Rainfall distribution in the city of St.John's: temporal distribution, spatial variation, frequency analysis, and Tropical Storm Gabrielle

Wadden, David (2002) Rainfall distribution in the city of St.John's: temporal distribution, spatial variation, frequency analysis, and Tropical Storm Gabrielle. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (8MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

The evaluation of rainfall distribution throughout the City of St. John's, Newfoundland, was performed: to investigate the temporal distribution of rainfall across the City; to compare the spatial variations of concurrent rainfall events at the City's three rain gage stations, to determine the most appropriate probability distribution for the frequency analysis of rainfall; to update the IDF curves for the City of St. John's; and to analyze the extreme rainfall event of September 19,2001, which resulted from Tropical Storm Gabrielle. -- The temporal distribution of rainfall in the City of St. John's was examined resulting in the determination of a family of probability curves (10% through 90%), which related percent storm rainfall to percent storm duration, for both single station rain gages and the Network Mean. The method utilized was similar to Huff (1967) except that storms were not grouped by the quartile which had the most rainfall accumulation but instead all storms were analyzed as a single group. The analysis indicated that the temporal distribution for the Network Mean was similar to the results obtained for each of the single station rain gages and that it was appropriate to represent the time distribution of rainfall, across the City of St. John's, by a Network Mean distribution that was applicable for all storm durations. The proposed Network Mean distribution was then compared to the AES Mean, Huff, and SCS temporal distributions. It was concluded that the 20% Network Mean distribution was the most appropriate for the City of St. John's in all cases except the 12-hour event where the AES Mean distribution should be used. -- The spatial variation of rainfall was analyzed using concurrent rainfall events, from the City's Windsor Lake, Ruby Line, and Bladder Avenue rain gage stations. The analyses indicated that the spatial variation of rainfall fluctuated across the City on a storm by storm basis and that, on average, the rainfall depths were greater in the Northeast at Windsor Lake. It was also shown that the data from the AES rain gage at the St. John's Airport could be combined with the data from Windsor Lake to provide an extended database for IDF analysis. -- The frequency analysis of annual extremes for the combined database of Windsor Lake and St. John's Airport was performed. The results indicated that the previously assumed AES Extreme Value Type 1 (EV1) distribution was no longer appropriate for the frequency analysis of annual rainfall extrema and that the Lognormal (LN) distribution was the best fit. Updated IDF curves were prepared, based on the combined database and the LN distribution, and it was found that the new curves, on average, gave slightly higher rainfall intensities for various return periods and durations. -- The rainfall event of September 19, 2001, resulting from Tropical Storm Gabrielle, was also examined. The temporal distribution of rainfall across the City for this event was uniform and best represented by the AES 12 hour distribution. The rainfall generated by Tropical Storm Gabrielle varied across the City with a maximum difference of 61.9mm between stations. The frequency analysis of this event indicated that the 2-hour, 6-hour, 12- hour, and 24-hour rainfall maxima all exceeded the 100 year return period.

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