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).

Abstract Long-term continuous monitoring in the Houston-Galveston area indicates that, during the past two decades (1993-2012), the overall land subsidence has been decreasing, while the groundwater head has been increasing. Subsidence in downtown Houston, the area along the Houston Ship Channel, and the coastal area of Galveston has almost ceased. Slight ground surface rebound has been observed at several sites along the Houston Ship Channel. Assuming that the hydraulic head in the aquifer will reach or exceed the preconsolidation level in the near future, will the subsidence in the Houston-Galveston area eventually cease? The key to answering this question is to identify if there is deep-seated subsidence in this area. This study investigated the recent subsidence observed at different depths in the Houston-Galveston area. The subsidence was recorded by using 13 borehole extensometers and 76 GPS antennas. Four of the antennas are mounted on the deep-anchored (549, 591, 661 and 936 m below the land surface) inner pipes of borehole extensometers. We conclude that recent subsidence (1993-2012) in the Houston-Galveston area was dominated by the compaction of sediments within 600 m below the land surface. Depending on the location of specific sites, the compaction occurred within the Chicot and part or all of the Evangeline aquifer. No measurable compaction was observed within the Jasper aquifer or within deeper strata. Recent GPS observations also suggest that there is currently no considerable lateral ground deformation or subsidence associated with deep-rooted faulting activities in the Houston-Galveston area.

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.

Abstract Each year from June through November, tropical cyclones are a common potential problem for those living in coastal communities along the southwest Louisiana and southeast Texas coasts. Developing from small tropical disturbances, tropical cyclone strength is determined by many factors: ocean temperature, upper and lower wind circulation, latitudinal position, etc. Ecological, geological, and economic effects of strong-to-devastating tropical cyclones on coastal areas are typically extreme. Since the 1860s, seven strong or greater tropical cyclones have struck the Louisiana-Texas coast. Their impact has made an indelible impression on the coastline as well as on the communities in the area

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.