ETL 1110-2-347
31 May 93
dynamic and static controls; Zone B describes the
where n = h2/H, the normalized height of the dens-
effects of purely dynamic controls; and Zone C
ity interface. If the densimetric Froude number is
describes the effect of purely static controls. For
equal to or greater than the value shown in
any combination of density interface height and
Equation 6, the dynamic control will be effective.
Froude number which characterizes a particular
channel, the height of a control device can be
(3) Height control device. If a dynamic control
determined. For example, if the normalized density
device of height td is placed on the channel bottom,
interface height n is less than 0.1, any channel
it acts to block the saline wedge and creates a sud-
Froude number will fall into Zone C where any
den constriction, increasing velocities at the barrier
static control device of height t = n will provide
and compressing the boundary layer. The control
device becomes an important factor in density cur-
adequate control. For values of n that are greater
rent shape and must be considered in the relation-
than 0.1 and Froude numbers that are less than or
ship described by Equation 6. Using the control
equal to 0.45, the normalized design height will be
device height, Equation 6 can be reformulated as:
in Zone A or Zone B, combined dynamic and static
devices and purely dynamic devices, respectively.
n) (1
t) (n
t) (2
t
n)
(1
2
(7)
Purely dynamic control devices, Zone B, of design
F0
(1 2t
n)
height 0.1 will provide adequate salinity control for
Froude numbers greater than 0.45 and values of n
greater than 0.1.
where t = td/H is the normalized barrier height.
Utilizing the parameters in Equation 7 for a given
12. Summary
combination of channel densimetric Froude number
and intrusion thickness, the corresponding height of
Both the static and dynamic control devices can be
a dynamic structure for halting intrusion can be
used as methods of halting salinity intrusion. The
predicted using the chart shown in Figure 6. Equa-
advantage of the static control method is that the
tion 7 provides a conservative estimate of the
structure does not require a specifically designed
densimetric Froude number, since assumptions are
shape. The disadvantage is that it relies on the
made concerning a uniform velocity profile over the
height of the density interface which, if sufficiently
control device and any mixing upstream of the
high, could begin to impede navigation or limit
device is neglected. It should also be pointed out
flood passage depending on its location in the chan-
here that for small densimetric Froude numbers,
nel. The advantages of the dynamic controls over
typically less than 0.1 in a river environment, the
the static controls is that a much lower height of the
efficiency of the control device needs further
structure will provide superior performance. The
investigation.
dynamic barrier height requirement will be approxi-
mately 80 percent of a similar static barrier to pro-
11. Saline Wedge Control Design
vide the same performance.
Considerations
There are three zones of design considerations
shown in Figure 6. Zone A is identified by Equa-
tion 7 and describes the combined effects of
1-5
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