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Description
Urbanization changes flow regimes and watershed characteristics that affect the way channels evolve, especially in watersheds with erodible soils and rapid, unregulated development. Direct human channel alterations such as soil infill, channel wall reinforcement, concrete lining, and check dams are used to control channel changes. CONservational Channel Evolution and Pollutant Transport System (CONCEPTS) has previously been used to model channel evolution in the Los Laureles Canyon Watershed in Tijuana Mexico, (LLCW) and estimate erosion and deposition and predicted high levels of erosion but little deposition. This study used cross section surveys, field observations, and digital terrain model (DTM) differencing to measure change. The DTM of difference was used to estimate erosion downstream of a concrete flume and to measure upstream migration. Fifty-four percent of cross sections showed signs of erosion and lowered in thalweg elevation. Forty percent of cross sections decreased in size, however most of them had direct channel alterations. The only watershed characteristic with a significant correlation was proximity to hardpoints. The DTM of difference showed channel widening is the is the greatest factor downstream of the flume but that upstream migration is occurring underneath the flume leading to its destruction and generating a significant amount of sediment. The channel downstream of the flume has transitioned from a phase of incision to widening. The other cross sections downstream of hardpoints were found to still be in the incision phase but evidence suggests they may be transitioning to widening. Channel incision is the dominant process downstream of other hardpoints. The processes most associated with deposition appears to be direct channel alterations. Most of the cross sections found to decrease in size had experienced infill or channel wall reinforcement. In one reach a check dam was installed that successfully captured sediment and reduced erosion in the area. The CONCEPTS model agreed with field measurements in many of the sources, however few of observed sinks were predicted by the model. The estimated erosion predicted by CONCEPTS was 31% lower than observed, and the predicted deposition was 474% less than observed. The CONCEPTS model is useful for predicting erosion but is limited when it comes to estimating deposition due the ubiquitous channel alterations occurring in the watershed. Most of the erosion occurring is downstream of hardpoints and 25% of the channels measured are generating most of the channel derived sediment. Channel widening, combined with check dams, and increased downstream clast size such as cobble or boulders may be an effective measure for reducing erosion and capturing sediment in some parts of the watershed though further investigation is required. Highly erodible semi-arid watersheds are highly susceptible to erosion downstream of concrete lined channels. When combined with rapid unregulated development, the changing flow regimes lead to channel destruction, upstream propagation, and destruction of the concrete. Direct human channel alterations make mechanistic predictions less reliable in this watershed. This research has applications in other developing watersheds and could be applied to help with planning to reduce impacts of rapid urbanization.
