EASTERN WASHINGTON
STORMWATER EFFECTIVENESS STUDIES
TECHNICAL EVALUATION REPORT (TER)
SAND FILTER SIDEWALK VAULT BMP
Study Classification: Structural BMP
Study Objective(s):  Evaluate Effectiveness  Develop New BMP
February 3, 2021
Prepared For:
Spokane County
Public Works Department
1116 W. Broadway Avenue
Spokane, Washington 99260
(509)477-3600
Prepared By:
Osborn Consulting, Inc.
101 S Stevens St.
Spokane, Washington 99201
(509)867-3654
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QAPP and TER Publication Information
The project Quality Assurance Project Plan (QAPP) and Technical Evaluation Report will be
stored
and
accessible
the
public
the
Spokane
County’s
website:
https://www.spokanecounty.org/918/Stormwater-Utility. For questions regarding the project,
please contact Matt Zarecor by email [email protected] or phone (509) 477-7255.
TER Authors and Contact Information
Aimee Navickis-Brasch, P.E., Ph.D.
Engineering Manager
Osborn Consulting, Inc.
101 S Stevens St.
Spokane, WA 99201
[email protected]
(509) 995-0557
Taylor Hoffman-Ballard, P.E.
Stormwater Engineer
Osborn Consulting, Inc.
101 S Stevens St.
Spokane, WA 99201
[email protected]
(952) 836-7863
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DOCUMENT HISTORY
This study was conducted following the QAPP which can be accessed at the link on the previous
page. The study started in October 2018 and the last data was collected in December 2020. The
draft Technical Evaluation Report (TER) was presented and submitted to the Technical Advisory
Group (TAG) in January 2021 for review and comment. Appendix Q of the TER contains a
summary of the TAG’s comments along with a summary of response to the comments including
how they were addressed in this document. The final TER was submitted to Ecology in January
2021.
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DISTRIBUTION LIST
Name, Title
Organization
Contact Information:
E-mail, Telephone
Jake Saxon,
Project Manager
Spokane County
[email protected]
509-477-7245
Matt Zarecor
Assistant County Engineer
Spokane County
[email protected]
509-477-7255
Bill Gale
Stormwater Utility Manager
Spokane County
[email protected]
509-477-7261
Ethan Murnin
Stormwater Engineer
Spokane County
[email protected]
509-477-7261
Bill Aukett
Stormwater Program
Manager
City of Moses Lake
[email protected]
509-764-3792
Shilo Sprouse
Stormwater Services
Program Manager
City of Pullman
[email protected]
509-432-9052
Nigel Pickering
Research Assoc. Professor
Washington State
University
[email protected]
509-335-8624
Drew Woodruff
City Engineer
City of West
Richland
[email protected]
509-967-5434
Doug Howie,
WQ Project Manager
Department of
Ecology
[email protected]
360-407-6444
Karen Dinicola,
WQ Project Manager
Department of
Ecology
[email protected]
360.407.6550
Brandi Lubliner,
WQ Project Manager
Department of
Ecology
[email protected]
360.407.7140
Adriane Borgias,
WQ Project Manager
Department of
Ecology
[email protected]
509-329-3515
Amanda Mars,
WQ Program – ERO
Department of
Ecology
[email protected]
509-329-3554
Aimee Navickis-Brasch,
Engineering Manager
Osborn Consulting,
Inc.
[email protected]
509-995-0557
Taylor Hoffman-Ballard,
Stormwater Engineer
Osborn Consulting,
Inc.
[email protected]
952-836-7863
Kathy Sattler, Laboratory
Project Manager
Anatek Labs
[email protected]
509-838-3999
Medhanie Tecle,
Engineering Manager
Materials Testing &
Consulting, Inc.
[email protected]
360-534-9777
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1.0
TABLE OF CONTENTS
DOCUMENT HISTORY ……………………………………………………………………………………………………………………….. III
DISTRIBUTION LIST …………………………………………………………………………………………………………………………… IV
1.0
TABLE OF CONTENTS …………………………………………………………………………………………………………………. V
2.0
EXECUTIVE SUMMARY ……………………………………………………………………………………………………………….. 1
3.0
INTRODUCTION ……………………………………………………………………………………………………………………………. 4
3.1
INTRODUCTION TO THE STRUCTURAL BMP …………………………………………………………………………………… 4
3.2
PROBLEM DESCRIPTION ……………………………………………………………………………………………………………… 8
3.3
PROJECT GOALS AND OBJECTIVES ……………………………………………………………………………………………….. 8
3.4
PROJECT OVERVIEW …………………………………………………………………………………………………………………… 9
3.5
STUDY LOCATION ……………………………………………………………………………………………………………………. 10
4.0
SAMPLING PROCEDURES ………………………………………………………………………………………………………….. 12
4.1
TYPES OF DATA COLLECTED ……………………………………………………………………………………………………… 12
4.2
SAMPLE COLLECTION PROCESS ………………………………………………………………………………………………….. 14
5.0 DATA QUALITY ASSESSMENT ………………………………………………………………………………………………….. 17
5.1
DATA VERIFICATION ………………………………………………………………………………………………………………… 17
5.2
DATA USABILITY ASSESSMENT ………………………………………………………………………………………………….. 18
6.0
RESULTS AND DISCUSSION ………………………………………………………………………………………………………. 21
6.1
BMP DESIGN AND MAINTENANCE GUIDANCE ……………………………………………………………………………… 21
6.2
WATER QUALITY DATA ANALYSIS …………………………………………………………………………………………….. 25
6.3
COMPARISON OF RESULTS TO TAPE TREATMENT PERFORMANCE GOALS ………………………………………… 30
6.4
BMP DESIGN FLOW …………………………………………………………………………………………………………………. 38
6.5
OPERATION AND MAINTENANCE (O&M) CYCLE ………………………………………………………………………….. 40
7.0
FUTURE ACTION RECOMMENDATIONS ………………………………………………………………………………….. 43
8.0
CONCLUSIONS ……………………………………………………………………………………………………………………………. 44
9.0
REFERENCES ………………………………………………………………………………………………………………………………. 45
10.0 APPENDICES ……………………………………………………………………………………………………………………………….. 46
APPENDIX A. LABORATORY ANALYTICAL REPORTS ………………………………………………………………………………….. 47
APPENDIX B. STORM REPORTS ……………………………………………………………………………………………………………….. 48
APPENDIX C. DATA QUALITY ASSESSMENT ……………………………………………………………………………………………… 49
APPENDIX D. STATISTICAL AND DATA ANALYSIS ……………………………………………………………………………………… 65
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2.0
EXECUTIVE SUMMARY
The focus of this study is to evaluate the stormwater treatment performance of a new potential best
management practice (BMP) referred to as the sand filter sidewalk vault. This BMP fits in a 4-foot
by 5-foot vault installed below the sidewalk that receives runoff through a curb cut located in the
street gutter. The sand filter media consists of 18-inches of coarse sand overlaid by a coconut coir
mat. This BMP is being evaluated to provide jurisdictions with more options for retrofit or
redevelopment projects located in built urban areas where available space (right of way) for new
BMPs is limited. The proposed BMP is a variation of the basic sand filter vault BMP as defined
by the Ecology stormwater manuals for Washington State. The primary differences between the
sand filter sidewalk vault and the basic sand filter BMPs include: the proposed BMP can be
installed underneath the sidewalk (usually within the existing right of way) and is designed to
accept runoff from a larger contributing basin area.
The goal for this study is to evaluate the effectiveness of the proposed BMP. Effectiveness will be
based upon:
• The ability of the BMP to infiltrate stormwater during the 6-month 24-hour storm event
without overflowing into the bypass system within the maintenance cycle.
• The efficacy of the BMP to reduce the concentrations of total suspended solids (TSS),
dissolved copper (Cu) and zinc (Zn), and oils, which will be evaluated to determine
whether the BMP can achieve the respective Ecology treatment goals.
The goals for this study were achieved by conducting flow-through column testing and field testing
of the BMP. The flow-through column testing was conducted to develop the BMP design and
maintenance guidance prior to installation in the field. This work is described in the study QAPP
Section 3.3. The field testing was conducted at a test site in Spokane County. Automated
equipment installed at the test site was used to collect composite influent and effluent water quality
samples, flow rate, temperature, and precipitation depth. The water quality parameters tested
included the required and screening parameters defined in Technology Assessment Protocol
Ecology (TAPE) for basic, dissolved metals, and oil treatment. The physiochemical properties of
the sand filter media and sediment accumulated on top of the media were also analyzed. The media
infiltration rate was measured continuously using the data collected from the automated equipment
and twice using a modified falling head test.
The test site was constructed in 2016 and field testing was conducted from October 2018 to
December 2020. In January of 2019 (three months after testing started), the BMP failed due to the
large quantity of solids deposited on top of the sand filter media which inhibited infiltration. The
sand filter sidewalk vault design was re-evaluated, and the test site was modified to provide a pre-
settling basin to reduce the quantity of solids entering the vault. The contributing basin area was
also reduced. In June of 2019, a new sand filter media was installed in the sidewalk vault. Sample
collection re-started in the summer of 2019 and continued through November 2020. Maintenance
was performed in February 2020 to improve infiltration.
Samples were collected from a total of 24 storm events. For each of these events, a storm report
was created, and the data was evaluated to determine whether the storm met the TAPE qualifying
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storm criteria and sample collection criteria. These criteria define the minimum storm depth and
duration, storm antecedent and post storm dry period, minimum number of aliquots, sample event
coverage, and minimum number of samples. The evaluation results determined that 11 storm
events met the TAPE criteria for qualifying conditions and 8 met all the qualifying conditions
except for one or two of the criteria. The remaining storm events did not meet the criteria for three
or more qualifying conditions.
The collected data was analyzed to determine the effectiveness of the proposed BMP. The specific
objectives completed to meet the study goal are summarized below along with the results.
Objective 1: Define the BMP design and maintenance guidance for the test site.
Prior to the start of the field study, the BMP design and maintenance guidance were developed for
the proposed BMP. The guidance was based on column testing results (QAPP Section 3.3) and
modified design guidance outlined in the Eastern Washington Stormwater Management Manual
for the basic sand filter BMP (Ecology, 2019). A specification was developed for a sand filter
media (and followed to construct the test site) with a design infiltration rate of 50 inches/hour and
a hydraulic loading of 1.6 gallons per minute. The target contributing basin area for each sand
filter sidewalk vault was 3,500 sq.ft. for a site with a mean annual precipitation (MAP) of 18-
inches. Reference Section 6.1 for a complete description of the guidance.
Objective 2: Determine the pollutant removal efficiency of the BMP by measuring and comparing
the pollutant concentrations in the influent and effluent.
The average pollutant removal efficiencies were calculated for TSS, TPH (oils), and dissolved
copper and zinc. The average pollutant removal efficiency was 53.4% (TSS); -17.8% and 28.0%
(dissolved copper and zinc respectively), and 16.6% (TPH). The results of the statistical analysis
indicated a statistically significant difference between influent and effluent concentrations for TSS
and dissolved zinc. However, the influent and effluent concentrations difference is statistically
insignificant for dissolved copper and TPH.
Objective 3: Determine whether the treatment performance goals were achieved by comparing
study results to TAPE goals and requirements.
The sand filter sidewalk vault BMP did not meet the TAPE treatment performance goals for Basic,
Dissolved Metals, or Oils Treatment.
Objective 4: Establish a design hydraulic loading rate in gallons per minute per square foot of the
sand filter surface area.
The design hydraulic loading rate of 1.6 gpm/sq.ft. was developed through column testing prior to
the start of this study. This hydraulic loading rate was assessed in comparison to the measured
loading rate, TSS treatment performance, and bypass flows to determine if the rate was appropriate
for the site. Based on this evaluation, the proposed 1.6 gpm/sq.ft. hydraulic loading rate was
recommended for the design of sand filter sidewalk vault.
Objective 5: Determine the maintenance cycle frequency using the results from infiltration testing.
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The maintenance frequency was determined using infiltration and precipitation data…