Rivers and Flooding Lab - student handout-2
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School
Colorado State University, Fort Collins *
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Course
101C
Subject
Geography
Date
Jan 9, 2024
Type
Pages
8
Uploaded by SargentWillpowerMagpie42
Rivers and Flooding Lab
Learning Objectives
After completing this lab, you should be able to:
1)
Design an experiment to learn more about river morphology and behavior.
2)
Calculate the recurrence interval and probability of a given discharge.
3)
Read and interpret a flood plain map to assess flood risk.
Introduction to Rivers and Flooding
Humans have inhabited areas adjacent to rivers for millennia. Traditionally people have settled along
floodplains because of the availability of fertile soils for agricultural purposes. As population increases,
and urban areas grow, development along flood plains changes the way rain and snow infiltrate the
ground, and runoff flows across the surface. Habitation next to rivers also puts people at risk for flood
hazards. Floods are the third deadliest natural hazard in the United States, with only tornados and
lightning strikes ranking higher. In this lab, you will be evaluating river forms and processes, and flood
potential in Fort Collins and on the CSU campus using discharge data to calculate flood recurrence
intervals and examining floodplain maps.
We live in the Cache la Poudre drainage basin, with the Cache la Poudre River flowing from its
headwaters
in Rocky Mountain National Park, down Poudre Canyon and through Fort Collins. The
Poudre River joins the South Platte River east of Greeley, Colorado, and the South Platte River joins the
North Platte River in Nebraska, to form the Platte River, which eventually flows into the Missouri River,
and finally the Mississippi River.
Channel planform
describes the two-dimensional pattern of the river channel as viewed on a mapor an
aerial photograph. The planform of a river reflects sediment available to be transported and stored, and
how the energy of the flowing water interacts with the channel boundaries. Many types of channel
planform exist, but the two most common planforms are
meandering and braided rivers
.
The Poudre River is a meandering river through Fort Collins, though it has a straight planform where it is
confined by rock walls in Poudre Canyon. Many boaters and fishermen are interested in the amount of
water flowing within the Poudre River in order to evaluate recreational uses. We talk about the volume of
water moving down a river over a specified time interval as
discharge (
Q
)
which 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
). The units of discharge are m
3
/s or ft
3
/s.
To describe the characteristics of floods, we discuss the flood
magnitude
(discharge or size of the flood,
in ft
3
/s), the
duration
(how long it lasted), and how common or rare it is (
recurrence interval and
annual probability
).
Flood recurrence intervals
(
RI
), describe the average number of years between
successive floods of a given size. The recurrence interval is a useful way to describe how often we might
expect a flood of at least a certain size. For example, a 100-year flood is a flood of a given size that
happens once in 100 years on average. The probability of a flood event of a given discharge occurring
during a particular year is called the
annual probability
. Annual probability is related to recurrence
interval by the following equation:
P=1/RI
where
P
= probability (between 0-1), and
RI
= recurrence interval
A low
P
or probability (or percentage) indicates that the flood will occur less frequently (high magnitude
floods are more rare) and a high
P
or probability indicates that it will occur frequently (low magnitude
floods are more common). As you can imagine, this information is important for engineering purposes, as
well as zoning for urban development along rivers. For example, many engineered structures such as
culverts or bridges are typically constructed to handle the 100-year discharge for the river they span. The
probability of the 100-year event occurring during any given year is 1/100 or 0.01 or 1% (low probability
of occurrence). Additionally, recurrence intervals are used to develop floodplain zoning maps in towns to
inform homeowners of flood risks and the need to purchase floodinsurance.
Increasing development within urban areas may change the magnitude, duration, and probability of
flooding. As the density of houses and impervious roadways continues to increase in Fort Collins, think
about how this might change the flood hazard. Urbanization played a large role in the destructiveness of
the 1997 Spring Creek flood.
Part 1: Experimenting with River Form and Behavior
In small groups, design an experiment to investigate some aspect of how rivers behave using the stream
table. You could investigate changes in channel form, sediment transport, erosion, deposition, etc., which
occur as a result of changes in discharge, stream gradient, grain size, etc. Whatever you choose to
investigate, you should design your experiment so that you have a clear hypothesis that you are testing
and you are only altering one variable at a time.
1) What is the question you are trying to address?
Investigating how flooding affects the flow of sediments
2)What is your hypothesis?
If there is flooding, there will be a change in sediment carried by the water because there is no
relationship between the volume of flow and volume of sediment carried
3) List 3 observations from your experiment:
1. There was more erosion
2. There was more deposition of sediments at the end of the flow
3. Larger sediment were carried down
4) List 3 conclusions from your experiment:
1. There is a relationship between the volume and velocity
2. Some of the sediment carried is from the erosion taking place
3. With water flow increase the sediment deposit increases
Part 2: Discharge Variability and Flooding Recurrence Intervals
Variability in the discharge of a river through time contributes to the form and characteristics of that river.
Flooding constitutes increases in discharge beyond what the banks of a river can handle. The figure below
shows daily discharge measurements in 2021 for the Cache la Poudre River gaging station in Old Town.
The gaging station is operated by the US Geological Survey, and can be accessed via the web at:
http://
waterdata.usgs.gov/nwis/uv?06752260
1)
Describe the changesin discharge of the Poudre River through the year.When was
discharge the highest? Lowest? What factors might contribute to the variability you
observe?
The discharge of the river was high in May through July because it received more rain during those
months. The lowest was in September-october because it doesn’t get so much rain then.
The next figure shows daily discharge measurements for the last ~44 years. You will notice that the
summer peak discharge varies greatly from year to year. We can use the peak discharge from each year to
calculate the
recurrence interval
and
probability
of a given discharge.
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The data on the next page are a subset of 40 years of data from a USGS gauging station on the Poudre
River. On the data table provided, the discharges are ranked in descending order, giving the highest
discharge a ranking of 1, and the second highest a ranking of 2, etc.
2)
Calculate the recurrence interval for the five blank years using the equation:
RI= (n+1)
m
where
n
= total number of years of data, in this case n=40,
m
= rank number of specific discharge
Date
Annual Peak
Dis charge (Q)
(ft
3
/s)
Rank
(m)
(years)
Recurrence Interval (RI)
6/18/95
3500
8
5.13
6/16/96
2670
18
2.28
6/9/97
3290
12
3.42
6/4/98
959
33
1.24
4/30/99
7710
2
20.50
5/6/00
785
36
1.14
5/30/01
713
40
1.03
5/31/02
751
38
1.08
5/30/03
2100
21
1.95
7/1/04
997
31
1.32
6/4/05
1720
26
1.58
10/30/05
1210
29
1.41
6/13/07
933
34
1.21
6/7/08
1830
23
1.78
6/22/09
1790
25
1.64
6/12/10
4570
6
6.83
6/9/11
3220
13
3.15
7/30/12
764
37
1.11
9/13/13
8140
1
41
5/31/14
5860
4
10.25
3)
Plot the RI vs. the Discharge values from the table above on the semi-log graph paper provided on the
next page. Logarithms are used to show several orders of magnitude of data. Draw a best-fit line to
your data.
Here is a resource explaining how to plot points:
https://serc.carleton.edu/mathyouneed/graphing/
plotting.html
And this page describes how to draw a best fit line:
https://serc.carleton.edu/mathyouneed/graphing/
bestfit.html
4) What is the relationship between recurrence interval (RI) and discharge (Q)?
Higher discharge has a higher RI and does not occur as often.
5) The Fall 2013 Front Range flooding caused devastation along much of the Front Range. What is the
probability (P) of a flood the same size as the 2013 flood on the Poudre River happening again next year?
(Remember P = 1/RI)
P=0.02 which means there's a 2% chance of happening a year.
6)
Use the graph to estimate the discharge associated with the 100-year flood on the Poudre River. You
will need to extrapolate a best-fit line. How does this discharge compare to that of Fall 2013?
100 year flood had a discharge of 8000 ft^3/s based on the line of best fit. This aligns with the flood in
2013, which was a 100 year flood
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Part 3: Examining Flood Maps
Obtain the
Fort Collins Flood Risk Map
. You can also view a GIS version of this map at:
https:// www.fcgov.com/utilities/what-we-do/stormwater/flooding/floodplain-maps-documents
1) What are the flood risk categories shown on the map legend? What are the flood recurrence
intervals associated with each flood risk designation?
High risk, Mordrate Risk, and Low Risk. The RI is not given
2) Reflecting on the RI calculations and discharge data in Part 1, why do you think the 500- year
floodplain is wider than the other floodplains?
Because the rarity and devastation of the flood shows that it would create a wide span of water.
3) Find W. Elizabeth St. on the map, just west of Shields. Why do you think Elizabeth Street is marked
as a flood zone on the map?
It decreases in elevation as the street goes on.
4) What happens when the water from W. Elizabeth enters campus? Does it get blocked like the map
may suggest?
It all goes into the man made lagoon but would not be blocked.
5) Examine the CSU Floodplain Map. Where are the most flood-prone places on campus? Are they the
areas you would have expected? Why or why not?
West and east sides of campus but not the campus itself. It is because of the increasing elevation and the
decreasing elevation.
6) Locate your place of residence on the floodplain map. (If you don’t live on the map, pick a location
you go to frequently.) Do you live in a flood zone? If so, what is the risk? If you live on campus, do
think that your dorm is susceptible to flood waters? In which RI floodplain do youlive?
I am not on the floor plan for flooding. I live in braiden hall on campus. I wouldn't think that my dorm is
susceptible to flooding.
7) Locate the intersection of N. Lemay and E.Vine Dr. If you were to buy property here, would it be in a
floodplain? If so, do you think flood insurance is required? What are some of the risks you may assume
as a homeowner in a floodplain?
In an area in moderate flood risk so you would need flood insurance.
8) If you lived on a floodplain with a 500 year RI, would you buy flood insurance? What is the
probability that the flood would occur in a given year, and damage your house? Based on this
probability, if a flood did occur while you lived there, would this be an acceptable risk to you?
I would buy flood insurance.
9) The flood recurrence intervals that you see on these maps are determined using data from stream
gauges, just as you did in Part 2. Are there any reasons you can think of why the likelihood of floods in
the future may be different than the likelihood determined from past data?
It would change based on seasonal flow with the fact that climate change is making everything wild
10) What is still unclear about calculating flood recurrence intervals or interpreting flood maps? What
questions do you have?
Nothing