2006-11-13_REPORT - M1986015 - Laserfiche WebLink

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It appears that the smaller overflow entered either just before ox shortly after the main breach. The smaller overflow produced a deep gash in the south wall that cut back all the way to the small rill (cxoss- channel) that was carrying water from the main cross-channel. But the depth of the cut was limited to about half the total depth of the pit. So it is pretty clear that if the small overflow was first it happened only minutes before the main breach. On the other hand, it could have occurred at just about the same time as the main breach but perhaps a couple of minutes after. Changes in a flash flood can occur very fast and the exact sequence of events is often hard to determine. The size of the gash in the smaller overflow would seem to indicate it occurred a few minutes before the main breach and when the main breach occurred, which probably filled the pit very quickly, the lake rose above the inflow elevation of the smaller overflow and stopped the erosion in that area. Thus, although the smaller overflow cut back quite a ways its depth was limited by the main breach and the rapid filling of the pit with water. Once the river flow went down to the point where repairs could begin, the contour of the breach was determined by plumb lining the water from a small boat. This provided an estimate of the volume of material needed to fill the breach. It also showed that the breach was not all the way to the full depth of the pit. This led to the conclusion that the breach was a blend of bank undercutting followed by headward erosion with an overflow. That pattern would be consistent with a flash flood pattern which usually cuts the bank first and then overflows into adjacent land. A breach caused by only overflow and headward erosion would probably have resulted ut a much deeper cut than occurred in this event. Nevertheless, the volume of fill needed to plug the hole was considerable. Backfilling began after consulting with the Corps of Engineers and the Division. Although it was difficult work due to the river rising and falling most every day, eventually it was repaired, but not quite fuushed with respect to achieving the thickness needed. Then the second flood occurred which removed about three- fouxths of the repair and filled the pit again. This second flood has similar characteristics to the first flood and, once again, the mid-channel sand bar was implicated as a major player in the second breach. The pit had been almost pumped down and the initial breach plug was holding well. Thus, the work had to begin again. By early September the breach was thoroughly plugged and dewatering began again. It took nearly two weeks to pump out the lake. For much of this time the operation was essentially closed. However, a new pit was begun to the north of the pit that was a lake. The dewatering from this new excavation went into the lake which was essentially the river until the breach repairs were completed. But subsequent flooding intertnittenfly closed this new shallow pit and even further expanded the exposed water and the size of the lake. The new pit though was shallow and easily pumped out once the water in the main lake went down. After the fast flood event of the sutxuner, examination of other accessible locations along the river not associated with the operation showed localized damages elsewhere, but also showed that streambank stabilization installed after the 1999 flood was secure. In fact, at the two locations where stabilization had occurred, both of which were subjected to far more intense forces from the 2006 floods than those that caused the breach, were in such good condition it was hard to tell a flood had even occurred. Keep in mind that the pit breach occurred on a normally low impact portion of the downstream bend of a meander; this is the very place where breaching should be least likely to occur. At the stabilized locations, other than fresh debris and fresh mud lines on the trunks of woody vegetation, it did not look like anything had happened. It is not known whether the breach could have been prevented if these structures had been in place on the downstream side of the meander, but examination of the locations where these structures were in place showed the overflow was mild and gentle rather than intense. Certainly if these stmctures had been in place undercutting of the bank and thinning of the setback would not have occurred and if overflow had happened it is less likely the breach would have been so easily created. The pit might have still filled with water, but it is less likely a physical connection to the river would have occurred. The installation of the structures were at locations that were clearly the most likely places for major overflowing and bank cutting to occur -the upstream side of the meander. It was considered the least likely scenario that the pit would be breached on the downstream side of the meander as the major forces of the bend in the river on the downstream side would impact streambanks further Transit Mix of Pueblo 2006 Annual Report M-1986-015 Pueblo East Pit Page 2 of 3