Army’s Design Channel Remains Intact

The Army’s navigation channel through the Sioux City metropolitan area appears to have survived the Great Flood of 2011 with only minimal damage.  Even though powerful currents pushed rocks off riprapped wing dams and knocked askew the wooden poles atop pile dikes, the dikes and revetments continue to hold their positions in the river.  The Mighty Missouri did not completely grind them down and carry them away.  Rather, the training structures are at this moment directing the Missouri’s flow through what the Army calls the “design channel.”

The most noticeable change to Sioux City’s waterfront is the sand left behind by the river.  Tall sand dunes now lie around tree trunks, on top of parking lots, and downstream of the pile dikes.  Although the floodwaters emanated from the Dakota reservoirs, which normally act as silt traps, the flooding Missouri still carried a heavy silt load below Gavin’s Point Dam.  As river levels fell in August and early September, the Missouri’s channel velocity slowed down.  Thus, the Missouri lost the ability to carry the suspended sediment in its waters.  Consequently, it dumped silt all along its course.  The tallest dunes are immediately next to the river.  At Sioux City, the boat ramp at the foot of Hamilton Boulevard is full of silt.  Several feet of creamy white sand has spread across the sloping concrete ramp.  The revetment that runs from the marina to the mouth of the Big Sioux River is also covered with sand.  From a distance, it appears as if the Missouri has scoured away all its rocks.  But upon closer inspection, it is apparent that silt has filled in the gaps between the rocks – creating a smooth, even bank line of soft muck.  The revetment’s otherwise pink quartzite stone has been turned a light yellow.  The mouth of the marina at Sioux City has been completely clogged with sand [see above photograph of the harbor entrance].  A channel, only inches deep and two-feet wide, is all that connects the marina with the main river.  There will be no boats passing in or out of the harbor for some time.  When the river drops to 40,000 cfs in early October, the sand embankment will grow in size, totally shutting off the marina from the river.  The biggest challenge facing Sioux City will be the removal of the deposited sand along the waterfront and in the marina.

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