The Science of River Flooding
Water is the essential substance of life as we know it, but in massive quantities, the same lifegiving liquid can cause devastation to the point that it takes life away.
Flooding can occur in a variety of weather conditions and bodies of water. For example, unusually heavy rain could bring about flooding from streams, creeks, rivers, lakes, ponds and other freshwater sources, while coastal areas can experience heavy flooding from powerful storms that cause storm surge, such as tropical storms and hurricanes.
River flooding is a complicated thing to track and forecast. When rain falls, the location of where exces runoff ends up depends on what river basin the area is part of. The river basin is defined as the total area that is drained by a river and its tributaries. In the U.S., there are 18 major river basins, which are then split up into smaller basins. The Mississippi River Basin here in the U.S., which spans from the Rocky Mountains to the Allegheny Mountains, is the fourth-largest river basin in the world.
Within these river basins, any runoff precipitation will eventually flow to the main river delta. On its way, however, an excess of rain or snowmelt or debris and ice jams can cause river flooding, which is officially defined by NOAA as “when river levels rise and overflow their banks or the edges of their main channel and inundate normally dry areas”.
To create an accurate flood model, hydrologists need to understand many aspects of the water cycle. First, the natural hydrologic cycle needs to be understood and simulated, which is a complex combination of several processes, including evaporation, transpiration, precipitation, infiltration, interflow, groundwater storage and runoff. Once a baseline has been modeled, hydrologists can begin to create accurate representations of possible impacts from increased inputs, such as precipitation.
Accurate representations of precipitation inputs are important to creating an accurate flood model. Directly related to the amount of precipitation is the amount of runoff, which can appear as overland flow, rain that falls directly on existing bodies of water, and interflow runoff, which is the flow of water below ground but above groundwater reservoirs.
The next step for flood modeling is to calculate a forecast hydrograph in units of discharge, which essentially shows how much runoff water will enter the river system over time. Streamflow measurements, which are taken on-location, are combined with the forecast hydrograph to capture the vital relationship between the discharge and stage, or water height, of a given location. Along with this, hydrologists analyze the routing of the river system, and account for how the water moves once it’s in the river and how the flood is modified due to storage and friction as the water moves downstream. Essentially, this takes into account the behavior of water upstream in calculations for water levels downstream.
As you can clearly see by this heavily condensed overview, flood-modeling is no easy task. It requires high level efforts across scientific disciplines, with complex challenges of integrating coding, observations, and theory into a final product.
Sources: NOAA, NWS
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