Using the Right Tool

By Ting Powell, Flood Engineer

Flooding poses a significant threat to many communities worldwide, often bringing devastation to homes, infrastructure, and lives. Traditional static flood maps have long been used to predict and prepare for these events. However, their limitations become evident when faced with real-time flood situations. 

You may have seen a map showing an X-year or X% flood zone (e.g., the 100-year or 1% flood zone), often referred to as Flood Insurance Rate Maps or FIRMs. This type of map estimates the flood extent that has an X% chance of being equaled or exceeded in any given year using a flood frequency analysis, which is a statistical technique that involves using historical observed data to calculate the statistical probability of a flood occurring. The goal of the FIRMs is to provide an understanding of the frequency and magnitude of expected flooding in a specific region over a year (e.g., the annual exceedance probability). These flood probability maps are commonly used in community planning, in the insurance industry, or by the public who want to understand the risk of an individual property flooding. However, when it comes to understanding and mapping an actual flood event, these static maps are limited in their application because they were derived from annual exceedance probability data.  

Figure 1: Example of a FIRM map.

Each flood event is unique. A static map conveying the probability of a flood occurring is unlikely to accurately capture the variations of the flood flow during an actual event at different locations across the terrain, even within a relatively small watershed. This is why the information provided by a FIRM map may not be helpful and can even be misleading during an actual flood. For example, let’s examine the lower San Jacinto River between Porter Heights and Highlands in Southeast Texas. This region experienced severe flooding in early May of this year (May 3 – 4, 2024). When we compared the corresponding flood frequency for the seven downstream gages (Figure 2) to the actual flood observations (Figure 3), we found that the flood frequency along this 30-mile stretch of river varied from just above a flood event with a statistical return period of 10 years to a return period of almost 100 years.  

Figure 2: The locations of seven gages compared along the lower San Jacinto River.

Figure 3: The comparison of the flood frequency for the May 3-4, 2024 flood event with the observed peak water levels (dashed red line) for the seven gages along the lower San Jacinto River

The above comparison highlights the potential significant variations in flood frequency at various locations within a watershed during a singular flood event. Each flood event is unique, exhibiting different magnitudes and peak times. To empower communities and emergency response agencies to prepare, plan, and recover from flooding, we need innovative technology that provides dynamic, live mapping feeds to show the real-time flood extent and depth. This information offers more accurate, up-to-date situational awareness of the flood’s impact on people, property, and critical infrastructure. That is where FloodMapp can help. One of our products, NowCast, does exactly that.  

NowCast is FloodMapp’s innovative, real-time flood mapping technology. It delivers dynamic, live feeds that accurately illustrate the current flood extent and depth. By leveraging advanced algorithms and vast data sets, NowCast is the perfect tool to provide unparalleled situational awareness during flood events. 

Let’s delve into the flood extents from NowCast at a couple of locations during the peak of the May 3 - 4, 2024 flood event, where observational imagery of actual flooding is also available for comparison. At the San Jacinto River at US 59 near Humble (Point 2 in Figure 2), the flood frequencies are 56.5 feet and 63 feet for the 10-year and 50-year floods, respectively. The flood peak for this event reached 58.7 feet, corresponding to a flood frequency somewhere between the 10-year and 50-year event. Figure 4 illustrates our NowCast result (in red), which matches well with a footage taken near the Fairfield Inn on US 59. In contrast, the 1% flood zone (in blue) is larger than the actual flood extent. 

Figure 4: Comparison between FloodMapp NowCast (in red shade), 1% flood zone (in blue shade) and the screenshots of video footage at Location 2, San Jacinto River @ US 59. The observational images were taken at Fairfield Inn & Suites Houston Humble.

At the San Jacinto River at Kingwood Country Club (Point 3 in Figure 2), the flood frequencies are 50.3 feet and 51.7 feet for the 50-year and 100-year floods, respectively. The peak flood level for the May 3-4, 2024 flood event was 50.8 feet, placing the flood frequency between a 50-year and 100-year event. Figure 5 shows our NowCast result (in red), which matches well with footage taken near the Kingwood Country Club. Again, the 1% flood zone (in blue) is overestimated compared to the actual flood. 

Figure 5: Comparison between FloodMapp NowCast (in red shade), 1% flood zone (in blue shade) and a screenshot of video footage at Location 3, San Jacinto River @ Kingwood Country Club.

Conclusion  

The recent flooding in southeast Texas demonstrates that even within a relatively small watershed, flood frequency at each gage can vary significantly. While static maps based on flood frequency analysis may be useful for long-term planning, they fall short in providing accurate, real-time intelligence during an actual flood. Every flood is unique.  

FloodMapp offers a dynamic street-level flood intelligence that gives you live updates on flood extent and depth, providing situational awareness on flood impact to people, property, and critical infrastructure. Request a demo today to experience the power of real-time flood data and protect your community with confidence.