River Hydraulics and Sediment Transport

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SAND TRANSPORT, DEPOSITION, AND EROSION Increasing the supply of mobile sand in the Central Platte River channel can help offset areas of erosion and help build sandbars. Sand transport is measured in tons for the 13- and 61-year period as total sediment transported, net deposition in tons, or cumulative sand augmented. The cumulative sand augmented is a measure of the sand input to the channel through the mechanical island leveling activities in some alternatives. Depositing sediment causes the channel bed to aggrade, while eroding sediment from the bed causes the channel to degrade. A summary of the cumulative sand transport, cumulative sand augmented from land management actions, and the net sand deposition along the Platte River after the First Increment of the Program is presented in table 5-28, and 61 years in table 5-29. The geomorphic study area has been subdivided into five reaches based on the dominant process of aggrading or degrading channel that is occurring in each subdivision. Variances in the performance of each alternative can be better understood when analyzed on a reach-by-reach basis. The first reach from RM 310 to 247 is aggrading for all alternatives with the Water Leasing Alternative depositing the least volume in both the short (13 years) and long term (61 years). The Wet Meadow and Water Emphasis Alternatives deposit the most volume of sand in the short term, and the Governance Committee, Scenario 1, and Wet Meadow Alternatives deposit the most volume of sand in the long term. From RM 247 to RM 234, the reach is degrading and the channel bed is consistently eroding under the Present Condition and all alternatives. The upstream end of this reach coincides with a large volume of irrigation return flow. In the short term (13 years), the alternatives with sand augmentation included in the land plan significantly offset the erosion. Although mechanical aspects of sand augmentation stop at year 13, the augmented sand that may have deposited at different locations in the river by year 13, is still available as an additional source over the long term. The reduction in erosion resulting from sand augmentation is therefore still apparent, but less significant over a 61-year period. Governance Committee Alternative, Scenario 1, which has no sand augmentation, shows the most volume of degradation in the short term exceeding even the Present Condition. In the long term (61 years), the Governance Committee, Scenario 1, and Water Emphasis Alternatives cause the greatest volume of erosion. The Water Leasing Alternative causes the least erosion in the short term (13 years) and the Wet Meadow Alternative causes the least erosion in the long term (61 years). In the next reach downstream, RM 234 to 206, the benefits of the land plan, shared by the Governance Committee, Scenario 2; Water Leasing; and Wet Meadow Alternatives, are apparent in the short term (13 years). This reach is aggrading for the land plan alternatives and degrading under the Present Condition, and the Governance Committee, Scenario 1, and Water Emphasis Alternatives. Over the long term (61 years), the reach is degrading under all alternatives, with the Governance Committee Alternative, Scenario 2, showing the least amount of erosion.