Moving Substrate in an Ephemeral Stream

A case study concerns a small bridge site in an arid area where ongoing inhibition of the transport of bed load sediment has caused chronic problems and maintenance issues for many years. The crossing of Guadalupe Arroyo by US-62-180 exhibits many unusual and important characteristics that are seldom seen in one place. Guadalupe Arroyo is an ephemeral stream in a very arid area. The stream originates high on the east slopes of the Guadalupe Mountains of Texas, where the watershed is subject to rainfall generated by orographic lift. The stream traverses several miles of arid land and ultimately disappears into a dry lake. It is subject to severe flash flooding because of the slope and orographic effects of the mountains, and it apparently transports large amounts of widely graded material (silt to boulder sized). A bridge across this stream was constructed for US-62-180 in 1959. Since that construction, the stream has exhibited symptoms of instability in the reach around the bridge, manifesting as chronic, severe aggradation accompanied by widening, avulsion, and bank erosion. Evidence exists of large magnitude transport of very large particles on a regular basis, to the extent of requiring protection of the piles from boulder impacts. Maintenance forces have continually removed accumulated bed material from the bridge opening and the reach immediately upstream. A large lens of bed material has accumulated upstream and extends approximately 1,000 ft (305 m) from the bridge. This site presents a rare opportunity to study an extreme case of the inadvertent inhibition of the transport of bed material in an ephemeral desert stream by the construction of an otherwise ordinary and innocuous highway bridge.