1. Introduction
Information about long-term homogeneity in trends of precipitation can help in making informed management decisions [
1]. Almost 90% of the storm rainfall in the urban watersheds is lost to surface runoff causing floods [
2]. Other than topography of a region, climatic factors are responsible for general and flash floods. For example, main- and mid-land regions experience flooding due to patterns of incoming and melting precipitation; whereas, the coastal regions face river backflow due to high tides or high waves resulting from tractive force of lunar tides. Due to climate change, it is advisable to identify the vulnerable areas and methods to mitigate the climate-induced hydrologic variation [
3]. Literature describes occurring of floods as a potential impact of climate change [
4,
5,
6,
7,
8,
9,
10].
Floods in the northern North coastal Atlantic regions are caused due to high tides, high waves, snowfall, snowmelt, and heavy rains. Climate change in the past years has resulted in irregular trends in the climate extremes including precipitation and temperature. Well defined seasonal floods have also changed the trends of their occurring [
4]. It has been debated that flooding may vary from year to year depending upon specific years’ patterns of climate extremes [
11,
12,
13].
Like many other coastal cities on Atlantic Ocean, the capital cities of the Atlantic Canadian provinces New Brunswick and Prince Edward Island; i.e., Fredericton and Charlottetown usually get storm warnings. The two cities are rapidly growing as they accommodate new immigrants, experience urban sprawl and extension in residential as well as commercial setups, situate on coastal belts, and experience similar climate. Fredericton faces regular floods and Charlottetown does not despite homogeneity in extreme events of precipitation and coastal location. Because of the location of Charlottetown in one of the areas in Canada’s most vulnerable to storm surges, concern over the likelihood of sea-level rise affecting the city in the future has been raised by the provincial and local governments, as well as climate change researchers. Charlottetown, with its highly concentrated urban infrastructure along the ocean front, has potential for being seriously affected by sea-level rise and by storm surges, which occur at the rate higher than anywhere else in Atlantic Canada [
14,
15]. A co-existence of precipitation extreme events and sea-level rise due to the storm surges may cause a situation of devastating flooding and the resultant socio-economic disasters. Flooding due to storm surges have been the subject of climate change studies, including a comprehensive study, completed in 2002, on the coastal impacts of sea-level rise [
14,
15,
16,
17]. Potential threats of the extreme precipitation have yet to be explored for their further possible combination with storm surges and resultant flood hazards in Charlottetown.
In general, flood hazard maps are produced based on floodplains analyzed using river level (i.e., stream gauge) and/or topographical data. Li et al. [
18] used GIS to processes topographical data; i.e., Digital Elevation Models (DEM), generated with remote sensing, for simulating the flood hazard range and flood-prone areas. They compared their results with flood maps generated from hydrological data as well and recommended that the method of flood hazard generation from GIS can be applied for flood hazard assessments. The literature also indicated that hydrodynamic models are more appropriate for detailed coastal flood vulnerability assessment, but GIS can be used for flood hazard assessment in large areas [
19,
20]. GIS analysis for flood hazard assessments is based on geographic/topographical information data, such as DEM [
18,
21,
22]. The most of the flood risk assessment models including GIS-based model for urban flood inundation (GUFIM) and the urban storm flood inundation simulation method (USISM) used GIS and DEM to analyze the urban flood hazard maps [
22,
23] as GIS can analyze the factors that have a greater impact on urban floods [
24,
25]. Lyu [
26] analyzed the relationship between flood risk and urbanization, and concluded that one of the significant influential factors of flooding was identified as the urbanization degree. It has been observed that GIS can be pivotal in delineating flood-prone areas without using river level data [
18]. Long-term time-series data can be analyzed for trends in climate extremes and GIS can be used to delineate watersheds and processes DEM for mapping flood-prone areas to visualize areas under flood hazards in regions where stream gauge data (river levels) are unavailable. The outcome of such study may help policymakers for future developments with regard to flood planning under climate change impacts. Therefore, the objective of this study was to investigate if such situations existed in the past with regard to precipitation events having potential for causing floods in Charlottetown.
3. Results and Discussion
The trend of simple daily intensity index (
SDII) showed significant decrease (
p < 0.001) in the daily precipitation intensity for Charlottetown (
Table 4). However, the decrease in the trend of
SDII for Fredericton was non-significant. This reflects a decreasing trend in the high intensity precipitations in Charlottetown over the past 100 years supported by the significant decrease (
p < 0.001) in the number of heavy precipitation days (
R10mm). However, maximum one-day precipitation and maximum five-day precipitation significantly increased in Charlottetown (
p < 0.001) and Fredericton (
p < 0.05), respectively. Although there were negative trends in total precipitation received per year by both the cities, the trend for Charlottetown was significant (
p < 0.001); nonetheless, it received overall more precipitation than Fredericton during the past century.
Despite non-significant trends, the mean values of number of consecutive dry days increased at the rate of almost 1.5 days per decade; however, there was an addition of half a day per decade in the number of consecutive wet days for the both cities with a slightly less than 0.1 mm/day decrease in the annual total precipitation per wet days (
Table 4 and
Table 5). Mean number of per annum days when precipitation was more than 10 and 20 mm termed as heavy and very heavy precipitation days, respectively, were almost the same for both the cities; i.e., 40 and 15, respectively. However, Fredericton experienced two time more very wet days (gauged by above 95th percentile of total annual precipitation) and only seven extremely wet days (gauged by above 99th percentile of total annual precipitation) than Charlottetown every year during the past century. The monthly maximum one-day and five-day precipitation averaged over a year for the both cities were almost similar; 57–60 and 89–90 mm, respectively. Charlottetown, however, received 65 mm more precipitation per year than Fredericton over the past century (
Table 5).
With apparently slight difference between the values of the indices shown in
Table 5, the mean values of extreme dry and extreme wet indices for the two cities were not statistically different from one another. The bars of the standard deviation values of the respective indices of the two cities overlapped visually (
Figure 2) and arithmetically (
Table 5) reflecting statistical similarities between the respective annual mean values of the four indices shown in the
Figure 2.
Frequency as well as intensity of the indices represented by
R20mm (very heavy precipitation), and heavy precipitation
R10mm heavy precipitation) shown in
Figure 1 as indicators of annual count of days when precipitation was equal to or exceeded 20 and 10 mm, respectively indicate similar soaking conditions of the two cities during the past century. This situation is also supported by the results of the simple daily intensity index denoted by
SDII, which is the annual total precipitation divided by the number of wet days when precipitation was equal to or more than 1 mm in the year (
Figure 3). The two cities did not only have the similar
SDII (e.g., 9.56 ± 1.13 versus 8.81 ± 0.98;
Table 5), but the indicators of the patterns of precipitation intensity, as represented by the maximum one-day precipitation (
Rx1day) and the maximum five-day precipitation (
Rx5day) followed the same trends of similarity between the two cities as shown in
Figure 3.
Charlottetown had the higher accumulated annual precipitation then Fredericton over the past century as shown in the black and white inset of
Figure 4. Never in the past 100 years, had Fredericton’s cumulative precipitation surpassed the values of Charlottetown showing that the latter remained wetter than the former from 1989 to 2018. During the past three decades, Charlottetown had thirty-three events when the precipitation exceeded 50 mm as compared to forty-six similar events (
R50mm) experienced by Fredericton as shown in the main
Figure 4. For twelve times, these events occurred more than once in a year, depicted by solid lines with multiple symbols (hollow circles) per line in Charlottetown as compared to fourteen times in Fredericton, depicted by solid lines with multiple symbols (close circles) per line, which had three of these intense events per annum during the years 1999 and 2013, and four during 2008 and 2012 (
Figure 4).
Results in the
Figure 5 presented with red stars show the similar extreme events results in ten devastating floods in Fredericton on these dates (
Table 3). Review and deep search of the available literature do not report flooding during these years of similar events in Charlottetown; however, the results of the statistical analyses (
Table 5) dictates occurrence of danger of floods shown by a strong correlation (
r = 0.770) for number of days when precipitation exceeded the marked threshold of return period (50 mm) in the two cities. These events can greatly increase overland flow and without designated areas of flood storage, this could also contribute to increased urban flooding if coincided with the sea-level rise producing storm surges. Thus, the water and sewer system in Charlottetown will have to contend with the increased urban run-off and coastal inundation at the same time [
15]. Moderate correlations were also calculated, and reported in
Table 5, for the indices defining intensity of precipitation including
R10mm,
SDII, and
R20mm (
r = 0.472, 0.400, and 0.383, respectively) and frequency of precipitation; i.e.,
TOTPRCP (
r = 0.469). The indications of similarities between the extreme climates of the two cities are also proven with the low values of
RMSE especially for these mentioned indices (
Table 5).
Historical records (
Table 2) show that all the major flood events in Fredericton occurred when one-day precipitation exceeded 50 mm. This means that during the past, the climate conditions of Charlottetown had potential to cause heavy flooding had its topographical and urban scenarios been similar to those of Fredericton. An obvious difference between the settings of the two cities is the absence of a river of the size of the Saint John to flow through Charlottetown. However, geographical location of Charlottetown in the Gulf of St. Lawrence and on the waterfront of Atlantic can never be ignored for surges in the days of high tides that may lead to a blocked drainage of city’s surface runoff during the events of higher intensity. McCulloch et al. [
15] investigated the effects of climate change induced sea-level rise and land subsidence on Prince Edward Island, focusing specifically Charlottetown and the Island’s North Shore and identified the coast of Prince Edward Island, particularly its northern shoreline, as one of the most sensitive regions to climate change and sea-level rise in Canada.
The situation may worsen as according to Vasseur and Catto [
41], the projections for the Atlantic region have indicated an increase in both the mean annual temperatures and annual precipitation rates to be 1.5–6 °C in summer and 2–4 °C between the winter by the year 2050 when rainfalls are expected to increase seasonally and are likely to contribute to more frequent precipitation events. For example, on 10 August 2018, a high tide-induced shorter but powerful bout of rain, hail and heavy wind hit Charlottetown and left it underwater (
Figure 4). It also left nearly 2500 residents, mostly in Charlottetown without power according to a newspaper reporting (
https://www.cbc.ca/news/canada/prince-edward-island/pei-charlottetown-heavy-rain-flooding-water-1.4781688). The reason was claimed to be the catch basins being clogged to create a major problem in terms of the water rising to a level where it interferes and flows into people’s properties. Discussions of officials of Public Works Department of Charlottetown revealed their experience and opinion that as per routine for this coastal city, storm water of this city goes to the harbor, but with the water coming in from the tide, it just backs up into the system that in some areas cannot handle the water (
Figure 6).
Armenakis et al. [
42] reported that in recent years, large Canadian cities have been exposed to devastating floods, resulting in damage to properties, loss of lives, the endangerment of exposed populations, the disruption of social and societal services, and substantial damage to the important urban critical infrastructure. Similar events are common for Fredericton and potentially possible for Charlottetown because of the topography of watersheds of these cities (
Figure 7). Southern coastal areas and water front of Charlottetown are also at risk for flood hazards due to the three pore points of its three watersheds (
Figure 1 and
Figure 7b) in contrast to the situation in Fredericton, which is prone to flood hazards in the vicinity of River Saint Johns (
Figure 7a and
Figure 8a). Severe floods reported for Fredericton might have been generated because of topography of this city having one watershed and all the water of its catchment being drained through one outlet only (
Figure 1 and
Figure 7a).
Another reason for frequent and devastating floods in Fredericton was varying slope of its only watershed. Slightly over 10% of its area is located at elevation above 100 m and over ~33% of the area is at elevation between 50 and 100 m (
Table 6). Rainfall received at higher elevation generate flash floods that accumulate in the lower elevations; i.e., 56.8% in case of Fredericton. In contrast, over 95% of the areas of the three watersheds of Charlottetown are prone to accumulation of flood water once an event is or will be generated. Such topographical differences between the watersheds of Fredericton and Charlottetown are responsible for the past devastating floods in the former and the potential floods of extreme nature in future climatic forecasts in the latter.
The land use maps the two cities show similar flood risks as the most of the urban areas of these cities exist around the streamlines and/or rivers passing through the urban settings (
Figure 8). Although, the residential areas of the two cities are well apart from potential flood hazard ranges but the most of the roadways of the two cities lie in flood prone areas portraying potential lockdown of the cities in the events of flooding.
These evidences could help policymakers establish for future developments. However, any development with regard to civil work and spending of resources would need further analyses with varying approaches. The approach introduced by Armenakis et al. [
42], that builds upon the methods for estimating spatial vulnerability, spatial hazard, and spatial risk considered in various studies [
43,
44], incorporates the higher spatial resolution and details determined from Earth observation data may also be adopted for further studying the risks of floods in Charlottetown in future endeavors. As suggested by Armenakis et al. [
42], this approach can be applied to the development of strategies for future flood risk reduction, risk-based land use planning, resilience, and capacity-building as in particular, it supports spatial decision-making and the development of disaster impact reduction strategies, and the overall effectiveness of disaster management.