MODEL: Sediment Grain-Size Depth of Residence

Technical Note DOER-N4

May 1999

MODEL: Sediment Grain-Size

Depth of Residence

PURPOSE: This technical note describes a screening-level computer model for determining the

water depth at which a normally distributed sand/coarse-silt sediment becomes mobile. The

mobilizing force is determined from a specified wave height, wave period, and superimposed

current.

BACKGROUND: The Corps of Engineers has several tools in existence or under development

that can predict the fate of sediments placed at nearshore and offshore locations (namely, STFATE

(Johnson and Fong 1993), MDFATE (Moritz and Randall 1995), and LTFATE (Scheffner et al.

1995; Scheffner 1996)). However, while MDFATE and LTFATE provide high-quality predictive

capabilities for the transport of dredged sediments, they require significant investments of time and

resources to operate effectively. This investment of time and resources is sometimes unwarranted

in the early phases of project development where quick estimates of sediment motion are all that is

desired. The numerical model described herein provides a simple tool for determining the limiting

depth at which incipient motion of a particular-sized sand particle occurs for a given set of

hydrodynamic conditions. Using this tool that provides the threshold depth for motion, one can

quickly determine that water depths much deeper than the threshold will provide a stable environ-

ment for the sediment, while water depths much more shallow will result in active motion of the

sediment. This tool, therefore, allows quick screening of the suitability of potential locations for

dredged-material placement.

The input data requirements of the model are wave height, wave period, tidal current, and the median

grain size value, D50. During the planning phase of a dredging project, the median grain-size value

of the in situ dredged material is either known or is estimated. The hydrodynamic conditions for a

coastal location can be estimated from the Corps of Engineers wave hindcast and tidal current

databases. These databases can be found on the U.S. Army Engineer Waterways Experiment

Station URL Internet site http://bigfoot.wes.army.mil/cetn.index.html. Therefore, all of the required

input data for the model can be readily obtained. In any particular case, the sensitivity of the

result to particular hydrodynamic or grain-size values can be obtained by methodically varying

the parameters.

Several important simplifying assumptions were made in the development of this model of which

the user should be aware. These are outlined in detail below along with a description of the

procedures used within the model to translate wave and current data into bed shear stresses and the

determination of incipient sediment motion.

THEORY: Most noncohesive (sand/coarse-silt) transport models have been developed for cur-

rent-only environments. The near-bottom current outside the boundary layer, coupled with the bed

roughness, is used to calculate a near-bottom shear stress. There is a critical stress value at which

a given grain size will begin to move as bed load. In the present application, waves often produce