Features Surface Water Water Issues
After the flood

Understanding why – and by how much – ground water levels rise during a flood can help us determine losses and prevent future damages. Knowledge of this effect is required when rebuilding and future building occurs, so that the risk of rising ground water may be taken into account.

Understanding why – and by how much – ground water levels rise during a flood can help us determine losses and prevent future damages. Knowledge of this effect is required when rebuilding and future building occurs, so that the risk of rising ground water may be taken into account.

This map, taken from the report entitled “Surface Materials of the Calgary Urban Area” (Fenton, Alberta Research Council, 1986), shows downtown Calgary and the extent of gravels (in orange). Map courtesy Fenton, Alberta Research Council Advertisement

After Calgary’s major flood of 2013, many homeowners near the river were surprised to see their basements filled with water given that the flood waters did not reach their lots.

Over 500 homes in Calgary were affected in this manner. Many people thought they were victims of a sewer backup. Perhaps one reason for claiming sewer backup is that damage caused by sewer backup is covered by the homeowners’ insurance, but damage caused by flooding is not.

Unfortunately, in most cases (over 90 per cent of the time, by one insurance adjuster’s estimate), the damage was not caused by sewer backup, but by rising ground water levels. Why do ground water levels rise during a flood and how much can they rise? These are important questions – necessary for both determination of previous losses and prevention of future damages.

Calgary, like many cities in Canada, is a river city. The earliest development, along with the downtown and relatively expensive residential development, has taken place near the Bow and Elbow rivers. These rivers are in ancient riverbed channels and are underlain by gravels, sometimes up to 30 metres thick. (See map showing surface materials of the city’s urban area.)

Does it matter whether your home is underlain by bedrock, clays or gravels? Yes, it does, and here is the reason: in gravels, the high permeability of the sediment means that the water level rise is almost as high, and almost as quick as the rise in water in the river. So if your lot is four metres above the normal river level – with the water table normally one metre below your basement – and the river rises two metres, your street might be dry but your basement might be surrounded by gravels that are saturated one metre above the basement floor. Basements are not usually “tanked,” so the water will enter.

Less permeable sediments show a time lag in ground water level rise after the river levels have risen, with the result that the flood water may start to recede before the ground water levels have matched their height.

Ground water levels will rise in floods, but we did not appear to have any continuous recordings of ground water levels in the gravels in Calgary. The Alberta provincial government has a network of observation wells in Alberta that read water levels on a continuous basis; none of these had been placed in the shallow gravels in Calgary. The City of Calgary does not measure water levels in the ground water either.

As an alternative, the author undertook measurement of ground water levels in a monitoring well in the gravels near the Bow River in June and July 2014. Flooding of the Bow River in 2014 did not occur, but the usual increase in water levels in the river was observed due to spring rains and snowmelt from the nearby Rocky Mountains. Data is available on river levels from federal government river-gauging stations.

The water levels in the monitoring well rise as much, or almost as much, as the rise in the river levels. A time lag of about one day is observed between a rise in river levels and a rise in ground water levels almost 290 metres away. The rise and time lag would be expected to vary depending on the distance away from the river and the type of sediments in the area.

If river levels rise by two to three metres, the ground water will likely rise nearly as much in areas underlain by gravels near the Bow River. The water levels in this monitoring well rose approximately 0.5 metres when the river level rose the same. If the river was to rise by one to two metres during a flood, ground water levels would likely rise by the same amount, bringing ground water levels above basement levels.

Can anything be done to prevent this? Basements can be tanked to prevent seepage, but this is probably easier to do at the construction stage than it is after the basement has been developed. One problem with sealing a basement is that unless a basement is designed to take the water pressure, the walls or the footings may collapse or heave, causing significant damage to the home. Personnel from the City of Calgary recommended installation of one-way valves on the sewer lines and installation of sump pumps. Installation of sump pumps is unlikely to be effective during a flood in areas underlain by gravels. The sump pumps are supposed to be tied to the storm water sewer system, a system that automatically shuts in during a flood, and there would be no place to drain the water. As every water well driller knows, the drawdown in water levels in a gravel at the slow pumping rates that a sump pump can do (less than 10 gallons per minute) will result in only negligible lowering of the water table along the perimeter of a house. Using reasonable values of aquifer parameters, pumping rates on the order of 100 gallons per minute are required. In addition, the water has to be disposed of – not an easy accomplishment when the storm sewers are closed. Pumping water onto the street next to your neighbour’s house may not be a welcome plan.

Knowledge of this effect is required when rebuilding and future building occurs, so that the risk of rising ground water may be taken into account.

Ken Hugo is a technical director and hydrogeologist with Groundwater Information Technologies (GRIT).