The Low Impact Development Site Planner is a web hosted tool that enables the use to quickly assess the feasibility of specific stormwater mitigation approaches including green infrastructure and conventional treatment systems. This presentation describes the architecture of the program and demonstrates its use on a typical project.

1. Development and Application of a Web-
Based Post-Construction Stormwater
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BMP Feasibility Screening Tool
Vaikko Allen, CPSWQ, LEED-AP
Director Stormwater Regulatory Management
Contech Engineered Solutions
vallen@conteches.com

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LID is Sweeping the Nation!!!
Photo credits: SvR Design Company, Seattle, WA

6. LID implementation doesn’t have to be this
be complicated!
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LID Design Hierarchy

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LID Site Planner – Purpose
 A fast, easy to use tool that follows a Low Impact Development
(LID) design approach and is consistent with regulations that
prioritize Green Infrastructure
 Helps minimize the cost and delay of redesigns by prompting
users to consider a wide range of common site constraints
early in the design process
 Captures specific site conditions precluding the use of
infeasible BMPs
 Allows flexibility to select flow through treatment controls
where runoff reduction is not feasible
 Provides a summary report with links to design guides,
standard details and specifications for stormwater
management approaches that are likely to be feasible and
approved on the project

9. LID Site Planner – Core Functionality
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 Collect site information
o Project name, project location, total disturbed area, total project area,
dominant land use
 Divide site into discrete drainage management areas
(DMA) with dominant land surface types
 Apply screening questions to identify feasible BMPs for
each DMA
 Capture and report constraints precluding more favorable
BMPs
 Summarize feasible BMPs and provide links to standard
specifications and typical drawings

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BMPs represented
Dominant Unit Process BMP
INFILTRATION Bioretention
INFILTRATION Permeable pavement
INFILTRATION Infiltration trench or gallery
INFILTRATION Drywell
RAINWATER HARVEST Rainwater Harvest
EVAPOTRANSPIRATION Green Roofs
FILTRATION Conventional Biofiltration
FILTRATION High Rate Biofiltration
FILTRATION Media Filter
GRAVITATIONAL SEPARATION Swale
GRAVITATIONAL SEPARATION Hydrodynamic Separator

11. Land surface type drives BMP selection
Considering LEED benefits and O&M concerns, Rainwater
Harvesting and Green Roof priority may be reversed
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BMP Selection Based on Land Surface Type
Unit Process INFILTRATION HARVEST ET FILTRATION SETTLING
Unit Operation Bioretention
Permeable
pavement
Infiltration
trench or
gallery
Drywell
Rainwater
Harvest
Green
Roofs
Biofiltration
High Rate
Biofiltration
Media
Filter
Swale Separator
Roofs X X X X X X X X X
Primary
Roads, High
Use Parking
Lots X P P P X X X X X
Landscaped
areas X X
Walkways,
courtyards X X X P P X X X X X
Secondary
roads,
parking lots X X P P P X X X X X
Notes:
X = Suitable BMP
P = Pretreatment required

12. BMP Feasibility Criteria Sources
 Orange County - 2011 Technical Guidance Document
 Ventura County – 2011 Technical Guidance Manual
 Delaware article 3.06.2 – Post construction BMP standards
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and specifications
 2009 MD Stormwater Design Manual, Chapter 3 -
Performance criteria for urban BMP design
 2012 Stormwater Management Manual for Western
Washington
 WEF MOP 23 – Design of Urban Stormwater Controls

13. Site Constraint Screening
Unit Process INFILTRATION HARVEST ET FILTRATION SETTLING
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Unit Operation Bioretention
Permeable
pavement
Infiltration
gallery
Drywell
Rainwater
Harvest
Green Roofs Biofiltration
HR
Biofilter
Filter Swale Separator
High Groundwater within 5-10' of surface
Sensitive Groundwater, <100' to extraction well
Expansive Soils
Contaminated soils
Low permeability soils (<0.5"/hr infiltration rate)
Lack of control over soil compaction during
construction
Adjacent to foundations and utilities
High spill potential PT PT PT PT PT PT PT PT
Slopes >5%
Potable water demand minimization Xeri Xeri Xeri
Fertilizer, pesticide, herbicide minimization O O O O
< 4% of EIA available for BMPs
Discharge to surface required (no stormdrain)
Existing plumbing or structure not supporting green
roof
Existing plumbing not supporting harvest
Insufficient supply or demand for harvested water
Maintenance equipment, access and expertise
availability
Ecological or geomorphic water balance sensitivity
Notes:
PT = Requires pretreatment
Xeri = Xeriscaping or using drought tolerant vegetation
O = Organic landscaping with native or adapted plants can minimize impacts

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Infiltration System Constraints
 Seasonal high groundwater within
5’ to 10’ of surface
 Sensitive groundwater,
groundwater extraction well within
100‘
 Expansive soils
 Contaminated soils or groundwater
 Potential BMP location adjacent to
foundation or utilities that may be
compromised by infiltration
 Low permeability soils <0.5 “/hr
infiltration rate or NRCS class C, D
This photo shows one of the houses
damaged by the 2005 Laguna Beach
Landslide. The house experienced an
undermining by the landslide but
remained intact. Photo by Pam Irvine,
California Geological Survey (CGS).

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Bioretention
A stormwater control measure that
is comprised of plants and
engineered soil located in a
landscape depression. The plants
and soil cleanse stormwater and
retain pollutants and stormwater is
infiltrated into native soils.
Conventional bioretention system
size is dictated by the permeability
of native soils and is commonly
>10% of the size of the
contributing impervious area.

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Permeable Pavement
Permeable hardscape materials
including asphalt, concrete and
paver block systems allow the
movement of water and air around
or through the paving material
while providing a durable surface
for pedestrian, bicycle and
vehicular traffic.
Infeasibility criteria
• High spill potential
• Heavy sediment and gross
solids loads
• High traffic roads and parking
lots
• Not available to treat roof runoff

17. Infiltration Gallery
Infiltration trenches and galleries store stormwater below grade in a stone matrix and/or a
perforated pipe, chamber or vault shaped vessel designed to release stormwater into native soils
over time.
Pretreatment Required
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Dry Well
A vertical stormwater injection well
used to access permeable soil
layers at some depth below the
surface.
Pretreatment Required

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Rainwater Harvesting
Capture of stormwater runoff from
roofs, streets and other impervious
areas for beneficial use as
irrigation or for other non-potable
indoor and outdoor uses.
Feasibility constraints:
• Rainwater harvest and use is
limited or prohibited due to
water rights conflicts
• Local building code or public
health restrictions prohibit
rainwater harvest and use
• Inadequate demand for
harvested water considering all
allowed uses including,
irrigation, indoor non-potable
use, cooling water makeup,
etc.

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Green Roof
A green roof intercepts and stores
rainwater within vegetation and a
growing medium located on a roof
surface.
Only applicable where land
surface type is “Roof”.
Feasibility Constraints:
• Existing roof will not support
additional weight of green roof
(Extensive roof up to 50# wet,
Intensive roof up to 150# wet)
• Green roof would require
supplemental irrigation in Arid
(<10"/annual precip) or Semi-arid
Climate (10-20" annual
precip)

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Conventional Biofilter
A stormwater control measure that
filters stormwater as it passes
through plants and engineered.
Biofiltration may allow incidental
infiltration it differs from
bioretention in that it contains an
under drain which collects treated
stormwater and discharges it from
the site. Conventional biofiltration
sizing is typically greater than 4%
of the contributing impervious
area.
Constraints:
• High spill potential requires
pretreatment
• >4% of contributing effective
impervious area required
• Site must be tolerant of
vegetation and wildlife

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High Rate Biofilter
High rate biofiltration systems are
typically <1/20th the size of
conventional biofiltration due to the
use of optimized soils that have
sustained infiltration rates of more
than 50”/hr. They may also be
configured to allow incidental
infiltration.
Constraints:
• Require 3’ minimum drop from
grade to outlet invert
• Site must be tolerant of
vegetation and wildlife
• Susceptible to spills and heavy
sediment and gross solids
loading

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Media Filter
Media filters use a bed of
engineered media without
vegetation to filter stormwater prior
to discharge downstream. They
are commonly installed below
grade.
Constraints:
• Regulatory approval as stand
alone BMP required unless
used as pretreatment
• Require 3’ minimum drop from
grade to outlet invert

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Swale
A swale is a long linear vegetated
ditch designed to provide a
residence time of at least 7-10
minutes for stormwater runoff.
Pollutants settle out and are
filtered as they travel the length of
the swale. Swales typically
provide significant volume
reduction through incidental
infiltration and evapotranspiration.
Constraints:
• Susceptible to spills and heavy
sediment and gross solids
loading
• Long linear space required
(100 ft min)

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Hydrodynamic
Separator
Hydrodynamic separators are flow
through treatment systems using
internal baffles, weirs, plates and
other flow directing features to
optimize removal of floating and
sinking pollutants. Most commonly
used as pretreatment in an LID
application.
Constraints:
• Typically a pretreatment BMP
• Regulatory approval as stand
alone treatment not assured
• Require 3’ minimum drop from
grade to outlet invert
• Not able to address dissolved
pollutants

26. Confidentiality
 Users can save and retrieve an unlimited number of their own
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projects
 Users cannot view others projects
 Basic login information auto-populated on new projects
 Contech support is available on request
o Where assistance is requested, questions directed to a local resource
based on project zip code or user information
 If requested, Contech can review project details and can assist
with feasibility screening and stormwater plan development

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Demonstration
• This is a Preliminary screening tool
• Precursor to detailed stormwater
management plans
• Local criteria supersede default criteria

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Demonstration

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Demonstration

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Demonstration

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Demonstration

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Demonstration

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Report Contains:
 Project information
 Contact information
 DMA information
 Selected BMP(s)
DMA Report
◦ Description
◦ Web link to specifications,
drawings, design information
 Non-selected feasible BMPs
◦ Description
 Infeasible BMPs
◦ Reasons for infeasibility

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