Figure 5. Simulation of the longitudinal profile of the March 1986 ice jam in the Middle Bottom area

March 1986 ice jam simulation

Ice-affected water levels - CR95_190018

CR95_19
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Figure 5. Simulation of the longitudinal profile of

the March 1986 ice jam in the Middle Bottom

area.

Using this information the HEC2 computer

water surface elevation at the nearest cross section

program (USACE 1990) was used to calculate the

to this site, 1567.44, was 1868.75 ft msl. Consider-

resulting water surface profile as shown in Figure

ing that no ice-related calibration of the open-

5. The area of zero ice thickness, appearing as

water-verified HEC2 data file was conducted, the

a nearly vertical line through the ice cover near

agreement of the ice jam simulation with the

HEC2 cross section 1552.7, corresponds to the

known water surface elevation is surprisingly

location of the Route 85 bridge. Because the HEC

good.

2 ice option is actually a modification of the stan-

Had we not been able to obtain information on

dard bridge option, ice cannot normally be simu-

the location of the toe of the jam, we would have

lated at cross sections where the bridge code

had little alternative but to assume that a frag-

appears in a data file. Since bridge widths are

mented ice cover existed throughout the river and

quite small relative to the river lengths typically

that it was free to thicken into an equilibrium jam

modeled with HEC2, the absence of ice in the

in response to forces imposed by the flowing wa-

bridge throat has a very localized effect on the

ter. As described earlier, this assumption would

computed water surface profile. Except in the im-

result in the maximum possible water levels for a

mediate vicinity of a bridge, the effect of deleting

given discharge. A simulation of this condition

the ice cover over such a short distance is normal-

was also run (Fig. 6), and the computed water sur-

ly negligible. Another option is to delete the

face elevation in the vicinity of the Borlaug Bridge

bridge from the simulation if the ice effects are de-

was 1870.13. Thus, assuming a fully developed ice

termined to be of greater significance.

jam below the known (or estimated) toe of the jam

In this jam, water reached the elevation of the

resulted in a computed water surface elevation

road at the Borlaug Bridge site in the Middle Bot-

1.38 ft higher than that computed using the known

tom. A Corps of Engineers survey found this loca-

toe location. Areas closer to the toe of the jam

tion, as identified in a photograph, to have an ele-

would have been more severely affected by the

vation of 1868.5. Although it is difficult to deter-

difference in assumptions, while points farther

mine accurately from the figure, the computed

upstream would have seen lesser or no effect.