Thoughts From Engineers: Global Work Group Takes Aim at Flood Science

The disastrous flood events in several world regions within the last several weeks, from the Hill Country of Texas to northern China, underline the particularly damaging and deadly flood events we now encounter routinely. Communities worldwide are working to get ahead, fortify and redesign infrastructure, and adapt. From cities such as Copenhagen, Denmark, that have redesigned urban areas to accommodate flooding to countries such as Japan enabling vast IoT-enabled flood data reporting networks for its largest cities to the United Kingdom developing a Flood Hydrology Roadmap for the next 25 years, significant initiatives are at work to bring some semblance of security to insecure times.

The World Climate Research Program—Global Energy and Water Exchanges Hydrometeorology Panel has plans for a Global Flood Crosscutting project to synthesize the results of global and regional research to identify the most critical scientific questions in flood science for accelerated and targeted investigation. Despite the many advances made to date, many unknowns persist with respect to the broad spectrum of issues relating to riverine flood risk analysis and management.

According to this group, these unknowns limit our ability to accurately gauge flood risk, identify likely flood impacts and develop effective flood-control measures in this time of climate change. If there’s an “angle” to this proposed research program, it’s best characterized as an interdisciplinary “whole Earth system” approach that recognizes the important interactions among atmosphere, land, oceans, biosphere and human activities.

As Samadi et al. point out in their article in Wiley Wires Water, “The Needs, Challenges and Priorities for Advancing Global Flood Research” (bit.ly/WiresWater), “(l)and use changes have been as significant in impacting the generation of flood risks as changes in climate.” As identified by this group of international researchers, below is a partial list of core areas in need of further investigation.

The Hydrologic and Physical Factors Driving Flooding

Broadly speaking, the mechanisms that generate floods include climatic, hydrologic and hydrodynamic processes that combine to determine timing, duration, intensity and flooding extent. Climatic factors such as atmospheric circulation patterns and hydrologic parameters such as intensity and distribution of precipitation are the primary drivers of flooding, but there could be others as well. The failure to recognize these other factors and integrate them into current simulations may lead to an oversimplification in flood predictions.

Data relating to soil moisture content, groundwater discharge, topography, human disturbance and land-use types, slope stability, snowpack, and other features of a watershed aren’t routinely considered in current flood-modeling analyses. For example, certain parameters may be more relevant than others under certain conditions or seasonal basis. However, the basic point is that the dynamics at work during flood events are complex.

Moreover, the extent to which these various “triggers” may combine differently for distinct types of catchments—from mountainous to rural, coastal and urban—also is unclear, as is the degree to which some flood-generating mechanisms may become more dominant over others under changing climate and land-use conditions. Hence, our understanding of the many complex nonlinear, hydrological, climatic and hydrodynamic processes and inputs is still fundamentally a work in progress.

Causation, Flood Drivers and Flood Development

Another source of uncertainty relates to the interplay among climatic and meteorological systems with watershed-specific attributes and processes and how one system affects another. For example, depending on location and intensity of precipitation within a watershed, temperature shifts, combined with catchment characteristics and conditions (e.g., soil moisture content, soil types and other variables), flood response can vary considerably. These processes can account for the differences in how floods develop in urban and rural landscapes. Understanding the mechanisms behind different system interactions will help identify the attributes typical of problematic situations.

The use of remote-sensing technology can help fill gaps in our knowledge by helping us understand causation, particularly when multiple flood-generating mechanisms could be relevant. For example, leveraging large-scale satellite data through NASA’s Global Flood Monitoring System and integrating these data with hydrological models can lend insight into the spatiotemporal variability of floods associated with different regions. Lidar data, ground-based weather systems and space-based instrumentation should help advance flood forecasting capabilities and our understanding of flood-generating dynamics. An emerging area of research is the development of hybrid models that merge data and large-scale computing capability with today’s physics-based models to represent flood dynamics more accurately.

The bottom line is the significant differences in how floods develop is the function of many factors and processes, operating at multiple scales. We see how small watersheds receiving intense precipitation often are at higher risk of flash flooding, while larger watersheds receiving steady rainfall over longer periods of time are prone to a gradual flooding risk. The effects of multiple sources of physical variability within a catchment are further influenced by land-use changes, which can significantly alter surface runoff and heighten flood risk. It’s no surprise that some of our most destructive floods have occurred in highly populated and heavily managed land areas with dense development and impervious surface areas.

What Might We Expect with a Changing Climate?

Land activities can significantly affect surface runoff and flooding. From the effects of deforestation on rainwater infiltration to urbanization, dam management or the effect of levees and flow-through dams on flooding, each can impact flood risk. The interactions among land use, climate, land management and hydrologic regimes is an increasingly key area of interest.

We need to understand how urbanization, agricultural development and other human activities may affect watershed hydrology—both surface and subsurface—in the context of changing climatic and meteorologic dynamics. By projecting how certain types of land development and other activities progress through time and cause significant change within a catchment, while simultaneously understanding how these scenarios may aggravate flood risk and shape flood response, we’ll be in a better position to take proactive measures and implement policy more in line with what a watershed can support under extreme weather conditions.