Seattle SDCI Tip #325 Enviornmentally Critical Areas CAM325

User Manual: CAM325

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Environmentally

Critical Areas:

Peat Settlement-Prone

Areas

Updated May 15, 2017

Peat settlement-prone environmentally critical areas

are areas of Seattle that contain substantial deposits

of peat-rich soils that are prone to settlement. These

areas receive protection under Seattle’s Environmen-

tally Critical Areas Ordinance (Seattle Municipal Code

25.09). If you are planning to develop property in a

peat settlement-prone area you should consult this Tip

to understand how these regulations may affect the

maintenance and redevelopment of the property.

PURPOSE OF ECA REGULATIONS

Environmentally Critical Area (ECA) regulations

provide protections for areas of Seattle that provide

significant habitat and environmental function or that

represent geologic hazards. The goal of these regula-

tions is to accommodate reasonable development in

the urbanized environment of Seattle while balancing

environmental and public safety concerns that arise in

these areas.

Peat-rich soils represent a potential geologic hazard

as they are highly compressible and are prone to set-

tlement (sinking of the ground surface) when loaded

by new structures and fill or when the groundwater

table is lowered. Peat is an accumulation of decaying

organic plant material that typically forms in wetland

environments. In Seattle, these deposits have formed

historically in low lying areas such as valleys and the

nearshore areas of lakes and their floodplains. As Se-

attle has developed, these areas were often filled and

now support development throughout the city.

Development in areas of peat-rich soils can thus im-

pact existing structures where development modifies

the groundwater table through temporary dewatering

(e.g. during construction), permanent dewatering

(e.g. basements that require intermittent or continuous

pumping), or the addition of new impervious surface

that prevents infiltration of stormwater.

The peat settlement-prone ECA designation identifies

areas of Seattle where development could have the

potential to cause settlement and provides develop-

ment standards to prevent new development from

causing settlement impacts on nearby properties.

ECA LOCATION

SDCI maintains maps of ECAs that are available to

the public through the SDCI website (www.seattle.

gov/dpd/toolsresources/) and at the Public Resource

Center (PRC), located on the 20th floor of Seattle

Municipal Tower at 700 Fifth Ave. in downtown Seattle.

The peat settlement-prone ECA designation is de-

signed to address area-wide impacts of new develop-

ment. Accordingly, the boundary of these areas will

not be modified based on project-specific geotechni-

cal reports.

Peat settlement-prone areas are categorized as Cat-

egory I or Category II areas. Category I areas repre-

sent a higher level of concern and are thus subject to

additional development standards.

DEVELOPMENT STANDARDS

The development standards for Peat ECAs are con-

tained in Seattle Municipal Code (SMC) 25.09.110.

Subsurface Development

New development below the annual high static

groundwater level (see clarification of terms) is

prohibited in Peat ECAs except for specific elements

detailed in SMC 25.09.110. Connections to a public

utility, such as drinking water or sewer, and required

structural components, such as shallow foundations

325

SDCI Tip #325—ECAs: Peat Settlement-Prone Areas page 2

LEGAL DISCLAIMER: This Tip should not be used as a substitute for codes and regulations. The applicant is responsible for compliance

with all code and rule requirements, whether or not described in this Tip.

or pilings, are specifically exempted. Applicants

proposing excavation more than 30 inches in depth

will be required to submit a geotechnical report that

includes testing to determine the annual high static

groundwater table on the site. Requirements for mea-

suring the annual high static groundwater level are

detailed in Director’s Rule (DR) 14-2008. The intent of

this regulation is to prevent the construction of base-

ments or other structures that may require permanent

groundwater pumping that could result in settlement.

Repair or alteration of an existing structure located

below the annual high static groundwater level may be

allowed if it does not increase the extent of the struc-

ture below the annual high static groundwater level.

Stormwater

In Category I Peat ECAs only, projects proposing new

impervious surface must provide an infiltration facility

to offset lost infiltration function. In limited circum-

stances, a soil amendment project may be substituted

to meet this requirement. Replaced or reconfigured

impervious surface does not have to be offset. In-

filtration facilities are stormwater facilities that allow

water to percolate into the underlying soil. Infiltration

facilities include rain gardens, infiltration trenches,

infiltration planters, or porous pavement. Descriptions

and requirements for designing infiltration facilities are

contained in the Flow Control Technical Requirements

Manual available at www.seattle.gov/sdci. Sizing for

these facilities to meet the requirements of the ECA

code is described in DR 14-2008.

Groundwater

Temporary construction dewatering and land-disturb-

ing activities in Peat ECAs may be limited or restricted

to prevent modification of the groundwater regime.

Groundwater collection systems (e.g., footing drains)

are also prohibited in Peat ECAs unless otherwise

required by law.

Parking, Height, and Floor Area Allowances

Where the high groundwater table makes a full story

of underground parking infeasible, parking require-

ments may be reduced in certain zones to the mini-

mum extent necessary to offset underground parking

potential lost due to these ECA regulations. Limited

additional height may be granted, floor area dedi-

cated to above-grade parking may be exempt, and

certain street-level development standards for parking

may be waived for development in commercial zones.

These allowances are detailed in SMC 23.47A.012

(Height), 23.47A.013 (Floor Area Ratio), and SMC

23.48.085 (Parking & Loading).

The determination regarding whether underground

parking is infeasible will be based on the geotechnical

report which will be required at the time of the Master

Use Permit applications.

Other Standards

The Director may waive compliance with some or all

of the peat ECA development standards for projects

in Category II peat settlement-prone areas if the ap-

plicant can demonstrate that the project has been

designed to avoid adverse impacts to off-site parcels

from peat settlement.

Small project waivers are not allowed in Peat ECAs.

CLARIFICATION OF TERMS

“Annual high static groundwater level” means the

highest elevation where the soil is saturated with the

main body of groundwater during any part of the year.

Links to electronic versions of SDCI Tips, Direc-

tor's Rules, and Forms are available on the "Tools

& Resources" page of our website at www.seattle.

gov/sdci. Paper copies of these documents are

available from our Public Resource Center, located

on the 20th floor of Seattle Municipal Tower at 700

Fifth Ave. in downtown Seattle, (206) 684-8467.

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