Ontario Power Generation's Response to the Joint Review

Attachment 1 to OPG letter, Albert Sweetnam to Dr. Stella Swanson, “Deep Geologic Repository Project for Low and Intermediate Level Waste – Submission of
Responses to the Final Sub-set of Package #4 Information Requests”, CD#: 00216-CORR-00531-00143.
IR# EIS Guidelines
Section
The response is provided in two parts:
a) Precipitation intensity; and
b) Probable Maximum Precipitation.
Information Request and Response
(a) Precipitation Intensity
The DGR environmental assessment (GOLDER 2011, Appendix D, Section D2.3.4) considers changes in climate
within the Great Lakes Basin, specifically changes in both mean precipitation and precipitation intensity. Forecasts
from the Intergovernmental Panel on Climate Change (IPCC), and recent papers by Warren et al. (2004), Mortsch et al.
(2003) and Kharin et al. (2007) were cited and the information presented. It was noted that Kharin et al. (2007)
suggests the 20-year return period for total 24-hour rainfall could increase in intensity by 10% to 20% by the 2071 to
2100 forecast period.
With respect to the Information Request for an updated evaluation, the literature was surveyed for recent literature on
precipitation intensity relevant to southern Ontario, and one new paper was identified:
Cheng et al. (2011) suggest an increase in frequency of future daily heavy rainfall events over the period 2081
to 2100 for four river basins in southern Ontario. Cheng et al. (2011) also predict increases in the return values
of annual maximum 3-day accumulated rainfall. More specifically, for the 100-year return period storm
(considered in the DGR stormwater management design), the 3-day accumulated rainfall totals are projected to
increase from about 100 mm currently observed to 130-170 mm for the period 2051-2100. The 24-hour
accumulated rainfall would be less than Cheng’s 3-day accumulated rainfall projection, but is not estimated in
the paper.
As outlined in OPG’s response to IR-EIS-04-130, the stormwater management pond is designed to:
� provide a minimum retention time of 24 hours for runoff from 25 mm of rainfall in 6 hours;
� ensure that the average annual total suspended solids concentration in effluent discharge does not exceed
40 mg/L;
� attenuate post-development peak outflow rates for 24-hour rainfall events with return periods ranging from 2 to
100 years to pre-development values; and
� safely convey the peak outflow rate from the 24-hour, 100-year rainfall event.
The current design of the stormwater management pond considers a 24-hour rainfall event with a return period of
100 years (110 mm) based on data from Goderich, Ontario. As stated in the response to IR-EIS-04-130, it is expected
that the rainfall amounts at Kincardine would be less than at Goderich. The design of the stormwater management
pond has extra capacity if rainfall intensity is higher, in part because of conservative assumptions leading to the current
design and the availability of 300 mm of planned freeboard on the sides of the pond. The capability of downstream
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