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An App and Volunteer Army Are Improving Local Tidal Flood Forecasts

Hundreds of people came out to map the king tides in coastal Virginia in November. Virginia Institute of Marine Science researcher Derek Loftis explains how that is helping the region better prepare as flooding worsens.

Written by Jessica Leber Published on Read time Approx. 5 minutes
VIMS king tide flood forecast used for field verification of flooding in Norfolk’s Hague on November 5.Courtesy of Virginia Institute of Marine Science

The king tide floods came on a sunny Sunday in November in Hampton Roads, a region of southern Virginia at the mouth of Chesapeake Bay. These floods happen every year when the tide is at its highest annual level, but this year more than 500 people went out to watch them.

On several coasts, from Vancouver to Hawaii, scientists and cities have been engaging citizens to photograph and document king tides as a way of understanding how flooding worsens as sea level rises. That’s one of the reasons why, a few years ago, the Virginia nonprofit Wetlands Watch created a publicly available app that crowdsources data by allowing organized groups to map floods, submitting GPS coordinates and photos to local project leaders.

Data from the app had already been used by the Hampton Roads city of Norfolk for a few years, but this fall, four local online, print and television media partners, working with the Virginia Institute for Marine Science (VIMS) at the College of William & Mary, spread the word to significantly expand the mapping project across the region. They called the event Catch the King.

Oceans Deeply spoke with VIMS assistant research scientist Derek Loftis about the citizen science event and how more than 50,000 data points collected in one morning will help improve the region’s flooding predictions.

Oceans Deeply: What are king tides like in Hampton Roads?

Derek Loftis: In the technical field, we refer to it as perigean tidal flooding, related to the moon being [nearest to] the earth. We knew long in advance that the king tide was going to happen on November 5.

Tidal flooding is a blue-sky flooding event. It’s not dangerous to go out in, like maybe a hurricane or a nor’easter might be. We still collect data during or after those events using debris lines as a proxy; but this is an event that would allow people to go out with their cellphones on a day that was honestly pretty bright, clear-sky weather. It was quite warm. Folks were able to take their cellphones and walk around. They were able to track the maximum flooding extent because we provided our hydrodynamic model forecast information and told folks approximately when the king tide would arrive in their county or city. It didn’t happen simultaneously everywhere. It happened over about a four-hour period.

Some of the interest behind this was to see how many folks we could get to go out and actually collect data. Because of the media presence and encouragement with this project, we had over 500 volunteers throughout 12 cities and counties in Hampton Roads, and they collected over 53,000 GPS data points during the event.

Oceans Deeply: What are you doing with this data?

Loftis: We basically can confirm whether or not there was actually flooding at the places the model indicated. During a tidal flooding event, there’s not really a lot of slope to the water. In many cases, like during a hurricane, the wind dictates where all the flooding occurs. But during the tidal flooding event, the water surface is relatively flat. It was almost uniformly 2ft [60cm] above mean sea level everywhere.

The mapping showed where people said our tidally connected waters flood during the king tide. We can actually use that for a term we call hydro-correction. We noticed all throughout Hampton Roads small creeks that were not in our model that we didn’t know could flood or did flood, mostly because the elevation information that we have is based on a technology called LIDAR (Light Detection and Ranging) … which is collected in an airplane.

Many of these places, like creeks, were occluded or blocked by heavy tree cover, and we didn’t even know they were there. As it turns out, those creeks were used as conduits to flood roads and neighborhoods that we couldn’t see with LIDAR, but we’re able to use the information collected by the users in a tidal flooding event to fix the LIDAR.

We hadn’t been able to address this before, mostly because it’s very cost-prohibitive to collect those kinds of data. But volunteers collected data for free, so in this case that was really the value: that we are able to improve our model’s accuracy for future flood events. That will have us a little bit better prepared for the next year. Not only that, but this new data they collected helped calibrate some new water-level sensors we just had installed.

Oceans Deeply: How did your model’s forecast do in general?

Loftis: We were excited because the model predicted this king tide accurately, and everyone saw that. That’s always the danger when you put out a forecast: You don’t know if it’s right until people say it is. If you can’t predict the tide correctly, there’s no way you get hurricane storm surge right.

Oceans Deeply: How is Hampton Roads experiencing the effects of sea-level rise?

Flooding data has already been used to calibrate a new water-level inundation sensor in Norfolk’s Hague. (Courtesy of Virginia Institute of Marine Science)

Loftis: If you look at Maine all the way down to Florida, the closer you get to the center of the U.S. eastern seaboard, the faster the rate of sea-level rise is. Norfolk is the highest rate along the U.S. eastern seaboard based on long-term data records.

The users of the sea-level-rise app and the water-level sensors, combined with all the federal military assets in Hampton Roads, all have really put it on the map in terms of how much flooding we experience, not only regionally but also nationally.

They have very long-term records at the Norfolk naval base. You can very clearly see that the sea level is rising, roughly 4.6mm [0.2in] per year. About half of that is related to land subsidence, and the other half of that is related to global sea-level rise trends.

Oceans Deeply: Are king tide flooding levels now a helpful predictor of what sea-level rise will look like in Hampton Roads later this century?

Loftis: If the past is any indication, they are … the king tide monitoring data revealed that tidal flooding in Hampton Roads is a widespread, persistent problem that is becoming more of a nuisance than the “nuisance flooding” moniker implies. When combined with old and new water-level sensor data records, they provide a rich new data history that helps demonstrate incontrovertibly that water levels are rising while providing a geospatial map of water levels that may shed some light on variation of rates in land subsidence in coastal areas.

This is a tidal flooding event, a mild flooding event, but it’s also the most frequent that we experience in Hampton Roads. In Virginia, the state legislature recently established a new center focused exactly on this kind of flooding event, called the Commonwealth Center for Recurrent Flooding Resiliency.

Oceans Deeply: Will you continue to crowdsource flooding data?

Flooding model predictions and app data collection points from the Catch the King event in the Hampton Roads region. (Courtesy of Virginia Institute of Marine Science)

Loftis: For next year, our hope is we can call on many of these same volunteers when the hurricane season kicks into gear. There’s definitely been an expressed interest to continue this type of approach to better understanding flooding that we experience in the region.

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