Report Galveston Bay: Flood Risk Reduction Barrier

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Report Galveston Bay

Bolivar Roads Flood Risk Reduction Barrier: Sketch design

Authors: S.N. Jonkman, M. van Ledden, K.T. Lendering, L. Mooyaart, K.J. Stoeten, P. de Vries, A. Willems, R. de Kort

Date: July 19, 2013 Version: final draft 1.0

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Report Galveston Bay: Flood Risk Reduction Barrier

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1. Introduction

1 . 1 Bac k gro u n d

After Hurricane Ike in 2008, Texas A&M has proposed the concept of the Coastal Spine or Ike dike (herein further referred to as Ike Dike) to reduce flood risk of the Galveston/Houston area. The Ike Dike comprises a coastal protection system across the Galveston and Bolivar Islands together with a storm surge barrier in the Bolivar Roads. The Bolivar Roads is the deep-draft entrance channel to the Port of Houston. Various studies have been carried out since 2008 resulting in conceptual ideas for the barrier and the adjacent coastal protection (see e.g. Davis et al., 2010, Stoeten, 2012).

Recently, Texas A&M and Delft University of Technology have agreed to mutually work on the aspect of flood risk reduction for coastal systems.. In this framework, Texas A&M University has requested Delft University of Technology (TU Delft) a high-level study of the Ike Dike system to reduce flood risk in the Galveston/Houston region. Special emphasis must be paid to alternative options of the storm surge barrier in the Bolivar Roads.

The study has been executed by staff from Delft University of Technology in the Netherlands with some experts from private sector in the Netherlands to address some key aspects and involve state-of-the art expertise with international barrier design. For a list of participants, the reader is referred to Appendix 1. It is emphasized that this study is a first step in a further conceptual and detailed design process of the entire Ike Dike system.

1 . 2 Obje c t iv e o f t h is s t u dy

The objective of this study is to provide suitable candidate solutions for a storm surge barrier in the Bolivar Roads as part of the Ike Dike system and provide recommendations for further research.

1 . 3 Appro ac h

The following steps have been carried out to achieve this objective:

Step 1: Data gathering and analysis of the system behavior

After a kickoff meeting, information has been gathered to conduct this study. Hydro-meteo information (surge, waves, currents, wind) and site specific details regarding the geotechnical conditions were analysed. Based on the various information sources, design conditions and functional requirements and boundary conditions have been set. Data collection has focused on the Ike Dike system alignment and the location of the storm surge barrier.

Step 2: Barrier design brainstorm workshop

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6 possible solutions for the a storm surge barrier in the Bolivar Roads. Pros and cons of various solutions (e.g. various types of gates, etc.) have been discussed and critical issues and subjects for further study were identified.

Step 3: Report barrier concepts and provide recommendations

Based on the outcomes of the workshop possible barrier design(s) have been documented along with their main characteristics. At a general level, attention has been paid to technical characteristics of the various components (foundation, gates, abutments, etc.), risk & reliability of the design, costs, operation & maintenance, etc. etc. As part of the reporting phase, impacts have been considered at a general level. Important aspects to be considered are the required tidal exchange for sufficient refreshment of the bay but also the navigational requirements.

1 . 4 Re p o rt Ou t li n e

This report is structured as follows. Chapter 2 provides an overview of the relevant input data. Chapter 3 describes the several barrier sketch designs and its design considerations. Conclusions and recommendations are given in Chapter 4.

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2. Data gathering and boundary conditions

2 . 1 S y s t e m de s c ri pt io n

Galveston Bay is a large natural estuary system located on the Upper Texas Coast (Figure 1). The estimated surface area of the Bay is 1554 square kilometer and the length of the bay-shoreline is about 374 kilometer (Phillips, 2004).

Galveston Bay is a micro-tidal wind dominated estuary system with an average depth of 3 meter (NGDC, 2007). Three channels connect Galveston Bay to the Gulf of Mexico, the largest being the Bolivar Roads inlet. The flow velocity within these channels remains below 2 ms-1_{under ordinary conditions (NOAA, 2013a). The average residence time of water is 40} days (Phillips, 2004). Tides are of a mixed type (both diurnal and semi-diurnal) with a tidal range of approximately 0.3 meter (1 feet) (See: Figure 2 and Table 1).

Figure 1 - Map (left) and DEM (right) of Galveston Bay. Vertical reference: NAVD88. Data-Source: (NGDC, 2007).

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Gauge Mean Range Diurnal Range Phase difference (est.)

Pleasure Pier (Open Coast) 1.16 feet 1.67 feet -

Eagle Point 1.02 feet 1.11 feet + 6 hour lag

Morgan’s Point 1.13 feet 1.31 feet + 6 hour lag

Figure 2 - Tidal propagation in Galveston Bay. Vertical Reference: MSL. (NOAA,2013)

The Galveston Bay is a highly valuable ecosystem. It is home to nursery and spawning grounds for many types of marine life including crabs, shrimp, oysters, and many varieties of fish thereby supporting a substantial fishing industry.

In addition to this, the Bay is the entrance to the Port of Houston, one of the world’s busiest ports and a large and vibrant component of the regional economy. A 2012 study by Martin Associates says ship channel-related businesses contribute 1,026,820 jobs throughout Texas. It is therefore of great importance that both the ecosystem and the navigation function of the Bay are preserved when constructing a barrier in the Bolivar Roads.

2 . 2 Hy drau lic de s ign c o n dit io n s

2 . 2 . 1 No rm al c o n dit io n s

In normal conditions the storm surge barrier has to allow sufficient tidal exchange to preserve the ecosystem of the Galveston Bay. Ruijs (2011) has made a two-dimensional computational model to estimate the effect of a storm surge barrier under these conditions. The results are shown in Table 1. The tide of the Mexican Gulf is of a mixed type, consisting

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9 of both (significant) diurnal as semi-diurnal components. Friction and inertia in the Galveston Bay can be neglected, meaning that the water levels inside Galveston Bay are about equal at a the same moment of time (“small-basin approximation”).

Table 1 Tidal response to partial closure (based on Ruijs, 2011)

Situation Cross-section (µ As*₎ _{Tidal response in} Galveston Bay

Current 28,000 m2

Bolivar Roads: 22, 000 m2 Other: 6, 000 m2

90% of incoming tide (estimate made for this study) 40% closed 16, 000 m2 Bolivar Roads: 13, 000 m2 Other: 3,000 m2 (Ruijs: 80% of original) 72% of incoming tide 60% closed 12,000 m2 Bolivar Roads: 9, 000 m2 Other: 3, 000 m2 (Ruijs: 61% of original) 55% of incoming tide * actually cross-section multiplied by flow coefficient

As a basic assumption for the brainstorm session, it has been assumed that a closure of 40% at maximum of the current opening of the Bolivar Roads can be allowed. For the design of the storm surge barrier this will be an important functional requirement, as larger openings will increase the costs for a storm surge barrier.

2 . 2 . 2 Hu rric an e c o n dit io n s

Storm surge on the open coast is a combination of wind setup, wave setup, barometric setup, coriolis setup and astronomic tide (Dean, 2004). Storm Surge within a semi-enclosed Bay is a superposition of open-coast surge (inflow), local wind setup and local wave setup (Valle-Levinson et al., 2002; Shen et al., 2006)

The average height of Bolivar Peninsula is about 1.0 – 1.5 meter MSL (NGDC, 2007). During extreme storm events, the barrier islands partially deflect the surge (Rego & Li, 2010). Most recently, Hurricane Ike (2008) produced significant overwash on both Galveston Island and Bolivar Peninsula. Such events are known to have occurred at least three times during the past century.

Storm surge within semi-enclosed bays is highly sensitive to the relative landfall location of the hurricane. Hurricanes making landfall left of a bay will force water into the system, increasing the surge (Figure 3: left). As the hurricane moves onshore, the wind direction shifts from East (Stage 1) through South (Stage 2) to West (Stage 3). Hurricanes making landfall east of a bay will force water out of the bay, depressing the surge (Figure 3: right). As the hurricane moves onshore, the wind direction shifts from East (Stage 1) through North (Stage 2) to West (Stage 3).

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Figure 3 - The influence of landfall location on storm surge within Semi-Enclosed Bays. (darker blue indicates areas with highest surge).

Computer simulations with SLOSH (Sea, Lake, and Overland Surges from Hurricanes) indicate that storm surge of over 11 meter is possible within the Upper Bay area. Storm surge on the open coast can reach as high as 7 meter (Gilmore & Englebretson, 1997). It must be noted that SLOSH is frequently used by emergency managers and provides a conservative scenario. Table 1 presents an overview of observed peak surges during recent historic events.

Table 1 – Recent historic observations of storm surge within Galveston Bay.

Hurricane Cat Landfall

Location Peak surge open coast Peak Surge North Bay Peak Surge South Bay Ike (2008) 2 0 km 4.5 meter 5.0 meter 3.5 meter Rita (2005) 5 120 km East 1.5 meter 1 meter 1.3 meter Alicia (1983) 3 50 km West 2.5 meter 4 meter 3 meter Carla (1961) 5 180 km West 3 meter 4 meter 3 meter Cindy (1963) 2 50 km East 0.8 meter -1 meter 1 meter “Surprise” (1943) 2 30 km East unknown -1.5 meter -1.5 meter

The slope of the coastal shelf near Galveston is about 1:2000 resulting in a dissipative wave environment. Jin et al. (2010) find that wave height is, in general, directly related to depth

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11 (including surge) and has less relation with the wind velocity. Table 2 and Table 3 (Jin, et al., 2012) present estimated near shore wave parameters1_{obtained through simulations with} SWAN. Table 4 presents estimated permissible water levels within the Bay (without flooding).

Table 2 – Hmax in meters at Rollover Bridge from Jin et al. (2012)..

Table 3 – Tmax in seconds at Rollover Pass Bridge from Jin et al. (2012).

Table 4 –Estimated (coarsely) permissible water levels within the Bay

Location Inundation when: (estimated)

Galveston Island (bay-side) WL > MSL + 1 meter Galveston Island (ocean-side) WL > MSL + 4.5 meter Texas City WL > MSL + 5 meter Houston Ship Channel WL > MSL + 4 meter Kemah (west bay) WL > MSL + 2 meter

2 . 3 Ge o t e c h n ic al de s ign c o n dit io n s

Geologic background

Drillers’ logs of wells provide information about the deeper soil layers in Galveston County (Petitt and Winslow, 1955). It reports the county is underlain by sequences of unconsolidated sands and clays; the so-called Beaumont clay formations. The sediments are mostly of alluvial or deltaic origin.

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12 Both the drillers’ logs and research into the hydrogeology of gulf coast aquifers report that between 0-150m below NAVD [0-500ft] predominantly clay layers are present. Between 150-215 m below NAVD [500-700 ft], a more sandy clay layer is present, the Alta Loma formation, which is part of the Beaumont clay formation.

Geotechnical information

In order to accurately design the foundation of a storm surge barrier near surface soil information is required. Borings logs that contain very detailed information about the upper soil layers are gathered in the TxSed Sediment Viewer database (Texas GLO, 2013).

Unfortunately, this database only contains accurate soil information for the Bolivar Roads Pass limited to a depth of NAVD-13m [43 ft]. As the Houston Shipping Channel (HSC) is already dredged until a depth of approximately NAVD-13m [43 ft] this information less relevant for the barrier design. There are two boring logs located northward along the HSC into the Galveston Bay (USACE16872-31 and USACE14872-32) that reach a depth of NAVD-19m [64 ft]. Both logs indicate mainly clay and clayey layers. Information about the soil layers below NAVD-19m [43 ft] is only available at the east end of Galveston Island. The reported logs located at the shoreline of the Bolivar Roads Pass are part of a reconnaissance study. This study is executed to investigate piling support in the Port of Galveston. The study, executed by McClelland Engineers (1985), contains six sample borings to explore local subsurface conditions that reach a depth of approximately 50m [165 ft] below NAVD. It gives a good impression of the soil layers in this location which may be representative for Bolivar Roads Pass. Figure 6 is an illustration of the generalized subsurface profile based on several boring locations (N-2, N-4 and N-5 in the source document).

The image gives a good impression of the deeper-lying soil layers. As can be seen, a thick stratum of very dense sand is present at a level of approximately NAVD-40m [131 ft]. McClelland Engineers (1985) report this soil layer as "an excellent bearing layer for high capacity piles". The test results state that the soil is able to resist a force of 6672 kN [1.5 · 106 lbf] by closed end pipe piles with a diameter of 0.61 m [24 in]. This is equal to a maximum soil bearing pressure of 24.6 · 103 kN/m2 [3563 Psi]. The strength of the cohesive soil layers is summarized in Table 2. The information is also adapted from the study performed by McClelland Engineers (1985).

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Table 2 - Soil layer classification and strength properties

Figure 4 - Subsurface soil conditions at the east end of Galveston Island (McClelland Eng., 1985).

2 . 4 Nav igat io n al de s ign c o n dit io n s

The Bolivar Roads is currently used for shipping. The traffic intensity is about 7,000 ships per year. This function will have to be maintained with a storm surge barrier.

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Figure 5 - Bolivar Roads Navigational Chart

Situation Width channel Depth channel

Current (Suezmax) 160 m 14m

Future objective (according to Davis et al., 2010) 180 m 20m

New Panamax 200 m 18m

2 . 5 Bo u n dary c o n dit io n s a n d as s u m pt io n s fo r t h e d e s ign

Based on the available information the following requirements, boundary conditions and assumptions for the design have been formulated.

Design life time: 100 years Hydraulics / surge reduction

The function of the storm surge barrier is to reduce storm surge level in Galveston Bay significantly during extreme conditions.

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 Maximum flood level at Houston Ship Canal: MSL + 4 m

 Maximum flood level at Bolivar Peninsula & Galveston Island, Bay Side: MSL + 1 m

 Maximum flood level at Texas City: MSL + 5 m

 Maximum flood level at Kemah (West): MSL + 2 m This can be done by:

1) Constructing a barrier, preventing the surge to enter Galveston Bay a. Water level Gulf of Mexico near Bolivar Roads: MSL + 5 m b. Water level Galveston Bay near Bolivar Roads: MSL -0.5 m c. Head over the barrier: 5.5 m

2) Constructing a reduction barrier, reducing inflow through the inlet. a. Allowable opening: (maximal) width 200m, depth 16 m b. Water level Gulf of Mexico near Bolivar Roads: MSL + 5 m c. Head: approximately 5 m

Assumption for subsidence and sea level rise during design period: 0.70 m Shipping: Channel width: 180 m; Channel depth: 20m

Environment: Required opening at Bolivar Roads to limit the effect on the environment: 60% of the current total, which is 13.000m2

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3. Barrier sketch designs

3 . 1 In t ro du c t io n

This chapter presents the barrier sketch designs based on the outcomes of a workshop with experts that was held on June 20, 2013, in Delft (the Netherlands). The list of participants is included in the appendix.

This chapter is organized as follows. First the system of the Galveston Bay, the Ike Dike and the role of the Bolivar Roads is discussed. Secondly the design considerations of a storm surge barrier at the Bolivar Roads are dealt with.

3 . 2 S y s t e m le v e l de s ign c o n s ide rat io n s

The Ike Dike project consists of a dike on the Galveston and Bolivar Islands and a barrier in the Bolivar Roads. It is assumed that during critical conditions water can only enter the bay through the Bolivar Roads, since it is assumed that other parts of the Ike Dike have been constructed to stop the surge. This assumption does not allow for water to overwash the Galveston and Bolivar Islands or to enter the bay through the openings to the North and South side of the bay. These openings also require closures during critical conditions. The design of these solutions for these closures is considered beyond the scope of this report. A storm surge barrier in the Bolivar Roads will increase the safety against floods in the Galveston Bay. But the (exact) amount of increase in protection that can be achieved does require more study. It is therefore required to determine the current flood protection level, including the dikes on the Galveston and Bolivar islands, based on hurricanes with different wind speeds, land fall locations, low-pressures and size. A probabilistic method can be applied to determine hurricane and surge probabilities. Together with a hydraulic (computational) model to estimate water levels and flood probabilities this will provide more insight in the current safety and risk levels and the influence of a barrier in the Bolivar Roads.

i. Location/alignment?

The optimal alignment of the storm surge barrier is subject for discussion. Different options have been explored. Firstly the shortest path from the Galveston Island to the Bolivar Peninsula can be an appropriate location for the storm surge barrier. But considering the subsoil and depth along the alignment it may be better to position the barrier deeper inside the bay. A drawback of locating the barrier more inside the bay is that an modifications to the entrance channels to Galveston could be required. Other factors play a role as well, such as the impact on environment and shipping but also the amount of exposure of waves from the Gulf of Mexico.

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Figure 6: Barrier locations (short (left) versus shallow (right))

ii. Open or (partially) closed?

The opening in the Bolivar Roads is relatively narrow compared to the large surface area of the Galveston Bay. So even if some water will enter through the Bolivar Roads the rise of water levels within the bay can still be limited. The costs of storm surge barriers are proportional to the horizontal head and the horizontal forces. So, if it would be allowed to reduce forces and head by not closing the barrier completely, there is an opportunity to reduce costs.

The storm surge barrier will consist of two sections; a navigational section (A in fig. 8, left) and a section for refreshing the Galveston Bay in normal conditions (herein further referred to as environmental section, B in fig. 8 left). Considering the navigational section unlimited headway is required for navigational purposes. If a partially closed environmental section of the barrier provides sufficient flood protection the navigational section could remain open (without barrier). This section will have to deal with large flows during Hurricane conditions and will therefore require a large bottom protection. It is noted that keeping a part of the barrier completely open can harm the image of the storm surge barrier in the perception of the general public.

It can therefore be considered to allow some opening vertically (similar as the Eastern Scheldt) For this purpose the permissible amount of overtopping / underflow should be investigated which allows for sufficient flood safety. When allowing overtopping and/or underflow the horizontal forces on the structure, including the foundation, will be minimized resulting in lower costs.

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iii Bay partition?

The set up of water inside the bay is the result of both the storm surge entering the bay and local wind set up inside the bay, which has a large influence on the water levels on the bay shores.

The wind set up is influenced by the water depth inside the bay and the fetch (distance over which the wind travels over water inside the bay). Another option to reduce the water levels inside the bay during a hurricane is to reduce the wind set up by making compartments inside the bay. This could be achieved by constructing an archipelago of islands or other man-made obstacles which splits the bay in two. On these islands there are possibilities for developing recreational and environmental areas and other functions. This could be consistent with the “Building with Nature” principles.

Figure 7: Partitioning of Galveston Bay to reduce wind set up

Because a hurricane has a swirling character changing the direction of the wind set up as it passes the exact orientation and influence of such a barrier requires further investigation.

3 . 3 Nav igat io n c h an n e l s e c t io n o f t h e barrie r

Since the barrier allows a large volume of water passing through the opening, there is a trade-off between applying a fully closable or open barrier for the navigational section and the amount of water to flow through the environmental section. If the navigational barrier fully blocks the surge, the environmental section may allow more water passage. Vice versa: if the navigational section is left open during all circumstances the environmental section must be able to block the larger part of the inflow.

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i. Open: current velocities and bottom protection

If the navigational channel is left open– without a flood gate but with a bottom protection – this opening of approximately 200 m (600 ft) will lead to a limited raise of the water level the Galveston Bay during a hurricane. The bay is relatively large compared to the navigational cross section (200m), so the water level in the bay will rise with only 1 m in case of a 12h storm surge2_{of 5 meters at the entrance of the Gulf of Mexico. This is a simple (and} conservative) estimate which should be further investigated.

In case of an open option breakwaters would need to be constructed along the navigational channel. Applying a rough bottom protection in this channel further reduces the flow velocities, which could initially be up to 10 m/s. As high flow velocities (up to 10 m/s) occur in the navigational section the stability of bottom protection has to be thoroughly examined and compared with the cost.

Although technically feasible and economically attractive, this solution could harm the image of the storm surge barrier as parts of it are always left open.

ii. Closed barrier: floating door

If required or desired, a barrier inside the navigational channel could be designed. The most logical gate types being a barge gate and a sector gate (as applied for the Maeslant barrier).

Sector gates Barge gate

These types of barrier both have a unlimited headway and allow for a large span. A barge gate consisting of a floating barrier with gates (allowing for an calmly closure) with venturi-fountain (for demping) is considered to be the best alternative, as the closure is relatively easy and simple. A major disadvantage of the sector gates is that it cannot deal with negative hydraulic heads, i.e. a situation in which the water level in Galveston Bay would be higher than on the gulf side. Since surges on the bay side can change rapidly during a hurricane, it is likely that load cases with these negative heads are likely. Also, for the

2_{Calculation: water head difference over barrier = resulting in a maximum flow velocity} = √ √ . Assumption: the channel is dredged down to MSL-20m, resulting in an inflow of or during a 12h storm surge. Divided by the bay’s surface of this induces a water level rise.

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20 closure of the barge gates the existing hydraulic heads can be used for moving the gate, whereas a mechanical system will be needed for the sector gates.

Figure 8: Navigational channel, closable (left) versus open with bottom protection (right)

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En v iro n m e n t al s e c t io n o f t h e barrie r

The decision whether or not the opening for tidal exchange should be closed completely or partially during critical conditions will have a large effect on the costs. Allowing overflow or underflowing during critical conditions could reduce construction costs significantly, because the horizontal loads on the structure could be minimized (instead of fully absorbing these loads). Further, full closure of the environmental opening induces large horizontal loads that have to be transferred to the soft soils providing a large technical challenge for the foundation design. The horizontal force on the structure is obviously an important driver for the overall costs.

For the barrier inside the environmental section two designs are considered, a caisson design and a floating gate design. Both designs aim on placing key structural and movable elements above the water level to allow inspection and maintenance.

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i. Caisson design

A caisson solution similar to the Eastern Scheldt Barrier in the Netherlands is considered. Two sub-options were considered. The first being a full closure with caisson and vertical lift gates on top of which an abutment is made which limits the amount of overtopping. The top of the gates lies at Mean Sea Level resulting. The other design considered is a partially closed barrier with the same caissons and lifting gates, without an abutment so overtopping over the structure is allowed.

Figure 9: Partially closed (left) vs fully closed (right) environmental barrier with vertical lift gates

Figure 10 Partially closed environemntal barrier with vertical lift gates

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22 For both designs the location of the highest point of the caisson with respect to wave impacts and water levels should be optimized. Whether or not overtopping is allowed depends on the allowable total amount of overtopped water and whether or not the navigational channel is left open or closed with a barrier.

Further, the foundation of the caissons is problematic due to the soft soils present. This may influence the choice for a fully closed barrier with large horizontal loads or partially closed barrier with lower horizontal loads. In both cases a foundation on the stronger sand layers is required, so a deep and expensive foundation up to -40 to -50m (140 ft deep) would be needed.

ii. Floating gate design (location of hinge)

Another more innovative option considered is the so-called hinge gate (also refered to as mailbox gate). We considered a heavy concrete flap gate of 20-30 m length hanging on two yokes. In normal conditions the flap is positioned horizontally. At high tide, however, the floating flap reaches the end of the slotted holes and because the axle stubs are eccentric relative to the pressure point, it creates a moment making the flap to tilt vertically. This effect may be enhanced by including ballast water in the hollow flap.

The yokes are founded on the bearing sand layer via inclined foundation piles. In this way, the soft clay layer with poor bearing capacities is avoided. It is a leaky system as the barrier is the combination of a top and bottom spillway. Advantage of this concept is its ability to be simply adjustable to the bathymetry.

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Figure 11: Floating gate (hinge gate) design

After further analysis it was found that this design is likely less feasible, as:

1) the gate will need to have a high weight (or actually torque) to function resulting in a gate thickness of several meters.

2) The gate has a natural frequency similar to the frequency of wind waves, leading to large movements (actually rotations) during operating, harming its closing function. The design of this innovative concept will therefore need further investigation and adjustments to cope with these issues.

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4. Conclusions and recommendations

Conclusions

In this study opportunities and technical challenges for a storm surge barrier in the Bolivar Roads were investigated. From this investigation the following can be concluded:

1) The optimal alignment of the storm surge barrier is a subject for discussion. The shortest path from the Galveston Island to the Bolivar Peninsula could be an

appropriate location for the storm surge barrier. But other factors play a role as well, such as the subsoil, the depth along the alignment and the impact on environment and shipping.

2) The relatively narrow opening at the Bolivar Roads and the large water surface of the Galveston Bay allow a large water volume to pass a storm surge barrier during critical conditions with limited raise of the water level in the Galveston Bay. As a result the structure of a storm surge reduction barrier can become less if some openings and “leakage” through under- and overflow are allowed. This is an opportunity to make the storm surge less expensive3_{and a barrier solution more feasible.}

3) The proposed barrier will consist of two sections; a navigational section (about 200m wide) and an environmental section (about 2800m wide) for refreshing the Galveston Bay in normal conditions. For these sections the following can be concluded:

o A barge and a sector gate (as the Maeslant barrier) are found to be feasible options for the navigational opening. Another option is to leave it open – without a flood gate but with a bottom protection – as just this opening of approximately 200 m (600 ft) will lead to a limited raise of the water level (< 1 m / 3 ft) in the Galveston Bay. However, it could be challenging to convince stakeholders that a partially open barrier is a robust solution.

o The opening for tidal exchange can be constructed with caissons with a vertical lift gate. Allowing overflowing or underflowing during critical

conditions could significantly reduce costs. Another more innovative concept is the hinge gate, but this needs further consideration.

The concepts that are proposed for these two sections of the barrier are summarized in figure 14.

3_{In general a rule of thumb is that the costs for a storm surge barrier are 1 million euro per m length}

(plus or minus a factor 2). So for the Bolivar Roads could be 3 billion Euros, so about 4 billion US $. Optimization of the design and reduction of costs will therefore be very important.

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Figure 12: Overview of the proposed solution(s) and concepts for the Bolivar Roads barrier.

4) Only limited soil investigation is done in the opening at the Bolivar Roads. Soil

investigations near the Bolivar Roads indicate a (very) soft clay layer at a depth of 10 to 20 meters (30 to 60 feet) below sea level. If this layer is present at the proposed location for the storm surge barrier, this will provide a large technical challenge for the foundation design.

Recommendations

The following recommendations are made: Further conceptual design of the barrier:

 Based on the preferred solution, a further conceptual design of the main elements of the barrier can be made. This could focus on various aspects such as structure, gates, foundation, bottom protection etc. and result in a conceptual design (incl. general dimensions) and a preliminary and very indicative cost estimate.

o When a partially opened storm surge barrier proves to be an interesting option, it could be studied whether it is more cost-effective to open the barrier horizontally (/have a part without a flood gate) or vertically (allow over- and/or underflow) or a combination.

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26 More in general

 Estimate the current flood protection level based on hurricanes of different wind speeds, land fall locations, low-pressure and size. A probabilistic method can be applied to determine hurricane probabilities and a hydraulic (computational) model can be applied to estimate corresponding water levels and flood probabilities. This information will provide the basis to assess the current safety and risk of the system, and the risk reduction achieved by various alternatives.

 Performe a system analysis to determine the influence of a fully closed and partially opened storm surge barrier on the flood protection level. Compare the cost of fully closed and partially opened storm surge barrier with flood protection level in Galveston Bay.

 Also, the optimal level of protection of the Ike Dike system and the barrier need to identified. This can be done based on a system-wide cost benefit analysis, for which the probabilistic hurricane – surge model will provide important inputs.

o Also the reliability of the closing elements of the barrier needs to be addressed further as part of the overall systems reliability concept.

 The issue of the amount of desired or allowed opening is important. A further optimization based on hydraulic models and structural , water quality and environmental considerations can be made.

 Other elements of the Ike Dike system, such as the coastal levees and barriers in other inlet in Galveston Bay need to be (conceptually) designed.

 Investigate other options to increase the flood protection level, such as

compartmenting Galveston Bay to reduce wind set up and other alignments of a flood protection system. Eventually, it is important to compare various alternatives (incl. do-nothing, Ike Dike and other alternatives).

 Since the foundation is a critical aspect for the structure and the costs, a further investigation of the subsoil at the Bolivar Roads is recommended.

5. Bibliography

Committee on Natural Disasters, 1984. Hurricane Alicia Galveston and Houston, Texas, Washington: National Acedemy Press.

Davis, Z., Flores, K., Szempruch, P., and Thomas, J. (2010). Design of the Galveston Bay Storm Surge Protection Barrier. BSc Thesis, Texas A&M University at Galveston.

Dean, R., 2004. Coastal Processes with Engineering Applications.

Gilmore, R. & Englebretson, R., 1997. Hurricane haven evaluation, Monetery: US Naval Research Labaratory.

Jin, C. et al., 2012. Site Specific Wave Parameters for Texas Coastal Bridges: Final Report, College Station: Texas A&M.

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27 Kennedy, A. et al., 2011. Inundation and destruction on the Bolivar Peninsula during hurricane Ike. J. of Waterway Port Coastal and Ocean Engineering, 127(132), pp. 132-141. McClelland Engineers (1985). Reconnaissance Study U.S. Navy Home Port Galveston, Texas. Report to Navel Facilities Engineering Command, Charleston, South Carolina.

Needham, H. F. & Keim, B. D., 2012. A Storm Surge Database for the U.S. Gulf Coast. International Journal of Climatology, 32(14), pp. 2108-2123.

NGDC, 2007. National Geophysical Data Center; Galveston, TX 1/3 arc-second MHW DEM, Galveston: NOAA.

NOAA, 2013a. Tides and Currents. [Online] Available at: http://tidesandcurrents.noaa.gov/

[Accessed 1 1 2013].

Petitt, B. and Winslow, A. G. (1955). Geology and ground water resources of Galveston County, Texas. Texas Board of Water Engineering Bulletin, (5502):219.

Phillips, J., 2004. A sediment budget for Galveston Bay, Kentucky: Department of Geography, University of Kentucky.

Rego, J. & Li, C., 2010. Storm surge propagation in Galveston Bay during Hurricane Ike. Journal of Marine Systems, 82(4), pp. 265-279.

Ruijs, M. (2011). The effects of the “Ike Dike” barriers on Galveston Bay, A 2D numerical modeling study on hydrodynamics and the implications for the water quality and morphology of Galveston Bay, TU Delft.

Shen, J., Wang, H., Sisson, M. & Gong, W., 2006. Storm tide simulation in the Chesapeake Bay using an unstructured grid model. Estuarine, Coastal and SHalf Science, Iss 68, pp.1-16. Stoeten, K. (2012). Applying best practices from the Delta Works and New Orleans to Galveston Bay.

Texas GLO (2013). TxSed Mapping Viewer. http://gisweb.glo.texas.gov/txsed/index.html. Valle-Levinson,A.,Wong,K. & Thompson-Bosley, K., 2002. Response of the lower Chesapeake Bay to forcing from Hurricane Floyd. Cont. Shelf Research, Issue 22, pp. 1715-1729.

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Appendix 1

List of workshop participants

Person Role / expertise Organisation

1 Prof. B. Jonkman Principall investigator TU Delft

2 W. Molenaar Msc Hydraulic structures TU Delft

3 K. Lendering Msc Flood defences TU Delft

4 A. van der Toorn Msc Hydraulic structures TU Delft

5 M. Voorendt Msc Hydraulic structures TU Delft

6 Prof. emer. H. de Ridder Barrier expert TU Delft

7 Dr. K.J. Bakker Geotechnical TU Delft

8 L. Mooyaart Msc Barriers TU Delft/RHDHV

9 Dr. M. van Ledden Hydraulics / barriers TU Delft/RHDHV

10 D.J. Peters Msc Structural engineer RHDHV

11 A. Willems Msc Reliability / barriers Iv-Infra

12 H. Vos Msc Barrier expert Iv-Infra

13 Prof. H. Vrijling Barrier expert TU Delft

14 K. Stoeten Bsc Msc student TU Delft

15 P. de Vries Bsc Msc student TU Delft

16 H. Janssen Msc Rijkswaterstaat Rijkswaterstaat

17 M. van Breukelen Bsc Msc student TU Delft

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Report Galveston Bay: Flood Risk Reduction Barrier