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Measuring ephemeral gully erosion rates and topographical thresholds in an urban watershed using unmanned aerial systems and structure from motion photogrammetric techniques

Napoleon Gudino‐Elizondo, Trent W. Biggs, Carlos Castillo, Ronald L. Bingner, Eddy J. Langendoen, Kristine T. Taniguchi, Thomas Kretzschmar, Yongping Yuan, Douglas Liden
Land degradation & development 2018 v.29 no.6 pp. 1896-1905
drainage, erodibility, gully erosion, models, photogrammetry, planning, pollution load, ravines, roads, runoff, sediment yield, sediments, shear stress, soil types, storms, urban areas, urban development, watersheds, Mexico
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.