6.0 SEDIMENT TRANSPORT

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1 Coastal Geomorphology Study November SEDIMENT TRANSPORT 6.1 SEDIMENT CHARACTERISTICS Bed Sediment Properties The size distribution of sediments on Roberts Bank was determined using previously published data (GeoSea Consulting, 1996). The distribution of surface grain size is shown in Figure 2-4. Most of the sediments on Roberts Bank are sand or silty sand. The surface sediments become finer in the upper flats near the +2 m contour. However, a number of test pits showed the underlying sediments were noticeably coarser and were mainly clean sand. Finer sediments are also found in portions of dredged turning basin near Deltaport. Although some of these sediments may be recently deposited, it is also likely that some of the sediments exposed by dredging were laid down at an earlier stage in the delta s development and are not representative of contemporary sedimentary environments. Further offshore, the sediments remain relatively coarse (medium sand with a typical D 50 of 0.25 mm to a depth of around 100m. At greater depths the sediments become noticeably finer with depth, becoming predominantly sandy-silt to silt Sediment Sources Potential sediment sources and inflows to Roberts Bank include: Fraser River inflows from the South Arm and Canoe Passage; littoral drift from erosion of cliffs at Point Roberts; onshore-offshore movement of sediments; re-mobilization of Roberts Bank tidal flats due to erosion and transport; and re-mobilization of dredge spoil dumped either in deep water on in shallow areas adjacent to structures. As discussed in Section 3.2, most of Roberts Bank is isolated from significant influxes of coarse sand loads from the main arm of the Fraser River. Most of the sand load is deposited at the mouth of the main arm off Sandheads and moves by gravity flows down the foreslope into deep water. The amount of sand discharged onto Roberts Bank via Canoe Passage is also very low.

2 Coastal Geomorphology Study November 2004 Based on previous sediment budget estimates, this load is in the order of 50,000 tonnes/year, which is negligible in comparison to the amount of sand stored in the tidal flats themselves. An approximate estimate of the sediment reservoir was determined by computing the mass of sand contained between the HW and LLW level in the tidal flats using the 2002 bathymetry. The estimated quantities are as follows: Canoe Passage to Causeway: Causeway to Tsawwassen Ferry Terminal: Tsawwassen Ferry Terminal to Point Roberts: 30 x 10 6 tonnes 20 x 10 6 tonnes 7 x 10 6 tonnes Fine sediment (silt and clay) is dispersed over a wide area of the Strait of Georgia in the Fraser River plume. However, most of the bed material in Roberts Bank tidal flats contains relatively little of this material, indicating it behaves as wash load and can be re-suspended by waves and currents. Construction of the Roberts Bank Causeway has clearly deflected the Fraser River plume from the inter-causeway area, which should have reduced fine sediment inputs to this portion of Roberts Bank. A number of studies have suggested sediment to Roberts Bank was previously supplied by northward moving littoral drift originating from the vicinity of Point Roberts (Luternauer and Murray, 1973; HayCo, 1996). Construction of the Tsawwassen Ferry Terminal would have obstructed this process. Therefore, under present conditions, most sediment transport on Roberts Bank involves re-distribution of material contained in the sediment reservoir of the tidal flats themselves, rather from external sources Inferred Patterns of Sediment Movement Delta Foreslope Several previous studies have indicated that net sediment movement is northward along the foreslope of Roberts Bank (Luternauer and Murray, 1973; Kostaschuk et al., 1993; GeoSea Consulting, 1995). Luternauer et al. (1977) reported bottom currents on the foreslope strong enough to generate bedforms offshore in deep water. These findings are in accordance with tidal

3 Coastal Geomorphology Study November 2004 current observations, which have shown that northward flood tidal currents are stronger than the southward ebbing tidal currents. Some concerns were expressed by Kostaschuk et al. (1993) on the stability of the foreslope due to the presence of a prominent dune field off Roberts Bank. However, subsequent studies have demonstrated that the dune field was actually dredge spoil placed in deep water (HayCo, 1996). Tidal Flats Sediment movement near the mouth of Canoe Passage is dominated by fluvial processes due to outflows from the distributary channel. As discussed in Section 3.3.1, there has been ongoing channel shifting near the mouth of Canoe Passage, which has caused periodic channel incision and erosion. Sediment tracer studies were carried out in 1982 to monitor sediment transport on Roberts Bank (McLaren and Buckingham, 1983). This involved placing fluorescent dyed sand at several sites on the flats and then observing the dispersal patterns. These results showed net sediment movement under normal tidal currents was very low on the shallow flats. However, the sediment tracer dispersed very rapidly in the vicinity of local tidal drainage channels, indicating transport was dominated by localized fluvial processes. GeoSea Consulting (1995) produced maps showing apparent sediment pathways on Roberts Bank that showed northward movement from deep water into the inter-causeway area and then development of a clockwise gyre onto the upper tidal flats. A similar gyre was proposed on the southeast side of the Tsawwassen Ferry Terminal. The assessment of sediment movement was based on a statistical analysis of surface sediment samples rather than an assessment of physical processes or direct observations. A number of significant limitations have been identified with the general approach of relying solely on grain size statistics to predict sediment transport vectors (Church, 1994). On this basis, we have decided that other physically based methods should be relied upon to infer sediment movement patterns at Roberts Bank. Experience at Deltaport suggests that actual sediment transport rates in most of the intercauseway area are low. For example, no significant infilling has occurred in the dredged approach channel or ship turning basin since its construction. High sediment transport rates have been observed on the bar surface at the head of the prominent drainage channel during flood tide

4 Coastal Geomorphology Study November 2004 conditions. However, this high transport occurs over a relatively short duration during flood tides and is relatively localized in extent. Further details on these processes are described in Appendix C, Section 6.4. The trench dredged by BC Hydro in 1959 for their power line crossing on the south side of the Tsawwassen Terminal provides further evidence on the relative magnitude of sediment transport on the tidal flats is relatively low. Although the excavation triggered drainage channel formation on the upper portion of the tidal flats, the trench does not appear to have experienced significant infilling. If high littoral transport was occurring it is likely that the trench would have infilled. Further discussion of this feature is contained in Appendix C, Section SEDIMENT TRANSPORT BY WAVES Method Sediment transport by waves was characterized using the methods developed by van Rijn (1989). The main advantage of this approach is that the equations are directly comparable to relations developed by van Rijn and others for steady currents in rivers and tidal areas. This provides a means of comparing the relative significance of wave and current induced transport. However, all methods that attempt to compute sediment transport from hydrodynamic conditions are subject to considerable uncertainty (Lehfedt, 2001). Therefore, the results of these computations have been used primarily as a tool for assessing relative changes in transport conditions under various scenarios. The general method is based on relating sediment mobility to the applied bed shear stress. The maximum bed shear stress (τ) due to waves was computed as follows: 2 τ = 1/2ρf w U 0 where, f w is the wave friction factor, U 0 is the maximum orbital velocity due to waves, and ρ is the density of seawater The wave friction factor was estimated using van Rijn (1989) from the relative roughness height (k s /A) and Reynolds number:

5 Coastal Geomorphology Study November 2004 Re = (U 0 A/υ) where, k s is the roughness height, A is the amplitude of water particles moving near the bottom, and υ is the kinematic viscosity. Test calculations suggested the flow will remain in the rough turbulent or transitional range under most conditions, indicating the friction factor is independent of the Reynolds number. The influence of eelgrass on friction was not directly considered. The bed mobility under wave action was characterized by van Rijn using three main parameters: N s 2 U 0 = particle Froude number ( s 1) gd 50 D ( s 1) g = [ D50 dimensionless grain diameter ν 1/ 3 * ] 2 T τ τ c = excess shear parameter τ c Z ws = sediment suspension parameter κ U * where, D 50 is the median grain size, (s-1) is the specific gravity, u 2 is the kinematic viscosity, w s is the particle fall velocity, and U * is the shear velocity. The parameter N s was used mainly to characterize the bed form regime. The particle parameter D * and excess shear parameter T are used to characterize initiation of motion and sediment transport rates by bed load. The suspension parameter Z describes the distribution of suspended sediment in the water column. The sediment transport parameters were computed at each grid point in the SWAN numerical wave model using the program TEC-PLOT. This provided a convenient means of producing contour plots of sediment mobility and sediment transport in order to assist in visualizing these

6 Coastal Geomorphology Study November 2004 processes. Additional post-processing was carried out by extracting values along selected profiles and cross section transects to make comparisons between runs Results Sediment mobility, expressed by van Rijn s (1989) T parameter, was computed for selected wind and wave frequencies at each grid point in the SWAN numerical model output for existing conditions. The computations were made for the conditions listed below in Table 6-1, which represent a frequency of exceedence of approximately 12 hrs/year. Figure 6-1 shows the spatial distribution of sediment mobility for SE and S waves at High Tide, Mean Tide and Low Tide conditions. Figure 6-2 shows similar plots for W and NW waves. It should be noted that the variations in sediment mobility generally follow similar pattern as the bed orbital velocities. In deep water where the bed velocities were virtually zero, conditions were below the threshold for sediment movement. Initiation of motion occurred at depths of approximately 7 to 10 m for incident wave heights greater than 1.5 m. Transport intensities peaked in depths of 3-4 m typically, then decreased shoreward due to the reduction in wave heights due to refraction, shoaling and attenuation. This variation is illustrated in Figure 6-3 for incident SE waves and Figure 6-4 for waves approaching from the south. Peak sediment concentrations were computed to reach up to a maximum of 200 mg/l for these conditions. Due to the oscillatory nature of the wave-generated currents, the high sediment concentrations do not necessarily generate significant net transport rates. They indicate considerable stirring up of the bed sediments, which may allow it to be advected by other tidal-generated currents. Table 6-1: Wave Conditions for Sediment Transport Calculations Direction Run Hs (m) Tp (sec) SE S SW W NW

Coastal Geomorphology Study - 80 - November 2004 Figure 6-1:

7 Coastal Geomorphology Study November 2004 Figure 6-1: Sediment Mobility SE and S Waves at Low Tide, Mean Tide and High Tide

8 Coastal Geomorphology Study November 2004 Figure 6-2: Sediment Mobility W and NW Waves at Low Tide, Mean Tide and High Tide

9 Coastal Geomorphology Study November 2004 Mobility Parameter T, Hs (m) Sediment Mobility, Bottom Velocity, and Wave Height Versus Depth for SE Waves Shields Para Wave Height Ums Ums (m/s) Water Depth (m) Figure 6-3 Variation of Sediment Mobility With Depth, SE Waves, East Side of Causeway Mobility Parameter T, Hs (m) Sediment Mobility, Bottom Velocity, and Wave Height Versus Depth Water Depth (m) Shields Para Wave Height Ums Ums (m/s) Figure 6-4 Variation of Sediment Mobility With Depth, S Waves, East Side of Causeway

10 Coastal Geomorphology Study November 2004 South East Waves Incident SE waves experience a substantial reduction in height due to refraction as they pass over the relatively shallow tidal flats (Figure 6-1). Refraction also causes the waves to change direction appreciably and become aligned with the tidal flat bathymetry. The highest sediment mobility (T> 6) occurred in depths of between 3 to 4 m west of the Deltaport Causeway near the point of initial wave breaking. The Tsawwassen Ferry Terminal and Deltaport Causeway shelter the inter-causeway area of Roberts Bank, greatly reducing velocities and sediment transport conditions. As a result, conditions were below the threshold for sediment movement in the ship turning basin. South Waves Incident S waves approach Roberts Bank virtually straight on, and experience less refraction effects than for other wave conditions (Figure 6-1). Therefore, even though the incident deep water wave height was less than for SE waves, the local sediment transport conditions on the shallow tidal flats were similar in magnitude or somewhat higher (Figures 6-3 and 6-4). Conditions in the ship turning basin in the inter-causeway area were well below the threshold for sediment movement. The greatest area of high sediment mobility occurred around Mean Tide on the lower portions of the tidal flats. At High Tide, the bed velocities were reduced due to the deeper water. At Low Tide, only a relatively small area of the lower tidal flats was exposed to wave breaking, although the transport intensities were highest. West Waves Incident West waves were refracted as they passed over the relatively shallow tidal flats and quickly aligned nearly straight on to the slope of the flats (Figure 6-2). The highest sediment mobility occurred west of the Roberts Bank Causeway and on the west side of the Tsawwassen Ferry Terminal. The east side of the inter-causeway area was sheltered by the Roberts Bank Causeway and was below the threshold for significant sediment movement. As in the other runs, the greatest extent of relatively high sediment transport intensities occurred at Mean Tide.

11 Coastal Geomorphology Study November 2004 North West Waves Incident NW waves were turned nearly 180 degrees by refraction in localized areas on the east side of the Roberts Bank Causeway (Figure 6-2). This effect and direct sheltering from the Causeway resulted in conditions being below the threshold for sediment movement in the intercauseway area. Relatively low intensity sediment movement occurred on the west side of the Causeway. 6.3 SEDIMENT TRANSPORT BY CURRENTS Method Sediment transport was estimated using two different methods: van Rijn s total load equation (van Rijn, 1989); and Engelund-Hansen equations for sand-bed channels (Engelund and Hansen, 1967). The use of sediment transport equations under tidally-varying flow conditions is probably somewhat more reliable than using them with waves. However, in general, the same qualifications should be considered when using the results. Therefore, the output of the sediment computations was primarily used as a tool for assessing relative changes in transport conditions in the study area under various scenarios. For van Rijn s method, the volumetric bedload transport rate (q s ) was estimated from the relation: q s = 1/ 2 3 / {( s 1) g} d50 D* T where, s is the specific gravity, d 50 is the median grain size, D * is a dimensionless grain size parameter and T is the Mobility Parameter, which was computed using the critical shear stress to initiate bed movement (τ c ) from Shields relation: τ τ c T = τ c

12 Coastal Geomorphology Study November 2004 The bed shear (τ) was estimated from the computed velocity (U) and bed friction factor (f) using the relation: 2 ρfu τ = 8 The suspended bed material load is then computed using the sediment transport equation, assuming the sediment concentration near the bed is equal to the bed load concentration. van Rijn provided a means for integrating the suspended load from the bed to the water surface in order to estimate the total load. The main advantage of this method is that it has been applied to both current and waves. van Rijn also claims the method provides a more reliable prediction of sediment transport than other methods. The main disadvantage is that it is computationally very complicated and it is not clear that the detailed computations are truly warranted given the uncertainties in the available data for developing and testing the equations. Limited field-testing of the equations on the sand-bed channel of the Fraser River have shown reasonably good results. The Engelund-Hansen equation can be expressed as: g s = 3 / 2 2 d 50 τ 0.05γ su g( sg 1) ( γ s γ ) d 50 where, γ s is the unit weight of sediment, U is the mean velocity, s g is the specific gravity and d 50 is the median grain size. The bed shear stress (τ) was calculated from the mean velocity and bed friction factor. The Engelund-Hansen equation has been demonstrated to provide surprisingly good predictions on a wide range of sand bed channels, despite its simplicity. A general limitation of sediment transport equations is in calculating the bed shear stress using the bed friction factor and mean velocity. It is difficult to estimate the friction factor precisely since its value depends on the presence or absence of bedforms or other bed irregularities. However, since we are mainly using the equations to identify relative changes (with and without project), this does not affect the overall results significantly.

13 Coastal Geomorphology Study November Fore Slope Initial computations were made using measured current velocities from two stations offshore from the Roberts Bank Causeway operated by Fisheries and Oceans Canada (DFO). Digital data were obtained from the Marine Environmental Data Service (MEDS). The station records are described in Table 6-2. Table 6-2: Current Observations Off Roberts Bank. Station Latitude Longitude Depth below LLW (m) Period of Record WES N W 4 Feb RB N W 65 Feb Sediment transport rates and sediment concentrations were computed at 15 minute intervals and then summed over the period of record to compute net monthly and annual transport rates. At station WES, the threshold for sediment movement was exceeded for 19% of the time, which corresponds to 68 days/year on average. However, the peak transport intensities (as expressed by the sediment mobility parameter T), were low (Table 6-3). For example at WES, the bed shear exceeded twice the threshold for movement for a total duration of 3.9 days/year. The net transport was towards the northwest at both sites, reflecting the dominance of flood tides. Sediment concentrations were very low, generally less than 50 mg/l. The computed annual transport rates during flood tide, ebb tide and the resulting net transport quantity are summarized below in Table 6-4. These results indicate net transport rates to the northwest of approximately 200,000 tonnes/year over a 1,000 m wide zone of the fore slope off from the Causeway. Table 6-3 Sediment Transport Intensities Table 6-4: Computed Annual Transport Rates Transport Parameter, Frequency (days/yr) T WES RB10 > > > >4 14 Transport Rate Tide (kg/m/year) WES RB10 Ebb nil 180,000 Flood 200, ,000 Net 200, ,000

14 Coastal Geomorphology Study November Inter-causeway Area Transport computations were made in the inter-causeway area using the hydrodynamic output from the 2-D hydrodynamic model. Figure 6-5 shows a plot of the sediment mobility parameter (T) at flooding and ebbing conditions during a Large Tide (December 27, 2003). The plots also show vectors of depth-averaged velocities to indicate the direction of transport. The sediment mobility and sediment transport capacity were computed assuming a median grain size of 0.2 mm. The plots illustrate that conditions are well below the threshold for sand transport in the navigation channel, ship turning basin and the vicinity of Deltaport. Sediment transport was also weak or below the threshold for movement over most areas of the tidal flats. These values were generally lower than estimates on the delta fore-slope. This is because the peak current speeds in the Strait of Georgia are generally considerably higher than on the shallow, vegetated tidal flats. Regions of higher transport conditions in the inter-causeway area were restricted to the tidal channels. Transport conditions were sufficiently high to develop dunes in the main channels, with sediment mobility values reaching up to 10 times the threshold of movement. The large main tidal channel was found to be active approximately 30 % of the time. 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 order to assess overall net transport over various tidal cycles. The net transport in the main tidal channel increased in the seaward direction during the ebb tide due to the increased flow being added to the channel. By comparison, the transport rate was virtually constant along the channel during the flooding tide. The calculations indicate there was a net seaward movement of sediment near the channel outlet and very little net overall transport near its landward end. The annual net sediment transport in the main tidal channel was estimated by integrating the loads over the year using the three representative tidal ranges and combining these with their frequency of occurrence. The net sediment transport down the channel varied between 3,000 and 15,000 tonnes/year using the Engelund-Hansen equation and between 500 tonnes/year and 3,000 tonnes/year using the van Rijn equation. The net transport rate was very sensitive to small

15 Coastal Geomorphology Study November 2004 velocity differences between ebb tide and flood tide. The net seaward transport provides further evidence that the channel is still in a state of adjustment and that headcutting has not ceased at this time.

Coastal Geomorphology Study - 89 - November 2004 Figure 6-5:

16 Coastal Geomorphology Study November 2004 Figure 6-5: Inter-causeway Sediment Mobility by Tidal Currents During Flood and Ebb Tide Large Tide

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

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