Erosion and deposition in the Petitcodiac River at Moncton, N. B.

Abstract

The Petitcodiac causeway, the second link between Moncton and Riverview in southeastern New Brunswick, has been the source of much controversy since its construction in 1968. Although it contains five flood gates, it is essentially a dam which has blocked most of the Petitcodiac River, causing some adverse effects. The impacts of the causeway include: (1) blockage and deposition of clay, silt, and fine sand, moved up river by the Bay of Fundy tides, and (2) subsequent erosion of this material due to the variation in flow rate caused by periodic extended openings of the causeway gates. Twenty nine bottom profiles were examined in cross-sections of the river. The profiles were taken at one location and examined for changes in cross-sectional area relative to change in discharge. These results were used to calculate an optimum range of flow rates whereby erosion and deposition are balanced. An average value for river velocity was calculated to be approximately 1.25 centimetres per second. This value, according to Hjulstrom's Diagram, will not erode much material, neither will it deposit any more silt than is already part of the riverbank. In essence, it balances erosion and deposition for the river along the reach studied. It will, however, continue to deposit the fine sand that is in suspension in the river. Although this conclusion is subject to some assumptions and restrictions, it indicates that the Petitcodiac River can approximate equilibrium conditions with controlled flow.