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Jelrrey W. Schwarz Massey, Semenor(, Schwarz, and Bailey Mobile Pxmix Concrete, Inc. Octobcr 10, 2001 Page 5 capacity is the increased development of channel bars. As channel bars encroach on the main flow of the river, there is an increased tendency for the river to meander. This results in bank erosion and a change in the river's sinuosity. In other words if the channel can't adjust vertically, it will adjust in a horizontal fashion. Channel iV(igration and Analysis o(the Failure of the Bank on May 5, 2001 The State of Colorado, Division of Minerals and Geology (the Division), as stated in their letter of September 14, 2001: believes that the bank jailure ojMay 5, 1001 and the damages to ofj-site properties and to the prevailing hydrologic balance was due to Mobile Premix's (MPC's) jailure to construct the Bul! Seep Relocation in the manner and at the location described in their permit documents. The Division estimated (inspection report 7/5/2001) that the west edge of the Bull Seep Drain Ditch was 60 fee[ from [he east edge of the South Platte River. To accotrtmodate a connection between the Bull Seep and the South Platte on May 5, 2001, a similar magnitude of material would require removal by riverine processes. It was reported that an adjacent landowner (Mr. Ken McIntosh) observed large slabs of concrete placed on the west side of the river bank sliding into the river. Once the concrete slid, the embankment failed rapidly and the historic 1973 channel connection to the Bull Seep Slough was reopened. This reported observation indicates that the bank failure was initiated on the river-side of the fill. Historical evidence indicates that the banks of the South Platte River from the Metro Structure (approximately one (1) mile upstream from the failed bank) downstream to the Brantner Diversion structure have been subjected to considerable stresses. Prior to the 1950 aerial photograph, the main South Platte River channel flowed approximately 1000 feet west of its present location. By 1950 the eastem channel had reopened (reference Figure 2.1) and a split flow condition ensued. By 1964 the western channel was fully abandoned and the eastem channel was the predominant thread. In-channel mining occurred in the area during the late 1970's (reference Figure 2.3) and the eastem channel became more entrenched. By the year 2000, Metro had constmcted a concrete grade control aeration structure at the location of the old abandoned channel. Active bank erosion is ongoing along both on the right (east) and the left (west) bank of the South Platte River below the Metro structure and above the Brantner Diversion stmcture. Much of this bank erosion is the response of the channel to increased sediment deposition along point bars and its increased tendency to meander. At one left bank location approximately 30 feet of bank erosion is reported to have occurred since 1999. Visual review of pre-1999 aerial photographs can identify a cottonwood tree located on the west bank. Based on a field review of current conditions, this tree has collapsed into the stream and the adjacent channel banks have retreated approximately 26 feet (see photos in Figure 2.5). The eroded bank is a "natural channel bank" and was not constructed of silts and loosely consolidated materials. Therefore, one might anticipate that the rate of bank erosion on a native bank will be slower than on a bank constructed of"fill".