Figure 3. Rill formation is enhanced in frost-susceptible soils as a result of freezethaw cycling.
Our research shows that rill development and sediment erosion by channelized flow on a thawed frost-susceptible soil
are increased between 1.2 and 4.9 times after just one FT cycle (Ferrick and Gatto in press). The precise magnitude of this
increase is directly related to soil moisture. Soil moisture is often high upon thaw and can be equally important in bank
failures. Our observations show substantial variations in soil moisture along a silt bank face and between banks with different
orientations. Total FT cycles, and freezing intensity and duration, also vary with bank orientation (Gatto and Ferrick in
press).
Recommendations
Watersheds that experience ground freezing have greatly enhanced soil erodibility and bank-failure susceptibility
following thaw. Much of the sediment load of upland rivers coincides with snowmelt as a result of overland erosion and bank
failure during the period of increased soil weakness due to freezethaw processes. The sediment load resulting from FT
affects most of the major navigable and non-navigable river systems and their tributaries, including the Mississippi, Missouri,
Illinois, Ohio, Susquehanna, Delaware, Columbia, and Sacramento, for which the Corps has navigation and flood-control
responsibilities. As a result, planning, engineering, and O&M activities that address sediment management should include
1.
Confirmation via FT or frozen ground mapping that FT impacts occur in the project area (see, for example,
Hershfield 1974);
2.
Measurements or estimates of seasonal changes in soil moisture and strength to define FT weakening (one method is
proposed in Gatto and Ferrick [in press]);
3.
Measurements or estimates of the rate of recovery of soil strength to pre-FT conditions (Gatto and Ferrick in press);
and
4.
Measurements or estimates of upland sediment eroded by overland flow and bank sediment that failed immediately
following spring thaw (Gatto and Ferrick in press).
ERDC/CRREL TN-04-2
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