Geomorphic Evolution Model For The Middle Rio Grande, Nm

GEOMORPHIC EVOLUTION MODEL FOR THE MIDDLE RIO GRANDE, NM

Drew C. Baird, Ph.D., P.E., D.WRE1, Joshua Sperry2, Andrew Schied2, Ari Posner Ph.D3., Nathan Holste M.S., P.E.1, and Pierre Y Julien Ph.D4. P.E.4

1Sedimentation and River Hydraulics Group, Technical Service Center, Bureau of Reclamation, P.O. Box 25007, Denver, Colorado 80225, 2Graduate Student, Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, CO 80521, 3River Analysis Group, Technical Services Division, Albuquerque Area Office, Bureau of Reclamation, 555 Broadway Blvd, NE, Albuquerque, NM 87102, 4Professor, Department of Civil and Environmental Engineering, Engineering Research Center Colorado State University, Fort Collins, CO 80521.

ABSTRACT The Middle Rio Grande (MRG) valley in New Mexico spans 232 miles from the mouth of White Rock Canyon south to Elephant Butte Reservoir. Large floods and sediment loads have played a significant role in shaping the MRG. Floods created rapid geomorphic evolution and channel avulsions caused by channel aggradation. Construction of the levee system in the 1930s narrowed the river corridor. Additional construction of dams, channelization, and floodway clearing, together with changes in precipitation and natural reduced sediment loads, has changed channel morphology.

With the reduced flooding and sediment load the MRG has a more predictable evolutionary process. This geomorphic evolution model builds upon the planform evolution model developed by Massong et. al. (2010). We propose an empirically based evolution model that describes channel evolution based on multiple distinct paths that is influenced by the relationship between sediment transport capacity and supply, and base level changes associated with water surface elevation trends in Elephant Butte Reservoir. Representative cross sections for each evolutionary stage are added to planform changes to complete the geomorphic evolution model. For the reaches of the MRG where the planform begins by “abandoning medial and point bars located within the active channel (Stage 4)” (Massong et al. 2010), there are two evolutionary paths: those channels that have excess transport capacity relative to supply and are degradational, and those that have a sediment supply that exceeds transport capacity. Channels which are degrading evolve towards migrating single thread planform while those that are aggrading evolve towards channel avulsions.

There are unique patterns of channel adjustment within the reservoir delta. When the reservoir water surface is held approximately constant, sediment that enters the reservoir creates a pivot point (point between the topset and foreset delta slope) and continues to deposit sediment upstream of the pivot point, which moves progressively upstream over time. When the reservoir water level rises, the pivot point moves upstream as does delta deposition resulting in the development of distributary channels forming a braided condition. Far upstream the channel may remain in its current planform but could experience channel aggradation such as stage A5 (Massong et. al 2010), likely with the formation of natural levees and the formation of sediment plugs. When the reservoir water surface falls, some of the delta deposits are eroded and transported downstream causing the pivot point to move downstream. The foreset slope, which was formerly underwater, becomes an over-steepened section of the channel profile that has a high local sediment transport capacity. The once braided channel begins to incise creating a single thread channel that over time may migrate laterally. The greatest incision occurs at the previously formed delta where there is the thickest zone of previously deposited sediment. The incision then starts to head cut upstream, which will cause channel degradation and potentially some narrowing of the main channel.