WSP03817

• File Number: 8102
• Description: Arkansas River Basin Basic Hydrology
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 5/7/1999
• Author: Inter Fluve Inc
• Title: Fluvial Geomorphic Assessment of Upper Arkansas River Final Report
• Water Supply Pro - Doc Type: Report/Study

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In order to assess relative channel stability within each of the Incipient Motion Sites, the
fraction of bed material mobilized at each of the selected recurrence intervals was
estimated. The 1996 bed sampling locations for bars, riffles, and pool features (Appendix
A) coincide with 1M Site 1, 1M Site 2, and 1M Site 5. By comparing the bed material
gradations in each of these features for the different sites, the fraction of bed material
mobilized for a certain hydrologic event can be estimated. The percentage of sediment
mobilized for each recurrence interval discharge, based upon various size distributions
present for the three types of bed material features located in each of the three reaches, is
shown on Figures 7.8, 7.9, and 7.10.

For the 2-year peak discharge at 1M Site I (Geomorphic Subreach 2), particles diameters
of3.6 inches or less, corresponding to 83% of bar material present, will be mobilized.
Approximately 69% of sediment comprising riffle and pool features will be mobilized at
the same discharge. At the 1 O-year flood, bed material less than 4.6 inches in diameter
will be mobilized, which corresponds to 89% of sediment within the bar features, and
82% of the material in riffle and pool features. The 100-year flood is capable of
mobilizing approximately 95% of all material in the various depositional environments,
as bed material particle diameters less than 5.4 inches are mobilized (Figure 7.8). At 1M
Site I, a majority of bed material moves at all significant flood flows. Between the 2-
and 5-year floods, pool and riffle features have mobilization rates close to that of the bar
features, which coincides with the bankfull recurrence interval.

In 1M Site 2, near the Lake Fork confluence, bed material particle sizes mobilized for the
range of peak discharges are smaller than those ofIM Site 1. The 2-year flood is capable
of mobilizing up to 2.3 inch diameter particles, which is approximately 46% of bed
material present within the bar features, 21 % of the riffles, and 58% of the pool substrate.
The 10-year flood can mobilize up to a 3.0 inch particle size, which is represented by
57% of bars material, 34% of riffle features, and 67% of the pool gradation. Under 100-
year flow conditions, 62% of the bar gradation, 41% of the riffle gradation, and 71% of
the pool gradation is less than 3.3 inches in diameter and hence is prone to mobilization
(Figure 7.9), At 1M Site 2, most of the bar and pool gradations are prone to mobilization
near the 1.5-year flood, which approximates the 1.3-year bankfull discharge.

1M Site 5 is characterized by relatively lower sediment mobility. For the 2-year flood,
particle diameters up to 1.7 inches can be mobilized, which correlates to 52% of bar
gradations, 46% of riffle features, and 39% of pool substrate. At the 10-year flood, the
critical grain sized mobilized is 2.1 inches, which reflects 57% of the bar gradation, 54%
of the riffles, and 45% of the pool gradations, The 1 OO-year flood is capable of
mobilizing 64% of bar gradations, which includes all particle sizes less than 2.6 inches in
diameter. Up to 63% and 54% of the riffle and pool features would be potentially
mobilized, respectively (Figure 7.10). At 1M Site 5, a majority of the bar gradation is
mobilized near the 2-year flood, whereas the majority of riffle and pool material
mobilization occurs between the 10- and 25-year flood.

May 7. 1999

Fluvial Geomorphological Assessment
Upper Arkansas River

Page 70