barrier were set at a 15 m height to avoid any overtopping. While west Galveston Island (B1) would be inundated for the condition with barriers along T2 and B3, the extent of inundation and surge levels throughout the remainder of the Houston-Galveston region were nearly equivalent to the fully enclosing barrier which includes additional defenses across T1, B1, and B2. 4.4. Sensitivity to erosion and sea level rise This section discusses results for the three scenarios outlined in Table 2, which include i) severe erosion to Galveston Bay's barrier islands (B1 and B3), ii) projected sea level conditions for the year 2050, and iii) projected sea level conditions for the year 2 100. The subset of storms evaluated, which include the weakest, smallest storm (1a), an intermediate storm (3c), and the largest, strongest storm (4e) from the suite of storms evaluated for baseline conditions provide an indication of potential variations in surge dynamics for these different scenarios relative to baseline conditions. Discussion of coastal and in-bay storm surge dynamics is provided for landfall location B, which results in the most severe storm surge response along the bay's coastline and throughout the Houston-Galveston region for baseline conditions; however, additional simulations at landfall locations A, C, and D demonstrated similar trends to that discussed in this section. Details regarding the erosion and sea level conditions can be referenced in the methodology section and Figs. 4 and 5. Fig. 16a shows variations in the volume of surge that flows across Galveston Bay's coastline for the different scenarios and corresponding

Fig. 15. Resulting peak surge for a storm that results in approximately 250-yr stillwater levels (storm 3c at landfall location B) when simulated against baseline (existing) conditions, a scenario with barriers along T2 and B3, and a fully enclosing barrier along the bay's entire coastline. Barriers are represented by black lines, and the southeast to northwest line on the bottom left represents the storms track. 175

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Fig. 16. Storm surge response and percent increases relative to baseline conditions for the smallest, weakest storm (1a), an intermediate storm size and intensity (3c), and the largest, strongest storm that were evaluated in this study at landfall location B. Peak surge is represented at a north point location.

Fig. 18. Minimum and maximum relative volume of surge that occurred across each coastline section (T1, B1, B2, T2, B3) for the smallest, weakest to largest, strongest storm evaluated at landfall B.

Fig. 17. Minimum and maximum volume of surge that flows across each coastline section (T1, B1, B2, T2, B3) for the smallest, weakest to largest, strongest storm evaluated at landfall B under baseline conditions and the severe erosion scenario. The grey dash represents results for the intermediate storm (3c).

sea level conditions. For the erosion scenario and 2 100 sea level conditions, a transition in the majority flow occurring across tidal inlets to barrier islands occurs at or below the storm size and intensity of the intermediate storm (3c) evaluated in this study, which corresponds to an event that produces 250 year surge levels throughout the study area. These findings indicate that the storm size and intensity threshold required to transition in the majority of flow occurring across tidal inlets to barrier islands reduces significantly when considering possible erosion and 2 100 sea level conditions, which should be taken into account when planning regional surge mitigation.

(larger, stronger) storms. The results shown in Fig. 18, confirm that the majority of flow will continue to occur across T2 and B3, with a relative increase (decrease) in importance for B2 (T2) under the additional scenarios evaluated. As demonstrated in Fig. 19, these results suggests that the volume of surge that flows across barrier islands can be relatively more important for present-day conditions due to erosion that can occur from a direct TC strike and will only become more important in the future. For the three scenarios evaluated, the relative volume of flow across barrier islands, as compared to tidal inlets, will increase from least to greatest for 2050 sea level conditions, the erosion scenario, and 2 100

4.5. Local surge within the bay Analysis in Sections 4.1 and 4.2 provide insight regarding coastal 176

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surge levels reached as high as 2.5 m and 3.5 m in the northwestern portion of the bay for wind intensities of 53.6 m/s (120 mph) and 71.53 m/s (160 mph), respectively. These local surge levels increased significantly along the western and northern tributaries of Galveston Bay; for example, maximum local surge levels along the western (Clear Creek) and northern tributaries (LSJR) were, respectively, 2.75 m and 3.5 m, for a 53.6 m/s (120 mph) wind intensity, and as high as 5 m, along both tributaries, for a 71.53 m/s (160 mph) wind intensity. The most intense storm (71.53 m/s – 160 mph), with a moderate Rmax of 41.3 km (right image in Fig. 20), was additionally simulated at landfalls C and D to highlight the different areas within the bay vulnerable to local surge due to tropical cyclone landfalls west of (B), directly at (C), and just east (D) of the bay. Fig. 21 compares the maximum surge levels that develop for baseline (existing) conditions versus an isolated bay for these storms. The peak surge impacts are fairly uniform across the bay for each respective landfall location under baseline conditions, with decreasing impacts to the bay as tropical cyclone landfall shifts to the east. However, for an isolated bay, sharp gradients in local surge levels resulted (Fig. 21) due to wind-fetch across the bay's shallow, smaller volume of water without coastal surge influences. For a direct hit (landfall C), northeasterly winds initially acted over the bay and became southwesterly as the tropical cyclone moved across the bay. As observed for Hurricane Ike (Rego and Li, 2010; Sebastian et al., 2014), a sloshing effect caused a reverse in direction of local surge as the tropical cyclone moved across the bay, with water initially being “pushed” from the northeast to the southwest, and then pushed back to the northeast from the opposite direction. Maximum local surge levels for landfall C were relatively low, with 2.75 m in both the southwest and northeast portions of the bay as compared to peak local surge levels as high as 5 m for landfall B. The lower local surge produced for landfall C was due to variations in wind direction, which did not allow for sustained winds to act towards a single portion of the bay, as well as the weaker winds of the eye of the tropical cyclone crossing directly over the bay. Despite local surge that could develop within the bay, results demonstrate that a coastal barrier preventing coastal surge from entering the bay would provide significant reductions in surge levels and extent. This is true for all landfall locations and portions within the bay, except for the local surge produced in the southern portion of the bay due to landfall D. For landfall D, sustained northeasterly winds acted across the bay and caused water to accumulate or pile up against the coastal barrier. Along the backside of the existing location of B2, peak local surge levels were shown to be as high as the surge that would develop for the same tropical cyclone and landfall location under baseline conditions (4.5 m). Nonetheless, this issue would be reduced with operational flood gates that could open to allow for the outflow of surge across the bay's primary tidal inlet, Bolivar Roads (T2), at critical moments along a tropical cyclone's track.

Fig. 19. Relative volume of surge across the tidal inlets (T1 and T2 – represented by solid lines) and the barrier islands (B1, B2, and B3 – represented by dashed lines) for the smallest, weakest storm (1a), an intermediate storm size and intensity (3c), and the largest, strongest storm evaluated at landfall B. 19a) represents changes compared to baseline conditions for the erosion scenario and 19b) represents changes for intermediate 2050 and 2 100 sea level projections.

surge and in-bay dynamics that can help guide surge mitigation strategies for the region, where a coastal barrier is currently being considered. However, even after protecting entirely from coastal surge that can flow from the Gulf of Mexico into Galveston Bay, wind fetch acting across the significant volume of water within the bay can lead to local storm surge. Local or residual surge levels and practical issues associated with a coastal barrier were evaluated by performing simulations against a mesh representing a barrier along the bay's coastline at an infinite height (Coastal Barrier Condition discussed in methodology section). While such a barrier would entirely prevent coastal surge from flowing into the bay, it could also impede a portion of local surge that would otherwise flow back into the Gulf of Mexico, and thus potentially result in increases to local surge levels. The range of local surge levels that could develop within Galveston Bay was evaluated for storms with increasing wind intensities (storms 3a to 3e) at landfall B, as shown in Fig. 20. As tropical cyclones approached landfall and moved inland, wind fetch began to act across the bay's extensive horizontal (east to west) and then vertical (south to north) extent, resulting in an increasing gradient of peak surge levels from the southeastern to northwestern portion of the bay. Local

5. Discussion This study provides the first characterization of the relative amounts

Fig. 20. Peak surge for a range of storm intensities simulated at landfall B for the coastal barrier condition. The edge of the mesh is indicated as a black line along the bay's coastline and represents a solid coastal barrier of an infinite height. The storm size is uniform for each intensity, with an Rmax of 41.4 km, and the southeast to northwest line represents the storms track. Note the increase in local surge generation within the Bay as storm intensity increases. 177

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Fig. 21. Peak surge for the coastal barrier condition exposed to a tropical cyclone with an Rmax of 41.4 km and maximum wind intensity of 71.53 m/s (160 mph) simulated at landfall's B, C, and D. Note that the scale for the isolated Bay results is 0–5 m as compared to the 0–10 m scale for the baseline results.

of surge that can flow across a bay's coastline for a range of tropical cyclone sizes and intensities. Various technical and practical questions were elucidated by evaluating a range of storms. For example, storm size (Rmax) was shown to have a greater influence on a tropical cyclone's volumetric surge that enters Galveston Bay as compared to wind intensity (Vmax). In addition, for varying landfall locations, storm sizes, and storm intensities, the volume of surge across Galveston Bay's coastline was shown to have a nearly direct correlation to the extent of inland areas inundated behind the bay's coastline. Such information can be useful for describing a given storm's regional impacts from a hydraulic perspective, and served as the basis or motivation for analyzing the relative volume of surge across the bay's different coastline sections. While this study focused on general trends in coastal storm surge dynamics owing to variations in storm landfall, size, and intensity while using an average forward speed and angle for the region, results will vary for different forward speeds and angles. Sensitivity to the surge dynamics evaluated in this study was performed for storms with different forward speed (Appendix B), given that the time-variable surge dynamics evaluated may vary with storm duration. In addition, variations in coastal and in-bay storm surge dynamics were evaluated for a severe erosion scenario based on post-Ike (2008) conditions and for future conditions by considering intermediate sea level rise scenarios for 2050 and 2 100. Understanding the sensitivity of coastal surge dynamics to sea level rise is particularly important for guiding effective surge mitigations strategies

for present-as well as future-conditions. Analysis of the relative volume of surge across Galveston Bay's different coastline sections provided insight that can help guide surge mitigation designs that have a reduced structural footprint, and thus lower costs and environmental impact. Specifically, for Galveston Bay, minor amounts of surge were found to enter the bay via T1, suggesting that a flood gate across this tidal inlet, which allows for fresh/salt-water exchange necessary for the health of the bay, is likely unnecessary. On the other hand, the majority of surge was found to flow across T2 and B3, indicating that surge defenses across these sections could achieve nearly the same surge reductions as a uniform coastal barrier across the bay's entire coastline. Analysis for the erosion scenario and future sea level conditions suggest that the majority of surge will continue to flow across these two coastline sections (T2 and B3), with a relative increase (decrease) in importance for B2 (T2) under the additional scenarios evaluated. This study also found a transition at which a greater amount of surge began to flow across Galveston Bay's barrier islands as compared to its tidal inlets. While the exact storm intensity and size necessary to reach this transition may be unique to Galveston Bay, other bay environments may exhibit this transition as well, which could be determined using similar analysis to that performed in this study. Also, while this transition may occur for extreme storms under baseline conditions, possible erosion of barrier islands and sea level rise can significantly reduce the storm size 178

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and intensity threshold required for this transition. This finding has important implications for whether stakeholders in a given bay environment may be more invested in protecting against small, weak storms where surge across tidal inlets dominate or large, intense storms where surge across barrier islands can become relatively more important. Where and to what height protection across tidal inlets versus barrier islands is provided can ultimately dictate whether, for example, 100- or 500-yr surge levels are protected against. Such information is crucial for bay environments considering surge mitigation designs, and cannot be obtained from FEMA's peak frequency levels, which are typically uniform along a given coastal stretch, such as a bay's coastline (FEMA, 2011). Finally, this study evaluated the levels of local surge that could develop within Galveston Bay. Analysis demonstrated that while a solid coastal barrier could provide significant reductions in the height and extent of surge impacts to the Houston-Galveston region, local surge, as high as 5 m, could still develop in different portions of the bay for very intense tropical cyclones. This finding suggests that multiple lines of defense, with surge protection across the bay's coastline as well as within the interior of the bay, may be required to achieve satisfactory surge protection for the region. Other bay environments may similarly require “multiple lines of defense” depending on their respective wind-fetch across the bays, bathymetry and inland topography, and tropical cyclone climatology. The methodologies used to understand coastal and local surge dynamics in this study can be applied in other bay and estuary environments to characterize complex surge dynamics and help guide surge mitigation strategies. This is particularly recommended for bays or estuaries considering surge mitigation strategies like the study region and other areas such as New York Harbor (Aerts et al., 2014) or for retrofitting already built coastal protection strategies such as Venice's Experimental Electromechanical Module (MOSE) project (Sharma et al., 2016).

rise for the years 2050 and 2 100. In addition, local surge was evaluated to understand the residual surge that can still develop within Galveston Bay when fully protected by a coastal barrier that prevents coastal storm surge from flowing into the bay. This numerical evaluation provides a technical perspective of coastal and in-bay storm surge bay dynamics that have not fully been explored before for baseline conditions or the additional erosion and sea level scenarios evaluated. Practical results from this research can aid in understanding regional storm surge impacts and help guide surge mitigation strategies in the study region. Similar analysis can be performed in other bay/estuary environments. While this study provided a general depiction of shifts in coastal and in-bay storm surge dynamics due to an erosion scenario, future work should provide a more detailed analysis of the potential erosion and breaches that can occur along barrier islands during tropical cyclone strikes using physics-based models that can provide time-variable sedimentary and topographic boundary conditions for a hydrodynamic storm surge model. Furthermore, this study utilized an intermediate projection of sea level conditions; however, future work should make use of probabilistic sea level rise predictions if computationally feasible in addition to changes in future geomorphologic conditions due to wave action and accretion of barrier islands. Finally, this study evaluated general trends in coastal storm surge dynamics across Galveston Bay's complex coastline primarily based on variations in tropical cyclone storm size, intensity, and landfall. While the sensitivity to variations in forward speed is briefly discussed in the appendix and can have a relatively small influence on coastal surge levels, future work should further evaluate how coastal and local surge dynamics vary for different tropical cyclone forward speeds and angles. Acknowledgments This research was supported by the Houston Endowment under the Severe Storm Prediction, Education, and Evacuation from Disasters (SSPEED) Center and the National Science Foundation PIRE grant OISE154837. C. Dawson also acknowledges the support of National Science Foundation grant ACT-1339801, the Extreme Science and Engineering Discovery Environment (XSEDE) grant TG-DMS080016N, and the Texas Advanced Computing Center for the use of their computing resources.

6. Conclusions This study provided a detailed analysis of the integrated volume of surge that can occur across Galveston Bay's complex coastline, which is made up of tidal inlets, barrier islands, and an existing seawall. This analysis was performed for baseline conditions, a severe erosion scenario representative of post-hurricane barrier island conditions, and sea level

Appendix A. Dimensional analysis of inundation area For general guidance or informative purposes, dimensional analysis was implemented to allow for estimates of inundation area in the HoustonGalveston region based on a given storm's size, intensity, and landfall location. This can be used for general insight, but should not be used for forecasting or evacuation decisions since, as discussed in the paper, estimates of inundation area can vary with a given storms forward speed (see Appendix B). Note that the dimensional analysis was performed for simulations of baseline conditions and thus does not include possible erosion of barrier islands, storms with slower forward speeds, or any increases in future sea level, which can result in significant increases in inundation area. For each landfall location, dimensional analysis was performed with the following dimensionless products: Π1 ¼

ζA R2max

(A.1)

Π2 ¼

Rmax *g 2 Vmax

(A.2)

where Π represents a given dimensional products, ζA is the dependent variable which represents inundated area in the Houston-Galveston region (km2), and g represents gravity. Using TC characteristics for the 80 storms simulated under baseline conditions (Table 1) and their corresponding inundation shown in Fig. 10, a polynomial basis function was fit to determine the relationship between the dimensionless products. With some rearranging, this allows for the estimation of inundated area in the Houston-Galveston region, as shown in the following equation:

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4 3

2
R

max *g 2 Vmax

6 6 6
 6 R *g max 6 2 6 Vmax 6 6 6 6
  2 Inundation Area km2 ¼ R2max CðLandfallÞ6 6 Rmax *g 6 2 Vmax 6 6 6 6
 6 R g max* 6 2 6 Vmax 4

3

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 5

(A.3)

1
where the right hand side

Rmax *g 2 Vmax

 terms were normalized and scaled by a mean value of 149.8 and standard deviation of 113.9. As expected, Eqn. (A.3)

indicates that increases in inundated area occur with increases in Rmax and Vmax. The fitted coefficients for each landfall location, or CðLandfallÞ in Eqn. (A.3), are shown below: Landfall Landfall Landfall Landfall

A: B: C: D:

(0.2706 (0.3585 (0.3045 (0.2471

1.148 1.554 1.316 1.057

1.061 1.549 1.286 0.9987

0.1885 0.368 0.2329 0.1352

0.7747) 0.8775) 0.6305) 0.4883)

Fig. A1 shows predictions of inundation area based on dimensional analysis compared to simulated results from ADCIRC þ SWAN. An overall coefficient of determination (R2) of 0.86 was achieved.

Fig. A.1. Simulated inundation area compared against predicted inundation area utilizing the dimensional function developed based on baseline conditions.

Appendix B. Sensitivity to storm forward speed A suite of storms were evaluated at Landfall B to understand the sensitivity of coastal and in-bay storm surge dynamics to variations in forward speed. This landfall location was chosen for sensitivity analysis since the most severe surge response was found for this landfall location compared to the other landfalls evaluated in the main text, and because a significant surge response is experienced along Galveston Bay's entire coastline for this landfall location. While this sensitivity analysis was considered relevant to the study given that the time-variable, coastal storm surge dynamics presented in the main text of the study may be sensitive to the duration of a storm, the authors would like to remind the reader that slower- and faster-moving storms have a lower-probability of occurrence relative to the average forward speed utilized in the main text. In this analysis, deviations to the forward speed of a storm with an intermediate storm size and a range of wind intensities (3a, 3c, 3e) were evaluated. The different forward speeds evaluated (Table B1) represent lower and upper-bound values historically observed in the study region (FEMA, 2011; Dorst, 2014). To develop the slower and faster versions of storms 3a, 3c and 3e, a single slow and fast storm was selected from FEMA's flood insurance study for the U.S. Gulf Coast (FEMA and USACE, 2011) that exhibited the same path, angle, and radius to maximum winds as storms 3a, 3c, and 3e. The selected slow and faster storm's wind speeds were then scaled similarly to that performed for the storms evaluated in the main text of this study (reference Section 3). Aside from having a slower or faster forward speed, the storms created have the same characteristics (angle, path, landfall location, maximum wind speeds, radius to maximum wind) of storms 3a, 3c, or 3e.

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Table B.1 Storms utilized to evaluate sensitivity of coastal and in-bay surge dynamics to variations in forward speed Storm ID

Vmax (m/s)

Rmax (km)

Pmin (mb)

6-hr Vf (m/s)

Angle of Approach (degrees from due North)

Faster 3a Slower Faster 3c Slower Faster 3e Slower

35.8 35.8 35.8 53.6 53.6 53.6 71.5 71.5 71.5

41 41 41 41 41 41 41 41 41

942 942 942 942 942 942 942 942 942

9.48 6.7 3.65 9.48 6.7 3.65 9.48 6.7 3.65

41 41 41 41 41 41 41 41 41

As demonstrated in Fig. B1 storms with an average forward speed result in peak surge levels along the coast (Fig. B1a) and inside the bay (Fig. B1b) that are generally equal to or greater than peak surge levels for faster and slower moving storms. For the weakest storms (3a) with a maximum wind speed of 35.7 m/s, minor differences are generally observed between peak surge levels along the coast and within the bay. For the faster moving storm, the intermediate (3c) and most intense (3e) winds, respectively, result in coastal surge levels that are 1.17–1.57 m lower, and in-bay surge levels that are 1.80–3.04 m lower with respect to an average forward speed. For the slower moving storm, the intermediate (3c) and most intense (3e) winds, respectively, result in coastal surge levels that are 0.97–1.18 m lower, and in-bay surge levels that are roughly the same as a storm with an average forward speed. These results indicate that faster moving storms begin to result in significant reductions in coastal and in-bay peak surge levels as wind intensities increase. Slower moving storms generally result in lower peak surge levels along the coast as wind intensity increases; however, in-bay surge levels are roughly the same as a storm with an average forward speed. This suggests that the highest surge levels are produced along the coast for an average forward speed for the study region owing to the balance or trade-off between effective wind speeds on the right hand side which increase (decrease) for faster (slower) moving storms, but have a decreased (increased) residence time for winds to act across the mildly sloping Gulf of Mexico continental shelf. These results agree with Irish et al. (2008) for very shallow continental shelf slopes, but are specific to the study location and will vary for locations with narrower or wider continental shelf slopes. Despite lower coastal surge levels produced for a slower moving storm, the increased residence time a storm's winds have to act across the bay increases overall surge levels within the bay to an equivalent level of a storm with an average forward speed.

Fig. B.1. Impact of forward speed on peak surge at reporting locations for a B.1a) coastal point location and B.1b) a north point location (reference Fig. 6 for location of points).

Fig. B2 demonstrates how varying forward speeds, for different wind intensities, impact the volume of surge across Galveston Bay's coastline and inundated area throughout the Houston-Galveston region. As shown in Fig. B2a, differences in the volume of surge for the weakest storm (3a) are small. For the faster moving storm, the intermediate (3c) and most intense (3e) winds, respectively, result in a volume of surge across the bay's coastline that is 4.07 km3 (41%) and 8.23 km3 (42%) less than the storm with an average forward speed. For the slower moving storm, the intermediate (3c) and most intense (3e) winds, result in a volume of surge that is nearly equivalent to that observed for the storms with an average forward speed. These results indicate that faster moving storms begin to result in significant reductions in the volume of surge entering Galveston Bay as wind intensities increase. This intuitively makes sense since lower surge levels are produced for faster moving storms (Fig. B1a) for a shorter duration. On the other hand, despite lower coastal peak surge levels (Fig. B1a), slower moving storms generally result in nearly equivalent volumes of water entering Galveston Bay as storms with an average forward speed owing to the increased duration of time in which elevated water levels are produced for the slower moving storms. Fig. B2b demonstrates that the inundated area across the Houston-Galveston region does not necessarily follow the same trends as that observed for the volume of surge entering Galveston Bay via its coastline. For example, the weakest storm (3a) demonstrates 21% and 29% increases in inundated area

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for faster and slower storms, respectively. For the intermediate (3c) and most intense (3e) winds, faster storms result in slightly lower amounts of inundated area (6 to 9%), and slower storms result in significant increases in inundated area (29–38%). Generally, these results indicate that in addition to the volume of coastal surge that enters the bay via its coastline, the forward speed and thus effective wind speed and resident time over Galveston Bay begins to become relatively important for determining the inundated area throughout the Houston-Galveston region. The findings in this appendix differ from a study by Rego and Li (2009) that specifically focused on variations in Hurricane Rita's forward speed and its impact on maximum surge levels and flooded inland volumes. However, the previous study focused on surge responses along an extensive stretch of the Louisiana coastline rather than a specific bay along the Texas coastline. Each environment has unique continental shelf slopes and coastline features that can impact the surge response. In addition, each study employed different hydrodynamic models and storm sets. Finally, Rego and Li (2009) focused on maximum surge values irrespective of location, while this study distinguished between specific coastal and in-bay locations, and the analysis of flooded area by Rego and Li (2009) was performed for a general coastline, while this study specifically focused on flooded areas behind a bay where the flood response can be more sensitive to a storm's forward speed. Hence, the two studies should be compared with these differences in consideration. The trends observed due to variations in storm forward speed in this study should not be generalized elsewhere as the dimensions of another bay environment and the slope of its continental shelf are unlikely to reflect that of this study's location.

Fig. B.2. Impact of forward speed on B. 2a) volume of surge across the bay's coastline and B. 2b) area inundated throughout the Houston-Galveston region.

Fig. B3 demonstrates variations in the relative volume of surge that enters via barrier islands versus tidal inlets for the different forward speed storms. Results suggests that faster and slower storms result in slight increases in the relative volume of surge across tidal inlets as compared to tidal inlets, with a maximum 10% change. This is primarily attributed to lower peak coastal surge levels for faster and slower moving storms (Fig. B1a), and 182

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thus less overwash along barrier islands. While the intention of this study is to provide a general description of coastal storm surge dynamics based on storm characteristics known to have the strongest influence in determining a storms surge response (landfall, storm intensity, storm size), the authors consider the insight provided with respect to storm forward speed in this appendix important given the integrated time-variable surge analysis performed in the study. Most importantly, this analysis found that the inundated area could increase from 29 to 38% for an intermediate (53.6 m/s) to extreme wind intensity (71.5 m/s), respectively, for a slower moving storm as compared to the average forward speed evaluated in the main text of this study.

Fig. B.3. Impact of forward speed on relative volume of surge that flows across tidal inlets (black) versus barrier islands (grey).

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