Currents - trdoer10038

needs to be taken into account). Further, currents do not appear to affect

the surge phase of the disposal (Bokuniewicz et al. 1978; Truitt 1986a).

However, water column currents and bottom slopes are important in slow

placement of sand caps where the currents and density flows can cause

some waste of capping material.

Long-term effects of currents at a prospective site may still need to be

investigated from the standpoint of potential erosion of the mound and cap

or potential recontamination of the site from adjacent sources. Storm-

induced currents are also of interest in the long-term stability of the site.

However, disposal operations are not conducted during storms, so the de-

signer does not need to consider storm-induced currents during disposal.

Measured current data can be supplemented by estimates for extreme

events using standard techniques; for example, see the Shore Protection

Manual (HQUSACE 1984). Selection of a nondispersive site in a rela-

tively low-energy environment normally results in a site with low bottom-

current velocity and little potential for erosion. However, in some cases,

particularly if the material is hydraulically placed, a thorough analysis of

the potential for resuspension and erosion is necessary. In the analysis of

erosion, the effects of self-armoring due to the winnowing away of finer

particles are a factor that increases erosion resistance over time but is

difficult to quantify.

The same technical approaches used to evaluate erosion potential

and/or magnitude and rate of erosion for purposes of cap design can be

used in screening and/or selecting sites. The process of screening and site

evaluation for erosion potential must consider current and wave conditions

for both ambient and episodic events such as storms. Conventional methods

for analysis of sediment transport can be used to evaluate erosion poten-

tial (Teeter 1988; Dortch et al. 1990). These methods can range from sim-

ple analytical techniques to numerical modeling (Scheffner et al. 1995).

Modeling evaluations will normally result in a varying rate of erosion for

various portions of a site or mounded feature (e.g., erosion would normally

be greater at the crest of a mound or at the corners of a mounded feature).

Erosion criteria for site screening should also be based on both ambient

and episodic events and should account for a varying rate of erosion over

the site. For projects in which no subsequent capping is anticipated for a

long time period (several decades or longer) or for which materials for cap

nourishment are not easily obtained, it is suggested that net cap erosion

over the major portion of the mound or deposit should not exceed 1 ft1 over

a period of 20 years of normal current/wave energies or for a 100-year

extreme event. The recommended criteria of 1 ft of erosion, 20-year ambi-

ent time interval, and 100-year return interval for storms is based on

engineering judgement, a common sense level of conservatism, and field

experience gained to date. One foot is a round number that can be measured

with some precision for most locations. Twenty and one hundred years as

The U.S. customary units of measurement are used in lieu of metric (SI) units for

those cases common in dredging practice. Metric (SI) units are used in this report when

consistent with standard usage. A table to convert from non-SI units of measurement to SI

units can be found on page xiv.

Chapter 4 Site Selection Considerations for Capping