Thursday, October 29, 2015

Joaquin Brought Historic Flooding and Death to South Carolina

from USGS

Very heavy rainfall fell across South Carolina during October 1–5, 2015, as a result of the low-pressure system that funneled tropical moisture from Hurricane Joaquin into the State. Though the Hurricane never made landfall the storm that hit the eastern half of South Carolina caused major flooding and destruction throughout the region.

Almost 27 inches of rain fell near Mount Pleasant. Other areas experienced more than 20 inches of rain. Seventeen people died. This week the U.S. Geological Survey (USGS) released its report: “Preliminary Peak Stage and Streamflow Data at Selected USGS Streamgaging Stations for the South Carolina Flood of October 2015.”

The USGS operates over 9,800 stream gauges nationwide with about 170 of them in South Carolina. Streamflow data collected from the stream gauges is used to better understand flooding, improve forecasting ability and document the extent of flooding that occurred. During flooding, streamflow data are vital for flood warning, forecasting, and emergency management. The long-term streamflow data are used to assess risk and changes in risk and to mitigate flooding through flood-plain management and in the design or repair of infrastructure.

The storm system caused significant widespread freshwater flooding throughout the State. USGS data shows the highest rainfall total of 26.9 inches near Mount Pleasant, and at the Charleston Airport rainfall totals set new records during the storm and its aftermath. Seventeen of the USGS streamgages recorded the highest peak streamflow and/or river height since those streamgages were installed. One of the streamgages, on the Black River at Kingstree, South Carolina, recorded its largest peak in the 87 years it has existed. An additional 15 USGS streamgages recorded peaks in the top 5 for their periods of record.

According to John Shelton, the USGS hydrologist who oversaw the agency’s field response and gauging operations in South Carolina, "This was absolutely an historic flood for South Carolina." The L.A. Times reported that in rural counties, conservative estimates of agricultural loses are expected to be at least $300 million, and total damages across the State will likely exceed $1 billion.

A long time reader emailed me to ask if she should have flood insurance. To help you decide you can enter your address in the FEMA web-site and see what flood zone (if any) you are in. Flood hazard areas are identified as a Special Flood Hazard Area (SFHA). This is the familiar base flood or 100-year flood zone. SFHAs are labeled as Zone A, AO, AH, etc. or Zone V, etc. Moderate flood hazard areas, labeled Zone B or Zone X are the areas between the limits of the base flood and the 0.2% annual-chance (or 500-year) flood zone. The areas of minimal flood hazard, which is where I live, are the areas outside the SFHA are labeled Zone C or Zone X and are unshaded on the maps.

The term "100-year flood" is misleading because it leads people to believe that it happens only once every 100 years. The truth is that an uncommonly big flood can happen any year. The term "100-year flood" is really a statistical designation, and there is a 1-in-100 chance that a flood this size will happen during any year. If you live on the designated floodplain, the chances are about 1 in 2 that you will experience a significant flood during your lifetime. In addition, development changes the flooding characteristics of the land, in many cases increasing it. So, I simply do not live in flood plains.

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