What’s the Deal with Parking Lot & Street Flooding?

When we experience flooding, many times it’s not homes and businesses which are affected but rather cars and trucks driving or parked in lots and on streets. There is actually a good reason that these areas fill up with water during heavy rainfall events. The creeks, bayous, sloughs and channels take rainwater from the surface and connect to rivers, bays, and eventually the oceans. During particularly heavy rainfalls, or prolonged rainfall events these “conduits”, as I refer to to them, can fill up creating a “traffic jam”. As water backs up, it slowly fills these conduits further and further until they back up into roadside ditches and storm sewer pipes. If this trend continues then the water from the surface and the water in these roadside ditches and storm sewers which have slowed down so much as to hardly be moving, or have simply backed up altogether, start to accumulate at the surface. Since the pull of gravity is pulling the water towards the center of the earth while also backing up from being unable to flow properly, the water collects and fills up the low spots wherever it can. The lowest spots on most properties will be the parking lots and/or the streets. Imagine if you will that the water in the roadside ditch is flowing in a “U”, “V” or a trapezoid shaped profile.

What this means is that as water fills up these shapes the volume increases exponentially. And once the water has filled up to the top of the ditch then there is little else for it to be bound by except higher ground or objects like curbs, traffic barriers, or even buildings. The same holds true to the profile of your neighborhood streets. The roadway would be the “bottom” of the area and then the curbs would be a step up to the then gradual rise of the yards cresting at the finished floors of homes and buildings. So therefore the roadways and the parking lots are the “secondary containers” for runoff if it cannot adequately or rapidly drain away. While this poses a problem to lower lying areas where cars and people can become inundated, the main purpose of this designed system is so that the buildings and infrastructure can remain largely unaffected by the gathering water. The larger problem is when too much rain falls in such a short period of time (rainfall rate in inches per hour) or rain falls for such a length of time in such quantities that the conduits cannot adequately or rapidly convey the runoff to the rivers, bays and oceans (total rainfall-including above the 10 year, 25 year, 100 year or 500 year levels). If either of these conditions, or in the worst cases both, do occur then local flooding is likely and widespread flooding is possible.

The nomenclature for these rainfall events is a bit confusing. But I will tackle that in another post. Suffice it to say that I am not a fan of flooded vehicles or flooding in low lying areas. In fact I flooded out my personal vehicle in a May 2016 flood event. But I’d rather deal with the consequences of that than have my house flood. The roadways that are filling up during heavy or prolonged rains are actually doing their job. Let me answer for those of you who are thinking this, what about the areas that fill up with water when it barely sprinkles? These areas, actually, are just where the surface has been compacted and sits lower than the surrounding area, or maybe there are drains or ditches/storm sewers which need to be cleaned and maintained. Regardless of the issue, those areas do need to get some extra attention in order to avoid damaging property unnecessarily.

At Integrity Stormwater, we help property owners, management companies and municipalities uncover potential hazards in their Stormwater systems, assist in repairing or maintaining their ditches and storm sewers, or consulting in ways to reduce flood impacts or how to remove them altogether if possible. For more information, please send us some information about your issue and let us come alongside you to be your expert eyes and ears for all things stormwater related. Click here to get started!

I’ll end with this, the old adage holds true and I’m a witness of it myself…”Turn around, don’t drown!”

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