QUANTIFYING THE GEOMORPHIC IMPACT OF A LARGE FLOOD ON THE RIO PUERCO ARROYO, NEW MEXICO, USING PRE- AND POST-EVENT AIRBORNE LASER MAPPING AND HYDRODYNAMIC MODELING

Large floods strongly impact river valleys by mobilizing high volumes of sediment, removing and burying vegetation, and significantly changing the topography of the channel and floodplain. A fundamental goal in fluvial hydrology and geomorphology is to understand these impacts at a quantitative and ultimately predictive level. The analysis of repeated high-resolution topographic surveys is a fundamental tool in the study of the morphological impacts that large floods have on rivers.

The lower Rio Puerco in New Mexico serves as an ideal location for the study of large floods on ephemeral rivers. In 2003, herbicides were sprayed on a section of the river to remove saltcedar (Tamarix ssp.), an invasive species introduced in the 1920s for erosion control. In 2006, severe floods caused extreme erosion on the sections of Rio Puerco that were sprayed with herbicides, while little to no erosion occurred in the areas that were not sprayed. The eroded sediment deposited on the channel and floodplain along 10 km downstream of the sprayed reach.

LiDAR surveys were obtained for sections of the Rio Puerco in 2005 and again in 2010, providing a unique multi-temporal data set that brackets a major flood event. We present the results of a topographic differencing study showing patterns of elevation change downstream of the sprayed reach, estimated volumes of mobilized sediment, and overbank flow directions. We also show results of high-resolution flow-routing calculations for the lower reaches of the study area, performed using the 2D hydrodynamic model ANUGA Hydro. These analyses are an initial step in the development of a numerical model for predicting the erosional consequences of major floods.