The Corps of Engineers undertook emergency repairs that included: dredging, con-
struction of a rock sill along the gap between the north jetty spur and detached
breakwater, and reconstruction of 61 lineal meters of the damaged breakwater. The
sill was constructed by infilling the scour hole with "C-stone" (quarryrun stone having
0.08 to 0.5 m diameter) to a depth of -6.5 m MLLW followed by a 2-m-thick cover
layer of "B-2 stone" (1.8-tonne stone) to give a final sill elevation of -4.5 m MLLW.
The sill placement is shown on Figure 3. Repairs were completed by May 1995 for
a total estimated cost of US
.16 million as detailed in Hughes and Schwichtenberg
Figure 3: Scour sill and potential toe scour region.
Future Scour Concerns
Placement of the rock sill effectively prevents future scour in the immediate vicinity
of the gap; but the sill also restricts the cross-sectional area of the gap, resulting in
the potential for increased flow velocities over the sill and farther downstream. Past
observations indicated that during normal conditions southerly flow through the gap
between the north jetty spur and detached breakwater would not be strong enough to
cause significant scour downstream of the rock sill. However, there was concern that
during storm conditions a powerful jet would form, causing scour and undermining of
the unprotected detached breakwater toe downstream of the sill. This could lead to
damage of the armor layer on the leeside slope and additional costly repairs. Figure 3
illustrates the potential scour region downstream of the new scour sill.
An estimate of the potential increase of flow velocity through the gap due to the
new sill can be calculated using a simple formulation for equilibrium discharge per
unit width. By assuming the bottom stress caused by a fully turbulent boundary