When a coastal storm makes landfall, which poses the greater flood risk: heavy rainfall or storm surge?
The answer can be both.
Coastal communities can experience multiple sources of flooding simultaneously. Storm surge pushes water inland while heavy rainfall generates runoff and increases riverine flows. Where these processes interact, the resulting compound flooding can create complex—and potentially devastating—conditions that cannot be fully understood by looking at each source independently.
Accurately representing those interactions presents an important challenge for engineers and flood risk managers.
The Challenge of Modeling Compound Flooding
Historically, compound flooding has often been simulated by loosely coupling a hydrologic/hydraulic watershed model with a separate coastal storm surge model. Each model represents an important component of the overall event, but separating those processes can make it difficult to fully capture the nonlinear interactions among rainfall-runoff, riverine flows, and storm surge.
The U.S. Army Engineer Research and Development Center (ERDC) developed an advanced version of the Adaptive Hydraulics Model (AdH) specifically to address this challenge.
Unlike approaches that model individual flood sources separately, AdH can simulate rainfall-runoff, riverine flows, and storm surge together in a single, monolithic simulation. This allows the model to directly represent how multiple flood sources interact as conditions change throughout an event.
Simulating Hurricane Harvey in Harris County
DynamicSolutions | DCCM used the advanced version of AdH to perform a monolithic simulation of flooding in Harris County, Texas, during Hurricane Harvey.
The analysis identified a 37-kilometer-long compound flooding transition zone within Buffalo Bayou where multiple flood sources interacted. Within a transition zone such as this, flooding cannot necessarily be attributed solely to rainfall, riverine flow, or coastal surge. Instead, water levels reflect the combined and nonlinear effects of multiple processes.
A monolithic modeling approach allows these interactions to occur within the same simulation, providing a more complete representation of the resulting water levels and flood conditions.
Looking Beyond Historical Events
The value of compound flood modeling extends beyond recreating—or hindcasting—past storms.
AdH can also be used to run stochastic coastal storm simulations to evaluate flood risk in coastal areas vulnerable to compound flooding. Rather than focusing solely on what occurred during a single historical event, these simulations can help evaluate potential flooding across a range of storm conditions.
For communities where coastal surge, river systems, and rainfall-driven runoff converge, understanding these interactions can provide important information for flood risk assessment, infrastructure planning, and resilience efforts.
As flood hazards become increasingly complex, the modeling tools used to understand them must be capable of representing that complexity. Integrated approaches such as AdH provide engineers and public agencies with a way to evaluate multiple flood mechanisms as the interconnected processes they are.
DynamicSolutions | DCCM helps public agencies and project partners understand complex flood processes through advanced hydrodynamic modeling and water resources engineering.
Explore DynamicSolutions | DCCM’s services or connect with our team to learn how our modeling expertise can support compound flood analysis, coastal and riverine flood risk assessment, and resilient infrastructure planning.
