2013 12-05 challenging sites-rvss

Stormwater Management on Challenging Sites

Sustainable design isn’t about doing something neat, it’s about doing
something right.

Agenda
•
•
•
•
•
•
•

Introductions/Housekeeping
Challenging Sites & Low Impact Development Defined
Water Balance Model
Managing Rainfall Anywhere
Managing Rainfall in (Very) Slowly Draining Soils: Porous Pavement
Managing Rainfall in Slow Draining Soils: Bioretention
Managing Rainfall on Water Quality & Quantity Constrained Sites

Challenging Sites
•
•
•
•

Steep slopes/landslides
High seasonal groundwater
High bedrock
Inadequate setbacks
(ex. Buildings too close together)
• Clay soils

Types of Site Constraints
Real: Water Quality & Quantity
• Water Quality: When ground or surface waters may be degraded
• Water Quantity: When infiltrating water may cause a problem (i.e.
landslides, flooded basements) or cannot be infiltrated (high water table,
high bedrock or other shallow impermeable layer)

Types of Site Constraints
Perceived: Slowly Draining Soils
…but, I have tight clay soils with no infiltration!

LID takes many forms in each phase of the project
(and still incorporates some gray infrastructure)
Planning

Construction

Design

Maintenance

Sustainability for all the places between the buildings

Best Management Practices (aka BMPs)
Non-structural vs. Structural

Low Impact Development: A Definition
“Low impact
development is a
stormwater
management and land
development strategy
applied at the parcel
and subdivision scale
that emphasizes
conservation and use
of on-site natural
features integrated
with engineered, smallscale hydrologic
controls to more closely
mimic predevelopment
hydrologic functions.”

Water Quality AFTER Development
Sediment (air particulates)
Nutrients
Feces
Other debris
Runoff volumes

Sediment/turbidity
Hydrocarbons
Heavy metals (particles & soluble)
Other chemicals
Runoff volume

Sediment/turbidity
fertilizers
pesticides
herbicides
Runoff volume
11

flow
base tration)
25% infil
ow
(shall

25% groundwater
(deep infiltration)

0.5% runoff

50% evaporation

100%
XX” average
annual rainfall

Water Balance BEFORE Development
Example: WESTERN OREGON

Water Balance BEFORE: Simplified

No
infiltration

Reduced
infiltration

98% runoff
2% evaporation

reduced evaporation

100% average
annual rainfall

Water Balance AFTER Development
Example: EVERYWHERE

Water Balance AFTER: Simplified

Runoff Volume & Duration
from a Watershed Perspective
• Additional volumes over
pre-developed rates
scour stream banks.
• Additional durations of
flow impact habitat
further.

Best Management Practices (aka BMPs)
Manage Rainfall vs. Runoff

Porous asphalt at the Port of Portland manages RAINFALL

Bioretention manages RUNOFF
from a much larger area than
itself

Managing Rainfall Anywhere
Doesn’t necessarily depend on:
• Soils types
• Setbacks

Contained Planters
(Over Impervious Area)

Contained Planters (Over Impervious Area)

Restore the Soil: Garden Areas

Compost Amended Slopes
Washington DOT
• Great for keeping soil in place on steep slopes, too!

http://www.wsdot.wa.gov/Design/Roadside/SoilBioengineering.htm

Lots of great information at
soilsforsalmon.org

Managing Rainfall
in Slowly Draining Soils
• Porous pavements are effective in soils draining as slow as 0.1
inches/hour

Porous Pavement
Definition
• An engineered stormwater facility that you can drive or walk on,
which preserves permeability to reduce environmental and social
impacts of conventional impervious pavements.

Types

Porous asphalt

Pervious
concrete

Grass-crete

Courtesy of MGH Associates

Courtesy of Fortis Construction

Manufactured
Permeable Pavers

Poured-in-place
Permeable
Pavers

Grid pavements

Porous Pavement Generalized Cross Section Detail
for Types on Previous Slide
• A similar cross section applies to all the types on the previous slide

Pervious pavement surface
Open-graded crushed aggregate
Non-woven geotextile fabric
Uncompacted native subgrade

OSU Extension
Fact Sheet

• http://extension.oregonstate.edu

Click here

OSU Extension’s Stormwater Solutions
Additional Resources
• Standard Details LID 5.XX
• http://extension.oregonstate.edu/stormwater/swamp-lid-details

Click here

Siting Criteria for Porous Pavements
• Porous Pavement Siting Criteria:
http://extension.oregonstate.edu/stormwater/porous-pavement-1
• NOT on expansive soils

Design Criteria
1.
2.
3.

1.
2.

Manage stormwater = hydrologic design
Support traffic loads = structural design
Prevent clogging
• Siting
• Pavement mix design/specifications
• Construction
• Maintenance
Figure out how to get Criteria 1 & 2, even if we can’t prevent
clogging = Belt & suspenders approach
Avoid UICs

Prevent Clogging – Siting
Hydraulic Isolation of the Surface
Oops! Clogging
(Construction)

Ridge

Impe
rviou
s
Asph
alt
• Must be “hydraulically isolated”. A source control term that means
runoff from some other area should not flow onto porous pavement.


Impervious
Asphalt
Courtesy of Cahill Associates

P
o
r
u
s
P
a
v
e
m
e
n
t


s
rviou
Impe te
re
Conc

Water Quantity

Impervious
Concrete

P
o
r
u
s
P
a
v
e
m
e
n
t

Relationship of water quality storm
and sediment transport/scouring
• The most frequent storm (predicted as the 6-month frequency storm,
but probably happens more often)
• small to very small sized storm,
• expected to scour pollutants off a runoff generating surface.
• Conclusion: Hydraulic isolation is KEY to low maintenance porous
pavements!

More Clogging Design Considerations
• Pavements should still be sloped at a minimum of 1% (1/8” drop per
horizontal foot) in case they clog
• Belt and suspenders approach to clogging

Not a UIC

• This configuration was used at Pringle Creek without being
considered a UIC, should it clog.

Design Considerations

• When porous pavement is installed next to impervious pavement,
install a liner the depth of the porous pavement to block flows that
could undermine the structural stability of the impervious pavement.

Design Considerations
Soil Animals
• Pringle Creek had gophers
digging holes through the
base rock of their pervious
concrete!
• Consider durable, nonpolluting screen or raised or
flushed curbs on edges

Extra rock storage
• Some projects infiltrate roof runoff below the pavement.

You MAY have created a UIC if…
•
•

The facility infiltrates &
You’re using a perforated pipe underground

UIC

Not a UIC

Porous Pavement
Hydrologic Model
• http://extension.oregonstate.edu/stormwater/porous-pavement-calculator
• Excel model good to determine depth of base rock needed to store
desired storm depth until it can infiltrate
• MUST compare this against depth of base rock needed for structural
stability (from your geotechnical engineer)
• Rock needed = greater of these two criteria

Simple Hydrologic Design Case Study
Assume:
• City of Turner
• All stormwater must be kept on site
• Protection from stream scouring desired = infiltrate the 2-year
design storm
• Hydraulic isolation
• Not in a flooded area

Hydrologic Design in Excel Calculator
Post-developed Model

Step 1: Enter rainfall depth.

• Infiltrating the 2-year, 24-hour storm is predicted to meet detention
requirements (attenuate flows) and protect streams.
• This is a rule of thumb only and varies by watershed from between 18month and 2.5-year frequency interval.

Working on Post-developed Worksheet
Step 1: Enter rainfall depth.

• NOOA Isopluvial
Map
• 2-year frequency
rainfall in tenths of
an inch

Step 2. Enter pavement area [sf].

Step 3: Enter storage rock area.

• For now, we’ll assume a simple configuration where we’re only
managing rainfall, so the default setting is OK.
• Storage rock area will fill in automatically by default.

Step 4. Enter runoff coefficient.

• There's no runoff from porous pavement usually, but for modeling,
we assume that an area of impervious pavement is draining to the
same size area of native soil underlying it, so we enter 0.9 - 0.98 for
imp surface (the C in Q=CIA rational method of predicting runoff)
• You don’t need to change this, but it’s here for those designers who
may want to.

Step 5. Enter native soil design infiltration rate

• Enter after performing infiltration testing in the soil and at the depth
where the porous pavement will be installed.

Porous Pavement
Infiltration Testing

Proposed Conditions

Existing Conditions (with Proposed Shown)

Prepare the Hole
1.
2.

Use a post hole digger to the proposed facility bottom elevation.
Stick a pencil, pin, or nail in the side of the hole at a height from the bottom
that will equal the maximum ponding depth (as predicted by the model,
which we’ll see later)

Prepare the Hole
3.
4.
5.

If in clay soils, scrape the sides of the hole a little
Clean out any loose material in bottom
Set up your field log on a piece of paper.

Time
[min]
0.00

[sec]
0.00

Dist between
pencil and top
of water
[in]
0.00

Measuring Water Drop
6.
7.
8.

If in clay soils, place a few inches of clean rock in the bottom
Fill the hole gently with water up to the bottom of the pin
Start a stopwatch right away!

9.

Wait until the water drops a little bit. If it's dropping fast, then you'll
want to make measurements more often than if it's dropping slow.
10. Log the time in min and seconds on a piece of paper and right
away…
11. measure the drop in water
& log it

Logging Data

Time
[min]
0.00
10

[sec]
0.00
52

Dist between
pencil and top
of water
[in]
0.00
1/4”

12. Log the time & water drop a few times before the hole empties.

Logging Data

Time
[min]
0.00
10
19
32

[sec]
0.00
52
15
34

Dist between
pencil and top
of water
[in]
0.00
3”
5-1/4”
7-1/2”

13. Before hole empties, repeat steps 7 – 12 two more times (i.e. refill
hole again and measure water drop).

14. Use the slowest rate you tested. (It should be somewhere in the last
round of refilling.)

Yellow cells are what you
collected in the field

Calculate Your Infiltration Rate

Use this rate. (Factor of Safety
optional.)

Confirm Vertical Separation
15. When you’re all done and you have your field tested infiltration
rate…
dig another 3’ down to look for your seasonal high water table. If
you don’t hit fragipan or bedrock in 2’ below, this location will work
if it actually infiltrated water.
This whole process can take around 4 - 8 hours from start to finish, not
counting presoaking.

What About Presoaking?
• Presoaking is a holdover from EPA guidance on soil infiltration testing
for septic systems, which are constantly wet because we’re
constantly adding liquids to the field.
• Probably not necessary, especially if you’re testing during the wet
season and/or following our guidance to fill the hole 3 times and take
the lowest rate.
• Regardless, presoaking is often required by jurisdictions and will only
make the design more conservative/safe. (In clay soils, presoaking can
really drive up costs!)

Step 6: Enter Storage Rock (i.e. Base Rock) Depth

• Ask your geotechnical engineer for a porous pavement section
recommendation on wet, uncompacted native subgrade for your traffic
loading (usually H-20, 16,000 lb/wheel)
• Most of the porous asphalt projects I’ve worked on in clay soils in Oregon
only needed 12” of rock for structural stability, so try that out as a first
guess.
• Many pervious concrete projects in clay soils are 6” of concrete on 6” of
base rock.

Step 7: Enter void ratio of
storage rock (i.e. base rock)

• Most open-graded rock has a 40% void ratio. You should be able to get
this from your rock supplier.

How to Test Void Ratio Yourself

Step 8: Enter overflow elevation above
bottom of storage rock

• Enter the depth of water allowed to pond by whatever overflow
control structure you employ.
• If there is no large storm overflow control structure, this value equals
the depth of the storage rock that can pond.
• Let’s try the simplest design first – no overflow.

Determining Ponding Depth
Level Sites

Sloped Sites

• Facility bottom must be flat.
• Yep, on sloped sites the cost of rock and excavation would make this
prohibitive, but many cost effective projects step these flat beds up
the hill, carving them into the existing contours to reduced
excavation, and use underground check dams to hold back the water.

Using Overflow Control

• Pipes need cover, so designers are inclined to put the perforated pipe
at the bottom of the facility.
• With no controls, ponding depth = 0! Nothing! Nada!
• Big waste of your money and will not help meet TMDLs.

Using Overflow Control

• Add a control structure with a raised outlet pipe to get ponding.
• Use an internal weir when the outlet pipe (shown on the right of the catch
basin) cannot meet cover requirements.
• CAUTION USING WEIRS: Make sure maintenance staff can still fit their
cleaning equipment into both sides of the weir!

Step 9: Check that Outflow Elevation doesn’t
exceed Storage Rock Depth

• Is this is FALSE, you’ve created a physically impossible situation.
Increase Depth of Storage Rock or decrease the Overflow Elevation
Above Bottom of Storage Rock.
• Design pavement so water doesn't overflow out the pavement
surface.

Step 10: Analyze Calculated Values -- Ponding

• From the first test, we see that during the 2-year design storm the
highest level in the rock (even with only 40% void ratio) that the water
reaches is 1.36 inches.
• The base rock empties out in 30 hours and is ready for the next storm.
• Another storm dropping 4.8 inches tomorrow could be stored in the
base rock voids remaining without infiltration.

Conclusion
For
• City of Turner
• All stormwater must be kept on site
• Protection from stream scouring desired = infiltrate the 2-year design
storm
• Hydraulic isolation
• Not in a flooded area
Porous pavement IS SUITABLE for this site.

Construction
Stabilize uphill areas

• Put erosion control measures into place, especially uphill from
pavement area.
• Soils must be hydraulically isolated during construction! No run on
from other areas or soil could clog!

Construction
Protect infiltration area from compaction

Photo credit: Rob Emanual

• Protect infiltration area from compaction throughout the
construction process.
• Excavate from the side

Construction
Protect infiltration area from compaction
Porous Pavement
Parking Lot

18” deep haul
road was
installed at
start of project

Building

• Lay down a haul road of 3 – 6” diameter rock to drive over. This significantly
reduces (spreads out) the wheel load on the native soil.
• 18” deep should work, but check with your geotechnical engineer.

Great Construction Guidance
especially in clay soils
• Much more detailed info is here:
http://www.psp.wa.gov/LID_manual.php

Construction
Install geotextile

•
•
•
•

Lay down geotextile fabric.
Keep it clean.
Overlap it at least 12”.
Run it up the sides of the excavation.

Construction
Prepare rock

Base rock
Choker course
• Get the rock delivered clean, usually 2% wash loss.
• Then, hose off rock on-site to a very clean standard (0.5% wash loss).
A little rock dust if OK.
• This will prevent clogging at the geotextile/rock interface.

Construction
Place rock

• Place the rock in lifts compacting lightly.
• Do NOT use vibratory equipment.
• Keep construction equipment off the bare soil by backing over area.
Dump first lift that you will then back over, dumping as you go.

Construction
A note about the choker course
• If installing porous asphalt, choker course will be a smaller, opengraded rock that chokes the larger rock below it, allowing you to roll
the asphalt out without getting waves in the pavement.
• This rock must be clean, too.
• Don’t make this deeper than it needs to be to lock larger rock in place
or this layer could start to roll, too.
• This course is not needed for porous concrete, which is not installed
with a roller.

Construction
Place surface and cut geotextile
• Place surface as directed by specifications.
• Cut geotextile from sides of trench.

Construction
Delineate parking

• Striping is OK. A small area of surface can be impervious. Use ODOT
Standard Spec water based paint.
• Different color pavers
• Wheel stops
• Plastic dots

Maintenance
• Vacuum trucks: hard to get suction on a pavement that’s open to the
air. Works when pavement is clogged.
• Leaf blowers: blow air across pavement, not down
• Pressure washer: direct water across pavement, not down

Maintenance
• Here’s a cool product: Storm-crete.com
• If it gets clogged, pick it up, flip it over, pressure wash directly
through the pours and put it back.

Maintenance
Will tree leaves clog my pavement?
• In 25-years of porous asphalt at the Morris Arboretum, they’ve have
had no clogging problem!

Courtesy of Cahill Associates

Cultural Practices
Clogging Prevention

Relative costs for Porous Pavements

$$$ Commercial
pavers

$$$ Flexible Pavements $$$ Grass-crete
(GrassPave )

Courtesy of MGH Associates

$$ Pervious
concrete

$$ Porous asphalt

$-$$ Homemade
pavers

$ Gravel

Cost Comparison to
Conventional Impervious Pavements
Regardless of type, porous pavement cost is offset by:
• Infrastructure typically needed for impervious pavements that’s not
needed for porous pavement: pipes, detention ponds, water
quality facilities, catch basins, manholes, and excavation
• Value added amenity
• Lower stormwater fees

Managing Runoff in Slowly Draining Soils
Bioretention
• Rain Gardens
• Stormwater Planters
• Green Streets or Private
Property

Types of Challenging Sites
Perceived
I have tight clay soils with no infiltration!

50%
infiltration

50% evaporation

100% rainfall
yearly avg
0.5% runoff

In Nature (in Western Oregon)
Clay Soils Infiltrate A Lot!

Siting Criteria for Infiltration
with Bioretention
• Choose the Right Rain Garden Decision Tree:
http://extension.oregonstate.edu/stormwater/choose-right-rain-garden
• Rain Garden & Stormwater Planter fact sheets:
http://extension.oregonstate.edu/stormwater/lid-fact-sheets

Clay Soils That Infiltrate Slowly
Design Solutions
If water is actually draining, you can:
• Make the rain garden bigger.
• Don’t make the rain garden deeper.

“Infiltration Rain Garden with Planting Soil”
• May be compost amended soils OR
• Bioretention soil mix

33

Rain Garden & Stormwater Planter
Excel Models
• http://extension.oregonstate.edu/stormwater/lid-infiltration-facilitycalculator-aka-rain-garden-calculator

Common Mistakes
“The Rock Burrito”
• Also, don’t try deepening with a rock trench below.

38

Choose the Right Testing Depth
• Depth depends on difference between existing and final grades as
well as type of rain garden!

Common Mistake
for “Rain Gardens with Planting Soil”
• Replacing or amending soil alone will not increase the infiltration rate
of the rain garden…

Amended soil

Native soil

Common Mistake
for “Rain Gardens with Planting Soil”
…unless you’re able to reach a different soil horizon.

Construction in Clay Soils
Porous Pavement & Infiltration Bioretention

31

Constructing Infiltration Facilities in Clay Soils
Protect Against Clogging
 Don’t let clay soils get exposed to rain

24

Construction in Clay Soils
Protect Against Compaction
 Compost amend soils if built with “shovels and friends”

24

Sediment Control for Sheet & Concentrated Flow
Wattles (are your friends!)

Managing Runoff Without Infiltration

Types of Constraints
Water Quality & Quantity Constraints
• Water Quality: When ground or surface waters may be degraded
• Water Quantity: When infiltrating water may cause a problem (i.e.
landslides, flooded basements) or cannot be infiltrated (high water table,
high bedrock or other shallow impermeable layer)

The “Go Anywhere” Lined Filtration Rain Garden

Lined on all sides with an
impermeable liner = “FlowThrough”
43

Lined/Filtration/Flow-Through
Stormwater Planters are Also Common

Use a Lined Filtration Facility when:
• Infiltration siting criteria cannot be met.
• Examples:
• Too close to structure
• Near sensitive area
• At the top of a steep slope
• Over an area with high seasonal groundwater, bedrock, or
fragipan (i.e. buried ash layer)

If at all possible, AVOID the Lined Filtration Rain
Garden because
• If you don’t significantly
reduce runoff volume leaving
the site, you’re not really
protecting water quality.
• Only delay runoff by 13
minutes*, so not sufficient to
meet flow control
requirements.

* Study on Portland’s standard of
managing the 10-year storm.

And, also because they’re EXPENSIVE!

43

…and prone to clogging

Don’t substitute a geotextile fabric
for this. It will probably clog.

43

Additional Construction Steps
for Lined Rain Gardens: Amend & Place Soils
 Place at a depth of 6” then
 Boot compact/light tamp or water compact
 Repeat until your soil is at the elevation you want

Stormwater Planter Excel Model
for No Infiltration
ENTER RAINFALL DEPTH
• Rainfall depth = 1 inch = Pollution reduction standard

for No Infiltration
ENTER INFILTRATION RATE
• Enter infiltration rate of Bioretention Soil Mix instead of “Native Soil
Infiltration Rate”
• Can test with ASTM D1557 Method B (85% compaction with boot compaction)
& ASTM D2434 (permeability testing)
(More Info at:
http://www.ecy.wa.gov/programs/wq/stormwater/bsmresultsguidelines.pdf )
OR
• Can assume to be 2 inch/hour (minimum) if using Portland’s standard mixes

for No Infiltration
ENTER ROCK TRENCH DEPTH
• May be anything that will keep the pipe covered.

LID Implementation Template
DRAFT

LID Implementation Template
DRAFT
• Download: http://greengirlpdx.com/Publications.htm#ImpGuide
• To become not a DRAFT someday when we get funding to facilitate a
Technical Advisory Committee.

Thank You!

Sustainable design isn’t about doing something neat, it’s about doing
something right.

Detention ponds are not LID or… why you must
reduce runoff volume to restore water quality
Postdeveloped
flow rate

Post-developed =< Pre-developed
Remember: Hydrologic
models used by
engineers grossly
overestimate this.

Predeveloped
flow rate

Detention ponds
are not low impact development

Post-developed > Pre-developed
Ponding begins

Detention ponds

Post-developed > Pre-developed
Ponding continues

Detention ponds

Rain stops
Ponding begins to empty

Detention ponds

Pre-developed flow out of pond
continues

Detention ponds

…and continues

Detention ponds

30 hours later, it’s ready for the
next storm.

2013 12-05 challenging sites-rvss

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