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

(J(j2.J!l'J

diven flow _ With this infonnation, magnitude and
frequency of bed-material transporting discharges can be
calculated directly for the reach of intcrest. The quantity of
bed material supplied to the affected rcach will dcpend
upon the strucrore built to store or divert flow. A reservoir
will usually trap all gravel sized material e.cept where
provisions for sluicing are incorporared into a dam. In
contrast, flow divcrsion strucrures typically rctain
significantly less coarse sediment than a dam.
Formulating a practical operational regime of channel
.maintenance flows from an analysis of the magnirude and
frequency of bed-material transport is not a simple
e"ercise. The ebannel maintenance regime will be part of
a development project wbose primary purpose is to store
and/or divert flo'" from the stream. Such projectS
inevitably involve significant economic, legal, and
operational constraints on the possible instream flow
tegime. A proposed regime of channel maintenance flow
will be evaluated and judged on whether the flows are (I)
efficient and (2) predietable. An efficient regime will
maintain the desired channel characteristics with the
minimum quantity of water possible. The regime of
channel maintenance flows also must be predictable. That
is, the channel maimenance discharge on a given day must
be specified. The numbers of times when mean daily flow
""'as within the range of the dominant bed.material
transponing discharge on a given date over the entire
period of record are shown in Figure 6a.f. for several of
the rivers considered in the study. A5 shown in Figure 6a-
f, eVen during that period each year when discharges are
mosllikely to occur within the range of the dominant bed-
material transporting flows, the probability of occurrence
on any given day is less that 50 percent. For most of the
snoW1ll"lt period, the probability of a dominant bed-
material c:ansPOrting discharge occurri.ng on a given day is
significantly less than 50 p=nt. Consequently, any
chanllel maintewmce regime structured to retain all
streamflows for a specific period of time each year, for
e"ample May IS-May 31, will be inefficient most of the
time, b=e the acmal streamflow is less than the range
of dominant bed-material transporting discharges. Such a
regill1e is predictable; however, it is also inefficient
because the flows retained in the channel during most years
will be insufficient to transpOrt appreciable quantities of
sediment. Altemativecbannel maintenance regimes that ate
more efficient are less predictable. In fact, efficiency and
predictability are in nearly all practical circumstances
mutually e"clusive. For the type of stream considered in
this study, gTavel-bed screams with snowmelt floods. any
channel tnaintenance regime will require a balancing of
efficiency and predictability.

ZIOd

0991 86v OL6:131

ANDREWS AND NANKERVIS 161

As water supplies becoll1e more e"tensively developed
and .competition for aquatic resources intensifies, it seems
ccrtain that greater efficiency will be demanded at the
expense of predictabiliry. Accordingly, operation of a
diversion or Stotage structure will become much more
complc" than is now the common practice. The principal
consequences of this shift will be a need to develop
sophisticated diversion structures which can bypass the
dominant bed-material transporting flows whenever they
occur.
A highly efficient and effective channel maintenance
flow regime would retain the historical magnitude and
frequency of bed-material transport utilizing the least
quantity of waler. One possible channel maintenanee
regime for Middle Boulder Creek would be the natural
flow whenever it is sufficient to move bed.material
particles. In Middle Boulder Creek, bed-material transport
begins at aboUl4.8 m'lsec, whieh is approximately 3 times
the mean annual discharge and one.half of the bankfull
discharge.
The resulting channel maintenance flow regimes are
compared with the namral flow in Figure 7 for 3 years,
1915 average runoff. 1954, below average runoff and
1957. well above average runoff. The channel maintenance
flow regime has essentially the same magnitude and
fre'l"enCY of bed-material tr.mspOrting discharge.
Over the entire period of record the proposed channel
maintenance flows equal 35 percent of the long-term mean
antlual nmoff. During 3 of the 85 years of record, natural
daily mean flows did not e"ceed 3 times the mean annual
discharge, and, therefore, under the proposed regime no
channel maintenance flow would have occurred. Con-
versely, during several of the largest runoff years, the
natural flow acceded 3 times the mean annual flow for
several weeks,

6. SUMMARY AND CONCLUSIONS

I. A method for determining a regime of ehannel
maintenance flo,,",s that will preserve the physical character-
istics of self-formed streams when an appreciable quantity
of the natural flow is diverted or Stored has been formu-
lated. The method relies upon identifying the magnitude
and frequency of bed.material transporting discharges in
the natural channel, and then, retaining these flows
following :my modification of the narura1 flow regime.
2. To demonstrate this method, the magnitude and
frequency of bed-material transporting discharges were
computed for 17 gravel-bed rivers typical throughout the
Rocky Moontain Region. The range of flow which
transported the vast majority of bed-material over a period

3~1^~3S lS3~Od vosn

H:III03MIL6.Ll-d3S