Annual transport rates at two locations on the fore-slope.

Transcription

1 Sediment Transport by Currents Fore-slope Sediment transport rates and sediment concentrations were computed from the hydrodynamic model runs as well as from direct measurements of current velocities at two DFO current meter sites offshore from the Roberts Bank causeway. The long-term time series of tidal current measurements was used to estimate frequencies of sediment movement and net transport rates over the year. Results are summarised in Table 7.1 At Location 1 (4 m below LLW) the threshold for sediment transport was exceeded 19 % of the time (68 days/yr), but the peak transport intensities were low (Table 7.1). For example the bed mobility exceeded twice the threshold for movement for a duration of only 3.9 days/yr. Net transport was towards the northwest at current meter sites, reflecting the dominance of flood tides. Annual transport rates during flood and ebb tides, and the resulting net transport quantity were also computed. These results indicate the net sand transport to the northwest is approximately 200,000 tonnes/yr across a 1 km wide zone of the fore-slope off the causeway. Estimates of sediment transport rates by theoretical equations are subject to considerable uncertainties. Good predictive equations may provide estimates that range between one half to two times the actual transport rates. Table 7.1 Annual transport rates at two locations on the fore-slope. Transport Frequency (days/year) Parameter (T) Location 1 Location 2 > > > >4 14 Inter-causeway Area Computations of sediment transport in the inter-causeway area from the 2D hydrodynamic model showed maximum shear stresses are well below the threshold for sand transport in the navigation channel, ship turning basin and around Deltaport, and they were weak or below the threshold over most of the tidal flats (Figure 7.19). The shear stresses were generally lower than estimates on the fore-slope because the peak current speeds in the Strait of Georgia are higher than on the shallow, vegetated tidal flats. Areas of high transport conditions in the inter-causeway area were restricted to the tidal channels, where conditions were sufficiently high to develop dunes. Environmental Assessment Application Page 205

2

3 The net sediment transport rate was computed at 15-minute time steps at a number of points in the inter-causeway area for three different tidal ranges (large tide, mean tide and neap tide). In the main tidal channel net transportation increased in the seaward direction during the ebb tide due to the increased flow in the channel. By comparison, the transport rate was virtually constant along the channel during the flood tide. There was a net seaward movement of sediment near the channel outlet and very little near its landward end. Annual net sediment transport was estimated by integrating the loads in three representative tidal ranges over the year, combined with their frequency of occurrence. The computed net seaward transport in the channel varied down the channel between 3,000 and 15,000 tonnes/yr using the sand-bed channels equation (Engelund and Hansen 1967) and between 500 tonnes/yr and 3,000 tonnes/year using the van Rijn equation (van Rijn 1989). The computed net transport rates were very sensitive to small changes in flow parameters. The net seaward transport provides further evidence that the channel is still in a state of adjustment, and that headcutting is still occurring Summary of Existing Coastal Geomorphology Trends The following qualitative assessment of the general morphological trends at Roberts Bank is based primarily on historical observations made over the last 75 years, and on an understanding of the physical processes occurring on Roberts Bank. It predicts likely changes on the tidal flats if the existing conditions persist; i.e. no further developments on Roberts Bank. Historical evidence and results from the numerical models indicate that sediment transport rates on most of Roberts Bank are relatively low. Furthermore, most long-term morphological changes are associated with tidal drainage channel formation in the inter-causeway area, or channel shifting near the outlet of Canoe Passage. Future morphologic changes in the vicinity of Deltaport are expected to be minor. However, it is apparent that the network of drainage channels that formed after the earlier terminal and causeway developments is still evolving and has not reached a final equilibrium state. In particular, the largest drainage channel (Figure 7.11) is still extending seaward and landward. The large area of deposition near the head of the channel also appears to be extending further landward and accreting to a higher elevation. Field investigations and analytical studies indicate the trunk channel will continue to migrate landward until reaching approximately elevation 2 m (chart datum). The zone of sand deposition at the Environmental Assessment Application Page 207

4 head of the channel will also shift landward. This should not impact native eelgrass (Zostera marina) since it is confined to lower elevations. There is a chance that the smaller drainage channel to the east, may also continue to grow, particularly if the outlet can by-pass the crest protection structure by joining with the larger channel along the creek. Only minor erosion is necessary to cause this development. 7.5 ASSESSMENT OF IMPACTS Analysis Framework The following geomorphological conditions were analysed to assess the significance of impacts to the environment: the extent of potential impacts on wave conditions; the extent of potential impacts on tidal currents; the extent of potential impacts on sediment transport conditions; and the potential for new drainage channel development in response to the Project. Based on experience and knowledge gained from earlier developments on Roberts Bank, the aforementioned geomorphological conditions have the potential to cause: local erosion or scour adjacent to new structures; impacts to overall delta stability caused by interruption of sediment transport; changes to deposition patterns in dredged basins and adjacent areas; and eelgrass erosion on the tidal flats due to new drainage channel development. These environmental effects are discussed in more detail below. Given the relatively small scale of the proposed development in comparison to previous developments, potential impacts were likely to be more localized, and the analysis required a relatively fine resolution to detect any impacts. The assessment of morphologic response to the Deltaport Third Berth proposals was based on two approaches. First the numerical wave and hydrodynamic models was used to generate representative hydrodynamic simulations of With Project and Without Project scenarios. The difference between the velocities and bed shear stresses in each scenario was mapped to present the area of potential impact. Additional sediment transport computations were then made Environmental Assessment Application Page 208

5 to further assess the effect of these hydraulic changes on sediment movement patterns. Secondly, independent of the numerical modeling, interpretative geomorphic assessments from the experience of earlier developments at the two terminals and the field observations were made Impacts on Wave Conditions Outputs from the two scenarios were compared by computing the changes between them at each point in the model grid. Two parameters were used to represent the project impacts: changes in bottom velocity (U 1 -U 0 ) and changes in sediment mobility (T 1 -T 0 ). The impact on waves from the southeast (Figure 7.20) and south at low, medium and high points of a mean tide indicate the impact zone was very localized, being limited virtually to the footprint of the main structures. The wave height was increased directly in front of the wharf extension. The wave height was reduced directly behind the wharf and along the side of the causeway directly behind the third berth wharf extension due to sheltering. No significant changes in velocity or sediment mobility due to waves occurred in the vicinity of the drainage channels or on the upper tidal flats Impacts on Tidal Currents The projected bathymetry after construction of Deltaport Third Berth was digitized and used to generate a new computational mesh for the inter-causeway area, with the project area added as a no-flow boundary. The model was re-run for a range of different tide conditions. The impacts of Deltaport expansion were very localized, and confined to the immediate flow field around the modified structures and the dredged channel. The absolute magnitude of the changes was also very small, and to illustrate these changes detailed comparisons of the local area around the project site were prepared for representative large and mean (Figures 7.21 and 7.22) tidal ranges during flood and ebb tides. During flood tides the end of the present wharf extends into the flow, inducing a zone of flow divergence and a weak re-circulating eddy behind it (Figure 7.21). Extending the wharf further displaces this flow pattern landward, but does not appear to change the overall pattern. During the ebb tide, the wharf causes the flow to accelerate around its end creating a zone of local flow concentration (Figure 7.22). Extension of the wharf creates higher flow concentration at the end of the structure. However, the absolute magnitude of the peak velocities remains very low and is typically less than 0.1 m/s. These results indicate that the primary impact of the proposed Deltaport expansion will be to shift the existing flow patterns shoreward. Environmental Assessment Application Page 209

6

7 The velocity differences between the two scenarios were computed at each node in the mesh, then plotted and contoured to illustrate the spatial extent of the project impacts (Figure 7.23). Velocity differences of less than 0.05 m/s were considered to be insignificant as they are within the tolerance of the numerical model computations. The impacts are highly localized and so small as to be near the tolerance of the numerical computations (apparent velocity differences far from the project are induced by numerical imprecision rather than actual project impacts) Impacts on Sediment Transport The magnitude of the wave bottom velocities and tidal current velocities in the vicinity of the proposed expansion area are well below the threshold for significant sediment transport of fine sand. Therefore, no significant alterations in sediment transport patterns are expected to occur. Furthermore, no significant local scour or infilling is anticipated adjacent to the wharf, navigation channel or in the tug basin Impacts on Tidal Drainage Channels The main excavation for the Deltaport Third Berth Project involves deepening the approach channel to the wharf extension (Figure 7.24). This excavation occurs in deep water where the existing ground level is -12 m CD. Based on our understanding of past tidal channel formation on Roberts Bank, there is no potential for significant headcutting to occur from an excavation at this depth. Furthermore, due to the large depths, the peak velocities generated by tidal currents in this area are very low; typically less than 0.2 m/s. Therefore, the risk of any significant new channel formation occurring from deepening the north end of the existing ship channel appears to be negligible. Preliminary plans show the tug basin extending into the existing crest protection, which is at 0 m CD. Some modification to the structure is anticipated, although details are not finalized. Excavation of the tug basin may involve cutting into portions of the tidal flats that are above LLW. Theoretically, there is potential for minor headcutting to be initiated, particularly if the crest protection was eliminated. However, given the limited extent of the excavation and its orientation with respect to the existing drainage patterns, the potential for headcutting is expected to be minor. Furthermore, this effect will be controlled if the crest protection structure is modified to tie-in with the tug basin. Environmental Assessment Application Page 211

8

9

10

11

12 7.5.6 Conclusions Based on the combined analysis of the numerical modelling and geomorphic investigations six conclusions regarding the proposed Deltaport Third Berth Project have been reached. Points 1 through 3 relate specifically to the proposed Project, while 4 through 6 are ongoing effects from existing developments at Roberts Bank and are not affects from the Project. 1. Impacts of the proposed Deltaport Third Berth Project on tidal currents and waves will be small and confined to the immediate flow field around the Deltaport Third Berth wharf extension and the dredged channel. Tidal velocities will increase locally along the front of the wharf extension, and decrease behind it. The magnitude of the velocities in the impact zone is well below the threshold for any significant sediment transport. No significant morphological impacts are expected to develop in response to changes in tidal flow magnitude or direction. 2. The end of the present wharf extends into the current flow, which causes zone of flow divergence and a weak eddy to form behind the wharf during flood tides. Extending the wharf shifts the flow pattern landward but does not appear to significantly change the overall pattern. During the ebb tide, flow accelerates and concentrates at the end of the wharf. Extending the wharf will create higher flow concentration at the end of the structure, however the absolute magnitude of the peak velocities remains very low and is typically less then 0.1 m/s, which is very similar to the present situation. 3. The risk of new tidal drainage channels forming in response to planned navigation dredging appears to be very low. The proposed channel excavation would occur in relatively deep water (below LLW and the tidal flats where headcutting is initiated) and will not induce significant headcutting on the tidal flats. There is some potential for localized disturbance of the tidal flats near the proposed tug basin, which could induce shallow, small-scale channels, although this can be controlled if the existing crest protection structure is modified. Further design information on the tug basin and re-aligning the crest protection structure needs to be reviewed and assessed for more certainty. Environmental Assessment Application Page 216

13 4. Previous developments on Roberts Bank induced long-term morphological changes on the tidal flats, which continue at the present time. The main changes involved initial headcutting on the tidal flats, expansion of eelgrass beds and the formation of a network of tidal drainage channels. 5. The large drainage channel in the middle of the inter-causeway area continues to expand in both seaward and landward directions. Sand from this channel is being transported up onto the tidal flats during flood tides to form a large sand lobe. It is expected that the channel will expand further shoreward before eventually stabilizing. Shoreward expansion is limited by the higher elevation of the tidal flats and the reduced amount of runoff that can be generated during the tidal exchange. 6. The crest protection structure appears to have effectively limited the growth of at least two of the smaller channels. However, the large channel near the centre of the inter-causeway area has partially by-passed the structure and continues to expand. This site, as well as similar experience at the smaller structure near the B.C. Ferries terminal, illustrates the difficulty of fully preventing channel formation when the contributing drainage area leading to the channel is large. This situation does not arise at the Third Berth Project Significance of Effects Based on the significance criteria for this assessment (the extent of changes to coastal processes and the potential for the development of new drainage channels), there are no significant effects of the Deltaport Third Berth Project on the coastal environment or processes at Roberts Bank. Changes continue to occur as a result of earlier developments on Roberts Bank, but this investigation into the introduction of the Deltaport Third Berth Project has not shown any increase in the magnitude or extent of these changes. Environmental Assessment Application Page 217

14 7.6 MITIGATION The results of this investigation show that the effects of the proposed Deltaport Third Berth Project on the hydrodynamics and coastal processes at Roberts Bank are essentially unchanged. Therefore, no specific mitigation measures are proposed. The wave and current climates in conjunction with the layout of the coastal developments drive sediment transport changes at Roberts Bank. Many design features were incorporated into the Deltaport Third Berth Project to minimise or avoid potential impacts on the hydrodynamics and coastal processes of Roberts Bank. These include: sloping rock revetment design to minimise reflection and propagation of waves; crest protection Subject to detailed engineering design on the wharf structure, the extension of the crest protection into the new Tug Basin will be assessed from a geomorphological perspective and planned and constructed in advance of dredging for the basin to mitigate for potential impacts on hydrodynamics and coastal processes. 7.7 RESIDUAL EFFECTS As there are no significant effects on coastal processes arising from the construction of the proposed Deltaport Third Berth there can be no residual effects after mitigation. Environmental Assessment Application Page 218