Assessment of Ephemeral Channel Cross-Section Morphology Following Pipeline Construction in Southern Arizona

Abstract

Morphologic change of ephemeral stream cross-sections is a natural component of fluvial geomorphology but disruptions to natural erosion and deposition by anthropogenic disturbances has the potential for cascading impacts down the channel corridor. The proximal impact of a natural gas pipeline construction on ephemeral stream cross-section geometry in southern Arizona was evaluated from July 2014 (pre-construction) to July 2016 (two years post construction). Cross-sections at three locations (upstream the pipeline Right-Of-Way (ROW)), through the middle of the ROW, and downstream of the ROW) were measured using Light Detection And Ranging (LIDAR) and field methods for 16 ephemeral streams. Results of both the LIDAR and field measurements indicated insignificant difference in cross-sectional area change between upstream, across, and downstream-ROW cross-sections [(F 2,64) = 0.341, p = 0.73; (F2,18)= 0.980, p = 0.395]. Sediment generated during pipeline construction appeared to have moved beyond the physical confines of the study site, which limited the assessment of larger-scale geomorphic impacts. Furthermore, the 2014-2016 study period experienced only small (high-recurrence frequency) precipitation events, indicating the absence of large flows capable of significant morphologic change. To further explain differences in cross-section area change between LIDAR datasets, a linear regression model was used to assess the predictive value of nine variables: year of measurement, drainage area, drainage density, basin slope, upstream-, across-, downstream-ROW cross-section locations, percent bare soil in basin, percent mesquite in basin, total precipitation, and number of storms with average precipitation above 25 mm/hour. Though the amount of bare soil in the basin and the second study period (February 2015-July 2016) at least partially explained the changes in cross-section area, the model was not a strong predictor of morphologic change during the 2014-2016 study period. The majority of the variability in cross-sectional area change in the study basins remained unexplained.