CSI: Investigating High Water Marks for Flooding
Many engineers enjoy a good episode of Crime Scene Investigation, where science and engineering help to reconstruct the details of one person's violation of another. But forensics plays a role in many fields outside of criminal justice. When large floods damage land and structures, owners can feel violated, with real value being stolen from their property. Although the "thief" cannot be captured and put away, it can be identified and quantified. While CSI draws us into the world of criminal forensics, flood forensics helps us quantify the extent of a flood -- the first step in planning the resistance to future flooding.
High-water marks are the primary evidence of flood forensics. They are the traces of physical evidence left on the ground, on structures, and on vegetation that tells the story of what happened while people were seeking safer ground. These details help us to reconstruct the flood, assess losses, and characterize the event concerning past events and manage water resources in the future.
Obvious and Not So Obvious
High-water marks take on many forms. Like a bloodstain at a crime scene, some marks are obvious. But the most subtle marks often produce the most important evidence. Obvious marks usually lead us closer to the scene of the “crime”, and subtle marks tend to give us the details needed to draw conclusions. For flood forensics, those conclusions include computing reliable peak water elevations, flow rates, and flood volumes.
Debris lines are often the most obvious marks. Large piles of sticks, leaves, grass, or trash will often assemble around a tree trunk, a wire fence, or a bridge column. Because these piles are easy to see and most resilient to washing away, they are often the first evidence we notice when driving down the road or hiking through the woods. But their size often makes it difficult to nail down the exact elevation of the water surface.
Smaller debris lines and seed lines are the next best evidence for which to search. Floodwaters have a way of stirring up small debris from the ground and seed-bearing plants and floating it to the surface where it sticks to buildings, window screens, and small rings around tree bark and poles. Even finer sediment in floodwaters can coat vegetation and structures, leaving a clear line between what was coated by muddy water and what remained above the waterline.
The Early Hours are Critical
Like criminal evidence, flood evidence is likely to disappear or become more obscure over time. Cleanup efforts after a flood, as well as natural wind and rain, will remove evidence quickly. To preserve the evidence, observers must record the information by measuring, describing, flagging, photographing, or even surveying the mark. Often, the elevation survey can be done later if efforts to flag the evidence are done quickly after the event.
Once acquired, elevations of high water can become evidence of flood damage. Comparisons of flood levels from one event to records of previous events can provide context. Differences in elevation from one location to another nearby location in the same event can be used to establish the slope of the water surface at its highest flow. Engineers and hydrologists can use the slope to determine the volume flow rate with surprising accuracy. When we collect solid evidence of flooding, we are often able to model future flooding events to inform better construction planning and emergency preparedness.
The encouraging news is that with a little bit of knowledge, anyone can become an experienced investigator in the art of finding high-water marks. Often, the evidence can be found right in our backyards. And high-water mark hunting can be a fun way to include the community in flood awareness.
(All photographs courtesy of the U.S. Geological Survey)
