Evaluation Of Beach Nourishment Sand Resources Along The Central Texas Coast

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 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

3 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.

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

4 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.

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.

5 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 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.

6 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 suspended-sediment 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).

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

7 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 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.

8 Conclusions

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/ResourceCenterPublicationsSLRCost_ 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.

9 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.

7. Leatherman, S.P., 1989, National assessment of beach nourishment requirements- associated with accelerated sea level rise: U.S. EPA Office of Policy, Planning, and Evaluation, http://yosemite.epa.gov/OAR/globalwarming.nsf/UniqueKeyLookup/JSAW5HJQMP/ $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.

10 11 Figure 2. Core introduction page.

12 Figure 3. Sample lithologic log.

13 Figure 4. Sample grain size data.

14 Figure 5. Core datasets in each of the detailed areas of study.

15 Figure 6. Fence diagram showing the shoreline deposits of the Central Texas region (4).

16 Figure 7. Reconstruction of fluvial channels on the continental shelf from Matagorda to North Padre (4).

17 Figure 8. Facies of the modern Brazos Delta.

18 20 15 10 5 0 0.01 0.1 1 10 100 1000 3000 Particle Size (µm)

NPI-T14-97-10cm, 05/20/03 13:31:33

20 15 10 5 0 -1.7 -0.2 1.3 2.8 4.3 5.8 7.3 8.8 10.3 11.8 13.3 14.8 16.3 Particle Size (Phi units)

NPI-T14-97-10cm, 05/20/03 13:31:33

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.

19 20 10 0 0.01 0.1 1 10 100 1000 3000 Particle Size (µm)

BDB-02-27cm, 12/17/02 09:12:26

20 10 0 -1.7 -0.2 1.3 2.8 4.3 5.8 7.3 8.8 10.3 11.8 13.3 14.8 16.3 Particle Size (Phi units)

BDB-02-27cm, 12/17/02 09:12:26

Figure 10. Comparison of representative samples from Brazos Delta cores showing the dominance of fine sediments there.

20 Figure 11. Map of the offshore Brazos region showing the location of Freeport Rocks Bathymetric High.

21 22 Figure 12. Representative lithologic log from Freeport Rocks Bathymetric High.

23 20 10

0 0.01 0.1 1 10 100 1000 3000 Particle Size (µm)

frbh-16-30cm, 08/01/03 15:12:42

0 -1.7 -0.2 1.3 2.8 4.3 5.8 7.3 8.8 10.3 11.8 13.3 14.8 16.3 Particle Size (Phi units)

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Figure 13. Comparison of near-surface samples from Freeport Rocks Bathymetric High.

24 Figure 14. Isopach (in msec) of the transgressive sand-prone delta mapped by Snow (8).