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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6.3.3. Channel Migration Rates and Cutoffs

Migration rates were measured on channel segments that displayed significant bankline
retreat during the time periods 1939-1956 and 1956-1996 (Figure 6.3). The rates
measured from 1939-1956 ranged from approximately 2 to 5 ft/yr. From 1956 to 1996,
however, six measured sites migrated between 5 and 13 ft/yr. Although the data set is
limited, the range in migration rate measurements indicates that since 1956, average
bankline migration rates have increased.

Bendway cutoffs were also measured for the same time periods. The radius of curvature
of a bend is a measure of the radius of an arc drawn through the bend; as the radius of
curvature decreases, the bend becomes tighter. As a bend tightens, it is naturally more
prone to cutoff due to its decreasing hydraulic efficiency, which causes deposition and
loss of cross-section area on the upstream limb. In coarse meandering rivers, bend way
migration rates are most rapid at a Rc/W value of approximately 2.5 (Nanson and Hicken,
1986; Harvey, 1989). Below that value, bends tend to be prone to cutoff. Data from the
project reach indicate that bendway cutoffs occurred at Rc/W values ranging from 2 to
3.5 during the 1939-1956 time period; between 1956 and 1996, measured cutoff Rc/W
values ranged from I to 4.5 (Figure 6.4). The relatively high Rc/W cutoff values for the
project reach relative to published data indicates the high sediment loads and changing
hydrology within the project reach and effects of local deposition ,md channel relocation.
The trend in the data suggests that the bendways cut off over a much wider range of
values during the 1959-1997 time period, which is indicative of rapid planform
adjustment relative to the 1939-1956 time period.

6.4. Effective Discharge and Morphologic Change

The assessment of channel response to changing sediment loads is based on a general
assumption that loads increased during active mining, and have progressively decreased
upon sediment control implementation on the California Gulch. Similarly, grazing has
been introduced to the system, but a detailed assessment of the timing and intensities of
grazing practices was not performed for this project. Field evidence and aerial
photography analysis indicate that both impacts have contributed to channel widening in
the project reach over the past 50 yrs. Flow augmentations can be similarly correlated to
progressive channel widening, and the availability of hydrologic data allow a more
detailed assessment of that impact.

Geomorphic adjustments of river channels reflect periods of active sediment transport. In
general, higher flows are capable of transporting larger sediment, although the relative
intrequency of those events limits their ability to drive channel adjustment. The concept
of "effective discharge" refers to a flow condition that is primarily responsible for
channel formation due to its combined transport ability and rate of occurrence. In stable
river systems, effective discharge is generally correlative to bankfull flow, which is that
discharge that fills the channel to the top of its banks, and thus is primarily responsible
for channel form. In unstable settings, however, channel adjustment makes bankfull flow
indicators suspect. Researchers have estimated that effective discharge within Rocky
Mountain gravel bed rivers is on the order of the 4% flow duration, or that flow which
occurs, on average 15 days out of the year (Andrews and Nankervis, 1995).

May 7, 1999

Fluvial Geomorphological Assessmen/
Upper Arkansas River

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