Estimating critical conditions for initiation of motion Evaluation of Erosion Potential

the predicted time rate of consolidation as compared with actual field

data.

Evaluation of Erosion Potential

If practical, capping should normally be conducted predominantly at

sites that are classified as nondispersive, i.e., sites with relatively little po-

tential for erosion. However, existing sites with more frequent potential

for erosion can be used for capping projects after completing studies of

the frequency of erosion of a specific capping material (considering grain

size, mound geometry and sediment cohesion) for expected wave and

current conditions (to include storms) over time predicted in the area. The

results from such a study will provide data that can be used to predict the

expected cumulative amount of erosion over time along with confidence

intervals on the answers. The estimated erosion amounts can then be used

to define the design cap thickness component for erosion protection re-

quired for a given length of time (say 20 to 100 years). Cap thickness

should be monitored periodically as well as after large storm events to ver-

ify cap stability and measure cap erosion rates. In addition, minimum

thicknesses for contaminant isolation should be predetermined. If monitor-

ing indicates that cap thickness has been reduced below the minimum val-

ues, contingency plans should be enacted to place additional capping

sediments.

The deposit of contaminated dredged material must also be stable

against excessive erosion and resuspension of sediment before placement

of the cap. The potential for resuspension and erosion depends on bottom-

current velocity, potential for wave-induced currents, sediment particle

size, and sediment cohesion. Site selection criteria as described above

would normally result in a site with low bottom-current velocity and little

potential for erosion during the window for placement of the contaminated

sediments and cap. However, if the contaminated sediment is hydrauli-

cally dredged, erosion potential is greatly increased due to the high water

content of the slurry (eventually this water content decreases, thus reduc-

ing erosion potential). In this case, a thorough analysis of the potential

for resuspension and erosion should be performed to estimate the short-

and long-term effects on resuspension potential. Conventional methods

for analysis of sediment transport are available to evaluate erosion poten-

tial (Teeter 1988; Dortch et al. 1990; Resio and Hands 1994; Scheffner

1991a,b). The first level of investigation of cap stability against erosion

involves examination of the normal wave and current regime to determine

if these cause measurable amounts of erosion. However, sites where day-

to-day waves and currents cause measurable amounts of erosion would be

poor sites for capping projects.

Estimating critical conditions for initiation of motion

in wave or current environment

For most sediment bed compositions, a critical stress value exists be-

low which no or negligible sediment movement occurs. Stress is the force

Chapter 8 Long-Term Cap Stability