Lab 2
Fluvial Geomorphology
September 9, 2012
Due: September 17, 2012
Field Measurement of Channel Cross-Sections, Channel Slope, and Roughness
Field site: Summit Creek in Mack Park, Smithfield, UT
Surveying Basics
Survey a channel cross section and survey the channel slope of Summit Creek in part of Mack
Park in Smithfield, UT, using the tape-and-level method, as described by Harrelson et al. (1994,
p. 26-32). Read “Surveying Basics” (Harrelson et al., 1994 p 13-25) before you begin the
activity.
Each team of 2 or 3 will survey one previously-marked cross-section determined by the
instructors. These cross-sections will be located by benchmarks in the field. Survey the cross
section generally following the strategy described by Harrelson et al. (p. 26-32) using a tape, rod,
and engineers level. Survey the entire topography of the cross section between each end point.
Also, survey key channel features such as the left and right edges of water. The instructors will
identify your back-sight location. The arbitrary elevation of the backsite is 100m. Your survey
rods are in meters, thus, you will survey and report all data in SI units.
Survey the longitudinal profile of the bed and of the water surface along the centerline of the
channel, for a length of approximately 30 channel widths. At each location where the bed
elevation is surveyed, note the water depth at each location so that you can determine the
elevation of the water surface. Make sure to collect a survey point at the location of your cross
section. The primary challenge here is that you will need to move the instrument several times in
order to survey reach slope over the required distance. In your notebook, check to see that your
survey “closes” to an acceptable tolerance.
When you are finished, plot the longitudinal profile in Excel or some other plotting software. In
both of your plots, show each data point that you survey and connect straight lines between
surveyed points. Make sure to note the location of your cross section on your longitudinal profile.
In the event that you survey any points that are clearly in error, develop a second plot in which
you censor these points from your final data set. Determine the slope of the bed profile and of the
water surface by fitting a linear least squares equation to your data.
Plot your cross sections looking downstream. Make each plot with a reasonable vertical
exaggeration and with no vertical exaggeration. Determine the water surface width, cross-section
area, wetted perimeter, and hydraulic radius (R = A/P) of your cross section. Also determine the
local slope of your cross-section using your longitudinal profile data immediately upstream and
downstream from your cross sections.
Throughout your work and analysis, refer to Chapters 5 and 6 of Harrelson et al. (1994).
In your notebook, be sure to include a location and site map so that someone else could repeat
your survey.
Field Determination of Channel Roughness
The discharge form of the Manning equation is:
Q=(AR0.67S0.5)/n
Where ‘n’ is the channel roughness.
We can rearrange this equation and solve for n, which is what you should do in this exercise.
You should record the stage of flow as measured at the stage plate on the Bridge. Use the 2nd
order polynomial best-fit line of the stage-discharge relation below to calculate the discharge
using the stage you recorded.
Using your cross-section measurements, determine the hydraulic radius (R) for the existing flow
condition. Determine the reach-average water slope (S). In the calculation of n, convert all of
your units to the SI system (Q,R) where necessary; S is unitless.
Assignment to be submitted:
1) A description of the site, referencing maps and diagrams, such that a survey crew could find
and reoccupy the site. The write-up should also include a written description such that a survey
crew could find and reoccupy the exact cross sections and resurvey them within the same
elevation reference system that you establish.
2) Include a general location map and a site map. Your maps should make reference to nearby
features that were part of the study area
3) A plot of the cross section that you survey
4) A plot of the longitudinal profile the you survey, including the location of the cross section on
that longitudinal profile
5) Calculations of wetted width, hydraulic radius, cross section area, and wetted perimeter of your
cross section, channel bed and water surface slope, and calculations of discharge and your backcalculated channel roughness value.