The Importance of Site Specific Measurements

Applying the Partheniades (piecewise excess-shear) method successfully requires estimating the shear thresholds and the erosion rates well. These parameters are site specific and can differ by five orders of magnitude between sites. Even within the same reach, τ_c, τ_mw, M, and M_mw can vary significantly between samples or at different depths. Therefore, the parameters can be determined experimentally (e.g. with a SEDFLUME apparatus or in situ jet measurements) or are calibration parameters, adjusted to replicate measured bed change. These parameters are very difficult to estimate a priori.

Briaud et al. (2001) summarize the situation:

"Today, no widely accepted correlation could be found (between cohesive erodibility and bulk soil parameters) after extensive literature reviews. If a correlation is likely to exist on one hand, and if it has not been found after forty years of effort on the other hand, the correlation must be complex…Considering all the problems associated with correlations, a direct measurement with the (erodibility testing) is favored." Briaud et al. (2001) Journal of Geotechnical and Geoenvironmental Engineering

Others concur:

"With the vast number of factors involved in the determination of the erodibility of cohesive soils, it becomes necessary to test cohesive soils for critical shear stress for erosion and deposition rather than using soil properties for predicting threshold values or using methods similar to those for coarse sediments." Huang et al. (2006) Erosion and Sedimentation Manual

"Unfortunately, the erodibility of cohesive sediment cannot be predicted on the basis of environmental parameters. As a consequence, researchers have developed various test apparatus to empirically measure sediment erodibility." Ravens (2007) ASCE Journal of Hydraulic Engineering

The cohesive literature is full of pronouncements that these parameters must either be measured or calibrated including: Roberts et al. (1998), McNeil et al (1996), Jepsen et al., (1997), Hanson (1996), Julian and Torres (2006), Hansen and Simon (2001), Kapen et al. (2007), Sanford and Maa (2001). Therefore, HEC-RAS does not include default parameters.

In the absence of robust calibration data, the Parthenaides method requires experimental data for reliable results. The SEDFLUME is the most common apparatus used to measure the cohesive parameters, usually computing parameters from Shelby tube samples. This device pushes a core of the cohesive bed material through the bottom of the flume. The Corp's sediment lab in ERDC, and several universities, can perform these experiments. ERDC's has a portable SEDFLUME that can deploy to a project site, avoiding sample disturbance during transport.