San Juan Creek Lateral Erosion Study

South Orange County, CA

A streambank lateral migration analysis was performed to assess the potential for future lateral migration of San Juan Creek. The study reaches from downstream of Ortega Highway to upstream of Planning Area No. 1 (PA-1) within boundary of Rancho Mission Viejo.  San Juan Creek is approximately twenty-seven miles long and encompasses a drainage area of 175 square miles at the river mouth.  San Juan Creek has a drainage area of approximately 108.5 square miles at San Juan Capistrano Corporate limits and 105 square miles at the Antonio Parkway bridge location which is representative of the study reach.  A study reach of approximately 12,000 feet in length were evaluated as part of this investigation, although the amount of frontage for the PA-1 area is not this long, the study extended upstream and downstream of the project area.  Base data from the watershed hydrology, geology, floodplain hydraulics, and geomorphology of the study reach established the context for the lateral stability assessment. These data were also used to define stream reaches with relatively uniform geomorphic and geographic characteristics. The recommended methodologies for the lateral erosion analysis incorporate both (1) qualitative geomorphic techniques along with (2) the application fluvial engineering analysis.  This general two-level type approach is typically applied in most river engineering and stream mechanics analysis. The comprehensive design procedure provided utilizes the different results from a variety of analyses to predict the lateral erosion through: (1) river reach hydraulic assessments, (2) horizontal velocity distribution within the floodplain, (3) measured lateral erosion rates through historic aerial photographic comparison, (4) geomorphic analyses predictions, (5) channel morphology trends and characteristics, (6) geologic conditions, (7) Field observations, (9) historical channel changes, (10) stream classification data,  (11) longitudinal profiles, (12) expected channel pattern, (13) stable bank slopes, (14) expected lateral erosion mechanism, (15) armoring potential, (16) equilibrium channel slope, (17) floodplain hydraulic modeling, (18) allowable velocity estimates, (19) channel morphology relationships – regime equations and hydraulic geometry, (20) sediment transport modeling continuous series of events with lateral erosion analysis.  The predicted streambank erosion buffer requires evaluation of both the single short event and the cumulative long term impacts on the lateral erosion in order to adequately a suitable buffer that addresses the different erosion mechanisms.

Creek Stabilization Alternatives Formulation to develop potential creek stabilization mitigation “alternatives” based on the results of the fluvial/geomorphic study and creek instability indicators associated with the study reach.   Alternatives included: (1) bank erosion treatments, (2) channel invert or grade stabilization requirements, (3) channel geometry, (4) channel profile, (5) geomorphic stream corridor restoration elements, and (6) bioengineering/vegetation bank stabilization.