ETL 1110-2-347
31 May 93
CONTROL METHODS FOR SALINITY INTRUSION IN WELL
STRATIFIED ESTUARIES AND WATERWAYS
1. Criteria outlined
channel where the velocity profile is strongly
affected by boundary shear is called the boundary
This engineer technical letter (ETL) outlines criteria
layer. The thickness of the boundary layer can be
that have been experimentally derived for use in
characterized by two parameters: the displacement
determination of the minimum height of a control
thickness
device used in preventing saltwater intrusion beyond
H
u
the control location. The guidance is based on
⌠ 1
δd
(1)
1
dz
⌡ H
laboratory experiments.
U
o
2. Lab experiments determine design
criteria
and the momentum thickness
The laboratory experiments were concerned with the
H
u
development of design criteria for engineering
⌠ 1
u
δm
(2)
1
dz
⌡ H
control structures to limit intrusion of salt water into
U
U
o
stratified estuaries and coastal waterways. Stratified
estuarine flow conditions arise when mixing forces
(tides and wind effects) are relatively weak with
respect to stratifying forces (a large flow of low-
where U = qf / H is the mean velocity
salinity water into a high-salinity water body).
6. Saline Wedge
3. Idealized condition
If the waterway flows to a saltwater bay or ocean
In an idealized steady-state condition, the ocean salt
with a density ps , the intrusion of salt in the water-
water intrudes, in the form of a salt wedge,
way occurs during the upstream movement of a
upstream against the direction of the overlying
definable saline layer underlying the freshwater
freshwater flow. The salt wedge creates a sharp
flow. This is called a saline wedge. With constant
interface separating the overlying freshwater and the
freshwater flow, water depth, and saltwater concen-
underlying saltwater layers.
trations, an arrested saline wedge forms. These
4. Control measures
In periods of lower freshwater flow, the salt water
can intrude against the freshwater flow for very
long distances, causing adverse economic and eco-
logical consequences. A control structure placed in
the path of the advancing salt wedge to disrupt the
balance of internal forces present in the flow can
limit the upstream movement of salt water beyond
the control point.
5. Boundary Layer Thickness
The freshwater flow within a waterway will have
density of pf and a discharge of Qf. Assuming that
the channel is of a uniform width B and depth H,
the discharge per unit width is qf = Qf / (BH). For
Figure 1. Velocity distribution profile of
practical situations, it is assumed that equilibrium
approach flow
exists and the steady profile resembles that shown
in Figure 1. The fluid layer near the bottom of the
Enclosure 1
1-1
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