What does Flood Factor’s presentation of uncertainty mean?

Learn about climate uncertainty and how it is represented on Flood Factor.

 

Climate modeling and changes in the environment

The First Street Foundation Flood Model is unique in that it presents risk of flooding with respect to a changing climate. The model explicitly accounts for climate uncertainty in a way that attempts to reflect current and future changing environmental factors associated with the mechanisms by which flooding occurs naturally. The factors taken into consideration as non-static climate inputs include: sea level rise, changing hurricane intensity and landfall locations, changing hurricane precipitation patterns and impact to river discharge landfall locations, and changing non-hurricane precipitation patterns and impact to river discharge.

 

Current climate conditions are estimated using the carbon emissions associated with the Intergovernmental Panel on Climate Change’s (IPCC) Representative Concentration Pathway (RCP) 4.5. This is a middle of the road scenario with respect to carbon concentrations in the atmosphere, representing some interventions but not the most aggressive scenario. This concentration pathway then informs the climate models that yield the changing environmental factors. The Flood Model used the output of an ensemble of 21 global climate models to identify three different potential values for each changing environmental factor - a high, a median, and a low. Flood Factor presents the depths of flooding and likelihood of flooding for the median scenario, while also displaying a range of values defined by the low and high scenarios. 

 

Uncertainty and accuracy

By presenting the uncertainty associated with the flood model outputs, the Flood Model acknowledges the inherent uncertainty associated with both climate projection and probability associated with flood frequency estimation. Providing a range in this way shows different possible realizations of the true climate today, which cannot always be known from a limited set of observations over a short period of time, and what the climate could be in the future. This treatment provides for a greater confidence in the results, as is common in statistical and scientific research, but atypical of common flood modeling outputs.

 

Uncertainty of depth

Flood Factor shows this range of uncertainty first with respect to the depth of flooding in a given year and return period event (ie the 2020 1 in 100 event). A median scenario. For instance, this property shows 7.8’ of flooding in the median scenario 1 in 100 event, but a range of 6.9’ (low) to 8.7’ (high).

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Uncertainty of probability

This range of depth also enables First Street to quantify a different likelihood of flooding to a certain depth in the low and high climate scenarios. Because the depth of water can greatly affect the damage done by a flood, First Street calculates the likelihood of flooding at all, to 6”, and to 12”. Using a statistical relationship between depth and probability, First Street calculates the probability of flooding 12” or more in the low, median and high climate scenarios. 

 

Learn more

Flood Factor and RCP Curves

How will different flood depths affect my property?

What are probabilities, and how do they affect my Flood Factor? 

How can sea level rise affect my property? 

 



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