Cherry Creek Sedimentation Study
George K. Cotton, P.E.
Summary of Study
In aggrading stream channels, improvement of the stream environment depends on understanding the sediment budget. This consists of determining the sources of sediment, transport rates, and the location of aggrading and scouring segments of the stream. Planning models can then combine information on basin hydrology, sediment transport, and topography to develop scenarios of basin development and the potential impact to stream health.
Prior to 1940, the hydrology of the Cherry Creek can be inferred from annocdotal information (new paper reports of floods, records from early explorations) and proxy data (e.g. tree ring data). Detailed hydrology records of the upper Cherry Creek basin begin in 1940 (at two gages: Melvin and Franktown), that were installed 10 years prior to the completion of Cherry Creek dam (and six years after the failure of Castlewood Canyon dam). Sedimentation (depletion estimates) studies have been conducted at about 10 years intervals for the reservoir since it was completed in 1950. The sedimentation calibration was based on the period between 1940 and 1965. Calibration was limited by the Melvin gage duration of the record. The gage was moved to the Arapahoe Road bridge in 1960, which is above Pine Creek, and then taken out of operation in 1969. In 1992 the CC-10 gage was put in operation by the Cherry Creek Basin Water Quality Authority at a location just below that of the original Melvin gage. To further complicate the sedimentation history of the reservoir the surveys since 1988 have been flawed and no data has been reported.
From 1940 to 1965, watershed conditions were very dry and the reach below Franktown saw only 1 in 5 days with flow in the Cherry Creek channel. Since 1992, the Cherry Creek has been wetter and base flows have increased significantly. However, the segment of the Cherry Creek below Parker looses flow. This flow loss is the result of water use from the alluvial aquifer in accordance with Colorado water administration on the recapture of return flows. At the same time, tributary inflow is increasing geometrically at a pace that is equivalent to urbanization of the land use.
The sediment transport calibration was based on observations of channel conditions at the Melvin gage made from aerial photographs (1937, 1955 and 1993); sediment surveys in 1950, 1961 and 1965; sediment samples from 1985, 2010 and new samples for the study; daily flow records from the Melvin gage; reconstructed flood 1965 inflow by the COE Omaha District; gage shift history for the Melvin gage; and, current and reconstructed channel geometry near the Melvin gage. Transport functions were restricted to those currently available in HEC-RAS 4 for non-cohesive bed material. Of the six transport functions initially screened, the Laursen (Copeland), Engelund-Hansen, and Ackers-White were selected for detailed evaluation. Ackers-White was found to significantly underpredict sediment inflows, while the Laursen (Copeland) equation was biased on the high end. Engelund-Hansen was selected on the basis of being relatively unbiased and accurate for two sedimentation periods. To support the general use of the calibrated equation, a synthetic data set that was prepared within the range of channel gradients and bed material found in the study reach and a simplified regression equation developed.
The sediment budget and for Cherry Creek channel is being developed for the five mile reach of the Cherry Creek between the Town of Parker and Valley Country Club. This reach maintains stream gradients of about 0.40% + 0.05% and an active channel width of 200 to 300 feet. Calibrated monthly stream flows for the period from 1995 to 2002 was provided by CCBWQA that accounts for the pattern of water use (both surface and alluvial), storm runoff and ground water exchange. The model provides the option of accounting for flow diversions from the Cherry Creek to the new Rueter-Hess reservoir. Results of this analysis show that during the CCBWQA simulation period of eight years, there was a sediment inflow of 1.1 million tons but only 0.5 million tons of sediment is transported to Cherry Creek reservoir. Deposition dominates the reach but some sub-reaches still have scour potential.
Equilibrium conditions will be assessed considering the interplay of four variables (discharge, sediment transport, bed material size, and stream gradient) relative to a stable proxy reach. For this reach, the upper stream segment near the Pine Lane bridge is used as a proxy scaling reach. This reach has modest stabilization measures that have allowed the stream to avoid scour or sedimentation problems that have been observed in other reaches.
Logical scenarios for stream management will need to consider future water resource development in the Cherry Creek and the resulting alluvial aquifer levels, stream base flows, and storm runoff volume. It is expected that management of sediment will be complex and may require the removal of sediment, unless sufficient stream power can be maintained through river training and water operations to transport sediment to the reservoir.
Author Information:
| George Cotton, PE George K Cotton Consulting, Inc. 11612 Pine Grove Lane Parker, CO 80138 Phone: (303) 840-0165 |
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