Canadian Dam Association Bulletin - Spring 2018 - 15

3. ALTERNATIVES

4. SELECTED ALTERNATE

A variety of alternative designs
were examined using a combination
of engineering desktop studies, review
of precedents, physical hydraulic modelling (PHM) and computational fluid
dynamics (CFD) modelling. Some of
the primary alternatives are summarized in Table 2. The key criteria from
Table 1 are highlighted to illustrate
why each option was eliminated.

After several iterations and design
workshops with both internal and
external advisors, a design involving
in-line orifice energy dissipaters was
selected as the preferred alternative.
The selected arrangement utilizes four
in-line orifice structures installed in
diversion tunnel 2 just prior to reservoir filling. The orifice design is based
on the Xiaolangdi tunnel spillway

design that has operated successfully
in China. The Site C arrangement (orifice spacing, orifice geometry, tunnel
bends and tunnel slopes) was optimized to minimize adverse hydraulic
phenomena in the converted tunnel.
This selected alternative is described
in more detail below.
During development of the orifice
conceptual design, the key screening
criteria outlined in Table 1 were closely

Table 2 - Summary of Eliminated Alternatives
Alternative

Reason for Eliminating

1. Regulating gate control
in one or both diversion
inlet structures

This option involves a hydraulic jump that would move down the tunnel as the head
increased. At lower reservoir levels, the jump would be submerged at the control
gate which could cause control gate vibrations. The jump location is also extremely
sensitive to tailwater level, discharge and tunnel roughness, all of which will have
uncertainty prior to operation.

2. Modify tunnel diameters
(e.g. one larger and one
smaller tunnel)

Significantly larger tunnel diameters were not considered feasible given the
geotechnical risk with the rock conditions at the site. The largest diameters
considered feasible would not allow for a small enough second tunnel to significantly
reduce the hydraulic risks.

3. Construct a 3rd tunnel
of a smaller diameter
to use during filling

This alternative has the lowest technical risk of the options identified; however, the
cost of this option was significant given the short duration of operation required
during reservoir filling.

4. Modified gate system in
the tunnel inlet structure
such as a drop shaft or
multiple gate openings

This alternative consists of several sub-alternates; however, the common themes
for eliminating from further consideration were gate vibration risks and risk of a
translating hydraulic jump. Constructing an alternate without compromising the
head loss during river diversion was also a challenge.

5. Separate inlet structure
feeding into the diversion
tunnels just for releases
during reservoir filling

The primary risks for this option included control gate vibrations with the junction
occurring just downstream of the main gate and risk of a translating hydraulic jump.

6. Separate low level outlet
through the south bank
RCC buttress

This alternative would require a temporary structure to be built in the middle of the
key foundation for the permanent south bank structures. The increased geotechnical
risk and cost eliminated this option from further consideration.

7. Outlet control

This alternative considered restricting tunnel flow by using a control gate or
modified geometry at one of the diversion outlet structures. The reason for
eliminating this alternate was the increased geotechnical risk caused by pressurizing
the tunnel up to a much higher level than experienced during diversion.

8. Force the jump to
occur in a desired
location (e.g. negative/
positive step, baffle sill,
roughened invert)

Modifications to the tunnel geometry to force a hydraulic jump at a pre-defined
location would have an impact on normal diversion operation such as an increased
overall tunnel head loss. The geometry modification could be retrofitted prior to
filling in the case of a baffle sill, for example, but this would introduce construction
schedule risk. The jump location would also be sensitive to both tailwater level and
discharge which would be variable as the reservoir rises.
CFD modelling was performed to examine this idea in more detail; however,
investigations of various alternatives did not find a stable hydraulic solution.

Canadian Dam Association * Spring 2018

15



Table of Contents for the Digital Edition of Canadian Dam Association Bulletin - Spring 2018

Board of Directors
President’s Message
Site C Clean Energy Project Diversion Tunnel Orifices for Energy Dissipation During Reservoir Filling
Application for 2018 CDA Scholarships
CDA Presents 2017 Scholarships
ICOLD Corner
CDA Conference 2018
EIC Awards Gala
Buyers’ Guide and Trade List
Canadian Dam Association Bulletin - Spring 2018 - 1
Canadian Dam Association Bulletin - Spring 2018 - cover1
Canadian Dam Association Bulletin - Spring 2018 - cover2
Canadian Dam Association Bulletin - Spring 2018 - 3
Canadian Dam Association Bulletin - Spring 2018 - 4
Canadian Dam Association Bulletin - Spring 2018 - 5
Canadian Dam Association Bulletin - Spring 2018 - 6
Canadian Dam Association Bulletin - Spring 2018 - Board of Directors
Canadian Dam Association Bulletin - Spring 2018 - President’s Message
Canadian Dam Association Bulletin - Spring 2018 - 9
Canadian Dam Association Bulletin - Spring 2018 - Site C Clean Energy Project Diversion Tunnel Orifices for Energy Dissipation During Reservoir Filling
Canadian Dam Association Bulletin - Spring 2018 - 11
Canadian Dam Association Bulletin - Spring 2018 - 12
Canadian Dam Association Bulletin - Spring 2018 - 13
Canadian Dam Association Bulletin - Spring 2018 - 14
Canadian Dam Association Bulletin - Spring 2018 - 15
Canadian Dam Association Bulletin - Spring 2018 - 16
Canadian Dam Association Bulletin - Spring 2018 - 17
Canadian Dam Association Bulletin - Spring 2018 - 18
Canadian Dam Association Bulletin - Spring 2018 - 19
Canadian Dam Association Bulletin - Spring 2018 - 20
Canadian Dam Association Bulletin - Spring 2018 - 21
Canadian Dam Association Bulletin - Spring 2018 - 22
Canadian Dam Association Bulletin - Spring 2018 - 23
Canadian Dam Association Bulletin - Spring 2018 - 24
Canadian Dam Association Bulletin - Spring 2018 - 25
Canadian Dam Association Bulletin - Spring 2018 - Application for 2018 CDA Scholarships
Canadian Dam Association Bulletin - Spring 2018 - 27
Canadian Dam Association Bulletin - Spring 2018 - CDA Presents 2017 Scholarships
Canadian Dam Association Bulletin - Spring 2018 - 29
Canadian Dam Association Bulletin - Spring 2018 - ICOLD Corner
Canadian Dam Association Bulletin - Spring 2018 - 31
Canadian Dam Association Bulletin - Spring 2018 - CDA Conference 2018
Canadian Dam Association Bulletin - Spring 2018 - 33
Canadian Dam Association Bulletin - Spring 2018 - EIC Awards Gala
Canadian Dam Association Bulletin - Spring 2018 - 35
Canadian Dam Association Bulletin - Spring 2018 - 36
Canadian Dam Association Bulletin - Spring 2018 - 37
Canadian Dam Association Bulletin - Spring 2018 - Buyers’ Guide and Trade List
Canadian Dam Association Bulletin - Spring 2018 - cover3
Canadian Dam Association Bulletin - Spring 2018 - cover4
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