Quantification of uncertainties in the 100-year flow at an ungaged site near a gaged station and its application in Georgia

- An equation is derived for evaluating uncertainties in the 100-year flow.
- A method for calculating the “1-percent plus” flow is proposed.
- The method is applied to a rural, mountainous watershed in north Georgia.
- The method will serve as a tool for evaluating uncertainties in flood risks.

SummaryThe Federal Emergency Management Agency has introduced the concept of the “1-percent plus” flow to incorporate various uncertainties in estimation of the 100-year or 1-percent flow. However, to the best of the authors' knowledge, no clear directions for calculating the 1-percent plus flow have been defined in the literature. Although information about standard errors of estimation and prediction is provided along with the regression equations that are often used to estimate the 1-percent flow at ungaged sites, uncertainty estimation becomes more complicated when there is a nearby gaged station because regression flows and the peak flow estimate from a gage analysis should be weighted to compute the weighted estimate of the 1-percent flow. In this study, an equation for calculating the 1-percent plus flow at an ungaged site near a gaged station is analytically derived. Also, a detailed process is introduced for calculating the 1-percent plus flow for an ungaged site near a gaged station in Georgia as an example and a case study is performed. This study provides engineers and practitioners with a method that helps them better assess flood risks and develop mitigation plans accordingly.