rameter is used to delineate general conditions for which ice momentum signifi-
cantly affects jam thickness profile. The parameter takes into account initial ice
conditions and the expected flow changes.
CONCLUSIONS
The study led to the following principal conclusions:
1. The flume experiments, which provide a detailed description of the observed
processes of jam formation, failure, and evolution, brought to light two jam failure
and reformation mechanisms--progressive and complete. Progressive jam failure
and reformation happens with lower initial discharge and lower discharge increases
relative to the discharge needed to completely destabilize the jam. It is character-
ized by a smooth progression of a shoving front downstream through the jam. Com-
plete jam failure, then jam reformation, occurred for initial discharges close to the
discharge necessary to completely destabilize the jam. It is characterized by the
entire jam mobilizing en masse, moving downstream, slamming into a downstream
barrier (e.g., an existing, stationary ice cover, or an ice boom), and reforming.
2. The flume experiments revealed that progressive and complete modes of jam
failure and reformation result in measured jam thicknesses that exceeded those
predicted using prior jam formulations, based on analyses of stationary jams. This
finding confirms that the momentum of the moving ice arrested during jam forma-
tion produces an important force that should be taken into account when estimat-
ing jam thickness for many conditions. Prior formulations underestimate jam thick-
ness because they do not include this force, or the interaction of the water and ice.
3. The numerical simulation model provides further quantitative information
illuminating the effects of ice momentum on jam thickness. Not only does ice
momentum result in greater average jam thicknesses than predicted using the sta-
tionary jam theory of prior formulations, it also induces a high degree of
nonuniformity in a jam thickness profile. The interaction of the ice movement and
water flow result in unsteady variations in water depth and velocity, as well as ice
thickness and velocity, throughout the entire simulated flow and jam. It is these
interactive effects that result in nonuniform jam thickness profiles. For this case of
zero ice velocity at the downstream boundary, the profiles are characterized by
greater thickness in the upstream reaches where ice velocity and, thus, momentum
are greatest. The use of a fully coupled solution technique preserves this interac-
tion of the variables. The loosely coupled solution results in some averaging or
smoothing of the variables and their effects upon each other. As the time step
decreases or the number of ice calculation cycles increases, the results of the loosely
coupled model approach those of the fully coupled model.
4. The dimensionless parameter Ω = (a/c)(∆Q/Qin) is useful for delineating con-
ditions when ice momentum should be taken into account for jam thickness esti-
mation. In Ω, a = (fiu2B)/8, and c = gsi(1p)(1si)k0λKpη2, so that (a/c) represents the
portion of the initial jam strength mobilized by the water shear stress on its under-
side. The parameter relates the ratio of average jam thickness, as determined by the
fully coupled model, to steady equilibrium thickness determined from stationary
jam models. The relationship is useful for establishing when changes in flow con-
ditions (i.e., hydrograph properties) will destabilize a jam and, through ice impact,
affect jam thickness profile. The parameter delineates the conditions when a fully
77
Previous Page
