DOWNLOAD: First guidance primer for application of Water Balance Methodology described how to establish performance targets that relate directly to the watershed objectives (Feb 2008)
Note to Readers:
In February 2008, the British Columbia Inter-Governmental Partnership released a document titled Beyond the Guidebook: Establish Watershed-Specific Runoff Capture Performance Targets to provide guidance in implementing RAINwater management solutions. The “event of record” was the Water Balance Partners Forum, hosted by the District of North Vancouver.
This was the first of three primers (in 2008, 2011 and 2014) that have documented the evolution of the Water Balance Methodology over time. It has been a building blocks process (as summarized in the image below). The foundation document is Stormwater Planning: A Guidebook for British Columbia, released in 2002.
The 2008 Primer provides context for the evolution and transition from “runoff capture” to “mimic flow-duration”. This represented a milestone leap forward in understanding.
Identify what needs to be done at the site scale to prevent stream erosion and protect stream health
In 2002, Stormwater Planning: A Guidebook for British Columbia introduced the concept of performance targets to facilitate implementation of the integrated strategy for managing the complete rainfall spectrum.
The Guidebook articulated a principle that performance targets at the watershed scale provide a starting point to guide the actions of local government in the right direction.
The Guidebook states that the objective is to translate those targets into appropriate site design criteria that then provide local government staff and developers with practical guidance for achieving the goal of stream protection.
The first building block resulted from an understanding of how to reduce runoff volume.
Application of Watershed Targets
“Establishing performance targets provides a quantifiable way of measuring success in protecting or restoring a watershed, and for identifying what needs to be done to achieve a certain level of protection for a given watershed,” stated Ted van der Gulik, Partnership Chair, when the Primer was released at the 2008 Water Balance Partners Forum.
“The litmus test for an acceptable Watershed Target is that the resulting RAINwater management solutions make sense, are affordable and result in net environmental benefits at a watershed scale.
“For a performance target to be implemented and effective, it must have feedback loops so that adjustments and course corrections can be made over time.”
Define What is Achievable
“To provide a starting point for early action, the Guidebook referenced the Water Balance Methodology to a healthy watershed, defined as one where the proportion of impervious area is below the 10% threshold for runoff volume,” added Kim Stephens, Program Manager for the Water Balance Model initiative.
He pointed out that the Guidebook also emphasized the need for flexibility in setting performance targets:
“Performance targets that are based on the characteristics of a healthy watershed, including targets for runoff volume, runoff rate, and any other indicators that may be used to define a target condition, should be used as a starting point.
Performance targets should be customized for individual watersheds and catchments, based on what is effective and affordable in the context of watershed-specific conditions(page 6-8).”
“A key principle stated in the Guidebook is to establish performance targets that relate directly to the watershed objectives,” concluded Kim Stephens.
Define a Baseline Condition
“There are many documented instances where the runoff is in excess or 10%. If the natural runoff volume exceeds 10% then the post development condition must also exceed 10%.”
Modeling Alternative Scenarios
“Scenario modeling is used to assess a range of performance targets, and evaluate options for achieving these targets,” continued Jim Dumont.
“Furthermore, scenario modeling involves consideration of the complete spectrum of rainfall events that typically occur in a year.”
“As discussed in the Guidebook, the key is to determine which scenario or blend of scenarios has the best ‘fit’ to address a range of watershed objectives.