Nearly a week after the impressive rainfall across much of South Carolina, several rivers in the Coastal Plain/Low Country region are at their crest or approaching their crest at major level flood.
Major level flooding means that extensive and flooding with some low-lying areas completely inundated. Structures may be completely submerged and large-scale evacuations may be necessary.
A location of particular concern is Edisto River near Givhans Ferry, SC. The last time the river was this high was on September 21, 1945. The flood of record is 17.5 ft which was set in 1925. The forecasts for this location are challenging because the river is hovering near the top of the rating curve.
While at this time the additional rainfall on forecast for Saturday (see graphic below) is not expected to significantly impact the crest level, it does create the potential for the river flow could go above the rating curve.
The USGS, the agency that maintains this river gauge, does not have a rating curve that extends beyond 16 ft and a flow of 25.6 cubic feet per second (cfs). This is shown in the NWS River forecast graphic above by no values above 25.6 cfs.
This lack of information corresponds to the gap of flow data in the area circled in orange on the USGS plots in comparison to the stage plots.
What is a rating curve?
The rating curve allows is a correlation between the amount of water in the river or river flow to the river height(stage). When a river exceeds the rating curve (or the amount of flow is higher than the rating curve), a high degree of uncertainty enters the forecasts as we limited in our understanding of how to correlate the amount of water increases (flow) to the stage because we are not sure how the water will spread horizontally outside of the river banks.
This correlation is traditional developed through statistical analysis, understanding of the river banks and overflows floodplain, as well as historical measurements – or when the USGS physically evaluates the flow for a specific height level with special instrumentation. The measurements are made every couple of months or as needed in flood events. The Asterisk on the USGS graphics is a measurement that was taken on Oct 7, 2015. They measured 24,200 cfs at a level of 15.6 ft. The video below shows how USGS takes measurements.
The rating curve correlation can also be developed with highly detailed topographic data and complex hydraulic river models. However this is not simple process or something that can be quickly done on the fly during a flood event.
The problem from a river forecasts perspective is the river models provide forecasts in a context of flow. So if we do not have a confidence in the correlation that is used to convert the forecast flow to stage, then our confidence in the forecasts of the river heights is decreased.
You might ask why can’t we use a rating curve from a nearby river gauge. Unfortunately, each river point is different since their river banks and overflow areas are different. So we are not able to “adjust” a rating curve using an upstream or downstream location. In addition not only is each location different, but each flood event can be different. If the river bank and overflow area is full of vegetation, the friction from the vegetation that will cause the movement of water to slow. This in turn causes the water to “back up” and result in a higher height for a given flow. Another challenge with rating curves is understanding what is going on with the bottom of the river channel. During a flood, dirt, rocks and debris are transported in the river. Initially, the fast moving water eats away at the bottom of the river channel which is a scouring effect. But then once the crest moves through, the water slows down and that dirt and rocks starts to deposit on the bottom of the river. This is the deposition process and actually allows for “less room” for water in the river channel. So really the rating curve changes through and event which is why it is important for the USGS to continuously take measurements to adjust the rating curve and improve the accuracy of flood forecasts.
Those with interests along rivers in the South Carolina Coastal Plain are urged to remain aware for changing water levels and respond according to local Emergency Management recommendations.
The team at WxIntegrations will continue to provide relevant information and resources to help you better understand and prepare by incorporating weather impacts into your planning and response.