Was it really a record flood? Part II

Historic South Carolina Flood

My last post was all about the rain. Now it’s all about the flood. How did our watersheds respond? Was it really a 1,000-year flood event?

What better place to start than the center of the state. A place that has annual stream records dating for over 123 years—beginning in 1892. This place is on the Congaree river near the heart of Columbia, a mile downstream from the confluence of the Broad and Saluda Rivers. While the Saluda River has some flood peak dampening from Lake Murray, the Broad River has limited reservoir storage. Here is the breakdown from the flood:

  • The river crested 31.8 ft on Sunday, October 4 with a corresponding flow of 185,000 cfs of flow.
  • To give you a little perspective, 5,000 cfs or so is the average flow for this river. So, at its peak the river’s flow was 37 times its normal flow.
  • This is the highest it has been since April 8, 1936, when the river peaked at 231,000 cubic feet per second with a stage of 33.3 ft.

Congaree River October 2015 Flooding

South Carolina USGS quickly published a wonderful summary of the gages.  17 USGS stream gages set new peak flow stream records. The map below shows where these records were shattered.

October 2015 Flood Records

It is easy to see this flood was historic and broke records, but I didn’t see anything in the USGS report about the recurrence interval of these floods. Many in the media were calling this a 1,000-year flood event [a 1,000-year flood event is a statistical value that means a flood of that size (or greater) has a 1 in 1000 chance of happening in any given year]. Was this correct?

I am going to go ahead and give away the answer—no. While rainfall broke statistical 1,000-year records, USGS data shows no sign that 1,000-year flood events occurred at any of the USGS stream gages. More stream gages throughout the state experienced 10-year to 50-year flood events (Dr. Robert Holmes, USGS National Flood Hazard Coordinator, Q and A).

I double-checked the data and reviewed with flood frequency statistics from earlier flood studies. USGS was correct. Here are a few I looked at with the approximate recurrence intervals for this flood:

  • Black River at Kingstree, SC: Greater than a 500-year storm event or .2% chance of exceedance
  • Edisto River Near Givhans, SC: 50-year storm event or 2% chance of exceedance
  • Enoree River Near Woodruff: 25-year storm event or 4% chance of exceedance
  • Lynches River at Effingham, SC: 100-year storm event or 1% chance of exceedance
  • Little Pee Dee river at Galivants Ferry, SC: Less than a 2-year storm event or 50% chance of exceedance
  • Congaree River, Columbia, SC: 25-year storm event or 4% chance of exceedance

Does this matter that some are calling it a 1,000-year flood event?  To me, it does. I am afraid that remembering this event as a 1,000-year flood event for the state absolves this event from reality to a statistical fantasy realm where no one lives, works and raises their families. A realm filled with locusts and wild beasts. A realm where we can go back to normal after the page is turned. There is no doubt we will turn the page in our State, but we don’t learn from fantasy tales. We learn from hard reality. We learn from remembering the bumps and the bruises. We adapt based on where we think the next hits are going to come from in the future.

My next post I will try to deconstruct and summarize some of the infrastructure damages from this flooding.  Part III of a series of posts about the October SC 2015 Flood