3.5 Net sediment deposition in the subaerial delta
3.5.2 Estimation of the weight of fine sediments trapped in the subaerial delta
The weight of fine sediments trapped permanently in the subaerial delta is first estimated in the upper delta and then in the lower delta plain. Fine sediments are mainly trapped on channel levees, lakeshores, delta plain and lake beds.
Estirnates of net sedimentation in the upper delta
CARSON ( 1994a) estimated th at the mean annual net sedimentation in the upper delta is 7 Mm3 on channel levees, 8 Mm3 in lake basins and 10 Mm3 onto lake beds. Deposition in lake basins includes sediment deposition on lakeshores, on lake deltas and shoals and on the delta plain. Carson converted the sediment volumes into a mass figure by using a mean dry bulk density of 1.2 g/cm3 and the resulting net deposition was 30 ± 5 Mt/y. As dry bulk densities of overbank and lake sediments have been deterrnined in this study, the weight of sediment has been recalculated using a mean dry bulk density of 1.38 g/cm3 in overbank sediments (deposits on channel levees and lake basins) and 1.1 g/cm3 for lake bed deposits (appendix 2.5). Based on these data, the mean annual weight of sediment trapped in the upper delta is 32
Mt. Of this, about 10 Mt is deposited on channel levees, 11 Mt in lake basins and 11 Mt on lake beds.
The difference between the two estimates (30 and 32 Mt/y) is not significant considering the uncertainty attached to the volumetrie estimate.
Estimates of net sedimentation in the lower delta plain
The net sediment deposition in the lower delta plain is now estimated as it was not included in CARSON' s study. It corresponds to the gross sedimentation Jess the amount of material eroded from channel banks and overbank deposits. The estimate is based on the sedimentation rates measured by PEARCE (1986, 1993, 1994) in one study area located in the outer delta, on the erosion rates estimated in this study (see chapter 3.4) and on the net sedimentation rates determined in lakes (this study; CORDES and McLENNAN, 1984b).
Sediment deposition
Based on Pearce's measurements, mean sedimentation rates have been estimated in each ecophase (appendix 2.6). Sedimentation rates in mudflats were not measured in the outer delta (PEARCE, 1994). It should be noted that sedimentation rates determined by PEARCE (1993, 1994) correspond to the annual sediment input into each ecophase and do not take into account the erosion rates.
Table 3.16: Estima te of the mean annual volume and weight of sediments deposited in the fluvial system in the lower delta (3872 km2). Based on data provided by CORDES et al., (1984), HIRST et al. (1987),
PEARCE ( 1994).
Ecosites in the outer A rea Eco phases Sedim. . Volume Weight delta (3872 km2)1
ecosite [% of ecosite area] rate [m3/y] [t/y]
[km2] [cm/y]
Point bars [2.5%] 97 Pioneer willow [25%] 5.2 1,261,000 1,740,000 Mature willow [ 10%] 2.0 194,000 267,700 Richardson' s willow + 0.18 113,490 156,600
others [65%]
Levees [5.5%]2 213 Sedge [20%] 2.7 1,150,200 1,587,300
Pioneer willow [ 10%] 1.7 362,100 499,700
Mature willow + 0.22 328,000 452,600
Richardson's [70%]
Lakeshores [64.1% ]3 2482 Sedge [25%] 1.2 7,446,000 10,275,500
Richardson' s willow 1.5 9,307,500 12,844,400 (lower) [25%]
Richardson's willow 0.02 248,200 342,500 (upper)+ others [50%]
TOTAL 20,411,000 28,166,300
1 Delta surface provided by LEWIS (1988). 2 Main levees + distributary channel levees. 3 Lakeshore area
=
100 - ether eco site are as - channel beds ( 10%) - lake beds ( 17.9% ).The volume of sediments deposited in each ecophase is calculated by multiplying the mean
-75-Chapter 3: Sedimentation in the Mackenzie Delta
sedimentation rate by the mean surface occupied by the ecophase in the lower plain (table 3.16). The knowledge of ecophase and ecosite distribution is limited as their surfaces were determined in only one area (Area 5 in PEARCE, 1994). However, the similarity of proportions of terrestrial surfaces covered by high, moderate and low sedimentation rates, determined with satellite images in four sites, (table 12 in PEARCE, 1994) suggests that the ecosite and ecophase distribution in Area 5 can be considered representative of the distribution in the outer delta. A few modifications have been perforrned (table 3.16) in order to take into account ecosites which are not present orunder-represented in Area 5:
• the area covered by point bars has been increased from 1% to 2.5% (mean surface covered by high sedimentation rate in areas studied by PEARCE, 1994),
• the area covered by main and distributary channel levees have been estimated at 5.5% (mean area in the subaerial delta) (CORDES et al., 1984; HIRST et al., 1988; PEARCE, 1994) as no data are available specifically for the outer delta.
The resulting gross sediment deposition in the fluvial system of the outer delta is estimated to be about 20 Mm3 or 28 Mt using a mean bulk density of 1.38 g/cm3 (table 3.16).
The volume of sediments deposited in lake beds ( 1.2 Mm3 or 1.4 Mt) is obtained by multiplying the mean net sedimentation rate by the surface area occupied by lakes (table 3.17). Two groups of lakes have been distinguished as their sedimentation rates are very different; the first one includes connected first order lakes (single connected lake or first lake in a sequence of connected lakes) and the second one contains the other connected lakes and the closure lakes. The weight of sediments deposited in lakes is determined by multiplying the sediment volumes by the dry bulk density of lake sediments ( 1.1 g/cm\
The volume of sediment deposited in connected lakes corresponds to the net volume and not to the gross volume, as sediment has been eroded from the lake systems (see chapters 3.3.4 and 3.4). As a rough estimation, the gross sediment volume deposited in lakes is around 2 Mt/y as the net volume in connected first order lakes is on average 50% of the total annual sediment input (chapter 3.4; table 3.17).
Table 3.17: Estimate of the mean annual volume and weight of sediments deposited in lake beds in the lower delta (3872 km2, LEWIS). Lake distribution and delta suiface provided by LEWIS (1988) and sedimentation rates by this study and by CORDES and McLENNAN ( 1984b).
Lake bcds Surface Net sedimentation Volume Weight
Outer delta (3872 km2)1 [km2] rate [cm/y] [m3/y] [t/year]
No-closure 1 order [3.5%] 136 0.372 503,200 554,000
Other no-closure + closure 558 0.133 725,400 798,000
[14.4%]
TOTAL [23.6%] 1,228,600 1,352,000
1 Delta surface provided by LEWIS (1988). 2 data from this study. 3 data from CORDES and McLENNAN (1984b).
The total sediment deposition (lake and fluvial system) in the lower delta plain is approximately 21 Mm3 or 29.4 Mt. The amount of sediments deposited in channel beds, which cover approximately 10% of the lower delta surface, is not included in the calculations as no data are available. It is assumed that long-term channel bed scour is in equilibrium with long-term bed deposition. Moreover, the amount of sediments deposited in channel beds is probably minor (2 Mm3/y) compare to the total volume deposited in the lower delta CARSON ( 1994a).
Sediment entrainment
The estimation of eroded material in the lower delta is difficult because of the lack of data. As channel sinuosity decreases in the outer delta (LEWIS, 1988), point bar deposition and associated bank scour are assumed to be limited. The relatively small bank erosion rates along main channels in the outer delta (LAPOINTE, 1986) and the small terrestrial surface covered by high sedimentation rates (PEARCE, 1994) suggest that sediment deposition in point bars is less important than in the upper delta. The mean channel retreat is 1.8 rn/y along Middle Channel (36 observations) and 0.7 rn/y along distributary channels (43 observations) (LAPOINTE, 1986). Assuming that Middle Channel has a length of 42 km approximately in the lower plain and a mean depth of 32 rn (TRA YNOR and DALLIMORE, 1992), the sediment loss is estimated to be around 2.4 Mm3/y. The sediment entrainment from distributary channel bank erosion is about 0.8 Mt considering a mean channel depth of 6 rn and a total channellength of 200 km. On average, the estimate of the channel bank erosion is about 3.2 ± 1.0 Mm3/y or 4.4 Mt/y.
Erosion processes do not affect only channel banks in the outer delta. They are very active on channel levees in the willow ecophase where between 20 and 60% of the annual sediment input appears to be eroded and reentrained in the network system (see chapter 3.4). Erosion rates in the sedge ecophase on channel levees and in the pioneer willow ecophase on point bars have not been estimated in this study but the range of erosion rates is likely similar to or even larger than the range observed in the willow willow ecophase and between 0 and 10% in the upper part.
~ 0.9
Figure 3.27: Left: Probability plot illustrating the simulated eroded volumes. Right: Rank correlation coefficiellts between the simulated eroded volumes and the simulated erosion rates in the various ecositeslecophases (see abbreviations in appen.dix 2.1 ).
The volume of material eroded from the different ecophases has been calculated by multiplying the gross sediment volume in each ecophase by the corresponding erosion rate. As each ecophase can be eroded at a different leve! depending on its location in the lower plain, it is difficult to calculate a mean volume of eroded material. To take into account the variability of erosion rates in each ecophase, a range of volumes was determined using @Risk (PALISADE, 1997) by calculating 2000 combinations of possible
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erosion rates. For each ecophase, the range of erosion rates was summarized by a triangular distribution (minimum, mode and maximum erosion rate) (table 3.18). The simulated volumes of eroded material range from 2.4 to 5.7 Mm3 and the mean of volumes is 3.9 Mm3 (fig. 3.27). A sensitivity analysis, which identifies the most significant parameters influencing the simulations, indicates that erosion rates in the Richardson's willow ecophase and the sedge ecophase on lakeshores (3a-Sr and 3a-C respectively) have the biggest influence on the estimation of the eroded volume in the lower plain (fig. 3.27). Although the erosion rates are much higher in the sedge and pioneer willow ecophases on levees and on point bars (table 3.18), they do not affect significantly the estimate of the eroded volume. In contrast, the erosion of sediments deposited on lakeshores, albeit lesser, is much more important for the estimate because of the large volume of sediment deposited in this ecosite.
Table 3.18: Estimates of the mean volumes eroded from the various ecosites and mean net volumes of sediments deposited in the outer delta. Based on the sedimentation rates measured by PEARCE (1994) and the range of erosion rates determined in this study (triangular distribution with minimum, mode and maximum erosion rates). resulting net deposition in the lower delta plain is about 14 Mm3 or 20 Mt.