Stage‐Frequency Analysis at a Major River Junction

Abstract

A coincident frequency analysis using the total probability theorem was performed for an 80‐mile (129‐km) reach of the Illinois River to evaluate existing stage‐frequency relationships and revisions caused by a proposed higher levee system. Frequency and duration relationships were derived to establish the upstream and downstream boundaries. A water surface profile model was applied to calculate stage at 210 points in the reach for given starting backwater and discharge conditions. The analytical process was calibrated to stage data in the reach through iterative adjustments in the boundary duration curves. Comparisons of the adopted results to previous analyses using simplified techniques showed a significant reduction in levee design profile and potential cost savings of $1,600,000 in levee construction.