This will be the last hydrology post for a while. Alas, the topic is a wee bit too current in my part of the world, and people are seeing the effects of the expansion of hardscaping on run-off speed and flow peaks. Granted, getting an average year’s rainfall in a month plays a major role, because no system is designed for that sort of thing, especially when you get four inches in an hour or so.
So, broadly speaking, when rain or snow falls, it either evaporates, sinks into the soil, or runs off. If it evaporates, it goes back into the atmosphere as part of the water cycle. If it sinks into the soil, some will remain in the upper layer, while some should percolate down into the sub soil, or even deeper. Plants will take a portion of that water and draw it into their systems, then return it to the atmosphere as they “breathe” (transpire, part of evapotranspiration). As long as there is space between the grains of sand, loam, or bits of clay in the soil, water will seep in. Once the soil is completely saturated, then the water flows along the surface of the soil and away. Plants, be they grasses, trees, or whatever, also slow the rate of water reaching the ground. That allows more to soak in and less to run off. Some of the water stays on the plants, as anyone who has walked through drew-soaked or rain-soaked grasses and shrubs knows. Or been dripped on by a tree long after the rain stopped.
When the moisture hits the ground and doesn’t sink in, you have runoff. This can occur when the rain falls too fast for even loose, air-filled soils to absorb, like hit Houston a few years ago. The combination of saturated soil and really heavy, fast-falling rain was too much, and all the water ran along the surface of the land, causing floods where they had not happened in quite a while. That happened up here, when an area west of the town of Hereford got between 4-7 inches of rain in an hour. The ground couldn’t absorb so much water so fast, and Tierra Blanca Creek became Tierra Blanca Raging River.
In general, much of the Texas Panhandle and adjacent New Mexico is saturated from thirty days of daily rain, much of that very wide-spread and heavy. We’ve gotten a year’s worth of moisture in a month. The rivers and streams are flowing like rivers, and are flooding areas that were considered outside the floodplain. The rainwater lakes, the playas, are all full. This is great, because some of that will go into the aquifer and help recharge it. It also keeps the soil around the playa moist, so irrigation isn’t as needed later on. This is the sort of high water that used to be considered normal, or as one retired farmer put it, “The river flooded. That’s what rivers do.” The down side is you can’t get equipment into the fields to either plant or harvest, because they will bog, and any seeds will float away, or drown.
The other kind of runoff happens when the surface cannot absorb water. Rain falling on nearly bare rock is one example, and is why you never, ever go hiking in slot canyons or narrow places out west during a rain. Or if there is a high risk of rain upstream. You get flash floods as all that rain races over the rock and joins streams and arroyos. I’ve seen a stream go from almost dry to six-feet deep in minutes. It was impressive. I was on the upper rim of the canyon. The rain was three miles away. Again, this is what you’d expect given the landscape. When people build and pave over the soil, you get the same effect. “Hardscaping” forces the rain to fall and go away. Instead of a curve of flow, you get a saw-tooth. And street flooding, and house flooding. Even if there are catchment basins and ponds, that much hard surface sheds a lot more water a lot faster than most people anticipate, unless they do extended runoff calculations.*
The current problem in several regional communities is that over the years, people have encroached on playas and built new subdivisions in low areas. Or perhaps the developer did not sit down with the city and say, “OK, if this is built up, and that other bit is built up, can the storm sewer and catchments handle the runoff from a two-inch-an-hour rain?” So when either we get a three-inch-an-hour rain, or an inch a day for ten days and more, the water can’t sink in anymore. There’s too little open land left. Those places “that never had water in them” have water in them again, along with the new houses. The planned lakes and drainage basins can’t handle the fast run-off from super-saturated ground, even with pumping, and you get street flooding, then neighborhood flooding. Those of us who shook our heads about developers building [there] are shaking our heads again. It’s not fun for the people with water in their homes, or who are trapped on islands, or who now have non-working septic systems. It’s also not fun for the people who have to try and remove that water so that the urban drainage systems work again. And it costs money and time and heartache.
What’s the solution? Well, what caused the problem? If it was lots of rain from a persistent weather system, well, you just have to wait it out. Prayer helps. If it was a single storm that dumped a huge amount of rain and then left, again, that’s just how weather happens. Cities and towns plan for an average plus X%, and that usually works. When it doesn’t, that’s when people get the big pumps out, or start calling insurance companies.
I personally would like to see developers required to do runoff calculations and plans their both their own area and the surrounding neighborhoods and shopping areas, and make the information available to the public as well as city planners. That would give a baseline idea about how much water’s going to have to be allowed for, and they can scale drainage or work with the municipality for storm-water disposal accordingly. In my dream world, no one would build in or close to the playas, and they would be turned into nice nature parks, or domesticated urban parks. But that takes a lot of potentially valuable property out of the tax network, limits what property owners can do with their land, and requires a very long civic or regional memory. Especially in places that have other geographic or political constraints. If you are like the town of Borger, with a cliff and river on one side, and rough land on another side, you’re going to build in flat spaces and hope for the best. Borger happens to be pretty well drained compared to Hereford or Sunray, but enough rain fast enough still causes a mess.
It’s a balancing act. If the county plans for the absolute rarest scenario, such as the year of 40″ of rain like 1941, but the thirty-year average is 20″, voters will not be happy about the expense and the restrictions for something that might not happen again. If the city plans for 22″ per year, batched with most of the rain in April-June, and all that rain falls in three weeks, voters will not be happy because of the flooding. Unless the city is Houston or another place where hurricane and tropical rains are not rare, planning for a four-inch-per-hour event doesn’t make much economic sense, at least not unless it happens semi-frequently so people remember.
Compounding the situation, low, flat places are often cheap, so things get built there for businesses and people who can’t afford the high ground (like St. Louis and other Missouri/Mississippi River cities). That’s also a problem, but a social one, not a topographic one.
*Baseline runoff calculations are not difficult. The books about hydrology, and the little pocket “rules of thumb” books include tables with the variables for percentages of ground cover and rainfall rates, and give you a rough starting place. I’m sure now there are computer programs that allow you to plug-n-chug the same thing with far greater detail, faster. It’s knowing how to use the numbers, and how best to apply them, that starts getting expensive. To be fair, several developers did plan for that, and did work with the land. Just not quite enough, as things turned out, in part because of later development that added more runoff.