NANFA-- Dam bypass

Fritz Rohde (
Mon, 18 Dec 2000 12:05:30 -0500

There is an alternative to typical fish passage structures on low-head
dams that has been effective in Minnesota and in Europe. It's the
construction of "rock arch rapids" structures. This is basically a rock
ramp (at a slope of 20' to every foot of elevation) that leads up to the
lip of the dam with a series of V-shaped groins with larger rocks
(boulders) that provide the fish a refuge on their movement upstream
over the dam. The currents are reduced close to the surface of the rock
which fish can handle. The Cape Fear River in southeastern North
Carolina is home to a number of anadromous fishes, including 2 species
of sturgeons, shad, and striped basss. For many years their upstream
migrations have been hindered/thwarted by three low-level dams on the
lower river. The USACOE here in Wilmington, North Carolina has tried
several techniques to facilitate upstream movement - fish ladders (not
effective!) and locking the fish through the lock chambers (somewhat
effective). There have been a number of proposals for their removal
since they no longer serve to help commercial traffic. Instead the
USACOE is funding the construction of one of these rock rubble ramps on
the lowermost dam to be built this summer. They will be funding several
scientific studies to see if the anadromous fishes as well as resident
fishes will use this structure to get over the dam. Stay tuned for
further developments.

Fritz Rohde
NC Marine Fisheries

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