FINAL HANDOUT

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Colorado State University, Fort Collins *

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362

Subject

Geology

Date

Dec 6, 2023

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pdf

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11

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GEOL 121 Name ___________________________ Lab Section ________________ Spring Creek Field Trip Learning Objectives After completing this lab, you should be able to: 1) Measure stream discharge using the Continuity Equation. 2) Describe the morphology of a stream, including bed material, bank material, channel dimensions, flood plain vegetation, and human alterations. 3) Interpret the characteristics of a stream to determine how a flood would affect the stream. 4) Use an inclinometer to measure the height of an object. Introduction On July 28, 1997 Fort Collins experienced an unusually large urban flood that was centered over the Spring Creek drainage, a tributary of the Cache la Poudre River. Over a 90-minute period on the eve- ning of July 28, up to 5 inches of rain fell within the Spring Creek basin, and a total of 21.3 cm (8.4 inches) of rain was measured in an area centered near Drake Road and Overland Trail, west of the CSU campus near Hughes Stadium. 1 Maximum instantaneous rainfall rates likely exceeded 12.7 – 15.2 cmper hour (5-6 inches per hour) at times. Because of numerous days of rain prior to July 28, infiltrationwas low, and incredible volumes of water accumulated as runoff, flowing downhill from approximately west to east, initiating the devastating flood in Fort Collins. The flood caused five deaths and millions of dollars of damage, including flooding of Morgan Library and the Lory Student Center, and has become a pivotal event in the development of new floodplain ordinances and flood hazard maps within the City of Fort Collins. Today, you will be visiting the area of Spring Creek at the College Ave. Bridge, just south of Dairy Queen on College Ave. to reconstruct the events of the 1997 Spring Creek Flood and to observe current stream processes. Your task is to: 1) Walk upstream and downstream along Spring Creek and observe elements of the landscape and human development that may have influenced and contributed to the flood hazard, as wellas current channel features. 2) Characterize current water flow and channel morphology conditions on Spring Creek. 3) Reconstruct flood conditions, including the maximum flow depth, using photographs of the flood and any current evidence of high water marks. 4) Draw a complete sketch of the area along Spring Creek that depicts the features that contrib- uted to the flood. 5) 5) Hypothesize why the flood was so damaging to this area. 1 Doesken, N.J., and McKee, T.B., 1998, An analysis of rainfall for the July 28, 1997 flood in Ft. Collins, CO, Cli- matology Report 98-1.
GEOL 121: Spring Creek Field Trip Figure 9.1 : Rainfall in Fort Collins from 4:00 p.m. on July 27, 1997 through 11:00 p.m. on July 28, 1997. Source: http:// ccc.atmos.colostate.edu/~odie/rain.html
1 GEOL 121: Spring Creek Field Trip Assignment Characterize current conditions in Spring Creek 1) (2 pts.) Estimate discharge in m 3 /s (rate of streamflow, measured in water volume per unit of time)by measuring width, depth, and velocity at one location along the stream. Discharge ( Q ) is calculated using the Continuity Equation as follows: Q = w * d * v or Q=vA , where w is average width of flow (m), d is average flow depth (m), and v is average velocity (m/s), and A is channel cross sectional area (m 2 ). Figure 9.2 : Stream discharge is the volume of water moving past a cross section of a river in a given amount of time. Source: USGS: https://water.usgs.gov/edu/streamflow2.html a) Pick a location along the stream for your discharge measurement. Measure the width of the streamat that site using a tape measure. Flow width (m): b) Keep your tape measure draped across the stream. Measure the water depth using a meter stick atseveral points across the stream to get an average depth of the stream at this location. Average flow depth (m):
2 GEOL 121: Spring Creek Field Trip c) Measure the water velocity using some kindof floatable object, Measure the time it takes to travel a certain distance. (Be sure to select a channel reach with some water current.) A channel reach is a section of the creek that you define byits constant channel geometry. Measure the water velocity three times, and take the average. Average flow velocity (m/s): d) Calculate the discharge at this location. Discharge (m 3 /s): 2) (1 pt.) Discuss two possible sources of error in your discharge estimate. 3) (1 pt.) If you were to a repeat your calculations at a different section of the stream, would you expectthe discharge to be different or would it be the same? Why
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