Flood Risk Zones
Flood risk zone maps have been prepared for sixteen communities in the province most prone to flooding. Floods occur when, because of high discharge, a river overtops its banks. This often occurs during spring run-off, and can be made worse if the channel is blocked by an ice jam. But heavy rain at any time can cause floods.
Floods can also occur near the coast when high tides, onshore winds and a storm surge combine, causing the flow of a river to back up.
The flood risk zone maps identify areas (in red) that are likely to be flooded once in any twenty-year interval. These areas are "designated floodways", and municipalities are advised that their zoning regulations should prohibit building in these areas. Areas identified in pink are the "floodway fringe" -- areas likely to be flooded once in a hundred-year interval. Buildings in these zones should be floodproofed.
Flooding intervals, like other recurrence intervals, do not imply that the events occur regularly every twenty or one hundred years; it is quite possible to experience twenty-year or even one hundred-year events in consecutive years.
The flood risk zone map for Badger shows that the flood-plain of the Exploits River and its tributaries is liable to flood. These floods are caused by ice-jams and high spring run-off, in which the ice-dams are occasionally dynamited to allow the floodwater to flow away. The map shows a number of residential streets inside the 100-year flood risk zone, or floodway fringe; there are even some streets within the 20-year flood risk zone, or designated floodway. From this it can be suggested that these streets were developed before the map was available, or before the community had regulations in place to minimize construction in the flood zone.
The map for Placentia shows a community built on a beach-ridge complex, very close to sea-level. Almost all the built-up area is in the 20-year flood zone, including the historic town centre. Floods here are caused by a combination of high tides, a storm surge and high wind and waves. As a result the water in South-East Arm backs up (the map does not show that the Arm extends from the bridge on Highway 100 to around the back of the town). To reduce flood damage an embankment has been built along the Placentia Bay shore, and a sea-wall along the harbour (the narrow part of the Arm next to the town centre, south of the bridge). If one of the effects of global warming is a rise in sea-level, then it will be increasingly difficult to protect Placentia from floods.
The built-up areas of Stephenville for the most part avoid the flood risk zones. Flooding here results from high discharge of Warm Creek (the stream draining Noels Pond) and its tributary, Blanche Brook, either during spring run-off, or when there is heavy rain, such as occurred 8 December 1990. It has been suggested that logging in the valleys behind the town has led to an increase in flooding. Removal of vegetation allows water to flow more rapidly overland to the streams.
Flood-risk maps offer no help to communities which suffer flooding from unexpected extreme events. For example, on 11 September 1995, heavy rain from Hurricane Luis caused damage to roads and buildings near Burin on the Burin Peninsula estimated at about $500,000.
Reference: Water Resources Atlas of Newfoundland.