North Poudre Sediment Study Final Report

• Project Type: Ag/Muni Grant
• Contract/PO #: C153779
• Applicant: North Poudre Irrigation Company
• Project Name: North Fork Poudre Sedimentation Study
• Title: North Fork Cache la Poudre River Sedimentation Study Final Report
• Date: 1/1/1999
• County: Larimer
• Water Conservation - Doc Type: Final Report

Modeling Results

Model estimates of scour or deposition (bed change) were compared with

resurveyed cross sections of the Tick Pool, such as that shown for Cross Section 13

(Figure 4). Because the geometry of the Ouzel Pool, formed largely of a stagnant water

eddy on the right bank of the channel, rendered it less applicable for one-dimensional

modeling, results for the Ouzel Pool are not presented. It is important to note that HEC-6

estimates average bed change for each cross section, and assumes that the bed change,

either scour or deposition, is uniform over the width of the cross section. GST ARS 2.0

does not provide cumulative bed change at each cross section, so vertical change in

thalweg elevation was compared 10 change along the thalweg from the resurveys. Much

of the comparison between models underutilizes the strengths ofGSTARS 2.0 which has

the capability of documenting bed change at every stream tube for every time step.

Flushing Flow 1

HEC-6 De/uult Simulations

Assuming limited tidd data are available for modeling, the default simulations

using HEC-6 were completed, Results indicate that 4-15% of the bed change that

occurred along the thalweg of Cross Sections 11-/6 was predicted by HEC-6 (Simulation

1, Table 3a), In other words. the model uniformly underpredicted the actual scour in the

Tick Pool when all default parameters were selected. By specifYing a moveable bed

limit. which prevents scour in the riffles, and the number of iterations of the governing

equations, 4-22% of the actual bed change within the Tick Pool thalweg was simulated

(Simulation 2), Graphically the results can be depicted as plots through the longitudinal

protile of the Tick Pool (Figure 5),

The default model setting for fine sedimenls allows deposition only, which is one

of the largest limitations of the default modeling. Increasing the amount of input data.

that is allowing for deposition and erosion, and providing all the theoretical information

that is required, provides a 66-170% match between observed and predicted (Simulation

3, Table 3a). Results greater than 100% occur when the model predicts scour below lhe

bedrock elevation. such as at Cross Section 13 (Figure 6),

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