What time of year does it flood?

Does it usually flood in summer or winter?  It depends on the climate but where I live, low evaporation and wetter catchments favour winter flooding while the intense rain from thunderstorms can lead to flooding during the warmer parts of the year.

Looking at some flow data tells an interesting story.  Plotting the annual peak flow and time of peak in polar coordinates makes it easy to see patterns in flood timing.  I’ve created some examples below.  The gauge on the Ovens River at Wangaratta has one of the longest flow records in Victoria with data available from 1885.  Figure 1  shows the annual flood magnitude as the length of a radial line and flood timing as an angle.  Jan 1 is straight up with months increasing in a clockwise direction.  The big flood of March 2012 is at about 2 o’clock.  Most floods have occurred between July and October.  Until 2011 there had never been a large flood between January and May in 132 years, then we had two in two years.

Wang_polar

Figure 1: Magnitude and timing of annual flood peaks, Ovens River at Wangaratta (1886-2015) (catchment area 5138 km2)

Let’s look at a stream which is influenced by urbanisation, but with some remaining undeveloped areas; Mullum Mullum Creek at Doncaster East (Figure 2).  It looks like large floods occur at any time.  The smaller catchment area and impervious surfaces mean thunderstorms are likely to lead to high hows in summer but there are also floods in winter.

Mulled_polar.jpeg

Figure 2: Magnitude and timing of annual flood peaks, Mullum Mullum Creek at Doncaster East (1980-2014) (catchment area 37 km2)

Gardiners Creek is more highly urbanised (229624A).  Similar to Mullum Mullm Creek, floods can occur and at any time with large floods possible in summer.

Gardiners_polar

Figure 3: Magnitude and timing of annual flood peaks, Gardiners Creek at Gardiner (1990-2014) (catchment area 96 km2)

Finally, a small highly urbanised creek in Canberra, Yarralumla Creek at Mawson (410753).  Flooding generally occurs late spring to early autumn.

Yarralumla_polar.jpeg

Figure 4: Magnitude and timing of annual flood peaks, Yarralumla Creek at Mawson (1970-2015) (catchment area 4.4 km2)

There are a few studies (listed below) that show that urbanisation increases summer runoff and flooding.  The increase in impervious surfaces makes catchments more responsive to intense summer rainfall even when they are dry before the rain starts.  Plus there is a contribution from urban irrigation and water leaks.

Prosdocimi, I., T. R. Kjeldsen and J. D. Miller (2015). Detection and attribution of urbanization effect on flood extremes using nonstationary flood-frequency models. Water Resources Research 51(6): 4244-4262 (link).

Townsend-Small, A., D. E. Pataki, H. Liu, Z. Li, Q. Wu and B. Thomas (2013). Increasing summer river discharge in southern California, USA, linked to urbanization. Geophysical Research Letters. 40(17): 4643–4647 (link).

Smith, B. K., J. A. Smith, M. L. Baeck, G. Villarini and D. B. Wright (2013). Spectrum of storm event hydrologic response in urban watersheds. Water Resources Research 49: 2649–2663 (link).

Information on how to plot polar graphs in R is available at this blog.

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