CHAPTER 3: CIRCULATION MODELS
TABLE OF CONTENTS
3.
CIRCULATION MODELS........................................................................................................................... 3-1
3.1.
WEST GALVESTON BAY CIRCULATION MODELS .................................................................................... 3-1
3.1.1.
2D SALINITY CIRCULATION MODEL ...................................................................................... 3-1
3.1.2.
3.1.3.
CHE CIRCULATION MODEL ................................................................................................ 3-3
ADCIRC MODEL ............................................................................................................ 3-3
3.1.3.1. RESULTS FROM THE ADCIRC MODEL ..................................................................... 3-4
3.1.4.
THE ROLE OF WIND ......................................................................................................... 3-8
LIST OF FIGURES
FIGURE 3.1
FIGURE 3.2
FIGURE 3.3
SALINITY CONTOURS IN WGB FROM THE TXBLEND-2D MODEL.............................................................. 3-2
ADCIRC MODELLING DOMAIN (A) MODEL MESH AND (B) MODEL BATHYMETRY......................................... 3-5
SAN LUIS PASS TIDE STATION WATER LEVELS ...................................................................................... 3-6
FIGURE 3.4
FIGURE 3.5
SAN LUIS PASS ADCIRC MODEL RESULTS .......................................................................................... 3-7
MAXIMUM SPRING FLOOD TIDE FOR WGB AS CALCULATED BY THE ADCIRC MODEL .................................... 3-9
FIGURE 3.6
FIGURE 3.7
FIGURE 3.8
MAXIMUM SPRING EBB TIDE FOR WGB AS CALCULATED BY THE ADCIRC MODEL ..................................... 3-10
WIND ROSE FOR THE WGB........................................................................................................... 3-11
AVERAGE VELOCITIES FOR ONE MONTH IN WGB AS CALCULATED BY THE ADCIRC MODEL. ......................... 3-12
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3.
Circulation Models
3.1.
West Galveston Bay Circulation Models
Despite the almost 70 years of dredging activities and port infrastructure development in the
Galveston Bay region, there have been limited studies on the water circulation in WGB. Only
two water circulation projects developed in recent years were found to be available specifically
for WGB: (1) a basic 2D salinity model developed by the Texas Water Development Board
(Matsumoto et al. 2005); and, (2) an inlet circulation model developed by CHE (2007) for San
Luis Pass.
3.1.1. 2D Salinity Circulation Model
A basic 2D salinity circulation model was developed by Matsumoto et al. (2005) in order to study
the effect of structures on the circulation and salinity in Galveston Bay. The study included water
circulation inputs entering WGB from the main Galveston Bay area and the effect of the Texas
City Dike (TCD) on water exchange and circulation in adjacent bays. The study applied the
TxBLEND-2D model, which was used to simulate the circulation and salinity patterns in
Galveston Bay over an 8-year period from 1989 to 1996. The TxBLEND model is currently used
on a daily basis to forecast flows and the data is fed into an oil-spill model that is used for oil
spill responses. The model can examine salinity conditions under selected monthly inflows,
which helps monitor the health of the ecological system in surrounding estuaries.
The study identified the residual water velocity vectors to determine net movement of bay water.
According to the report, net flows through bay sections and passes were computed to find major
pathways of freshwater through the Galveston Bay system. The report cannot provide exact
estimates of salinity and flows associated with water circulation, but it can provide fair estimates
of the system’s response to different salinity scenarios. For example, one of these scenarios
was to examine the effect of the TCD on the salinity and circulation patterns in the WGB
system. Although the model has limitations, it can be used to see the general circulation flows
occurring in WGB (Junji Matsumoto, personal communication, 2011).
Matsumoto et al. (2005) found that the total annual freshwater inflow to Galveston Bay averaged
17.0 million acre-feet (approximately 5.54 trillion gallons) over the 8-year study period, of which
5 percent flows through Rollover Pass, 20 percent through San Luis Pass, and 75 percent
through the entrance channel to the Gulf (Bolivar Roads). Approximately 9 percent of
freshwater flows go into West Bay through the TCD section (mainly the Texas City Channel)
and 8 percent through the Galveston Harbor Channel, making up 17 percent of the flow that
goes through WGB from Bolivar Roads.
An important conclusion is the scenario resulting from removing the TCD and its effects on
circulation. The dike has been blamed for historical changes in circulation and shoreline retreat
in WGB. The report shows that, contrary to expectations, flows in WGB would not increase
noticeably if the Texas City Dike were removed. However, the composition of salinity values
would change. In the dike removal scenario, net flow through the Bolivar Roads entrance
channel will decrease by 1 percent and net flow through West Bay, San Luis Pass, and Rollover
Pass will increase only by 0.6 percent.
Hypothetically, if the dike were removed, the extent of the WGB flows would change from the
existing condition of 8 percent flowing through the Galveston Harbor Channel and 9 percent
through the TCD section to 7 percent through the Galveston Harbor Channel and 11 percent
into Galveston Bay. Removal would also have an effect on salinity in both wet and dry periods
with the major effect occurring in and near the Texas City Ship Channel and in WGB, where
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salinity would decrease by 4 parts per thousand (ppt) in the turning basin and by 2 ppt in midWGB. Salinity in Galveston Bay and Trinity Bay may also be decreased by 0.6 ppt. In summary,
according to the model, the impacts of the dike on the general water circulation and salinities of
WGB are limited.
The importance of this 2-D model for WGB is that it also measures the effects of the average
net water flow from Bolivar Roads and San Luis Pass into WGB. According to the model, the net
water flows coming from both inlets intersect and mix at the middle of WGB during high tides
and return separately to the inlets during ebb tides. The net flood tide flow coming from Bolivar
Roads under normal conditions does not reach San Luis Pass and the net flood tide flow
coming from San Luis Pass does not reach the Bolivar Roads area during average flows. An
area of intersection occurs in the middle of WGB next to Carancahua oyster reef just north of
Jamaica Beach and may move a short distance east or west, depending on the wind and other
conditions. Figure 3.1 shows the salinity values in ppt measured for October 19, 2004, during
model runs for that specific area.
Salinity Contours in ppt.
Flow Intersection
Area
Bolivar Roads
Galveston Island
San Luis Pass
Figure 3.1
Salinity Contours in WGB from the TxBLEND-2D Model
Notice the mixing zone in the middle of WGB measured on October 19, 2005. The zone is an intersection of the
flows coming from San Luis Pass on the west and Bolivar Roads on the east. Source: Matsumoto et al. 2005.
The salinity data shows that the mixing area tends to have higher salinities, at least at the
beginning of the mixing periods near Carancahua Reef. The data suggests that, in the middle of
WGB, both inlets have important roles in water circulation and provide net water flows over the
entire WGB area. According to the model, Chocolate Bayou appears to receive direct flows from
San Luis Pass but has lower salinity values as the tidal flows are mixed with fresh water from
the bayou. The bays and lakes located west of WGB (Christmas, Bastrop, and Drum bays and
Oyster Lake) receive minimum influence from the inlets, and salinity values appear to be
controlled by evaporation, rain, or water flows coming from Big Slough or Oyster Creek.
The results of this TxBLEND-2D model also suggest that WGB tends to have two major salinity
circulation cells dictated by the flow coming from both inlets and that human infrastructure such
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as the GIWW, does not have significant impacts on circulation. The GIWW may locally increase
or decrease water velocities, but, overall, the model suggests that salinities are dictated by the
mixing of the flows.
3.1.2. CHE Circulation Model
A local water circulation model was developed by CHE (2007) for San Luis Pass and a portion
of WGB. CHE used different models, including the 2-D ADCIRC and the SWAN model, which
transforms waves from offshore to San Luis Pass and provides input to wave-generated current
modelling through the CHE’s proprietary sediment transport pathway model, MORPHO.
MORPHO is used in conjunction with the lagrangian sediment transport model called
LAGRSED. CHE also used other hydrodynamic models to analyze the dynamics and quantify
sediment input for San Luis Pass. All the models were calibrated with field data.
These models evaluated the water circulation and sediment transport occurring at San Luis
Pass. The study called “San Luis Pass – Phase 2 Inlet Management Study and “Treasure Island
Shoreline Stabilization Study” estimated sediment dynamics in the pass to determine if
sediments could be used for beneficial use of dredge material (BUDM). The data collection and
analysis were an integral part of the assessment of the sediment sources and erosion control
methods in the restoration of the Texas General Land Office (GLO) and Texas Parks and
Wildlife Department (TPWD) West Bay Bird Island project. Using the results of the models,
sediments from the San Luis Pass flood delta area were dredged for the restoration of the
Island. The models helped propose a dredging or borrow site that would recover naturally
according to sediment transport and circulation. The borrow site was identified on the borders of
a flood tide channel, which recovered to previous conditions a few months after dredging.
Knowing the success of these modelling efforts and the hydrodynamic data already available for
the area, Atkins subcontracted CHE to extend the hydrodynamic modelling to the rest of WGB,
here in referred as CHE (2011b). The objective was to run the ADCIRC model for WGB and
develop a validated water circulation flow model. The model was also needed to determine how
the flow works in conjunction with the geologic evolution model within the boundaries of the
sedimentary basins and also, if possible, to determine the potential connection between
shoreline retreat processes and other environmental changes observed in the system. Special
emphasis was focused on the role of the water circulation coming from the inlets to test the
results of the 2D salinity model developed by Matsumoto et al. (2005).
3.1.3. ADCIRC Model
The ADCIRC model is a 2D numerical model traditionally used for large-scale, region-sized
projects. ADCIRC simulates depth-averaged current velocity vectors and water level fluctuations
forced by tidal harmonics at the boundaries of the modelling domain and wind inputs over the
entire modelling domain. The ADCIRC modelling procedure includes construction of the finite
element modelling domain, development of the boundary and input parameters, model
calibration, model verification, simulation of modelling scenarios, and analysis of the output
parameters. In general, the ADCIRC model for the entire WGB was run on maximum spring tide
flood flow for a period of 1 month.
ADCIRC requires the construction of a modelling grid for the development of the input
conditions, model calibration and model verification. Figure 3.2 shows the modelling grid and
bathymetry of the modelling domain for both San Luis Pass and Bolivar Roads, which are the
main sources of tidal flows to WGB. The grid was extended offshore in the Gulf to provide input
to the circulation processes along Galveston Island.
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The NOAA tide station at San Luis Pass is located next to the inlet connecting WGB on the bay
side of Follets Islands. The values for the San Luis Pass tide station for the last 2 years are
presented on Figure 3.3. Average tide values range in the order of ±1 feet for the San Luis Inlet
(+1 feet in high tide to –1 feet in low tide); however, wind induces stronger tidal changes by
creating temporarily elevated or lower tide values (USACE Staff 2011), mainly on the northwest
side of WGB.
3.1.3.1. Results from the ADCIRC Model
This section describes the tidal induced current flow patterns coming from the inlet conditions as
determined by the ADCIRC model (CHE 2011b). The flow patterns for this model do not include
any wave-induced hydrodynamics and only the tidal inputs coming from Bolivar Roads and San
Luis Pass. The tidal-induced currents selected for this study were for maximum spring flood
flows and maximum spring ebb flows. These periods of maximum flows provided a model that
illustrates how the water circulates in WGB under maximum conditions.
For the San Luis Pass Inlet, Figure 3.4 shows that maximum velocities can be 1.0 to 1.3 m/sec
along the deepest channel of the Pass during the highest velocities. The highest velocities are
concentrated in the main tidal channels, although velocities of more than 0.5 m/sec occur on
and around the channels next to the flood delta shoals, which have enough energy to transport
sediments along the shoals. Historic aerial photos show that the channels with higher flows
have resulted in meandering channels and shallow sediment shoals. These higher velocity flows
seem to be concentrated along or near the shorelines of Mud Island, West Bay Bird Island, and
several of the shoals in the flood delta, including along the bay side of the southern tip of
Galveston Island. In addition, these high velocity flow channels indicate that the abovementioned areas have a high potential for erosion and shoreline retreat (Figure 3.4).
The flow in San Luis Pass moves through a main channel that is from -22 feet deep at the
entrance of the inlet to -1- to -2-foot-deep contour in several areas of the flood delta shoals.
Larger areas of higher velocities appear to occur more on the flood tide than the ebb tide,
suggesting that sediment transport may be biased in the bay direction. During the maximum
tide, current velocities are reduced quickly at the distal area of the flood delta where the
sediment bars end and the WGB bottom begins and the current velocities decrease from 0.5
m/sec to an average of 0.2 m/sec or less.
At Bolivar Roads, tidal current flow circulation is more complex since it includes tides for the
entire Galveston Bay system. Figures 3.5 and 3.6 show the maximum tidal current velocities
during the flood and ebb periods; which are as much as 0.8 m/sec and are in a wider area than
the San Luis Pass Channel. According to the model results for WGB, currents with the highest
velocities are coming from the Bolivar Roads area and concentrate along the Houston Ship
Channel between the two jetties. The currents are separated into two areas as they enter WGB
from the east side. One of these areas includes the Galveston Harbor Channel at the gap
between the Texas City Dike and Pelican Island. High velocities (0.5 m/sec) are observed at the
Port of Galveston Harbor Channel and then up to 0.7 m/sec between North Deer and Tiki
Islands.
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Figure 3.2
ADCIRC Modelling Domain (a) Model Mesh and (b) Model Bathymetry
Source: CHE 2011b.
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Figure 3.3
San Luis Pass Tide Station Water Levels
Water elevation data observed at the San Luis Pass Tide Station from May 2009 to May 2011. Peaks in the data
represent elevated tide values influenced by wind. Source: TCOON 2011.
Lesser maximum velocities of up to 0.4 m/sec are observed between Pelican Island and the
Texas City Dike. The majority of the flow moves through the Galveston Harbor Channel during
(maximum) flood flows. During ebb flow, flow patterns are reversed with flows up to 0.6 m/s in
the Galveston Harbor Channel to up to 0.3 m/sec between North Dear and Tiki Islands. The ebb
and flood tide flows make the Galveston Harbor Channel the most active flow channel in the
WGB East Basin.
Circulation between Pelican and North Deer Islands is controlled by the shallow depths of the
bay, with maximum velocities of up to 0.3 m/sec. Medium maximum velocities are observed
flowing in between North Deer Island and Galveston Island with maximum values of up to 0.4
m/sec. The model suggests that higher velocities in the area are along the GIWW between
North Deer and Tiki Islands, with values reaching up to 0.7 m/sec during both ebb and flood
flows.
The results of the model show that velocity magnitudes from both inlets reduce rapidly upon
entering WGB. Generally the open waters of WGB the maximum potential flow velocities do not
exceed 0.18 m/sec anywhere at any time. This means that the velocities associated with tidal
flows in the middle of WGB are very low. In addition, the results show that during flood flow, the
velocities coming from San Luis Pass and Bolivar Roads tend to converge near the Carancahua
reef area (Figure 3.5). The convergence zone spans the width of the entire WGB from north to
south. Tidal flood flows coming from San Luis Pass move to the northeast whereas, flows from
Bolivar Roads and the Galveston Harbor Channel flow southwest, with flows from both inlets
following the morphology of WGB towards the convergence point. Conversely, on ebb tides
(Figure 3.6), the flow tends to diverge from the same location with the flows in opposite
directions to flood tides. This convergence-divergence zone is supported by the results of
Matsumoto et al (2005) using the 2D TxBLEND salinity model. The two circulation models
concur that WGB appears to be hydraulically divided into two circulation cells. The cells almost
mimic the morphologic basins identified in Chapter 1 in this report. The WGB West and East
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Basins are independently controlled from the hydrodynamic point of view by the tide flows
coming from their closest inlets.
a
b
Figure 3.4
San Luis Pass ADCIRC Model Results
San Luis Pass ADCIRC model results for (a) max spring flood and (b) max spring ebb tides. Source: CHE 2007.
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3.1.4. The Role of Wind
The wind rose prepared by CHE (2011b) for the WGB (Figure 3.7) shows wind directions in the
WGB area. Winds up to 30 miles/hour are common during different periods of the year with
speeds between 10 to 20 miles/hour being the most common. Winds from the southeast, southsoutheast and east-southeast are the predominant winds on the Texas upper coast. About half
of the year the predominant winds range from the south to the east. Winds from the north to the
east-northeast are also very important in the winter. The combination of strong winds, coming
mainly from the easterly direction, has been interpreted as a source or shoreline retreat/erosion
in the bay system (Gibeaut et al. 2003a).
In order to characterize the details in the circulation cells and the influence of the wind, the tidal
water velocity vectors obtained from the ADCIRC model were averaged over a one month long
period for an even number of tidal cycles without wind effects. The resulting average velocity is
shown on Figure 3.8. This figure illustrates that the average tidal water velocities have generally
small magnitudes in the center of WGB; indicating that tidal velocities are essentially almost
zero throughout the center of the bay (excluding the areas near the inlets); and, that the
predominant winds (see Figure 3.7) are the main forces that control the movement of the tidal
currents and wind generated waves (fetch effect) over WGB.
In order to identify, in more detail, the flow circulation cells in the previously identified basins, the
model was run to represent the maximum possible flood and ebb flows and the role of the wind
in these flows. The results of the ADCIRC model show that the effect of the wind differs
between seasons. In general, under normal conditions, as the flow moves away from the San
Luis Pass or the Galveston Harbor Channel, flow is reduced in velocity and the wind energy
takes over. During the summer, wind creates waves and currents that break against the
northern shorelines of the bay; in the winter, it creates currents and waves that break against
Galveston Island.
The results of the ADCIRC model also suggest that during the summer, the effect of the wind in
the WGB West Basin appears slightly more important than in the WGB East Basin. This may be
due to its larger size and greater depths and fetch. During the winter, the wind effect appears to
be almost the same for both basins.
In conclusion, based on the information observed in the model, two circulation cells appear to
control the water circulation in WGB. The area near Carancahua Reef, in the middle of the bay,
appears to be the mixing zone for water flowing from the East and West Basins. WGB is a winddominated system, where wind direction and speeds appear to play a more important role in
water circulation in WGB. It is important to notice that for the San Luis Pass inlet, larger areas of
higher velocities appear to occur more during flood tides than ebb tides, suggesting that
sediment transport may be greater in the bay direction than toward the Gulf.
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CARANCAHUA
REEF
Figure 3.5
Maximum Spring Flood Tide for WGB as Calculated by the ADCIRC Model
Source: CHE 2011b.
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CARANCAHUA
REEF
Figure 3.6
Maximum Spring Ebb Tide for WGB as Calculated by the ADCIRC Model
Source: CHE 2011b.
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Figure 3.7
Wind Rose for the WGB
The graph represents wind data collected from the Galveston Pleasure Pier between 1997 and 2009. Source: CHE
2007.
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Wind Rose
Figure 3.8
Average Velocities for One Month in WGB as Calculated by the ADCIRC Model.
The values show almost no net flow, which means that the wind is the main controlling factor for water circulation in WGB. Source: CHE 2011b.
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