Chapter A - Introduction - City of Pickering

property.
COMMENT MESPA RESPONSE (December 2011) CITY RESPONSE (October 2012) RESPONSE
• Surface Water Hydrology (Section 5.0)
Hydrologic Model
The uncalibrated existing conditions hydrologic model that was developed
for the Seaton MESP (AMEC Model) varied considerably from the
TRCA’s approved existing conditions calibrated Duffins Creek model,
which was updated by Aquafor Beech in 2002 (Aquafor 2002 Model).
Aquafor’s 2002 Model was further discretized by AMEC into 126 different
subcatchments, with the main parameters that affect the calibration of a
model being changed using different methodologies (i.e. CN vs. CN*,
different equation used for time to peak, additional route channel
commands, etc.) and no re-calibration of the model was completed. While
the need for the extensive discretization completed for the existing
conditions model is unclear, the methodologies used to derive
parameters should be consistent for the entire Duffins Creek model and
should use methodologies, which are appropriate for Southern Ontario.
The time to peak equation used in the calibrated Aquafor 2002 Model
was Watt & Chow’s method, which was developed utilizing streamflow
and precipitation data from Southern Ontario. Through comparing results
from the Aquafor 2002 Model with actual streamflow data, it was
confirmed that this method gave good results for the Duffins Creek
watershed. Notwithstanding that, the MESP states that AMEC used the
HYMO equation to calculate the time to peak in their analysis. It is
unclear why this equation was used instead of Watt & Chow’s method
given that the OTTHYMO-89 and VO2 literature, as well as other reports,
state that the HYMO equation gives erroneous results in Southern
Ontario unless it is calibrated using local streamflow and precipitation
data.
The reason for the discrepancies given in the literature is that the
regression constants in the HYMO equation were developed using data
collected from farm fields in the southern United States. Given the
inherent difficulties with transposing methods derived from differing
climatic and physiographic regions to Southern Ontario, it does not
appear as though the proponent has completed the rigorous technical
analysis required to support the use of the HYMO method in the Duffins
Creek watershed.
whether the roof leaders are connected and the flows
are conveyed by clean water collector to a centralized
infiltration facility. This detail will be worked out through
the NFSSRs. The text has been clarified to make this
distinction.
An update to the hydrologic model has been undertaken
as per discussions at technical meetings with TRCA and
the City.
Briefly, the approach agreed upon involves using the
2002 existing conditions model as the base for the 2 –
100 year and Regional Storm event modelling, with
minor changes only to boundaries within the Seaton
lands, based on more current mapping, and the creation
of discrete subcatchments for each of the different
subwatersheds, to enable accurate comparisons at
varying points in the watershed. All changes have been
made using the 2002 methodology for parameterization
and retaining the CN values from the 2002 model as per
direction from TRCA and the City of Pickering. The one
variable changed for calibration purposes, as discussed
with the TRCA and City, was that N has been lowered
from 1.5 to 1.45 in order to reproduce the currently
approved flow rates as per the requirements of the
TRCA and the City of Pickering.
In addition, the models have all been validated by
assessing their performance with additional suitable
historical rainfall and streamflow records found within
the data provided by TRCA (2002 to present).
In terms of the Regional Storm model for future
development conditions, discussions with the TRCA and
City of Pickering staff have occurred throughout 2011 to
identify a modelling approach and alternative potential
management scenarios, and review updated existing
conditions model results. These discussions are
ongoing and will continue over the next few months.
The conclusion of these discussions and analyses will
make specific recommendations regarding a Regional
Storm Control Strategy, if needed to minimize or reduce
risks to human life and property due to flooding.
No further action required from the SLG. All
issues related to the previously submitted
hydrology model is being dealt with through the
2012 Duffins Hydrology Update.
The SLG will be responsible for achieving the
required level of SWM control, as recommended
in the hydrology update.
frontages less than 12m, is acknowledged.
The PRMS model showed that having
disconnected roof leaders was a “tool” in
the “LID toolbox” that would help to achieve
the required mitigation. Going forward, the
on-going PRMS modelling is looking at the
potential implication to removing
disconnected roof leaders for lots less than
12m frontage.
REPORT CHAPTER
AND SECTION
No response required. Chapter B5.0
Unless there is a clearly demonstrated need to change the method used
to calculate the time to peak, the proponent should use the Watt & Chow
method to ensure consistency with the rest of the Duffins Creek model
The higher-discretized model has been used for the
future land use with SWM condition to allow for peak
flow calculations at each SWM facility and channels
The Sernas Group Inc., Stonybrook Consulting Inc. December 2011, Revised February 2013 Master Environmental Servicing Plan - Amendment
SPL Beatty, Bird and Hale Limited, Earthfx Inc. 07161 Seaton Community, City of Pickering
AMEC Earth & Environmental, R.J. Burnside Appendix A7 – Page 7
Amos Environment + Planning