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Field work (21 march)

The class will be broken into groups. You must work with different people than you worked with for the weather station measurements. We will visit two sites, with a set of tasks to be done at each site. Each group is responsible for the following:

Site 1 (below a small dam):

(1) Locate and survey one cross-section; identify bankfull stage IF that can be identified in the stream section we are studying. Be sure to measure the left and right edge of water (LEW and REW, respectively), and remember the hydrologist always faces downstream.

(2) Measure discharge at your designated cross-section. You should try to have no more than 10% of the flow in any one section. Since we only have one current meter, just make one measurement in each section for 10 seconds at 0.6 of the depth.

(3) At the deepest point in the stream (“thalweg”): (a) measure the velocity with a current meter at 0.2, 0.4, 0.6, and 0.8 times the depth; do this for a minimum of 40 seconds and use the rating table provided to you; (b) measure the velocity using v = (2gh)0.5, where h is the rise in water level against some obstruction, such as the stadia rod; and (c) measure the velocity using a float over a distance of around 5 m.

(4) Measure the water surface slope along a short reach centered on your cross-section using a clinometer.

(5) Estimate a value for Manning's "n" for the area immediately around your cross-section. Guidance for this should come from the tabulated values in the handout (Chow, 1959).

(6) Do a Wolman pebble count (minimum of 100 pebbles) along your cross-section. Samples should be systematically spaced at a distance that is roughly equal to the second or third largest particle in your cross-section. Note that you will need several transects across the stream in order to obtain the necessary sample size (each additional transect should be about one meter upstream of the previous count). Sampling transects should extend from bankfull to bankfull.(7) Collect the information to classify the stream in the vicinity of your cross-section according to: (i) Rosgen (1994) and (ii) Montgomery and Buffington (1993, 1997).

Site 2 (more natural reach):

(1) Locate and do a rough survey (maximum of 20 points) of one cross-section. Locate the cross-section in a location that would be good for measuring discharge. Be sure to identify current water level and bankfull in your survey.

(2) Measure the water surface slope along a short reach centered on your cross-section using a clinometer.

(3) Estimate a value for Manning's "n" for the area immediately around your cross-section. Guidance for this should come from the tabulated values in the handout (Chow, 1959).

(4) Collect the information to classify the stream in the vicinity of your cross-section according to: (i) Rosgen (1994) and (ii) Montgomery and Buffington (1993, 1997).

(5) Each individual should make a field sketch (plan view) of the channel reach under study. This sketch should include the banks and an indication of the adjacent topography and vegetation. Be sure to include, along with other important features: (i) the approximate location and size of the major habitat units within the study reach; and (ii) the approximate location of each cross-section. To facilitate the drawing of the sketch maps, each group should leave their string up until the entire class has completed all portions of this exercise.

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Source:  OpenStax, Field measurements in hydrology. OpenStax CNX. Jul 29, 2009 Download for free at http://cnx.org/content/col10769/1.1
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