Dam Influenced Channel Incision: The Lower Trinity River in Texas, USA

Wednesday, 17 December 2014
Virginia B Smith, University of Texas at Austin, Austin, TX, United States and David C Mohrig, Univ of Texas at Austin, Austin, TX, United States
Reservoirs behind dams act as deposition sites for much of the bed-material load being transported by rivers. As a result, the water exiting dams is relatively free of sediment and the river flow is well below the transport capacity for bed-material. Because of this, rivers flowing downstream from dams tend to erode into their beds. This occurrence is well documented in gravel-bed rivers, but has not been as completely studied in sand-bed channels, such as the lower Trinity River, Texas. Sediment mining from the bed of a gravel river acts to coarsen the surface layer until the armoring shuts off any further bed erosion. This armoring control on the sediment discharge is not effective in a sand bed river. The abundant supply of sediment in a sand bed alluvial river results in a unique response: the river bed is scoured until sediment transport capacity is reached. In the lower Trinity River the consequences of this scouring and bed-sediment mining are channel bed lowering, channel wall steepening, and reduced rates of lateral migration, as well as bed-sediment coarsening and deflation in the total volume of sediment constituting bars. The process of bed incision produces a convex long profile for the river segment influenced by the dam. After 40 years of impoundment the channel immediately downstream of the dam has incised five to seven meters and dam-influenced adjustments to the geomorphology of the river are observed for 50 to 60 river kilometers downstream. The channel downstream of this zone appears unaffected by the dam. Over time the river bed continues to erode and the zone of dam influence expands downstream. In this paper we present a one-dimensional morphodynamic model that estimates the adjustment in channel profile elevation through time due to the dam’s retention of sediment. Model output matches the field measurements of physical changes to the river channel. Results of the model and physical observations explain the sediment transport dynamics associated with the lower Trinity River downstream of Livingston Dam.