Sedimentation and Siltation

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21 Φεβ 2014 (πριν από 3 χρόνια και 5 μήνες)

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Request Number 12: Fish and Fish Habitat – Sedimentation and Siltation

References: EIS, Vol 2, Figure 11.8.9; EIS, Vol 2

JRP Context: The Proponent has identified a typical pattern of reservoir surface turbidity during spring
freshet which illustrates tributary sediment inputs and shows the spatial distribution of near-surface
turbidity based on the 2007 freshet. Figure 11.8.9 illustrates that there will be changes to turbidity levels
within the proposed reservoir. The EIS further states that sediment inputs from erosion of newly
inundated areas outside of the active Peace River channel and sedimentation caused by deposition of
suspended sediments would alter existing clean riverbed materials. It does not appear that potential
effects from turbidity on fish and fish habitat have been assessed.

JRP Question: Provide an assessment of the effects of sedimentation and siltation on fish and fish
habitat. Identify how sedimentation may impact the eventual success of the fish habitat compensation
plan.

BC Hydro Response

The effects assessment for Fish and Fish Habitat in the EIS as amended took into account the potential
effects of turbidity and sedimentation, as described below. As well, habitat compensation works in the
Site C reservoir would take into account predicted sediment deposition rates.


Sedimentation

Sedimentation commonly refers to the deposition of sediment on the bottom of a waterbody. Sediment
deposition can affect fish and fish habitat. Here, sediment deposition refers to the deposition of all particle
sizes of sediment.

Sediment deposition is predicted to occur in the reservoir. It is estimated that after the first 10 years of
operation the deposition thicknesses would range from about 0.1 m in the main reservoir to over 2 m at
the Halfway confluence and adjacent to some shoreline segments (further information is provided in EIS
Section 11.8.5.1 Suspended Sediment Dynamics in the Reservoir). The Project is not expected to result
in any changes in channel erosion or sediment deposition downstream of the dam, other than in the reach
immediately downstream of the dam under extreme high flow conditions (described in EIS Section
11.8.5.3 Channel Erosion and Deposition Patterns Downstream of the Site C Dam Site).

The potential effect of sediment deposition on fish and fish habitat in the Site C reservoir was assessed
using best available information, modelling, and professional investigation and analysis, as described
below. The effects of sediment deposition on primary production in the form of periphyton (algae on the
bottom of the reservoir) in the Site C reservoir was modelled (described in Appendix P Part 2
Hydrodynamic, Water Quality and Productivity Modelling for the Site C Project). Periphyton production
and biomass was estimated using a two-dimensional hydrodynamic and water quality model (CE-QUAL-
W2). CE-QUAL-W2 includes a module of the periphyton cycle that considers burial in the bottom
sediment layer. Sediment inputs to the reservoir from EIS Section 11.8 were used as an input to CE-
QUAL-W2.

The potential effects of sediment deposition on fish habitat and the fish community composition in the
reservoir were assessed (described in EIS Section 12.4.1.2 Change in Habitat Due to the Construction
Headpond and Reservoir Filling, and EIS Section 12.4.2.1 Transformation of Reservoir Habitat During
Reservoir Operation). Sediment deposition is predicted to affect species composition in the short term (1
to 10 years), as species that are able to rapidly exploit new habitats, that are tolerant of perturbations to
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the aquatic environment (e.g., elevated suspended sediment concentrations and sedimentation of clean
bed materials), and that presently utilize tributary habitats would quickly dominate the system.


Siltation

Siltation commonly refers to the input of fine sized particles (i.e., <63 micro m) to the waterbody. These
fine particles typically remain in suspension and contribute to the turbidity of the water. Total Suspended
Solids (TSS) is a measure of turbidity that is commonly employed.

The potential effects of changes in turbidity on fish and fish habitat were assessed using best available
information, modelling, and professional investigation and analysis, as described below. The effect of
turbidity on primary production in the form of phytoplankton (algae suspended in the water column) in the
Site C reservoir and Peace River downstream of Site C was modelled (described in Volume 2 Appendix
P2). Phytoplankton production and biomass was estimated using the phytoplankton module in CE-QUAL-
W2, which takes into account TSS. CE-QUAL-W2 was calibrated with field data and sediment input
predictions to the reservoir from EIS Section 11.8.

The potential effects of turbidity on fish habitat and the fish community composition in the reservoir were
assessed. In the reservoir, the seasonal pattern of suspended sediment affects the competitive
interaction among fish species and the predicted composition of the fish community. In the Peace River,
changes to the seasonal pattern of suspended sediments would contribute to shifting the transition zone
of the clear coldwater fish community and the turbid coolwater turbid water fish community downstream
from the present location near the confluence of the Pine River. Further description is available EIS
Volume 2 Section 12 and in Volume 2 Appendix P3 Future Aquatic Conditions in the Peace River.

Fish Habitat Compensation Plan

As described in the response to question 11 above, BC Hydro proposed to adopt an adaptive and
evidence-based approach in the design of fish habitat compensation plan. Compensation works in the
Site C reservoir would take into account predicted and observed sediment deposition rates so that
sedimentation would not limit the effectiveness of habitat compensation.