Cohesive mound movement

Kraus and Larson (1988) found that in some large wave tank cases, the local

slope of a mound of noncohesive material exceeded the angle of repose due to

constant waves and water levels. Therefore, the concept of slope failure was

incorporated in LTFATE to ensure stability of the dredged material mound by

employing an algorithm developed by Larson and Kraus (1989). The algorithm

is based on laboratory studies conducted by Allen (1970), who investigated

steepening of slopes consisting of granular solids. Allen (1970) recognized two

limiting slopes, the angle of initial yield and the residual angle after shearing. If

the slope exceeds the angle of initial yield, material is redistributed along the

slope through avalanching, and a new stable slope is attained, known as the

residual angle after shearing.

Cohesive mound movement

An improved cohesive sediment transport model has recently been incor-

porated into LTFATE to account for transport of fine-grained material, i.e., silts

and clays. Fine-grained sediments are hydraulically transported almost entirely

in suspension rather than as bed load; therefore, the Ackers-White equations are

not applicable for these conditions. The cohesive sediment transport model

requires bottom shear stress as input. The total bottom shear stress due to cur-

rents and waves is determined using the combined current/wave >perceived

velocity=, Vwc (Bijker 1971; Swart 1976) and bottom roughness parameters. This

method for calculating shear stress, like most others, is influenced by bottom

roughness parameters. These parameters are frequently not available for the

study area, and the results may change significantly depending on their values.

Bottom roughnesses for typical ocean sediments can be used in lieu of actual

data.

The factors influencing the resistance of a cohesive sediment bed to erosion

may be best described by Ariathurai and Krone (1976) as: (a) the types of clay

minerals that constitute the bed; (b) structure of the bed (which in turn depends

on the environment in which the aggregates that formed the bed were deposited),

time, temperature, and the rate of gel formation; (c) the chemical composition of

the pore and eroding fluids; (d) stress history, i.e., the maximum overburden

pressure the bed had experienced and the time at various stress levels; and

(e) organic matter and its state of oxidation. It is obvious from this description

that the resistance of the bed to erosion will be different not only from site to

site, but also potentially with depth at a given location. Therefore, erosion

potential is usually considered a site-specific function of shear stress (and

sometimes depth). Methods have been developed to determine erosion based on

stresses, but these equations require parameters whose values are site specific. A

commonly used method of relating erosion to shear stress has been incorporated

into LTFATE. This method relates erosion as a function of shear stress to some

exponential power. The equation for the erosion rate in grams/square

centimeter/second is:

Appendix F LTFATE Model