Mass Wasting, Streams, and Floods

Learning Objectives:

  • Obtain and analyze peak stream-flow data from the United States Geological Survey.
  • Calculate Recurrence Intervals for different peak stream-flows over time.

Points: 32 points total.

Flood Stage and Flooding

Flooding is a common and a serious problem on and along our nation’s waterways. Flood stage is reached when the water level in a stream overflows its banks. Floodplains are popular sites for development but are best left for playgrounds, golf courses, and the like.

Have you ever heard someone say, “that flood was a 1 in 100-year flood”? What does this mean? Does it mean that a flood will only occur every 100 years, and that we are safe the other 99? The short answer is No; on average, we can expect a flood of this size or greater to occur within any 100-year period. However, we cannot predict whether it will occur in any particular year, only that each year has a 1 in 100 (1%) chance of occurring in any year. The timing between these major flooding events, or any major geologic event (drought, volcanic eruption, earthquake, tsunami, etc.), is referred to as a recurrence interval. This is the average time period within which a given flood event will be equaled or exceeded once.

In order to better understand stream behavior, the USGS has installed thousands of stream gauges throughout the country, locations with a permanent water level indicator and recorder. Data from these stations can be used to make flood frequency curves, which are useful in making flood control decisions.

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  • Attributions
  • Figure 1: “Inundation of the State Capitol, City of Sacramento, 1862” (Public Domain; A. Rosenfield via Calisphere)

 

Activity 1:

Go to the current stream flow data at the USGS website. https://waterwatch.usgs.gov/?id=ww_current

  1. Which states currently have floods? (look at the key at the bottom of the map) (2pts)
  2. Pick out a single state and click on it.  Choose a single stream gage and click on it.
    1. What is the 8-digit stream gage number and name of the gage you chose? (1 point)
    1. What is the date and stage of the most recent data? (Hint: look in the pop-up window on the map) (2pts)
    1. What is the highest stage ever recorded and when did that happen? (Hint: check the Peak tab at the top of the pop-up window) (2pts)
    1. What is the highest discharge ever recorded and when did that happen? (2pts)
    1. What data is available? (Hint: Go back to the Summary tab and click “Streamgage inventory” the bottom of the pop-up) (1 point)
    1. When did data collection begin at your streamgage? (1pts)
    1. What is something interesting you found about your stream gage site? (2 pts)

 

Activity 2: Recurrence Intervals and the Russian River

Recall that a recurrence interval (or return period) is based on the probability that a given event, in this case a flood of specific magnitude, will be equaled or exceeded in any given year. For example, a “1 in 100-year flood event” means on average, we can expect a flood of this size or greater to occur within any 100-year period. However, we cannot predict it will occur in any particular year, only that each year has a 1 in 100 (1%) chance of occurring in any year.

 Data for this activity was collected from the USGS peak streamflow for the Russian River, located in northern California south of Ukiah. The chart below includes the 20 largest discharge events for Russian River at USGS station 11467000 from February 28, 1940 – February 27, 2019.

 Step 1: To create a flood frequency graph, we must calculate the recurrence interval. First, however, we need to rank the flood events on the chart below. The bigger the stream flow (cfs, read as cubic feet per second), the higher the discharge.  A 1 signifies the highest discharge event and a 20 the lowest discharge event (see table below).

Date Stream flow (cfs) Flood Rank Recurrence Interval (years) Probability (%)
February 28, 1940 88,400      
February 6, 1942 67,800      
January 22, 1943 69,200      
December 23, 1955 90,100      
February 25, 1958 68,700      
February 1, 1963 71,800
December 23, 1964 93,400      
January 5, 1966 77,000      
January 21, 1967 68,400      
January 14, 1969 68,600      
January 24, 1970 72,900      
January 17, 1974 74,000      
February 13, 1975 67,300      
December 20, 1981 67,200      
January 27, 1983 71,900      
February 18, 1986 102,000 1 80 1.25
January 9, 1995 93,900      
January 1, 1997 82,100      
January 1, 2006 86,000      
February 27, 2019 72,000      
Table 1: 20 largest discharge events for Russian River at USGS station 11467000 from February 28, 1940 – February 27, 2019.

 

 

Step 2: Calculate the recurrence interval for each discharge event using the following equation:

Where,

RI = Recurrence Interval (in years)

n = number of years of record (in this case, 79)

m = rank of flood (see table)

Example: Feb. 18, 1986

RI = (79 + 1) / 1

RI = 80 (Note: It is ok to round to the nearest tenth)

 

 

 Step 3: Calculate the probability of each discharge event:

Where,

RI = Recurrence Interval (from Step 2)

Example: Feb. 18, 1986

Probability = (1/80) * 100

Probability = 1.2% (Note: round to the nearest tenth)

What does this calculation signify? On average a flood event of this magnitude, discharge of 102,000 cfs (cubic feet per second), occurs every 80 years. This does not preclude the event from happening every year, but the probability of that is small (~1.25%).

 

Step 4: Now that the table has been completed, plot the discharge against the recurrence interval on the graph below. After plotting each point, use a ruler or other straightedge to draw a best fit line.

What is a best fit line? It is a straight line on a graph that shows the general direction that a group of points appear to be heading, however it does not connect all points on the graph.

 

Step 5: Answer the following questions based on Table 1 and Figure 2 from above.

  1. On which date did a flood event have a recurrence interval of 10? (1 pt)
    1. 1/5/1966
    2. 1/17/1974
    3. 12/20/1981
    4. 2/18/1986

 

  1. Of the following dated flood events, which one would you expect to happen more often? (1 pt)
    1. 12/23/1955
    2. 2/25/1958
    3. 2/18/1986
    4. 1/1/2006

 

  1. Observe your best fit line. What approximate discharge would be associated with a 50-year recurrence interval? Hint: If your best fit line does not get close to one of these numbers, you should consider redrawing it to better capture your data points as plotted. (1 pt)
    1. 82,000 cfs
    2. 88,000 cfs
    3. 94,750 cfs
    4. 98,000 cfs

 

  1. Is it possible that a flood with a similar discharge to that of the event from 2/27/2019 could happen again in the next 10 years?

 

  1. Yes or no? (1 pt)

 

  1. Why or why not? (3 pts)

 

 

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  • Attributions
  • Table 1: “Peak Streamflow for the Russian River” (Public Domain; Chloe Branciforte and Emily Haddad via USGS/NWIS)
  • Figure 2: “Recurrence Interval Graph” (CC-BY 4.0; Chloe Branciforte, own work)