A numerical study of storm surge and inundation in the Croatan–Albemarle–Pamlico Estuary System

Estuarine, Coastal and Shelf Science 59 (2004) 121e137

A numerical study of storm surge and inundation in the CroataneAlbemarleePamlico Estuary System Machuan Peng, Lian Xie), Leonard J. Pietrafesa Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Box 8206, Raleigh, NC 27695-8208, USA Received 5 July 2002; accepted 31 July 2003

Abstract An integrated storm surge and inundation modeling system is used to simulate the storm surge and inundation in the CroataneAlbemarleePamlico Estuary System in eastern North Carolina under the influence of 10 hypothetical Category 2 and 3 hurricanes representing typical historical hurricane scenarios in the study region. The integrated storm surge and inundation modeling system is numerically stable in the complex and shallow CAPES environment under hurricane forcing conditions. For an assumed northward or northeastward moving Category 3 hurricane with a translation speed of 25 km/h, the peak storm surge occurs along the western Pamlico Sound and western Albemarle Sound. The most severe flooding as measured by inundation area is in the Pamlico River mouth region where the flooding area reached 500 km2. In general, a more intense or larger hurricane (lower minimum central pressure, MCP or larger radius of maximum wind, RMW) produces higher storm surge and a larger inundation area in the entire region. For the cases considered in this study, the storm surge height and inundation area are more sensitive to MCP than to RMW. Slower translation speed produces higher storm surge, and thus larger inundation area, but the sensitivity of storm surge to storm translation speed can be vastly different for different storms. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: storm surge; inundation; numerical modeling; coastal flooding; hurricane

1. Introduction The CroataneAlbemarleePamlico Estuary System (CAPES) (Fig. 1) in eastern North Carolina is the largest lagoon system and the second largest estuary (next to Chesapeake Bay) in the United States (Pietrafesa et al., 1986). It covers a total area of approximately 5500 km2. The CAPES comprises primarily two major bodies of water, Pamlico Sound (w120 km!40 km) and Albemarle Sound (w70 km!20 km), which are linked by the relatively small Croatan and Roanoke Sounds (Pietrafesa et al., 1986). The average water depth of the CAPES is only 4.5 m, though the water depth ranges from 2 m around the perimeter or near the shoals to approximately 7 m in the deepest basin. The CAPES is separated from the Atlantic Ocean by the Outer Banks, a chain of emergent bar-built islands on their east sides.

) Corresponding author. E-mail address: [email protected] (L. Xie). 0272-7714/03/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2003.07.010

Three narrow inlets, Oregon, Hatteras and Ocracoke Inlets, along the Outer Banks, contribute to the water exchange between the CAPES and the Atlantic Ocean. The CAPES faces frequent threats from tropical and extra-tropical cyclones. Damages associated with these cyclones derive from strong winds, storm surge, and inland flooding. As a function of the combination of environmental factors and the socioeconomic structure of the region impacted, the impact can vary tremendously. However, amongst the above suite of factors, coastal and inland flooding is known as the most dangerous to human life and property. The purpose of this study is to investigate the feasibility of modeling hurricane-induced currents, storm surge and inundation in the CAPES using a three-dimensional computer model and to improve our understanding of the hydrodynamic responses of the CAPES to the passages of hurricanes. Numerical studies of hurricane-induced storm surge and circulation in the CAPES began in the mid-1970s. These studies were mainly published in technical reports (Amein and Airan, 1976; Pietrafesa et al.,

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M. Peng et al. / Estuarine, Coastal and Shelf Science 59 (2004) 121e137 36.5N

Atlantic Ocean E♦

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Fig. 1. The geographic location of the CroataneAlbemarleePamlico Estuary System and adjacent shelf region. Letters A, B, C, D and E denote the locations of Hatteras, Frisco, Buxton, Avon and Duck Pier, respectively.

1986) and student theses (e.g., Lin, 1992). Amein and Airan (1976) constructed a two-dimensional (depth averaged) model for the Pamlico Sound. Pietrafesa et al. (1986) developed a three-dimensional linear shallowwater model for the Pamlico Sound, which was later configured for the Albemarle Sound by Lin (1992). While the efforts of Pietrafesa et al. and Lin showed that the earlier 2-D models underestimated the storm surges in the Pamlico Sound, and the 3-D model agreed much better with actual point observations of high water, their model did not include interactive coupling with the coastal ocean and thus could not be used to study estuaryeocean water mass exchange. Moreover, Pietrafesa et al. (1986) and Lin (1992) did not include nonlinear advection terms and thermodynamic equations in the 3-D shallow-water model. Thus, their model could not be used to study the density driven circulation in the system. To overcome these drawbacks, Xie and Pietrafesa (1999) configured the nonlinear, three-dimensional Princeton Ocean Model (POM) for the entire CAPES and its adjacent coastal shelf, and studied the water mass exchange between the CAPES and the coastal ocean. A major limitation for all the hydrodynamic models configured for the CAPES is that they were not designed to simulate inundation and retreat or wetting and drying, respectively. Without a proper mathematical treatment of the physics of the inundation and drying processes, hydrodynamic models can become computationally unstable in shallow estuarine environments under strong wind and tidal forcing conditions. These limitations

hampered the application of hydrodynamic models to the CAPES under severe weather conditions, such as during hurricanes. Hubbert and McInnes (1999) introduced an inundation and drying scheme (hereafter referred to as the HM scheme) and incorporated it into a two-dimensional storm surge model. They simulated the storm surge and inundation produced by a tropical cyclone and two separate cold fronts on the Australian coast and found that the model results were realistic, and mathematically well-behaved for moderate variations in grid resolution. Xie et al. (2003) introduced a modified version of the HM scheme that incorporates mass conservation and the flexibility to choose inundation speed based on three-dimensional flow fields. This new scheme was incorporated into a three-dimensional storm surge model. Experiments under idealized geometry and forcing conditions revealed the need to impose mass conservation and properly set inundation speed for inundation modeling in closed or semi-closed coastal systems, such as lakes and sounds. In this study, the coupled inundation and three-dimensional storm surge modeling system of Xie et al. (2003) will be applied to a realistic coastal system, the CAPES, to simulate the storm surge and inundation in and around CAPES under realistic and typical hurricane forcing conditions. The rest of the manuscript is organized as following. In Section 2, the integrated storm surge and inundation modeling system and the configuration of the modeling system for the CAPES are briefly described. Section 3

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describes the hurricane wind model and experimental design. Results are presented in Section 4. Section 5 discusses the results from a set of sensitivity experiments. Section 6 presents the simulation of Hurricane Emily’s (1993) storm surge and inundation. Conclusions are given in Section 7.

2. Model configuration in the CAPES The integrated storm surge and inundation modeling system described in Xie et al. (2003) will be used in this study. For convenience, we will briefly describe the modeling system here. It includes a three-dimensional storm surge component and an inundation component. The storm surge component is essentially the same model that has been configured for the CAPES by Xie and Pietrafesa (1999). It is based on the Princeton Ocean Model (POM) (Mellor, 1996), which uses a terrainfollowing sigma (s) coordinate in the vertical plane and a staggered Arakawa C grid in the horizontal plane. An embedded second moment turbulence closure sub-model is used to compute the vertical mixing coefficients. The model uses a free surface, and thus allows explicit prediction of sea level change. In POM, the horizontal finite differencing is explicit whereas the vertical differencing is implicit. The latter eliminates time-step constraints on the vertical resolution and permits the use of fine vertical resolution near the surface and in shallowwater regions. A three time level leapfrog scheme is used for temporal integration. Atmospheric forcing for the model is computed externally. It is imposed on the system via surface boundary conditions, which include wind stress and pressure perturbation. Additional surface boundary conditions in the form of heat, moisture and radiation fluxes can also be imposed, but are not used for storm surge modeling. Lateral boundary conditions include an open boundary in the coastal ocean that permits surface waves to propagate out of the model domain to prevent energy accumulation along the boundary. On the land side of the model domain, runoff from major rivers can be prescribed. The inundation scheme used in this study is the modified HM scheme described in Xie et al. (2003). To determine if a land grid point will be inundated, the height of the water at the grid cell adjacent to the coastline is compared to the topographic height next to it on land. If the water is higher than the adjacent land, then flooding is possible and a second criterion is examined. The distance in the x and y directions over which water could travel in a single time step at each mass grid cell which is adjacent to the coast is computed using the inundation speed at that location. If the distance is larger than the grid size, then the grid

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cell turns into water. Otherwise, flooding will not occur at this time. One of the modifications made to the HM scheme is in the choice of the inundation speed. In the HM scheme, the inundation speed is determined by vertically averaged currents, whereas in the modified scheme, the inundation speed is computed from the surface current derived from a three-dimensional model. The procedure for draining in the modified HM scheme also differs from the original HM scheme. In the HM scheme, a draining speed determined by the vertically averaged current adjacent to the coast is used. While a draining speed can be uniquely determined in a coastal region of a simple, verge-shaped geometry, determining a unique draining speed is not practical in complex estuaries. For a shallow estuary, even under moderate wind stresses, water at interior cells (cells surrounded by water) can be drained. In this case, it is impossible to determine a unique adjacent grid cell since all grid cells around the interior grid cell are adjacent grid cells. Thus, the use of a draining velocity, and hence the accumulated distance traveled by retreating water, loses meaning for interior cells. Therefore, in the modified HM scheme, draining occurs when water depth falls below a preset threshold value. Another difference between the modified HM scheme and the HM scheme is that a mass balance constraint is imposed on the inundation process in the modified HM scheme. Flooding at one location is at the expense of water mass elsewhere. This process lowers the water level in the original water body. Similarly, when one area dries out, the water that was there must be accounted for in the new system. This process increases the sea level in the new system. Though the addition to and deduction from the water volume by flooding and draining may not be important for adjusting sea level change on the continental shelf and other regions that are connected to an open ocean (where one can assume the availability of an infinite amount of external source of water mass), mass balance should not be ignored in a closed or nearly closed shallow-water system. In our model, an iterative mass rebalancing procedure is implemented whenever the procedure of flooding or draining occurs. If a study domain is an enclosed system, when a land cell is flooded or a wet cell is drained, the water mass in the system must remain unchanged. If a study domain is a nearly closed estuary like the CAPES, the flux through the inlets has to be further considered in the model (Peng, 2002; Xie et al., 2003). In this study, the storm surge and inundation modeling system is configured for the CAPES and its adjacent shelf (75.25e77.25(W, 34.75e36.5(N). In the current setting, the total water mass in the CAPES system is forced to equal the initial water mass, plus all lateral inputs through rivers (or barrier island inlets). Three barrier island inlets (Oregon, Hatteras, and

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Ocracoke Inlets) are explicitly resolved by the current model grids. The horizontal resolution is determined by a 325-m grid size in both x and y directions, while four sigma levels are used in the vertical. The bottom topography of the system was obtained from the GEODAS publication (version 4.0.7 at http://www.ngdc.noaa.gov/ mgg/gdas), and a minimum depth of 1 m was given to the grid cell where the mean water depth is less than 1 m. The land relief and bathymetry of the CAPES are shown in Fig. 2.

where r is the air density, P is the atmospheric pressure at radius r, Pc is the hurricane central pressure, Pn is the ambient pressure, A and B are scaling parameters, and Vw# is the hurricane pressure gradient induced wind velocity. Those parameters are set to: Pn ¼ 1010 mb, r ¼ 1:2 kg=m3 , B ¼ 1:9, A ¼ ðRmax ÞB , where Rmax is the radius of maximum wind (RMW), which is 51 km and 46 km (Hsu and Yan, 1998), respectively, for the hypothetical Category 3 and 2 hurricanes. The wind speed used in this study is the combination of Eq. (2) and hurricane translation speed VH:

3. Winds and experimental settings

~w # þ V ~H ~w ¼ V V

3.1. Hurricane tracks and structure

Hurricane wind stress is computed using the conventional bulk formula:

The 10 most intense (according to the Saffir Simpson scale) hurricanes that affected the CAPES in the 20th century are listed in Table 1. These storms had either northward or northeastward tracks (Fig. 3a). For convenience, we will examine the response of the CAPES system to 10 hypothetical Category 2 and 3 hurricanes which pass through the CAPES system from the S or SW on 10 linear tracks (Fig. 3b). The hurricane pressure field and surface wind velocity induced by the pressure gradient were modeled following Holland (1980):

~w KV ~w t ¼ rCd KV

P ¼ Pc þ ðPn
Pc Þ expð
A=rB Þ 1=2

ð4Þ

where Cd is the drag coefficient, which is assumed to vary over water with wind speed (Large and Pond, 1981): 8 ~w K KV ~w KR10 m=s > 0:49 þ 0:065KV > < ~w K!10 m=s 1:14 3%KV ð5Þ 103 Cd ¼ ~ ~ > 0:62 þ 1:56KVw K 1%KVw K!10 m=s > : ~w K!1 m=s 2:18 KV The 10 hypothetical hurricanes are assumed to move at a translation speed of 25 km/h. The central pressure was kept at 960 and 970 mb for Category 3 and 2 hurricanes, respectively. The hurricane pressure field given by Eq. (1) is applied as a forcing term in the

ð1Þ

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ð3Þ

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Fig. 2. The land relief (m) and bathymetry (m) of the CroataneAlbemarleePamlico Estuary System and the adjacent shelf region. The dashed line indicates the border of the eight sub-zones (A, B, C, D, E, F, G, H).

M. Peng et al. / Estuarine, Coastal and Shelf Science 59 (2004) 121e137 Table 1 Top 10 most severe landfall and coast-brushing hurricanes in the CAPES in the 20th century ranked according to the Saffir Simpson scale (from NOAA National Hurricane Center database at http:// www.nhc.noaa.gov) Rank

Name

Category

1 2 3 4 5 6 7 8 9 10

Helene (1958) Bonnie (1998) Hurricane #2 (1924) Emily (1993) Donna (1960) Floyd (1999) Lone (1955) Gloria (1985) Barbara (1953) Hurricane #13 (1933)

3 3 3 3 2 2 2 2 2 2

Note: the category of these storms is valid for the time of the closest approach to the CAPES area.

momentum equation, so the sea level change induced by atmospheric pressure gradient is incorporated by the model (Hubbert et al., 1990). The astronomical tide was not considered in the CAPES because the amplitude, or half of the range, of M2 tide (the major partial tide) at the three inlets is just around 0.6 m, and the amplitude damps rapidly when the tide gets into the CAPES (Pietrafesa et al., 1986; Peng, 2002). For convenience of presenting the pattern of flood during the passage of the hurricane, the CAPES is sub-divided into eight subzones as shown in Fig. 2.

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4. Results 4.1. Cases 1e5: northward moving hurricanes Case 1: First, consider the case where the hurricane traveled northward to the west of the CAPES (Track 1, Fig. 4a). As the hurricane moved northward along 78.0(W, high surge levels occurred along the coastline on the western side of Pamlico and Albemarle Sounds. Storm surge in the Pamlico River region (Zone B) reached up to 3 m, producing approximately 350 km2 area of flooding in Zone B. Significant flooding also occurred along the coast of northern Albemarle Sound. Southern Pamlico Sound and the Outer Banks did not experience large inundation as evidenced by the negative sea levels developing over region H as shown in Fig. 4a. Inundation in Zones A and E is also minor despite the large surge heights and this is due mainly to the relatively steep terrain adjacent to the western end of southern Pamlico Sound and the Albemarle Sound. Case 2: The hurricane traveled along the western border of the CAPES in Case 2. The extent of storm surge and inundation in this case was generally greater than in Case 1. This is clearly indicated in Figs. 4b and 5. The inundation area in Zone B reached over 500 km2. Significant flooding also took place in Zones A, C, E, and F where the inundation area in all sub-zones exceeded 100 km2.

Fig. 3. (a) The tracks of the 10 most severe landfall or coast-brushing hurricanes in the CAPES according to the Saffir Simpson scale. The numbers in the figure correspond to the hurricanes listed in Table 1. (b) The 10 hypothetical hurricane tracks. The numbers denote the experimental cases, with 4 indicating the starting position of each hurricane.

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Fig. 4. (a) The storm surge and inundation situation as a hypothetical Category 3 hurricane center moves to four different locations along Track 1. The dashed line is for sub-zone border and the contour is for sea surface elevation (m). (b) Same as (a), except for Case 2. (c) Same as (a), except for Case 3. (d) Same as (a), except for Case 4. (e) Same as (a), except for Case 5. (f) Same as (a), except for Case 6. (g) Same as (a), except for Case 7. (h) Same as (a), except for Case 8. (i) Same as (a), except for Case 9. (j) Same as (a), except for Case 10.

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Fig. 4 (continued )

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Fig. 4 (continued )

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Fig. 4 (continued )

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Fig. 4 (continued )

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Case 3: The hurricane traveled through the central portion of the CAPES in Case 3. High storm surge occurred in both eastern and western Pamlico and Albemarle Sounds. As shown in Figs. 4c and 5, over 3 m of surge was realized in Zones A, B, E, F and G. The most severe inundation took place in Zones A, D, E and F. Case 4: In this case (Fig. 4d), the storm was moving northward about 50 km offshore Cape Hatteras (Fig. 3b). Storm surge and inundation occurred mainly along the Outer Banks and Zone D. Significant inundation occurred in Zone D, where the inundation area was about 350 km2. Both the storm surge and the inundation area in Zone H increased slightly compared with Case 3. Zones A and F were also inundated, but the total area of the flooding was much smaller than in Case 3. The maximum storm surge in the eastern portion of the CAPES (Zones D, F, G and H) reached as high as 3 m. Case 5: The storm stayed about 100 km offshore in this case (Fig. 4e). While the peak storm surge was generally lower than 2 m in all sub-zones, Zones A and D still experienced significant inundation due to flat land nearby. Among all northward moving cases, the results show that storms that traveled through the central portion of the CAPES (Tracks 2 and 3) produced the highest storm

5.0

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surge and inundation for the entire system. Generally, lower surge and smaller inundation area were expected for hurricanes that traveled further to the west or to the east. However, as to a specific sub-zone, storm surge and inundation area could occur even if the hurricane did not move across the CAPES. For instance, in Case 4, the storm stayed offshore, but the storm surge in the eastern part of Pamlico Sound was the greatest among all northward cases in the vicinity of the boundary between Zones G and H. Both storm surge and inundation in Zone D reached peak values in Case 4. Even in Case 5, significant inundation still existed in Zone D although the hurricane stayed offshore due to wind directions that are favorable to storm surge in this region. 4.2. Cases 6e10: northeastward moving hurricanes Case 6: In this case (Fig. 4f) the storm track was along the northwestern border of the CAPES and passed though the western end of Albemarle Sound. The maximum storm surge took place in the western part of Pamlico Sound, around Pamlico River mouth, and in the northeastern part of Albemarle Sound. The peak storm surge in Zones B, C, F and G reached 3 m, and major flooding occurred in Zones B, D and F where

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Fig. 5. The maximum storm surge and the total inundation area for each track. The uppercase and lowercase letters of A, B, C, D, E, F, G, H on the horizontal axis indicate, respectively, the storm surge and inundation area in the eight sub-zones with the lighter color bar for sea surface elevation and the darker for inundation area. The number on the vertical axis is for storm surge (m) and inundation area (!120 km2). These results are for Category 3 hurricanes.

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more than 250 km2 of land was flooded in each subzone. Inundation also took place in Zones C, E and G, albeit less severely. Case 7: The storm moved along the western coast of Pamlico Sound and continued northeastward through central Albemarle Sound (Fig. 4g). In this case, storm surge and inundation were greater than in Case 6 in all of the sub-zones except for Zone B, where the peak surge decreased from 3.5 m to 3.0 m, and in Zone F, where the inundation area decreased from 350 km2 to 250 km2. The storm surge in the southeastern shore of Pamlico Sound, Zone H, reached 3 m, significantly higher than in Case 6. In this case, the inundation areas in Zones B and D exceeded 350 km2. Case 8: The track passed through the major southwestenortheast axis of Pamlico Sound in this case (Fig. 4h). The maximum storm surge reached 3 m in every sub-zone except for Zone A where the peak surge was about 2.3 m. Flooding occurred in all sub-zones, and inundation areas exceeded 300 km2 in Zones B, D and E. Case 9: The storm stayed offshore and while the hurricane eye did not make landfall, the storm remained near the coast (Fig. 4i). In this case, storm surge and inundation decreased in all sub-zones except in Zone A where both the peak surge and the inundation area increased. In Zone A, the maximum storm surge exceeded 3.5 m and the flooding area reached 400 km2. Case 10: In this case (Fig. 4j), the storm track stayed more than 50 km offshore and again the eye did not make landfall. The extent of storm surge and inundation was less compared with all other cases. The peak surge in all sub-zones was below 2 m, except for Zone A where the peak surge reached 2.8 m. No flooding took place in Zones C, E, F and G. Still, significant flooding occurred in Zone A (Fig. 5). In the northeastward moving cases (Tracks 6e10), storms following the two central paths (Tracks 7 and 8) induced the most severe storm surge and inundation in the entire system. Storm surges in almost every sub-zone exceeded 3 m, and significant flooding appeared in all sub-zones. When the track of the hurricane eye did not cross the CAPES, high surge and inundation of large adjacent land could still occur in a given sub-zone, although the overall level of storm surge and flooding in the whole system were lower. For example, when the hurricane moved along one of the offshore tracks (Track 9), the storm surge and flooding area in Zone A were above 3.5 m and greater than 400 km2, respectively. The surge and inundation remained significant in Zone A even in Case 10, where the storm stayed quite a distance offshore. It should be noted that Zones F, G, H and part of Zone A included the barrier islands. The total land area is limited in these Zones. The inundation area as shown in Fig. 5 cannot be directly compared to that in other sub-zones. For example, the land area in Zone H is only

about 50 km2. Thus, if the flooding area in Zone H is about 35 km2 as in Case 4, more than half of the barrier island in this sub-zone is flooded.

5. Sensitivity experiments 5.1. Sensitivity to hurricane MCP and RMW The storm surge and inundation produced by Category 3 hurricanes are presented in Section 4. In this section, we will study the storm surge and flooding area by Category 2 hurricanes. Category 2 hurricanes are expected to produce lower peak surges in each sub-zone than the Category 3 hurricanes. This is not only because the hurricane MCP is higher for Category 2 hurricanes (10 mb difference between the two categories in this study), but also because the RMW is smaller for the hypothetical Category 2 hurricanes. Results for each track are not shown individually, but the maximum storm surge and the total inundation area in the subzones are illustrated in Fig. 6. In all the simulations carried out so far, the hurricane’s MCP and RMW were set to 960 mb and 51 km, respectively, for all the Category 3 hurricanes and 970 mb and 46 km, respectively, for Category 2 hurricanes. These settings are based on the study by Hsu and Yan (1998) which indicated that the MCP for Category 3 and 2 hurricanes range, respectively, from 945 to 964 mb and from 965 to 979 mb, and the mean RMW for Category 3 and 2 hurricanes are 51 km and 46 km, respectively. In real hurricane cases, the actual MCP and RMW for the same category of storms can vary significantly. In the following section, we will evaluate the sensitivity of storm surge and inundation area to changes in MCP and RMW. Five cases were analyzed to investigate the sensitivity of the model results to changes in MCP and another five cases were studied to evaluate the model sensitivity to changes in RMW. In the former five cases, RMW was fixed at 50 km while the hurricane central pressure changed from 975 to 955 mb. In the latter five cases, hurricane central pressure was fixed at 965 mb while the RMW ranged from 40 to 60 km. In all 10 cases, the hurricane moved along Track 3 as shown in Fig. 3b at a speed of 25 km/h. The results (Fig. 7) show that both storm surge and inundation area increased as the hurricane MCP decreased or as RMW increased. Within the ranges of MCP and RMW, the effect of changing hurricane MCP is greater than that of RMW on storm surge and inundation for every sub-zone in the CAPES. For example, in the first five cases when the hurricane MCP decreased from 975 to 955 mb, the difference of the maximum storm surge in Zone E increased up to 1.5 m

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Aa

965 mb 45 km

4.0

3.0

4.0

965 mb 50 km

4.0

3.0

5.0

965 mb 55 km

4.0

3.0

5.0

965 mb 60 km

4.0

Aa

Bb

Cc

Dd

Ee

Ff

Gg

Hh

Fig. 7. The effects of hurricane minimum central pressure (MCP) and radius of maximum wind (RMW) on the maximum storm surge and total inundation area. The RMW is fixed at 50 km for the left five panels and the hurricane MCP varies from 975 to 955 mb. The MCP is set to 965 mb for the right five panels and the RMW range is from 40 to 60 km. The indication of uppercase and lowercase letters and the ratio scale for the vertical axis are the same as in Fig. 5. The hypothetical hurricanes move along Track 3 at a speed of 25 km/h for all 10 cases.

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M. Peng et al. / Estuarine, Coastal and Shelf Science 59 (2004) 121e137

Storm surge (m) and inundation area (× 200 square km)

5.0

White Bar: Storm Surge Black Bar: Inundation Area

4.0

3.0

2.0

1.0

0

10

20 30 40 Hurricane translation speed (km/h)

50

60

Fig. 8. Peak storm surge and inundation area in Zone A as a function of storm translation speed. The storm is a Category 3 hurricane moving along Track 3.

while in the cases when the RMW changed from 60 to 40 km, the peak surge decreased by less than about half a meter. 5.2. Sensitivity to hurricane translation speed Hurricane translation speed is another factor that affects the storm surge and inundation area. As the translation speed may vary greatly from one hurricane to another within the same storm intensity category, it is important to understand how sensitive the CAPES system is to changes in storm translation speed. In Section 5.1, the hurricane translation speed was set to 25 km/h for all cases. This speed falls between the ‘‘slow’’ and ‘‘fast’’ hurricane speed defined by the Lake and Overland Surges from Hurricanes (SLOSH) model (Jelesnianski et al., 1992). Actual hurricane translation speed can be faster or slower. For instance, in the 10 most severe landfalling or near-shore hurricanes in the CAPES region (Fig. 3), the hurricane translation speed could be over 30 km/h like that of Hurricane Donna (1960), or less than 20 km/h like that of Hurricane Emily (1993) or could even stall near the coast such as Hurricane Dennis (1999) did. To investigate the sensitivity of storm surge and inundation to hurricane translation speed, four cases with storm speeds of 50, 25, 12.5 and 6.25 km/h were examined for a hypothetical Category 3 hurricane moving along Track 3. The results show that slower translation speed produces higher storm surge and larger inundation area in all sub-zones. This is consistent with the findings of Jelesnianski et al. (1992). It is important to note that the inundation area is not only determined by peak surge height, but also by the topography in the coastal region. In the specific case

examined here, although the peak surge increased almost linearly as the storm speed decreased, the inundation area did not show any linear relationship with storm speed. For example, in Zone A, the peak surges and inundation areas for the above cases were 2.3, 3.5, 4.5, and 4.8 m and 140, 420, 860 and 940 km2, respectively (Fig. 8). When the translation speed decreased from 50 to 25 km/h the storm surge increased by 1.0e1.5 m and the inundation area almost tripled in most of the sub-zones. When the translation speed decreased from 25 to 12.5 km/h, the peak surge increased by 0.5e1.0 m and the inundation area nearly doubled. When the translation speed decreased from 12.5 to 6.25 km/h, the changes in peak surge and inundation area were much less dramatic. In this case, the storm surge increased by less than 0.5 m in all zones and the inundation area increased by only about 15%.

6. Hurricane Emily (1993) Some observed hurricane tracks are similar to one or the combination of several of the 10 hypothetical tracks that have been studied in the last section. Hurricane Emily (1993), for instance, moved along a track similar to Track 4 when it was close to the CAPES. It passed along the North Carolina coast within 50 km of the Outer Banks from August 31 to September 1 in 1993 (Pasch and Rappaport, 1995). The best track position and pressure for the period while the storm was offshore NC are given in Table 2. Houston et al. (1999) found that the parametric hurricane wind model used in the National Weather Service Sea, Lake, and Overland Surge from Hurricanes (SLOSH) model underestimated the peak wind speed of Hurricane Emily by 15% which resulted in a significant underestimation of the peak surge in the Pamlico Sound. The inflow angle for the SLOSH hurricane wind also tended to be less than that from post-storm analysis. On the other hand, Pietrafesa et al. (1997) showed that a three-dimensional storm surge model driven by Holland (1980) hurricane wind model [Eqs. (1)e(3)] using a inflow angle determined by B ¼ 1:9 and Emily’s ‘‘best track’’ was able to predict the peak surge on the sound side of the Outer Banks produced by Hurricane Emily. In this study, Emily’s best track data (Table 2) together with a radius of maximum wind (43 km) were used in the hurricane wind model [Eqs. (1)e(3)] to compute the wind and wind stress field. The simulated storm surge and inundation from 08/31/2000Z to 09/01/0200Z are shown at every 2 h in Fig. 9. As we will show, even using an axis-symmetric hurricane wind model, the three-dimensional storm surge model based on POM was able to accurately simulate the peak surge associated with Hurricane Emily on the sound side of the Outer Banks.

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M. Peng et al. / Estuarine, Coastal and Shelf Science 59 (2004) 121e137 Table 2 Time, location and pressure of Hurricane Emily (1993) Time

Latitude

Longitude

Pressure (mb)

08/31/00 08/31/06 08/31/12 08/31/18 09/01/00 09/01/06 09/01/12 09/01/18

32.40 32.90 33.60 34.50 35.60 36.60 37.50 38.20


73.00
73.80
74.70
75.20
74.90
74.40
72.70
70.70

972 970 965 962 960 962 965 969

At 08/31/2000Z, major storm surge occurred around Cedar Island in southern Pamlico Sound and along the south section of the Outer Banks. Part of Cedar Island and most of the barrier island in this region were inundated. Two hours later, the flooding area moved northeastward along the barrier island and the storm surge on Hatteras Island exceeded 2 m at Frisco, Buxton and Avon (see Fig. 1 for locations). At 09/01/0000Z, the storm surge near Hatteras Island continued to rise and exceeded 3 m at Buxton. At this time, the surge in the southern Pamlico Sound decreased, but the inundation remained at Cedar Island and extended all the way to Hatteras Island. At 09/01/0200Z, storm surge and inundation began to decrease in all regions, but the storm surge at Avon was still over 2 m.

The U.S. Geological Survey and National Hurricane Center documented that the location of the highest water marks (HWM) produced by Hurricane Emily was along the sound side on southern Hatteras Island, the night of August 31. Buxton had the highest HWM and significant flooding occurred across the barrier island. The observed HWMs with uncertainty ranges at Hatteras, Frisco, Buxton, Avon and Duck Pier and the simulated peak surges at the corresponding positions are shown in Fig. 10a. In general, the simulated peak surges are within the observed ranges of HWMs. Fig. 10b shows the observed and simulated time series of storm surge (sea level minus tides) at Buxton from 08/ 31/1800Z to 09/01/0600. There is a remarkable agreement between the observed storm surge and the modeled storm surge at Buxton.

7. Conclusions The integrated storm surge and inundation model (Xie et al., 2003) was applied to simulate hurricaneinduced storm surge and inundation in the Croatane AlbemarleePamlico Estuary System (CAPES). The storm surge and inundation patterns induced by 10

36.5N

09/01/0000Z Emily 1993

−1

1

−1

1

−1

−1

1

36.0N

−1

−2

08/31/2000Z Emily 1993

1

−1

−1

−2−1

−2

35.5N

−2

−1 1

−1

2

1

2

1

35.0N

2

1

36.5N

−1 1

08/31/2200Z Emily 1993 1

−1 1

36.0N

09/01/0200Z Emily 1993

−1

−1 −2

−2

35.5N

−1 −2 −1

1

35.0N

2

77.0W

76.5W

21

2

1

1

1

1

76.0W

75.5W

77.0W

76.5W

76.0W

75.5W

Fig. 9. Simulated storm surge (m) and inundation at 08/31/2230Z, 09/01/0000Z, 09/01/0130Z, and 09/01/0300Z when Hurricane Emily brushes the CAPES coast in 1993.

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M. Peng et al. / Estuarine, Coastal and Shelf Science 59 (2004) 121e137 3.5

3.5

Storm Surge (m)

Observed Simulated 3

3

2.5

2.5

2

2

1.5

1.5

1

1

0.5

0.5

a 0

b Hatteras

Frisco

Buxton

Avon

Duck Pier

0

08/31 19 20 21 22 23 09/01 01 02 03 04 05 06

Fig. 10. (a) The comparison of observed highest water mark range (rectangular boxes) at Hatteras, Frisco, Buxton, Avon and Duck Pier and simulated highest storm surge (horizontal line) at corresponding locations. (b) Observed and simulated time series of storm surge at Buxton from 08/ 31/1800Z to 09/01/0600Z. The historic data used in the figure are taken from the U.S. Geological Survey (1993) and the National Hurricane Center’s (1993) preliminary report for Hurricane Emily (1993), and only the ‘‘excellent’’ highest water marks are chosen. The locations of Hatteras, Frisco, Buxton, Avon and Duck Pier are shown in Fig. 1.

hypothetical Category 2 and 3 hurricanes that pass over or nearby the CAPES are studied. The main results are: (1) Incorporation of the inundation scheme into the storm surge model ensured mathematical tractability thus enabling the simulation of storm surge and inundation in the CAPES. For an assumed Category 3 hurricane with a translation speed of 25 km/h, Tracks 2 and 3 induced the most severe storm surge and inundation among the five northward tracks. In those two cases, the peak storm surge reached 3 m in most regions. In some areas such as western Pamlico Sound and western Albemarle Sound, it exceeded 4 m. The most severe flooding region measured by inundation area is located around the Pamlico River mouth, where the flooding area reached 500 km2. For the northeastward tracks (Tracks 6e10), the two central tracks (Tracks 7 and 8) induced the most severe storm surge and inundation. The peak storm surge in almost every sub-zone exceeded 3 m, and significant flooding occurred in all sub-zones. Generally, hurricanes moving along this group of tracks generate higher storm surges and larger inundation areas than those moving along northward tracks (Tracks 1e5). (2) Along any given storm track, the decrease of hurricane minimum central pressure (MCP) or increase of hurricane radius of maximum wind (RMW)

lead to more severe storm surges and inundation across the entire region as one might expect. However, within the range of 955!MCP!975 mb and 40!RMW!60 km, both storm surge and inundation are more sensitive to changes in MCP than to RMW. (3) Storm surge heights and inundation areas in all subregions increased as hurricane translation speed decreased. For Track 3, when the storm translation speed decreased from 50 to 25 km/h, the peak storm surge increased as much as 1.5 m, and the inundation area almost tripled in some regions. When the translation speed decreased from 25 to 12.5 km/h, the peak surge increased by 1.0 m in some regions and the inundation area nearly doubled. When the translation speed dropped from 12.5 to 6.25 km/h, the increase of surge and inundation area was much more modest compared to faster moving storms. This is easy to understand, because for a storm with a lower translation speed, the same percentage decrease in its translation speed represents a smaller decrease in actual speed. Although the model was able to accurately reproduce the peak surge on the sound side of the Outer Banks during the passage of Hurricane Emily of 1993, realtime storm surge forecasting will always contain a high degree of uncertainty. Accurately predicting detailed

M. Peng et al. / Estuarine, Coastal and Shelf Science 59 (2004) 121e137

wind distribution is still impractical due to uncertainties in hurricane track and intensity forecasts, as is accurately accessing the storm surge and inundation area. Most storm surge models are driven by theoretical hurricane wind models such as Holland (1980) and SLOSH winds, using forecasted storm track and intensity. As a result, the accuracy of storm surge forecasting, to a large degree, is limited by the accuracy of hurricane forcing. Until the skills of hurricane track and intensity forecasts become more reliable, storm surge forecasts based on a single forecasted track will inevitably contain large uncertainty. To reduce uncertainty, one may resort to probabilistic storm surge and inundation forecasts based on a large ensemble of storm tracks and intensity probabilities.

Acknowledgements This study was supported by the National Oceanic and Atmospheric Administration under Grant #NA060CO373-001 through the NOAA Charleston Coastal Services Center in collaboration with Waterstone Enterprise Strategies and Technologies Inc. in Boulder, CO. We appreciate the comments from two anonymous reviewers, which helped to improve the final paper.

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Hsu, S.A., Yan, Z., 1998. A note on the radius of maximum wind for hurricanes. Journal of Coastal Research 14(2), 667e668. Hubbert, G.D., Holland, G.J., Leslie, L.M., Manton, M.J., 1990. A real-time system for forecasting tropical cyclone storm surges. Weather and Forecasting 6, 86e97. Hubbert, G.D., McInnes, K.L., 1999. A storm surge model for coastal planning and impact studies. Journal of Coastal Research 15(1), 168e185. Jelesnianski, C.P., Chen, J., Shaffer, W.A., 1992. SLOSH: sea, lake, and overland surges from hurricanes. NOAA Tech. Report NWS 48, 77 pp. Large, W.G., Pond, S., 1981. Open ocean momentum fluxes in moderate to strong winds. Journal of Physical Oceanography 11, 324e336. Lin, G.-Q., 1992. A numerical model of the hydrodynamics of AlbemarleePamlico Sounds system, North Carolina. M.S. thesis, North Carolina State University, Raleigh, N.C., 118 pp. Mellor, G.L., 1996. User’s Guide for a Three Dimensional, Primitive Equation, Numerical Ocean Model. Princeton University, Princeton, NJ. National Hurricane Center, 1993. Preliminary Report of Hurricane Emily. Available from ftp://ftp.nhc.noaa.gov/pub/storm_archives/ atlantic/prelimat/atl1993/emily. Pasch, R.J., Rappaport, E.N., 1995. Atlantic hurricane season of 1993. Monthly Weather Review 123, 871e886. Peng, M., 2002. A numerical modeling study of the hydrodynamics of the CroataneAlbemarleePamlico Estuary System, North Carolina (a dissertation at http://www.lib.ncsu.edu/etd/public/etd165492721023850/etd.pdf). Pietrafesa, L.J., Janowitz, G.S., Chao, T.-Y., Weisberg, R.H., Askari, F., Noble, E., 1986. The physical oceanography of the Pamlico Sound. UNC Sea Grant Publication UNC-WP-86-5, Raleigh, N.C. Pietrafesa, L.J., Xie, L., Morrison, J., Janowitz, G.S., Pelissier, J., Keeter, K., Neuherz, R., 1997. Numerical modeling of the storm surge in and around the CroataneAlbemarleePamlico estuary system by hurricane Emily of August 1993. Mausam 48, 567e578. U.S. Geological Survey, 1993. Water-Supply Paper 2499: Storm Surge and Flood of Hurricane Emily on Hatteras Island, North Carolina. Xie, L., Pietrafesa, L.J., 1999. Systemwide modeling of wind and density driven circulation in CroataneAlbemarleePamlico Estuary system Part 1: model configuration and testing. Journal of Coastal Research 15(4), 1163e1177. Xie, L., Pietrafesa, L.J., Peng, M., 2003. An integrated storm surge and inundation modeling system for lakes, estuaries and coastal ocean. Journal of Coastal Research, in press.