Spring Creek Field Trip: Stream Discharge and Flood Analysis

Name: Colton Warner Lab Section 12 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 cm per hour (5-6 inches per hour) at times. Because of numerous days of rain prior to July 28, infiltration was 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 well as 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) Hypoth esize 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

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

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 stream at that site using a tape measure. Flow width (m): 310 cm b) Keep your tape measure draped across the stream. Measure the water depth using a meter stick at several points across the stream to get an average depth of the stream at this location. Average flow depth (m): 57 cm

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GEOL 121: Spring Creek Field Trip c) Measure the water velocity using some kind of 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 by its constant channel geometry. Measure the water velocity three times, and take the average. Average flow velocity (m/s): 0.11m/s d) Calculate the discharge at this location. Discharge (m 3 /s): 0.19437m 3 /s 2) (1 pt.) Discuss two possible sources of error in your discharge estimate. 1. All of our measurements could be off including our velocity measurement. 2. Could have measured in the wrong spots using the wrong methods. 3) (1 pt.) If you were to a repeat your calculations at a different section of the stream, would you expect the discharge to be different or would it be the same? Why? I believe it would be the same because even if the velocity decreases, the width or depth would increase meaning the discharge should stay the same.

4) (2 pts.) Test your hypothesis by calculating the discharge at two additional locations along the stream. a) Site 2: Flow width (m): 3.40m Average flow depth (m): 0.29m Average velocity (m/s): 5/56 = 0.089m/s Discharge (m 3 /s): 0.087754m/s b) Site 3: Flow width (m): 2.47m Average flow depth (m): 0.49m Average velocity (m/s): 31.25/5 = 0.16m/s Discharge (m 3 /s): 0.1936m/s 5) (1 pt.) Was your hypothesis correct? Why or why not? Our hypothesis was not correct , because the three different discharges were way more different than expected.

GEOL 121: Spring Creek Field Trip 6) (4 pts.) The shape, or morphology, of stream channels is influenced by the history of streamflows (climate), sediment supply (climate and geology), the gradient (or slope) of the stream, and, in many cases, human alterations. The gradient measures how steep the stream is, and therefore how much en- ergy it has to transport sediment and erode its banks during floods. Channel gradient is also important during low-flow conditions, and influences the size of the sediment deposited in and along the chan- nel, as well as the types of aquatic organisms that can find habitat in the stream. Gradient has units of meters per meter or feet per mile, and is calculated as rise/run. Gradient can also be visually estimated using qualitative categories (steep, moderate, flat). Select two short reaches of Spring Creek that appear to have different gradients from each other. Draw a plan view of each reach and qualitatively estimate the gradient using one of the categories listed above (steep, moderate, flat). Then describe the morphology of these two reaches, including the characteristics of the 1) bed material (size of sediment, abundance of algae), 2) bank material (natural versus human-placed sediment, size of bank material, vegetation), 3) channel dimensions, 4) flood- plain vegetation, and 5) human alterations. Give quantitative measurements where possible. Clearly label features on the space provided below. Reach 1: Beginning of the creek, moderate, rock/mud, wider, glassy, cement/cement rocks, small sediment size, lots of algae, human made. Reach 2: End of the creek by the dam, Little to no algae, slower stream, smaller sediments, human made dam, 80:20 man made.

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GEOL 121: Spring Creek Field Trip Reconstruct the Spring Creek flood Figure 9.3: Flooding on Spring Creek near College Ave. Source: CSU Libraries Archive: http://www.colostate.edu/fea- tures/flood97-photos2.aspx Figure 9.4: Flooding on Spring Creek near College Ave. Source: CSU Libraries Archive: http://www.colostate.edu/fea- tures/flood97-photos2.aspx

GEOL 121: Spring Creek Field Trip Figure 9.5: Flooding on Spring Creek near College Ave. Note the high water line indicated by the green arrows. Source: City of Fort Collins Figure 9.6: Flooding on Spring Creek near College Ave. Note the high-water line indicated by the green arrow. Source: City of Fort Collins .

GEOL 121: Spring Creek Field Trip 7) (1 pts.) Look at the pictures on the preceeding pages of Spring Creek during the 1997 flood, and try to match current features that you see with those in the photos. Also note that certain features that are visible in the pictures are no longer present. Select two of the photos and write a short paragraph describing what they show about the Spring Creek flood. In figure 9.5 it shows the Dairy Queen right next to the creek. This picture shows the high-water line against the Dairy Queen which happened to be halfway up Dairy Queen’s wall. I also notice a bunch of sediments scattered throughout the Dairy Queen parking lot showing how the water carried these sediments all the way up to the Dairy Queen. In figure 9.6 it shows the water rushing under the bridge after the great flood had happened. It also shows a high-water line against a tree with the creek in the background. Even the water in this picture is quite high compared to what it looked like when we went. 8) (2 pts.) Why was the flood so damaging in the area of Spring Creek at College Avenue? Come up with at least three reasons. a. The bridge became a barrier clogging the flood, not letting the water flow freely which created a large dam and pooled the water, making it rise. b. There was a trailer park in the area that got flooded, ruining many peoples homes

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GEOL 121: Spring Creek Field Trip c. Poor planning for floods around the area by the creek.

GEOL 121: Spring Creek Field Trip 9) (1 pt.) What changes do you see in the area that indicate the City of Fort Collins is trying to remedy flood hazards along Spring Creek? There are now many man made rocks all along the bank of the creek. There is also new retaining walls and levies that raise the flood plain. We have also made concrete channels leading to the creek. Also, on the railroad nearby we’ve built more culvers. You will now reconstruct flow conditions at this site during the Spring Creek flood. Use the inclinom eter on the Brunton compass or the level and tape to help calculate the depth of water in this area. 10) (2 pt.) What was the maximum depth of water in this area, in meters? Include a diagram showing your method of calculating depth of water. 0.4m + 0.45m + 1.81m + 3.29m = 5.95m

GEOL 121: Spring Creek Field Trip The estimated discharge ( Q ) at this site during the 1997 flood was approximately 140 m 3 /s (5000 ft 3/ s), or about twice as large as the 500-year flood! Based on the photos of this site, we can infer that flow velocity ( v ) was low here during the flood, because of water backing up behind the College Avenue bridge. 11) (2 pt.) Assuming that flow velocity was approximately 0.5 m/s, use the equation Q=w*d*v to esti- mate the flow width during the flood. This will be a rough estimate, but should give you some idea of how far away from the channel the flood extended (remember that CSU was flooded). Show all your work. Flow width during flood (meters) (or width of ponding at the park): 5.95m (1 pt.) Discuss two possible sources of error in your width estimate. 1. Measurements could be inaccurate and differ from person to person. 2. The water back then could’ve been flowing more than we know.

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Spring Creek Field Trip - Colton Warner

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