NON-VEGETATED FILTRATION SWALE EFFECTIVENESS STUDY
Non-Vegetated Filtration Swale Effectiveness Study | Fact Sheet
Study Goal and Background
The goal of this study was to evaluate the
effectiveness of a non-vegetated filtration swale
BMP. Effectiveness was based upon whether the
BMP could provide basic treatment (80% reduction
of total suspended solids) in accordance with
Ecology treatment performance goals.
Constructing a non-vegetated filtration swale is
highly desirable for locations with hot and dry
summers or in areas where dry periods cause
grass to become dormant or where supplemental
water is needed to establish vegetation. A non-
vegetated BMP will benefit multiple Washington
State Permittees by providing a BMP option that
does not require irrigation. This fact sheet is a
summary of the information found in the Non-
Vegetated Filtration Swale Effectiveness Study
Technical Evaluation Report.
Study Description
The study goal was accomplished through controlled tests conducted at a test site in West
Richland. Four swale design alternatives (alternatives) were tested in 200-foot-long swales at
the site followed by one final swale design alternative (final alternative) as shown in Figure 1.
The final alternative was selected based on the treatment performance of the four alternatives.
A cross-section of the final alternative swale design is shown in Figure 2.
Figure 2: Final Swale Alternative Cross Section
Figure 1: TEST SWALE AND SAMPLE LOCATIONS
NON-VEGETATED FILTRATION SWALE EFFECTIVENESS STUDY
Non-Vegetated Filtration Swale Effectiveness Study | Fact Sheet
Treatment performance was measured from
samples collected from each alternative, which
were analyzed for total suspended solids (TSS). An
influent distribution system mixed and pumped
synthetic stormwater to the swale at the design
flow rate to simulate a storm event (as shown in
Figure 3).
As the synthetic stormwater flowed through the
swale, grab samples (shown in Figure 4) were
collected in eight sample locations that were
spaced at 25-foot increments along the swale.
After each simulated storm event, an amount of
TSS equivalent to one year of loading was
distributed to the swale to stress-test the swale
and determine when the swale would require
maintenance.
The travel time for stormwater to flow through the
swale was recorded at each sample location. The
measured travel time was then used to estimate the velocity of flow through the treatment
layer. This information was used to inform the velocity limits for the swale design guidance.
FIGURE 3: INFLUENT DISTRIBUTION SYSTEM AT TEST SWALE
Figure 4: GRAB SAMPLE FROM TEST SWALE
NON-VEGETATED FILTRATION SWALE EFFECTIVENESS STUDY
Non-Vegetated Filtration Swale Effectiveness Study | Fact Sheet
Study Location
The test site location was south of the City of West Richland Public Works Building and adjacent
to a gravel parking lot (as seen in Figure 5). An existing 430-foot-long swale at the test site was
retrofitted into the two 200-foot-long test swales. The controlled tests were conducted during
the dry season; therefore, no runoff from the gravel parking lot contributed to the test swales.
Results
The initial percent removals for the final alternative indicated that 84.5–87.8% removal of TSS
was achieved for the first simulated year, at the sample location at 200 feet from the start of
the swale. However, percent removal decreased for the following two simulated years, which
was likely due to modifications to the swale needed near the last sample port, due to observed
erosion from a grade break immediately downstream of the swale. As a result, the samples
collected at the last sample port (200 feet) were discarded and statistical trendline analysis
was used to determine how the swale would have performed if the swale modifications had
not occurred. This analysis is shown in Table 1 and indicates that the swale met performance
goals for the first two years. Since the treatment performance dropped below 80% for the third
year, it is likely that maintenance would need to be performed sometime around the third year
to restore treatment performance. Further testing needs to be done to confirm the
maintenance procedures and schedule.
Figure 5: Test Swale Location
NON-VEGETATED FILTRATION SWALE EFFECTIVENESS STUDY
Non-Vegetated Filtration Swale Effectiveness Study | Fact Sheet
Table 1: Final Alternative Water Quality Results from Trendline Analysis1
Location in Swale
Year 1
Year 2
Year 3
25 FEET
58.5%
-13.2%
11.9%
50 FEET
62.8%
1.00%
21.4%
75 FEET
67.0%
15.3%
30.9%
100 FEET
71.3%
29.5%
40.4%
125 FEET
75.5%
43.8%
49.9%
150 FEET
79.8%
58.0%
59.4%
175 FEET
84.0%
72.3%
68.9%
200 FEET
88.3%
86.5%
78.4%
1. Results shown are concentrations developed using trendlines.
The percent removal results in Table 1 were compared to the TAPE treatment performance
goals for TSS using the bootstrap statistical analysis to predict the treatment performance of
the swale. Years 1 and 2 represent the performance of the swale before maintenance is needed.
However, only two data points were available, causing the result of the bootstrap analysis to
be equivalent to the lower of the two removal efficiencies. The evaluation of removal
efficiencies calculated for years 1–3 added one data point and indicated the swale would meet
the TAPE treatment performance goal for all three simulated years.
The measured travel time for flow to travel through the
swale was 50 minutes, from which a design velocity of
0.066 ft/sec was calculated. It is anticipated that
treatment will be provided by a swale 200 feet long if the
velocity and residence time are less than or equal to the
values measured during the study.
Future Action Recommendations
Submit the swale for Conditional Level Use
Designation, so the performance of the swale can be
further evaluated in the field for actual storm events.
Perform additional field testing to understand
effective maintenance activities to restore the swale
treatment performance every two to three years and
the frequency at which more minor action items
such as removal of sediment and debris from inlets,
weed control, etc., should be performed.
Perform additional field testing to understand the impact that a catch basin or forebay
at the inlet would have on treatment performance and maintenance cycle of the swale.
Lead Entity:
City of West Richland
Contributing Entity:
City of Richland
City of Kennewick
City of Pasco
City of Walla Walla
Walla Walla County
City of Moses Lake
City of Pullman
Idaho Dept. of Environmental
Quality
Washington Dept. of Ecology
This
study
was
conducted
support the lead and participating
entities in meeting NPDES MS4
Phase II Permit Requirements for S8
Monitoring and Assessment.