Flood Generation

Floods are among the most predictable and manageable hazards, if people are well informed and plan accordingly.  This lab introduces several tools and techniques related to flood risk management: hydrograph and snowpack analysis, flood frequency analysis, and floodplain maps.  We will look at these in the context the Cowichan River and the city of Duncan, which it regularly floods.

A. Flood Generation

Flood prediction requires an understanding of how much water will be delivered to a river by its watershed, and how quickly that water will make its way downstream as a flood wave.  In Canada, floods usually result from either rain or snowmelt.  Weather forecasting helps to predict when big rain events will occur.  The risk of a snowmelt flood (or ‘freshet’) depends on how much snow has accumulated, and how quickly it melts. 

The relative importance of rain and snowmelt can be readily assessed by examining a stream’s annual hydrograph, which tracks stream flow over the course of a year.  Rain events show up as abrupt spikes, while snowmelt floods are more prolonged and typically occur in late spring or early summer.  In Canada, stream flow data and hydrographs can be easily obtained from the Water Survey Canada web site.

In watersheds where snow may be significant, it can be monitored using snow pillows.  These sensors measure the weight of the snowpack and convert it to an equivalent depth of water.  In BC, a network of snow pillows is maintained by the Ministry of Environment’s River Forecast Centre.  You can view live snowpack data from their web site.

The Cowichan River is located on southern Vancouver Island.  Its watershed occupies approximately 1200 km2, including steep, mountainous headwaters with elevations that exceed 1000 m.  These surround Cowichan Lake, a large water body that helps to moderate flows in the river downstream.  The lake level is controlled in the summer by a weir at its outlet, to ensure that there is enough water for lake recreation, for salmon in the river, and for water supply to the pulp mill at Crofton.  (This water budgeting is very tricky.)  In the winter rainy season, the lake rises and the weir is opened, allowing natural flows in the river below.  Duncan and its surrounding area are in a low floodplain area close to sea level.

Figure 1 below shows the annual hydrograph for the Cowichan River at Duncan.  It includes mean, maximum and minimum daily flows for the period of record (1960 to 2017), as well as data from the 2016-17 water year (from Oct. 1 to Sep. 30).  Use it to answer the following questions.

  1. What was the largest flow recorded at this stream gauging site in 2016-17?  When did it occur?           

/ 2

  1. How does this compare to the largest flows in a typical year?  Or of all time?                    / 2

Figure 1: Annual hydrograph for the Cowichan River at Duncan.  Mean, maximum and minimum daily flows are from 1960-2017.  The red line shows actual daily flows from Oct. 1, 2016 to Sep. 30, 2017.  Source: Water Survey Canada.

Go to the River Forecast Centre’s web site and look up this year’s snowpack data (https://governmentofbc.maps.arcgis.com/apps/webappviewer/index.html?id=c15768bf73494f5da04b1aac6793bd2e) at Jump Creek.  This snow pillow is located at 1134 m elevation, near a tributary stream above Cowichan Lake. 

  1. How does the past year’s (2019-2020) snow water accumulation compare to historical average, minimum and maximum?                                                                                            / 3
  1. Around what dates does snowmelt typically begin and end?                                             / 2
  1. Considering this and Figure 1, are high flows in the Cowichan River dominated by rain events or snowmelt?  Give specific evidence to support your answer.                                       / 2

B. Flood Frequency

Another approach to managing flood risk involves predicting the likelihood of future floods based on a river’s past behaviour.  This technique, called flood frequency analysis, cannot predict exactly when a flood will happen, but it provides a reasonable estimate of how often floods of different sizes can be expected. 

Flood frequency analysis takes the largest recorded river flow for every year available, and ranks them from highest to lowest.  Each flow is assigned a calculated ‘return period’ – the number of years expected to elapse between occurrences of that flow.  Floods follow a normal magnitude-frequency relation: big ones are less common.  The largest flow in the data set is given a return period equal to the number of years in the record (often an underestimate).  Smaller floods get ever smaller return periods (which are usually more accurate).

Figure 2 below shows flood frequency data for the Cowichan River at Duncan, based on daily flow records from 1960 to 2017.  The x-axis (return period) is logarithmic, allowing a straight best-fit line to be drawn through the data points.  This line is used to estimate discharges or return periods as needed.

Figure 2:  Flood frequency curve for the Cowichan River at Duncan, 1960-2017.  Data source:    Water Survey Canada.

  1. Based on the best-fit line, how often can a flow of 220 m3/s be expected in the Cowichan River at Duncan?  How about 600 m3/s?                                                                             / 2
  1. What is the estimated 250-year flood for the Cowichan River at Duncan?                          / 1

C. Floodplain Maps

Floodplain maps are made by combining topographic maps with flood-frequency analysis.  They require knowledge of how often rivers flood to certain heights, and what areas next to the river are low enough to be inundated.  In BC, floodplain maps follow a conventional format.  See the last page of this lab for a sample map.  The general idea is to read the isograms (bold lines that estimate the flood height), and compare them to the ground elevation (shown as contours and spot elevations) to assess flood risk for a given location.

Use the provided floodplain map (Cowichan and Koksilah Rivers & Tributaries at Duncan, Sheet 3, 1997) to answer the questions below. 

East (downstream) of the Highway 1 bridge over the Cowichan River, you will see two labeled dikes: the Duncan City Dyke and the District of North Cowichan South Dyke (which turns into the South Side Spur Dyke). If you look closely at the dykes, you’ll see little numbers – normally with tiny arrows – that are pointing to the maximum elevations of the dykes.

  1. Starting at Highway 1, move east and provide the progression of elevations on the Duncan City Dyke as it stretches through town.                                                             / 1
  1. Again, starting at Highway 1, move east and provide the progression of elevations of the District of North Cowichan South Dyke (there are two elevation on the South Side Spur Dyke, too, but the numbers are pretty hard to read due to overprinting).                                               / 1
  1. East (downstream) of the Highway 1 bridge over the Cowichan River, there are a number of artificial flood-control dykes, mostly built in the 1980s.  Are the dykes high enough to contain the 200-year flood?  Explain.                                                                                                / 2
  1. Knowing that the Cowichan River flows from the west (left) to the east (right) on this map, use the isogram boxes to determine the range of estimated depth of flooding at the 200-year flood stage at the following location:                                                                                                           / 3

On Boys Road between Allenby and the E&N Rail line: _________________________

At the Tertiary Treatment Sewage Ponds: _____________________________________

In the largest sports field south of Marchmont Road: _____________________________

  1. Which of the above locations places the most people in jeopardy from a flood and why?         /2

A few days ago, the federal government announced $24.2 million to reduce the impact of climate change on the Cowichan watershed’s ability to deal with increased winter storms and summer drought (Cowichan Watershed Resiliency Program). Why has this funding be allocated now? What does it aim to do (in your answer, please specifically reference both the summer and winter water supply issues in the Cowichan). Here is one article about it, though there are many: https://www.cowichanvalleycitizen.com/news/feds-pump-24-2-million-into-flood-mitigation-efforts-in-cowichan-watershed/

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