2012 LID Technical Guidance Manual for Puget Sound

Technical Guidance Manual for Puget Sound

December 2012

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Low Impact Development Technical Guidance Manual for Puget Sound

WSU Puyallup Research & Extension Center 2606 West Pioneer Puyallup, WA 98371 www.pierce.wsu.edu

December 2012

326 East D Street Tacoma, WA 98421 360.464.1232 [email protected]

Author and project co-lead: Curtis Hinman, Washington State University Extension Faculty Project co-lead: Bruce Wulkan, Puget Sound Partnership Design and layout: AHBL Illustrations: AHBL (except where noted) Research, facilitation and external review: VEDA Consulting

Publication No. PSP 2012-3


Acknowledgements Technical Advisory Committee Bruce Barker Catherine Benotto Mark Buehrer Art Castle Rich Geiger Kathy Gwilym Kas Kinkead Tim Kurtz Alice Lancaster Chris May David McDonald Matt Miller Ed O’Brien Tracy Tackett Bruce Wulkan Craig Young

MGS Webber Thompson 2020 Engineers Building Industry Association of WA Mason Conservation District SvR Design Cascade Design Collaborative City of Portland Herrera Kitsap County City of Seattle Associated Earth Sciences Department of Ecology City of Seattle Puget Sound Partnership Snohomish County

Contributors Michael Bledsoe Patrick Carey Jeff Cox Drena Donofrio Rick Gagliano Erica Guttman Robin Kirschbaum Andy Marks David McDonald Ed O’Brien Robert Pine David Smith Paul Thompson Brian Taylor

Pervious Concrete Inc. Hadj Designs Triad Associates Seattle Public Utilities PIN Foundations Washington State University Extension HDR Puget Sound Concrete Specification Council Seattle Public Utilities Department of Ecology Pine and Swallow Environmental Interlocking Concrete Pavement Institute Urban Forestry Services, Inc. AMEC

Funding Portions of this project were produced with support from the Puget Sound Partnership. Washington State University Extension contributed in-kind support for this project.


Table of Contents Glossary ........................................................................................................................................................................................................................................i 1. Introduction ..............................................................................................................................................................................................................................1 1.1 1.2 1.3 1.4

Puget Sound Hydrology..................................................................................................................................................................................2 Impacts of Urbanization.................................................................................................................................................................................2 Current Stormwater Management.........................................................................................................................................................9 Low Impact Development..........................................................................................................................................................................10

2. Site Assessment .....................................................................................................................................................................................................................17 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10

Stormwater Site Plans..................................................................................................................................................................................18 Soil and Subsurface Hydrology Characterization.....................................................................................................................22 Hydrologic Patterns and Features.........................................................................................................................................................27 Native Forest and Soil Protection Areas.......................................................................................................................................27 Wetlands .................................................................................................................................................................................28 Riparian Management Areas (RMA)...................................................................................................................................................29 Streams ..................................................................................................................................................................................... 30 Floodplains ....................................................................................................................................................................................30 Sub-basin Delineation....................................................................................................................................................................................31 Site Mapping Process.............................................................................................................................................................................32

3. Site Planning & Layout .................................................................................................................................................................................................35 3.1 3.2 3.3 3.4

Urban Redevelopment and Infill..............................................................................................................................................................37 New Suburban Development...................................................................................................................................................................47 Commercial Development...........................................................................................................................................................................69 Road Crossings....................................................................................................................................................................................................72

4. Vegetation & Soil Protection & Reforestation .......................................................................................................................73 4.1 4.2 4.3

Native Vegetation and Soil Protection................................................................................................................................................75 Re-establishing Native Vegetation.......................................................................................................................................................80 Maintenance of Protected Areas.............................................................................................................................................................82

5. Precision Site Preparation, Construction & Inspection of LID Facilities ......................................83 5.1 5.2 5.3 5.4

Precision Site Preparation...........................................................................................................................................................................84 Techniques to Minimize Site Disturbance..............................................................................................................................86 Inspection of LID Facilities...........................................................................................................................................................................86 Construction Sequencing of LID Facilities.........................................................................................................................................90

Table of Contents, cont. 6. Integrated Management Practices ......................................................................................................................................................97 6.1 6.2 6.3 6.4 6.5 6.6 6.7

Bioretention ...........................................................................................................................................................................98 Amending Construction Site Soils....................................................................................................................................................149 Permeable Pavement..................................................................................................................................................................................161 Urban and Suburban Trees....................................................................................................................................................................203 Vegetated Roofs..............................................................................................................................................................................................217 Minimal Excavation Foundation Systems......................................................................................................................................227 Roof Rainwater Collection Systems...................................................................................................................................................233

7. Washington State Department of Ecology Low Impact Development Design & Flow Modeling Guidance ................................................................................................................................................................................................243 PART 1: Guidance for ModelinG with the current wwhM 3..........................................................................................................................244 7.1 Bioretention .....................................................................................................................................................................244 7.2 Soil Quality and Depth..................................................................................................................................................................................246 7.3 Permeable Pavement...................................................................................................................................................................................246 7.4 Tree Retention and Planting..................................................................................................................................................................251 7.5 Vegetated Roofs...............................................................................................................................................................................................252 7.6 Minimal Excavation Foundations........................................................................................................................................................253 7.7 Rainwater Harvesting...................................................................................................................................................................................253 7.8 Dispersion ...............................................................................................................................................................................................253 PART 2: SuMMary of wwhM 2012 repreSentation of lid practiceS...............................................................................................254 7.9 Bioretention .....................................................................................................................................................................254 7.10 Post-Construction Soil Quality and Depth......................................................................................................................................255 7.11 Permeable Pavement...................................................................................................................................................................................255 7.12 Newly Planted and Retained Trees.................................................................................................................................................................255 7.13 Vegetated Roofs...............................................................................................................................................................................................255 7.14 Minimal Excavation Foundations........................................................................................................................................................255 7.15 Full Dispersion....................................................................................................................................................................................................255 7.16 Rainwater Harvesting...................................................................................................................................................................................255 PART 3: lid SizinG tool for weStern waShinGton lowlandS (GSi-calc)...................................................................................256 7.18 GSI-Calc Applicability..................................................................................................................................................................................256 7.19 GSI-Calc Development................................................................................................................................................................................258 7.20 GSI BMPs and Design Requirements..............................................................................................................................................259 7.21 Limitations and Future Updates............................................................................................................................................................260

Table of Contents, cont. Appendix 1 - Bioretention Plant List .............................................................................................................................................261 Appendix 2 - Street Tree List ...................................................................................................................................................................283 Appendix 3 - Laboratory Procedures for Determining Bioretention Soil Mix Saturated Hydraulic Conductivity .......................................................................................299 Appendix 4 - Maintenance of Low Impact Development Facilities ...................................................301 References ...............................................................................................................................................................................................................................330


Manual Intent The purpose of this manual is to provide stormwater managers and site designers with a common understanding of LID goals, objectives, specifications for individual practices, and flow reduction and water quality treatment credits that are applicable to the Puget Sound region. In addition to the guidelines for specific practices, this manual provides research and data related to those practices to help managers and designers make informed decisions when adapting LID applications to their jurisdictions. Low impact development is a relatively new and evolving stormwater management approach; accordingly, this document will evolve and be periodically updated as additional research becomes available and professionals in the region gain more practical experience. This is a technical manual and the information provided is targeted for engineers, planners, landscape architects, technical staff to policy makers, and developers.

Manual Organization Chapter one of the manual sets the context for the LID approach with an introduction to Puget Sound lowland hydrology and the effects of urban development on streams, wetlands, and Puget Sound. Chapter one also establishes the goals and objectives for LID in the context of the reissued municipal NPDES general stormwater permits for western Washington and aligns with the 2012 Stormwater Management Manual for Western Washington. Chapters on site assessment, planning and layout, vegetation protection, and precision clearing and grading follow chapter one and emphasize the importance of planning and protecting native vegetation and soils in the LID approach. Chapter six provides general guidance for seven integrated management practices (IMPs) as well as detailed construction and material specifications for many of the IMPs. Chapter seven provides the flow credits in the Western Washington Hydrology Model (or equivalent continuous simulation model) that will allow engineers to eliminate or reduce the size of conventional flow control facilities when using LID practices. Finally, several appendices include lists of plants appropriate for LID applications and guidelines for maintenance. Bolded words within the text of the manual are defined in the glossary of terms.


Glossary Advection:

The transfer or change of a property of the atmosphere (e.g., humidity) by the horizontal movement of a mass of fluid (e.g., air current).

Allelopathic:

The suppression of growth of one plant species as a result of the release of a toxic substance by another plant species.

Alluvium:

Unconsolidated clay, silt, sand, or gravel deposited by running water in the bed of a stream or on its flood plain.

Ammonification:

The process in which organic forms of nitrogen (e.g., nitrogen present in dead plant material compounds) are converted to ammonium (NH4+) by decomposing bacteria.

AASHTO H-20:

The load representing a truck used in design of highways and bridges. The basic design truck is a single unit weighing 40 kips. A kip (often called a kilopound) represents 1,000 pound-force. The subsequent HS20 designation represents higher loads typical of tractor-semi-trailer combinations.

Bankful discharge:

The stream discharge that fills the channel to the top of the banks and just begins to spread onto the floodplain. Bankful discharge occurs on average 1-1.5 years in undisturbed watersheds and is the flow that primarily controls the shape and form in natural channels.

Bedload:

Sediment particles that are transported as a result of shear stress created by flowing water, and which move along, and are in frequent contact with, the streambed.

Bioretention cells:

Shallow depressions accepting stormwater from small contributing areas with plants and a soil media designed to provide a specific saturated hydraulic conductivity and pollutant removal characteristics and support healthy plants. A variety of plants are used in bioretention areas, including trees, shrubs, grasses, and/or other herbaceous plants. Bioretention cells may or may not have an under-drain and are not designed as a conveyance system.

Bioretention swales:

Incorporate the same design features as bioretention cells; however, bioretention swales are designed as part of a conveyance system and have relatively gentle side slopes and flow depths that are generally less than 12 inches.

Biotic integrity:

The condition where the biologic or living community of an aquatic or terrestrial system is unimpaired and the compliment of species diversity and richness expected for that system is present.

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i

Bole:

The trunk of a tree.

California Bearing Ratio:

A test using a plunger of a specific area to penetrate a soil sample to determine the load bearing strength of a road subgrade.

Crown projection:

The perimeter of a tree’s crown (outer most extent of the branches and foliage) projected vertically to the ground.

Cation exchange capacity:

The amount of exchangeable cations that a soil can adsorb at pH 7.0 expressed in terms of milliequivalents per 100 grams of soil (me/100 g).

Compost maturity:

A term used to define the effect that compost has on plant growth. Mature compost will enhance plant growth; immature compost can inhibit plant growth.

Compost stability:

The level of microbial activity in compost that is measured by the amount of carbon dioxide produced by a sample in a sealed container over a given period of time.

Critical shear stress:

Lift and drag forces that move sediment particles. The forces are created as faster moving water flows past slower water.

Denitrification:

The reduction of nitrate (commonly by bacteria) to di-nitrogen gas.

Desorb:

To remove (a sorbed substance) by the reverse of adsorption or absorption.

Diurnal oxygen fluctuations:

The fluctuation in dissolved oxygen in water as photosynthetic activity increases during the day and decreases during the night.

Exfiltration:

The movement of soil water from an infiltration IMP to the surrounding soil.

Endocrine disruptors:

Substances that stop the production or block the transmission of hormones in the body.

Effective impervious area (EIA): The subset of total impervious area that is hydrologically connected via sheet flow or discrete conveyance to a drainage system or receiving body of water. Washington State Department of Ecology considers impervious areas in residential development to be ineffective if the runoff is dispersed through at least 100 feet of native vegetation using approved dispersion techniques. Evapotranspiration:

The collective term for the processes of water returning to the atmosphere via intercepted and evaporation from plant surfaces and transpiration through plant leaves.

Exudates:

Substances exuded from plant roots that can alter the chemical, physical and biological structure of the surrounding soil.

Friable:

The soil property of consistence describing the resistance of material to deformation or rupture. Consistence refers to the degree of cohesion or adhesion of the soil mass and is strongly affected by the moisture content of the soil. A friable soil is easily broken apart.

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glossary

Gravimetric sampling:

Methods used the quantitative determination of an analyte based on the mass of a solid. Isolation of the analyte for measurement is usually achieved by precipitation, volatilization or drying.

Hydrologically functional landscape:

A term used to describe a design approach for the built environment that attempts to more closely mimic the overland and subsurface flow, infiltration, storage, evapotranspiration, and time of concentration characteristic of the native landscape of the area.

Hydroperiod:

The seasonal occurrence of flooding and/or soil saturation that encompasses the depth, frequency, duration, and seasonal pattern of inundation.

In-line bioretention facility:

A bioretention area that has a separate inlet and outlet.

Invert:

The lowest point on the inside of a sewer or other conduit.

Los Angeles (LA) Abrasion:

The standard L.A. abrasion test subjects a coarse aggregate sample (retained on the No. 12 or 1.70 mm sieve) to abrasion, impact, and grinding in a rotating steel drum containing a specified number of steel spheres. After being subjected to the rotating drum, the weight of aggregate that is retained on a No. 12 (1.70 mm) sieve is subtracted from the original weight to obtain a percentage of the total aggregate weight that has broken down and passed through the No. 12 (1.70 mm) sieve. Therefore, an L.A. abrasion loss value of 40 indicates that 40 percent of the original sample passed through the No. 12 (1.70 mm) sieve. The standard Los Angeles abrasion test is: AASHTO T 96 or ASTM C 131: Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine.

Liquefaction:

The temporary transformation of a soil mass of soil or sediment into a fluid mass. Liquefaction occurs when the cohesion of particles in the soil or sediment is lost.

Mycorrhizal:

The symbiotic association of the mycelium of a fungus with the roots of a seed plant.

Native Soil and vegetation protection areas:

Areas covered by vegetation that will not be subject to land disturbing activity or compaction (clearing, grading, storage, stockpiling, vehicles, etc.) that are fenced and continuously protected from impacts throughout the construction process and protected post-construction through zoning or other legal agreement.

Nitrification:

The process in which ammonium is converted to nitrite and then nitrate by specialized bacteria.

Off-line bioretention facility:

A bioretention area where water enters and exits through the same location.

Phytoremediation:

The utilization of vascular plants, algae and fungi to control, breakdown, or remove wastes, or to encourage degradation of contaminants in the rhizosphere (the region surrounding the root of the plant).

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Potholing:

Excavating a hole in the ground to observe buried utilities or facilities. Potholes are typically excavated using a backhoe or by hand, depending on the environment.

Precision site preparation:

The process where mass clearing and grading (that increases the probability of high sediment loads released from the property, excessive soil compaction and sediment management expense) is replaced with more targeted clearing and grading and sequencing that protects native soils and vegetation, minimizes exposure soil and reduces soil compaction.

Rain Garden:

A non-engineered, shallow landscape depression with native soil or a soil mix and plants that is designed to capture stormwater from small, adjacent contributing areas.

Saturated hydraulic conductivity:

The ability of a fluid to flow through a porous medium under saturated conditions and is determined by the size and shape of the pore spaces in the medium and their degree of interconnection and also by the viscosity of the fluid. Hydraulic conductivity can be expressed as the volume of fluid that will move in unit time under a unit hydraulic gradient through a unit area measured at right angles to the direction of flow.

Seral stage:

Any stage of development or series of changes occurring in the ecological succession of an ecosystem or plant community from a disturbed, un-vegetated state to a climax plant community.

Soil stratigraphy:

The sequence, spacing, composition, and spatial distribution of sedimentary deposits and soil strata (layers).

Soil bulk density:

The ratio of the mass of a given soil sample to the bulk volume of the sample.

Stage excursions:

A post-development departure, either higher or lower, from the water depth existing under a given set of conditions in the pre-development state.

Stemflow:

The portion of precipitation falling on a tree (or other plant) that is intercepted by the foliage and branches and flows along the stems and trunk of the tree and to the ground.

Time of concentration:

The time that surface runoff takes to reach the outlet of a sub-basin or drainage area from the most hydraulically distant point in that drainage area.

Threshold discharge area:

An onsite area draining to a single natural discharge location or multiple natural discharge locations that combine within one-quarter mile downstream (as determined by the shortest flow path).

Throughfall:

The portion of precipitation that falls within the tree crown perimeter and falls to the ground without bring intercepted by the foliage or branches or is intercepted and then drips from the foliage or branches to the ground.

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glossary

Total impervious area (TIA):

The total area of surfaces on a developed site that inhibit infiltration of stormwater. The surfaces include, but are not limited to, conventional asphalt or concrete roads, driveways, parking lots, sidewalks or alleys, and rooftops.

Transmissivity:

A term that relates to movement of water through an aquifer. Transmissivity is equal to the product of the aquifer’s permeability and thickness (m2/sec).

Tree crown dripline:

The outer most perimeter of a tree crown defined on the ground by the dripping of water vertically from the leaves of tree canopy perimeter.

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Commonly Used Acronyms AASHTO:

American Association of State Highway and Transportation Officials

ASTM:

American Society for Testing and Materials

BSM

Bioretention Soil Media

CEC:

Cation Exchange Capacity

CRZ:

Critical Root Zone

IMPs:

Integrated Management Practices

Ksat:

Saturated Hydraulic Conductivity

OGFC:

Open Graded Friction Courses

PIT:

Pilot Infiltration Test

SBSS:

Sand-based Structural Soils

SWMMWW:

Stormwater Management Manual for Western Washington

TMECC:

Test Methods for Examination of Composting and Compost

WWHM:

Western Washington Hydrologic Model

Metric Equivalents 1 centimeter

0.39 inches

1 meter

39.37 inches

1 millimeter

0.039 inches

1 hectare

2.27 acres

1 liter

4.23 gallons

1 cubic meter

31.31 cubic feet

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acronyms & metric equivalents

Introduction

1 CHAPTER

Puget Sound Hydrology Current Stormwater Management Impacts of Urbanization Low Impact Development Definition Low Impact Development Goals & Objectives Low Impact Development in the Watershed Context

1.1 1.2 1.3 1.4 1.5 1.6

1.1 Puget Sound Hydrology Native forests and prairies of the Puget Sound lowlands intercept, store, and slowly release and convey precipitation through complex pathways. Water budget studies of wet coniferous forests in western Washington, British Columbia, and the United Kingdom indicate that approximately 40 percent of the annual rainfall is intercepted by foliage and evaporated during the rainy season. Bauer and Mastin (1997) found that interception and evaporation from vegetation during the winter months (approximately 50 percent) far exceeded estimates for western Washington, and attributed the high rate to the large surface area provided by evergreen trees, relatively warm winter temperatures, and the advective evaporation of precipitation. Bidlake and Payne (2001) and Calder (1990) also found that the aerodynamically rough forest canopy and advection energy supported evaporation rates of intercepted precipitation that were higher than previously estimated radiationbased potential evapotranspiration.

“Water budget studies of wet coniferous forests in western Washington, British Columbia, and the United Kingdom indicate that approximately 40 percent of the annual rainfall is intercepted by foliage and evaporated during the rainy season.”

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1

1.2 Impacts of Urbanization

Native soils also play a critical role in storing and conveying Pacific Northwest (PNW) rainfall. Typically, 2-4 feet of soil, high in organic material and biologically active near the surface, overlays the subsurface geology. Solar radiation and air movement provide energy to evaporate surface soil moisture that contributes to the overall evapotranspiration component. Soil biota and organic matter chemically and physically bind mineral particles into stable aggregates that build soil structure, increase soil porosity, and provide 20-30 percent of active water storage by volume. Shallow subsurface flow (interflow) moves slowly down slope or down gradient and, depending on the soil characteristics, may take many hours, days or weeks to move through these upper soil layers to receiving waters. Depending on the underlying soil type and geology, 10-40 percent of the annual precipitation moves to deeper groundwater (Bauer and Mastin, 1997). For most storm events in the region, the gentle rainfall intensities are less than the combined capacity of native Puget Sound forests and soils to intercept and store the precipitation; as a result, overland flow does not occur or is minimal (Booth, Hartley and Jackson, 2002). Instead, the storm flow moves downslope below the surface at a much slower rate than overland flow and displaces antecedent, subsurface water in areas near streams, lakes and wetlands (Bauer and Mastin, 1997). The displaced soil water adjacent to water bodies contributes to stream flows or wetland and lake levels rather than the entire watershed.

figure 1.1 Satellite images of Puget Sound urbanization in 1970 and 1996. Dark color in the lowland areas indicates clearing of vegetation and development. Source: American Forests

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introduction

As storms and the wet season progress, available soil storage capacity declines and the saturated or contributing areas near receiving waters increase as does the response to storm events (Booth et al., 2002). Watershed hydrology is dynamic, and changes depending on numerous antecedent conditions and seasonality.

1.2 Impacts of Urbanization The conversion of the U.S. landscape to urban development is occurring rapidly. From 1954 to 1997 the urban land area grew from approximately 18.6 to 74 million acres, and during the latter part of that time period (1982 to 1997) the population grew by 15 percent while developed land increased by 34 percent, or 25 million acres. Analyses of 22 metropolitan areas revealed that 95 percent of building permits were on green field sites (EPA, 2006). The transition from a native landscape to a built environment increases the impervious surface coverage of roads, parking areas, sidewalks, rooftops, and landscaping. These changes reduce, disrupt or entirely eliminate native vegetation, upper soil layers, shallow depressions, and native drainage patterns

1.2 Impacts of Urbanization

1

PRE-DEVELOPMENT FOREST • During winter months much of the precipitation is intercepted by the forest canopy and evaporated while transpiration is relatively inactive. • Shallow subsurface flow (interflow) moves slowly down slope over many hours, days or weeks to receiving water.

PRECIPITATION

EVAPOTRANSPIRATION 40-50%

• Surface runoff is minimal. • As winter progresses, the interflow component of stream flow increases. • During the summer and fall, streams are maintained primarily by glacial melt water and/or groundwater flow.