Hydraulic Geometry and Channel Roughness

Les Hasbargen
,
SUNY College at Oneonta
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Summary

In this field exercise, students measure channel cross sections in a rough (alluvial) and smooth bedrock-floored reach in a local stream. The students test the hypothesis that increasing roughness decreases average stream velocity, and so the depth and/or width must increase for the rougher bed.Students gain insight into how bank and bed roughness influences water flow, and thus can also influence flood heights.

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Context

Audience

I have used this exercise for both an upper division undergraduate geomorphology course, as well as in a general education course for non-science majors.

Skills and concepts that students must have mastered

Students should have basic algebra skills. In addition, they must have been exposed to basic discharge equation for stream flow, Q = VA = WDV, where Q is discharge (volume/time), V is velocity (length/time), A is cross sectional area, and D is water depth. They should also have been introduced to the idea that the same discharge can have different velocity and or cross section area, and that channel boundaries strongly influence friction.

How the activity is situated in the course

This is a stand-alone activity.

Goals

Content/concepts goals for this activity

This activity introduces students to fundamental controls on friction and stream velocity, and shows them how this can influence flood heights, for instance. Students learn simple measurement techniques, practice graphing skills, and learn to recognize "roughness elements" in stream channels.

Higher order thinking skills goals for this activity

Higher order thinking skills for this exercise include: hypothesis development and testing, quantitative assessment of stream cross section areas, and comparative analysis.

Other skills goals for this activity

Students learn how to record field data, and how to work together in groups with assigned roles (booker, shooter, rod person, etc).

Description of the activity/assignment

In this field exercise, students measure channel cross sections in a rough alluvial and smooth bedrock-floored reach in a local stream. This exercise is hypothesis-driven. The hypothesis states that increasing roughness decreases average stream velocity, and so the depth and/or width must increase for the rougher bed. Working in groups, students use hand levels, tape measures, and surveying rods to document channel geometry. They then must reduce the data and plot it as distance-elevation on a chart. Finally, they analyze and compare the results to the predicted roughness-hydraulic geometry relation. Students learn simple surveying methods to collect field data. Students compare real world data to theoretical predictions. Students gain insight into how bank and bed roughness influences water flow, and thus can also influence flood heights.
Designed for a geomorphology course
Integrates geomorphology into a core course in geology
Designed for an introductory geology course
Uses geomorphology to solve problems in other fields

Determining whether students have met the goals

Assessment of student efforts is based on a ranking system for categories (a rubric), given below. Each category receives a rank from 0 to 4, where 0 = unacceptable; 1 = poor, needs a lot of improvement, but passing; 2 = fair, but could use some improvement; 3 = good, and only has very minor problems; 4 = excellent; needs no improvement
i. Hypothesis statement
ii. Field Measurements
iii. Data reduction
iv. Analysis and conclusions/summary