In recent years, there is a worldwide interest to perform probabilistic flood hazard assessments (PFHAs) to support a risk-informed decision making framework for critical infrastructure, instead of traditional deterministic flood assessments. A first step in performing a PFHA is to classify the uncertainty of the input parameters into two categories: aleatoric (or uncertainty due to chance) and epistemic (or uncertainty due to lack of knowledge). This paper discusses a PFHA study performed for a small (~1 square mile), partly impervious watershed in Eastern Tennessee. 

The characteristics of the design storms – their durations, depths, and temporal patterns – and antecedent moisture conditions (based on the seasonality of the storms), are selected as having aleatoric uncertainty. To model the rainfall-runoff process, the Green-Ampt methodology is applied to estimate infiltration losses, and the SCS unit hydrograph method is used to transform the computed sub-basin runoff rates into flow hydrographs, with the input parameters having epistemic uncertainty. A climate change factor for rainfall intensities is considered for the study area, with epistemic uncertainty.

Two sets of computations or “loops,” one nested inside the other, are incorporated in selecting model parameters. For each set of epistemic parameter computations (outer loop), a series of aleatoric parameter computations (inner loop) are performed. The outer loop consists of twenty different sets of input variables that are selected with a Latin hypercube sampling approach. The selection of the input variables for the inner loop is performed by using a stratified sampling approach. After the input variables are selected, flood flows are estimated with HEC-HMS. The probabilistic flood hazard curves for different locations within the watershed are estimated from the model results using the total probability theorem, which can be used to estimate probabilistic flood levels at locations of critical infrastructure within the watershed.

Probabilistic Flood Hazard Assessment for a Small Watershed in Eastern Tennessee

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[Paper Proceedings](https://doi.org/10.1061/9780784483466)