Thursday, February 3, 2011

Icejam Update

What do you get when you add an inch of heavy rainfall to a frozen creekbed?  Check below for the answer.  Icejams are fairly common in mid to late winter along Captina Creek, especially in downstream areas west of Powhatan Point.  Icejams happen when heavy rains raise water levels in frozen streams causing the ice to fracture into blocks of different size which are then pushed downstream by the current.  As chronicled in recent posts, Captina Creek has experienced at least a month of sub-freezing temperatures that sometimes dipped near 0F.  Successive nights with single digit temperatures will result in formation of ice sheets over the creek which then thicken in some cases to 12 inches or greater. 

Ice jams can be hazardous to property along the creek especially when flow rates are above average.  As the blocks move downstream they will begin to accumulate and pile atop each other on bends and obstructions in the creek like bridge crossings.  If the jam becomes large enough, it will begin to back water upstream creating a flood hazard above the jam.  Additionally if a jam suddenly gives way, areas downstream can be flash flooded with little notice.  For this reason a flood warning was issued by the National Weather Service for Captina Creek yesterday primarily for areas east of Town Hill Rd and SR148.   

A view of the floodplain along SR 148 entering Steinersville. 
An ice jam beginning under the bridge near the Clair-Mar golf
course caused water to slowly back upstream into the mouth
of Cat Run.

Ice blocks piled by creek waters on the bank
just upstream from the confluence with Cat Run
next to CR 56.

A closeup view of the jam beginning under the SR148 bridge
near Clair-Mar.
Looking downstream along the golf course.  The entire flood-
plain is packed in ice causing the creek to spill onto some of the

Looking upstream from the same location.  The actual blockage
was further downstream along Cove Rd. where the ice sheet
had remained unbroken over the creek not allowing the
blocks to move downstream.

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