Watershed-Scale Flood Damage Reduction Using LID BMPs - Does Green Infrastructure for New Development and Redevelopment Significantly Reduce Existing River Flood Risks

River Flood Loss Avoidance (Deferred Damages) with
Green Infrastructure / Low Impact Development BMPs
in New Development and Redevelopment in the US.
A study by Atkins for the U.S. EPA evaluates flood damage reduction across North America using Low Impact Development (LID) Stormwater Best Management Practices (BMPs). The 2015 report "Flood Loss Avoidance Benefits of Green Infrastructure (GI) for Stormwater Management" is available at this link and evaluates benefits of implementing LID BMPs in new development and redevelopment over 20 years.

Flood Losses Avoided in the Year 2040 for Various Zero Damage Thresholds
What is the value of estimated flood damage reduction from 2020 to 2040 by constructing GI / LIDs? Savings range from $63M to $136M per year in 2040, depending on whether river flood damages are assumed to start above 10 year events or more frequently above 5 year events.

Over the 20 year implementation period the deferred flood losses increase from $0 in 2020 to an average of $100 million (2011 dollars) for 5 and 10 year zero damage thresholds.

Deferred Flood Losses with LID BMP Implementation
for Stormwater Management in New Developments
After the first 10 year s of implementation the flood losses avoided averages approximately $50 million. The table to the right indicates the increase in flood reduction benefits over the green infrastructure implementation period.

Key question: is the losses avoidance significant and is there value in implementing green infrastructure to achieve river flood damage reduction?

To explore whether deferred damages are significant, lets compare the average loss reduction over the 20 year implementation with average losses in North America. Munich RE's NatCatService estimates losses in North America for meteorologic events and hydrological events. The chart at right shows that over the past 10 year from 2008 to 2017 the average losses per year we approximately $75 billion in 2017 dollars. (we could net out non-US areas like Canada to make this more apples to apples).

Munich RE Catastrophic and Relevant Event Overall Losses
Inflation Adjusted and Normalized 
The deferred flood losses over the 20 year period with LID implementation are $50 million in 2011 dollars - according to the Bureau of Labor Statistics consumer price index, the dollar experienced an average inflation rate of 1.42% per year. Therefore the annual LID loss reduction amount in 2017 is 8.8% higher than deferred losses in 2011, or $54 million (2017 USD).

Deferred annual flood losses of $54 million represents only 0.07 % out of $75 billion in overall annual losses. This suggests that near-term river flood damage reduction is not a core benefit of green infrastructure implementation.

The cost of expanded green infrastructure implementation is not assessed in the Atkins study as noted here:

"The costs of GI implementation are not included in this document. Nevertheless, new development and redevelopment already require stormwater management expenditures, either on-site or downstream; therefore, GI could be used to meet those requirements fully or partially for little or no
additional cost compared to overall construction costs. This study does not assume retrofitting of existing imperviousness. Retrofitting, in addition to implementation on new development and redevelopment, would be expected to generate more flood loss avoidance benefits but would incur
additional costs."

Given that green infrastructure stormwater controls are assumed to be included in the base cost of development or redevelopment, the micro-sized river flood loss reduction benefit comes at no addition cost, which could indicate good 'value' - however given the almost insignificant percentage of overall flood damages averted, one could question whether river flood risk reduction should be prescribed as a low impact development benefit at all. It would appear that core benefits of green infrastructure are instead the environmental ones related to water quality improvements and erosion risk reduction.

Today it is popular to promote green infrastructure citing its multi-faceted triple-bottom-line (TBL) benefits on many aspects of the environment. It would appear that riverine flood risk reduction is not one of those benefits to consider in the TBL assessment. Many groups and Canadian municipalities have noted impacts of green infrastructure on existing utilities and foundations as a dis-benefit / adverse impact of green infrastructure measures that typically infiltrate runoff into the ground. Many of these negative impacts were recently summarized in the Ontario Society of Professional Engineers' (OSPE) comments on Ontario's draft Watershed Planning Guidance which had promoted the role of green infrastructure for flood control :


The OSPE comments note:

"While green infrastructure has recognized roles in achieving watershed outcomes, including
water balance and water quality management in greenfield developments, the above statement
is inconsistent with numerous studies that discount the flood-control benefits of green
infrastructure. Numerous studies have demonstrated that green infrastructure does not provide
a flood risk reduction benefit."

To support this statement, OSPE cited numerous Master Plans, Master Drainage Plans, Class Environmental Assessment studies, Best Practice documents, local university research, and municipality and water industry comments on the Ontario Ministry of the Environment and Climate Change's draft Low Impact Development guideline.