Depending on how hard or how long it rains will affect how much flooding occurs on your property. A powerful thunderstorm can bring large amounts of rain in a short period of time making it hard for the ground to absorb water as fast as it is falling. On the other hand, a steady rain that occurs over a long period of time - such as a few days - can cause the ground to become saturated making it hard for the ground to continue to absorb the rain.
New subdivisions are designed to carry rain water across properties at specific locations, also known as overland flow routes. Overland flow routes are shown on lot grading drawings. Unfortunately, many older subdivisions were designed and constructed without overland flow routes.
When a subdivision is first developed, yards are properly sloped to the designed drainage patterns. As people move into these homes or time passes, people “add personality” to their individual lots such as a pool, fence, gardens, etc. These items can become obstructions and make it hard or impossible for the water to exit the property as it was designed. This is when yard flooding can affect neighbouring yards or water can make its way into basements. These deviations from the approved drainage plan for your lot and/or subdivision may also be contradictory to local by-laws.
Flooding in London
This is a photo taken December 2008 when an accumulation of snow melted, the ground was saturated and/or frozen, and water could not escape from the backyards. At first glance, it appears this is a terrible flooding situation but, at closer inspection, things could have been a lot worse. Although the backyards have a substantial amount of water, it is not entering the homes. Proper grading has forced the water to stay in the backyard and not enter the home through a basement window or another opening.