Look Upstream to Gauge Flash Flood Exposure
Many property owners believe their building is safe from flooding. That’s understandable if it sits outside an identified flood zone and has never suffered inundation. It’s even more so when the area has endured intense rainfalls in the past, but the building has stayed dry inside. Unfortunately, however, distant changes have the potential to put previously safe buildings under inches or even feet of water when a flash flood races by.
A classic example of the effect of upstream changes occurred during the catastrophic flash flooding that swept across much of Vermont in July 2023. Some of the properties which passed the subjective evaluations of flood zone and previous record were in fact badly flooded, leaving their owners unhappily surprised. reThought Flood has examined several of the losses to figure out why “safe” buildings flooded, and identified a critical factor.
Many of the flooded buildings are located along the streams that funnel water down from higher elevations. While the water is close to hand, normal watershed runoff had not in the past caused a damaging inflow, even during severe storms. So what was different this summer?
In short, upstream changes caused downstream losses. Changes of all sorts can increase the level of downriver runoff. New developments are a major factor. Landscape modifications with retaining walls redirect runoff, creating the potential for formerly friendly streams to burst their banks during a downpour. FEMA-delineated floodways set within stream boundaries severely restrict construction that would impact downstream occupancies. However, any change in a watershed will impact runoff flows and velocities. In combination with an intense rainfall, many otherwise acceptable changes may lead to a massive increase in flash-flood impacts at lower elevations.
Road bridges are often at risk. Few are built higher than necessary to surmount the peak water flows anticipated at the time of their construction. But as upstream development changes runoff potentials, many bridges’ original clearances become insufficient. Upwards of 1,285 bridges were washed away or severely damaged during the serious Vermont flooding of 1927. The total number of bridge losses this time around has not yet been added up, but just the small town of Marshfield, in Washington County, lost four bridges. They will no doubt be built back higher.
Trees are also a major source of increased flash-flood potential. Those growing along embankments, some seeded or planted as long as a century ago, have grown substantially. That makes them more vulnerable to soil erosion and collapse due to floods which would have been easily survived when the trees were smaller. As the waters rise due to changes upstream, the threat may be compounded. It is fair to predict that every tree along a stream will eventually splash down into it.
Some will flow downstream. Fallen trees – and anything else that might be picked up by floodwaters, from trash cans to trailers, mobile homes and even buildings – become a serious potential obstruction of culverts, drains, and bridges. When its natural course is blocked, flowing water simply redirects around the obstruction, like Google Maps routing in action. The water then may erode structures including bridge and roadway supports, as well as building foundations. It likewise adds more floodwater-diverting solids to the stream of detritus careening downstream, and creates instant grade modifications that may put your building at risk.
When a floating mobile home lodges between overhanging trees on the banks of the stream at your property’s edge, it may send the flowing water directly into your structure. The resulting flood loss is much less likely to have happened without the new trailer park built last summer just a mile upstream.
reThought Floods’ Flood Resilience Score is the first sensible step towards assessing the potential impact of changed upstream grade conditions and topology. It can reveal developments which contribute to the occurrence of previously unlikely flood events. No one can predict where a floating trailer will end up (or – much worse – a floating, venting propane tank), but a thorough risk evaluation that considers upstream exposures will lead to a much better understanding of flood risk in otherwise unthreatened locations. That, in turn, delivers much better-risk-adjusted insurance prices.
Sources:
https://www.weather.gov/media/btv/events/1927Flood.pdf