I saw a vivid demonstration of the event known as a flash flood recently, as Tropical Storm Hermine moved inland. The center of the storm passed about 50 miles west of our location on September 7th.

We started getting rain from Hermine on the 6th, although only 0.1 inches that first day. On the 7th, we received 3.2 inches and on the 8th, 5.2 inches. These numbers are measured with a rain gauge at our house. Other areas in central Texas received over 12 inches.

When I got home on the 8th, I checked the rain gauge and it read 3.6 inches. I recorded the value and emptied it, thinking that we might get more and I didn’t want it to overflow. A little later that evening, we had a very heavy downpour that lasted about half an hour. When it was over, I checked the rain gauge again and it showed 1.6 inches, so the event was producing about 3.2 inches of rain per hour.

As I was walking back to house, I realized I could hear the roar of rushing water. Our drive/road way has a low water crossing at a wet weather creek and the sound was coming from that direction. The road was fine when I first got home, so I drove down to see what it looked like.

The first photo shows the view back to where I left the car. I didn’t try to drive across the culvert because I couldn’t tell how deep the water was. The culvert is about 3 feet high. The water came up to my knees when I waded across and I realize now that it was very dangerous for me to cross. The current was strong and swift and exerted quite a force. A car would have been swept away.

The culvert is not designed to handle this much water. There are four pipes, each about 2 feet in diameter, but they are clearly overwhelmed. The culvert actually acts more like a dam and you can see the water mounding up on the left side of the photo.

This photo shows the same area, taken on September 12th, for comparison. You can see how the current swept away the rocks that are normally piled up against culvert, as riprap, meant to control erosion. With a current this strong, the riprap failed and I’ve got some work to do to put it back.

The third photo is a view downstream of the culvert, where we have a couple of ponds that hold water for a while after a rain, but eventually dry up. The area is completely flooded.

Here’s the same area, as it normally appears, for comparison.

As quickly as the water rose, hence the name “flash” flood, it receded. About 20 minutes after the first photo was taken, the water was no longer spilling over the road. These floods are dangerous because they can catch you by surprise.

There are several factors involved in creating the flash flood: terrain, topography, and the amount of rain. In this case, a 1.6 inch rain event is not large. By itself, it would not have caused this flood. However, it occurred in a short period of half an hour, and after the ground was already saturated by almost 7 inches of rain in the previous days. This was an ideal combination for a flash flood.