Flood Risk and Vulnerability Analysis Project - Atlantic Climate ...

Development of Projected Intensity-Duration-Frequency Curves
for Corner Brook and Goulds/Petty Harbour, Newfoundland May 16, 2012
4.4 Use of Composite Historical IDF for St. John’s Area
A second set of historical IDF curves were used to project IDF values for the St. John’s
area. This was done to account for a large precipitation event that took place in 2002,
after the St. John’s A station was taken offline. In order to include the effect of this
storm, data from the St. John’s A station with data was combined from a station at
Windsor Lake, approximately 1.6 kilometer from the St. John’s A station (updated data
provided by WRMD). The record for the St. John’s A station runs from 1949 through
1996, and the record for the Windsor Lake station runs from 1999 through 2010. The
combined record from these two stations is referred to as the St. John’s Composite.
It is important for any analysis of extreme events that all large storms are taken into
account, though any analyst should exercise caution combining data from multiple
stations. Without a significant period of overlap in the data record for two gauges, no
accurate assumptions can be made about the homogeneity of the data recorded by the
gauges. There are a number of reasons that a single gauge with a long record provides
much higher-quality data than a composite dataset from multiple gauges:
- If two gauges are placed in separate climatic zones, different elevations or regions
with varying land cover, they may not experience the same storms.
- Even if two stations are in close proximity they may return very different results for
the same storm because, during precipitation events, rainfall is not deposited in a
spatially uniform manner.
- There are many environmental factors that can impact the measurements taken by
a precipitation gauge including proximity to buildings, localized wind patterns, and
local obstructions.
- Gauges of the same type may not be calibrated in an identical way or maintained to
the same standard of operation.
The projected IDFs in this report are developed based on the baseline historical IDFs at
both the Environment Canada St. John’s A site and for the St. John’s Composite
record.
4.5 Fitting Models
For this work the generalized extreme value (GEV) distribution was fit to the historical
monthly precipitation maxima using potential predictor variables. The GEV distribution
can assume three possible distribution types, known as the Gumbel, Fréchet and
Weibull. The Gumbel is the distribution used by Environment Canada to estimate IDF
curves and it is a relatively ―light-tailed‖ distribution with a fixed shape. Using the
Frechet variant of the GEV would allow a model to be fit to the shape parameter, which
AMEC Environment & Infrastructure 21