around an existing ice jam site, thereby reduc-
ing the water level behind the jam and conse-
quent flooding. It, therefore, also reduces the
water pressure head exerted on the jam, and
delays jam failure. A bypass channel can be
used alone, as was proposed at Port Jervis,
New York, or in conjunction with an ice control
structure (e.g., Cazenovia Creek structure, Fig.
6c and d).
Constructed of heavy riprap or similar to
timber cribs, spur dikes protrude into the flow
Figure 7. Example of rock-filled crib, Cherryfield, Maine.
to about one-third of the river width. They
serve the same purpose as the timber cribs (Fig.
Modified ice booms
Booms are ice holding devices, such as sub-
marine nets, attached to steel cables anchored on
both banks. For ice breakup control, they resist
Dredging is a jam enhancement method that
the downstream movement of an ice run, pro-
can be used primarily at existing ice jam sites.
viding time for downstream reaches to clear
By deepening the river channel, flow velocities
themselves of ice. They can be used in conjunc-
and slope of the energy grade line are reduced.
tion with an ice control weir or small dam and
The stability of ice jams is increased, thereby
are usually removed at the end of the ice season.
delaying jam blow out (jam release). It also in-
creases local ice storage (Fig. 9).
Rock-filled timber cribs
Approximately 8 to 16 ft at the base, cribs
should protrude about 10 ft above normal water
Such techniques may be useful in controlling
surface elevation. They temporarily hold an ice
ice jamming when flood control or hydropower
jam at an existing ice jam site and delay ice runs.
projects exist on an ice-jam prone river and
Such timber cribs have been built at Cherryfield,
their flow release can be controlled over a suffi-
Maine (Fig. 7).
cient range. Depending on the existing loca-
tions of ice jams with respect to the projects,
and the type of ice jams, project operations can
be altered to mitigate ice jams and their flood-
ing. Output flow at a project can be reduced
A bypass channel is built to carry flow
Figure 8. Spur dikes and rock-filled timber cribs for ice control.
Figure 9. River bed excavation for ice jam enhancement.