HEALTH AND SANITATION 31
Smarter stormwater management and flood mitigation for a changing climate
By
Mike Flenniken
Flooding devastates homes, overwhelms infrastructure and compromises public health. By designing resilient systems that manage stormwater more effectively, communities can better protect people and property while reducing long-term costs.
As areas are redeveloped and the landscape changes, such impervious surfaces as pavement and roofs that prevent precipitation from naturally soaking into the ground become more prevalent. Stormwater management is the effort to repurpose and / or control, when possible, the runoff of rainwater or melted snow into areas where it can benefit communities and not cause flooding or other negative conditions.
Ideally, stormwater is absorbed into the soil, where it is filtered and replenishes aquifers, or flows into streams and rivers. However, in cases when heavy rainwater saturates the ground, excess moisture runs across the surface and into storm sewers and road ditches. This water often carries debris, chemicals, bacteria, eroded soil and other pollutants, and introduces them into streams, rivers, lakes or wetlands, and causes flooding in extreme cases.
Best practices for mitigation IAPMO vice-president of Technical Services and Research Christoph Lohr, P. E., said of all the factors that determine a building’ s resiliency, properly sized storm drains are the most important.
Definitions on the difference between stormwater and rainwater can vary, but rainwater is generally accepted as natural precipitation that has not been contaminated by use. IAPMO’ s Water Efficiency and Sanitation Standard( WE • Stand ®), the first American National Standard focused entirely on efficient water use in buildings while maintaining safety, defines stormwater as“ Natural precipitation that lands on a manmade, above-ground surface and can be collected on site for beneficial uses.”
In 2024, IAPMO and the American Society of Plumbing Engineers( ASPE) co-published White Paper 2-2024: Capacities of Stacks and Horizontal Drains in Storm Drainage Systems.
The sizing for storm drainage systems is dependent upon flow capacity equations used to calculate velocities and flow rates in pipe conduits. One of the variables in the equations is the coefficient of roughness. The white paper explores how the roughness of different types of material will change the computational results that are dependent upon the roughness coefficient of the pipe.
With some model codes used in the Southeast US mismatching pipe material with their sizing tables, the potential for roof collapse or pipe-bursting inside the building increases. It is vital for cities and states to take a second look.
“ Storm drainage systems play a vital role in building resiliency as they convey water off the roof”, said Lohr, who co-authored the white paper.“ Studying storm piping systems is important to make sure these systems are not undersized, as the weight of water on a roof could exceed design parameters and fail, which could be catastrophic. With changing weather patterns, particularly rainfall rates increasing in many locations, revisiting assumptions and validating the equations of how storm piping systems are sized is vital. We are thankful to our partners at ASPE for collaborating with us on this white paper, which shows the depth of plumbing expertise exhibited by both organisations and their dedication to public safety.”
Daniel Figola, director of National Engineering for Advanced Drainage Systems, Inc., and a member of ARCSA International’ s board of directors, said the best practices for stormwater mitigation are changing and adapting. The US Environmental Protection Agency( EPA) requires certain types of waste or runoff to be detained on site before being released to prevent pollution and erosion, which changed the practice of moving that water offsite as quickly as possible.
With climate change leading to more intense rain events as well as droughts, Figola said conveyance piping and detention systems are often undersized based on the new rainfall patterns. Infiltration practices such as traditional permeable pavements and rain gardens are more geared toward runoff control than flood control.
“ When you’ re getting these unanticipated rainfall events you almost need a belt-and-suspenders approach in my opinion, where you need to have all of those components. You need to properly size the pipes carrying the water, you need to properly size your detention systems and ideally should be reusing that water or infiltrating a component of it to reduce that downstream runoff and recharge groundwater tables,” he said.
David Dexter, P. E., an engineer and master plumber who chaired IAPMO’ s stormwater pipe sizing task group, said regulations prohibit property owners from discharging more off the site than would occur if the site were in its natural condition, which presents a problem given the present landscape.
“ We have made hard surfaces all over the place, so the water has nowhere to go and we wind up having floods. You have to find some means to get that water to go into the ground or retain it where it can eventually evaporate or leach into the ground over a period of time as compared to allowing it to escape your property,” he said.
David Viola, IAPMO CEO In the spirit of sharing the unique experiences that shape the plumbing industries in our respective nations, the following article examines the increasing need for resilient water infrastructure as storms grow more intense and frequent. Written by IAPMO Senior Communications specialist Mike Flenniken, it is the next in a regular series of similar articles that will run in this magazine.
Dave Viola IAPMO CEO
“ Catching and
infiltrating the stormwater in smaller quantities helps to reduce peak flow rates and keeps the land from being overwhelmed.”
May 2026 Volume 32 I Number 03 www. plumbingafrica. co. za