Saturday, September 6, 2008

Flash Flood

Last Sunday evening dark clouds filled the sky to the north of us, lightning flashed, and thunder rumbled. I got all excited and convinced my husband to take me and Desert Boy out into the desert to watch nature's show. We spent over half an hour watching the clouds rush by, dumping their contents on the mountain range and desert to the north. We felt a few sprinkles, but we were pretty much right on the edge of the storm and could see blue skies to the south.

Eventually it got dark and we went home. My husband was happy that part of the range where our cattle graze was getting some moisture. We didn't think much more about the storm.

The next day we learned that the storm had packed a punch in an area to the northwest of us. Torrents of rain had fallen in a short time, overwhelming a small creek. Four people live on that small creek in two houses separated by several miles. Both reported that their roads had been washed out by the flood water.

I wanted to find out more so I asked one of the folks who lives by the creek, Blake. He said that he had a foot bridge over a the dry streambed. It had survived all the previous floods, but this one had washed it away and he couldn't find it. The high water mark was two to three feet higher than the bridge. That meant a wall of water about 10 feet high and 20 feet deep came rushing down the normally dry creekbed--probably more than 200 cubic feet per second of water.

The water flowed right over this main road, eating away at the road and surrounding banks. The main part of the flood came between 8:30 and 9:00 p.m. that Sunday evening, although Blake noted that the water kept flowing all night long.

This is the other side of the same road, where water flowing over the road eroded sand and dirt that had been covering most of this culvert. The culvert was nowhere near large enough to handle the quick flash of water.

The water came so fast that some animals, like this little fawn, didn't have a chance to get out of the way. My husband says that in the 1992 flood in a nearby creek, 20 cattle died in the gully when they didn't have a chance to get out of the way of the powerful water.

These old cars had previously been put in the stream channel as flood control, to help slow it down and trap sediment. Not exactly high tech flood control. I scoffed when I saw them previously, but they were still in pretty much the same positions as the last time I saw them, earlier in the summer.

The new flood control structures, installed by the ranch this year using modern design methods, didn't fare so well. The three concrete structures, each weighing about 50 tons and designed for 200 cubic feet per second of water, were washed out. This one had gullies on both sides. When I visited it yesterday, some water was running over the main part of the flood control structure, so at least it is working again. The other two aren't so fortunate.

At first this flood control structure didn't look too bad. It has a gully along the right side of it and extra sediment at the bottom. You can see that the stream channel is dry again.

From the top it doesn't look so good--the stream took away the sediment from underneath the structure, so any water that flows this way will just bypass the structure, and erode more of the sand and dirt. There's a lot of backhoe work to be done to make it functional again. 

The third flood control structure may not be salvageable without a lot more than backhoe work. One whole side was washed out, causing the very heavy concrete to tip over sideways. 

Here's another view of it. A pool of water still sits at the base of it, but no water is flowing anymore. The flash flood came and went, leaving just signs of its passing. To the left of the flood control structure you can see the bank that was cut out by the flood waters. The structures were installed to prevent this downcutting, but even they have their limits.

Out of the streambed, I found many more signs of the flood, like cracked and flaking mud. I could see animal tracks in some of the mud.

When the torrents of rain came down, the water started flowing downhill in what hydrologists term "overland flow," when the precipitation rate exceeded the infiltration rate. The water just flowed in a sheet, carrying debris with it, until it got into or formed rivulets that became larger and larger and eventually reached the formerly dry stream channel. The debris pictured here was about two inches high on a relatively flat part of the desert.

Next time I see the clouds open up, I will definitely be thinking more about where that water is going. The flash flood that came down over this little section of desert didn't get any news coverage because so few people live out here and relatively little damage was done. In a city, this amount of water would probably have endangered many people's lives. Flash floods are part of the desert's ecosystem, distributing seeds, reworking the desert terrain, and providing a quick energy input to some plants and animals. We don't get flash floods often, but when they do come they leave reminders of their force for years and even decades.


The Incredible Woody said...

Wow! A wall of water is a scary thought!

Sarah said...

Don't mess with water!


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