Cohesive mound movement (Cont.)

J Jcr

where

A0 and m = site-specific parameters

J = shear stress due to currents and waves

Jcr = site-specific critical shear stress below which no erosion occurs

(which can reasonably be set to 5 dynes/cm2 if site data are not

available)

Jr = a reference shear stress (assumed to be 1 dyne/cm2)

Most research on cohesive sediment erosion has been performed in laboratory

settings at moderate shear stresses less than 20 dynes/cm2 (Lavelle, Mofjeld, and

Baker 1984). The method incorporated into LTFATE was developed for

moderate stresses. Data for high shear stresses are sparse, and the experimental

methods are still under development (McNeil, Taylor, and Lick 1996). Despite

this, a lot can be determined by using the moderate shear equations in high-shear

regions. It would appear from bathymetry measurements in high-shear regions

that the above equation can adequately simulate these conditions.

It should be noted that the values of the site-specific parameters used in these

methods can vary significantly. Experimentally determined values of A0 range

over several orders of magnitude from 1 10-9 to 5 10-6 (g/cm2/sec) and m

ranges from 1 to 5 (Lavelle, Mofjeld, and Baker 1984). The experimental range

of exponent m values coupled with the equation for J demonstrate that the rela-

tionship between velocity and erosion is highly nonlinear (J is a function of V2

and is a function of Jm resulting in is a function of V2m). Therefore, the rare

storm events will produce most of the cohesive sediment erosion for a given

year. This is well known to occur in many rivers, lakes, and nearshore environ-

ments. Some studies on San Francisco Bay sediments suggest that m ranges

from 1-2 for these sediments, assuming they have had long compaction periods

(Parthenaides 1965). The higher values of m are reserved for freshwater lake

and river sediments. For application of LTFATE, erosion tests should be per-

formed on site sediments. If at all possible, values for A0 and m should be

determined from laboratory experiments on sediment cores extracted from the

study area. If no such data are available, values for A0 and m can be set to 7.6

10-8 g/cm2/sec and 2, respectively. These values will produce a decent conserva-

tive (i.e., high) estimate for erosion potential. They were developed for recently

deposited sediments at the New York Bight Mud Dump site. They will produce

a conservative estimate because they are for recently deposited, and therefore

more easily resuspended, sediments.

Appendix F LTFATE Model