Seeing the Invisible: RAPID Technology Reveals Ephemeral Floodwater Marks
The Challenge: High-water marks are among the most critical, yet perishable, types of data following a flood. They reveal the disaster's true depth and extent, informing future flood risk assessments and mitigation strategies. However, these vital indicators can fade rapidly due to drying, environmental exposure, and cleanup efforts, often becoming invisible to the naked eye within weeks. When reconnaissance teams cannot safely access disaster zones immediately after flooding, this essential information is frequently lost forever, hampering efforts to understand flood behavior and protect communities.
Researchers pilot a drone with cameras over the Ahr River near a damaged bridge
A New Approach: A research team, supported by the National Science Foundation through the Geotechnical Extreme Event Reconnaissance (GEER) Association, deployed to Germany following the devastating July 2021 Western European floods to test an innovative approach. The team utilized advanced multispectral and thermal imaging cameras provided by the RAPID Facility to detect high-water marks that had become invisible to human observers. Working in the town of Mayschoss along the Ahr River nearly four weeks after the flood—long after high summer temperatures, solar radiation, and cleanup efforts had made water lines barely detectable—the researchers collected ground-based multispectral imagery of flood-damaged buildings.
Data & Discovery: The breakthrough came during data processing when researchers discovered that "invisible" high-water marks left distinct spectral signatures in the multispectral imagery. The key finding was that trace amounts of residual moisture in building materials created a clear signal in the blue wavelength band (443-507 nm) of the multispectral camera. By developing a technique that combined the blue and red-edge light bands (705-729 nm), the team could computationally isolate and enhance these faded signatures, making invisible watermarks "reappear" with centimeter-level accuracy. The method revealed that materials below the flood line retained higher moisture content and different reflectance characteristics even weeks after the event, creating a detectable contrast with areas above the high-water mark.
Impact: This research, published in Natural Hazards Review, demonstrates for the first time that invisible high-water marks can be reliably detected using advanced remote sensing technology weeks after a flood event. This breakthrough transforms post-flood reconnaissance by enabling rapid, safe, and accurate mapping of flood extent even when visual evidence has disappeared. The technique can be deployed using UAVs to survey entire communities that are otherwise inaccessible, dramatically expanding the ability to collect critical flood data for hazard assessment and mitigation planning. By providing cutting-edge multispectral instrumentation and field expertise, the RAPID Facility enabled this proof-of-concept study that fundamentally improves our ability to learn from floods and create more resilient communities.
Citation: Gardner, M., Nichols, E., Stark, N., Lemnitzer, A., & Frost, D. (2023). Multispectral Imaging for Identification of High-Water Marks in Postdisaster Flood Reconnaissance. Natural Hazards Review, 24(2), 06023002. https://doi.org/10.1061/NHREFO.NHENG-1735