dynamic and static controls; Zone B describes the

where *n *= *h*2/*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 *t*d 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.

(1

2

(7)

Purely dynamic control devices, Zone B, of design

(1 2*t*

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 *= *t*d/*H *is the normalized barrier height.

Utilizing the parameters in Equation 7 for a given

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-

vide the same performance.

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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