E.1.8 Surface runoff discharge

less than 100 mg/L for sediments dredged from estuarine environments and less

than a few grams per liter for sediments dredged from freshwater environments.

Since effluent is normally discharged from a hydraulically filled CDF over a

time period of weeks while dredged material is being disposed in the CDF,

the discharge can be assumed continuous for purposes of mixing-zone

calculation.

E.1.8 Surface runoff discharge

Runoff flowrate from a CDF is a function of the site conditions prior to a

precipitation event, the intensity and duration of the precipitation event, and the

degree to which water is controlled by ponding during and immediately

following the precipitation event. Discharges of surface runoff normally occur

over a period of days following an event. However, in northern latitudes here

may be no runoff for long periods during freezing temperatures, followed by

high runoff over a relatively short period during thawing.

E.2 Applicability of Models and Techniques

E.2.1 General considerations

General considerations for applicability of models for a variety of

discharges, including discrete barge and hopper discharges, are discussed in the

Inland Testing Manual (ITM). Only those considerations applicable to CDF

discharges are discussed here.

E.2.2 Considerations for tidally influenced rivers and estuaries

The assumptions necessary for evaluation of mixing are more difficult to

satisfy in estuaries and the tidally influenced portions of rivers. The assumption

that velocities in the water body near the mixing zone can be represented by a

single mean velocity parallel to the bank is usually a reasonable one in the

nontidally influenced portion of a river. However, it is not always acceptable in

estuaries. Typically the downstream section of an estuary exhibits horizontal

circulation patterns, so that the horizontal water velocity and direction vary with

distance parallel to the bank, distance perpendicular to the bank, and time.

Under these conditions, water near the mixing zone may not always travel

parallel to the bank. Therefore, simple mixing-zone equations may not be

applicable to the wide, open low-velocity sections of estuaries.

Also, mixing-zone equations are not theoretically applicable as the mean

velocity tends to zero. This is because the equations are dependent upon the

process of advection, which does not exist in the absence of a flow velocity, and

also because the primary source of dispersion is assumed to be the turbulence

caused by the horizontal movement of water. However, in a real water body, as

the velocity tends to zero, the primary sources of turbulence and dispersion are

the wind and waves.

Appendix E Evaluation of Mixing in Surface Waters