The Formation and Future of the Upper Texas Coast: A Geologist Answers Questions about Sand, Storms, and Living by the Sea

5. Coping with Coastal Change

5 Coping with Coastal Change Can beach erosion be stopped—and should we try? Eroding Beaches or Retreating Shorelines? Before we get started in our discussion, we need to set the record straight about “beach erosion.” To a coastal geologist, the term erosion implies that sediment is being lost from the system. A geologist thinks about where the sediment is going and whether it is truly being removed from the coastal system or simply redistributed. Mountains erode, and much of the sediment they shed ultimately ends up in sedimentary basins. The fact is, our beaches are not eroding at a rate of several feet per year beThe road that goes nowhere. C O P I N G W I T H C O A S T A L C H A N G E 69 cause sand is not being lost from the system anywhere near that rapidly. The shoreline is simply migrating landward. Sand that is moved offshore during storms is, for the most part, not moving seaward of the shoreface. Hence, that sand remains within the longshore transport system and ultimately finds it way back onshore. Even the sand that is being stored in back-barrier wash-over fans and tidal deltas will eventually be reincorporated back into the longshore transport system as these features are overtaken by the advancing shoreline. In fact, the rate at which sand is redistributed is fast enough that it happens in our lifetimes. For example, think about the changes that occurred off Surfside Beach when the Brazos River was diverted. Within a few decades sand from the old delta was deposited in the new delta (chapter 3). If we remove sand from an area to nourish a beach, we need to be certain that we are not taking sand that will within decades be needed to maintain the coastal profile by natural forces. Otherwise, we are creating problems that our children and grandchildren will inherit. Wherebeacheshavebeenallowedtomigratelandwardundertheforces of nature, they have remained fairly much the same in terms of their width and profile. The Galveston State Park is a good example. Only where man has interfered with the natural migration of the shoreline have beaches significantly shrunk, such as along the Galveston Seawall. In this book I use the terms landward migration of the shoreline or coastal retreat, unless significant amounts of sediment are being removed from the system; in that case the term erosion is appropriate. Surfside Beach, where the sand supply of the Brazos River was diverted, is an example of an eroding beach. The beaches east of Rollover Pass are another example of eroding beaches. Current rates of shoreline retreat along the upper Texas coast average between 3 and 5 feet per year. There are a few shorelines that are not changing significantly and other areas where they are changing much faster. The Bureau of Economic Geology (BEG) at the University of Texas is the agency responsible for monitoring rates of shoreline change in Texas (http://www.beg.uTexas.edu/coastal/intro.htm). In the past, scientists there used aerial photographs taken at different times over the past several decades to estimate rates of shoreline movement. Now they use a sophisticated method known as LIDAR (Airborne Light Detection and Ranging), which uses an airborne laser to measure very subtle changes in coastal elevation and beach location. The method is accurate to within inches, and surveys can be repeated at regular intervals. 70 C H A P T E R 5 Figure 5.2 shows the most recent estimates of shoreline change along the upper Texas coast between High Island and Cedar Lakes. Two areas are experiencing seaward growth, the areas on either side of the Bolivar Roads jetties and the Brazos River delta. Humans have altered both. The Bolivar Roads jetties trap sand that moves within the longshore transport system. The U.S. Army Corps of Engineers’ 1929 alteration of the course of the Brazos River shifted the sand supply of the river from the Surfside areas to a new location 6 miles to the west (fig. 3.4). Three areas of the coast are experiencing rates of landward retreat in excess of 5 feet per year. One of these is located just west of Rollover Pass at the east end of Bolivar Peninsula. The opening of the pass trapped sand that was being transported west by longshore currents. This in turn resulted in increased rates of erosion...