Unforeseen geomorphic consequences of modifying coastal river systems

Capital works projects, particularly the modification of coastal rivers, are becoming increasingly significant to economic activities worldwide as a response to climate-driven changes and urbanization. The economic benefits of channel modification projects can be realized quickly, but at decadal timescales the altered movement of sediments in the river channel can lead to harmful and costly morphologic changes that were not initially considered. An example of this is the closure of the San Bernard River mouth, located on the central coast of Texas, which was clogged by sediments in the 1990s as a result of two major capital works projects in the area: the diversion of the Brazos River channel (1929) and the construction of the Gulf Intracoastal Waterway (GIWW) (1940s). The objective of this study was to document the delayed geomorphic responses to the projects and provide a snapshot of the flow of water and sediment between engineered channels using measurements collected in situ. Results showed that the San Bernard River played only a peripheral role in the evolution of its river mouth. During low discharge conditions the GIWW was revealed to be the main conduit for river flow with speeds up to 38 cm/s, as the barge canal bisects the San Bernard 2 km inland of its river mouth. As a result, flow speeds and discharges in the terminal limb of the San Bernard were relatively small leading to accumulation of sediments and a reduction of the erosive ability of the river at its mouth. As a result of reduced flow, the river mouth became clogged with beach sediment transported along shore from the nearby Brazos River delta which had been diverted from its natural pathway to within 6 km of the San Bernard.  Since the San Bernard River is unable to maintain its own mouth, consistent dredging has been required to connect the river to the sea, incurring costs of over $10 million to this point, with more maintenance projects funded into the future.  To optimize the cost-benefit framework of channel modification projects, the long-term impact of hydrodynamic changes to sediment-transport fields must be considered as managers continue to adapt to ever-changing coastal zones.