Stream restoration projects are usually intended to improve fluvial processes and stability and to restore ecological functions. Langhammer et al. (2023) evaluated three of these projects near Prague, Czech Republic, for up to six years after completion using unmanned aerial vehicles (UAVs).1 The UAVs were equipped with 16–20 megapixel cameras and flown at 60–75 m twice per year in the spring and fall. Using a Structure from Motion algorithm, the aerial surveys provided 2 cm and 5 cm per pixel resolution for 2D and 3D digital models, respectively, to evaluate changes in channel geometry. The three streams were in relatively flat (1% to 1.5% slope) urban corridors with typical restrictions on the restoration area such as roads, sidewalks and bridges, with available floodplains of 30 m to 100 m. Plans were to increase the stream lengths 17% to 32%, but the actual (“as built”) lengths were only increased 7% to 12%. Similarly, the sinuosity was supposed to be increased 8% to 16%, but in reality increased only 4% to 8%.
The authors suggest that these reductions in actual complexity significantly limit the hydroecological benefits relative to the plans. Bank erosion was evident where the constructed channel geometry was substantially different than the planned one. A flood event on one stream before the vegetation could be established resulted in continuing bank erosion. Several ponds originally hydrologically connected to one stream were subsequently isolated after restoration, resulting in eutrophication, heavy reed growth and dry periods. The lack of shade was an issue on all restored streams, although some trees had been planted in places. The UAV surveys also detected construction activities that severely disturbed parts of the restoration work, as well as evidence of sewage spills resulting in heavy eutrophication in one stream. Nine restoration parameters were derived from the UAV surveys and helped identify stream reaches most in need of follow-up repair. Overall, the authors suggest that this relatively inexpensive system using UAV-derived data can be very useful in evaluating the success of stream restoration projects after completion.
Sometimes timing is everything in studying streams. In 1975 and 1978 there were cross-sectional surveys of the Powder River in the northern Great Plains of the United States.2 It is a perennial, meandering, free-flowing river subject to periodic flooding from naturally occurring events such a snow melt. In May 1978 there was the second largest known flood of about five times the bank full rate of 160 to 170 m3 s-1, followed by an invasion of non-native Russian olive (Eleagnus augustifolia) along the river. Because Russian olive forms much denser, stiffer stands than the native willows and cottonwoods, the effects on stream morphology were studied using subsequent surveys on land and by Light Detection and Radar (LiDAR) technology over a 90 km stretch of the river. With theoretically greater sediment trapping in the Russian olive stands, the authors hypothesized that the point bars would have lower slopes and higher elevations. It turned out the width of Russian olive stands did correlate well with decreased point bar slopes, but not with elevation. The authors consequently proposed that the point bar morphology controls the colonization by Russian olive, which is more likely to have seeds deposited and plants establish on less steep areas. A comparison of the ground survey elevations to that derived from the lidar data indicated that they correlated closely. The largest deviations between the two methods were where the slopes were steepest, such as on cut banks, but overall they matched well.
References:
- Langhammer, J., T. Lendzioch, and J. Solc. 2023. Use of UAV Monitoring to Identify Factors Limiting the Sustainability of Stream Restoration Projects. Hydrology 2023, 10, 48. https://doi.org/10.3390/hydrology10020048
- Moody, J. A. and D. M. Schook. 2022. Ecogeomorphic interactions of Russian olives (Elaeagnus angustifolia) and point-bar morphology along Powder River, Montana, USA. River Res Applic. 2023;39:1094–1109. DOI: 10.1002/rra.4139.
About the Expert
Rich McLaughlin, Ph.D., received a B.S. in natural resource management at Virginia Tech and studied soils and soil chemistry at Purdue University for his master’s degree and doctoral degree. He has retired after 30 years as a professor and extension specialist in the Crop and Soil Sciences Department at North Carolina State University, specializing in erosion, sediment and turbidity control. He remains involved with the department as professor emeritus.
