Characterizing Roadway Runoff and Hydraulic Performance of Engineered
Media for Treating Contaminated Runoff
Abstract
The objective of this joint field and laboratory study is to
characterize the delivery and infiltration of nutrient loads from
roadway runoff within roadway shoulders. Roadway runoff can be a major
non-point source of nutrients entering surface and groundwater
resources. Vegetated swales equipped with engineered media (vegetated
filter strips) placed between roadways and receiving water bodies may
hold promise to remedy contaminated runoff. However, performance of
roadside filters is dependent upon hydraulic conditions as a first-order
control of infiltration into filters. Variation of flows and
infiltration rates were observed at distances of 0 to 6 m along
vegetated shoulders and embankments using field-scale roadway models.
Two laboratory test beds (experimental and control) drained by 300
drainage ports were designed according to the typical standard cross
section of roadways in Florida. The experimental bed was filled with a
surface layer of sandy soil (0.3 m depth, D50 = 0.27 mm) underlain by
engineered media (0.7 m depth, D50 = 0.3 mm), while the control bed was
filled with sandy soil (1 m depth, D50 = 0.27 mm). All media was
compacted to 1400 – 1550 kg/m3 and beds were vegetated with Argentine
Bahia grasses. Runoff was introduced from a 3 m impervious roadway
section into a 1.5 m vegetated shoulder (6.0% slope) followed by 4.5 m
vegetated embankment (16.0% slope). Storms designed by frequency
analysis of 30 years of 15-min rainfall data gauged in North-Central
Florida were introduced to the systems using a rainfall simulator with
maximum precipitation rate of over 300 mm/hr. To determine dynamic
nutrient contents, five synoptic samples of roadway runoff were
collected at three locations over 10 storm events. Samples were analyzed
for nutrient content and storm characteristics (precipitation, runoff)
were determined from gauged information observed at study sites. Field
information was integrated to design dynamic nutrient content of runoff
introduced by synthetic storms.