4.1.2 Method of Filling

Figure 4-1. Schematic of supernatant water interaction in an active confined

disposal facility affecting effluent quality

CDFs are typically designed to retain virtually all the solid fraction of dredged

material. However, all solids cannot be retained during the disposal process, and

associated contaminants are transported in dissolved form and with the particles in

the effluent. The only solids in the effluent are typically very fine-grained and are

widely dispersed so that any accumulation on the bottom of the receiving water

body is negligible. Therefore, effluent typically has the potential for water

column effects only, and evaluation of benthic effects related to effluent is usually

not appropriate.

The duration of effluent discharges will roughly correspond to the time

required to complete the dredging operation and may vary from days to months.

Effluent discharges may occur from a few hours per day up to 24 hours per day,

depending on project conditions.

It is important to distinguish intentional release of ponded water during filling

and subsequent management of the CDF from runoff released from the CDF

following precipitation. Precipitation runoff is another contaminant pathway and

will require separate evaluation if there is a reason to believe that contaminants

might be released (Chapter 5).

4.1.2 Method of Filling

The techniques for evaluation of effluent discharges described here are

specifically designed for the case of hydraulic disposal of material into CDFs with

the effluent discharge to receiving waters occurring from an outlet pipe or weir

structure or structures. Hydraulic disposal can be in the form of direct pipeline

inflow from cutterhead or similar hydraulic suction dredges, intermittent hydraulic

placement from hopper dredge pumpout operations, or intermittent hydraulic

placement by reslurrying material from barges (which may have been filled by

mechanical dredges). Such disposal operations would normally have an effluent

discharge flowrate roughly equal to that of the inflow.

Some CDFs may be designed to allow flow of effluent water through filter

cells or permeable dike sections. The techniques described here may be applied to

4-2

Chapter 4

Effluent During Disposal Operations