Abstract
In 2012, the U.S. Geological Survey and the Oregon
Department of Transportation began a cooperative study
to demonstrate use of the Stochastic Empirical Loading
and Dilution Model (SELDM) for runoff-quality analyses
in Oregon. SELDM can be used to estimate stormflows,
constituent concentrations, and loads from the area upstream
of a stormflow discharge site, from the site of interest and in
the receiving waters downstream of the discharge. SELDM
also can be used to assess the potential effectiveness of
best management practices (BMP) for mitigating potential
effects of runoff in receiving waters. Nominally, SELDM is
a highway-runoff model, but it is well suited for analysis of
runoff from other land uses as well.
This report provides case studies and examples to
demonstrate stochastic-runoff modeling concepts and to
demonstrate application of the model. Basin characteristics
from six Oregon highway study sites were used to
demonstrate various applications of the model. The highway
catchment and upstream basin drainage areas of these study
sites ranged from 3.85 to 11.83 acres and from 0.16 to
6.56 square miles, respectively. The upstream basins of two
sites are urbanized, and the remaining four sites are less than
5 percent impervious.
SELDM facilitates analysis by providing precipitation,
pre-storm streamflow, and other variables by region or from
hydrologically similar sites. In Oregon, there can be large
variations in precipitation and streamflow among nearby
sites. Therefore, spatially interpolated geographic information
system data layers containing storm-event precipitation and
pre-storm streamflow statistics specific to Oregon were created
for the study using Kriging techniques.
Concentrations and loads of cadmium, chloride,
chromium, copper, iron, lead, nickel, phosphorus, and zinc
were simulated at the six Oregon highway study sites by using
statistics from sites in other areas of the country. Water‑quality
datasets measured at hydrologically similar basins in the
vicinity of the study sites in Oregon were selected and
compiled to estimate stormflow-quality statistics for the
upstream basins. The quality of highway runoff and some
upstream stormflow constituents were simulated by using
statistical moments (average, standard deviation, and skew) of
the logarithms of data. Some upstream stormflow constituents
were simulated by using transport curves, which are relations
between stormflow and constituent concentrations.
Stochastic analyses were done by using SELDM to
demonstrate use of the model and to illustrate the types of
information that stochastic analyses may provide:
- An analysis was done to demonstrate use of dilution
factors as an initial reconnaissance tool for comparing
relative risk among sites.
- An analysis of hardness-dependent, water-quality criteria
was done to illustrate the effects of variations in hardness
and flow on the application and interpretation of such
criteria. This analysis shows that hardness-dependent
criteria can vary by an order of magnitude among storm
events because hardness is diluted by stormflows.
- An analysis of uncertainties in input and output values
was done to demonstrate that properly selected robust
datasets are needed to represent conditions at a site of
interest. This analysis shows that the rate of water-quality
exceedances that are measured or simulated may depend
on sample size and the luck of the draw.
- An analysis was done to demonstrate that SELDM and other Monte Carlo models may generate extreme values from input statistics, which may or may not be feasible based on physicochemical or hydrological limits.
- An analysis of BMP modeling methods was done to demonstrate use of the model for estimating treatment requirements for meeting water-quality objectives.
- An analysis of the use of grab sampling and nonstochastic upstream modeling methods was done to evaluate the potential effects on modeling outcomes.
Additional analyses using surrogate water-quality
datasets for the upstream basin and highway catchment were
provided for six Oregon study sites to illustrate the risk-based
information that SELDM will produce. These analyses show
that the potential effects of highway runoff on receiving-water
quality downstream of the outfall depends on the ratio of
drainage areas (dilution), the quality of the receiving water
upstream of the highway, and the concentration of the criteria
of the constituent of interest. These analyses also show that
the probability of exceeding a water-quality criterion may
depend on the input statistics used, thus careful selection of
representative values is important.
U.S. Department of the Interior |
U.S. Geological Survey