Figure 16. Volume of ice in the jam retained by the cylindrical-pier
ICS as a function of river discharge. Transport losses reduce 10,000,000
ft3 pre-breakup ice supply to an initial jam volume of 7,000,000 ft3 at 2000
cfs. Ice melting, plus washouts through the ICS above 8000 cfs, reduce
ice-jam volume as discharge increases throughout an event.
as one-dimensional, steady, and gradually varied (spa-
at Mill Road and Leydecker Road. We input the 5-ft-
diameter 10-ft-tall 12-ft-gap cylindrical-pier ICS
tially), as did its predecessor HEC-2. At each cross sec-
tion, it can include an ice cover of known thickness or
using the HEC-RAS feature "multiple blocked obstruc-
solve for the thickness of an ice jam according to stan-
tions."
dard theory (U.S. Army 1998b). The latter is a steady-
All model runs used subcritical flow. The down-
state theory that treats the ice as a granular material with
stream rating curve derived from the output of the HEC-
no cohesion. The program solves the one-dimensional
2 deck (RS 308.00, U.S. Army 1986b). Several cali-
force balance for the jam, where the under-ice water
bration flows were used to compare computed water-
shear and jam self-weight are resisted by the shear
surface elevations (WSE's) with physical-model and
strength of ice at the banks. The ice jam floats on the
field data. Final runs were made using the 100-year (1%-
water and adds a rough top surface, resulting in water
exceedence) open-water flow of 15,600 cfs and ice-jam
levels that are much higher than for open water at the
flows of 400013,000 cfs.
same discharge. The user can constrain the jam to the
main channel (appropriate for treed floodplains) and