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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.
E4
Appendix E Evaluation of Mixing in Surface Waters

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