The safety aspects of detention basins and their design have, for a long time, given me cause for concern. I’m probably not the only one. In my practice, primarily as a hydraulic engineer, hydrologist, and stormwater manager, I have acted both as a designer and a reviewer of projects that are directly concerned with stormwater runoff.
One aspect of this discipline is public and professional concern about safety when wet but shallow, or normally dry, detention basins suddenly fill up during a storm—sometimes to the surprise of the local, non-technically oriented populous. The very stormwater event the basin is intended to suppress causes the low-lying pasture or pond, which seems to have a strange-looking concrete or metal structure located at one end, suddenly to become a medium-sized lake.
The subject of how to keep people out of stormwater management areas has been long and hotly disputed. To fence or not to fence is a serious question. A fence could thwart an attempt by a less-than-agile senior citizen to rescue a toddler who managed to get into a life-threatening situation on the wrong side of the fence, which was intended to keep the tyke out of harm’s way. No barrier is proof against the ability of a youngster to access a detention pond.
So, having thus conceded the possibility, even the likelihood, that it can happen, what can we do, as designers or reviewers, to reduce the possibility of a tragic accident? We can never totally eliminate such an event; however, safety ledges are one (not perfect) solution. The subject of safety ledges was brought to my attention by an official public notice, which was published recently in my local newspaper to meet legal requirements—at a cost of more than $1,700!
The entire ordinance is many thousands of words long and discusses just about all aspects of stormwater management, including a table that details the precipitation for "design storm distribution [no frequency mentioned] and cumulative rainfall to be designed for at five-minute intervals."
As an aside, the rainfall is shown to five significant figures—time of storm: 80 minutes; cumulative rainfall: 1.08340 inches! With hydrology being such an inexact science, invoking five or six significant figures may give the locals a false impression of what we engineers can safely design for. A case in point was Hurricane Katrina and the devastation it caused.
A section titled "Safety standards for stormwater management basins" lists, among other details, the requirements ordered to "protect public safety through the proper design and operation of stormwater management basins." It applies only to new basins. Apparently old, and perhaps substandard designs, are grandfathered in and need not be upgraded to meet more modern, stricter standards.
Approximately 15 requirements are published. They include a discussion about trash rack designs, overflow grates, and design provisions for escaping from a wet pond or from a normally dry pond with water in it. The safety ledge design for the subject town includes a cross-section diagram (Figure 1) showing that for a detention basin with a permanent water level more than 30 inches deep, a 4- to 6-foot-wide ledge be provided so that a pre-school child would be able to perch on it and not drown if he or she were to fall or wander intentionally into the wet place.
There is a considerable amount of additional detail concerning detention basins and their design in the rest of the ordinance announcement, including information on the methods to use in making stormwater calculations; design and performance standards for stormwater management measures; stormwater management requirements for major developments (which are presumably more stringent than for minor developments); non-structural stormwater management strategies; erosion control, recharge, and runoff quantity standards; and stormwater quality standards.
However, not much additional technical information is provided about the design of safety ledges. I think some useful design features could be added. For example, the interior side slopes of the detention basin could be shown. All that appears in the sketch is a notation, "Slope to be stable," and that the safety ledge itself should "slope gently" for drainage purposes.
Perhaps in another column I’ll address other aspects of what may be a typical municipal ordinance for stormwater standards and practices.