Study Introduction
This project is one of eight Eastern Washington stormwater effectiveness
studies conducted to meet Spokane County’s NPDES MS4 2014-2019
permit requirements (S8. Monitoring and Assessment). An effectiveness
study evaluates the effectiveness of permit-required stormwater management
program activities and best management practices (BMPs). The purpose of this
study was to evaluate the stormwater treatment performance of a new sand
filter BMP, the sand filter sidewalk vault. The results of the study were intended
to be used to justify the development of a new BMP approved for general use on
future projects, if Technology Assessment Protocol – Ecology (TAPE) treatment
performance goals were met.
Constructing BMPs at sites with space constraints is a challenge for projects
located in built urban areas, where the right-of-way abuts property lines or
where there is limited space between the sidewalk and roadway. The proposed
sand filter sidewalk vault BMP is a variation of the basic sand filter vault
BMP defined by the Ecology stormwater manuals for Washington State. The
4-foot by 5-foot sand filter sidewalk vault is installed below the sidewalk and
receives runoff through a curb cut located in the street gutter. Since the BMP
is contained and provides treatment within the vault, it can be connected to
existing (or new) storm drain networks. This BMP could eliminate (or reduce) the
need for constructing a treatment BMP downstream and subsequently reduce
the overall cost of stormwater management on future projects.
Preliminary BMP Design and Maintenance Guidance
Prior to the field study, BMP design and maintenance guidance was developed
for the proposed sand filter sidewalk vault BMP. The guidance was based on the
results from column testing and modifying the design guidance defined in the
Eastern Washington Stormwater Management Manual for the basic sand filter
BMP (Ecology, 2019). The column test included simulated rainfall events using
a synthetic stormwater solution. The column testing system was designed to be
representative of the new BMP constructed in the field. Results from the column
test indicated that a design infiltration rate of 50 inches/hour and a hydraulic
loading of 1.6 gallons/minute were appropriate for the sand filter media.
Water Quality Sampling
Automated monitoring equipment was installed at the sand filter sidewalk
vault BMP site in 2018 to collect composite influent and effluent water quality
samples, flow rate, temperature, and precipitation depth. Through two wet
seasons, a total of 24 storm events were collected. The storm events were
evaluated to determine whether the TAPE storm event guidelines and sample
collection requirements were met. The water quality data obtained was used to
evaluate the reduction of the following stormwater runoff target pollutants: total
suspended solids (TSS), dissolved copper and zinc, and oils.
STUDY LOCATION:
The test site is located near the intersection
of East Hawthorne Road and US 2 (Newport
Highway) in Spokane, Washington. At this
location, the land use is a mix of residential
and commercial. The contributing basin area is
approximately 0.41 acres, of which 0.32 acres
is impervious surfaces (roadway and sidewalks)
and 0.09 acres is pervious surfaces (lawns).
The contributing roadway is primarily an urban
arterial with some runoff from a residential road.
STUDY
LOCATION
RECOMMENDED SAND FILTER
SIDEWALK VAULT
APRIL 2021
SPOKANE COUNTY | OSBORN CONSULTING
SPOKANE COUNTY
SAND FILTER SIDEWALK VAULT BMP
EFFECTIVENESS STUDY
Water Quality Treatment Performance
The water quality treatment performance of the sand filter sidewalk vault was
evaluated against Ecology treatment performance goals for basic, dissolved
metals, and oil treatment using the bootstrap statistical method per TAPE. The
results are summarized in the table below:
1 Treatment performance criteria shown reflects the criteria for typical influent
concentration ranges at the site.
2 Insignificant difference was found between influent and effluent concentrations.
Operation and Maintenance Cycle
The infiltration and sediment accumulation data collected during the study
provided an estimate of operation and maintenance frequency for the sand filter
sidewalk vault. An analysis of infiltration over time indicated that maintenance
would be required twice per year for a basin area of 0.41 acres and once every
2.5 years for an area of 0.08 acres. An estimate of sediment accumulation rate
indicated a rate of 1.66 lb sediment/inch precipitation could be anticipated.
Recommendations
If the study is repeated, the test site should be modified to include a catch basin
with a sump upstream of the BMP (see Recommended Sand Filter Sidewalk
Vault figure on previous page) to reduce the sediment accumulation rate, which
will ultimately reduce the maintenance cycle frequency. Including a sump in
the BMP design allows a vactor truck to clean the catch basin, a practice that
maintenance staff often perform.
Based on the PSD results, the sand filter media can be improved by reducing
the media pore size distribution (i.e. the different sizes of empty spaces in the
media). For example, instead of using coarse sand media, a mix of medium
grain and coarse sand could be used to reduce the pore size distribution.
Alternatively, adding high carbon fly ash to the sand mix will reduce the media
pore size distribution while still allowing for high infiltration rates and enhance
the dissolved metals removal.
ECOLOGY TREATMENT PERFORMANCE GOALS
Performance
Goal
Pollutant
Treatment
Performance
Criteria1
Study Water
Quality Data
Results
Treatment
Performance
Goal
Achieved
Basic
Treatment
Total
Suspended
Solids (TSS)
Effluent
<20 mg/L
31.9 mg/L
Dissolved
Metals
Treatment
Dissolved
Copper (Cu)
30%
Reduction
— 2
Dissolved Zinc
(Zn)
60%
Reduction
16.7%
Oil Treatment
NWTPH-Dx
Daily average
effluent conc.
< 10 mg/L
— 2
COLUMN TESTING
CONFIGURATION
SAND FILTER VAULT
& DRY WELL
SPOKANE COUNTY CONTACT:
Jake Saxon, Project Manager
509.477.7245
[email protected]
OSBORN CONSULTING RESEARCH TEAM:
Aimee Navickis-Brasch, PhD, PE
509.867.3654 Ext. 301
[email protected]
Taylor Hoffman-Ballard, PE
509.867.3654 Ext. 302
[email protected]
Nicole Chen, EIT
206.628.9133 Ext. 232
[email protected]
SPOKANE COUNTY | OSBORN CONSULTING
APRIL 2021