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

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152 EFFECTIVE DISCHARGE AND CHANNEL-MAINTENANCE FLOWS

ch3J1Del and adjacent floodplain. as well as the impairment
of recreational and esthel1C resources commonly occur
fOlloWing subslantial modification to the magnitude and
frequency of natural streamflows.
The need and desire to mi.nirmze or avoid alteration of
the st= channel and floodplain downstream from water
storage and diversion facilities arise in nearly every
install"". Typically, the loss of aquatic and riparian
resources involve significanl COSlS and foregone opportUni-
ties. In many instances, the stream and riparian resources
are subject to state and federal SlalUles, e.g. National
Environmenlal Policy Act of 1969, Clean Water Act of
1972, Endangered Species ACI of 1973, Federal Land
Policy and Management Act of 1976, and N "ional Fotest
M3Mgement Act of 1976. State and federal courtS have
provided water rights for streamflows needed 10 achieve
the objectives for which lands were reserved from the
public domain. i.e. National Parks, Indian Reservalions.
etc. Thus. the design and operation of streamflow storage
and diversion facilities invariably will include an analysis
of the relation belween flows and rhe nature of rhe Slteam
channel and floodplain. That is, what quantity and
distribution of streamflows thrnughoul rhe y= are
required, together with a given contribution of sediment, to
COllStruct and maintain rhe channel and riparian resoun:es
of a particular reach? (See Roscen et al. 1986 and Reiser
er al. 1989 for examples of instream flow regimes
formulated to maintain desired channel features.)
Conver.;ely. what portion of rhe narurnl streamflow may be
stored or divened without causing an un:!CCeprable
impairment of fluvial resources? These questiollS conc.:rn
rhe fundamental issues of fluvial geomorphology and river
mechanics. Clearly, the answers will have great practical
significance in the management of waler resources.
The concept of a dominant discharge has existed for
deeades in the hydrologie Iiterarure. The design of canal
channels which would carry a specific discharge and a
sediment load of silt. clay and fine sand without deposition
or erosion was studied intensively in the 19th century,
[A.ckers. 1972] _ The relation between discharge and the
characteristics of a natural cnannel are much more
complex. uOPCJ(d turd MaddCJck [1953]. Stream discharge
varies over a collSiderable range from extreme low flow to
extreme flood. In addition. the range of sediment panicle
sizes in a naturally stream channel is typieaIly much
greater. The characteristics of naturally fonned stream
channels reflect this variability. Thus. One must eOllSider a
range and frequency of occurrence for the channel-forming
flows rather rhao a single discharge.
WolTTlilll and Miller [1960] developed a conceprual model
to describe the influence of flow magnitude and frequency

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Effective discharge

DISCHARGE

Fig.!. Wolman-Miller model showing lbe rd.tive importance of
magnimde and frequency of sU'eamflows_

on the relative sediment transpOrTing effectiveness of
various discharges in narura! channels. This model has
become one of the fundamental paradigms of geomorphol-
ogy. The Wolman-Miller tIIodel is illustrated by Figure 1.
The variation in sediment transpOrT with discharge is
represented by curve A. Sediment transpOrT begins when
discharge artains a threshold value, and then inereases
rapidly and continuously _ The threshold discharge depends
to a substantial degree on the size of sediment panicles
composing !be channel bed and banks. Relatively small
sediment panicles. silt and fme sand, will be transponed to
some eXlent even at very small discharge in most river
channels. Conversely. relatively large sediment partides,
gravel and boulders, rypically will only be moved by the
quite large and uncommon discharges. Because Iranspoll
rate is strongly influeneed by panicle size. it is usually
desirable to eaIculate separate sediment transport relariollS
for each particle size class. The frequency of discharges in
a given s!ream reach is described by curve B, which
represents all discharges from extreme low flow to extreme
flood. The produet of these two re1arions gives a !bird
eurve. C, which is the frequency distribution of seditnenl
tnUlSpott, i.e. the percent of time a specifie sediment
transpon rate has or will occur. Integrating the frequency
of sediment transpon rates over a period of time, typieally
one year, equals rhe lotal quantity of sediment transponed.
The principal result of this model is that most of the
sedimenl transpOrTed over a period of time is carried by a
range of intermediate discharges. Discharges less thalI the
threshold required for initiation of sediment motion
rnnspon no sediment. Discharges only somewhat greater
than the threshold discharge may be quite common, but
transport sediment at such a small rate thai they cartY only

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