ML20151H159
| ML20151H159 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 04/12/1988 |
| From: | Booth D KING COUNTY, WA |
| To: | |
| Shared Package | |
| ML20151H012 | List:
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| References | |
| NUDOCS 8808010163 | |
| Download: ML20151H159 (6) | |
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( LAND-USE PLANNING IN THE MITIGATION OF SEISMIC HAZARD by Derek B. Booth King County Basin planning Seattle, Washington INTRODUCTION Geologic hazards can be either ignored, avoided, or prepared for. Land-use planning efforts seek to reject the alternative of ignorance. Of the remaining two choices, "planning" typically has favored avoidance over structural or engineering solutions. Yet the tools and procedures of planning can also be used to identify when and where more active measures should be applied within an existing zoning framework.
In the Puget Sound region, King County has adopted what is probably the most comprehensive code dealing specifically with seismic hazards on a site-by-site basis. The County's "Sensitive Areas ordinance", adopted in 1979, provided for the delineation of potential seismic hazard areas and the mo^hanism to require additional site-specific study and design for developments proposed in such areas. The
' ordinance also adopted equivalent regulations for landslide.
orosion, and coal-mine hazard areas in an effort to avoid the worst consequences of development in geologically hazardous areas. The King County Comprehensive plan, adopted in *985, rraffirmed the intent of the ordinance in two policies:
"E-308 In areas with severe seismic hazards, special building design and construction measures should be us"d to minimize the risk of structural damage, fire and injury to occupants, and to prevent post-seismic collapse.
'E-309 prior to development in severe seismic hazard areas, builders should conduct special studies to evaluate seismic risks and should use appropriate measures to reduce the risks."
From a planning standpoint, the focus of these policies are essentially reactive and reflect the prevailing local attitude towards seismic rish. Developments are conditioned or modified once proposed, but the underlying zoning limitations are not altered as a result of ha.1ard designation. This approach stands in contrast with the treatment afforded other types of geologic hazards, such as landsliding or coal mine subsidence, where the restrictions
{ in many cases are tantamount to a prohibition on any development. Seismic risk in the puget Scund region is generally perceived as a ha:ard that can be mitigated by l k '
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(' appropriate engineering techniques.
There are several components to any regulatory effort designed to mitigate a geologic hazard. These include:
--Definition of the hazard;
--Characterization of a hazardous set of conditions;
--Delineation of the hazard zones on a map;
--Screening of proposed development; and
--Review and conditioning of projects.
Because this procedure outlines King County's implementation of its seismic hazard policy, and because the principles should be generally applicable to any municipality's approach to these risks, the steps are described below in greater detail.
IIAZARD DEFINITION Seismic hazards come in a variety of forms. They include the "direct hazards", such as ground shaking, rupture, and failure (including landsliding and liquifaction); and the "indirect hazards", such as floods, fires, and tsunamis. Not all of these categories will be
' relevant concerns in all regions; because planning efforts typically lag at least one earthquake (or more) behind the empirical data, past experience is usually available to guide the choice of relevant concerns in a particular region.
In the puget Sound area, effects from the 1949 and 1965 earthquakes suggest that ground failure and the effects of direct shaking on buildings are of primary concern.
Landslides and evidence of liquifaction were reported in several localities as well.
CIIARACTERIZATION O_F_ llAZARDOUS CONDITIONS The identification of hazardous conditions and the !
delineation of their areal extent is guided first by the scale of the desired product. On a continental scale, the determining factors include tectonic province and the distribution of known or inferred earthquakes, i r r e s pe e r. i v e of the theoretical understanding of their occurrence. Within a region of "high" seismic risk, such as the Puget Sound area, the hazard zones will depend on the type of seismic hazards judged relevant. The direct effects of shaking are measured by the intensity of the earthquake, which in turn depends on both geographic and geologic factors. Indirect effects will also depend in pt.rt on the inLeasity, together with more specific requirements (e.g., tsunamis or floods
/' obviously require low elevations and proximity to water).
For a given earthquake, energy will radiate outwards from the focus, ideally producing concentric shells of ever-l l
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decreasing seismic effects. Yet any plot of earthquake damage after a single event will show variability in this simple pattern: regions where the damage, and thus the intensity, is as high as areas much closer to the epicenter, and sites where those effects appear anomalously low relative to their neighbors. Land-use planning, when applied to seismic hazards, is primarily the attempt to recognize and prepare for those areas where the intensity of a quake vill be anomalously high.
The conditions that will control the variability of earthquake-related damage include:
--proximity to active faults,
--soil type and soil conditions, l
--site inclination, and
--subsurface focusing of earthquake energy.
Any of these factors could in theory be made a part of ;
the basis for seismic :onation of an area (i.e. the '
discrimination of areas of differing seismic hazard or risk).
In practice, some of these determinants are more applicable or usable than others. In King County, only soil conditions and site inclination are used. Earthquakes here are 1 relatively deep-seated and no surface trace of active faults in this part of the puget Lowland have been identified, so
{ proximity to known faults is nowhere relevant. The modeling of earthquake focusing is neither complete nor universally :
accepted, especially prior to 1979 when the ordinance governing seismic hazard zones was created. In cor.trast, soll types have been long accepted as a primary determinant of earthquake damage, both from the amplification of earthquake energy passing through thick unconsolidated sediments and from the potential for liquifaction. Slope inclination reflects the potential for increased landsliding of incipiently unstable soil masses during and immediately following an earthquake, observed most recently during the 1965 event. The identification of landslide-prone areas is !
itself an exercise in multiple determinants, of which slope inclination is only one factor, tMPPING Of HAZARD ZONES Ideally, the representation of seismic hazard zones would combine the various factors that determine the potential level of the hazard. For a given region or sub-region, where the likelihood of an earthquake of a given magnitude was roughly constant across the entire area, that hazard level might be quantified by the maximum hori:ontal ground acceleration for a quake of a given energy release.
The resulting product would be contour map delineating '
( several such categories.
In practice, the data are rarely available to make such estimates, although such information is becoming rapidly more l
( available. Instead, a simple "good-bad" discrimination is made, typically on the basis of whether any of the i unfavorable factors is present at a site.
The source of data to identify those factors can be a major weakness of the final ha:ard map. Municipalities generally do not have the resources to create their own maps of seismically susceptible soils, and so they must rely on
' existing soils or geologic mapping. Typically these existing ,
maps were not specifically intended to identify seismically 4
hazardous soils; they may also lack adequate information on the depth of the deposit. So although an complete data ,
source would show and identify the known types of seismic '
hazards, including artificial fills, recent alluvial soils, i
low-density organic soils, thick unconsolidated deposits, areas of potential focusing, and landslide susceptibility, more commonly the information available consists of surface soil types and slope information only. ~
In spite of these deficiencies, the actual determinants of seismic response correlate fairly well with available information. Deep, unconsolidated deposits are most common beneath surfaces of alluvial sediment, which typically include areas of loose, organic soil as well. Saturation of i these sediments is also common. Steeper slopes correlate i fairly well with landslide hazard. Yet use of existing mapping may also identify areas where no credible seismic ha ard exists, such as shallow pockets of peat on an l undulating till surface or moderate-gradient hillsicpes underlain by competent bedrock.
SCREENING OF DEVELOPMENT PROPOSALS Once a map is prepared, a mechanism must be established '
to screen and divert affected development proposals from the standard permitting process. In King Ccunty, that authority s was created by the Sensitive Areas ordinance, which required t j
that virtually all proposals requiring a permit be checked i j
against the final map showing "hazardous" and "non-hazardous" areas. The process is quite straightforward; the location of (
- the project is checked on a 1
- 62,500 map of the hazard zone I t
1 by the intake permit technician (in the case of building permits) or lead planner (in the case of subdivisions or other large projects).
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! If the proJoct lands within the i
j hazard zone, it is referred to a geotechnical specialist for ;
further review and conditions. About 10% of the land area of ;
j the developing portions of the County (i.e. outside of the eastern tree farms and National Forests) is so categori:od.
i j BEVIEW AND CONDITION!No oE PROPOSALS i )
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Once a project has been identified as lying in a seismic ha:ard zone, the technical reviewer must choose among several options:
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--Because of the project, no concern is warranted (e.g.,
a kitchen'remodel or a pole-supported carport).
--Because of the location, no concern is warranted (e.g., not actually in the hazard zone because of mapping error or map-reading error).
--The project lies in a seismic hazard zone, but the seismicity is the least of the project's concerns (e.g., excessive depth to bearing soil or active landslide threat). This category is by far the most common in the seismic hazard zones in King county.
--The seismic hasard is in fact a significant concern for the project and will not be addressed by other, more pressing needs.
Assuming that authority has been established, a municipality will typically proceed in a similar fashion for either of the last two options, where conditions or requirements beyond the standard zoning and building codes are deemed necessary. The applicant will be directed to hire a professional consultant to design a solution, which will be reviewed (usually) and approved by the municipality.
For seismic hazards an King County, typical proposed mitigation have included subgrade replacement or improved site drainage. In most cases they represent engineering
( solutions to other, non-seismic problems at the site, which have the additional consequence of reducing the seismic hazard to a level equivalent to "non-hasardous" sites.
In onl:. a few cases is a seismic-specific structural solution deemed necessary. The need in these cases generally transcends the information on the seismic hazard mapping, because they depend on additional knowledge by either the municipality or the consulting engineer about the depth of unconsolidated deposits, the historical association of the site's vicinity with high earthquake damage, or the peculiar nature of the structure.
LIMITATIONS 0._E LAND-US_E PLANNING J_t! SEISMIC llAZARD MITIGATION Because the scope of a planning technique is ultimately a function of the supporting ordinances, its limits will depend on the specific municipality that applies it. In King County, recognition of seismic hazard zones is not an avenue to disallou development. The ordinance provided the means to croate a map identifying areas associated with higher-than-typical seismic risk and the ongoing authority to require additional projects.
study and mitigation of that risk for specific Yet no project has been denied exclusively for reasons of seismic risk in this area, i
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Seismic conation also has not been used in this area ta date as a factor in long-range land-use.
Only in one area, for an as-yet unadopted communuity plan, has a density reduction been proposed that takes seismic hazard into 1
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( account as one of several determining factors. Three reasons probably underlie this general inattention. First, the conditions that yield high seismic hazard also correlcte well with other, more immediate land-use constraints, such as '
flooding in alluvial valleys or landsliding on steep slopes.
Second, seismic hazards are widely believed to be adequately manageable using structural and engineering techniques.
Under such conditions, denial of property use is considered an extreme and indefensible approach. Even reduction of the density of development has not been considered a warranted step in light of this hazard alone. 7 Finally, the area has experienced no major earthquakes in almost 25 years. Their absence has fostered little i support for more extreme land-use controls in seismic hazard enes, because there is very little first-hand experience,
- either popular or professional, on the success of the less
,' extreme development restrictions and techniques applied to date. It will be interesting to see, at some future date, if our relative complacency has been warranted. '
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