Stormwater Management Manual, Low
Impact Development Code, and Public Works
Design Manual Public Workshop
April 10, 2014
City of Tacoma Stormwater Management
Manual Updates Workshop
Stormwater Management Manual – Why?
• Phase I National Pollutant Discharge Elimination System
(NPDES) Municipal Stormwater Permit (Permit) requires a
program to prevent and control the impacts of runoff from new
development, redevelopment, and construction activities. The
City of Tacoma is electing to provide a Stormwater Management
Manual equivalent to the Washington State Department of
Ecology (Ecology’s) 2012 Surface Water Management Manual for
Western Washington (2012 SWMMWW).
Stormwater Management Manual History
• October 1995 Manual
• Developed in response to1995 NPDES Permit (Designed to be equivalent to
Ecology’s 1992 Manual)
• January 2003 Manual
• Developed in response to extension of 1995 NPDES Permit (Designed to be
equivalent to Ecology’s 2001 Manual)
• September 2008 Manual
• Developed in response to 2007-2012 NPDES Permit (Designed to be
equivalent to Ecology’s 2005)
• February 1, 2012 Manual – Current Manual
• Developed in response to local community concerns and questions
(developers, engineers, City staff).
• Draft 2015 Manual – Currently in Development
• Developing in response to 2013-2018 NPDES Permit (Designed to be
equivalent to Ecology’s 2012 Manual)
• Draft due to Ecology June 30, 2014
• Must be adopted by June 30, 2015
Changes to the Manual
1.
2.
3.
Changes required to comply with 2013 NPDES Permit.
Edits and reorganization for clarity.
Changes to update City of Tacoma specific requirements –
conveyance design and infrastructure protection.
Volume 1 Changes
Change – Minimum Requirements
2012 SWMM – 12 Minimum
Requirements
1. Preparation of Stormwater Site
Plans*
2.
Construction Stormwater
Pollution Prevention*
3.
Source Control of Pollution*
4.
Preservation of Natural Drainage
Systems and Outfalls*
5.
Onsite Stormwater Management*
6.
Runoff Treatment*
7.
Flow Control*
8.
Wetlands Protection*
9. Basin/Watershed Planning
10. Operation and Maintenance*
11. Offsite Analysis and Mitigation
12. Financial Liability
Proposed SWMM – 10 Minimum
Requirements
1. Preparation of Stormwater
Site Plans*
2. Construction Stormwater
Pollution Prevention*
3. Source Control of Pollution*
4. Preservation of Natural
Drainage Systems and
Outfalls*
5. Onsite Stormwater
Management*
6. Runoff Treatment*
7. Flow Control*
8. Wetlands Protection*
9. Operation and Maintenance*
10. Offsite Analysis and
Mitigation
*Denotes Ecology required Minimum Requirements
Why only 9 Minimum Requirements?
• Basin/Watershed Planning was removed for clarity. The
applicant is not allowed to develop their own basin/watershed
plan. This is a plan the City would develop and then the
applicant would follow (for example the South Tacoma
Groundwater Protection District). These specific basin
requirements will be described in the SWMM text or as part of
a separate policy document.
• Financial Liability was removed because it was not used.
Though financial liability is used in the work order process; its
inclusion in the SWMM was confusing. The City will still
require project cost information in order to determine which
Minimum Requirements apply to a project but at this point,
does not specifically bond for requirements of the SWMM.
Exemptions – Pavement Maintenance
• 2012 SWMM
• Road Maintenance
• Parking Lot Maintenance
• Proposed SWMM
• Combines road
maintenance and parking
lot maintenance into
pavement maintenance.
• Includes those practices
that are considered
redevelopment and
describes when various
surfaces are considered
new hard surfaces and
when they are considered
replaced hard surfaces
Why – Pavement Maintenance?
• Ecology revised the exemption from road maintenance to
pavement maintenance to ensure exemptions associated with
roads would also be associated with other types of paving, such
as parking lots.
• The applicability to new and replaced hard surfaces was
included in this section for clarity. The definition for new and
replaced surfaces is also contained in the glossary.
New Development
(Permit Compliance Revision)
• All new development shall be required to comply with Minimum
Requirement #2 and #11. Minimum Requirements #10 and #12 #9 may
apply to any project.
• The following new development shall comply with Minimum Requirement
#1 through #5, and Minimum requirement #10 (qualitative analysis only) for
the new and replaced impervious hard surfaces and the land disturbed:
• Creates or adds Results in 2,000 square feet, or greater, of new, replaced, or new plus
replaced impervious hard surface area, or
• Has land disturbing activity of 7,000 square feet or greater.
• The following new development shall comply with Minimum Requirement
#1 through #10 for the new and replaced impervious hard surfaces and
converted pervious surfaces vegetation areas:
• Creates or adds Results in 5,000 square feet or more greater of new plus replaced
impervious hard surface area, or
• Converts ¾ acres, or more, of native vegetation to lawn or landscaped areas, or
• Converts 2.5 acres, or more, of native vegetation to pasture.
Big Changes in New Development and
Redevelopment Sections
(Permit Compliance Revision)
• Redevelopment has similar wording changes to new
development.
• All new development sites will be required to mitigate for both
new and replaced hard surfaces regardless of monetary or
space thresholds. (New development has less than 35%
existing impervious surface coverage).
• New Terms:
• Hard Surface – An impervious surface, a permeable pavement, or a
vegetated roof.
• Vegetation – As related to applicability of the Minimum Requirements,
vegetation shall mean native vegetation, pasture, scrub/shrub, uncultivated
vegetation, or unmaintained non-native vegetation (e.g., Himalayan
blackberries, scotch broom).
Equivalent Areas
2012 SWMM
• The City allows the
Minimum Requirements to
be applied to an equivalent
area (flow and pollution
characteristics) within the
same site. For public road
projects, the area must drain
to the same receiving
waters.
Proposed SWMM
• Minimum Requirements may
be applied to an equivalent
area that drains to the same
receiving water.
Minimum Requirement #1
(Permit Compliance Revision)
• Stormwater Site Plans shall use site-appropriate development
principles, as required by Tacoma Municipal Code, to retain
native vegetation and minimize impervious surfaces to the
extent feasible.
• These site-appropriate development principles will be
developed with the Low Impact Code update.
Minimum Requirement #2
(Permit Compliance Revision)
• There are now 13 elements of a SWPPP (previously there were
12).
• Element #13: Protect BMPs – This Element intends to protect
BMPs from sedimentation that occurs during construction; and
includes protecting low impact development features from
sedimentation and compaction.
Minimum Requirement #5
(Permit Compliance Revision)
• The requirement varies significantly based upon final discharge
location.
• Discharges to Non-Flow Control Exempt Waterbodies (Leach, Flett, Swan
Creek, Joe’s Creek, other ditches and creeks)
• Flow Control Exempt Waterbodies (Puget Sound, First Creek, Puyallup
River)
Hydrology
Typically:
 Green – flow control
exempt.
 Blue – Non-Flow
Control Exempt
17
MR #5 – Not Flow Control Exempt
• Projects triggering only MR#1-#5 and #10, shall either:
• Use onsite stormwater management BMPs from List #1 OR
• Demonstrate compliance with the LID Performance Standard. Rain
Gardens cannot be used to achieve the standard but bioretention can.
• Projects triggering MR #1-#10 shall either:
• Use onsite stormwater management BMPs from List #2 OR
• Demonstrate compliance with the LID Performance Standard. Rain
Gardens cannot be used to achieve the standard but bioretention can.
List #1 – MR #1-5 (Not Flow Control
Exempt)
• BMPs must be chosen in the listed order of preference:
• Lawn and Landscaped Areas
• Soil Quality and Depth (BMP L613) if feasible.
• Roofs:
1.
Full Dispersion or Downspout Infiltration
2.
Rain Gardens or Bioretention
3.
Downspout Dispersion
4.
Perforated Stub-Outs
5.
Collect and Convey
• Other Hard Surfaces
1. Full Dispersion
2.
Permeable Pavement or Rain Gardens or Bioretention
3.
Sheet Flow Dispersion
4.
Collect and Convey
Rain Garden vs. Bioretention
• Rain Garden - A non-engineered shallow landscaped depression
for managing stormwater. Only used to meet intent of MR #5.
• Bioretention – Engineered facilities that store and treat
stormwater. Can be used to meet MR #6 and MR#7.
Low Impact Development Performance
Standard
• Stormwater discharges shall match developed discharge
durations to predeveloped durations for the range of
predeveloped discharge rates from 8% of the 2-year peak flow
to 50% of the 2-year peak flow. Refer the Minimum
Requirement #7 for the predeveloped condition to be
modeled. Projects that must also meet Minimum Requirements
#7 must match flow durations between 8% of the 2-year
through the full 50-year flow.
• Addresses the lower, more frequent stormwater flows.
LID Performance Standard
List #2 – MR#1-10 (Not Flow Control
Exempt)
• BMPs must be chosen in the listed order of preference:
• Lawn and Landscaped Areas
• Soil Quality and Depth (BMP L613) if feasible.
• Roofs:
1.
Full Dispersion or Downspout Infiltration
2.
Bioretention
3. Downspout Dispersion
4.
Perforated Stub-Outs
5.
Collect and Convey
• Other Hard Surfaces
1.
Full Dispersion
2.
Permeable Pavement
3.
Bioretention
4.
Sheet Flow Dispersion
5.
Collect and Convey
MR #5 – Flow Control Exempt
• There is no order of preference (applicants may use options from
List #1 or List #2 or the LID Performance standard):
• Lawn and Landscaped Areas
• Soil Quality and Depth (BMP L613) if feasible.
• Roofs:
• Downspout Infiltration OR
• Rain Gardens OR
• Downspout Dispersion OR
• Perforated Stub-Outs OR
• Collect and Convey (only if infiltration, dispersion, and perforated stubouts
are not feasible)
• Other Hard Surfaces
• Concentrated Flow Dispersion OR
• Sheet Flow Dispersion OR
• Rain Gardens
Minimum Requirement #6
(Permit Compliance Revision)
• Projects in which the total of effective pollution-generating
impervious hard surface is 5,000 square feet or more in a
threshold discharge area of the project.
• The term effective is removed to ensure pollution generating
permeable surfaces are included in the water quality
thresholds.
• Effective impervious surface are those impervious surfaces that
are connected via sheet flow or discrete conveyances to a
drainage system. (Effective impervious will apply to flow
control).
Minimum Requirement #7
(Permit Compliance Revision)
• Projects in which the total of effective impervious surfaces is
10,000 square feet or more in a threshold discharge area, or
• Projects that convert ¾ acres of more of native vegetation to
lawn or landscape, or convert 2.5 acres or more of native
vegetation to pasture in a threshold discharge area, and from
which there is a surface discharge in natural or man-made
conveyance systems from the site, or
• Projects that through a combination of effective impervious
surfaces and converted pervious surfaces hard surfaces and
converted vegetation areas, cause a 0.15 cfs increase in the
100-year flow frequency from a threshold discharge area as
estimated using the Western Washington Hydrology Model or
other approved model (assuming a 15-minute time step).
Comparison will be between existing and proposed conditions.
Minimum Requirement #7
(COT Revisions)
• 4 Types of Flow Control Standards
Standard Requirement – Forested Condition
2. Standard Requirement – Existing Condition (new section)
3. Infrastructure Protection Requirement
4. Flow Control Exempt Waterbodies
1.
MR #7 – Standard Requirement –
Forested Condition
• Applies in Leach Creek, Flett Creek, Joe’s Creek and discharges
to gulches or creeks in Western Slopes and Northeast Tacoma
Watersheds.
• Same as previous manual:
• Stormwater discharges shall match developed discharge rates
from 50% of the 2-year peak flow up to the full 50-year peak
flow.
• The predeveloped condition to be matched shall be a
forested land cover.
MR #7 – Standard Requirement –
Existing Condition
• Applies to discharges to gulches or creeks in North Tacoma,
Thea Foss, Tideflats, Lower Puyallup (except Puyallup River)
Watersheds.
• Stormwater discharges shall match developed discharge
durations to existing condition durations for the range of
predeveloped discharge rates from 50% of the 2-year flow up
to the full 50-year peak flow.
• The pre-developed condition to be matched is the existing land
cover.
Standard Requirement – Existing
Conditions – Rationale
• Permit states: The predeveloped conditions to be matched shall
be forested land cover unless: The drainage area of the
immediate stream and all subsequent downstream basins have
had at least 40% total impervious surface since 1985. In this
case, the predeveloped conditions to be matched shall be the
existing land cover condition.
• Ecology determined which areas can be considered 40%
impervious since 1985.
• A stream or channel will typically equalize in 10-20 years. It is
believed that those streams that have existed in highly urbanized
areas have adjusted to the existing land cover conditions so
maintenance of those existing conditions should suffice to
ensure no additional degradation.
MR #7 – Flow Control Exempt Waterbodies –
Puyallup River
• If the following requirements are met, flow control is not required
for projects that discharge directly or indirectly to the Puyallup River.
If all any of the following requirements are not met, flow control per
the Standard Requirement – Existing Condition in Section 3.4.7.3.2 is
required and the quantitative analysis is not required.
• Direct discharges to the exempt receiving water does not result in diversion
of a stream.
• Flow splitting devices are applies to route natural runoff volumes from the
project site to any downstream Type 5 stream or category IV wetland.
• The project site must discharge through a conveyance system comprised of
manmade conveyance elements.
• The conveyance system between the project site and the exempt receiving
water shall have sufficient capacity to convey discharges from future build-out
conditions of the site and the existing conditions from non-project areas. MR
#11 (infrastructure protection) applies from the project to ¼ mile
downstream from the site.
MR #7 - Infrastructure Protection
(COT Revision)
The determination of when a downstream analysis is required and the type of analysis required is based upon the project
scope as outlined below:
a.
For projects that do not add any impervious surfaces, a quantitative analysis of the downstream system is not
required.
b.
For projects that add less than 10,000 square feet of impervious surface, the increase of stormwater discharges
from the site shall not be more than 5% of the capacity in the downstream system. The analysis of the existing
system shall be based upon the pipe with the lowest capacity within ¼ mile downstream from the discharge
location or to a City-identified trunk main, whichever is less. If it is determined that the discharges from the site
will be equal to or greater than 5% of the lowest capacity pipe, a quantitative analysis is required (see c. below).
c.
For projects that add 10,000 square feet or greater of impervious surface or that cause a 0.15 cfs increase in the
100-year, flow frequency from a threshold discharge area as estimated using WWHM (assuming a 15-minute time
step), a quantitative downstream analysis per Minimum Requirement #10 shall be required to verify if the existing
system has capacity.
d.
The applicant may resolve the downstream capacity problem or provide onsite infiltration or detention. Where
infiltration or detention is provided, stormwater discharges for the developed condition shall not exceed the
discharges under existing conditions. The applicant is not required to match flow durations but can match flow
frequencies. If onsite detention or infiltration is proposed instead of upsizing the downstream system, the owner
must provide a signed letter stating that they understand the proposal and accept the operation and maintenance
of the onsite system.
The City of Tacoma, Environmental Services is currently in the process of modeling the entire City of Tacoma system for
capacity. If the project is located in a part of the City that has a developed capacity model, the applicant is not required
to complete a downstream quantitative analysis. The applicant should contact the City of Tacoma - Site Development
Group to determine if a capacity model has been developed. If the analysis shows a capacity concern within a ¼ mile
downstream of the project site, the applicant can resolve the downstream capacity problem or provide onsite infiltration
or detention (per c. above). The applicant may elect to provide their own model per Minimum Requirement #10 instead
of utilizing the results of the City of Tacoma model.
Minimum Requirement #8 – Wetlands Protection
(Permit Compliance Revision)
• Discharges to wetlands shall maintain the hydrologic conditions,
hydrophytic vegetation, and substrate characteristics necessary
to support existing and designated uses.
• Projects shall comply with Guide Sheets #1 through #3 in
Appendix D. The hydrologic analysis shall use the existing land
cover unless otherwise directed.
MR #8 – Guide Sheet #1
• Criteria that excludes wetlands from serving as a treatment or
flow control BMP.
• It is a category 1 wetland.
• The wetland provides a high level of many functions (Category 1 and II
wetlands)
• The wetland provides habitat for threatened or endangered species.
MR #8 – Guide Sheet #2
• Criteria for include wetlands as a treatment or flow control
BMP.
• It is a Category IV wetland or a Category III with a habitat score of 19 or
•
•
•
•
less.
There will be no net loss of function as result of the modification.
There is no breeding population of any native amphibian species.
The hydrologic function can be improved.
The wetland lies in the natural routing of the runoff and the discharge
follows the natural routing.
MR #8 – Guide Sheet #3a
• Protecting Functions and Values of Wetlands
• Includes items such as maintaining buffers, retaining native vegetation,
providing fencing, limiting access, and avoiding compaction.
MR #8 – Guide Sheet 3b
• Applies to full build-out conditions of the wetland’s watershed
as well as full build-out conditions of the project.
• Criterion 1: total volume of water into a wetland during a
single precipitation event should not be more than 20% higher
or lower than the pre-project volumes.
• Criterion 2: total volume of water into a wetland on a monthly
basis should not be more than 15% higher or lower than the
pre-project volumes.
MR# 9 – Operation and Maintenance
• Basin/watershed planning was removed.
(COT revision)
• The operation and maintenance checklists were updated to
include information from the Low Impact Development
Operation and Maintenance Manual.
MR#12 – Financial Liability
• This section was removed.
(COT Revision)
Adjustments – New Section
• Adjustments to the Minimum Requirements may be requested,
in writing, to allow a reduction or modification of a
requirement or to permit an alternative requirement. This
applicant shall submit all requests to Environmental Services.
Application for an adjustment to a Minimum Requirement shall
be made in writing and include documentation that:
• The adjustment provides substantially equivalent environmental
protection.
• Based on Sound Engineering Practices, the objectives of safety, function,
environmental protection and facility maintenance are met.
Appendices
• Revised Stormwater Site Plan and Construction Stormwater
Pollution Prevention Short Forms to make more user-friendly.
• Updated O&M Checklists per SWMM and LID O&M Manual.
Volume 2 – Stormwater Management for
Construction Sites
(Permit Compliance Revision)
• 13 Elements instead of 12. Element #13 – Protect BMPs was
•
•
•
•
added to ensure BMPs are protected from sedimentation
caused by construction activities. This BMP also includes
protecting LID features by ensuring heavy machinery is not
used in these features.
Stake and Wire Fence (BMP C104) is removed
Vegetation Filtration (BMP C236) – new BMP.
Small Project Construction Stormwater Pollution Prevention
(BMP C180) was removed.
Changes to BMP specifications were made throughout volume
for consistency with Ecology manual.
Volume 3 – Onsite Stormwater Management,
Flow Control, and Conveyance
• Rain Gardens were moved to Volume 6.
• Perforated Stub-out BMP L604 was added.
• Flow credits were updated in each section to match Ecology
flow control guidance. WWHM modeling is required in most
cases.
• Flow credits are not applicable to sites just needing to meet MR #1-5,
when MR#1-5 thresholds are met, MR#1-5 must be used on all applicable
surfaces.
• New Appendix to describe when a soils report is needed for
each BMP and what is needed in that report.
• Covenant and easement agreement for private facilities has
been added to the manual.
Volume 3 – Soils Reports
• All projects using infiltration to meet MR #6, MR #7, or MR#8
must complete a site specific soils report.
• All projects using infiltration to obtain flow credits must submit
a site specific soils report.
• In-situ testing to determine infiltration rates is required:
• For determining infeasibility of a rain garden or permeable pavement.
• For design of infiltration facilities to meet MR#6, MR#7, or MR#8
Vol. 3 – Design Saturated Hydraulic
Conductivity
(Permit Compliance Revision)
2012 SWMM
Proposed SWMM
• USDA Soil Textural
• USDA Soil Textural Class
Classification
• ASTM Gradation Testing
• In-Situ Infiltration
Measurements
• Soil Grain Size Analysis
Method (D10, D60, D90)
• Large Scale Pilot Infiltration
Test (PIT)
• Small Scale PIT
• Drainage Area less than 1 acre
• Testing for dispersed LID
facilities
• Soil Grain Size Analysis
Method (D10, D60, D90)
• Soils unconsolidated by glacial
advance.
Vol. 3 – Bypass and Offsite Inflow
• Added guidance for bypass and offsite inflow.
• Conditions for allowing bypass:
• Runoff from the bypass area and the flow control facility converge within ¼
•
•
•
•
mile.
The flow control facility is designed to compensate for the uncontrolled
bypass area such that the net effect is the same with or without bypass.
The 100-year peak discharge from the bypass will not exceed 0.4 cfs.
Runoff from the bypass area will not create an adverse impact downstream.
Water quality requirements are met.
• Offsite inflow:
• If the existing 100-year peak flow rate from any upstream offsite area is
greater than 50% of the 100-year developed peak flow (undetained) then
the runoff from the offsite area must not flow to the onsite flow control
facility.
Vol. 3 – Conveyance Design
(COT revision)
• Allow private onsite system design to be modeled at 10-year
event rather than 25-year event.
• For pipe systems, for the 100-year, 24-hour design storm, if
overtopping of the system occurs, the applicant shall show the
extent of the impacts on neighboring properties and the rightof-way. The applicant may be required to provide mitigation for
localized flooding.
Vol. 3 – Pipes and Structures (COT Revision)
• CPEP pipe will no longer be allowed for pipes within the Public
Right of Way.
• In sag conditions, combination inlets (WSDOT Standard Plan B25.20-01) will be required.
Volume 4 – Source Control BMPs (Permit Compliance Revision)
• BMPs to consider for all commercial and industrial activities
was expanded to include additional good housekeeping
activities and ensure consistency amongst the NPDES permits.
Volume 5 – Water Quality Treatment
BMPs
(Permit Compliance Revision)
2012 SWMM
• Enhanced treatment is
required for the following
project sites that discharge
to fish-bearing streams, lakes,
or to waters or conveyance
systems tributary to fishbearing streams or lakes:
• Industrial Sites
• Commercial Sites
• Multi-Family Sites, and
• High AADT Roads
Proposed SWMM
• Enhanced treatment is required for
the following land use types that
meet the requirements for
stormwater treatment and:
• Discharge stormwater directly or
indirectly to fresh waters designated
for aquatic life use or have an existing
aquatic life use
• Use infiltration strictly for flow control
– not treatment – and the discharge is
within ¼ mile of a fresh water
designated for aquatic life use or that
has an existing aquatic life use:
• Industrial sites
• Commercial Sites
• Multi-family sites
• High AADT roads
Vol. 5 – Enhanced Treatment
• Aquatic Life Use – Waterbodies as defined in WAC 173-201A-
600 and WAC 173-201A-602.
• Categories include:
• Char spawning and rearing.
• Core summer salmonid habitat.
• Salmonid spawning, rearing, and migration.
• Salmonid rearing and migration only.
• Non-anadromous interior redband trout.
• Indigenous warm water species.
• All surface waters of the state (fresh waters) not specifically
named in WAC 173-201A-602 are to be protected.
Volume 5 – Water Quality BMPs
• Media Filter Drain (previously Ecology Embankment) added to
manual as BMP for phosphorus treatment, enhanced treatment,
and basic treatment.
• Compost Amended Filter now called Compost Amended
Vegetated Filter Strip (CAVFS).
• Narrow Area Filter Strip no longer allowed as a treatment BMP
Volume 6 – Low Impact Development
• Rain Gardens moved from Volume 3 to Volume 6.
• Rain garden, bioretention, and permeable pavement BMPs are
associated with infeasibility criteria to determine when the
BMPs are required for compliance with MR #5.
Glossary
• New terms or revisions to terms including:
• Aquatic Life Use
• Bituminous Surface Treatment
• Chip Seal
• City Block
• Converted Vegetation Areas
• Effective Impervious Surface
• Hard Surface
• Land Disturbing Activity
• Landscaped Areas
• Lawn Areas
• New Development
• New Hard Surface
• New Impervious Surface
• Pollution Generating Hard Surface – Now includes sports fields
• Pollution Generating Impervious Surface
• Project
• Replaced hard surface
• Vegetation
Provide Comments – Please!
• Via Email to swmupdates@cityoftacoma.org
• Subject Line – 2015 SWMM Updates
• Include Volume #, Section #, Page #
• Include recommended changes when feasible.
• All comments must be received by April 18,
2014.
Contact Information
Mieke Hoppin, PE
253.502.2105
mhoppin@cityoftacoma.org
General Stormwater and Wastewater Questions
stormandsewer@cityoftacoma.org
253.591.5218
Permit Intake Center – 3rd Floor TMB