Stochastic Empirical Loading and Dilution Model for Analysis of Flows, Concentrations, and Loads of Highway Runoff Constituents

Abstract

In cooperation with FHWA, the U.S. Geological Survey developed the stochastic empirical loading and dilution model (SELDM) to supersede the 1990 FHWA runoff quality model. The SELDM tool is designed to transform disparate and complex scientific data into meaningful information about the adverse risks of runoff on receiving waters, the potential need for mitigation measures, and the potential effectiveness of such measures for reducing such risks. The SELDM tool is easy to use because much of the information and data needed to run it are embedded in the model and obtained by defining the site location and five simple basin properties. Information and data from thousands of sites across the country were compiled to facilitate the use of the SELDM tool. A case study illustrates how to use the SELDM tool for conducting the types of sensitivity analyses needed to properly assess water quality risks. For example, the use of deterministic values to model upstream stormflows instead of representative variations in prestorm flow and runoff may substantially overestimate the proportion of highway runoff in downstream flows. Also, the risks for total phosphorus excursions are substantially affected by the selected criteria and the modeling methods used. For example, if a single deterministic concentration is used rather than a stochastic population of values to model upstream concentrations, then the percentage of water quality excursions in the downstream receiving waters may depend entirely on the selected upstream concentration.

Comparison of SELDM Simulated Total-Phosphorus Concentrations with Ecological Impervious-Area Criteria

“…SELDM can simulate three stormwater-treatment mechanisms, including runoff hydrograph extension, volume reduction, and waterquality treatment, using a trapezoidal distribution with correlation to the inflow values (Granato 2013(Granato , 2014Granato and Jones 2014). Three types of generic BMPs were simulated: one with hydrograph extension only, one with flow reduction only, and one comprehensive BMP with hydrograph extension, flow reduction, and water-quality treatment.…”

Approaches for assessing long-term annual yields of highway and urban runoff in selected areas of California with the Stochastic Empirical Loading and Dilution Model (SELDM)

“…SELDM was developed for simulating stormwater event mean concentrations (EMCs) to indicate the risk for stormwater flows, concentrations, and loads to be above user-selected water-quality goals and to evaluate the potential effectiveness of mitigation measures to reduce such risks (Granato, 2013(Granato, , 2014Granato and Jones, 2014, 2019. The runoff coefficient statistics are a function of the imperviousness of the site of interest.…”

Simulating Runoff Quality with the Highway Runoff Database and the Stochastic Empirical Loading and Dilution Model

“…As such, it provides practice-ready information about runoff quality and the need for mitigation measures to reduce the potential for adverse effects of runoff on receiving water quality. For example, the population of geometric mean concentrations for TSS and total phosphorus (TP) from different sites in the HRDB are shown in Figure 4; geometric mean concentrations are discussed because highwayrunoff quality commonly is analyzed using the lognormal distribution (7,10,12,14,(16)(17)(18)(19)(20)(21). Geometric means for TSS and TP range from 1.58 to 1,379 mg/L and 0.017 to 2.82 mg/L with median values of 42 and 0.12 mg/L, respectively.…”