Guidelines for Assigning Erodibility Parameters to Soil Horizons for SITES analyses
Recorded on October 12, 2010 (120 minutes)

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Content & Objectives: The SITES computer program provides engineers with a tool for predicting the formation and progression of a headcut in an auxiliary spillway during a design storm.  The predicted performance given by SITES includes the extent and speed of development of a headcut that may develop in the auxiliary spillway.   The predicted erosion in the spillway depends on (1) the duration and quantity of design flow through the spillway during the design storm (design hydrograph) and (2) the erodibility parameters of the soil and bedrock horizons in the spillway model. 

The erodibility factors used in SITES are the headcut erodibility index Kh and the detachment rate coefficient, kd.  This Webinar provides guidance primarily on selection of Kh and kd parameters for soils.  Soils are vastly more erodible than bedrock and the erosion potential of soil horizons is extremely important in the predicted performance of any spillway that is not situated solely on bedrock.  The Webinar does not provide specific training on the actual use of the computer program SITES.  The training in the Webinar is confined to guidance related to the selection of the two erodibility factors for soil.  Bedrock is not explicitly included in the Webinar training, but some of the same principles in selecting the Kh and kd parameters for soils also apply to bedrock materials.  Information is extracted from NRCS publications on the SITES program as well as supplemental guidance developed by the Webinar trainer, Mr. McCook.

Professional Development Hours (PDHs): Each paid registrant (whether viewing the live or archived version) will receive a link to the access the webinar and the post-event assessment tool.  Upon successful completion of the assessment, you will be able to print a certificate of attendance for 2 professional development hours.

Danny K. McCook, P.E. is an independent consulting engineer specializing in the geotechnical analysis and design of small earthen embankments. In his 41 year career with the Natural Resources Conservation Service (he retired from there in October 2008), he prepared geotechnical design reports for hundreds of embankment projects across the US, He is the author of a variety of NRCS publications and training manuals on geotechnical topics. He has presented papers at 16 previous ASDSO National and regional meetings. He presented two webinars for ASDSO on review of embankment projects and common problem soils and conditions. He received a BS degree from Texas Tech University in 1965 and a MS degree from the University of Nebraska in 1967. He is a registered PE in Texas and a member of ASCE and ASDSO.


  1. Overview of SITES computer program, history and development
  2. Three phase erosion in a spillway
    1. Surface erosion
    2. Concentrated flow erosion (downcutting)
    3. Headcut advance
  3. Basics of modeling a spillway cross-section for SITES
    1. Guidance on 2-D models for spillways
    2. Importance of documentation for model
    3. Important parameters for each horizon in model
  4. Detachment rate parameter, kd
    1. Algorithms for computing the kd parameter
    2. Parameters used in algorithm and correlations
      1. Percent clay and PI correlations
      2. Dry density and percent clay correlations
    3. Range of values of kd parameter
    4. Relationship to flume measurements
    5. Jet erosion test
    6. Summary spreadsheet for computing kd value from percent clay and dry density.
  5. Headcut Erodibility Factor Kh (General)
    1. Classical equation for computing
    2. Importance of material strength number for soils
    3. Summary of available tools
      1. Spreadsheet
      2. Guidance in Chapter 52
      3. Soils Catalogue
      4. Supplemental Guidance (Draft Appendix 52D)
    4. Typical values of Kh parameter for soils
    5. Tool (spreadsheet) for showing relationship between Kh value and rate of headcut migration for given values of unit flow and overfall height.
  6. Headcut Erodibility Factor for Cohesive Soils
    1. Relationship between Material Strength Number and consistency
    2. Importance of unconfined compressive strength and empirical tools for estimating.
    3. Measurements of unconfined compressive strength lab and field
    4. Typical Range of Values
    5. Soils Catalogue
  7. Headcut Erodibility Factor for Cohesionless Soils
    1. Relationship between Material Strength Number and relative density
    2. Importance of relative density and empirical tools for estimating.
    3. Measurements and of relative density lab and field
    4. Typical Range of Values
    5. Soils Catalogue
  8. Summary