Potential control measures for erosion

short-term cap is needed for a 50-ft mound, the designers might find a 25-

year erosion thickness; 3.0 ft provides a reasonable tradeoff between risk

and cost.

Another critical factor in selecting a design erosion thickness may be

the cost and difficulty in finding capping material. For example, assume

the project is one where the desire is to place a cap that would ideally

never have to be repaired, or one for which the renourishment interval

would be on the order of decades because of the difficulty and cost in

obtaining additional cap material. For such a project, a fairly long period

erosion thickness, say 100 years, might be selected (perhaps adding some

additional thickness for annual erosion if it is significant). However, if

the cost of such a project becomes too high and capping sand is relatively

available, then a shorter return period thickness, say 30 to 50 years (with-

out adding annual erosion rates), might be more acceptable.

As a starting point, past practice in engineering structure design pro-

vides some guidance. Many Corps projects are designed with 50-year

lives. However, because a capped project is, at least for now, assumed to

require maintenance for a considerably longer time, a 100-year erosion

thickness seems to be a reasonable starting point. First, because of our

limited knowledge of historical storm data, it is difficult to predict with

confidence storm conditions for return periods much greater than 100 to

200 years. Second, providing a cap thickness sufficient to resist storms

with intervals greater than 100 to 200 years would probably be much too

expensive. For projects where additional material is likely to be added in

the near future, a 20-year return erosion thickness seems to be reasonable.

The thickness of the erosion layer should also be capable of withstanding

multiple years of annual erosion; a minimum of 10 years is suggested for

caps designed for a long-term cap.

Additional cap should be placed when the average thickness of the cap

has been reduced such that the design year return period erosion thickness

would also remove some to all of the cap thickness that accounts for bio-

turbation. This is suggested because it is expected that a major storm that

causes significant amounts of erosion will also remove any established

biological community that is able to bioturbate a significant thickness of

material (typically 10 to 20 cm). It is also assumed that the thickness of

cap lost in a major storm will be repaired prior to recolonization by signifi-

cant numbers of organisms that bioturbate to a substantial depth (greater

than 1 year).

Potential control measures for erosion

If cap erosion is considered to be a problem, armoring with larger

diameter material (coarse sand, gravel, riprap) or geotextiles may be con-

sidered as engineering approaches to overcome or protect against this

problem. Procedures for design of caps composed of nonsediment compo-

nents is available in the EPA guidance document for in situ capping projects

(Palermo et al. 1996).

Chapter 8 Long-Term Cap Stability