WSPC04651

• File Number: 8062
• Description: Federal Water Rights - Colorado Litigation - National Forest ISF Claims - Division 2
• State: CO
• Basin: Statewide
• Date: 9/17/1997
• Title: Materials of Interest - Effective Discharge and the Design of Channel Maintenance Flows for Gravel-Bed Rivers
• Water Supply Pro - Doc Type: Report/Study

002\)92

154 EFFECTIVE DISCHARGE AND CHANNEL.MAINTESANCE FLOWS

regime of strearnflows necessary 10 move the several sizes
of panicles presene in the bed and banks may Dot be
sufficient [0 maimain the aquatic and riparian resources one
might desire to preserve in a given str= teach.
Additional mechanical or fluid dyn3II1.ical processes as well
as biological actIVIty may be essential to fotlIl and maintain
panicular channel resources. Thus, me streamflow regime
thai preserves the magnitude and frequency of transport
rate is a necessary. almough perhaps not a sufficicnt
condition for ID.1intaining all channel resources.

2. CHARACTERISTICS OF RIVER REACHES
STUDIED

The basic hypomesis of the Wolman-Millcr model and
the principle which this study relies upon to design a
regime of cb.annel maintenance flow is that similar river
channels will have similar magnitudes and frequencies of
transport rates. Conversely, differeDces betWeen stream
channels are a consequence. wholly or in pan. of different
magnitudes and frequencies of transport rate. Accordingly,
the appropriate test of this hypothesis musr COncern a group
or class of identifiable, i.e. similar, rivers and streams. For
this study, single-thread gravel-bed rivers with mobile beds
and stable banks have been selected, because they are
common throughout the mountainous areas of the western
United Stales and fre<juently subject to e>>tens!ve flow
regulation and diversion.
Andr&.'s [1984J detennilled the hydraulic geometry
relation of 24 gaging stations located on gravel-bed river.;
in Colorado. Most of the information required to determine
the magnitude and frequency of transport had been
collected at these gaging stations. Eleven streams consid-
elM in the previous study were eliminated for various
reasons, including a relatively short period of record,
appreeiable quantities of sand in the bed-tnaterial, or a
complicated stage-discharge relation affected by the
backwater of a tributary. Four river reaches nOI included
in the previous iDvestigation have been added.
All of the stream reaches studied are in mountainous
pans of Colorndo and are typical of aJluvial gravel-bed
streams throughout the Rocky MOUl1tain region. Several
geomorphic factOr>, including bed and bank tnaterial,
floodplain development and long-term stahility of the
riverbed elevation were considered whel selecting the
study reaches. The channel bed and banks were composed
primarily of sediment transported and deposited by fluvial
action. Many re8l:hes contain a small fraction of vet)'
coar.;e material, including boulders, which did not appear
to be moved by the stream e>>""pt perhaps during the most
c>>treme floods. A well.defined floodplain indicative of the

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bankfull elevation was an essential characteristic of all
reaches selected for study. Although the floodplain was
weJl-<lefmed along all streams, floodplains frequently were
discontinuous and had limited areal e>>tent. In each study
area, three to five cross sections were surveyed in
relatively straight parts of the reach. LongiludiDaI profiles
of the water surface and bankfull elevation were also
surveyed through a reach of approximately 30 cl1ilD11el
widths that included a streamflow.gaging station. The
bankfull discharge of the reach was determined from the
bankfull longitudinal profile and the stage-discharge
relation at the gage. The bankfull hydraulic characteristics
of the several cross sections were calculated using the
surveyed bankfull cross sections, the bankfull discharge,
and mean reach slope. The size distribution of the riverbed
surface was determined by a r:mdOlD sampling method
[Wolman, 1954J. The study reaches and their associated
hydraulic and bed.material transport: characterislics are
summarized in Table I.
Bankfull discharges range from 0.70 to 85.2 m'/sec.
Median bed-material sizes vary from 24 to 91 mm, and
bankfull water surface slopes vary from 0,0014 to 0.26.
Computed mean annual bed-material loads are generally
small relative to the coDtributing drainage area and vary
from 0.02 to 16610nikm'/year.
The seventeen roaches ....ere selected for study based
upon a welI.defined bankfull channel in the vicinity of the
gaging station, as weU as an appreciable period of record
uninterrupted by flow diversion and storage. The narura!
streamflow regime of approximately one-half of the study
reaches has been altered to some degree by flow depletion.
In all instances, these developments have existed for
several decades durmg which the channels will have
adjusted to the altered regime. The period of records
sbowtl in lable 1 refer to a gene:rally constant e>>tent of
streamflow alteration,

3, COMPUTATION OF BED-MATERIAL
TRANSPORT RATE

Parker er at, [1982J formulated an empirical bedload
transport function for poorly-sorted of gravellllld cobbles,
The Parker bedload funCtion is

W', "" 0,0025 exp (14.2 (q,; - I) - 9.28 (q, - I)'];

0.95 < <Pi < 1.65 (la)
and

W', =! 1.2 [1 - (0.822/t;)"'];

01>, > 1.65

(Ib)

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