# HY8:Scour Hole Geometry

The scour hole geometry presented in this screen represents the local scour at the outlet of structures based on soil and flow data and culvert geometry. Chapter 5 of FHWA publication HEC 14, Hydraulic Design of Energy Dissipators for Culverts and Channels, dated July 2006, presents the general concept and equations used by the program to compute the scour hole geometry for cohesive and cohesionless materials.

**NOTE**—A soil analysis should be performed prior to running this option of the program.

For Cohesive soils, the program requires the following parameters:

- Time to Peak—Enter the value obtained in the 'HYDROLOGY' option of HY-8 (If unknown enter 30 minutes).
- Saturated Shear Strength—Obtained by performing test no. ASTM D211-66-76.
- Plasticity Index—Obtained by performing test no. ASTM D423-36.

For Cohesionless soils, the program requires the following parameters:

- Time to Peak—Enter the value obtained in the 'HYDROLOGY' option of HY-8 (If unknown enter 30 minutes).
- D16, D84—Soil particle diameters which represent percent of particles finer.

## Note on Time to Peak

The time of scour is estimated based upon knowledge of peak flow duration. Lacking this knowledge, it is recommended that a time of 30 minutes be used in Equation 5.1. The tests indicate that approximately 2/3 to 3/4 of the maximum scour depth occurs in the first 30 minutes of the flow duration. The exponents for the time parameter in Table 5.1 reflect the relatively flat part of the scour-time relationship (t > 30 minutes) and are not applicable for the first 30 minutes of the scour process.