It has been almost two weeks since Hurricane Matthew entered US waters, and communities up and down the south east coast continue to clean-up and rebuild from the devastation. Meanwhile, the U. S. Geological Survey is continuing its Hurricane Matthew efforts, which started days before the fierce storm hit Florida.

The USGS has many responsibilities before and during major storm’s like Matthew, such as placing storm-tide sensors along coastlines to measure storm surge, installing rapid deployment gauges in critical areas for flood monitoring, performing stream flow measurements in rivers and creeks, water quality monitoring during heavy rains and flood events, and maintaining an extensive network of streamgages that could be damaged or flooded during severe weather.

However, a large portion of the USGS’ work takes places after the storm has passed and the flood waters have begun to recede. This is when the USGS begins to find, flag and survey high-water marks.

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Using High-Water Marks to determine flood height

During a flood event, rising waters are laden with floating debris that can stick to trees, buildings, or other structures. Once flood waters recede from their highest peak, the line of debris left behind is a high-water mark and these delicate lines will indicate to scientists the highest point the flood reached. However, these marks are fragile and easily destroyed – both by people cleaning up and by natural weathering – so collecting them is a time-sensitive effort.

After most major floods, USGS often partners with the Federal Emergency Management Agency and other state and federal agencies to flag and survey high-water mark elevations in areas that were flooded. For Hurricane Matthew, USGS is working closely with FEMA to determine the extent and severity of the flooding Matthew caused

What are High-Water Marks used for?

The data associated with HWMs has many different uses and the of location of the HWM plays a role in how the information is used.

“There are some differences between high-water marks caused from coastal surge and inland river flooding, and the data can be used for different reasons,” said Chad Wagner, USGS hydrologist. “One of the uses for coastal high-water marks is the National Hurricane Center can incorporate the data directly into their storm surge models, which can be used to verify how accurate their storm surge predictions were, and also to calibrate the models for improved accuracy in the future.”

High-water marks connected to inland river flooding and coastal flooding can be used for future flood forecasting, predicting the severity of future floods and also for delineating the FEMA floodplain maps.

The HWM data collected from Hurricane Matthew will allow FEMA to revises its current maps for the affected areas. This data is also part of the flood frequency calculations that FEMA uses to identify areas that are likely to experience high water in the event of a flood that has a 1 percent chance of happening in any given year. These floods, known as 100-year floods, are the most common severe inundation events, and serve as the foundation for flood management planning.

Another significant use for these high-water marks is the USGS Flood Inundation Mapping effort. A flood inundation map library is a set of maps that shows where flooding may occur over a range of water levels in the community’s local stream or river.

Inundation maps are one factor used to determine where changes should take place in building codes to help communities be more resilient; where evacuation routes should be; where (and how high) a bridge or road should be; and other community planning efforts. Once these flood inundation maps are complete, they’re uploaded to the USGS Flood Inundation Mapper, which allows users to explore the full set of inundation maps that shows where flooding could occur given a selected stream condition. Users can also access historical flood information and potential loss estimates based on the severity of the flood.

Hunting for High-Water Marks

Depending on the severity and area affected by a flood, USGS crews can collect dozens, hundreds, or even thousands of high-water marks. Hurricane Matthew’s unusual approach skimming up the eastern seaboard created a rather large area the USGS must target for HWMs. With coastal surges from Florida to North Carolina as well as severe inland flooding in North Carolina, USGS’ goal was to obtain almost 300 miles of HWM data across the four states. 

So far, more than 830 HWMs have been collected in Florida, Georgia, South Carolina, and North Carolina, and at least another 100 are expected to be found. The bulk of the HWMs have been surveyed in Florida, Georgia and South Carolina, while Matthew’s slow departure back to the Atlantic Ocean and the severe flooding North Caronia has been dealing with slowed their efforts. However, crews across North Carolina are now able to access previously flooded areas, and the HWM campaign is expected to be finished next week.

How High and Fast was the Water?

Finding and surveying high-water marks is a major portion of USGS’ work after major storms, but, consolidating and analyzing the vast amount of data the USGS collects before, during and after a storm is also a large post-storm effort. Some of this data includes peak of record flows USGS streamgages record during heavy rains and flooding.

Hurricane Matthew brought a tremendous amount of rain to most of the south east, setting almost 40 peak records, with river flows peaking at record-high levels in North Carolina, South Carolina and Florida. Most of these records were set in North Carolina because their already saturated ground couldn’t handle the additional rain, resulting in severe flooding across central and eastern parts of the Tar Heel State.

There was a combination of storm surge and rainfall that affected the coast and inland rivers.  In North Carolina, most of the flooding was due to rainfall from the hurricane which raised levels in the Tar, Neuse, Cape Fear, Lumber and Waccamaw River Basins resulting in record peaks at some locations. This was also the case on the Waccamaw River in South Carolina and the St. Johns River in Florida. The floodwaters are now receding or have receded in most of these areas.

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How is peak flow determined?  

Gauge height or stream stage indicates how high the water is, while streamflow is the quantity of water passing a location along a stream measured in cubic feet per second. A regulation basketball is about a cubic foot. Both are measured at streamgages across the country. Stage is measured and recorded continuously by electronic instruments to an accuracy of 0.01 foot. Streamflow information must be determined from streamflow measurements, and subsequent relationships developed between stage and flow. Both stage and streamflow from many gauging stations is transmitted hourly, or more frequently by satellite or telephone telemetry to USGS computers and made available to the public on the web at http://waterdata.usgs.gov/nwis/rt.

A View State by State

Stream flows were already higher than normal in North Carolina due to rain that fell before the passage of Hurricane Matthew. In the central and eastern part of the state, wet soils had limited capacity to absorb more rainfall. As a result, North Carolina waters reached record heights and flows.

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In North Carolina there are more than 260 streamgages operating across the state.  Current provisional data indicates that 28 sites in North Carolina had new record peaks, with 18 of those being gauges that have recorded data for decades. One has records that date back more than 70 years. A period of record is the timespan that USGS has been measuring flows at those sites, also known as the gauges’ period of record. The USGS has been measuring streamflow data at some sites across the country since the 1890s.

For example, on the Neuse River at USGS streamgage locations near Goldsboro and at Kinston, both recorded provisional peaks in excess of the Hurricane Floyd peaks of 1999. The Neuse River near  Goldsboro streamgage recorded a stage of 29.74 feet  October 12^th  and the streamgage on the Neuse River at Kinston recorded waters reaching a stage of approximately 28.31 feet  October 14^th. USGS crews measured streamflow at each of these two sites on the Neuse River near the peak stage to update the relationship between river stage and streamflow and to provide information needed for the river flood forecasts.

South Carolina operates more than 170 streamgages, and five peaks of records were noted, one of them on the Little Pee Dee River at Galivants Ferry, which was provisionally recorded as having a discharge rating of 65,000 cubic feet per second, and a gauge height of 17 feet October 12^th.  The Waccamaw River, AIW and Buck Creek also recorded peaks.

In Florida five peaks of record were noted, including on the St. Johns River at Jacksonville.  Other locations included, Blue Springs near Orange City, Little Creek at Green Swamp Road near Clermont, and Pottsburg Creek near South Jacksonville.  The streamgage a Lake Jessup Outlet near Sanford recorded the gauge height peak only. Across the state, USGS operates more than 550 streamgages.

To learn more about floods, peak flows, and high water marks visit the USGS Water Science School Surface Water Information website. To learn more about flood frequencies check out 100-Year Flood-It’s All about Chance.