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Heavy rain caused localized flooding

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The sun is shining! After the amount of rain that has fallen across the area since Saturday, it will take a while to dry things out across the state. While this particular rain event wasn’t historic or record-breaking, it was serious and damaging to many low-lying spots around the Triangle.

satellite image

Satellite image of low pressure circulating over North Carolina the morning of April 25, 2017

I was asked what caused such massive amounts of precipitation over the last few days, and here is the simple, short-term answer: Saturday through Sunday, we saw a frontal system pass bringing the first round of rain. Almost right on its heels, a slow-moving low pressure system that developed over the Deep South arrived and creeped across North Carolina. Drawing moisture from the ocean, it had a large amount available to dump on much of the state.

This isn’t the first time a situation like this has occurred, and it won’t be the last. In fact, we should probably be grateful that it’s not January — at least, those of us who aren’t fans of snow. Instead of snow drifts, we are dealing with the aftermath of localized flooding. Maybe that’s not much better, but the higher elevations recover and get back to business quicker after a heavy rain than after a heavy snow. The lower elevations may take a bit more time.

One infamous low-lying area in Wake County is Crabtree Creek near Highway 70/Glenwood Avenue, which is home to Crabtree Valley Mall. Anyone who has lived around here for any length of time knows that when we see four-plus inches of rain fall in a short period, Crabtree Valley is probably going to flood.

Crabtree Creek water level

Graph of water level of Crabtree Creek at Highway 70 in Raleigh during April 22-24 extreme rain event

The mall opened in 1972, and the first historic peak discharge listed at the Crabtree Creek and Highway 70 gauge operated by the U.S. Geological Survey is listed as June 29, 1973. I found it odd that the next peak flow listed wasn’t until 1996, so I inquired about the history of that station.

According to Doug Smith at the USGS Raleigh office, that gauging station “was operated as a partial record site and a few flow measurements were made at the site beginning in 1972. There were no continuous data collected at that site before February 1988.” There have been at least 10 flood-level or above-peak stream flow events at that gauge since September 6, 1996 (Hurricane Fran).

What contributes to Crabtree’s flooding issues? First and most obviously is that it is a low point in the geography of Raleigh. Second, the amount of urban development surrounding it has grown exponentially over the decades. The more impervious the surfaces created around the creek, the faster precipitation flows directly into the creek leading to faster water level rise and flash flooding. Concrete, asphalt, and buildings surrounding a creek with very little open green space and absorbant soil to slow the flow are a major cause of Crabtree Valley’s troubles.

Another potential factor there and elsewhere is storm drain blockages. When you see areas that don’t often flood, or those that do, flood more quickly, it’s quite possible that nearby storm drains are unable to do their jobs. Storm drains should divert the flow of water away from roads and into locations where the water can safely collect and eventually disperse. Precipitation that falls too fast for the storm drains to do their jobs is a possibility, but another is blockages caused by debris and litter. A simple rule of living with storm drains: don’t put anything down there except storm water. That water flows to streams which flow to rivers which flow to the ocean. Those drains need to be kept clean for that reason at the very least.

Some people would attribute rain events like this one to climate change, and they might be partially right. I have written here before about studies that point to a warming climate as a cause for extreme rain events. Below are links to a couple of relatively recent stories about two such studies. There is no question that climate models predict dramatic warming and related weather events such as heavy single-day rainfall, prolonged drought, and temperature extremes. I would be remiss if I did not acknowledge the documented questions about the accuracy of the climate models and such predictions and their potential fallacies.

Whether flooding is caused by poor choices in urban development, blocked storm drains, slow moving low pressure systems, or climate change, we need do what we can to mitigate the risk. There are some things we have no control over such as the weather. Other things we have pretty good control over include storm drains and choosing to use pervious materials in and around urban development. Considering storm water runoff when deciding where to build both residential and commercial developments can help avoid issues like those faced by our neighbors along Crabtree Creek in Raleigh.

We can’t go back in time and rethink where to put that mall, but we can avoid repeating that developer’s mistake.


Additonal reading not included in the hyperlinks above:

“Study finds more extreme storms ahead for California”

“Extreme downpours could increase fivefold across parts of the U.S.”