8110

• UCREFRP Catalog Number: 8110
• Author: FLO Engineering, I.
• Title: Green River Flooded Bottomlands Investigation Ouray Wildlife Refuge and Canyonlands National Park, Utah-Final Report.
• USFW Year: 1996.
• USFW - Doc Type: Breckenridge, CO.
• Copyright Material: NO

peak at Green River was 39,000 cfs. The average bankfull discharge at the Ouray reach is now 20,300 cfs with a ..
return period of 3.4 years, which is slightly greater than the mean annual peak. The average bankfull discharge in the ,.,
Canyonlands reach is 39,000 cfs with a return period of IS years. Under pre-l 963 discharges, overbank areas were
flooded one out of every three years, assuming the cross sections have not changed.

The investigation ofbottomlands flooding at Ouray and the backwaters in Canyonlands was conducted
through the hydraulic prediction of water surface using the Corps of Engineers HEC-2 step backwater model. A total
of 36 cross sections in the Ouray study reach and 22 cross sections in the Canyonlands reach were surveyed three
times over the course of the 1995 runoff season hydro graph to create a data base for the HEC-2 model. The results of
the HEC-2 analysis included the prediction of the bankfull discharge and flood inundation area as a function of
discharge. The analysis of flood levels for Canyonlands focused on flooding available side canyon backwater habitat
that is occupied by razorback larval fish during the downstream drift cycle. The Ouray investigation focused on the
potential for inundating historic floodplains with overbank flows.

The results of the Ouray inundation analysis indicate that overbank flooding is initiated at discharges ranging
from 15,800 cfs to 22,700 cfs under existing conditions with levees. The exception is Old Charley Wash, which has a
small side channel which initiates flooding at approximately 13,000 cfs. Currently, some application flooding is also
available with the use of existing inlet structures which are operational at discharges on the order of 3,000 to 4,000
cfs. If levees were removed, inundation may be initiated at most locations with discharges of approximately 18,600
cfs. This discharge corresponds to a return period of 2.6 years and an average annual duration of 4.4 days. Similarly,
if 2 to 3 foot deep side channels were excavated at appropriate locations to connect the bottomlands to the river,
flooding could be initiated at all Ouray bottomlands with a discharge of 13,000 cfs. The 13,000 cfs discharge has a
return period of 1.5 years and an average annual duration of I I days. Through a combination oflevee removal, side
channel excavation and application flooding, it would be practical to flood over 5,400 acres at discharges on the order
of the. 1.5 year return period. This would correspond to significant flooding every two out of three years on the
average and would provide flooding frequency and duration similar to pre- I 963 conditions. The maximum possible
area of inundation is approximately 10,000 acres, which currently occurs for a discharge over 37,000 cfs (100-year ..
event). ,.,

In Canyonlands, flooding of the side canyon back-water areas initiates at a discharge of approximately 7,000
cfs. Above 7,000 cfs, a linear increase in the flooded acreage occurs until bankfull discharge, approximately 39,000
cfs, is reached. There is no optimum discharge for the area of inundation of the Canyonlands backwater habitat; a
higher discharge results in more floodable acreage. At discharges in excess of bankfull, the floodplain would start to
become inundated, resulting in a significant increase in slow velocity, out-of-c,hannel habitat. It was estimated that
500 acres of floodplain became inundated between 30,000 (5-year) and 53,000 cfs (IOO-year).

The opportunity to restore flooded bottomlands habitat through a combination of increased water surface
elevations, prolonged peak flow duration, lower bank or levee heights and constructed inlets is discussed. Restoration
sites must have appropriate flood frequency and duration and also have the ability to be drained to be effective for
endangered fish nursery habitat. Flow augmentation on the order of several thousand cfs during the peak runoff
period would increase the frequency of flooding at Ouray to one out of every two years on the average. Inundation of
flooded bottomlands can be further enhanced at bankfull discharge by removing levees. Finally, construction of inlets
and outlets would allow flooding at discharges less than bankfull. This would increase the frequency of flooding and
the flood duration at many of the Ouray sites. Recommendations on future studies and a discussion of how to proceed
with restoration at Ouray are presented. Construction activities to restore flooding must address channel stability
issues, potential increase in flood levels, sediment deposition, and potential changes in channel morphology.

In Clillyonlands, enhancing the flooding of side canyon backwater areas can be accomplished only through
flow augmentation. It is not practical to restore or enhance flooding through physical alteration of the backwater
habitat areas. Flood enhancement in the backwater areas would result from the increase in the frequency and/or the
duration of inundation. Since the side canyon backwater flooding is initiated as low as 7,000 cfs, increasing the
frequency and duration of inundation through flow augmentation should be investigated. Average bankfull discharge
(39,000 cfs) is almost twice the current mean annual peak discharge and currently only occurs on the average of one ..
out of every fifteen years. Therefore, flow augmentation to achieve overbank flows is probably impractical. ,.,

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