"Understand How Water Reaches the Stream and Design for Interflow", urges Department of Fisheries and Oceans
Note to Reader:
In the 1990s, the pioneering work of Richard Horner and Chris May at the University of Washington provided a science-based understanding of the importance of ‘changes in hydrology’. Their findings provided a springboard to reinvent urban hydrology in British Columbia.
This reinvention resulted in the rainfall capture approach and water balance methodology for establishing performance targets. This breakthrough is explained in Stormwater Planning: A Guidebook for British Columbia, released in June 2002. A key to the breakthrough was first developing the concept of the rainfall spectrum, and then translating the concept into an integrating strategy as illustrated by the graphic below.
Time to Take Another Leap Forward
“Rainwater management has developed far beyond the simplistic assumptions that created the detention ponds of the 1980s. It is now time to take another leap forward, albeit by moving sideways, and recognize near surface lateral water flow, otherwise known as ‘interflow’,” states Alan Jonsson, Habitat Engineer with Fisheries and Oceans Canada.
If rainfall is captured to reduce site discharge, how does the water then get to the stream and what are the processes and timelines? Reproduced from Beyond the Guidebook 2010: Implementing a New Culture for Urban Watershed Protection and Restoration in British Columbia, the graphic below shows the generalized flow patterns of natural and post developed conditions.
Recognize the Importance of Interflow
“Interflow is often the dominant drainage path in glaciated landscapes of British Columbia. Even undeveloped sites that are founded on till and bedrock rarely show overland flow because of interflow pathways. Interflow has been traced flowing at velocities that are 1/200th as fast as channel flows on a similar gradient. It is not hard to imagine the beneficial effect that this has in prolonging flows from rainfall to first-order streams,” explains Alan Jonsson.
“Unlike deeper aquifer fed groundwater, interflow water is often rich in dissolved organic carbon and other nutrients. It is this flow that feeds hundreds of small ephemeral streams throughout the Lower Mainland (in the southwest corner of British Columbia) where more than half the population resides. Such streams provide important salmonid food supply and rearing habitat. In some cases, they may even support Coho spawning.”
“When we acknowledge the role of interflow and its incredible ability to absorb and slowly discharge precipitation, we are led to the realization: a watershed’s hydrology can be severely degraded without any increase in impervious area. All that is required is a loss of functional soil layer and/or the addition of ditches or perforated pipes and presto, one ‘urbanized’ watershed. Conventional watershed health metrics such as total impervious area can under estimate impacts where interflow dominates.”
Incorporate Interflow in Engineering Design
“Unfortunately, it is a rare thing to find a rainwater management practitioner that ever ‘thinks sideways’. How many times have we all heard ‘There’s no infiltration on this site’? The challenge for engineers is to determine the influence of interflow on a site and then design and implement techniques that replace or restore it.
Our present patterns of land development often seem perfectly suited to ensuring the elimination of interflow. Utility trenches, basements, discontinuous soil and highly compacted soils all work together to deprive small streams of water. Without a significant change in development practices and standards, based on watershed-specific understandings, we cannot maintain stream health and productivity.”
“The lesson is that the interflow system is an incredibly important and yet fragile component of a watershed. It is critical for maintaining stream health and our fishery resource. Where the system is still operating it must be protected; where human activity will cause an alteration to its function, then replacement systems must be created that will mimic its operation to prevent any additional impacts to the stream and our resource,” concludes Alan Jonsson.
To Learn More:
“Watersheds are not all created equal. And when we begin to examine them, we find that they function in all kinds of different ways. And what I often see missing in most engineering methodologies is an understanding of how a particular watershed actually functions,” stated Will Marsh.
After that, click on Conservation Hydrology to read the story behind the graphic below. Conservation Hydrology emphasizes the need in many areas for human development designs to move from drainage to retainage.
Posted August 2011