Stormwater Flow Control, Stormwater Treatment, and Green Stormwater Infrastructure City of Salem

Stormwater Flow Control,
Stormwater Treatment, and Green
Stormwater Infrastructure
City of Salem
Stormwater Advisory Committee
April 10, 2013
2
Tonight’s Workshop
• The Committee
• Overview of Schedule
• Looking at Washington State Regulations
▫ Phase I Municipal Stormwater Permit
▫ Low Impact Development (LID)
• LID Requirements in Oregon Phase I Permit
• Salem Draft Code & Draft Design Standards
• Design Storms and Salem’s Stormwater
Philosophy
3
Committee Charter
Key Points
• Purpose: Provide advice to PW Director
• Two tasks: SW Regulations & SW Master Plan
• Limited duration: Two years
• Meetings: Open
• Member viewpoints: Not required to represent a
constituency
4
Schedule
• Stormwater Code
▫ Began public process - January 2013
▫ Outreach to begin - June/July 2013
 HBA
 Chamber
 Neighborhood Associations
▫ Council Work Session - August/September 2013
▫ Council Decision - NLT November 2013
▫ Code effective - January 2014
• Stormwater Master Plan – Begin Summer 2013
5
A Quick History:
Phase I Permits in Washington State
• Permit #1: Issued July 5, 1995
▫ Issued by Water Quality Management Area
▫ All Phase I permits were the same
▫ Expired July 5, 2000
• Permit #2: Issued January 17, 2007
▫ Influenced by Tri-County ESA
▫ Required equivalence with SW Management
Manual for Western Washington (Ecology 2005)
▫ Appealed
6
A Quick History:
Phase I Permits in Washington State
• Appeal of 2007 Permit (Part 1)
▫ Equivalency determination lacked public process
• Appeal of 2007 Permit (Part 2)
▫ Permit fails to require maximum on-site
dispersion and infiltration through use of LID
techniques
7
A Quick History:
Phase I Permits in Washington State
• 2007 Permit Modified #1: Issued June 17, 2009
▫ Listed approved manuals & ordinances
▫ Includes language that Phase I permittees:
“….must require non-structural preventive actions and
source reduction approaches including Low Impact
Development Techniques (LID), to minimize the
creation of impervious surfaces, and measures to
minimize the disturbance of soils and vegetation
where feasible.”
8
A Quick History:
Phase I Permits in Washington State
• 2007 Permit Modified #2: Issued Sept 1, 2010
▫ Equivalency determination for Clark County
• Appeal of 2007 Permit (Part 3)
▫ Clark County is not equivalent
▫ “The Board finds that the Agreed Order rests on
no science as to the comparability of its mitigation
metric in relation to the Phase I Permit’s flow
control approach….”
9
A Quick History:
Phase I Permits in Washington State
• Permit #3: Issued August 1, 2012
▫ One-year permit with minimal changes
• Permit #4: Issued August 1, 2012
▫ Effective August 1, 2013
▫ Includes minimum requirements for projects
▫ Incorporates LID requirements developed in
2009-11
10
Selected Project Requirements in
Washington State Phase I Permit (2013)
• Natural Vegetation
▫ Retain the duff layer, native top soil, and natural
vegetation in an undisturbed state to the
maximum degree practicable.
• Phase Projects
▫ Phase development projects to the maximum
degree practicable and take into account seasonal
work limitations
• Protect Low Impact Development BMPs
▫ Protect from sedimentation, compaction, erosion
11
Selected Project Requirements in
Washington State Phase I Permit
• Natural Drainage
▫ Natural drainage patterns shall be maintained, and
discharges from the project site shall occur at the
natural location, to the maximum extent practicable.
• Downstream Impacts
▫ Runoff discharged from the project site must not cause
a significant adverse impact to downstream receiving
waters and down gradient properties
• Wetlands
▫ Discharges to wetlands shall maintain hydrologic
conditions, hydrophytic vegetation, and substrate
characteristics necessary to support existing &
designated uses
12
Selected Project Requirements in
Washington State Phase I Permit
• Stormwater Treatment Requirements
▫ Threshold is 5,000 ft2 of “pollution-generating
hard surface”
▫ Basic Treatment
▫ Oil Control Treatment
▫ Phosphorous Treatment
▫ “Enhanced” Treatment
At a minimum, 91% of the total runoff volume, as estimated by
an approved continuous runoff model, must pass through the
treatment facility(ies) at or below the approved hydraulic
loading rate for the facility(ies).
13
Selected Project Requirements in
Washington State Phase I Permit
• Stormwater Flow Control Requirements
▫ Threshold is 10,000 ft2 effective impervious
▫ Match discharge durations to pre-developed
durations for the range of pre-developed rates
▫ Pre-developed condition shall be a forested land
cover unless:
 Information indicates the site was prairie prior to
settlement; or
 The drainage area of the immediate stream and all
subsequent downstream basins have had at least 40%
total impervious area since 1985
14
Low Impact Development
Requirements in Washington State
• LID is Required
▫ Permittee must require On-site Stormwater
Management BMPs in accordance with the
following project thresholds, standards, and lists
to infiltrate, disperse, and retain stormwater
runoff on-site to the extent feasible without
causing flooding or erosion impacts.
15
Low Impact Development
Requirements in Washington State
• Must consider BMPs in order
• Must use first BMP considered feasible
16
Low Impact Development
Requirements in Washington State
Lawn/Landscape
Post-construction soil
quality & depth
17
Low Impact Development
Requirements in Washington State
Lawn/Landscape
Roofs
Post-construction soil
quality & depth
Full dispersion or
downspout full
infiltration
Rain gardens or
bioretention ≥ 5% of area
Downspout dispersion
Perforated stub-out
connections
18
Low Impact Development
Requirements in Washington State
Lawn/Landscape
Roofs
Other Hard Surfaces
Post-construction soil
quality & depth
Full dispersion or
downspout full
infiltration
Full dispersion
Rain gardens or
bioretention ≥ 5% of area
Permeable pavement or
rain gardens or
bioretention
Downspout dispersion
Sheet flow dispersion or
concentrated flow
dispersion
Perforated stub-out
connections
19
A Quick History:
Phase I Permits in Washington State
• Permit #3: Issued August 1, 2012
▫ One-year permit with minimal changes
• Permit #4: Issued August 1, 2012
▫ Effective August 1, 2013
▫ Includes minimum requirements for projects
▫ Incorporates LID requirements developed in
2009-11
• Appealed: By Phase I & II permittees, others
▫ Burdensome & unreasonable requirements
▫ LID requirements unlawful, unjust, unreasonable,
and/or impracticable
20
Phase I Permit Requirements for Salem
Post-Construction Site Runoff
• Thresholds
▫ SFR Projects @ 1,300 ft2 new/replaced
▫ Other Projects @ 10,000 ft2 new/replaced
• Capture & treat 80% of annual average runoff
volume
• Reduce site specific post-development runoff
volume, duration, and rates
• Minimize hydrological & water quality impacts
21
Phase I Permit Requirements for Salem
Low Impact Development
• Incorporate site-specific management practices
to mimic natural surface or predevelopment
hydrologic functions as much as practicable
• Optimize on-site retention based on site
conditions
• Prioritize and include implementation of LID,
Green Infrastructure, or equivalent planning,
design, and construction approaches.
22
Salem Draft Code/Design Standards
• In draft code
▫ Green stormwater infrastructure shall be used to
the maximum extent feasible
• In draft design standards, if meet criteria:
▫
▫
▫
▫
SW Planters
Rain Gardens
Vegetated Filter Strips
Vegetated Swales
If built per specifications, single facility will meet
treatment & flow control requirements
23
Salem Draft Code/Design Standards
• Alternative Stormwater Treatment Systems
▫
▫
▫
▫
Stormwater Treatment Wetland/Gravel Wetland
Green Roof
Permeable Paving
Rainwater Harvesting
• Manufactured Treatment Technology
• Structural Stormwater Flow Control
• Soil Amendment – Top 18”
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Design Storms
Water Quality Storm
• Requirement: Treat 80% average annual runoff
• Standard 1.38”/24 hr type 1A distribution
Flow Control for Flooding
• Requirement: Mitigate downstream flooding
• Standard match peak flows 2-yr, 10-yr, 25-yr
Flow Control for Hydromodification (anticipated)
• Requirement: Mitigate stream erosion potential
• Typical Standard: Match flow duration from half 2yr to 10-yr
Water Quality Design Storm
Permit Requirement 80% of average annual runoff
Water Quality Storm =1.38”/24 hrs type 1A distribution
For 10,000sqft
Volume = 56 cf
Flow = 0.1 cfs
Type 1A Storm
0.08
0.07
Rainfall Depth (in)
0.06
0.05
0.04
0.03
0.02
0.01
24.0, 0.01
0.00
0.0
5.0
10.0
15.0
Time (hrs)
20.0
25.0
30.0
Flow Control
Flood Control
Purpose: Flood Control
Method: Match peaks
•2-yr/24-hour storm
•10-yr/24-hour storm
•25-yr/24-hour storm
Result: Increased duration of
effective discharge
Effective Discharge
The Hydro-mod
Problem
Increased erosion in urban
streams
Aquatic Habitat
Water Quality
Aesthetics
Flooding risk
Infrastructure risk
Compliance risk
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Benefits of LID
For Developers
• More attractive, sustainable neighborhoods that sell faster and for a premium
• Reduces stormwater utility fees
• Reduces the cost of clearing, excavation, compaction, erosion control, and infrastructure construction
• Can provide more buildable lots by distributing stormwater management around the site in small facilities
instead of building a single large detention pond
For the
Environment
• Protects water
quality
• Maintains natural
stream flows in
rivers, creeks and
wetlands
• Provides and
protects fish and
wildlife habitat
• Improves air quality,
heat island, and
noise reduction
Graphic: courtesy of GreenWorks
29
Benefits of LID
For Cities
• Helps meet regulatory requirements, including the Federal Clean Water Act (MS4 permits
and TMDL plans), Endangered Species Act, Safe Drinking Water Act, and state land use
planning goals 5 and 6
For Communities
• Helps prevent flooding and
reduces the cost of
associated damage
• Helps maintain clean
drinking water supplies
• Can lower cost of streets,
curbs, gutters and other
infrastructure
• Increases the aesthetics of
neighborhoods
• Reduces long-term
maintenance costs
STORMWATER MANAGEMENT DESIGN STANDARDS
CASE STUDY- SEQUOIA VILLAGE
The 9.6 acre Sequoia Village site is zoned high density residential. This zoning requires a minimum of 30 dwelling units per acre. The
original site design accommodated 276 condominium units. Parking exceeds code requirements with 523 stalls and is provided via private
garages on the ground floor of the buildings and as surface stalls. WES current design requirements for stormwater flow control and water
quality treatment were met using underground facilities. As a case study, the development was then re-designed using the WES proposed
design standards which emphasize a Low Impact Development Approach (LIDA) in order to evaluate the cost difference between LIDA and
the current standards. Under the proposed standards, the same number of units is provided and the use of LIDA facilities saves the
developer $53,000 in estimated construction costs.
Current Standards
Element
Current Standards Proposed LID
Standards
Total Units
276
276
Parking
523 Stalls
(exceeds
requirement)
456 Stalls (meets
requirement)
Detention/I 368 feet of 48”
nfiltration
perforated pipe
Requiremen
t
18,652 ft of
Vegetated Swales
and 917 sq ft of
water quality
planters
WQ
CDS Media
Treatment
Filtration System:
Requiremen 70 filter cartridges
t
Met with swales
and wq facilities
listed under
detention above.
Total
$796,976
Stormwater
Manageme
nt
Constructio
n Costs
Cost
Difference
$743,912
$53,064 in
cost savings
Proposed Standards
STORMWATER MANAGEMENT DESIGN STANDARDS
CASE STUDY – HAPPY VALLEY TOWN CENTER
The 15.62 acre Happy Valley Town Center site is a commercial development. The project was originally designed and constructed as a 121,095 sq ft shopping
center with 484 parking stalls. All commercial developments within the City of Happy Valley are required to have 15% of their parking area landscaped. WES
current stormwater design requirements were met using underground detention and water quality treatment facilities. As a case study, the project was then redesigned using the WES proposed design standards which emphasize a Low Impact Development Approach (LIDA) techniques in order to evaluate the cost
difference between LIDA and the current standards. Under the proposed standards, the same square footage of shopping area and parking stalls were provided
and LIDA stormwater facilities were integrated into the landscaping requirement. If LIDA had been utilized during the design and construction of this project, it
would have resulted in an estimated $60,000 savings in construction cost to the developer
Current Standards
Element
Current
Standards
Total Building Area
Proposed
LIDA
Standards
121,000 sq. ft. 121,000 sq ft.
Parking
484
Detention/Infiltration 33,000 cu ft
Requirement
underground
detention:
420 ft of 120inch metal
pipe + 200 ft
of 18” perf.
pipe.
WQ Treatment
Three
Requirement
Downstream
Defenders
Total Stormwater
Management
Construction Costs
Cost Difference
$716,000
484
16,050 sq ft of
Vegetated
swales/plante
rs.
Met with
swales and
wq facilities
listed under
detention
above.
$656,000
$60,000 in
cost
savings
Proposed Standards
STORMWATER MANAGEMENT DESIGN STANDARDS
CASE STUDY – GREENBRIAR SUBDIVISION
The 15.62 acre Greenbriar residential community was originally developed with 59 single family lots averaging 7,100 sq feet in size. The
WES stormwater design requirements for water quality and detention were met with a traditional detention/water quality pond. As a case
study, the project was then re-designed using the WES proposed design standards which emphasize a Low Impact Development Approach
(LIDA) in order to evaluate the cost difference between LID A and the current standards. Under the proposed standards, the number of
buildable lots rose to from 59 to 61. The overall lot yield was increased by using water quality swales distributed throughout the
development and along the road-right-of way rather than a traditional treatment pond. A detention facility was still needed but required
less land area to build. If LIDA had been utilized during the design and construction of this project, it would have resulted in an additional
cost of $384.00 per lot in construction cost to the developer but a net gain of two additional buildable lots.
.
Current Standards
Element
Current
Standards
Proposed LIDA
Standards
Total Dwelling
Units
59
61
Detention/Infilt
ration
Requirement
0.33 Acre
Detention/Wat
er Quality Pond
0.078 Acre
Detention Pond
Water Quality
Requirement
See above
Total
Stormwater
Management
Construction
Costs
Cost Difference
$299,365
$5,073 per lot
(59 lots)
0.091 Acres
total of LIDA
Water Quality
Swales
(Distributed)
$332,933
$5,457 per lot
(61 lots)
+ $33,558
= TWO
Additional
Buildable
lots
Proposed Standards
33
Salem’s Stormwater Philosophy
Manage stormwater quality and quantity to protect the environment,
public health, and safety in an equitable and cost effective manner
• Meet permit requirements
• Utilize Green Infrastructure where feasible to meet both
water quality and flow control requirements
• Provide rate credits to developments that manage
stormwater on site
• Prevent flooding impacts to downstream properties