Water Balance Methodology
Prevent erosion and sustain 'environmental flows' in urban streams.
Funded by federal, provincial and local governments to support stream protection and climate change adaptation strategies, the Water Balance Model family of online tools (waterbalance.ca) includes the Water Balance Express for Landowners (Express). The Express helps landowners quantify how well rain gardens on their properties slow, sink and spread rainwater runoff and do their share to meet pre-set watershed targets for volume, infiltration and flow. The tool is integrated with Google Maps/Earth and the land use zoning of partner local governments.
Stream Health
The Express is populated with watershed targets determined by applying the Water Balance Methodology. The flow-duration relationship is the cornerstone of the methodology. By maintaining flow-duration, stream erosion is not increased during wet weather and ‘environmental flows’ are sustained during dry weather. When homeowners slow, sink and spread rainwater runoff on their property, urban streams benefit.
The new Water Sustainability Act will establish regulations pertaining to ‘environmental flows’ within the next few years. The Express is a key part of the long-term solution for protecting stream health.
Protection of streams and fish is an important public expectation, creating a driving force altering our perceptions, aspirations, and treatment of the urban landscape. Water Balance Methodology objectives start with the stream and end with the stream, thereby providing a true measure of success for environmental protection:
Maintain stream flows,
Improve water quality,
Provide flood protection,
Prevent unforeseen impacts,
Prevent stream erosion, and
Replace the lost interflow system.
The Water Balance Methodology (Methodology) is based upon watershed and stream function and operation. Understanding how precipitation makes its way to the stream allow us to assess how a watershed and stream operate while analytically demonstrating impacts of development and the effectiveness of mitigation works. The table summarizes the natural Water Balance of many west coast watersheds.
Rainwater Management
The objective of Rainwater Management is to mimic the natural Water Balance and flow pathways naturally occurring in a watershed. This approach provides a level of assurance that:
Excess water will not be directed to the ground and would avoid potentially adverse impacts of excessive groundwater levels and discharges in areas lower in the watershed.
Summer flows will be maintained with an operating interflow system.
Downstream properties will not suffer an increased risk of flooding or flood damages.
The Methodology establishes watershed targets with verifiable calculations and mitigation systems are optimized for both cost and function. Analysis begins with calibration of continuous simulation models using long term climate records. The effects of urban development are then estimated and required mitigation measures are sized. The rain garden detail below illustrates application of the three performance targets for design - the origin of the illustrated detail is the Metro Vancouver Source Control Guidelines.
