UC Santa Barbara

Urbanization can lead to stream channel erosion and ecological degradation. The majority of studies have focused on the impacts of urban development on channel morphology in developed regions, such as the United States and Europe, where urbanization is typically characterized by watershed-scale land alterations, such as the conversion of undeveloped land to impervious urban areas. This dissertation focuses on a rapidly developing, semi-arid region, Los Laureles Canyon watershed (LLCW), located in Tijuana, Mexico, which is characterized by steep slopes and highly erodible material. Urban development in Tijuana has led to excessive hillslope and channel erosion, and subsequent infrastructure failure of homes, water main pipes, and unpaved roads and sedimentation of the downstream Tijuana Estuary in San Diego, CA. The main objectives of this dissertation are to investigate the impact of urbanization and in-channel alterations on stream channel evolution, highlight channel sources and sinks of sediment, and evaluate the overall importance of channel erosion on the sediment budget of LLCW for future sediment mitigation plans. First, traditional geomorphic survey methods and Structure-from-Motion (SfM) photogrammetry techniques were utilized to describe the spatial patterns in stream channel geometry in LLCW and to provide a regional comparison of channel erosion in Tijuana, MX to reference and urbanized watersheds in southern California. Channels in Tijuana are statistically larger than urban and reference channels in southern California and major hotspots of erosion are located downstream of hardpoints, or non-erodible features. Second, to quantitatively evaluate channel evolution and to determine the driving mechanisms to channel instability downstream of hardpoints, field data were used to develop a computational model of channel evolution, CONCEPTS (CONservational Channel Evolution and Pollutant Transport System), for LLCW. A scenario analysis was conducted to quantitatively assess the impact of urban channel alterations, including hardpoint installation, slope alteration, bed composition change, and vegetation removal, on channel incision, widening, and sediment load. Hardpoints prevented incision in the upstream direction by serving as grade control, and only caused local channel instabilities downstream. Channel erosion is caused mainly by the destruction of the natural channel, including channel burial, straightening, steepening, and removal of riparian vegetation, often performed in the process of turning channels into roads. Reformation of an enlarged river reach that is disconnected from the floodplain, leads to higher flow depths constrained in the channel, larger shear stresses, and accelerated channel incision. Lastly, a watershed-scale model of hillslope processes, AnnAGNPS, integrated with CONCEPTS was developed for LLCW to determine the spatial pattern of channel sources and sinks of sediment in the watershed and evaluate the overall importance of channel processes on the sediment budget for future sediment mitigation plans. Channel erosion contributes approximately 60% of the total sediment budget and only a third of the entire stream channel network is generating 90% of the channel-derived sediment load. This indicates that channel erosion is a dominant source of sediment in LLCW and targeted stream stabilization measures could potentially reduce a large proportion of sediment load to the Tijuana Estuary. However, coarsening of the bed alone may not decrease mean annual channel-derived sediment yield, as armoring of the bed can decrease channel incision but channel widening may be exacerbated. Overall, urbanization of the valley floor and alterations to the stream channel have led to constrained and enlarged stream channels. This dissertation provides an example of the use of a variety of geomorphic field methods, including traditional topographic survey methods and Structure-from-Motion (SfM) photogrammetry techniques, paired with a comprehensive modelling framework to provide an understanding of the driving mechanisms of channel instability and the overall importance of channel processes on the sediment budget to support local and federal sediment management plans in a rapidly developing, semi-arid region.