Finding Onsite Restoration Solutions for Hydrologic Issues Caused by Off-site Development: Two Case Studies of Hardwood Swamps in Ontario, Canada

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Heather Beam , Heather Whitehouse

Publication Date:

Two case studies are explored where off-site development altered wetland hydroperiods and threatened the persistence of deciduous swamp communities (Mixedwood Plains ecozone). The project objectives were to: compare existing hydroperiod conditions against the suitable hydroperiod for the swamp type (using feature-based water balance analysis), identify mitigation measure(s) to bring the hydroperiod closer to suitable conditions, and obtain buy-in from agencies and stakeholders. In the first case study, the outlet of an off-site storm water management pond was constructed near the boundary of a wetland complex that contains provincially rare Bur Oak swamp. This increased surface water contributions to the wetland by 158% and exceeded the flood tolerance of Bur Oak. The best solution given the limited space available (only 30m between the wetland and proposed on-site development) was to divert the first 7.5mm storm event into a diversion swale while allowing the >7.5mm storm event to enter the wetland complex. In the second case study, preliminary studies suggested upstream residential development caused a substantial increase in surface water contributions to a Swamp Maple-Hardwood swamp. The site-level water balance raised concerns that the swamp may be converting to an open marsh community. The case became more complicated when it was discovered the swamp was partially groundwater dependent, was not riparian, and behaved as a cascading system. The best mitigation measure was to direct all surface water flows from on-site development away from the feature. For both cases, the future vegetation communities are discussed along with preliminary observations of current swamp health.

Resource Type:
Conference Presentation, SER2021

Pre-approved for CECs under SER's CERP program