Channel Bottom Conditions
Background & Theory
The majority of deep sea vessels destined for Fraser Port are required to schedule their inbound and outbound times to the tide cycle. Avadepth, an acronym for available depths, was first developed in 1986 for the Canadian Coast Guard, and is designed to assist the mariner in determining the maximum draft and the best sailing times on the river to safely transit the Fraser River - South Arm Channel.
A complete redevelopment of the reporting system was undertaken to increase flexibility and to bring the program in line with current computer technology. The reporting system now enables clients to recalculate depth forecasts to incorporate any significant change in river discharge that can occur daily during periods of heavy rain or during "freshet" - a term used to describe the rising water levels and increased flow conditions during spring run-off of a melting snowpack.
Theory of Operation
Avadepth calculates the depth at any point along the river channel by adding together the effective channel sounding (surveyed depth below nominal low-water level), and the tidal assist (height of the water surface above low-water level), subject to the following time-sensitive restrictions:
- Channel soundings vary from time to time due to the effects of sedimentation, scour and dredging operations. Avadepth maintains a time-series database of minimum soundings for every one-kilometre interval along the channel, based on historical survey data, and automatically selects the soundings data in effect on any date specified by the user.
- The minimum effective sounding for any chainage interval depends on whether the channel is being used for one-lane or two-lane navigation.
- Tidal assistance varies from hour to hour, its magnitude being determined by the daily tide cycles and by the flow-rate in the river channel; depths are substantially greater in flood periods than in drought periods. Tide levels for any time and date are predicted in tide-tables developed by Institute of Ocean Sciences (IOS), Fisheries and Oceans Canada, with multiple figures provided to address several possible flow-rate conditions.
- For any time and date, Avadepth selects a flow-rate scenario, either by prompting the user or by examining a suitable hydrographic time-series; this permits selection of the most appropriate flow-rate to be used for selecting tide-table records.