Factor of Safety Essential in Stormwater Design
As noted in mid-March, we’ve been reaching out to experts for contributions on key issues — and are looking forward to featuring their commentaries as they come in.
As always, feel free to comment, and/or get in touch with us if you’d like to join our team of Guest Contributors.
Guest Commentary, submitted by
John A. Miller, P.E., CFM
Water Resources Engineer
Certified Floodplain Manager
To be perfectly blunt, I am very glad that some site engineers did not become structural engineers. Why? Because, if they had, we would have buildings, bridges and other structures falling down around us. Too many site engineers do not consider Factor of Safety. Let me explain.
Factor of Safety in structural engineering is used to compensate for variations of materials, oversight in production and assembly, construction workmanship, simplification in design and future unknown loads that a structure may experience beyond its initial design. Think of a commercial building, where a room is converted from office space to file storage. The dead load or weight of the contents of the room dramatically increases with the addition of dozens of file cabinets, filled to bursting. In most cases this change of use does not lead to failure, as the original design anticipated, using a Factor of Safety, that an alteration might be made at some time in the future. (NB: As a professional engineer, I suggest that if you are making this magnitude of change, you consult a structural engineer to verify the structure’s ability to support the increased load).
A site engineer may say that using a Factor of Safety in stormwater management design is “over-engineering,” since the consequences of failure or partial failure are not as significant. And to boot, I have often heard a site engineer testify that his or her design produces “better than before conditions” when meeting the applicable regulations (unfortunately, for many reviews I have conducted, in truth, the design doesn’t even meet the minimum standards.)
There are a number of consequences for failing to design stormwater management systems with a proper Factor of Safety. For structural engineers, injury or death and major financial loss can occur at the design site. “Luckily” for site engineers and their clients, the consequences may be less clear. Stormwater runoff is not so easy to see as a building falling down, and the damage can occur further downstream, maybe in another municipality or even another state. Unintentional results such as stream channel erosion, degradation of water quality, or a drop of the groundwater table can all arise from disregarding the incorporation of a Factor of Safety. In the most extreme cases, flooding can result in death and major financial loss on downstream properties.
To design with a Factor of Safety, the reviewing agency or board would be more confident with the inaccuracies in the Curve Numbers or runoff coefficients, time of concentration, the accounting of the change in soils with construction compaction and other variables used in modeling the design or the modeling technique itself. One has to ask, has the engineer considered changes in rainfall intensity and totals and antecedent watershed conditions? Is the design fully mimicking the preexisting conditions for peak runoff, water quality treatment and groundwater recharge through the range of future precipitation? And what about excess runoff volume - has that been managed? Is there sufficient Factor of Safety if one or a few of the engineer’s generalizations is off the mark or misunderstood?
So, the next time you hear testimony, even by a licensed professional engineer, that the design is “better than current conditions,” take a moment and ponder if sufficient Factor of Safety is built in to make that so. Don’t take the regulations for granted. Ask yourself and then the engineer - does the design adequately consider and mitigate the water resource impacts from the current proposed and future land use changes and variability in precipitation? Is there a Factor of Safety incorporated into the design to protect the areas downstream of the site?
John A. Miller, P.E., CFM is an Associate Water Resources Engineer with Princeton Hydro, LLC in Ringoes, New Jersey.
