Citation:

Gudino-Elizondo, N., T. Biggs, C. Castillo, R. Bingner, E. Langendoen, K. Taniguchi, T. Kretzschmar, Y. Yuan, AND D. Liden. Measuring ephemeral gully erosion rates and topographical thresholds in an urban watershed using unmanned aerial systems and structure from motion photogrammetric techniques. Land Degradation & Development. John Wiley & Sons, Inc., Hoboken, NJ, 29(6):1896-1905, (2018). https://doi.org/10.1002/ldr.2976

Impact/Purpose:

Excessive flooding and sedimentation threaten both ecosystems and human populations. On the US-Mexico border, urbanization has increased runoff and sedimentation loads. In the Tijuana-San Diego region, the Tijuana Estuary in the United States suffers from “excessive sedimentation”, and determining the source of the sediment and mitigating its production is a primary management goal of the US EPA and other cross-border agencies. Gully erosion has been identified as one of the major sediment sources. However, there are few studies assessing gully erosion in urban settings. This study is intended to fill the gap of quantifying gully erosion in environments where gullies are rapidly repaired, and in urban areas where microtopographic complexity complicates the delineation of contributing areas. This study used unmanned aerial vehicles (UAVs) and Structure-from-Motion (SfM) photogrammetric techniques to quantify gully erosion in the Los Laureles Canyon watershed, a rapidly urbanizing watershed in Tijuana, Mexico. Following a storm event, the gully network extent was mapped using an orthomosaic (0.038 m pixel size); the local slope and watershed area contributing to each gully head were mapped with a Digital Surface Model (0.3 m pixel size). Gullies formed almost exclusively on unpaved roads which had erodible soils and concentrated flow. Management practices (e.g. road maintenance that fill gullies after large storms) contributed to total sediment production at the watershed scale. Sediment production from gully erosion was higher and threshold values of slope and drainage area for gully incision were lower than ephemeral gullies reported for agricultural settings. This indicates high vulnerability of unpaved roads to gully erosion which is consistent with high soil- erodibility and low critical shear stress measured in the laboratory with a mini jet-erosion- test device.

Description:

Both rural and urban development can lead to accelerated gully erosion. Quantify gully erosion is challenging in environments where gullies are rapidly repaired and in urban areas where microtopographic complexity complicates the delineation of contributing areas. This study used unmanned aerial systems and structure from motion photogrammetric techniques to quantify gully erosion in the Los Laureles Canyon Watershed, a rapidly urbanizing watershed in Tijuana, Mexico. Following a storm event, the gully network extent was mapped using an orthomosaic (0.038‐m pixel size); the local slope and watershed area contributing to each gully head were mapped with a digital surface model (0.3‐m pixel size). Gullies formed almost exclusively on unpaved roads, which had erodible soils and concentrated flow. Management practices (e.g., road maintenance that fill gullies after large storms) contributed to total sediment production at the watershed scale. Sediment production from gully erosion was higher, and threshold values of slope and drainage area for gully incision were lower than ephemeral gullies reported for agricultural settings. This indicates high vulnerability to gully erosion, which is consistent with high soil erodibility and low critical shear stress measured in the laboratory with a mini‐jet‐erosion test device. Future studies that evaluate effects of different soil types on gully erosion rates on unpaved roads, as well as model effects of management practices such as road paving and their impact on runoff, soil erosion, and sediment loads, are crucial for proper sediment management and planning in urban watersheds.

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