It’s not a matter of if, but when.
Harriston has had 13 floods throughout its history, and according to the Maitland Valley Conservation Authority, that will happen again.
According to an MVCA report, “Put simply, Harriston is built on top of the river, and most homes are located within the flood plain.”
Harriston’s earliest recorded flood was in 1912 and several of its major floods happened before 1950. In each of those instances, the floods affected roads, houses and stores.
“Since, 1950, although the flood risk remained the same, Harriston has been fortunate to experience only relatively minor flood events,” the report stated. “The flood risk for Harriston is larger than most communities in southern Ontario. Flooding in Harriston is caused by the restricted river flow through the community and the speed at which the water runs off the land into the river.”
Preliminary numbers indicate a regional flood would cause $15-million in damage, not including infrastructure damage to roads, sidewalks and bridges.
There are several structures in the floodway (deep and fast moving water in a flood) in Harriston, which can expect significant damage in a flood. Also, emergency operations will be greatly affected during a flood because the arterial roads are also located in the floodway.
MVCA date shows the climate has changed over the last 30 years. Generally, there is an increase in total precipitation, an increase in days above freezing and an increase in the intensity of rainfall. Those climactic factors lead to a much greater flood risk, particularly for towns like Harriston, which are prone to flooding from intense rain.
Measures can be taken to reduce flooding or reduce losses during a flood in Harriston, including: removing flood prone structures, education and communication, flood warning improvements, developing natural infrastructure, removing structures from the floodway, improving the hydraulics of the river through town and possibly rerouting the river.
The MVCA is taking steps to improve its flood forecasting. It has submitted an application to the Ministry of Natural Resources for rain gauges upstream of the town, which would significantly increase the authority’s ability to model and forecast floods.
Steve Jackson and Phil Beard and local MVCA representative John Cox were at a recent meeting in Minto to explain Harriston’s flood risks.
Jackson talked about the history of flooding, the potential of future flooding, and ways risk could be alleviated as well, as the potential of an improved flood warning system.
Jackson said, “Basically , from pre-1910 we have no documented history of flooding.”
He considered that odd compared to other areas where there was significant flooding particularly, in the 1880s.
“What we can draw from that is that something has happened in the watershed since 1910 that has led to flooding.”
Factors could include deforestation, cropping, or other things, Jackson said.
He called the most recent flood in December “nuisance flooding” compared to the significant flooding that has happened in the past. In the 1920s, he said severe floods caused damage to streets, homes, stores and factory.
“Those, in fact, are the same homes, stores and factories that exist right now,” he added.
In the 1930s, streets and stores were flooded, while in the 1940s, Jackson said water levels reached the old Harriston post office in the centre of the town. It is now the site of the clock tower.
He said flooding in the 1970s and 1980s was a bit different, with evacuations of a trailer park. In 2000, the trailer park was evacuated again.
“The important thing to remember is that the flood risk hasn’t changed; we’ve just been very lucky. For the past 60 years, we haven’t really seen the potential flooding in Harriston … and that’s more by luck than anything else.”
He noted that regional flood levels are a standard outline by the province of Ontario. Those levels simulate the impact of Hurricane Hazel (1954) above the watershed affecting the river in Harriston.
In the higher potential floods, Harriston, “in all probability, emergency crews would not be able to reach people trapped in their homes, because the roads would be flooded.”
The other thing to consider is the floodway itself. He said anything within that area will sustain significant damage in a regional flood. That path heads through the centre of downtown Harriston.
Jackson said it is not a matter of if Harriston experiences a regional flood – but when, and how many times.
“When the regional flood hits, it will impact 213 houses and 29 commercial buildings. We have an average of 1.4 metres of water for most houses. That’s a significant amount of water and basements will be completely inundated. Damage will include hot water heaters, electrical, insulation walls … everything, Jackson said.
A study was done in 1977 on damage estimates.
The first really heavy rains in the area were in Listowel in August 1883, which brought about six inches of rain in 24 hours, which is a very significant amount of rain, Jackson said.
Hurricane Hazel, in 1954, provided three inches of rain over a day and a half, then 8 inches of rain came down in 12 hours.
“That is our model, for a regional event.” Jackson said.
In 2004, Peterborough experienced 7-inches of rain in five hours. If the Peterborough flood happened in the Harriston area, flooding would be far greater, Jackson said.
One of the issues is the MVCA has few rain gauges in its watershed. That is because the system was set up based on large, widespread events.
In Molesworth (between Listowel and Wingham) in July 2005, gauges recorded 7 inches of rain over 10 hours. Volunteers however recorded as much as 10 in some locations.
“That far exceeds what happened in Hurricane Hazel quite significantly,” Jackson said. “The Hurricane Hazel rain event is not something that’s absolutely impossible. We’re seeing it all over the province. Really, it’s just been luck it hasn’t been in the Harriston watershed.
The flooding in December amounted to 4.3 inches (including snowmelt) over 36 hours.
The record rainfall is 30.8 inches in four hours in Smethport, Pennsylvania, in July 1942. Slopes were washed down to bedrock and 15 deaths were attributed to that flood.
“That was not a tropical event,” he said. It was just related to a conductive thunderstorm system.
That location is not really that far away.
“When I hear people say flood systems are designed for 8 inches of rain, and that we’ll never get what happened during Hurricane Hazel, we’re seeing it all over the place. We’re going to far exceed it.”
He then showed council the Molesworth storm on radar imaging.
Jackson said if the storm track had been slightly north, within six or eight hours, much of Harriston would have been under water with no real opportunity to get to people because of high water flows.
He had noted earlier that Harriston’s flood risk has not changed. In the past 30 years, he suggested the flood risk is increasing.
Jackson said the MVCA commissioned a climate change study, which has shown an increase in precipitation, an increase in the number of days above 0C creating floods such as the one last year.
“We’re also seeing that rain before, which used to fall over a period of 8 to 10 to 20 hours, is now falling over 2 or 3 hours.”
For river systems like Harriston, which has a very small watershed, those intense thunderstorms are going to be what cause the problems. He said flooding is not a winter or spring event, it can happen any time of year.
Jackson said the combination of more intense rainfalls and greater temperature fluctuations in the winter, means more flooding.
Instead of thinking that the flood of last December will happen every 10 or 20 years, he suggested thinking should be if Harriston gets through the winter without one of them it should consider itself fortunate.
“As time goes on (that degree of flooding) will become more normal … for better or worse.”
His point is not if Harriston if will get such flooding again, it will be if it is next week or twice in the next five years.
“Based on the statistics there’s a 10% chance we will far exceed the flooding of last year.”
He anticipated that percentage will increase as the climate continues to shift during the winter.
Jackson said another issue to consider is all of Harriston’s arterial roads cross through the floodplain. “Chances are those roads will have significant damage.”
He said if there is a good chance of certain structures being prone to flooding, “It makes sense to get those structures out of there. Instead of engineering out the risk, he said the option could be simply to take the people out of the risk and there isn’t a problem anymore.”
He explained “The problem is not that the river floods, it’s that people live where the river is supposed to flood. Basically, the town of Harriston is built in the river’s floodplain. You have to realize we are trespassing on the river.”
Another main issue is education and communication, and getting residents to understand how flood prone their structures are.
The other issue is improving flood warnings to the community.
There also needs to be serious consideration to determine why, before 1910, there were no documented floods and what changed in that era
“The areas in Minto and Wellington North acted like sponges for the watershed – now they do not,” he said.
He noted the final options, were presented in the study created in the 1970s. Those included improving the hydraulics of the river, to have the water move through the town faster – or, physically rerouting the river.
“I’m not suggesting these are what need to be done. But they were options on the table 30 years ago – and it still hasn’t changed.”
He noted the MVCA’s role is not to provide direct intervention, but to provide the information to the agencies that would respond.
The MVCA issues four types of bulletins based on the weather conditions. “We provide the information and the agencies provide the response.”
Jackson said the MVCA’s flood forecasting system was set up in the 1970s when flooding was generally predictable, in the spring and over a long period of time. He compared storm events to gentle giants, that came across the entire watershed.
While one rain gauge would service a large area, now they are finding significant variations even between individual gauges.
Another major factor is that the MVCA’s funding has been frozen, thus making it difficult to make needed changes.
For Harriston, and several other communities, Jackson said there is a need to look away from traditional methods of flood forecasting based on river levels, to one based on precipitation and snow models, and to use the river flows to act as feedback to determine the accuracy of the information.
Using precipitation as a model, information can reach the municipality and other agencies that much sooner.
He noted one other option is to initiate a river watch. Unfortunately the current river gauge is downstream of Harriston, and past the junction of another river.
The catchment area is 112 square kilometres, which he said may seem big, but in the scheme of things is not, Jackson said.
He said the North Maitland River has very little forest cover. In September, a group including town and MVCA staff met to consider options to mitigate the flood hazards.
He said the group felt it important to also hold an open house to share some of that information and options.
Mayor David Anderson said Minto council had already endorsed the MVCA application for additional gauges.
Jackson added that “even with the best information possible, the MVCA can give Harriston very little flood warning time.”
