Assignment 2/3
Fluvial Geomorphology
Due September 24, 2012
Field determination of the bankfull channel and bankfull discharge on a singlethread meandering channel
September 15, 2012
Big Creek near Randolph (USGS gage 10023000)
Background:
“Bankfull stage” and bankfull discharge” are terms fundamental to the vocabulary of
fluvial geomorphology. The purpose of this exercise is to teach you the procedure for
determining bankfull stage and bankfull discharge of a channel based on the availability
of information available at an adjacent gaging station.
Review either “A Field Guide to Field Identification of Bankfull Stage in the Western
United States,” “Identifying Bankfull Stage in Forested Streams in the Eastern United
States,” or “Guide to Identification of Bankfull Stage in the Northeastern United States,”
all of which are available at the USFS Stream Systems Technology Center’s website
home page http://www.stream.fs.fed.us/publications/videos.html#northeast
Big Creek originates from a spring on the east slope of the Monte Cristo Range and flows
into the Bear River approximately 20 mi downstream. USGS gaging station 10023000,
established in 1939, is located approximately two-thirds of the way between the source
and the Bear River in an area where the valley flat is relatively wide and the floodplain
has formed by lateral accretion. The watershed is heavily grazed.
The watershed is primarily underlain by the Tertiary Wasatch formation, although
Paleozoic limestones outcrop along the crest of the Monte Cristo Range (Fig. 1).
Throughout the field trip, you will note reddish, clayey hillslopes broken by outcrops of
conglomerates and sandstones. Thus, a wide range of sediment sizes, ranging between
clay and cobbles, is produced by erosion of the bedrock.
At the gage, the channel has a single-thread and meanders across its floodplain, very
similar to the planform of Watts Branch, where the concepts of bankfull stage and active
floodplain were first observed and described. Thus, the Big Creek gage site is an
excellent outdoor classroom to begin our consideration of channel form.
Procedure for Conducting a Channel Geometry Survey:
A. Assembly and evaluation of stream flow data
It is best to first assemble regional and local stream flow data, because channel form is
affected by recent catastrophic events or droughts, as well as average, long-term
conditions. Data are readily available at gaging stations, and determination of bankfull
discharge near a gage is an efficient and easy process whose results can sometimes be
extrapolated to nearby watersheds.
Thursday’s stream flow of approximately 9.2 ft3/s (Fig. 2) is still receding from high
water that occurred in the 2011 water year (Fig 1, 2, and 3). The peak runoff from spring
snowmelt occurred on May 1 with a discharge of 17 ft3/s, which is roughly 60% of the
median peak discharge shown by the yellow line on Fig. 3. The peak stream flow of 2011
was 159 ft3/s (stage > 6.53 ft). High flows in 2011 were caused by near record snowpack
and has only been exceeded once; in 1957 with a flow of 337 ft3/s (Fig 4).
The largest peak flows in the past 20 years occurred on May 10, 1998, when the
instantaneous peak flow was 102 ft3/s (stage = 5.96 ft), and last year when the
instantaneous peak flow was 152 ft3/s (stage > 6.25 ft). The mean daily peak flow for
these two respective dates were 98 ft3/s and 152 ft3/s (Fig. 5). Long-term stream flow
records suggest there may be a general pattern of declining total annual stream flow and
declining peak flows, because there were several large floods between 100 and 150 ft3/s
between 1950 and 1971 (Fig. 4).
There are two types of peak flows in this watershed. Most floods occur during spring
snowmelt, between March 1 and June 30 (Fig 6). In general, larger snowmelt floods
occur in years of large total runoff (Fig. 7). In years with small snow pack and small total
runoff, the annual peak flow is sometimes generated by thunderstorms. These summer or
fall floods are typically of short duration (Fig. 6 and 7).
The 2-yr recurrence flood for the period 1939 to 2011 is 56 ft3/s (Fig. 8). Flow duration
curves for different periods indicate the relative stability or “flashiness” of runoff. Look
at Fig. 9 and think about the shape of these curves and how they differ for relatively dry
periods with low peak stream flow, wet periods with large peak stream flow, and the
years following the wet periods.
B. Relation between stream flow and channel form
The rating relations for this gage allow one to link discharge with the water surface
elevation (or stage) at which these flows occur. The present rating relation in use by the
USGS is Rating 11. Comparisons with previous rating relations may give insights to the
style and channel change, if occurring. For example, comparing rating relation 10 to
rating 9 shows that there may have been slight bed aggradation at low flow (Fig. 10).
And comparison of rating 11 to rating 10 shows that there may have been some bed
degradation during the high flows in the spring of 2011. It is difficult to compare high
flows between ratings 11 and 10 because few high flows were measured for rating
relation 10, however, there may be a slight trend showing that flood stages are occurring
higher at higher elevations than they did during the period for which rating relation 10
was constructed. You can look back at the flood frequency graph (Fig. 7) and determine
the approximate stage at which floods of various recurrences occur. This information is
useful in calibrating your eye to the height at which bankfull stage might occur. Rating 10
is provided as Table 1.
C. Field activities
C.1. Determination of bankfull stage:
Your team should walk along the stream channel, noting all locations where you think
there is a good indication of the elevation of bankfull stage. Your goal is to identify the
elevation of the topographic break from the vertical or steep channel bank to the flatter
alluvial surface of the adjacent floodplain. The challenge that you face is distinguishing
between those places where the channel is adjacent to the active floodplain and those
places where the channel is adjacent to higher terraces or non-alluvial surfaces. Figure 12
shows the channel just below bankfull stage. You are trying to identify the geomorphic
evidence for the bankfull stage in the vicinity of the water surface depicted in this
photograph.
After you have placed flagging at every location where you think the bankfull stage is
indicated, your next task is to determine the elevation of each flag and the distance along
the centerline of each point. Thus, you will need to keep track of the incremental and
cumulative distance along the centerline of your survey ( just like we had planned on
doing on the Logan River). Use an engineers level and survey the elevation of each flag
in relation to the elevation benchmarks that have been established in the field and which
are marked. Also survey the elevation of the channel bed and note the depth of water at
all grade breaks in channel bed topography. Use the techniques described in Harrelson et
al (1994, Stream channel reference sites: an illustrated guide to field technique: U. S.
Forest Service General Technical Report RM-245) to complete this surveying activity.
Table 2 lists the elevations of each of these benchmarks, and the site map (Fig. 13) shows
the location of each of these benchmarks.
Develop a table listing the channel bed elevation, the water surface elevation, the
elevation of each flag, and its distance downstream or upstream from the gage.
C.2. Examination of evidence for channel migration and change:
We have resurveyed four cross-sections during the past few years (Fig. 13). These crosssections are located in the field. Each team will survey at least 1 cross section in order to
determine the nature, magnitude, and rate of channel change at these locations. Describe
the evidence for channel migration and floodplain formation? Is the channel’s
migration rate faster or smaller than that of previous years?
C.3. Mapping (each person does this for themselves):
On Figure 8, draw the outer boundary of the alluvial valley.
Examine the channel bed material. Decide how many types of bed material exist. Map the
distribution of each type of bed material. Describe each type.
Map the distribution of pools and riffles, making “R” for the center of each distinct riffle
and “P” for the center of each distinct pool.
D. Office activities (each person does this for themselves)
You will enter the data from your longitudinal profile survey of the bed, water surface,
and bankfull stage indicators into a spreadsheet/plotting program, plotting the distance
along the centerline as the x-axis and the elevation as the y-axis.
You will develop a plot where you connect each bed elevation point from your survey
and all of your water surface elevation points. The distance downstream should be
referenced to the location of the stream gage, with the gage location being 0, with all
distances upstream being negative, and all distances downstream being positive. Your
plot should also include the elevation and distances of your bankfull indicators similar to
your water surface and bed elevation data. Fit a linear least squares best fit line through
all of your bankfull indicator points. The slope of this line is the slope at bankfull stage.
Determine the elevation of the point where this best-fit line crosses the stage plate.
Determine the elevation of this point and look at the rating relation to determine the
discharge of this point. This discharge is the bankfull discharge.
On the same graph, plot the longitudinal profile with your bed and water surface
elevations. Fit a linear least squares best-fit line through these data as well. Comment on
the differences between the slope and shape of the longitudinal profiles of the bed, water
surface, and bankfull stage.
Plot your channel cross-section data in a spreadsheet with distance from the left endpoint
(looking downstream) on the x-axis, and elevation on the y-axis. Also plot the data from
all the other years of surveys. These will be provided on the class website. Describe the
evidence for channel migration and floodplain formation? Is the channel’s migration rate
faster or smaller than that of previous years? Are your results compatible with the
concepts of the lateral floodplain accretion models?
You will turn in copies of your field book sheets, data collected, spreadsheet analysis of
the data, the site map with alluvial boundary delineated, bed material size, and
pools/riffles delineated, and a write up/discussion of the lab.