Observing Short-Term Geomorphic Change in a Human-Modified River Using Terrestrial Repeat Photographs and Traditional Surveys: Uncompahgre River, Colorado, USA
The Uncompahgre River in Ouray, CO, was modified in 1996 from a braided river system to a meandering river channel. Large boulders of riprap were placed along designed meanders to prevent erosion and enable the development of permanent human structures on the flood plain. Deposition of gravel bars in the modified channel occurs annually during the summer. This gravel is "mined" by the City of Ouray; however, the effects of this excavation and the original modification were never assessed.
This study provides an assessment by quantifying cross-sectional area change, cumulative grain-size distributions, shear stresses, slopes, and sinuosities using traditional survey methods. In addition, volume change of a gravel bar inside the modified channel was estimated using extreme oblique photographs (>45 degrees from nadir) that were obtained from nearby cliffs. Close-range photogrammetry was used in the natural channel downstream to evaluate photogrammetric methods using different lenses, image sensors, and camera geometries.
Both traditional and photogrammetric methods clearly indicated significant deposition in the modified channel, whereas erosion occurred directly downstream from the modified channel, but did not occur at a reach 1.5 km downstream. In the natural channel, no cross-sectional area change occurred, grains were poorly sorted, and the longitudinal slope was ~four times steeper than the modified channel. Shear stress ratios were used as an erosion threshold, which did not correlate with actual cross-sectional area change, but a decrease in shear stress ratios from May 2011 to September 2011 were associated with erosion. Average RMSE values for DEMs created from extremeoblique photographs of a gravel bar in May 2011 and September 2011 were 0.140 m and 0.324 m, respectively. Using a DEM of difference with a t-statistic filter revealed that 115m3 of gravel was deposited.
The Uncompahgre River showed similar geomorphic characteristics to other rivers in southwest Colorado, however, the slope of the natural and modified channels were much steeper than other rivers. Extreme-oblique photography and unconventional sensors both yielded reliable results, showing that these atypical techniques can be used in terrestrial photogrammetric applications such as, post-restoration assessments, as long as proper base-to-height ratios are achieved.