Vancouver Port Authority Northwest Hydraulic Consultants Ltd/Triton Consultants Ltd. Roberts Bank Container Expansion File: 33863 Coastal Geomorphology Study – Appendix C - 46 - November 2004 6.0 EXISTING DRAINAGE CHANNELS 6.1 CHANNEL NETWORK Figure C6-1 shows the present network of channels in the inter-causeway area. As described previously in Section 5.4, four main channel systems initially formed at the head of the original dredge basin in 1969 and appear to be sites of preferential channel formation when the ship turning basin was dredged in 1982. Channels 3 and 4 did not continue to develop, and are presently small and largely obscured by eelgrass. Channel 2, which was initially the largest channel, has been partially arrested by the crest protection structure and its overall development appears to have stalled. We have focused our attention on Channel 1 and Channel 2, which each have a network of smaller channels draining into a single trunk channel. The following sections describe these two channel systems in detail as well as the physical processes modifying them. The channel system is characterised by a trunk (main) channel that extends from the crest protection structure upwards onto the tidal flats. Two main tributaries and a series of smaller channels deliver flow that drains from the tidal flats on the ebbing tide, to the upslope end of the trunk channel. The channels draining through and around the sediment lobe have a meandering planform and are connected in a dendritic pattern. Small channels seem to drain water off portions of the tidal flats on the ebbing tide, conveying flow into the main channels. At low tide, the larger channels have defined banks while the smallest channels appear as shallow depressions in the tidal flat surface. The planform shape of the trunk channel is generally straight and the uppermost end terminates bluntly at a sandy bar deposit. The main trunk of Channel 1 has a width of approximately 90 m near the crest protection structure, and extends for a length of about 700 m. The trunk channel of Channel 1 splits at the crest protection structure, running parallel to the crest for a further 370 m before draining into the ship turning basin. The main trunk of Channel 2 has a width of approximately 40 m near the crest protection structure and extends 350 m up onto the tidal flats. In planform, the channel systems are similar to ‘dendritic’ networks described by Howard (1967) and are analogous to a typical terrestrial-fluvial drainage network. This similarity allows the Vancouver Port Authority Northwest Hydraulic Consultants Ltd/Triton Consultants Ltd. Roberts Bank Container Expansion File: 33863 Coastal Geomorphology Study – Appendix C - 48 - November 2004 adaptation of standard channel network descriptions (Strahler, 1957). Using this approach, the smallest tributary channels that could be identified on the orthophotos are designated as Order 1 streams. When two first order streams converge, the resulting larger channel is termed Order 2. Following this approach, the main trunk of Channel 1 is a fifth order stream and the main trunk of Channel 2 is a fourth order stream. Table C6-1 summarizes some other key network parameters, including drainage density (total stream length/total area), stream bifurcation ratio and average stream length. Figure C6-2 compares the drainage density of the two largest drainage networks with values reported on other basins throughout the world. The drainage density of Channel 1 is comparable to other basins and demonstrates that although the physical processes on Roberts Bank are very complex due to the varying effects of tides and waves, the overall channel network follows similar rules of behaviour and scaling relationships as conventional basins. The drainage density of Channel 2 plots near the lower limit of the graph, indicating the degree of channel development is much lower than other drainage networks. This is believed to be due to the influence of the crest protection structure which has partially arrested further channel development by preventing headcutting from occurring. Table C6-1: Drainage Network Characteristics Parameter Units Network 1 Network 2 Σ Length of Channels (m) 8,318 1,577 Area of Basin (m2) 2,887,036 143,960 Drainage Density (Σ -1 2.9 1.1 Length/Area) (km ) Average 1st Order (m) 25 45.3 stream length 2nd Order (m) 58 53.5 in each stream 3rd Order (m) 98 684 order 4th Order (m) 920 -- Average 1st Order (m) 16 18 stream width 2nd Order (m) 4 11 in each stream 3rd Order (m) 2 8 order 4th Order (m) 1 -- Order 1/Order 4.1 3.75 Bifurcation Order 2/Order 3.4 4 Ratio Order 3/Order 5-- 4 Vancouver Port Authority Northwest Hydraulic Consultants Ltd/Triton Consultants Ltd. Roberts Bank Container Expansion File: 33863 Coastal Geomorphology Study – Appendix C - 50 - November 2004 The network characteristics also provide a means for estimating the type of channels that can develop in defined drainage areas on the flats. For example, the drainage density and bifurcation ratios provide a means to determine the upper limit of Stream Order that can develop in a defined drainage area. For example, based on the observed network geometry in Channel 1, it can be shown that a drainage area of at least 0.25 km2 is required to generate a Third Order stream channel on the tidal flats (assuming a drainage density of 3 km/km2 and a bifurcation ratio of 4). Such channels would have a typical width of 4 m and a length of 100 m. Based on the present topography in the inter-tidal area, the approximate limiting elevation range for Third Order streams appears to be below El. +2m. Larger channels representative of Fourth Order or Fifth Order streams would not be expected to develop at higher elevation areas because the contributing drainage area is too small to sustain them. This finding is in agreement with the observed conditions on the higher portions of the tidal flats where only minor First Order and Second Order channels can be found. 6.2 CHANNEL CROSS-SECTION The shape of the channel cross-section plays an important role in defining the drainage basin area. Figure C6-3 shows a typical cross-section surveyed during a low tide (RB/LB indicate right bank/left bank from the viewpoint of the ebbing flow). The section shows raised levees on either side of the channel, and a difference in water level elevation between the channel flow and the adjacent eelgrass beds. There is typically no vegetation in the channel or on top of the levees but outside the levees thick beds of eelgrass cover the tide flats. Eelgrass is sensitive to drying, so the beds thin shoreward, becoming sparse to non-existent above the 2 m elevation band. The absence of eelgrass within the channel zone is presumably because the in-channel flow velocities are too high and the tops of the levees are dry for a longer portion of the tidal cycle. At the outer margin of the levees, eelgrass is sparse and appears to be in the process of being buried by the levee as sediment is transported out of the channel and deposited on the eelgrass beds. At lower tide levels the levees restrict flow from entering the channel from the surrounding eelgrass beds, except at specific points. Figure C6-4A shows a levee crevasse (breakthrough) with a small splay of sediment that has been deposited into the channel. Photo C6-4B shows a Vancouver Port Authority Northwest Hydraulic Consultants Ltd/Triton Consultants Ltd. Roberts Bank Container Expansion File: 33863 Coastal Geomorphology Study – Appendix C - 51 - November 2004 levee prograding into the eelgrass bed with a sparse growth of eelgrass at the outer margin that appears to be in the process of being overwhelmed by the depositing sediment. Tidal-flat Channel Cross Section 1.0 RB LB Water Level 0.8 Water Level 0.6 0.4 0.2 Elevation (m) 0.0 0 20 40 60 80 100 120 Distance (m) Figure C6-3: Cross Section of Tidal Channel The levees described here are analogous to landforms found in other river systems, for example anastomosing rivers and birdsfoot type deltas. In both cases, overbank flow slows as it flows over the shallow banks and suspended sediment is deposited and trapped by vegetation growing along the channel margin. These levees slope gently from the channel bank into the flood basin outside the channel (Reinech and Singh, 1973, p.244). The term ‘flood basin’ describes the flood plain of an anastomosing river and the coastal bay of a delta. The analogous flood basin on the tidal flats at Roberts Bank is the eelgrass beds. 6.3 DISCHARGE MAGNITUDE Estimates of the tidally-varying discharge in the drainage channels were made by (1) direct field measurements, (2) by two dimensional numerical modelling simulations and (3) by simplified tidal prism computations. Discharge and sediment transport are bi-directional, flowing up the tidal-flat slope as well as down. The discharge in the drainage channels is governed primarily by the tidal prism of the contributing drainage area, which in turn, is governed by the tidal range in the Strait of Georgia, modified by resistance and storage effects from eelgrass and local tidal flat topography. The range in flow magnitude and sediment transport is relatively limited due to the restricted range of the tide. Consequently, the upper limit to the flow magnitude in the drainage channels is approached several times each month. Measurements of discharges were made in Channel 1 periodically during the tidal cycle on three separate dates. These measurements Vancouver Port Authority Northwest Hydraulic Consultants Ltd/Triton Consultants Ltd.