Evaluation of Beach Nourishment Sand Resources along the Central
Texas Coast
http://gulf.rice.edu/coastal
August 29, 2003
Report to the Texas General Land Office
Julia Smith Wellner and John Anderson
Department of Earth Science
Rice University
Houston, TX, 77251-1892
713-348-4880
http://rapidfire.sci.gsfc.nasa.gov/realtime
1
Hurricane Claudette approaching Central Texas, July 2003.
2
Introduction
Texas beaches are experiencing rates of erosion that are among the highest in the
continental U.S., averaging 5-6 ft/yr along the east Texas coast (1) and exceeding 10
ft/yr in certain locations. While this is close to the long-term (thousands of years) rate
of coastal retreat, waterfront property owners are convinced that this erosion is
unprecedented and they are demanding action. In the near future the problem will
shift from one that concerns mostly beachfront property owners to one that threatens
the economy of the state, and thus all Texans. The causes of the erosion include rising
sea level, land subsidence, and a paucity of sand sources. Unfortunately, property
owners and governmental agencies have had to turn to engineering solutions whose
efficacy is not yet proven.
The Texas inner continental shelf is comprised dominantly of mud; this is due in
part to the circulation patterns in the Gulf of Mexico, pushing fine-grain sediments to
the Central and East Texas coasts. In Texas, it is not a simple case of pumping sand
back onshore to replenish eroding beaches, as is the case in Florida and Alabama.
Therefore, sand nourishment for Texas is more expensive than in other coastal areas (2).
The underlying geology of coastal systems plays a large role in the development
of those coastal systems (3). The antecedent topography, i.e., the base over which
coastal systems develop, controls what type of features will develop and how thick
3
those features will be. The erodability of the underlying substrate is a factor in how
easily sediment can be obtained for a coastal system and what type of sediment will be
produced. The antecedent topography and character of the substrate together control
barrier island thickness, shoreface profile and thickness, and shoreline erosion rates.
These geologic factors also control the location of offshore sand resources. For this
reason, putting sand resource studies in the context of the geological setting of the area
is critical for developing an ability to locate sand sources. Our sand resource studies
follow on two decades of work on the Texas coast and continental shelf
(http://gulf.rice.edu).
We have undertaken a study to compile lithologic data from the Central Texas
inner continental shelf building on the work we already have completed for East Texas.
These data have been collected over two decades by the Rice University Coastal
Research Program. For the Central Texas study, data from 200 cores have been put on a
web site (http://gulf.rice.edu/coastal) in order to facilitate distribution to any
interested parties. The goal of this work is not to identify specific borrow sites. Rather,
we have developed the geologic framework to allow for future development of
individual resources. This report is meant to serve as an introduction to our web page.
Because the purpose of this project is in large part to disseminate knowledge about the
coastal geology and sand resources, the web site itself is our true final report.
4
Methods
Sedimentary cores were collected from 1985 to 2003 as part of this and other
projects. They were collected by various techniques including vibracoring, pneumatic
hammer coring, and rotary drill coring. The cores are stored in the Rice University
chilled storage facility. Each of the over 200 sedimentary cores from the Central Texas
coastal study area was described and broken into sedimentary units according to grain
size, grain type, and fossil content. Detailed grain size analysis was run on a surface
sample for each core and on each of the other units sampled in the core. The grain size
data was collected using a laser particle size analyzer that is able to measure the entire
range of sediment particle sizes, from clay to sand.
The web site for the Central Texas project was constructed differently than our
first sand resource web page. Many people who used the East Texas data stated that
the ability to import the data into Arc or other GIS based software would add to its
functionality. Therefore, we have partnered with the University of Texas Bureau of
Economic Geology to create an ArcIMS based web page
(http://inet1.beg.utexas.edu/website/ricecore/viewer.htm). The Central Texas data is
hosted on their web site with appropriate links back and forth to the Rice University
coastal pages. While this means that the East and Central Texas datasets cannot be
viewed in the exact same way, we believe that the added value of the GIS-based page is
worth the separation.
5
The database is entered through a map showing the cores (Fig. 1). At this
page, the user may choose a core to examine; a page showing the available information
appears (Fig. 2). From this page, the user can choose to look at either the lithologic log
(Fig. 3), which can be downloaded as a picture file, or the grain size data (Fig. 4), which
can be downloaded as a Microsoft PowerPoint file. Thus, all of the data is available to
any persons who are interested in the sand sources in the region.
Characteristics of the Central Texas Coast
For the purposes of this report the large study area will be broken into two parts,
the north-eastern area around the Brazos River and the south-western area from
Matagorda Peninsula to North Padre Island (Fig. 5). Neither of these areas is populated
as densely as the Galveston region, nor do they suffer from as much erosion.
The area from Matagorda to North Padre Island was studied with a dataset of
105 cores (Fig. 5) and is characterized by prograding clastic shoreline deposits (Fig. 6).
Predominant wind direction is from the southeast, setting up onshore directed waves
which strike the east and south Texas shelves at oblique angles. These asymmetric
waves generate longshore currents that transport fine sands to the central Texas shelf.
The sandy shoreline deposits are stratigraphically above the Pleistocene exposure
6
surface. The depth of the paleo-exposure surface is controlled by the presence or
absence of paleo-river channels. Thus, the thickness of sandy shoreface deposits is
controlled by the location of old river channels (Fig. 7).
The Matagorda-North Padre area shoreline differs from that in the Galveston
region. The east Texas shoreface is currently backstepping with marine sediments
onlapping shoreface sediments. The Central Texas shoreface is prograding slowly (4).
Thus, the shorelines in the two areas are responding differently to rising sea level and to
storm activity. This is due to a low sand supply across east Texas, longshore transport
of sands from east Texas to central Texas, and the steeper gradients across central Texas
versus east Texas.
The northeastern part of the study area (Fig. 5) is dominated by the Brazos delta.
The Brazos River leads all Texas rivers in rates of flow. The average annual suspendedsediment yield of the Brazos is the highest of all rivers in Texas, 39 metric tons per
square kilometer (5). The sediments of the Brazos River have a distinctive red color and
are characterized by fine grain sizes; most of the sediment load is of clay-size. The
Brazos Delta is approximately 35 km2 in area and extends seaward to water depths of
20 meters (Fig. 8; 6). The delta has formed since 1929, when the Brazos River was
diverted by the U.S. Corps of Engineers in an effort to reduce flooding and shoaling at
Freeport. A significant amount of sediment was eroded from the old delta and
deposited at the new delta following river diversion (6).
7
Potential Borrow Sites
The volume of sand needed to protect Texas shorelines from a potential one
meter of sea level rise by 2100 exceeds 917 million cubic yards (7). Significant sand
resources will need to be identified if the coastal communities choose to continue
fighting to preserve the current shorelines. There are only limited near-shore, shallow
sand sites that could potentially be used as borrow sites. Therefore, for ongoing sand
nourishment other sites will need to be considered. There are sand source sites that
could be viable in different economic settings.
The shorelines of the Matagorda-North Padre area are prograding in an overall
sense, but there are still local areas of eroding shorelines. Despite the relative
abundance of sand along the Central Texas shorelines, there are not obvious large
sources of sand that could be used for beach nourishment. The sand that is in the
shoreface sits above the wave base (Fig. 6); thus, removing this sediment would
destabilize the beach around it. Additionally, the sand may be finer than would be
preferred for a nourishment project (Fig. 9)
The Brazos River provides a large quantity of sediment to the Gulf of Mexico and
thus the area around Freeport is not suffering from serious erosion. Most of the deltaic
8
deposits are extremely fine-grained (Fig. 10). Therefore, while they are protecting the
coast from erosion, they are not suitable for nourishment projects in beach areas.
Freeport Rocks Bathymetric High is a shoal seaward of the Brazos Delta (Fig. 11).
This feature is similar in some ways to Sabine and Heald Banks that are offshore East
Texas and have been considered in the past as sand borrow sites, although it is in
deeper water. The cores from this feature do contain large thicknesses of sand (Fig. 12).
However, most of the sand is in mixed layers that have high silt and mud percentages
(Fig. 13).
One very large sand-prone body was identified in this study. This feature is a
delta formed by the Colorado River during the last transgression (Fig. 14; 8). The delta
sits in on an area of the shelf that is dominated by the Texas Mud Blanket. The delta
formed by the Colorado fluvial system flushing sequestered sand out of its system
during the rise in sea level. The delta contains an estimated 10.8 km3 of sand (8). This
huge volume of sandy sediment potentially could serve as a sand-borrow site under
different economic conditions.
Conclusions
9
In this study we have compiled lithologic data from across Central Texas into a
single database. The dataset includes lithologic logs from over 200 core sites and over
400 grain size measurements. This data has been made available to the entire coastal
community on a GIS-based web page. Now that the general geologic framework for the
Central Texas shorelines has been established the data can be used as a first step in
developing individual sand resources.
References
1. Heinz Center Report, 2000, Evaluation of erosion hazards: Federal Emergency
Management Agency, http://www.heinzcenter.org.
2. Titus, J.G., Park, R.A., Leatherman, S.P., Weggel, J.R., Greene, M.S., Mausel, P.W.,
Brown, S., Gaunt, G., Trehan, M., and Yohe, G., 1991, Greenhouse effect and sea level
rise: the cost of holding back the sea: Coastal Management, v. 19, p. 171-204,
http://yosemite.epa.gov/OAR/globalwarming.nsf/content/ResourceCenterPublicatio
nsSLRCost_of_Holding.html.
3. Rodriguez, A.B., Wellner, J.S., and Anderson, J.B., 2002, Geologic controls on coastal
erosion: Fourth Texas Coastal Issues Conference, Corpus Christi, Texas,
http://www.glo.state.tx.us/coastal/cic2002/index.html.
4. Fassell, M.L., 1999, Late Quaternary marine deposits, offshore central Texas:
processes controlling geometry, distribution, and preservation potential: M.A. Thesis,
Rice University, Houston, TX, 177 p.
5. Curtis, W.F., Culbertson, J.K., and Chase, E.B., 1973, Fluvial-sediment discharge to
the Oceans from the Conterminous United States: United States Department of the
Interior, US Geological Survey Circular 670, Reston, VA, 17 p.
6. Rodriguez, A.B., Hamilton, M.D., and Anderson, J.B., 2000, Facies and evolution of
the modern Brazos Delta, Texas: wave versus flood influence: Journal of Sedimentary
Research, v. 70, n. 2, p. 283-295.
10
7. Leatherman, S.P., 1989, National assessment of beach nourishment requirementsassociated with accelerated sea level rise: U.S. EPA Office of Policy, Planning, and
Evaluation,
http://yosemite.epa.gov/OAR/globalwarming.nsf/UniqueKeyLookup/JSAW5HJQM
P/$File/rtc_leatherman_nourishment.pdf.
8. Snow, J.N., 1998, Late Quaternary highstand and transgressive deltas of the ancestral
Colorado River: eustatic and climatic controls on deposition: M.A. Thesis, Rice
University, Houston, TX, 138 p.
11
12
Figure 2. Core introduction page.
13
Figure 3. Sample lithologic log.
14
Figure 4. Sample grain size data.
15
Figure 5. Core datasets in each of the detailed areas of study.
16
17
Figure 6. Fence diagram showing the shoreline deposits of the Central Texas region (4).
18
Figure 7. Reconstruction of fluvial channels on the continental shelf from Matagorda to
North Padre (4).
19
Figure 8. Facies of the modern Brazos Delta.
20
Volume (%)
Particle Size Distribution
20
15
10
5
0
0.01
0.1
1
MAI_T3-21-20cm, 05/08/03 14:33:42
MAP_T1-6-30cm, 05/08/03 16:06:34
MAP_T1-5-20cm, 05/08/03 17:14:52
MAI_T5-31-20cm, 05/08/03 18:26:52
MAI_T5-32-10cm, 05/08/03 18:35:34
MAI_T5-33-20cm, 05/08/03 19:03:52
MUI_T13-91-20cm, 05/13/03 14:15:21
NPI_T15-101-20cm, 05/19/03 15:10:16
NPI_T14-98-30cm, 05/20/03 11:11:18
10
100
1000
3000
Particle Size (µm)
MAP_T1-7-20cm, 05/08/03 14:49:13
MAI_T4-28-30cm, 05/08/03 15:27:57
MAP_T2-13-20cm, 05/08/03 16:58:36
MAP_T2-10-20cm, 05/08/03 17:03:04
MAP_T2-12-20cm, 05/08/03 17:46:51
MAP_T2-14-20cm, 05/08/03 17:42:15
MAI_T5-34-10cm, 05/08/03 18:31:19
MAI_T5-30-10cm, 05/08/03 18:39:57
MAI_T5-30-10cm-b, 05/08/03 18:44:31
MAI_T3-20-20cm, 05/08/03 18:50:02
MAP_T1-1-20cm, 05/13/03 13:43:16
MAI_T3-19-20cm, 05/13/03 14:10:58
MAP_T2-9-30cm, 05/08/03 15:53:49
NPI_T15-103-10cm, 05/19/03 14:19:48
NPI_T15-100-10cm, 05/19/03 16:06:17 NPI_T15-99-30cm, 05/19/03 17:04:48
NPI-T14-97-10cm, 05/20/03 13:31:33
Volume (%)
Particle Size Distribution
20
15
10
5
0
-1.7
-0.2
1.3
2.8
MAI_T3-21-20cm, 05/08/03 14:33:42
MAP_T1-6-30cm, 05/08/03 16:06:34
MAP_T1-5-20cm, 05/08/03 17:14:52
MAI_T5-31-20cm, 05/08/03 18:26:52
MAI_T5-32-10cm, 05/08/03 18:35:34
MAI_T5-33-20cm, 05/08/03 19:03:52
MUI_T13-91-20cm, 05/13/03 14:15:21
NPI_T15-101-20cm, 05/19/03 15:10:16
NPI_T14-98-30cm, 05/20/03 11:11:18
4.3
5.8
7.3
8.8
10.3
Particle Size (Phi units)
MAP_T1-7-20cm, 05/08/03 14:49:13
MAP_T2-13-20cm, 05/08/03 16:58:36
MAP_T2-12-20cm, 05/08/03 17:46:51
MAI_T5-34-10cm, 05/08/03 18:31:19
MAI_T5-30-10cm-b, 05/08/03 18:44:31
MAP_T1-1-20cm, 05/13/03 13:43:16
MAP_T2-9-30cm, 05/08/03 15:53:49
NPI_T15-100-10cm, 05/19/03 16:06:17
NPI-T14-97-10cm, 05/20/03 13:31:33
11.8
13.3
14.8
16.3
MAI_T4-28-30cm, 05/08/03 15:27:57
MAP_T2-10-20cm, 05/08/03 17:03:04
MAP_T2-14-20cm, 05/08/03 17:42:15
MAI_T5-30-10cm, 05/08/03 18:39:57
MAI_T3-20-20cm, 05/08/03 18:50:02
MAI_T3-19-20cm, 05/13/03 14:10:58
NPI_T15-103-10cm, 05/19/03 14:19:48
NPI_T15-99-30cm, 05/19/03 17:04:48
Figure 9. Comparison of shoreface sands from Matagorda-North Padre indicating the
overall fine grain size of the sands. Graphs are in volume percent for both phi and
microns.
21
22
Volume (%)
Particle Size Distribution
20
10
0
0.01
0.1
1
Volume (%)
BD-93-7-25cm, 01/08/03 13:24:04
BD-5AB-12cm, 01/15/03 22:59:05
BD-2AB-30cm, 01/10/03 13:41:15
BDP-10-1/2-18cm, 10/08/02 10:09:42
10
Particle Size (µm)
BD-93-3-26cm, 01/08/03 13:18:32
BD-OY-C6-15cm, 01/15/03 23:30:05
BDL-01-5cm, 07/25/02 17:17:59
BDB-02-27cm, 12/17/02 09:12:26
100
1000
3000
BD-93-1-20cm, 01/08/03 11:53:50
BD-1AB-18cm, 01/08/03 13:45:15
BDBR-02-20cm, 01/08/03 11:17:21
Particle Size Distribution
20
10
0
-1.7
-0.2
1.3
2.8
BD-93-7-25cm, 01/08/03 13:24:04
BD-5AB-12cm, 01/15/03 22:59:05
BD-2AB-30cm, 01/10/03 13:41:15
BDP-10-1/2-18cm, 10/08/02 10:09:42
4.3
5.8
7.3
8.8
10.3
Particle Size (Phi units)
BD-93-3-26cm, 01/08/03 13:18:32
BD-OY-C6-15cm, 01/15/03 23:30:05
BDL-01-5cm, 07/25/02 17:17:59
BDB-02-27cm, 12/17/02 09:12:26
11.8
13.3
14.8
16.3
BD-93-1-20cm, 01/08/03 11:53:50
BD-1AB-18cm, 01/08/03 13:45:15
BDBR-02-20cm, 01/08/03 11:17:21
Figure 10. Comparison of representative samples from Brazos Delta cores showing the
dominance of fine sediments there.
23
Figure 11. Map of the offshore Brazos region showing the location of Freeport Rocks
Bathymetric High.
24
25
Figure 12. Representative lithologic log from Freeport Rocks Bathymetric High.
26
Volume (%)
Particle Size Distribution
20
10
0
0.01
0.1
1
10
Particle Size (µm)
frbh-12-10cm, 08/04/03 15:17:19
frbh-25-20cm, 08/05/03 16:01:18
frbh-28-10cm, 08/05/03 16:33:59
frbh-35-30cm, 08/05/03 15:12:09
Volume (%)
frbh-11-10cm, 08/04/03 15:06:52
frbh-2-10cm, 08/01/03 15:48:54
frbh-27-20cm, 08/05/03 15:23:18
frbh-37-20cm, 08/04/03 16:14:34
100
1000
3000
frbh-13-10cm, 08/01/03 14:40:30
frbh-26-13cm, 08/05/03 15:33:19
frbh-29-10cm, 08/05/03 16:17:16
frbh-16-30cm, 08/01/03 15:12:42
Particle Size Distribution
20
10
0
-1.7
-0.2
1.3
frbh-11-10cm, 08/04/03 15:06:52
frbh-2-10cm, 08/01/03 15:48:54
frbh-27-20cm, 08/05/03 15:23:18
frbh-37-20cm, 08/04/03 16:14:34
2.8
4.3
5.8
7.3
8.8
10.3
Particle Size (Phi units)
frbh-12-10cm, 08/04/03 15:17:19
frbh-25-20cm, 08/05/03 16:01:18
frbh-28-10cm, 08/05/03 16:33:59
frbh-35-30cm, 08/05/03 15:12:09
11.8
13.3
14.8
16.3
frbh-13-10cm, 08/01/03 14:40:30
frbh-26-13cm, 08/05/03 15:33:19
frbh-29-10cm, 08/05/03 16:17:16
frbh-16-30cm, 08/01/03 15:12:42
Figure 13. Comparison of near-surface samples from Freeport Rocks Bathymetric High.
27
Figure 14. Isopach (in msec) of the transgressive sand-prone delta mapped by Snow (8).
28