Reliability-Based Water Quality Assessment with Load Resistance Factor Design: Application to TMDL

Abstract

Effective load reduction strategies rely on an accurate Total Maximum Daily Load (TMDL) calculation, which assesses pollutant loading from various sources. Quantification of uncertainty is a critical part of the TMDL process. To account for uncertainty, this paper adapts the load resistance factor design (LRFD), a reliability-based framework, to water quality assessment and the TMDL process. The LRFD replaces the traditional lumped margin of safety (MOS) with design factors that reflect the magnitude and distribution of uncertainty among the various contaminant loads. In addition, it produces load reduction estimates to meet management objectives with a contaminant-specific frequency-based target. The LRFD is computationally efficient and flexible in that, to compute the design factors, the procedure can use measurement data, analytical solutions or model simulation results, as well as full or marginal probability distributions. The applicability of the proposed approach is demonstrated with two case studies.