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A Historical Perspective
on
Regional
Channel Geometry Curves
William W. Emmett
Regional channel geometry curves are now
being developed in many parts of the United States
and some understanding of the early data used to develop the original
curves may be useful. Channel geometry and bankfull
discharge concepts had their origin in the 1950s. A little
background from the era when bankfull discharge was first being determined and bankfull values of hydraulic and channel geometry were first
being utilized, is helpful in understanding what early data may
be useful in the preparation of present day "regional curves."
As a general
rule, because channels are constantly changing to maintain equilibrium
with the water and sediment being supplied to them, historical data
may have limited utility in developing regional curves. For
example, in relatively stable environments with slow climate change,
30-year-old data may be useful, while in environments with rapid
land-use change, 3-year-old data may be useless.
In the late
1950s, Luna Leopold, then Chief Hydrologist of the U.S. Geological
Survey, requested that some USGS Districts conduct bankfull
channel-geometry surveys. A total of 13 surveys were made,
from Tennessee and Kentucky to Massachusetts and the Dakotas. Referred
to as the "original 13 surveys," these sites provided data for the
dimensionless rating curve first published in Emmett and Leopold
(1963), repeated as Figure 7-10 in Leopold, Wolman,
and Miler (1964) where the data are captioned as “eastern half of
United States,” and repeated as Figure 16-37 in Dunne and Leopold
(1978) where the data are captioned as “eastern United States.”
A listing of the gaging stations is provided
in Emmett and Leopold (1963), an internal USGS publication, but
these data were not further used in preparation of distinct "regional
curves," so this reference to Eastern United States has no connection
to Eastern United States as used in some regional curves.
In the early 1960s, some USGS personnel provided site-inspection
data of bankfull stage (published as Table
7-13 in Leopold, Wolman, and Miller [1964]),
but these data were not further used in preparation of distinct
"regional curves."
The first comprehensive
set of bankfull values of hydraulic and
channel geometry is the early 1970s data provided in Emmett’s 1975
publication, “The Channels and Waters of the Upper Salmon River
Area.” These data are almost always referred as "Idaho," "Upper
Salmon River, Idaho,"or "Emmett, 1975."
Because much of this area is pristine, these data should still be
valid.
In the mid-1970s, Leopold assembled bankfull values of hydraulic and channel geometry near his
residence at Pinedale, Wyoming;
these data are referred as "Upper Green River, Wyoming," generally
cited as Dunne and Leopold (1978), and these data should still be
valid. Some site information of the Upper
Green River
data is provided in Dunne and Leopold (1978) and use of both the
Upper
Salmon River
data and the Upper
Green River data is sprinkled liberally throughout
the book.
Leopold also assembled other data sets;
these were used in Dunne and Leopold (1978) as (a) "San Francisco Bay region" and (b) either "Eastern
United States,"
"Southeastern
Pennsylvania,"
or "Pennsylvania.”
Data for San Francisco Bay were from near Leopold's residence in
Berkeley (a listing of sites was not separately maintained), and
data variously labeled as for Eastern United States or Pennsylvania
came from our channel-geometry files compiled from eastern sites
while we were residents of Washington, D.C., and from data from
the Brandywine area of Pennsylvania (an exact listing of sites was
not maintained).
These west and east coast sites from Dunne
and Leopold (1978) are very insightful, giving a comparison of rainfall-runoff
channels to the snowmelt-runoff channels of Idaho and Wyoming.
Because of the lack of specificity of sites, the west and east coast
sites cannot be further used in preparation of present-day regional
curves; however, the published curves should show similarity to
curves developed for sites in the generalized west or east coast
areas.
The most important
criteria in developing any type of bankfull
relations are the correct field identification of bankfull
(floodplain) and the surveying procedure for determining bankfull stage. It has long been recognized that a single
cross section is inadequate in determining the floodplain elevation.
That's why the original USGS channel-geometry instructions (USGS,
1959), Leopold, Wolman, and Miller’s classic
1964 book on fluvial processes, the first detailed field work (pages
35-36, (Emmett, 1975), and the oft-cited procedure (pages 653-655,
Dunne and Leopold, 1978), all state the necessity of a longitudinal
profile of the floodplain.
Where the longitudinal
profile passes a given cross-section, that's the elevation of bankfull at the section; better yet when there's a gage, where
the long profile passes the gage section, the stage reading of the
longitudinal profile is bankfull stage.
Absent a longitudinal profile, each cross section is likely to have
a unique value of bankfull discharge.
And we know that when we have many cross sections, we really have
but one bankfull discharge, not a different one for each section.
The use of
a single cross section became so pervasive that it led to mentioning
techniques of channel-geometry surveys in two important Forest Service
publications: (1) “Stream Channel Reference Sites: An Illustrated
Guide to Field Techniques” (Harrelson et. al. 1994), and (2) “A
Guide to Field Identification of Bankfull Stage in the Western United States” (USFS 1994).
I played a role in both of these efforts, largely to ensure that
we begin to do things consistently.
Proper identification
of bankfull stage is critical to the development
of channel geometry curves. Bankfull
stage is that stage at which stream water just begins to overtop
the floodplain; the floodplain is defined as that relatively flat,
depositional surface adjacent to the stream that is being built
and rebuilt by the stream in the present hydrologic regime.
This definition separates the floodplain from within-channel bars
and berms, and from nearby terraces.
Bankfull identification is further complicated
when there's improper use of so-called "bankfull indicators." I am not enamored with the often
used phrase "...indicators to take the place of the floodplain,"
but am not opposed to the phrase ".... indicators which help us
find the floodplain."
A river channel
is formed by a range of flows. The bankfull
discharge is a surrogate for this range of flows. The recent
video by the U.S. Forest Service (2003), “Identifying Bankfull
Stage in Forested Streams in the Eastern United States”, in which
I was the technical coordinator, emphasizes these latter points.
Copies are available from the Stream Systems Technology Center
upon request. Use it in your endeavor to correctly identify
bankfull stage in the field and to develop meaningful regional
channel geometry curves.
References
Dunne, T. and L.B. Leopold, 1978.
Water in Environmental Planning.
W.H. Freeman, New York, 818p.
Emmett, W. W., 1975. The
channels and waters of the upper Salmon River Area, Idaho. U.
S. Geological Survey Professional Paper 870-A, p. 1-116 and i-viii.
Emmett, W. W. and Leopold, L. B., 1963.
A dimensionless rating curve.
U. S. Geological Survey, Water Resources
Division Bulletin, August, 1963, p. 22-24.
Harrelson, C.C., C. Rawlins, and J.
Potyondy, 1994.
Stream Channel Reference Sites: An Illustrated Guide to Field
Techniques. USDA Forest Service Rocky Mountain Forest and Range Experiment
Station General Technical Report RM-245, 67 p.
Leopold, L.B., M.G. Wolman, and J.P. Miller, 1964. Fluvial Processes in Geomorphology. W. H. Freeman and
Company, San Francisco, 522 p.
U. S.
Geological Survey, 1959. Surface Water Branch Memorandum 60.12,
August 13, 1959.
USDA Forest Service (Leopold,
L. B., Emmett, W. W., Silvey, H. L., and
Rosgen, D. L.), 1994. A guide to field identification of bankfull stage in the western United States. Stream Systems Technology Center, VHS video, 31 minutes.
USDA Forest Service (Wolman, M.G, Emmett, W. W., Verry,
E.S., Marion, D.A., Swift, L.W., Jr., Kappesser,
G.B.), 2003. Identifying
Bankfull Stage in Forested Streams in
the Eastern
United States. Stream Systems Technology Center, VHS video, 46 minutes.
Bill Emmett, formerly Research
Hydrologist, U.S. Geological Survey, is presently a consulting
hydrologist in Littleton, CO;
w.emmett@worldnet.att.net; (303) 795-7510.
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