reThought Flood

California Dreaming (About Drains)

On the heels of the California storms earlier this month, the Los Angeles Times took a good look at LA’s storm water drainage systems. They found the “Byzantine flood control system” to be in “good so far” condition. But what of the future? As the climate changes, the system may not be able to cope, the newspaper reports.

Fifty-one miles of the Los Angeles River were channelized between 1936 and 1960. This massive and impressive feat of engineering was undertaken in response to flooding nearly 90 years ago, events which also prompted the national Flood Control Act of 1936. The familiar channel remains a vital component of the City of Angels’ flood protection system, but it was designed for a time both when floods were less severe, and when the materials used to construct drainage systems were very different. 

A river with a bridge and a bridge Description automatically generatedHow old is your city?  When was the stormwater drainage piping installed, and where? Pipes last anywhere between 20 and 100 years (and in a few circumstances, even longer). Municipal authorities usually have the good sense to check the condition of the oldest of the underground freshwater mains in their care, but occasionally not until they fail. Storm sewers, however, get much less attention. They are installed and almost instantly forgotten, which at least is a testament to the reliability of some of them.

We count on storm water flowing away from our built environment. Drainage protects our homes, businesses, and developed lands. Heavy flows across roads and surfaces are generally undesirable, since at best they erode the surface, and at worst they flood buildings and sweep people and even vehicles away. Municipal drainage systems prevent this by using roads and curbs to direct water into catch basins and roadside storm grates. The runoff  then flows through piping installed underground before being discharged somewhere safe. These systems usually work well, but just because they do so out of sight, our surface water drainage systems must not be put out of mind, for at least three reasons.

  1. Pipes don’t last forever.  

At best, the pipes that make up a drainage system will fail in place. That causes water to leak into the surrounding ground. Such seepage may sound okay, but any new source of groundwater is likely to create sinkholes, damage buildings’ foundations, penetrate basements, or even spread pollutants. Worse, piping failures may result in backups, and consequently the failure of the drainage system to remove water as intended. When water doesn’t go down the drain, it flows off the roads and into the built environment comprising our homes and businesses. So, not okay.

With luck (and it’s far from certain), reliable piping was actually installed correctly. If so, it will last for decades. Maybe a century. But over time many different materials have been used. Early water systems, both for supply and removal, were made with drilled wooden pipes. Cast iron was an improvement, but brick assemblies were much more common before the twentieth century. By then, reinforced concrete or corrugated steel were used, but the old wood, brick, and iron networks remained. Many survive in old neighborhoods, waiting to fail. Some of the more modern materials are not far behind.

Since the 1970s, plastics including CPVC, FRP, and HDPE have been used to channel water underground. This selection of modern materials and good installation should ensure that stormwater handling systems will retain their capacity for many decades, or even longer if maintained. Most failures will be caused by poor installation or external effects – shifting ground or a backhoe through the system – both of which can and should be managed.  


  1. Changes to the water load can overwhelm perfect drains.

With every new development, new construction, new resurfacing of old surfaces – almost any change to the physical environment which covers soil or other existing, porous surfaces with something else – the flow of water delivered to storm sewers increases. The rule is this: what happens anywhere in the built system affects everyone else in the same system, whether upstream, level, or downstream. 

In other words, if the old pipes are not big enough, the excess water will find other ways to get to the ocean. 

Recent property developments should – at the very least – be provided with detention basins designed to ensure that localized, high-intensity runoff is, in effect, metered into the storm sewer system. Such pooling systems ensure storms don’t overwhelm drainage systems, causing water to find other routes. Unfortunately they didn’t become mandatory before the late 1980s, and in many municipalities, very much later.  

  1. Expectations are usually exceeded in the long run. 

Finally, and with the greatest impact, every system of any kind is designed to cope with an expected maximum demand. It’s usually expressed in terms of exceedance probabilities. In the case of drainage systems, design engineers consider the probability of a rainfall event exceeding the capacity of the system they’re creating. 

Design choices for such systems generally range from 50-year probability storms to those which can be expected as often as every five years. Those numbers reflect storms that have an annual chance of occurring of between 2% and 20% respectively. 

Responsible municipalities choose their level of risk tolerance based on the expected age of their systems, potential increased demand through development, and (hopefully!) some estimate of future increases. Decisions will of course also be influenced by budgets and political priorities. It’s worth noting in this context that FEMA, the Federal Emergency Management Association (which measures the nation’s flood resilience and vulnerability), gives significant credit in its Community Rating System (CRS) for Stormwater Management to drainage infrastructure built based on 50 to 100 year flood assumptions. 

Changing weather

Another, less easily managed factor may mean drainage system loads exceed expectations: climate change. Right now, storms are increasing in frequency and severity in most parts of the USA. Science attributes these increases to warming of the atmosphere, because warmer air holds more water vapor, and drops it more quickly when conditions are right. 

A map of the united states Description automatically generatedAs storm intensities increase – as seen in 2023 in New York, Vermont, and Massachusetts, and already this year in California –  the potential to overwhelm our municipalities’ always-aging storm runoff infrastructure rises in step. In these instances, the past is certainly not an indicator of future performance (atmospheric, that is.)

Taken together, reasons 1, 2, and 3 create enormous uncertainty for government decision-makers (not to mention insurance companies). Communities must take all of these factors into account if their built environment and – more importantly – their citizens are to be protected against the storms of the future. 

Implementing long-term drainage solutions is a huge challenge, stretching both design and funding. In the interim, personal safety and defense, along with financial risk management (insurance!), best assure that we will not only survive, but thrive during the stormy times ahead. 

Ed Haas

Ed Haas joined reThought Insurance in October 2019 as property risk and CAT modeling consultant. Ed was formerly with Marsh Risk Consulting for over twenty years as Senior VP, with a focus on natural hazard modeling for flood, wind, earthquake, and other perils. Work included managing data for modeiing, interpreting results, and applying them to insurance and risk management programs. The work included site assessments at a wide variety of unique properties throughout the US and internationally. Ed was the Risk Consulting leader in the Marsh Real Estate Practice and served as lead risk advisor to the largest clients. Responsibilities included insurance program design in regard to selection of limits and deductibles, as well as designing and managing appropriate risk control programs. Specification for new construction with regard to CAT perils was a key contribution to these clients, Ed’s career started with FM Global, and has held P&C broker’s and professional engineer licenses.