WC139 NovDec 2024 - Magazine - Page 35
for both dry weather flow (DWF) and wet weather flow (WWF),
to accurately represent the hydraulics and existing conditions of
the system. For the DWF calibration, the sewershed was divided
into six sub areas based on land-use, location, and wastewater
profiles. For future scenarios, projected growth data was spatially
distributed across the sewersheds, represented by property parcels
based on future land-use and zoning.
Willow Trunk Sewershed
The InfoWorks ICM model was used for existing and future
service level assessments, as well as the development of design
criteria for the separated Willow Trunk sewers and other options
such as Green Rainwater Infrastructure (GRI) and nature-based
solutions. The design criteria development considers the proposed trunk sewer’s function, impacts on local sewers’ service
level, design storm duration, and the joint scenario of coastal and
rainfall events. To better understand the current function of the
sewer system, a comparison of the model results and service call
data from the City was completed to assess what impacts service
level and residents the most. For the joint scenario of coastal
and rainfall events, the joint-probability was assessed to define
joint-scenarios that are equivalent in probability to certain target
design events. The intention is to develop a set of design criteria
that is risk-based and practical.
Based on the defined design criteria, an option assessment
was completed with GHD’s InDeGo, a geo-spatial multi-criteria-analysis (MCA) tool, that was used to help with streamlining
the decision-making and assessment process. InDeGo assigns
and computes ratings based on criteria determined by stakeholders and designers. The ratings are applied based on parameters
such as distances, densities, or pre-defined boundaries. For
example, ratings can be applied based on distance from a park
or number of cells with a certain flood depth. The ratings will
then be multiplied by a predefined weighting percentage. The
weight-adjusted ratings will then be aggregated for the cells. The
aggregated ratings are essentially the suitability scores, which
allow users to assess if a certain location/area is highly suitable or
unsuitable based on the defined criteria.
The InDeGo model is being used to optimize alignment and
site selections based on a series of criteria, such as land-use,
existing utilities, proximity to parks, and infiltration potential.
These priority-weighted criteria are geospatially represented in
the InDeGo model to produce a heat map of optimized GRI
locations and trunk alignments.
This study is currently on-going, with an anticipated completion date of Summer 2025. The outcomes of this study will
include a conceptual design of the Willow Trunk sewers, phasing
strategy for sewer construction, as well as options and design
criteria for GRI and large-scale nature-based solutions that are
integral to the trunk sizing. The project team is currently work-
WAT E R C A N A D A . N E T
ing on assessing and optimizing a suite of solutions that are most
suitable based on the City’s objectives. The proposed suite of
solutions will be tested in the calibrated model to confirm their
benefits and impacts.
Based on our current understanding of the system and objectives, the solutions will include key components such as strategic
GRI placements, coastal adaptation designs, and a ‘tight pipe’. A
tight pipe is a pressurized section of gravity storm sewer that is
designed to bypass the low-lying area that may be susceptible to
flood risks due to sea level rise.
Tight Pipe Concept
Altogether, the implementation and integration of the new
storm trunk with other watershed-wide solutions will improve
the service level, resilience, and adaptability of the drainage and
sewer system. The Willow Trunk Feasibility Study and Conceptual Design is a flagship project that presents a holistic approach
to sewer planning and GRI integration, which is enabled by advanced modelling using InfoWorks ICM, complex GIS analysis,
and geo-spatial MCA.
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