A USGS guide for finding and interpreting high-water marks

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High-water marks provide valuable data for understanding recent and historical flood events. The proper collection and recording of high-water mark data from perishable and preserved evidence informs flood assessments, research, and water resource management. This video provides guidance for skilled high-water mark identification, including marks left behind in natural and man-made environments by tranquil and rapid flowing water. Additionally, this video also presents pitfalls and challenges associated with various types of flood evidence that help hydrographers identify the best high-water marks and assess the uncertainty associated with a given mark. Proficient high-water mark data collection contributes to better understanding of the flooding process and reduces risk through greater ability to estimate flood probability.

For a more detailed explanation of these methods be sure to read the Techniques and Methods Manual: “Identifying and preserving high-water mark data” in the link below.
https://pubs.er.usgs.gov/publication/tm3A24

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Image Dimensions: 1280 x 720

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Length: 00:16:43

Location Taken: US

Transcript

- - Imagine you're a flood detective on the scene, reconstructing what happened after a flood. The goal is to recreate the flow that occurred and your main form of evidence is high-water marks. The evidence isn't always apparent so you should always take a second to stop, look up and visualize the flood. This will allow you to step back and gain perspective regarding the clues in front of you. And while you could find yourself in any one of many possible flood and measurement scenarios, it is most important to understand the nature of the system in front of you, and to record the best data possible. Regardless of the circumstances. The USGS Flood Response Team, and their high-water mark mapping efforts, are integral to our nation's disaster recovery. Expertise in high-water mark hunting equips you, the hydrographer, with the knowledge and tools necessary for verifying stream gage records and supplying valuable data for flood characterization and modeling. The water surface profiles that the high-water marks imply are our main interest. These are used to verify hydraulic model analysis at targeted reaches of rivers, streams and floodplains. These hydraulic models are then used for the construction of flood inundation maps which provide crucial information on decision making for land-use management, emergency response, flood insurance rate determination, and a myriad of flood mitigation efforts. While field experience is the best training for high-water mark hunting, this video and the Techniques and Methods Manual are designed to supplement actual investigations, and accelerate the process of learning the art of identifying high-water marks and the science of evaluating high-water mark usefulness. In this video, we only briefly discuss the different types of high-water marks. However, the Techniques and Methods Manual that you can find on the video description provides excellent in depth and detailed examples of the different types of high-water marks along with the nuances and shortcomings. Be sure to familiarize yourself with these descriptions as found in the manual. When searching for high-water marks in the field, you'll commonly come across wash lines, cut lines, debris lines, and seed lines. The first step is always to stop, look up, and visualize the flood. This will help you to understand and visualize what kind of flow occurred at your location. As the type of evidence left behind in its quality will differ between tranquil and rapid flow events. However, be aware that both scenarios can be present at the same site. It is up to you, the hydrographer, to locate the best high-water mark evidence. Tranquil water high-water marks. High water marks left behind by tranquil water have a smooth trend. Typically, with much smaller uncertainties than those left behind by rapid flow. For this reason, tranquil water high-water marks, if available, may be more useful that rapid water high-water marks. Even in swift moving floods, tranquil areas can frequently dot the stream edge where changes in channel geometry or large flow obstructions have created regions of ineffective flow. Rapid water high-water marks. In higher velocity stream reeaches, water often transports higher sediment loads and debris. Leaving behind high-water marks that are characteristic of high energy flows. Rapid water high-water marks often come with greater uncertainty, because of wave action and run-up on flow obstructions. However, rapid water high-water marks may represent the best available evidence for many flood peaks, and ultimately form an important class of high-water marks. Rules of thumb when finding high-water marks: One. Because high-water marks can be highly perishable, time is of the essence. Once the flood has passed and it is safe to do so, visit the site as soon as possible to flag the high-water marks. They can then be surveyed later. Two. If possible, avoid swift water areas which can cause varying amounts of pile-up. The flow velocity causes marks to be higher on the upstream side of objects and lower on the downstream side. Low velocity areas which are usually areas away from the main channel and near the waters edge or close to the ground, tend to have better quality marks. Three. Avoid high water marks on small bushes and trees in areas with substantial flow. Vegetation may get bent down during a flood and stand back up after the flood creating an artificially high-water surface. Four. Fences or window screens are usually good sources for high-water marks. Although, these sources should still be verified by other high-water marks in the surrounding area. Five. Building interiors and man-made enclosures like utility boxes will sometimes act as Stilling Wells. Capturing good quality high-water marks. However, field personnel should still try to verify that the inside and outside water levels have equalized before taking a measurement. Six. More high-water marks are always better than fewer especially if the marks are poor, the slope is steep, or the high-water marks are being used in the determination of a flood profile. To further illustrate these points, let's go into the field with USGS data chief, Chris Smith. - Okay, we're out today looking for high-water marks. We just had a flood, that came down here about a week ago. Some vegetation's now grown up around our high-water marks that we're looking for. You gotta look around and see where you're going to start and so what you want to do is stop, look up, look around, look at the big picture. You want to see is there any evidence so that you can then get initial depth of this flow over this floodplain. And here, this is where I would start. We have debris stuck in this grass, so we know that the flow here was, at least, six to eight inches deep. So I'll start here and then I'll go up-slope, up on the bank to see where the high-water marks are deposited. So let's go upstream a little bit. Let's see if we can find additional evidence. Now, I'm starting to pick up this heavy debris in these areas, and that's a good clue. That's a good clue that the water got up at least this high, but whether or not this is the crest, what we want to do is find additional evidence. Either really fine debris and, in this case, we've got really fine debris right in here. This may be the peak here, but I don't see... It could be a little bit higher. I'm not going to mark this one yet. What I'm going to do is look for additional high-water marks in this area. If this is the best one, I'll come back and mark it. So let's go upstream. I can see, again, heavy debris here. We're seeing it here, again. We're four inches deep here. I'm going to move up-slope. I'm seeing really heavy debris here and, now, this is kind of what I'm looking for. I'm looking for this really fine debris and I can see here, I can see a color change between the soil and this. This would, to me, is really the one I want to mark in this area, because it it has the evidence that... Because the color, because the really fine debris, I really feel confident that this is the... this is the height of the water. So I would consider this an excellent high-water mark. Well, let's go ahead and look upstream, cause typically we want to find a water surface profile from high-water marks. Sometimes, every five feet, sometimes every 10 feet, every 20 feet Depending on how big the flow was, and how wide the river is. In this case, we're going to go up probably about every 10 feet and see if we can find high-water marks. Again, this is this is a beautiful area to pick high-water marks. This is one of the easiest ones I've seen. The heavy debris, the really fine debris that changed in texture of soil and the interface with the really small debris. It's just, again, this would be an excellent mark and if I can get this nail in the ground. I'm going to put it there. A person can always put the rod right next to it. This is such a good debris line. So I'll move upstream another ten feet. Now, this is getting a little bit harder. It's been a week after the flow event. The grass is starting to grow and, so, it's a little bit more difficult to find this debris line, but let me step back. And we can look at kind of the line that I'm creating with these high-water marks. And let me use that as a guide, and then I can come back and I can stay on that line and then find evidence from that. And I'm seeing that right in here. Again, I have that really fine debris. And I feel like this one I wouldn't consider it excellent. I'd consider this good, because I'm not exactly sure if we got all the way. But I think we're within a couple hundredths of a foot. - [Bruce] Flagging. Hydrographers use various types of markers to preserve the elevation of evidence left by a flood. Thus, ensuring the record lasts until the high-water marks can be surveyed even after the evidence has been lost. If the surveying will be done in the future or by a different hydrographer, markers and flagging will aid in locating those marks again. Typical markers include stakes, nails, bright colored surveyed flagging tape, wire flags, USGS marking tabs, paint/permanent ink markers, and chiseled marks. For each high-water mark, paper or electronic notes should list a unique identifier, the type of mark, for example, a seed line or debris snag, the uncertainty of the mark, the approximate location and any comments or additional information on observations about landowners, hazards or nearby landmarks. In addition to recording high-water mark data, field crews can save time and resources by using a surveyor's Total Station or GNSS instruments to survey and plot high-water marks during the search visit. A common USGS short hand standard for uncertainty is listed in this table. Water marks within plus or minus 500ths of a foot are considered excellent. Within plus or minus a tenth of a foot are considered good. Plus or minus two tenths of a foot are fair. Plus or minus four tenths of a foot are poor. And more than plus or minus four tenths of a foot are very poor. Peak stage verification. One of the more common scenarios that hydrographers can find themselves in is collecting high-water marks for peak stage verification at a stream gaging station. When verifying records at a stream gage, you should look in areas with the same topographic features as at the gage. By surveying in an area with similar slope, channel shape and vegetation, you ensure that you are representing the same conditions that generated the reading at the gage. This is crucial, because stream gage records are held to a plus or minus one 100ths of a foot, and, therefore, the best quality high-water marks most representative of the gaging station conditions are needed to verify stage. You may want to check the Stilling Well and crest-stage gage for mud or seed lines. As two to three high-water marks will improve the confidence of the gage rating. Indirect streamflow measurements. Indirect streamflow measurements use high-water marks to reconstruct the peak surface water profile at a location. Many variables come into play when taking indirect streamflow measurements. Streambed slope, channel roughness, channel variations, reach length, ideal cross-sections and general efficiency are all factors that determine the number and location of data points. Experience and training is critical to being able to read the scene. Therefore, in order to have a solid grasp of the process of defining the water surface across a reach, you should work with an experienced hydrographer and take advantage of the many papers that highlight methods for collecting accurate data. In general, low-gradient streams in the plains and coastal regions may only need ten high-water marks per hundred feet. While high-gradient streams like those in mountainous regions can require 20 to 30 marks per hundred feet of reach to account for the frequently changing water surface. The actual number of high-water marks required at any given site will, of course, be site dependent. Flood inundation studies. Flood inundation studies are typically concerned with spatial extent of flooding in a defined area. So vertical and horizontal accuracy are important. But while a location like a small rural stream reach might only require five to ten high water marks to adequately define the extent of flooded water surface, urban areas tend to be much more complex requiring many more measurements. In particular, while man-made structures are good for collecting high-water marks, because they often act as Stilling Wells. It is important to check for hydraulic connectivity to ensure the accuracy of such high-water marks. Additionally, the distance to ground, not just the altitude or elevation, is an important data element to collect in urban areas. The primary reasons for collecting height above ground are: One. Helping surveyors locate the flagged marks. And two. Providing preliminary elevation data to emergency managers which allows their GIS experts to calculate a rough inundation map from a digital elevation model. This helps them to get the data out quickly. By using the principles outlined in this video, and taking the time to stop, look up, and visualize the flood, you're well on your way to honing your skills to successfully find and interpret high-water marks. It's now up to you to get into the field and supplement this knowledge with real world experience and training. If you wish to learn more about identifying and interpreting high-water marks, be sure to read the Techniques and Methods Manual. You can find a link to it in the description of this video.