ETL 1110-2-347
31 May 93
form of control. The buoyancy forces of the saline
structure to block an advancing saltwater wedge for
protection of industrial and municipal fresh water
wedge are sufficient to cause overtopping of the
supply areas. This type of control structure pro-
device with height equal to z2. Therefore, the opti-
vides static control of the advancing wedge. It is
mum height of the device is greater than z2 but less
basically a bottom barrier having an arbitrary shape
than h2, where z2 and h2 have been calculated by
but higher than the height of the local wedge (the
an appropriate method.
height of the density interface in Figure 2). For this
type of structure any shape can work, including a
c. Dynamic control. Dynamic control offers a
wide, broad-crested, underwater barrier similar to
more efficient method to limit salinity intrusion.
that shown in Figure 3.
The typical hydrodynamics of the saline wedge and
freshwater flow are shown in Figure 4. The
(1) This type of structure was used in the
approaching freshwater flow profile has a new-bed
Mississippi River in 1988 during a low river dis-
boundary layer of flow speed smaller than the main
charge period to block saltwater intrusion from the
flow speed. This area of deficient velocity allows a
Gulf and protect freshwater intakes for the city of
saline wedge to more easily intrude along the bed
New Orleans. The underwater dam or sill was
of a channel. The concept of dynamic control,
constructed of dredged material from an area
according to Jirka and Arita (1987), is to introduce
upstream of the barrier location. The sill was con-
into the path of the saline wedge an obstruction that
structed quickly and inexpensively provided ade-
both blocks the wedge and provides a sudden con-
quate control of the saltwater intrusion (Soileau,
striction of the approach flow, altering the near-
Garrett, and Thibodeaux 1989).
bottom boundary layer and eliminating the zone of
low flow speed. The typical ambient velocity dis-
(2) The basic requirement of this type of con-
tribution profile of the approach flow, shown in
trol method is that the barrier must be constructed
Figures 1 and 4, depicts the zone of low velocity at
of a height that is equal to or greater than the inter-
the near-bottom boundary layer. The end result of
face height h2 as shown in Figure 3. An alternative
the dynamic control process is to produce a wedge
to this would be to make the structure equal to the
shape similar to that depicted in Figure 5. Experi-
height of the zero velocity line z2 since the flow of
mental results from the study by Jirka and Suther-
the advancing saltwater wedge occurs up to this
land (in preparation) have shown that the average
height. A structure of this height can provide ade-
ratio of dynamic-to-static control device height is
quate blockage of the saltwater supply and eliminate
0.8. Therefore, the height of the dynamic control
the intrusion of the wedge. Therefore, the required
device needs only to be 80 percent of the height of
static control barrier height td would be equal to z2,
a static device to achieve the same performance in
the zero velocity point in a somewhat less conserva-
controlling salinity intrusion.
tive design.
(1) Preliminary design criteria. A variety of
(3) Extensive laboratory research (Jirka and
devices can be used to produce the desired result.
Sutherland in preparation) provides accurate design
These devices range from structures resembling a
criteria for the heights of the static control barriers.
sudden-step barrier or sharp-edged barrier to an
The experiments determined that the tentative
elaborate method of suction at the crest of the
design height for which control is obtained can be
barrier. The basic criterion for dynamic control is
expressed as:
that the height of the device or obstruction td,
placed in the channel to arrest the saline wedge, be
td ≥ 1.2z2
(4)
on the order of the dimension of the boundary layer
thickness. For a typical velocity profile, similar to
that shown in Figure 1, both the boundary momen-
and
tum thickness and the displacement thickness are
estimated to be approximately equal to one-tenth of
td ≥ 0.7h2
(5)
the water depth (H).
(2) Density current shape. In addition to the
These equations clearly point out that simply block-
height of the dynamic control barrier, local forces
ing the movement of the wedge is not a sufficient
must balance to maintain the shape of the density
1-3
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