Maybe some rain, and some notes on Savannah’s rain climatology and flooding

Siegfried may not be happy about it, but over night mid-town Savannah got about .15″ of badly needed rain. Some more may be on the way, but unfortunately at the moment most seems to be skirting the area to the north …

It should be over by mid-afternoon, some potential for thunderstorms but potential seems weak, here the forecast for lightning between 7:20 and 8:20 am …

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On a related note, the Savannah City Council is almost certainly going to impose a “stormwater utility fee” on residents. Several city officials have said that the flooding this summer is because of higher rain totals as justification, with Mayor Johnson saying in a Facebook post this Sunday:

In August, the National Weather Service recorded 11.70 inches of rainfall in #Savannah. The 30-year average for that month is actually 6.79 so we almost doubled that.

This is a classic example of misleading statistics.

The question isn’t the average, or how much above the average a given value may be. It is how unusual (the “return period”) the event is. And an 11.7 inch monthly rain total isn’t that unusual for the Savannah area. June to September is the wet season in Savannah, mostly due to tropical systems and moisture streaming up from the Gulf Totals over a foot are not unusual, August 1995 had over 17 inches! In 1971, 1944, 1919, 1961, 1972, and 1986, the month of August had totals over 12 inches. If we look at the month of September, 1924 had almost 23 inches (22.88″ to be exact). September of 1950, 1945, 1947, 1953, and 1975 all had totals over 12 inches. If we look at July there is a similar story: over the last 120 years, there were eight examples of July totals that were over 12 inches. Five Junes in history (1963, 1999, 1992, 1941, and 2019) have totals over 12 inches. Even October 1994 had almost 20 inches (19.84″).

So the Mayor and other City Council members and staff are absolutely, without a doubt wrong when they say recent rain totals are unusual. They aren’t.

Urban flooding is a complex subject. The climatology is just the start, not only rain totals but peak rain rates, soil moisture levels, and system saturation. Just as important is the land surface (topography and surface characteristics) and drainage system that controls where that water goes and if it backs up, causing flooding. In an urban area the land surface is altered by construction that is nominally controlled by zoning and development decisions, with the drainage systems likewise controlled by the local government through both construction and maintenance.

While climate change is real, the problem with flooding in Savannah has little to do with changing climate. The area does have a difficult hydrology, but no more difficult than other areas that have managed to keep flooding under control. The problem is more likely to be with urban planning, infrastructure design, and maintenance.