Stormwater Management on Challenging Sites

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

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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 ConstraintsReal: 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 ConstraintsPerceived: 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)

Sustainability for all the places between the buildings

Planning Design

Construction Maintenance

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, small-scale hydrologic controls to more closely mimic predevelopment hydrologic functions.”

Water Quality AFTER Development

Sediment (air particulates)Nutrients

FecesOther debris

Runoff volumes

Sediment/turbidityHydrocarbons

Heavy metals (particles & soluble)Other chemicalsRunoff volume

Sediment/turbidityfertilizerspesticidesherbicides

Runoff volume

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Water Balance BEFORE DevelopmentExample: WESTERN OREGON

25% base flow

(shallow infiltration)

50%

eva

pora

tion

0.5% runoff

100%

XX”

aver

age

annu

al ra

infa

ll

25%

gro

undw

ater

(dee

p in

filtr

ation

)

Water Balance BEFORE: Simplified

Water Balance AFTER DevelopmentExample: EVERYWHERE

redu

ced

evap

o-ra

tion

98% runoff

2% e

vapo

ratio

n

Redu

ced

infil

trati

on

100%

ave

rage

annu

al ra

infa

ll

No

infiltration

Water Balance AFTER: Simplified

Runoff Volume & Durationfrom a Watershed Perspective

• Additional volumes over pre-developed rates scour stream banks.

• Additional durations of flow impact habitat further.

Detention

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

Bioretention manages RUNOFF from a much larger area than itself

Porous asphalt at the Port of Portland manages RAINFALL

Managing Rainfall Anywhere

Doesn’t necessarily depend on:• Soils types• Setbacks

Minimize Impervious Pavement

Green Roofs

Ecoroof/Green Roof

Contained Planters (Over Impervious Area)

Contained Planters (Over Impervious Area)

Restored Soils

Restore the Soil: Lawn Areas

Restore the Soil: Garden Areas

Compost Amended SlopesWashington DOT

• Great for keeping soil in place on steep slopes, too!

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

Lots of great information atsoilsforsalmon.org

Tree Canopy

Tree Canopy

Managing Rainfallin Slowly Draining Soils

• Porous pavements are effective in soils draining as slow as 0.1 inches/hour

Porous PavementDefinition

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

Porous Pavement

Types

Courtesy of MGH Associates

Porous asphaltPerviousconcrete

Courtesy of Fortis Construction

Manufactured Permeable Pavers

Poured-in-place Permeable

Pavers

Grid pavements

Grass-crete

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 ExtensionFact Sheet

• http://extension.oregonstate.edu/stormwater/sites/default/files/Porous%20Pavement.pdf

Click here

OSU Extension’s Stormwater Solutions Additional Resources

• Standard Details LID 5.XX• http://extension.oregonstate.e

du/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

POROUS PAVEMENT DESIGN

Design Criteria

1. Manage stormwater = hydrologic design2. Support traffic loads = structural design3. Prevent clogging

• Siting• Pavement mix design/specifications• Construction• Maintenance

4. Figure out how to get Criteria 1 & 2, even if we can’t prevent clogging = Belt & suspenders approach

5. Avoid UICs

Prevent Clogging – SitingHydraulic Isolation of the Surface

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

Impervious Asphalt

RidgeOops! Clogging(Construction)

Prevent Clogging – SitingHydraulic Isolation of the Surface

Courtesy of Cahill Associates

Impervious Asphalt

Porous Pavement

Prevent Clogging – SitingHydraulic Isolation of the Surface

Impervious

Concrete

Water Quantity Hydraulic Isolation of the Surface

Impervious Concrete

Porous Pavement

Prevent Clogging – SitingHydraulic Isolation of the Surface

Relationship of water quality stormand 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

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

Not a UICNot a UIC

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 ConsiderationsSoil Animals

• Pringle Creek had gophers digging holes through the base rock of their pervious concrete!

• Consider durable, non-polluting 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 &The facility infiltrates &• You’re using a perforated pipe undergroundYou’re using a perforated pipe underground

Not a UICNot a UICUICUIC

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 CalculatorPost-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 18-

month and 2.5-year frequency interval.

Working on Post-developed WorksheetStep 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 PavementInfiltration Testing

Infiltration TestingProposed Conditions

Infiltration TestingExisting Conditions (with Proposed Shown)

Infiltration TestingPrepare the Hole

1. Use a post hole digger to the proposed facility bottom elevation. 2. 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)

Infiltration TestingPrepare the Hole

3. If in clay soils, scrape the sides of the hole a little4. Clean out any loose material in bottom5. Set up your field log on a piece of paper.

Time

Dist between pencil and top

of water[min] [sec] [in]0.00 0.00 0.00

               

Infiltration TestingMeasuring Water Drop

6. If in clay soils, place a few inches of clean rock in the bottom7. Fill the hole gently with water up to the bottom of the pin8. Start a stopwatch right away!

Infiltration TestingMeasuring 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

Infiltration TestingLogging Data

Time

Dist between pencil and top

of water[min] [sec] [in]0.00 0.00 0.0010  52 1/4”

          

Infiltration TestingMeasuring Water Drop

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

Infiltration TestingLogging Data

Time

Dist between pencil and top

of water[min] [sec] [in]0.00 0.00 0.0010  52 3” 19 15 5-1/4”32 34 7-1/2” 

Infiltration TestingMeasuring Water Drop

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

Infiltration TestingMeasuring Water Drop

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

Infiltration TestingCalculate Your Infiltration Rate

Use this rate. (Factor of Safety optional.)

Yellow cells are what you collected in the field

Infiltration TestingConfirm 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.

Infiltration TestingWhat 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 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.

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 abovebottom 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 DepthLevel Sites

Determining Ponding DepthSloped 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.

Determining Ponding DepthUsing 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.

Determining Ponding DepthUsing 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 areaPorous pavement IS SUITABLE for this site.

ConstructionStabilize 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!

ConstructionProtect infiltration area from compaction

• Protect infiltration area from compaction throughout the construction process.

• Excavate from the side

Photo credit: Rob Emanual

ConstructionProtect infiltration area from compaction

• 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.

18” deep haul road was

installed at start of project

Building

Porous Pavement Parking Lot

Great Construction Guidanceespecially in clay soils

• Much more detailed info is here: http://www.psp.wa.gov/LID_manual.php

ConstructionInstall geotextile

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

ConstructionPrepare rock

• 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.

Base rock Choker course

ConstructionPlace 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.

ConstructionA note about the choker course

• If installing porous asphalt, choker course will be a smaller, open-graded 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.

ConstructionPlace surface and cut geotextile

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

ConstructionDelineate 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.

MaintenanceWill 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 PracticesClogging Prevention

Cultural PracticesClogging Prevention

Relative costs for Porous Pavements

$-$$ Homemade pavers

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

$$$ Commercial pavers

$ GravelCourtesy of MGH Associates

$$ Porous asphalt$$ Perviousconcrete

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 SoilsBioretention

• Rain Gardens• Stormwater Planters• Green Streets or Private

Property

Types of Challenging SitesPerceived

I have tight clay soils with no infiltration!

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

50%

evap

ora

tion

0.5% runoff

100%

rain

fall

yearl

y a

vg

50%

infi

ltra

tion

Siting Criteria for Infiltrationwith 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 SlowlyDesign 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”

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• May be compost amended soils OR• Bioretention soil mix

Rain Garden & Stormwater PlanterExcel Models

• http://extension.oregonstate.edu/stormwater/lid-infiltration-facility-calculator-aka-rain-garden-calculator

Common Mistakes“The Rock Burrito”

38

• Also, don’t try deepening with a rock trench below.

Infiltration TestingChoose 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…

Native soil

Amended soil

Common Mistake for “Rain Gardens with Planting Soil”

…unless you’re able to reach a different soil horizon.

Construction in Clay SoilsPorous Pavement & Infiltration Bioretention

31

Constructing Infiltration Facilities in Clay SoilsProtect Against Clogging

Don’t let clay soils get exposed to rain

24

Construction in Clay SoilsProtect 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 ConstraintsWater 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

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Lined on all sides with an impermeable liner =

“Flow-Through”

Lined/Filtration/Flow-ThroughStormwater 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

43

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

clog.

Additional Construction Stepsfor 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 Modelfor No Infiltration

ENTER RAINFALL DEPTH• Rainfall depth = 1 inch = Pollution reduction standard

Stormwater Planter Excel Modelfor 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

Stormwater Planter Excel Modelfor No Infiltration

ENTER ROCK TRENCH DEPTH• May be anything that will keep the pipe covered.

LID Implementation TemplateDRAFT

LID Implementation TemplateDRAFT

• Download: http://greengirlpdx.com/Publications.htm#ImpGuide

• To become not a DRAFT someday when we get funding to facilitate a Technical Advisory Committee.

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

Thank You!

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

Pre-developed flow rate

Post-developed flow rate

Post-developed =< Pre-developed

Remember: Hydrologic models used by

engineers grossly overestimate this.

Detention pondsare not low impact development

Post-developed > Pre-developedPonding begins

Detention pondsare not low impact development

Post-developed > Pre-developedPonding continues

Detention pondsare not low impact development

Post-developed > Pre-developedPonding continues

Detention pondsare not low impact development

Rain stopsPonding begins to empty

Detention pondsare not low impact development

Pre-developed flow out of pond continues

Detention pondsare not low impact development

…and continues

Detention pondsare not low impact development

…and continues

Detention pondsare not low impact development

…and continues

Detention pondsare not low impact development

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