Urban hydromodification in Southern California: Impacts and management implications

The purpose of this thesis was to evaluate predictive methods and protection strategies for southern California stream channels subject to urbanization-induced hydrologic modification. Field surveys conducted between October 2001 and May 2005 at stream reaches in Los Angeles and Ventura Counties in semi-arid Southern California were evaluated for impacts from adjacent urbanization and to test the shear stress model for sediment mobilization. Survey sites were located on ephemeral and intermittent channels with contributing catchment areas ranging in size from approximately 1 mi2 to 75 mi 2. The predictive ability of equations for mean bed shear stress (tau) and critical shear stress (tauc) was evaluated using stream channel geometry and cross-sectional area changes. Calculated depths for bed instability (tau/tauc > 2.5) were significantly greater than field-estimated bankfull depths at most sites, yet field data indicate ongoing erosion and unstable channel conditions. Predictions of these equations alone are therefore not an effective evaluative tool at these sites. Calculated error in shear stress ratios ranged from 49--115%. Continuous simulation hydrologic model evaluations of shear stress ratio (tau/tauc) durations showed that the duration of erosive shear stress was increased up to 20% at two sites using regional detention basin flow controls. Stream erosion model predictions should incorporate thorough uncertainty evaluations, and present results as potential response ranges rather than clear response thresholds. Incorporation of uncertainty in response models, interpretations, and regulatory requirements would improve the probability of effective stream protection, and reduce the implementation of ineffective or potentially damaging strategies.; Scientific uncertainties regarding urban hydromodification (Urban HM) affect the development and implementation of federal Clean Water Act (CWA) requirements to protect stream channels and habitat. Despite decades of study, scientific understanding of stream system processes remains primarily qualitative. Stream response to Urban HM has not been well studied in semi-arid southern California. Federal and state regulations acknowledge stream degradation from Urban HM, but lack of scientific understanding limits specificity of regulatory requirements. Municipal Stormwater Permits and CWA Section 401 Water Quality Certifications primarily regulate Urban HM in California. Regulatory agencies pass the responsibility to resolve uncertainties to local jurisdictions through requirements to develop specific protective strategies and acceptable protective criteria. Stream protection from Urban HM would be greatly improved by a cooperative effort among regulators and local implementing authorities.