Civil Engineering Association

The purpose of this manual is to provide guidance for the analysis of sediment areas and arroyos in the SSCAFCA jurisdictional area, including methods for evaluating the potential effect of proposed structures or activities on the vertical and lateral stability of the arroyos and drainageways, and for establishing the Lateral Erosion Envelope (LEE) line. The LEE line is a boundary along an arroyo or drainageway that would have a low possibility of being disturbed by erosion, scour, or lateral migration of a natural (unlined) arroyo by storms up to and including the 100-year storm.
This Design Guide describes basic concepts related to the physical processes that control arroyo behavior, provides engineering tools that can be used to quantitatively analyze specific processes, and provides guidance on combining these tools to predict short- and long-term arroyo behavior.
The manual also provides guidance on selecting countermeasures to mitigate potential erosion or flooding problems in key areas, Chapter 2 describes the basic geomorphic and watershed processes that are important in the SSCAFCA area. Chapter 3 provides the individual tools with which to analyze specific aspects of arroyo dynamics, including the following:
• the Incised Channel Evolution Model that explains the progression through which most arroyos evolve in an urbanizing environment,
• methods for assessing hydraulic and sediment transport conditions in the arroyos,
• methods for using that information to assess vertical and lateral adjustability,
• a method for identifying the location of the LEE line, and
• methods for quantifying local scour at various types of structures that are common in the
SSCAFCA area,
Chapter 4 describes the general solution procedures that should be used to combine the specific tools that are provided in Chapter 3 to perform an overall assessment of arroyo stability, progressing from qualitative evaluation of pertinent information through quantitative analysis. Chapter 4 also provides guidance in interpreting the results of the quantitative analysis to identify the nature of the hazard (i.e., flooding erosion or sedimentation) to developed properties along the arroyo, and methods of identifying appropriate methods to mitigate those hazards.
Finally,Chapter 5 provides general guidance on selecting and designing erosion barriers that may be incorporated with the LEE line to accomplish the dual goals of maintaining natural or naturalistic conditions while limiting the width of the erosion limit corridor. A set of example problems are provided in Appendix E to illustrate the use of the various relationships.