Applying probable annual risk to a regional stormwater management program

The Northeast Ohio Regional Sewer District (District) services more than one million residents across 60 communities within the Greater Cleveland area and began implementing a Regional Stormwater Management Program (RSMP) starting in 2016.  

Utilizing SWMM model results from the 1, 2, 5, 10, 25, 50, and 100-yr design storms, the District applies asset management principles to assign flood risks to each stormwater asset along their regional stormwater system (RSS), as well as any building, transportation, or utility (BTU) along the RSS. By also assigning the probability of each design storm when calculating the flood risk of each asset, the District has developed an innovative approach to assigning flooding risk that objectively identifies a problem’s severity and helps to assess whether it is an isolated or systemic problem.  

The District defines an asset’s probable annual risk (PAR) as an asset’s flood risk multiplied by the design storm’s annual probability, then sums the PAR score for each of the seven previously noted design storms. The higher the PAR score the higher the flood risk, and the goal (like golf) is to have a PAR score at or below zero. The PAR score can be scaled down to an individual asset, grouped to a problem area, or scaled up to a service area wide summary. The application of the PAR score has successfully been adopted and utilized throughout the District’s RSMP, given its scalability and objectivity when comparing hundreds of stormwater problem across the stormwater service area (SWSA).  

Case studies and lessons learned will be shared on how the PAR score has been utilized for advanced stormwater planning contracts that evaluate and phase large-scale complex projects, prioritizing a construction stormwater program, supporting a flood warning program’s urgent storm event response, and developing reports for various stakeholders.   

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