Abstract A 3D ground penetrating radar (GPR) survey, using three different frequency antennae, was undertaken to image buried steel culverts at the University of Houston’s La Marque Geophysical Observatory 30 miles south of Houston, Texas. The four culverts, under study, support a road crossing one of the area’s bayous. A 32 m by 4.5 m survey grid was designed on the road above the culverts and data were collected with 100 MHz, 250 MHz, and 1 GHz antennae. We used an orthogonal acquisition geometry for the three surveys. Inline sampling was from 1.0 cm to 10 cm (from 1 GHz to 100 MHz antenna) with inline and crossline spacings ranging from 0.2 m to 0.5 m. We used an initial velocity of 0.1 m/ns (from previous CMP work at the site) for the display purposes. The main objective of the study was to analyze the effect of different frequency antennae on the resultant GPR images. We are also interested in the accuracy and resolution of the various images, in addition to developing an optimal processing flow. The data were initially processed with standard steps that included gain enhancement, dewow and temporal-filtering, background suppression, and 2D migration. Various radar velocities were tried in the 2D migration and ultimately 0.12 m/ns was used. The data are complicated by multipathing from the surface and between culverts (from modeling). Some of this is ameliorated via deconvolution. The top of each of the four culverts was evident in the GPR images acquired with the 250 MHz and 100 MHz antennas. For 1 GHz, the top of the culvert was not clear due to the signal’s attenuation. The 250 MHz shielded antenna provides a vertical resolution of about 0.1 m and is the choice to image the culverts. The 100 MHz antenna provided an increment in depth of penetration, but at the expense of a substantially diminished resolution (0.25 m).

Galveston Island and Bolivar Peninsula have experienced a well-documented history of shoreline and bay shoreline change ranging from +3.63 m/yr to −1.95 m/yr. By integrating core, Light detection and ranging (LIDAR), and coastal change data, we develop a sand budget that attempts to quantify long-term sand sources (e.g., fluvial sand cannibalization through transgression) and sinks (washover fans, offshore sand bodies, and flood-tidal deltas). These results are then considered in light of anthropogenic influences (e.g., beach-nourishment projects, coastal engineering structures, and dredging operations) in an attempt to relate natural versus human influence on coastal change. Our findings suggest that hurricane washover (Galveston Island = 0.4 m/100 yr; Bolivar Peninsula varies from 0.154 m/100 yr to 0.095 m/100 yr) and offshore sand deposits are minimal long-term sand sinks. Flood-tidal deltas, however, appear to be major locations for natural sand sequestration. We also conclude that damming of rivers has had minimal impact on the upper Texas coast, although hard structures, such as the Galveston seawall and its groins, have exacerbated erosion along a shoreline that is currently sand starved. The impact of hard structures has mainly been one of trapping sand in locations where that sand would not have naturally accreted. Sand supply is minimal, so understanding and developing a better sand budget for the Texas coast are vital to sustainability.

Seismic data and sediment cores from the Galveston estuary complex were used to reconstruct the evolution of the estuary during the Holocene. These data show that the estuary complex has had a history of rapid and dramatic change in response to (1) sea-level rise across the irregular topography of the ancestral Trinity River valley and (2) changes in climate, which regulated sediment supply to the estuary. In general, the valley morphology consists of a deep incision near the center and broad, terraced flanks. As sea level rose during the Holocene and flooded the valley, the shape of the estuary changed from narrow and deep to wide and rounded. As sea level rose to the elevation of the relatively flat fluvial terraces, these areas were flooded rapidly, resulting in expansions in bay area and dramatic reorganization of bay environments. The most notable changes were up-valley shifts in the bayhead delta of tens of kilometers in a few centuries. Radiocarbon ages indicate that these events took place ca. 9600, ca. 8500, and between ca. 7700 and 7400 yr B.P. The early flooding events occurred when sea level was rising rapidly (average 4.2 mm/yr; Milliken et al., this volume, Chapter 1), perhaps episodically. The ca. 8200 yr B.P. flooding surface corresponds to a prominent terrace at −14 m. The ca. 7700–7400 yr B.P. flooding episode was the most dramatic in terms of its impact on the estuary setting. Following this event, the area of the estuary increased by ~30%. This flooding event occurred as the rate of sea-level rise was starting to decrease. The level of this flooding surface also corresponds to a terrace at ~–10 m, but the magnitude of flooding is too large to be explained entirely by flooding of this terrace. At the same time, Matagorda Bay to the west and Sabine Lake to the east experienced similar dramatic flooding events. This event occurred when the climate of east-central Texas was in transition from cool and moist to warm and dry, and the vegetation cover of the region was undergoing a reduction in forest and an increase in grasslands. Hence, the ca. 7700–7400 event was likely amplified by a reduction in sediment supply to the estuary triggered by this regional climatic change coupled with an increase in sediment accommodation space caused by flooding of a terrace. Following the ca. 7700–7400 yr B.P. flooding event, the estuary setting changed relatively little as the rate of sea-level rise decreased to less than 2.0 mm/yr. By ca. 2600 cal yr B.P., the modern Trinity bayhead delta had begun to form. Circa 1600 cal yr B.P., the delta experienced a phase of rapid growth. This more recent episode of delta growth may have resulted from an increase in the rate of sediment supply, perhaps associated with human occupation and agriculture in the drainage basin. More recent anthropogenic changes include accelerated subsidence due to groundwater and hydrocarbon extraction. These changes are occurring at rates that approach those that occurred during prior flooding events of the Holocene. Thus, the Galveston estuary complex could be on the verge of another flooding event that would mainly impact the Trinity bayhead delta and low-lying areas around the delta.

Abstract A 2,700-km high-resolution seismic-reflection data set, acquired in recent years, has helped resolve some old problems concerning the age of Quaternary formations along the east Texas coast, and has resulted in mapping of the Trinity/Sabine incised valley. The seismic data were used in conjunction with oil company platform borings and recently acquired sediment cores to examine the stratigraphy of the incised-valley fill and to map the distribution of sand bodies on the shelf. The Trinity/Sabine valley has experienced at least two episodes of incision and infilling. The earliest incision occurred during δ 18 0 substage 5d and the latest reincision occurred during δ 18 0 stage 2. The late Wisconsinan-Holocene transgressive deposits that now fill the valley include the following facies (from bottom to top): fluvial; upper estuary/bayhead delta; middle estuary; lower estuary/tidal inlet/tidal delta; and offshore marine deposits. Backstepping parasequences, indicating an episodic rise in sea level, characterize the valley fill. Sandbody formation and preservation on the shelf also has been influenced strongly by the episodic nature of the late Wisconsinan-Holocene sea-level rise. Sabine Bank, the largest of the sand bodies and the only one studied in detail, is a reworked coastal lithosome that rests on the ravinement surface. Inner-shelf muds contain few discrete storm beds. Relatively thick (<75 cm) amalgamated storm deposits are restricted to sand banks and the incised valley. The modern Sabine Lake and Galveston Bay estuaries formed initially by flooding of the Sabine and Trinity valleys approximately 8 ka. The subsequent flooding event, which inundated the broad, shallow meander portions of the valleys, occurred approximately 4 ka and appears to have been rapid. Extant coastal systems of the study area incorporate a wide range of environments, including barriers, strandplains, chenier plains, and tidal inlets. The systems formed predominantly during the stillstand of the past 3,500 years. Galveston Island and Bolivar Peninsula were derived from offshore sand sources. Progradation of the coastal barriers ceased with the exhaustion of the sand supply.

Abstract Ground failures, ranging from long tension cracks or fissures to surface faults, are caused by man-induced water-level declines in more than 14 areas in the contiguous United States. These failures are associated with land subsidence caused by compaction of underlying unconsolidated sediment. Fissures, which range in length from dekameters to kilometers, typically open only a few centimeters by displacement but are eroded by surface runoff into gullies 1 to 2 m wide and 2 to 3 m deep. Surface faults commonly attain scarp heights of 0.5 m and lengths of 1 km; the highest and longest scarps are 1 m and 16.7 km, respectively. Scarps grow by aseismic creep at rates approximately ranging from 4 to 60 mm/yr; modern fault movement is high angle and normal. Fault movement commonly correlates with seasonal water-level fluctuations, and examples of seasonal water-level recoveries halting fault movement have been reported. The greatest economic impact from ground failure is in the Houston-Galveston, Texas, metropolitan region where more than 86 surface faults have caused millions of dollars of damage and losses of property value. Most ground failures probably are caused by localized differential compaction, although this mechanism has not been demonstrated everywhere. Earth fissures formed by this mechanism are caused by stretching related to bending of the overburden that overlies the differentially compacting zone. Surface faults form when differential compaction is discrete across preexisting faults. Fissures that form complex polygonal patterns probably are caused by tension induced by capillary stresses in the zone above a declining water table. Ground failures can be predicted either by determining potential areas of differential compaction or by monitoring surface deformation in areas of ongoing water-level decline. Potential ground-failure sites can be resolved by either technique to within a few dekameters.