Since 2010, the City of Toronto has implemented many effective schemes in resolving the chronic flooding problem of the Hoggs Hollow neighborhood. In 1954, Toronto suffered one of its worst flooding caused by hurricane Hazel. Our soil engineer, Vic Wood lived through that disastrous period. He remembered vividly that the part of the Don River that flowed through Hoggs Hollow swelled so high that it flooded all of the houses in the neighborhood. Hoggs Hollow had a history of flooding until about five years ago when the water diversion projects started to take effect and the water level of the river became visibly controlled.
A few builders in that area, without knowing the high water table and flooding issues, went ahead and excavated to whatever depth they desired and found nothing but problems afterwards. Water had to be pumped out of their basements almost continually.
Our lot backed onto the Don River.
Vic Wood actually had a record on hand showing how high the water table could be on our lot. He warned us though that it was not hard data and should be used with a substantial safety factor. We collected fifteen core-drill samples to a depth of twelve feet and found water at eleven feet. We excavated to a depth of nine feet ten inches and stopped there. Except in one corner where the soil was damp, the rest of the excavation was bone dry. And this was done in the month of March when the water table should be high during the spring thaw.
Our structural engineer Lawrence Tse, Vic Wood and myself got together and designed a foundation that could survive flooding as bad as the one caused by hurricane Hazel. The small area with the damp soil was walled off and not included in the basement area. A matrix of weeping tiles was laid inside the foundation walls under the concrete floor and drained into two large sump pits on different sides of the house. The standard footing weeping tile was installed on the outside around the footings. Twelve inches of two inch clear crushed stone was placed underneath the concrete floor slab to hold the water if any. Plastic pipe sleeves three inches in diameter were inserted vertically into the crushed stone. The sleeves were braced to prevent being knocked down during the pour. Then a six inch thick concrete slab was poured. When the concrete was cured, the top of the pipes were grinded off flush with the floor slab. The perforation of the floor might seem ridiculous when you wanted to prevent water from coming in. The concept was rather counter-intuitive. The actual explanation was more scientific. Despite the claim of some builders and waterproofers, no basement could be made waterproof indefinitely – not in the sense of water-tightness of a submarine. A house had too many joints and components that were not welded together. A house settled and shrank. Joints would eventually open up. Waterproofing material that could stretch would age, harden and become less flexible. Eventually, it would crack. So our idea of waterproofing was actually water redirection.
During a flood or swelling of the water table, a seamless concrete floor in the basement became the wall of a pressure vessel against the tremendous hydrostatic pressure built up underneath. The water would find any cracks and holes to get into the house. Instead, we perforated the floor to relieve the water pressure so it would have time to enter the weeping tiles and eventually drain into the sump pits.
Three years after my client moved in, a flash rainstorm raised the water level of the Don River so high that it breached its banks. The water came up to the backyard! The backyard was normally about ten feet above the river level. Most of the basements on the street got flooded. Our house escaped the misfortune. In its fifteen years of existence, the basement remained dry.
Because our basement floor was more than a foot above the normal water table, the sump pumps would come on only during heavy rain. The rest of the time they were silent.
Walk Out Basement – when flood water overshot the top of the lawn in the backyard, nothing could stop the water from entering the basement when there was a pair of doors eight feet below the lawn.
Underground Parking – during a rainstorm, all the sump pumps were working overtime. You would not want extra load of water rushing down the driveway slope into the basement parking garage. In any case, relying on mechanical means to get rid of water was only the last resort.
Consideration from the city to raise the roof another twelve inches so the entire house could be raised by the same amount. This would have left two feet between the basement floor and the normal water table. But the city would never had permitted building in a flood plain any differently than building on a usual street. In fact buildings in a flood plain should ideally be raised on stilts. But zoning regulations made no attempt to differentiate.
Permission to cut down trees that were detrimental to the proper performance of drainage pipes and weeping tiles. Due to the water rich soil, trees in the area developed very extensive root system. Roots would enter the drainage pipes and weeping tiles and clog them up. Some of the pipes were buried deep underground making repairs extremely costly.
The building code should allow homeowners to do away with eavestroughs. Hoggs Hollow was in a river valley covered with trees. Eavestroughs and downspouts were constantly filled with leaves. When they were blocked, water ended up inside the house. What the building code should really require homeowners to do was to grade their properties properly (see section Construction Management) and install drainage channels along the property lines.
We witnessed many disputes amongst neighbors about site drainage. Neighbors tend to blame each other when their basements get flooded. Read about sideyard issues and the need for neighborly cooperation.