While nobody would dispute the immense value of wetlands as important natural heritage features in our landscape, their role for flood attenuation is questionable. This is due to the fact that the geographic extent of wetlands is very limited, especially compared to the Laurel Creek watershed analyzed as part of ICCA's report. The headwater subwatersheds upstream of the Waterloo Special Policy Area (SPA) has percentages of wetland of 14% and 17.6%. Many urban catchments in southern Ontario have wetland percentages in the low singe digits (see Kitchener, Guelph, Fergus, Brantford, Mississauga, Markham examples in the slide deck below). Considering rural basins in the Grand River watershed, 86% of subcatchments have a lower percentage of wetlands than the 17.6% Laurel Creek headwater subcatchment. As a result, the effectiveness of wetlands for typical flood attenuation is overstated.
Mapping and analysis is shown in the slide deck below and that was prepared to advise Green Communities Canada on wetland flood attenuation feasibility as part of its Urban Flood Collaborative:
Other reports have evaluated the role of wetlands for flood control and their cost effectiveness including "The Use of Wetlands for Flood Attenuation", the at this link. The citation is as follows:
Williams, L., Harrison, S. and O’Hagan A M. (2012) The use of wetlands for flood
attenuation. Report for An Taisce by Aquatic Services Unit, University College Cork.
The report notes important issues such as (see Section 5) "Conflicts between flood attenuation and other wetland functions" which notes:
"Protecting biodiversity and enhancing the flood attenuation potential of a wetland are
often thought to be compatible and the two objectives are often conflated. However,
the biodiversity of a given wetland is largely driven by its particular hydrological nature,
which may conflict with flood management. Conflict between flood management and
biodiversity objectives on floodplains can arise with respect to the duration and
seasonality of flooding (Morris et al., 2004). Flood management generally requires the
storage of flood water during the period of peak flows followed by evacuation of flood
water as soon as possible in order to secure the storage facility for re-use. Biodiversity
objectives, however, usually require some retention of water beyond the flood period.
The management of wetlands for birds illustrates this problem. Areas of shallow, smallscale
flooding within floodplains are of critical importance for breeding wading birds. "
It also speaks to Cost Effectiveness (see Section 7). On cost-benefit case studies:
"The idea that wetlands can provide cost-effective solutions to flood management
pervades conceptual literature, but it is difficult to find detailed evidence to support
this. Four international examples are provided, for which some degree of the project’s
cost-benefit analysis detail could be found. There was evidence that schemes do show
cost-effectiveness, but there is also evidence to the contrary. Evidence is confounded
by a lack of detail in how costs and benefits are accounted for and whether these take a
wider ecosystem services approach. Arguments in support of NFM schemes utilising
wetlands draw strongly on the financial benefits in terms of reductions in the cost of
downstream flood damages, however, the cost of implementing such schemes has not
been found to be fully reported. Whilst capital costs of engineering works; maintenance
and so on are often reported, it is unclear if, and how, the cost of aspects such as a
community engagement process, land purchase, and ongoing management for
biodiversity goals are included. As already discussed above, many of these projects may
not be feasible under traditional cost-benefit scenarios, and would not have reached
implementation without some form of additional funding. Where possible the examples
in Table 3 showed how the projects were implemented due to funding from, for
example, WWF, EU LIFE funds, EcoFund Foundation (Poland) and Green Action Fund’
(Polish NGO) (ECOFLOOD, 2006)."
The Toronto and Region Conservation Authority has also identified wetland sensitivities to alterations in the water balance (i.e., how much water is stored in a wetland and when), which would limit the ability to increase flood storage and peak flow attenuation using existing natural wetlands. This is described in their report "Wetland Water Balance Risk Evaluation, Toronto and Region Conservation Authority, 2017" at this link.
The TRCA report notes:
"Wetlands provide many essential ecosystem services in urban and urbanizing areas. The focus
of the Risk Evaluation is on protecting the ecology of a wetland by assessing the risk of a
proposal to the maintenance of hydrological conditions."
"...altering hydrology has the potential to alter the capacity of a
wetland to provide several ecosystem services that are of importance at a watershed scale. "
The report notes further that only where the wetland has low ecological function could alterations to hydroperiods (i.e., changing how much and when water is stored in the wetland) be considered:
" In some cases where the existing level of wetland service provision or ecological function is low,
it may be acceptable for there to be a divergence between the pre- and post-development
hydroperiod such that the ecological function or other wetland services are enhanced."
The Insurance Bureau of Canada, Intact Centre for Climate Adaptation and International Institute for Sustainable Development released a report on natural infrastructure for flood resiliency called "Combatting Canada’s Rising Flood Costs: Natural infrastructure is an underutilized option", available here. Several case studies are presented including one that evaluates the benefit/cost of a reservoir storage / wetland facility called Pelly's Lake in Manitoba.
IBC and report authors conducted 'meta-analysis' to assess the wetland's flood damage reduction benefits - as shown on the following images, flood attenuation was reported as 20% of total benefits:
The net benefit-cost ratio for flood control only would be below unity.
Meanwhile a 2017 report on the same site reported significantly lower local flood damage reduction values and benefits considering an adjacent Manitoba watershed and literature values. Instead of flood damage reduction representing 20% of the benefits, as IBC calculated, the original analysis, published in Applied Water Science indicates flood benefits of 0.03% to 4% of total benefits.
This comparison shows that local flood damage information (adjacent watershed damages in Manitoba) can result in significantly lower benefits compared to 'meta-analysis' generic literature values, based on world-wide estimated benefits. As Williams, Harrison, and O’Hagan wrote on cost effectiveness, "Evidence is confounded by a lack of detail in how costs and benefits are accounted for.."