Defining dirty water: How much, how strong, how long, and how often?

Mike Gregory


When characterizing water quality constituents, either from a measured dataset or modeling output, it is common to report only the peak concentration or total load. In addition to these basic statistics, there are many ways to quantify observed or computed water quality timeseries data. A broader classification (and the corresponding type of analysis involved) includes the following:

  • Amount of pollutant over a specified period (loading analysis);
  • Magnitude of pollutant concentrations and mass loading rates (statistical analysis);
  • Cumulative time of threshold exceedance (duration analysis); and Frequency of occurrence (event analysis).

These analyses have direct parallels to water quantity analysis or design applications, for example:

  • Percentiles, trends, and other statistical measures to describe a given timeseries or quantify the average recurrence intervals of specific events;
  • Flow, depth, and velocity duration curves for water quantity (e.g., Flow Duration Curve, FDC) have as their equivalent water quality application a Pollutant Duration Curve (PDC) and Loading Duration Curve (LDC); and
  • A Hydrologic Response Unit (HRU) approach that translates the rainfall to a runoff response for individual surface cover types can also be applied to water quality, by generating landscape-specific pollutant concentrations and loads in response to rainfall. 

This paper presents analytical methods and performance metrics to characterize long-term water quality timeseries. Case studies make use of sediment concentrations (mg/L) and loading rates (lb/s) for the following applications: USGS measurements: 32-year observation in the Atchafalaya River in Louisiana, including total suspended sediment (TSS) as well as the finer particulate size fraction (<63 microns); and SWMM5 model results: 21-year simulation at a development site in Florida, including a pre-development scenario and proposed alternatives that represent uncontrolled (piped discharge), dry, and wet retention pond.

 Click here to watch recorded presentation on YouTube.

Permanent link: