Canadian Dam Association Bulletin - Spring 2018 - 17

Table 3 - Performance of Selected Alternate on Key Evaluation Criteria
Criteria Name

Preferred Arrangement Performance

Hydraulic Jump

Energy is dissipated via viscous losses at the shear plane between the main contracted flow
and the surrounding flow recirculation at each of the four orifice locations. A hydraulic jump
will not occur.

Control Gate
Vibration

The energy is dissipated within the tunnel downstream of each orifice. No regulation at the inlet
gate is required which minimizes gate vibration risk.

Impact on
Diversion
Head loss

The orifices will be constructed just prior to reservoir filling in one of the two diversion tunnels
which will be isolated and dewatered during low flow periods. Diversion head loss is, therefore,
not affected. Schedule risk is introduced but was determined to be manageable.

Constraints
on Upstream
Hydro Plants

There will be minimal constraints on the upstream plants consisting of a short period of above
average inflows required at the start of reservoir filling, otherwise no constraints are imposed.

Geotechnical
Risk

The energy dissipation is spread across four locations and all are located upstream of the future
tunnel plug, therefore, geotechnical risk is minimized.

Sensitivity

Small changes in tailwater level, tunnel roughness and discharge do not affect the feasibility
of the proposed option. The orifice sizing was carefully evaluated by physical model
testing to ensure the minimum downstream flow can be passed at the minimum operating
reservoir elevation.

TDG

The orifice option involves submerged energy dissipation. Therefore, no increase to dissolved gas
pressure above baseline levels is anticipated.

considered. Table 3 summarizes how
the proposed arrangement addresses
each of these key screening criteria.

5. ORIFICE DESIGN OVERVIEW
A literature search found several
projects that have utilized multiple
orifice reducers or sudden expansions as an effective means of dissipating excess energy in the diversion
facilities of large hydro projects. Mica
Dam (Russell and Ball 1967) and the
Xiaolangdi Hydro project in China
(Zhang and Chai 2001 and Lin 2003)
were two examples of relevant reference designs that were successfully
put into practice. At Mica, two tunnel plugs each containing three steel
lined conduits were constructed in
one of the diversion tunnels after the
diversion period. This arrangement,
along with some modifications to the
tunnel inlet and outlet, allowed for
dissipation of a large amount of energy
in the expansion chamber between
the tunnel plugs during reservoir
filling. At Xiaolangdi, thin orifices
constructed of concrete and steel were
retrofitted into the diversion tunnels
to convert the tunnels for continued
use during permanent operations.
Canadian Dam Association * Spring 2018

The application at the Xiaolangdi
Hydro project in China owned by the
Yellow River Water & Hydroelectric
Power Development Corporation was
the most applicable reference project found given the constraints and
requirements for Site C. At Xiaolangdi,
three orifices were constructed in
series in each of three diversion tunnels to convert the diversion tunnels
into spillway tunnels to form part of
the overall spillway arrangement for
the project. The orifices are the main
energy dissipating devices with radial
gates downstream from the third orifice in a chamber to control discharge
through each of the converted diversion tunnels.
The biggest technical design
challenge with the orifice reducer
arrangement is the potential for cavitation leading to damage to the orifices or the tunnel liner. For the Site
C arrangement, the main concern is
the lack of downstream pressure due
to the relatively low tunnel submergence. The cavitation performance,
orifice cross-sectional geometry and
hydraulic losses were initially evaluated based on the criteria presented
in reference papers for the Xiaolangdi

project (Zhang and Chai 2001 and Lin
2003). Once the preliminary layout
and feasibility had been established
both CFD and PHM were utilized to
confirm and optimize the design.
The conclusion of the optimization
process was that four orifices provided a good balance between cost
and cavitation performance. The
final arrangement was conservative
in terms of cavitation performance
because the cavitation criteria used
were intended for design of permanent structures such as the Xiaolangdi
tunnel spillway. For Site C, the orifice
tunnel will only be in operation for
approximately 2 to 4 weeks during
reservoir filling, therefore experiencing some cavitation within the flow
is likely acceptable.
To maintain economical sizes
for the orifices, the reservoir could
be filled up to elevation 425 m by
increasing Site C reservoir inflows
using flow regulation at the upstream
BC Hydro facilities for a short duration. Tunnel 1 (non-orifice tunnel)
would then be closed with the reservoir at elevation 425 m and Tunnel 2
(converted orifice tunnel) would operate un-regulated with the orifices
17



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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