Wednesday, December 28, 2005

flood

The creek near my house on December 22. It's even higher right now.

Today we are having floods throughout my state. We're experiencing what is called a "pineapple express" where a warm, wet air mass moves up from the southwest and drenches us with rain. When a pineapple express hits on top of low-elevation snow, that's when we get the biggest floods. Right now actually the rivers are bit a lower than predicted, but plenty of them are over the banks and causing headaches for drivers and home-owners in low-lying areas. And forecasters are predicting two more rain events by the new year.

So what is a flood? For most people, a flood is when a river overtops its banks and threatens or damages homes, roads, and property. For scientists however, only the first part is necessary. A flood is any water level that overtops the banks of the stream or other water body and submerges normally dry ground.

How often do floods occur? Using the scientific definition, most natural streams spill out of their channels every 1-2 years. In urban areas where parking lots, rooftops, and storm sewers have increased impervious surface area, reduced the infiltration capacity of the soil, and routed water rapidly to the stream, floods often occur multiple times per year. Similarly, agricultural areas with extensive tile drainage systems may experience greater magnitude floods more frequently than under natural conditions.

What is meant by a 10-year flood? How about a 500-year flood? A 10 year flood has a discharge that has the probability of occuring once every 10 years. The same idea with a 500-year flood. So a 10 year flood has a 10% probability each year, while a 500 year flood has a 0.2% probability. Frequently, these numbers are misunderstood. There have been numerous instances of journalists, politicians, and the general public saying things like: "Last year we had a 500 year flood, so we won't have another one for a long time." That's wrong. If you had a 500 year flood last year, this year you have the same probability of having one (0.2%). Thus, you can have 10 year floods 4 years in a row, or 500 year floods twice in a decade. But on average, you will have 1 10-year flood in a 10-year period.

How are these probabilities calculated? The probability of a flood of a given magnitude is calculated by looking at the historical record of streamflow for a given site. The highest flows for each year are sorted, ranked, and divided by the number of years in the record plus 1. The magnitude of the flows and their exceedance probabilities are then to fit a curve, which can be interpolated or extrapolated to get probabilities not represented in the original data.

How accurate are the calculations? While these calculations tend to be fairly robust for high probability events, there are several problems with estimates of low probability event magnitudes. First off, the records at most gauging stations are no more than 40-70 years long, which means that magnitudes of 100-year and 500-year floods are almost always beyond the range of observed data. And slight variations in how the extrapolation is done can lead to very different estimates of the streamflow. Compounding these uncertainties, very few watersheds (particularly those of interest to the general public) have been free of land-use changes during the period of historical data. Any changes within the watershed (urbanization, road building, agricultural drainage, reservoirs) will change the way water is routed to the stream, and consequently, change the distribution of flood magnitudes. Finally, the United States Geological Survey has current gauging sites on ~7000 streams in the United States. That means that the flood frequency calculations for many sites are based on regionally-calibrated models, not from data on the stream itself.

Filling in the picture: Based on data from nearby USGS gages, my little stream is experiencing a 1-2 year flood right now. But as you can see, the channel isn't very natural (note the submerged saplings from a restoration project a year ago) and my stream gets extensive storm sewer runoff, some of which comes from outside its natural watershed. When you combine those factors, it could be that the stream and surrounding areas (as well as all the plants and critters) are now getting deluged on an annual basis with what might once have occurred (on average) once every 5-10 years.

For more information: USGS National Streamflow Information Program; Floods Q&A; Flood Frequency Analysis Tutorial (math-intensive)

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