30 July 2004
wall and align with and enter the lock chamber. The performance or efficiency of the
upper approach lock guard wall is greatly impacted by two major factors: outdraft
and draw towards the guard wall. Outdraft is defined as the flow in the upper lock
approach that cannot be passed under or through the guard wall, and thereby moves
across the upper lock approach or around the end of the guard wall and towards the
dam. The outdraft, if severe, tends to move the head (bow) of the tow out of
alignment with the guard wall and requires that the pilot increase maneuvering. Draw
towards the wall is defined as the flow in the upper lock approach that moves towards
and under the guard wall towards the dam. Excessive draw towards the guard wall
can cause downbound tows to strike the wall at excessive speeds and could cause
damage to the barges and/or guard wall. Furthermore, if the draw towards the guard
wall is excessive, it could inhibit an upbound tow from departing the lock and
proceeding upstream. Upper approach guard walls should be designed to minimize
the negative impacts that outdraft and draw towards the wall can have on the
navigability of tows entering and leaving the lock chamber. By balancing and
minimizing the outdraft and draw towards the wall, pilot maneuvering for tow entry
and exit can be kept to a minimum, thus decreasing passage time for tows and
providing a safer project.
Generally speaking, there are three basic types of guard walls normally used in upper
lock approaches. These are multi-cell (ported or non-ported), long span, and floating
guard walls. Multi-cell guard walls consist of a series of circular driven-sheet-pile
cells spaced about 50 feet on center with a concrete cap connecting them. The
openings between the cells and below the concrete cap are the ports. The ports in the
guard wall allow the flow in the lock approach to move through and under the guard
wall towards the dam. The flow through the ports can be altered with draft curtains
between each of the cells. Long span guard walls are basically the same as multi-cell
guard walls with span widths typically around 100 to 125 feet. The cap connecting
the cells is normally a pre-cast concrete beam that serves as the rubbing surface.
Draft curtains can also be attached to the beam to regulate flow thru the guard wall.
Floating guard walls are generally a large hollow concrete pontoon. Floating walls
are generally about 30 to 40 feet wide and draft about 10 feet. Flow under these
walls may also be adjusted using draft curtains attached to the bottom of the pontoon.
The approach configuration is based on the lock's design tow size. The appropriate
approach width for the tow must first be determined. A general guideline that can be
used to establish the approach width is to consider an angle of approach to the lock of
12 to 15, as shown in Figure 1.