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, τc, τmw, M, and Mmw 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.

Measuring Cohesive Thresholds and Rates

There are two major methods for measuring these data.  The SEDFLUME and a Jet Test.  SEDFLUME (McNeil et al. 1996, Hayter et al, 2014) or other flume apparatus uses a sample (usually a Shelby tube) of extracted sediment, transported to a laboratory apparatus (or the USACE ERDC has a portable lab that they can transport to the site).  Then the sample is extracted into a controlled flow filed with increasing shear stress to find the critical shear stress and erodibility of the sample.  This is commonly used for bed sediments that cannot be reached

Defaults and Literature

Recognizing that these data are extremely site specific and span five orders of magnitude, users still often want to know the possible range and starting points for their calibration.

HEC 6 included a default critical shear stress of 0.2 lbs/ft2.  It does not include defaults for the other parameters because they are site specific (Thomas, personal communication).

This came from the USSR irrigation data for clay channels in Chow's Open Channel hydraulics Text.  These relationships (data are lost) report the critical shear stress as a function of the void ratio, and - to a lesser extent the activity of the clay - and vary by more than an order of magnitude (0.02-0.8 lb/ft2). 

The USDA-ARS (Agricultural Research Center of the US Department of Agriculture) has a well developed database of the jet test data they have collected over the years.  It is common to plot the erodibility against the crical shear stress (kd-tc space) because these parameters are inversely related.  Hanson and Simon (2001) all analyzed 47 samples from three US States and plotted these data with the familiar erodibility zones.

They computed the following relationship between shear stress and erodibility:

Then, Simon et al. (2010) revisited the analysis with 882 from a more divers geographical distribution and developed a new relationship between erodibility and critical shear stress:  

But it is also worth noting that - even for a narrow band of shear stresses - the erodibilities vary by about three orders of magnitude.



Hanson, G.J. and Simon, A. (2001), Erodibility of cohesive streambeds in the loess area of the midwestern USA†. Hydrol. Process., 15: 23-38. https://doi.org/10.1002/hyp.149

McNeil, J, Taylor, C., and Lick, W. (1996) Measurements of Erosion of Undisturbed Bottom Sediments with Depth, ASCE Journal of Hydraulic Engineering, 122(6), 316-324. doi.org/10.1061/(ASCE)0733-9429(1996)122:6(316)

Hayter, E. Gustavson, K., Ells, S., Gallani, J., Wolve, J., Dekker, T., and Redeer, T., (2014) Technical guidelines on performing a Sediment Erosion and Deposition Assessment (SEDA) at Superfund sites, USACE ERDC TR-14-9, 183p https://usace.contentdm.oclc.org/digital/collection/p266001coll1/id/3861/