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Joint Probability Road Map for Flooding
Coastal area flooding arises from oceanographic (storm surges plus tides and/or waves), fluvial (increased river discharge), and/or pluvial (direct surface runoff) sources. The adverse consequences of a flood can be disproportionately large when these different sources occur concurrently or in close succession, a phenomenon that is known as “compound flooding”. Two distinct mechanisms storm surges and heavy precipitation, either through direct runoff or increased river discharge can lead to flooding in coastal areas. If they occur concurrently (or in close succession) the adverse consequences can be greatly exacerbated.
Canada is surrounded by oceans on three sides out of four. About one third of the population resides in coastal counties. Impacts of flooding in these usually low-lying, densely populated, and highly developed regions, can be devastating with wide-ranging social, economic and environmental consequences. Historically, local storm tides wreaked havoc in the Bay of Fundy. With the tidal system resonating at close to the 12-hour, 25-minute dominant lunar tide of the Atlantic Ocean, variations in normal Fundy tides are in the main caused by astronomical factors. The strongest Fundy tides occur when the three elements-anomalistic, synodical, and tropical monthly cycles-peak simultaneously. With continuing global sea-level rise and regional crustal subsidence, the possible recurrence of destructive storm tides has grave implications for property owners and settlements in the Fundy coastal zone.
Sine there are very few regulation procedure guidelines, conventional, the typical procedure is to combine the flood lines from different factors (but not actually calculate the joint probabilities) This procedure entails: calculate 1 in 100 year precipitation (flow) + 2 year storm surge (flood line 1), and 1 in 100 year storm surge (air pressure) + 2 year precipitation (flood line 2). Then take the envelope of flood line 1 and flood line 2 as the design flood line. This approach is obviously insufficient to understand the factors leading to the life destroying flooding. For engineering safety purpose, it is imperative to get to know how theses factors get progressively superposed and lead to the devastating compound flooding.
Our objectives are to: understand what kind of atmosphere systems are leading to correlations in the extreme precipitation and storm surge, and where it would be expected to be a factor across Nova Scotia by quantifying the correlation at specific locations. The input data are measured data with long record and close vicinity, along with numerical model output in case where measured data are not available. For a point-location test example: Precipitation: rainfall time series and measured flows from St Croix or precipitation closer to Saint John; Storm surge: Saint John tide gauge. The goal of this exercise is to obtain initial results for this specific location and obtain an answer to the question “is there any correlation between storm surges and precipitation at this location”?
