Stormwater management for challenging sites. This slide show was used for a class presented on Dec 5, 2013 at the Rogue Valley Sewer Services in Central Point, OR.
6. 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)
7. Types of Site Constraints
Perceived: Slowly Draining Soils
…but, I have tight clay soils with no infiltration!
8. 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
10. 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.”
11. 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
12. 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
18. 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
28. Compost Amended Slopes
Washington DOT
• Great for keeping soil in place on steep slopes, too!
http://www.wsdot.wa.gov/Design/Roadside/SoilBioengineering.htm
32. Managing Rainfall
in Slowly Draining Soils
• Porous pavements are effective in soils draining as slow as 0.1
inches/hour
33. 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.
36. 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
38. OSU Extension’s Stormwater Solutions
Additional Resources
• Standard Details LID 5.XX
• http://extension.oregonstate.edu/stormwater/swamp-lid-details
Click here
39. Siting Criteria for Porous Pavements
• Porous Pavement Siting Criteria:
http://extension.oregonstate.edu/stormwater/porous-pavement-1
• NOT on expansive soils
41. 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
42. 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.
43. Prevent Clogging – Siting
Hydraulic Isolation of the Surface
Impervious
Asphalt
Courtesy of Cahill Associates
P
o
r
u
s
P
a
v
e
m
e
n
t
44. Prevent Clogging – Siting
Hydraulic Isolation of the Surface
s
rviou
Impe te
re
Conc
47. 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!
48. 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.
49. 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.
50. 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
52. You MAY have created a UIC if…
•
•
The facility infiltrates &
You’re using a perforated pipe underground
UIC
Not a UIC
53. 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
54. 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
56. 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.
57. Working on Post-developed Worksheet
Step 1: Enter rainfall depth.
• NOOA Isopluvial
Map
• 2-year frequency
rainfall in tenths of
an inch
59. 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.
60. 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.
61. 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.
65. Infiltration Testing
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)
66. Infiltration Testing
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
67. Infiltration Testing
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!
68. Infiltration Testing
Measuring Water Drop
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
72. Infiltration Testing
Measuring Water Drop
13. Before hole empties, repeat steps 7 – 12 two more times (i.e. refill
hole again and measure water drop).
74. Yellow cells are what you
collected in the field
Infiltration Testing
Calculate Your Infiltration Rate
Use this rate. (Factor of Safety
optional.)
75. Infiltration Testing
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.
76. Infiltration Testing
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!)
77. 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.
78. 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.
79. 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.
81. 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.
83. Determining Ponding Depth
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.
84. Determining Ponding Depth
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.
85. Determining Ponding Depth
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!
86. 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.
87. 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.
88. 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.
89. 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!
90. Construction
Protect infiltration area from compaction
Photo credit: Rob Emanual
• Protect infiltration area from compaction throughout the
construction process.
• Excavate from the side
91. 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.
94. 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.
95. 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.
96. 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.
97. Construction
Place surface and cut geotextile
• Place surface as directed by specifications.
• Cut geotextile from sides of trench.
98. 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
99. 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
100. 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.
101. 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
105. 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
106. Managing Runoff in Slowly Draining Soils
Bioretention
• Rain Gardens
• Stormwater Planters
• Green Streets or Private
Property
107. Types of Challenging Sites
Perceived
I have tight clay soils with no infiltration!
109. 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
110. 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.
111. “Infiltration Rain Garden with Planting Soil”
• May be compost amended soils OR
• Bioretention soil mix
33
114. Infiltration Testing
Choose the Right Testing Depth
• Depth depends on difference between existing and final grades as
well as type of rain garden!
115. 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
116. Common Mistake
for “Rain Gardens with Planting Soil”
…unless you’re able to reach a different soil horizon.
122. 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)
123. The “Go Anywhere” Lined Filtration Rain Garden
Lined on all sides with an
impermeable liner = “FlowThrough”
43
125. 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)
126. 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.
128. …and prone to clogging
Don’t substitute a geotextile fabric
for this. It will probably clog.
43
129. 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
130. Stormwater Planter Excel Model
for No Infiltration
ENTER RAINFALL DEPTH
• Rainfall depth = 1 inch = Pollution reduction standard
131. Stormwater Planter Excel Model
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
132. Stormwater Planter Excel Model
for No Infiltration
ENTER ROCK TRENCH DEPTH
• May be anything that will keep the pipe covered.
134. 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.
136. 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
137. Detention ponds
are not low impact development
Post-developed > Pre-developed
Ponding begins
138. Detention ponds
are not low impact development
Post-developed > Pre-developed
Ponding continues
139. Detention ponds
are not low impact development
Post-developed > Pre-developed
Ponding continues
145. Detention ponds
are not low impact development
30 hours later, it’s ready for the
next storm.
Notes de l'éditeur
Non-Structural:
Preventative
Restorative
Early planning
Structural:
Mitigative
Engineered/Designed
After the fact
“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, small-scale hydrologic controls to more closely mimic predevelopment hydrologic functions.”
0-10% Coverage: Streams are structurally sound with good biodiversity and habitat
11-25% Coverage: Streambanks become unstable, biodiversity is lost, & water quality is degraded
>25% Coverage: Stream life is lost & water quality is poor
Non-Structural:
Preventative
Restorative
Early planning
Structural:
Mitigative
Engineered/Designed
After the fact
Reminder: Let’s talk about cost savings
Many people think that you can just add some storage rock below the infiltration rain garden, but this just adds depth and won’t be empty in time for the next storm.
Wattles are cheap and versatile; they can be used to prevent erosion from concentrated flows down the middle of this facility, for sheet flows, or encircling an inlet or outlet. To be effective, they must be staked into the ground, and overlapped a few inches at the ends.