MIL-HDBK-1005/16
is submerged and away from shore. If a pipe has a simple open
end, then better mixing can be attained by installing a diffuser
that induces jet flow from one or more ports. The permit should
include the dilution or mixing effect to help the FOTW to achieve
compliance. For a new discharge, these same issues may arise.
The regulatory agency will often have a major role in requiring
the level of dispersion and mixing necessary to reduce the
potential environmental effects.
9.2.3
Sizing/Capacity. The design of the outfall should be
conducted for peak hour flow at the maximum anticipated stage of
the receiving water. If the receiving water is tidal, then both
high and low conditions should be evaluated. The outfall
pipeline size should follow typical pipeline design guides for
these types of structures. If a diffuser is installed, then
special mixing models are used to select the port sizes and
spacing. While pipelines typically have flows of less than 10
fps (3 m/s), diffuser ports require velocities greater than 10
fps (3 m/s) to achieve jet mixing. The ports on diffusers should
be directed slightly upward so flow does not impinge on the water
body bottom. Port sizes should be 2 inches (5 cm) in diameter or
greater to ensure that the ports will not be clogged by scaling
or barnacles.
Pipes can be made of various materials, but most
outfalls constructed today use either ductile iron or plastic.
Corrosion protection should be considered if applicable (see
Section 12). High-density polyethylene (HDPE) pipe is often used
for submerged outfalls that are buried (see par. 9.2.5). Note
that HDPE is specified by its outer diameter, while other types
of plastic or iron pipe are specified by their inner diameter.
9.2.4
Outfall Depth. Most outfalls are submerged to avoid
visibility and interference with water body usage. Outfalls are
located away from the shoreline to minimize contact with shallow
water benthic organisms and to provide better mixing, as long as
they do not interfere with dredged navigation channels. The
deeper the outfall, the better the mixing. Outfalls need to be
at least 8 feet (2.4 m) deep to provide mixing opportunity.
Consider extending the pipeline to deeper water if an existing
outfall is in shallow water, is having permit compliance issues,
and could be extended. Shallow outfalls do not need to be
modified if the regulatory authorities do not require it.
9.2.5
Outfall Protection. Outfall pipelines should be
anchored in place in the receiving water to prevent movement.
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