How High Water Marks Help Fill Gaps in Flood Knowledge

Note: the post below was condensed and adapted from a longer USGS article. The U.S. Geological Survey (USGS) records continuous stream stage (height of a river) and streamflow (how much water is flowing) with thousands of gages throughout the nation. But how can they determine flood height where no gages exist?

Simple, USGS correlates high water marks after floods with peak gage heights during floods.

“If, for example, records show that stream stage reached 17 feet during a storm, a high-water mark will show the hydrologist what a stage of 17 feet means in terms of how high the water was on the riverbanks and surrounding land.”

USGS Water School Website

Finding high water marks on or near buildings is easy. You look for the mud line or the edge of debris fields. The same principle applies in nature. Sometimes it’s easy.

But they’re not always that obvious.

Can You Spot High Water Marks In Pictures Below?

Below are two pictures used by USGS for demonstration purposes. They took the pictures a few days after a record storm. High-water marks show in both pictures, although a hydrologist would only regard one of the marks as being reliable.

The pictures below are close-ups of the high-water indicators in the top pictures. Did you spot them?

The left picture shows a poison ivy vine with the bottom leaves covered in dried mud. Where the mud stops shows how high the floodwaters reached.

The right-side picture shows a limb that hangs over the same creek. During a flood, rapidly-moving water carries leaves and pine needles, etc.! They stick on limbs that are partially submerged. When the stream recedes the signs remain. The top of the leaves and pine straw indicate how high the creek was during the storm.

The mud on the vine is a much better high-water mark than the tree limb, though. During high water, the fast-moving water will cause partially submerged limbs to move up and down. Therefore, hydrologists would not use the limb to estimate high water.

How High Water Marks Are Used

Planning Development

Documenting high water marks helps plan development near floodplains. If you know that water reaches a certain mark on the bank every few years, you certainly don’t want people building homes and businesses there.

Determining Extent and Severity of Flooding

Gages can determine the height of a flood. But high water marks can also show the width and extent relative to topography.

After most major flood events, USGS partners with FEMA and other state and federal agencies to flag and survey high water marks in areas that flooded. USGS did so here after Harvey to determine the extent and severity of the flooding.

Prediction of Future Floods

Forecasters can use the data associated with high-water marks to predict the severity of future floods, delineate flood zones, and update current maps that may account for changes in upstream conditions.

Flood Frequency Calculations

High-water mark data is also part of the flood-frequency (or recurrence interval) calculations that FEMA uses to identify areas that are likely to experience a 1 percent chance of happening in any given year. These floods, known as 100-year floods, serve as the foundation for flood management planning.

Inundation Mapping

Another significant use: Flood-Inundation Mapping. A flood-inundation map shows the extent and depth of flooding that occurred in various communities as a result of a major storm or flood event. Inundation maps help determine things like:

• Changes needed in building codes
• Evacuation routes
• Heights of bridges and roads.

Once inundation maps are complete, USGS documents them and makes them publicly available online.

Recreating Data from Damaged Gages

If a flood knocks out a gage, as it did with the one at US59 bridge and the Kingwood Country Club on the San Jacinto West Fork, high-water marks can provide maximum height of a flood after the fact. If the cross section of the river is known (or surveyed), hydrologists can even back-calculate the flow rate.

Posted by Bob Rehak on 7/16/2020 with thanks to USGS and Diane Cooper

1052 Days after Hurricane Harvey and 301 since Imelda