The implications of naturally occurring ice-related scour and deposition on the success of in-channel contaminated
sediment remediation projects are clear. If jams are known to occur in the vicinity of the project, the remediation design must
consider ice impacts to avoid contaminant dispersal. Conversely, ice-related sediment deposition could be used to reduce
open-water scouring of remediation projects.
Review of hydrometeorological data
On northeastern rivers, fairly steady low flow is typical of the early-to-midwinter freezeup period. Midwinter thaws will
cause spikes in the hydrograph that may or may not result in breakup, depending on various factors, including ice thickness,
ice strength, and initial ice cover freezeup level. With the onset of spring thaw, stage and discharge rise in advance of ice
breakup. At one extreme, the period between the start of rise and ice release may be as short as one day, usually resulting in
dynamic and destructive ice events. Breakups of this type almost always involve significant rainfall, which accelerates
snowmelt and causes rapid runoff into streams and rivers. At the other extreme, a gradual warming trend may prolong the
hydrograph rise period up to several weeks, resulting in an extremely mild, thermally driven breakup.
Important components of a hydrometeorological data review include daily average air temperature, precipitation, and
snowpack data, as well as river stage and discharge data. Ice thickness can be estimated from records of daily average air
temperature and accumulated freezing degree-days (AFDD) (White 2004). Stage and discharge hydrographs, combined with
AFDD-estimated ice thickness, can be used to identify likely dynamic ice breakup events associated with ice jams (Tuthill et
al. 1996, 2003). ERDC-CRREL has assembled historic daily air temperature data for all National Weather Service first-order
weather stations in the United States and calculated the annual series of AFDD from these data.1 If available, historic stage
and discharge hydrographs are extremely useful indicators of the nature and timing of ice breakup. It should be noted that the
accuracy of discharge data is questionable when the rating curve is ice-affected.
Field inspection of study reach
A low-water field inspection of the river supplements the analyses described above. Field observations test initial
assumptions on ice jamming locations, ice source reaches, and channel features and structures, and may offer the best clues to
ice effects on sediment stability. If time and resources allow, a program of field observations during the winter season can be
extremely useful in understanding freezeup and breakup processes.
Considerable research has addressed the effect of open water floods on vegetation and streambank stability (e.g.,
Fischenich and Allen 2000). Though less commonly exploited, the type of bank vegetation or its absence can reveal the
nature and severity of ice action (Ettema and Daly 2004). Gouged or abraded river bank material may also provide evidence
of severe ice runs in the recent past. Upstream of a known ice jam location, eroded gaps in the bank may show where flow
escaped the main channel into the floodplain. Similarly, sections of riverbank may have scoured where overbank flow
returned to the main channel downstream of the ice jam location. Figure 5 shows overbank flooding and sediment deposition
from a 2003 ice jam on the Androscoggin River in Maine.
Ice scars on trees along riverbanks often provide the best evidence of past dynamic ice runs and ice jams. Maximum tree
scar elevations usually result from ice jams, either as they form or release. A single tree can experience multiple ice scarring
events with intervening periods of bark healing. By inspecting the scarred surface of the tree and counting annual growth
rings, one can determine the years that the ice events occurred, and, to some extent, estimate the events' relative severity.
Figure 6 shows an ice-scarred tree and a sawn section therefrom along the Grasse River. Graphical analyses of tree scars
further illustrate historical ice jam elevations and longitudinal extent, as seen in Figure 7.
Conclusions
Ice and sediment interaction processes are not well understood and are not commonly considered in riverine remediation
or channel stabilization projects. Thus, the risk of ice-scour-induced failure of in-channel contaminated sediment remediation
projects and subsequent release of contaminated sediments is real, but often neglected. Evaluation of the potential for ice
impacts on sediment stability is critical for northern rivers that experience ice effects, particularly since industrialization has
taken place for up to several hundred years in some cases. This technical note presents a method to evaluate whether ice
impacts on sediment stability should be considered in the design of a riverine contaminated sediment remediation project. If
1
These data will be available on line in the near future. Contact Steven.F.Daly@erdc.usace.army.mil.
ERDC/CRREL TN-05-1
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