Innovation in Global Power - Parsons Brinckerhoff

Hydropower – New Technologies, New Considerations
Major Flood. The PMF would have a duration of approximately
72 hours and a peak discharge of 5,776.7 m 3 /s (204,000 ft 3 /s).
It would occur typically during a springtime precipitation
run-off event, although it could occur any time of year,
particularly during the summer and fall hurricane season for
the southeast USA. Because the upper nappe associated
with this event impacts the bottom of the walkway above
the arch, thereby reducing its scour potential, it was not
studied. However, the major flood just below the PMF with
5,245.0 m 3 /s (185,225 ft 3 /s) was studied. During this event,
approximately 3,127.6 m 3 /s (110,448 ft 3 /s) overtops the arch.
This is the maximum discharge at which free flow occurs, and
thus was assumed to have the highest scour potential. The
reservoir elevation for this event, 560.3 m (1,838.2 feet),
was more than 6.4 m (21.2 feet) above the arch crest. The
resulting plunge jet was 2.0 m (6.6 feet) thick at issuance and
5.9 m (19.4 feet) thick at impact, and it hit the plunge pool
45.9 m (151 feet) from the toe of the arch with an applied
power (neglecting aeration) of 2,780 kW/m 2 . In this case, the
jet began to break up about 18.2 m (60 feet) below the
point of issuance, or more than 27.4 m (90 feet) above the
water surface, so the spray that hit the water did so with
only a fraction of the jet’s original power. By the time it
reached the bottom of the pool, the applied power of the
jet was reduced to 303 kW/m 2 , which is well below the
445 kW/m 2 minimum threshold value for erodibility of the
bedrock (Figure 4).
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In summary, the total stream power values at the base of the
arch varied from 97 kW/m 2 to 303 kW/m 2 , values that are
well below the 405 ft 3 /s and 445 kW/m 2 minimum threshold
values of the concrete and bedrock, respectively.
Conclusions
Based on our investigation, we determined that Santeetlah
Dam meets all safety requirements. Overtopping of the
arch section up to the PMF will not cause scour, which could
otherwise lead to undermining and possible dam failure.
Insofar as remediation-whether it be extending the apron
or constructing a toe dam and permanent plunge pool,
both of which would have cost several hundred thousand
dollars-we demonstrated that none was needed.
This state-of-the-art approach, used successfully to evaluate
the scour potential of hydraulic discharge chutes, is applicable
to other scour impact areas subjected to free falling water.
Related Web Sites:
• Federal Energy Regulatory Commission: http://www.ferc.gov
/industries/hydropower/safety/guidelines/eng-guide.asp
Figure 4: Scour Estimation for
Santeetlah Dam Using Annandale
and Ing Method.
Jay Greska, P.E., is a lead engineer with PB’s Hydropower & Water Resources Group in Boston. He joined the firm in 2006 and has more than 20 years of experience in
roadway design, bridge hydraulics, stormwater management and hydraulic modeling of large dams.
Bryce N. Mochrie, P.E., is a senior principal engineer with PB’s Hydropower & Water Resources Group in Boston. He joined the firm in 2000 and has more than 35 years
of experience in the civil/structural design, inspection and licensing of water resource and power projects.
PB Network #68 / August 2008 40