FEMA 54 / March 1984

ElevatedResidentialStructures

Federal Emergency Management Agency

ElevatedResidentialStructures

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AcknowledgmentsMany people contributed valuable assistance to the prepara-tion of this manual. We wish to acknowlege especially theguidance provided by Melita Rodeck and, later, John Gam-bel, the Federal Emergency Management Agency's technicalrepresentatives in this work. John Gambel's advice wasparticularly valuable in determining the final content andform of this manual. In addition, this project would nothave been possible without the help of Richard W. Krimm,Assistant Associate Director of the Federal EmergencyManagement Agency's Office of Natural and TechnologicalHazards, who saw the importance of increasing architects'involvement in flood damage mitigation efforts. Finally,Ray Fox provided a wealth of useful advice in addition tohis technical services throughout the course of the project.Prepared by

The American Institute of Architects Foundation1735 New York Avenue, N.W.Washington, D.C. 20006

Charles R. Ince, Jr., PresidentEarle W. Kennett, Administrator, Research

Donald E. Geis, Program Director and Project ManagerKaren N. Smith, Administrative ManagerPaul K. McClure, Editor

Technical Consultants

Raymond R. FoxProfessor of Civil EngineeringThe George Washington UniversityWashington, D.C.

Mark RiebauAssistant Chief of Floodplain and Shoreland ManagementWisconsin Bureau of Water Regulation and ZoningMadison, Wisconsin

Cost Consultant

Daniel Mann Johnson & Mendenhall, Architects-EngineersWashington, D.C.Paul Brott, Vice PresidentErnest Posch, Estimator

Graphics and Book Design

Assarsson Design CompanyWashington, D.C.Allan G. Assarsson, PresidentMark P. Jarvinen, Layout and Graphic DesignJeffery Banner, Graphic Design

Photographs

Raymond R. Fox, Dames & Moore, pp. 64 and 124 andFigures 4.23 and 4.26; Federal Emergency ManagementAgency, p. 1 and Figure 4.1; U.S. Geological Survey,pp. vi, 122, and 123; U.S. Department of Housing andUrban Development, p. 118 and Figure 2.3; PhilipSchmidt, U.S. Department of Housing and Urban Devel-opment, Figure 2.7; National Park Service, pp. iv and112; Spencer Rogers, p. 18; Rosenthal Art Slides, Figures3.1 and 3.2; AIA Library, Figures 3.3 and 3.4; U.S.Army Corps of Engineers, pp. 3 and 115;Davis andAssociates, Figure 4.49; Pittsburgh City Planning Depart-ment, p. 4; PARNG Photo, p. 2; and James K. M. Cheng,p. 98.

Most of the photographs and design data in the Recent DesignExamples section were supplied by the designers of thebuildings shown there. All other photographs were takenby Donald E. Geis of The American Institute of ArchitectsFoundation.

Disclaimer

The statements contained in this manual are those of TheAmerican Institute of Architects Foundation and do notnecessarily reflect the views of the U.S. Government ingeneral or the Federal Emergency Management Agencyin particular. The U.S. Government, FEMA, and TheAmerican Institute of Architects Foundation make nowarranty, express or implied, and assume no responsibilityfor the accuracy or completeness of the information herein.

This manual was prepared under Contract No. EMC-C-0579with the Federal Emergency Management Agency.

The Design Studies section of the manual was developed onthe basis of background data and design concepts submittedfor the 1976 version of this manual by Zane Yost andAssociates, Bridgeport, Connecticut; KEF Corporation,Metairie, Louisiana; Keck and Keck Architects, Chicago;Duval/Johlic Architects-Planners, San Francisco; LouisianaState University, Department of Architecture; Rhode IslandSchool of Design; University of California at Los Angeles,School of Architecture and Urban Planning; and Universityof Miami, School of Architecture.

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Table of Contents

ACKNOWLEDGMENTS ii

PREFACE v

ENVIRONMENTAL AND REGULATORY FACTORS 1FLOODING AND THE BUILT ENVIRONMENT 1

Riverine Flooding El Coastal FloodingFLOODPLAIN MANAGEMENT 4

National Flood Insurance Program El Base Flood Elevations E A and V Zones

SITE ANALYSIS AND DESIGN 8SITE SELECTION AND ANALYSIS 9SITE DESIGN 13

Site Flooding Characteristics E Access and Egress E Vegetation El Flood WaterDrainage and Storage [1 Dune Protection

ARCHITECTURAL DESIGN EXAMPLES 18DESIGN STUDIES 22

Bridgeport El Charleston and Newport E San Francisco C] ChicagoAESTHETIC CONSIDERATIONS 35RECENT DESIGN EXAMPLES 45

Logan House El Summerwood on the Sound E Breakers Condominium ElCampus-by-the-Sea Facility E Starboard Village E Gull Point Condominiums

DESIGN AND CONSTRUCTION GUIDELINES 64FOUNDATIONS 65

Fill El Elevated Foundations E Shear Walls El Posts E Piles E Piers E BracingFRAMING CONSTRUCTION AND CONNECTIONS 80

Framing Methods E Floor Beams El Cantilevers El Concrete Flooring Systems EFloor Joists E Subflooring E Wall Sheathing and Bracing E1 Roof Connections

RELATED DESIGN CONSIDERATIONS 92Glass Protection El Utilities and Mechanical Equipment El Building Materials ElInsulation E Breakaway Walls El Retrofitting Existing Structures

COST ANALYSIS 98

RESOURCE MATERIALS 112GLOSSARY 113SOURCES OF DESIGN INFORMATION 116FEMA REGIONAL OFFICES 118STATE COORDINATING OFFICES FOR THE NFIP 120PERFORMANCE CRITERIA 125REFERENCES 136

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PrefaceWhenever possible, residential structures shouldnot be located in flood-prone areas. Flooding inthese areas is virtually assured at some point inthe future, bringing with it the potential forproperty damage-no matter how well a structureis designed-as well as danger to building occu-pants. However, it is not always possible toavoid flood-prone areas. This manual is fordesigners, developers, builders, and others whowish to build elevated residential structures inflood-prone areas prudently.

The readers of this manual are assumed to haveknowledge of conventional residential constructionpractice; the manual is limited to the special designissues confronted in elevated construction.

This is a revision of a manual of the same title pub-lished in 1976 by the Federal Insurance Admini-stration. This revision reflects changes since 1976in floodplain management techniques and regu-lations, improvements in construction materialsand practice, increases in construction costs, andadditions to the relevant literature. This revisionalso contains increased information on elevatingstructures in coastal areas, although all the tech-niques described here apply to both coastal andriverine areas unless otherwise stated.

A second document, published by the FederalEmergency Management Agency (FEMA), DesignGuidelines for Flood Damage Reduction, supple-ments this manual's discussion of elevated residen-tial structures with information on the full range ofother floodplain management strategies.

A third document, Design and ConstructionManual for Residential Buildings in Coastal HighHazard Areas, is published jointly by FEMA andthe U.S. Department of Housing and Urban Devel-opment. It provides structural engineering guide-lines and other information on designing structuresin coastal areas subject to severe wind and velocitywave forces. Structures in such areas should not bedesigned without consulting it.

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ENVIRONMENTAL AND REGULATORY FACTORS

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Flooding and theBuilt EnvironmentRivers and seacoasts have always been focal pointsfor development. Access to water has provideddrinking supplies and sanitation, an importantsource of energy, and a valuable part of the trans-portation system. Recreational opportunities andaesthetic enjoyment further stimulate watersidedevelopment.

This development pattern, however, leads to a con-flict between the natural and built environments.The need for direct access to water places humansettlements in low-lying areas that are subject toperiodic flooding by rivers and the sea. In theUnited States, more than six million dwellings anda large number of nonresidential buildings arecurrently located in the nation's 160 million acresof floodplains. Flooding of these floodplains isresponsible for more damage to the built environ-ment than any other type of natural disaster. Thetotal flood damage in 1978, for example, was anestimated $3.8 billion. The following year,Hurricane Frederic alone caused $1.8 billion indamages.

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RIVERINE FLOODING

Floods are part of the natural hydrologic process.Riverine flooding is associated with a river'swatershed, which is the natural drainage basin thatconveys water runoff from rain and melting snow.Water that is not absorbed by soil or vegetationseeks surface drainage lines, following local topo-graphy and creating rivers and other streams.Flooding results when flow of runoff is greaterthan the carrying capacity of watershed streams.

Riverine flooding usually involves a slow buildupof water and a gradual inundation of surroundingland. However, flash flooding, a quick and intenseoverflow with high water velocities, can result froma combination of steep slopes, a short drainagebasin, and a high proportion of surfaces imperviousto water and unable to absorb runoff.

In addition to the direct threat to buildings,development in riverine floodplains alters naturaltopography, modifying drainage patterns andusually increasing storm water runoff. Develop-ment also displaces much of the natural vegetationthat formerly absorbed water and decreases thepermeability of the soil by covering it with build-ings or with nonporous surfaces for roads, side-walks, and parking. The effect of these changes isto increase the severity of flooding throughout theriverine environment.

COASTAL FLOODING

Coastal flooding is generally due to severe ocean-based storm systems. Hurricanes, tropical storms,and extratropical storms such as "northeasters" arethe principal causes, with flooding occurring wheristorm tides are higher than the normal high tide,and are accompaniled by water moving at relativ elyhigh velocity and velocity wave action. Themaximum intensity of a storm tide occurs athigh tide, so storms that persist through severaltides are the most severe.

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The velocity and range of coastal floods vary inpart with the severity of the storm that inducesthem. The damaging effects of coastal floodingare caused by a combination of the higher waterlevels of the storm tide and the rain, winds, waves,erosion, and battering by debris.

The extent and nature of coastal flooding is alsorelated to physiographic features of the terrain andthe characteristics of the adjoining body of water.Pacific coastal areas are vulnerable principally toearthquakes, tsunamis (seismically induced tidalwaves) and other natural forces that can triggerexcessive erosion, mud slides, and flash flooding.Great Lakes coastal areas are subject to erosion andsevere winter storms. The Atlantic and Gulf Coastsare consistently exposed to the forces of hurri-canes, lesser tropical storms, and northeasters.

Coastal flooding is most frequent on the Atlanticand Gulf Coasts, which are made up of a successionof barrier islands, beaches, and dunes. Thesephysiographic elements are maintained in dynamicbalance as sand is moved by wind, waves, andocean currents. This self-replenishing beach-dunesystem takes the brunt of the force of stormsurges and helps buffer inland areas.

In coastal areas the removal of beach sand and theleveling of dunes, along with the construction ofseawalls, jetties and piers, are common practice.These can help destroy the shoreline's natural protec-tion system, exacerbating the impact of stormsurges and high winds.

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Floodplain ManagementThere have long been attempts to moderate theimpact of riverine flooding, with major federalefforts in the United States since 1936. Untilrecently, these efforts have been concentrated onflood control measures devised to reduce oreliminate flooding itself-chiefly dams, levees andsimilar structural works. Despite a number ofpositive results, these measures have not succeededin reducing flood damage significantly.

Since the mid-1960s, therefore, federal policieshave reflected a recognition that structural worksneed to be complemented by nonstructural mea-sures. Rather than trying solely to prevent floods,current floodplain management programs addressthe need to reduce the losses incurred wheninevitable flooding does happen.

Elevating residential structures above the reach offlood waters, the subject of this manual, is onlyone of several floodplain management techniquescurrently used to reduce flood damage. Forexample, construction is prohibited in criticalfloodplain areas (termed floodways) unless it hasbeen determined that construction will not in-crease flood levels elsewhere. Where buildings arealready located in these critical areas, they caneither be relocated out of the flood area, elevated,or floodproofed to reduce the damage they willsuffer in a flood. Buildings that are badly damagedby flooding can be razed or floodproofed ratherthan being restored to their original, vulnerablecondition. Vacant land in flood-prone areas can bepurchased by the local community and reserved forrecreation, farming, or other safe uses.

These and other floodplain management teclni-ques (discussed in Design Guidelines for FloodDamage Reduction, cited in the Preface) can beused in a coordinated way to respond to eachcommunity's various needs, resources, and floodhazards. Elevated residential structures, if used atsites appropriate for them, can be useful com-ponents of effective floodplain management.

NATIONAL FLOOD INSURANCE PROGRAM

The National Flood Insurance Program (NFIP) isthe federal government's principal administrativemechanism for reducing flood damage. Estab-lished by Congress in 1968, the NFIP is adminis-tered by the Federal Emergency ManagementAgency (FEMIA). The NFIP insures buildings andtheir contents in flood-prone areas, where conven-tional insurance had, prior to the NFIP, beengenerally unavailable.

The NFIP provides this insurance only in com-munities that agree to implement comprehensiveland-use planning and management to reduce thelikelihood of flood damage in their jurisdictions.Community response to this incentive generallyinvolves the adoption of zoning, building code, anddevelopment regulations that place various require-ments and restrictions on new construction and onsubstantial improvements to existing construction.

Aote that somne local gouernirnents hate adoptedcodes and zoning ordinances that are considerablymore restrictive than the ininilnu rns required byFEDORA. The result i's that familiarity uith designrequirements in one c mu nitv cannot be reliedon elsewhere.

The rate structure of the NFIP's insurance pre-miums reinforces the intent of these regulationsby charging higher insurance rates for buildingssubject to greater hazard. These insurance rates areset primarily on the basis of designated hazardzones and the elevation of the building or structurein relation to the level of flooding likely to occurin each zone. This differential rate structureprovides a significant financial incentive to locatebuildings in less hazardous zones or to increasebuildings' flood safety by elevating them higherthan the N FIP's minimum elevations.

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It is thus vital to be aware of the NFIP rate struc-ture, as well as local regulations, when siting anddesigning new development or substantial improve-ments to existing construction. This informationcan be obtained from local insurance agents, publicofficials, and regional FEMA offices.

BASE FLOOD ELEVATIONS (BFE's), A ZONES,AND V ZONES

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Figure 1.1. Flood Insurance Rate Map

ON POST-TYPE FOUNDATIONA Zones V Zones

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The NFIP and related local and state regulationsdefine likely flood levels on the basis of the "100-year" flood, which is the flood that has a onepercent chance of being equalled or exceededduring any given year. Over a 30-year period, thereis at least a 26 percent chance that this "base"flood will occur.

The base flood elevations (BFE's), or likely flood-ing levels, at different sites in a community duringthe 100-year flood are determined on the basis ofhistoric records, climatic patterns, and hydrologicand hydraulic data. A community's BFE's aremapped on flood insurance rate maps (FIRM's),which are provided by FEMA for use by localfloodplain managers and FEMA officials (seeFigure 1.1).

FIRM's generally show flood-prone areas as eitherA Zones or V Zones. Riverine flood-prone areasand coastal flood-prone areas subject to stormsurges with velocity waves of less than three feetduring the 100-year flood are generally classed as AZones. FEMA's design standards (see Figures 1.2and 1.3) for A Zones call for the top of a building'slowest floor (including basements) to be elevatedto or above the BFE. "Coastal high hazard areas"are shown on FIRM's as V Zones. The V Zone isthe portion of the floodplain subject to stormsurges with velocity waves of three feet or moreduring the 100-year flood. FEMA standards forV Zones require the lowest portion of thestructural members supporting the lowest floor tobe elevated on pilings or other columns to or abovethe BFE. In addition, the space below the lowestfloor in a V Zone must not be used for human

Figure 1.2. Elevation Requirements for Post-Type Foundations6

habitation and must be free of obstructions.

NFIP requirements for A and V Zones as of.Januarv1984 are summarized in Figure 1.4.

Note that FIRM's are based on a variety of assump-tions about expected flood severity, developmentpatterns, etc. The actual level of flooding from a100-year flood may be significantly greater. Inaddition, the "500-year" flood level, which wouldbe significantly greater than the 100-year flood's,could conceivably occur once or even more oftenduring a building's lifetime. These uncertaintiesare further reasons for locating buildings in lesshazardous zones or elevating them higher than theNFIP's minimum elevations.

ON SLAB FOUNDATIONA Zoneslowest floor

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Figure 1.4. Key Floodplain Requirements of the National Flood Insurance Program as of January 1984.

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BOTH A AND V ZONES (Numbered and Unnumbered)

- All structural components must be adequately connected and anchored to prevent flotation, collapse, or permanentlateral movement of the building during floods.

- Building materials and utility equipment must be resistant to flood damage. All machinery and equipment servicingthe building must be elevated to or above the Base Flood Elevation (BFE), including furnaces, heat pumps, hot waterheaters, air-conditioners, washers, dryers, refrigerators and similar appliances, elevator lift machinery, and electricaljunction and circuit breaker boxes.

- Any space designed for human habitation must be elevated to or above the BFE, including bedroom; bathroom; kitch-en; dining, living, family, and recreation room; and office, professional studio, and commercial occupancy.

- Uses permitted in spaces below the BFE are vehicular parking, limited storage, and building access (stairs, stairwells,and elevator shafts only, subject to design requirements described below for walls).

A ZONES (A1-A30)

- Buildings must be elevated such that the lowest floor (including basement) is elevated to or above the BFE on fill,posts, piers, columns, or extended walls.

- Where fully enclosed space exists below the BFE, walls must be designed to minimize buildup of flood loads byallowing water to automatically enter, flow through (in higher velocity flooding), and drain from the enclosed area.For low velocity conditions, vents, louvers, or valves can be used to equalize flood levels inside and outside enclosedspaces. For high velocity conditions, breakaway walls (see below) or permanent openings should be used.

V ZONES (V1-V30)

- Buildings must be elevated on pilings or columns such that the bottom of the structural member supporting the lowestfloor is elevated to or above the BFE.

- Buildings must be certified by a registered professional architect or engineer to be securely fastened to adequatelyanchored pilings or columns to withstand velocity flow and wave wash.

- Space below the lowest floor must be free of obstruction or enclosed with breakaway walls (i.e., walls designed andconstructed to collapse under velocity flow conditions without jeopardizing the building's structural support.

- Fill may not be used for structural support.- No construction is allowed seaward of the mean high tide line.

SITE ANALYSIS AND DESIGN

Site Selection and AnalysisSITE SELECTION

Whenever possible, site selection should avoidflood-prone areas. If this is not possible it shouldbe recognized that the risk and severity of floodinggenerally decreases with the distance from theriver channel or from coastal waters. However, thisis not always the case, so it is important to checkthe level of expected floods in relation to theproposed site. If the base flood elevation (BFE)has not been determined, it would be wise to con-sult local flood history data before making a finalsite selection.

The regulations of the National Flood InsuranceProgram (NFIP) specifically prohibit building orlandfill in a floodway, if such has been designated,if the results would obstruct the flow of floodwaters and thereby increase flood heights.Similarly, building in a coastal high hazard areais also not permitted unless the structure is land-ward of the mean high tide level.

Development should be diverted away fromidentified mudslide or erosion-prone areas. Onlywhere site and soil investigation and proposed con-struction standards assure complete safety forfuture residents should such sites be considered.

Overall, customary site selection criteria should beused to evaluate the suitability of a site. Drain-age, height of the water table, soil and rock forma-tions, topography, water supply, and sewagedisposal capability should be considered alongwith economic and planning criteria such ascost, access, and compatible land use.

SITE ANALYSIS

The site elements of primary importance foranalyzing an elevated residential project areflooding, soil, and wind characteristics.

Flooding Characteristics

Floodwaters impose hydrostatic forces and hydro-:dynamic forces. Hydrostatic forces result from the

static mass of water at any point of flood watercontact with a structure. They are equal in alldirections and always act perpendicular to thesurface on which they are applied. Hydrostaticloads can act vertically on structural memberssuch as floors and decks, and can act laterally onupright structural members such as walls, piers, andfoundations (see Figure 2.1).

Hydrodynamic forces result from the flow offlood water around a structure, including a drag

____________________effect along the sides of the structure and eddiesFigure 2.1. Hydrostatic Forces or negative pressures on the structure's down-

stream side (Figure 2.2). These are more commonin flash floods, coastal floods, and when floodwater is wind-driven.

A number of hydrologic factors must be evaluatedin the design of an elevated structure:

- Depth of expected flooding and, in coastalareas, height of wave crests, which will deter-mine the required elevation of a building andthe hydrostatic forces to be expected.

- Frequency of flooding, which is the amountof time between occurences of damagingfloods. This will have an important influ-ence on site selection.

Figure 2.2. Hydrodynamic Forces- Duration of flooding, which affects the length

of time a building may be inaccessible, as wellas the saturation of soils and building materi-als.

- Velocity of flood waters and waves, whichinfluences both horizontal hydrodynamicloads on building elements exposed to thewater and debris impact loads from water-borne objects.

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- Rate of rise, which indicates how rapidlywater depth increases during flooding. Thisdetermines warning time before a flood,which will influence the need for access andegress routes elevated above floodwaters andwhether valuable possessions can be keptunderneath the structure and moved onlywhen flooding is imminent. Flash flood areasoften receive little or no warning of flooding.

Another hydrologic factor is ice, which in northernclimates can cause serious damage to structures ifflooding should occur during the spring before theice melts. In some cases winddriven ice or ice jamshave completely demolished bridges, homes, andbusinesses, snapping large trees and pushingbuildings completely off their foundations.Floating debris can be equally dangerous in thisregard. There is little that can be done to avoidthese phenomena short of avoiding sites wherethey are especially likely to occur.

Hydrologic data concerning a site, including bothtechnical studies and historical records, can oftenbe provided by the local or state government andfederal agencies such as the Federal EmergencyManagement Agency, the U.S. Army Corps ofEngineers, and the U.S. Geological Survey. Ifneeded information is not available from thesesources, engineers familiar with hydrologic andhydraulic techniques can analyze the floodingpotential.

Soil Characteristics

The characteristics of the soil in a flood area-soilbearing capacity, for example-can be importantin determining an appropriate design. Highlyerodable soil would not be desirable for use as fillin elevating a structure in a high velocity areaunless the fill is properly protected. When erosionremoves soils supporting building foundations, thefoundations can fail (see Figure 2.3).

Figure 2.3. Erosion Caused ThisFoundation to Collapse.

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Soil data can be obtained from soil survey reportspublished by the Soil Conservation Service of theU.S. Department of Agriculture. It may bedesirable to consult a qualified soils engineerfamiliar with the soils at the site.

Large-scale topographic maps of ground elevationscan be used to determine natural drainage patterns,mudslide- and erosion-prone areas, and the feasibi-lity of using fill. Local or state agencies or the U.S.Geological Survey can often supply this informa-tion. Detailed topographic maps (2-foot contourintervals or less) must usually be developed as partof the site-specific investigation and are necessaryfor developing grading and landscaping plans.

Winds

Buildings elevated off the ground can be morevulnerable than other buildings to wind (see Figure2.4). Data on expected winds appear in buildingcodes and Standard A58.1 of the AmericanNational Standards Institute. Design and Con-struction Manual for Residential Buildings inCoastal High Hazard Areas, cited in the Preface,discusses designing for wind in coastal areas.

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Site DesignSite design for elevated structures should followstandard planning criteria applicable to any sitework. Typical factors to consider include slopes,natural grades, drainage, vegetation, orientation,zoning, and location of surrounding buildings, aswell as expected direction of flood flow.

SITE FLOODING CHARACTERISTICS

Buildings should be positioned in the area of thesite that will experience the lowest flood levels andvelocities. In coastal areas, this means as far backfrom the beach as possible and, if feasible, behinddunes. Buildings should be oriented to presenttheir smallest cross-sections to the flow offloodwater. This reduces the surface area on whichflood and storm forces can act. __

When multiple buildings are to be placed on the Q : j0 0same site, the objective of site design is the same asfor an individual building. One approach is to -disperse buildings throughout the site, applyingthe criteria discussed above to each building. An 0 0 0 Oalternative to such dispersal, when local zoning ordinances allow (e.g., a planned unit developmentordinance), is to group buildings in clusters on thesafest parts of the site, leaving the more vulnerable areas open. This approach not only reduces flooddamage but can also allow greater flexibility inprotecting the natural features on the site (see * |E JFigure 2.5). -C 0)___ J_ __Adjacent buildings, bulkheads, or other structures Figure 2.5. Planned Unit Developmentshould also be considered in site layout, both for Ordinances Allow Greater Flexibility intheir potential to screen and divert flood waters Site Designand water-borne debris and for their potential tobecome floating debris themselves. Bulkheadsalso tend to divert flood waters around their ends,adversely affecting adjacent sites.

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ACCESS AND EGRESS

Access to and egress from a building can be facili-tated by locating parking and driveways-as well asthe building-in the area of a site least likely to beflooded. Access and egress are important duringflooding to ensure that building occupants canevacuate and that police and fire protection andother critical services can continue to be provided.

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Figure 2.6. Site Design to Reduce Flood Hazards

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In new developments, roads should be locatedto approach buildings from the direction awayfrom the floodplain, so that access roads willbe less likely to be blocked by flood waters anddebris (Figure 2.7). To reduce potential erosion,siltation, and runoff problems, roads should notdisrupt drainage patterns, and road crossingsshould have adequate bridge openings and culvertsto permit the unimpeded flow of water. If roadsare to be raised, the slope of embankments shouldbe minimized and open faces stabilized withground cover or terracing.

VEGETATION

Vegetation aids in slowing the rate of storm waterrunoff by holding water, thus allowing it to filterinto the ground or evaporate gradually. Inaddition, vegetation helps prevent erosion andsedimentation from flooding. Natural vegetationshould be retained wherever practical, and newplantings should be introduced in locations thatwill be most affected by runoff.

Crushed stone can be used to control erosion underlow-lying elevated structures and other locationswhere vegetation is difficult to maintain.

Larger bushes and trees can be sited to deflectfloating debris away from elevated foundations.Landscaping can also be used to screen elevatedfoundations from view. Trees, plantings, fencing,etc., can all provide this dual function of utilityand aesthetics.

FLOOD WATER DRAINAGE AND STORAGE

Good site drainage in riverine areas allows floodwaters to recede from a site without eroding it orleaving standing water that causes damage tostructural elements or health hazards from stagnantwater.

Water enters a riverine site either from precipita-tion or as surface runoff from upstream portions ofthe watershed. What happens to this water can bea major determinant of the degree of flooding and

Figure 2.7. Improperly Sited Streets Can BlockEmergency Egress and Access

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the amount of flood damage. Site developmentthat increases the volume of storm water runoffcan increase flooding levels. Ideally, runoff ratesafter development should not exceed the ratesbefore development.

Site design should work to protect the individualsite as well as to minimize increased flood levelselsewhere. A number of key factors such as theamount of nonporous surface and the amount ofon-site surface water storage can in part determinethe ability of a site to absorb water. Land-useregulations in some communities require devel-opers to defray part of the cost of developingregional water retention sites to offset the effectsof development.

On the site, open channels can be used both todivert water away from erodable areas, such asshort steep slopes, and to collect and transportwater runoff to larger drainage courses. Channelswith grass cover are appropriate where the channelgradient and consequent water velocity are low;they then serve as percolation trenches by allowinggradual infiltration while water is being trans-ported. Where vegetation cannot be established,concrete and asphalt paving or riprap can be usedas channel linings. However, such linings canincrease the velocity of runoff, and considerationshould be given to velocity checks to control therate of flow.

On some sites it may be possible to use fillmaterial-from either on-site or off-site-toimprove drainage and control runoff. Special con-sideration should be given to soil conditions andslope stability, as well as flood water velocities andduration, to avoid erosion during flooding. Whenrestructuring topography, exposed cut and fillslopes, as well as borrow and stockpile areas,should be protected. Runoff should be divertedfrom the face of slopes, and slopes should bestabilized with ground cover or retaining walls.

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DUNE PROTECTION

Dunes provide a natural shoreline defense againststorm surges and waves. Most coastal communi-ties require that construction be behind theprimary dune and that dunes not be cut orbreached by site features such as walkways orbeach access roads. Cross-over walkways shouldbe provided (see Figure 2.8).

Existing dunes should be maintained throughvegetation and sand fencing, which limit windlosses and promote further dune growth. If nodunes exist and the beach is sufficiently wide,successive tiers of sand fencing can induce duneformation; some communities require this beforea residence can be built.

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Many of the twentieth century's most importantbuildings have been elevated residential structures.The rise of modern architecture, inspired by theraised houses of Le Corbusier in the 1920s, wasmade possible by structural innovations. The VillaSavoie at Poissy (1929), for example, is liftedabove the ground on pilotis, freeing the lower levelfor parking and affording a spatial continuity withthe landscape (Figures 3.1 and 3.2). In hisTowards A New Architecture Le Corbusier wasexultant about the possibilities of elevated design:

The house on columns! The house used tobe sunk in the ground: dark and often humidrooms. Reinforced concrete offers us thecolumns. The house is in the air, above theground; the garden passes under the house.

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Figure 3.1

Since the Villa Savoie was centered on a dome-likerise in a large pasture, Corbusier did not need toconcern himself with the problem of flooding.Other masters of modern architecture, however,have used the principles of elevated residentialdesign to create aesthetically satisfying and func-tionally sound responses to hazardous flood condi-tions.

Mies van der Rohe's Farnsworth House (1950),considered one of the great icons of modern archi-tecture, owes at least some of its appearance to itsflood-prone site (Figures 3.3 and 3.4). Built alongthe Fox River in rural Illinois, the house wasdesigned to accommodate a body of water thatoverflows its banks each spring. Mies' solution tothe problem was to raise the plane of the first floorabove the flood level, creating his first clear-spanbuilding. The resulting structure seems to floatabove its site.

Good design and good flood protection must con-tinue to be treated together. Good design entailseffective use of the site and careful considerationof the needs of the surrounding neighborhood andcommunity. The best houses provide a cleartransition from ground to dwelling, integrating thefoundation with the rest of the structure. Creativelandscaping with trees, shrubs, and fences canenhance the appearance of elevated structures bysoftening the effect of potentially harsh or barren

Figure 3.3

20

exposures. Inventive landscaping also helps tocontrol erosion and protect the dwelling from theimpact of debris anrd high velocity flooding.Effective use of terracing and level changes canhelp achieve continuity with the surrounding areasand, equally important, provide a sense of varietyby indicating the different functions that occursimultaneously on a single site.

Such site considerations are but one part of a totalelevated design scheme. The following examplesare concerned with some of the many otherimportant factors involved in floodproof design.

Figure 3.4

21

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Design StudiesIThe ioiflming desigi studics xvvru dcvclopud b! anumbe)r of' aruhli tcinAral fl'lrls and aruch itecturalSchlOOis Iising" th ini'ormation presented in this

B;RIDGE lPORT, CO NNS( rlcl r\\ itll anl cI( latiot ri (t Iiri Inciit I ofi Io c I albo\egra(li. thl arlct c t , ( a\(i t d s i ji (d t ~ I c t l uxI( to)WhifsOlc- aroundi*I ra]iStIud ettral social duck(Fiigurcs 3.5 and 3.6). Parking il, Ioila)cd bin.aththu tl ik. \cccsu. to thu duuk and to thu town-IIoiist's iSl puroititti b)\ staiur mlidi a timnbr ramp.TIb iampt puoroides ai ess lor chtildire. the hantli-tdti(p(i aid fli ttle ch!\ . i)tirill hinxes of floodinig.thc rampt (daii also bt) lisud iou (ldi\lig' ailtoniloil(salin] rescuie Nebicics p) tbe dueck lux (1 ,Stee I irdersru-ting on uoncrete ptiers sup)port b)oth1 the souialdvuk and thu toxuihousus (F'iguur 3.7). The tleckhas a doulible floor conistructioi. allowing atiduddinsulatioii andl purotrtinu wtiliti scrxiues.

Figure 3.5

22

Access Ramp

Figure 3.6

Attirk Storage

Fsood1 L~eve! .l.1 S > r

T~~~~~~~~~~~~~~~~~~~~~~~~~~~ 04-e^ Concrete .E

PIej

Figure 3.7

A

23

I :,

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CHARLESTOWN AND NEWPORT, RHODEISLAND

The architect here has chosen two case studyareas, Newport and Charlestown, Rhode Island,with distinctly different cultural and naturalconditions that affect flood design considerations.Newport is a compact commercial and recreationcenter that has many residences along the water'sedge. The area studied in Newport is a protectedharbor with access from Rhode Island Sound intoNarragansett Bay. The portion of Charlestownthat is the second study area is a beachfront areawith vacation house development. Most develop-ment is in a coastal A Zone. Both study areas havehigh development pressures.

In both areas historic, scenic and communityvalues influence the design of elevated structures.In Newport the close proximity of a HistoricDistrict injects height, bulk, material, and sizeconsiderations into any planned development.(In the case of historic structures in floodplainslisted on the National Register of Historic Places ora state inventory of historic places, restoration maybe accomplished without elevating the first floorthrough a variance procedure.) Similarly inCharlestown, simply elevating structures, withoutregard for the natural environment, could produceungainly and visually distracting elements. It isnecessary in flood area design to not only meetengineering requirements, but to also be cogni-zant of the visual effect such design will have onthe prevailing character of the area.

Charlestown

An inventory of critical natural factors was madeto determine how and where development shouldtake place in the Charlestown floodplain. As aresult, specific land area within the floodplain wasdeemed acceptable for residential development.The analysis then proceeded to the evaluation ofmethods of elevation appropriate to the develop-ment area.

24

Base Flood Level

Figure 3.8

For numerous functional and aesthetic reasons,earthfill with heavy stone revetment was chosenas the method for elevating residential structuresin Charlestown (Figure 3.8). The homes wereclustered to keep down the cost of fill and becausethe land available for safe building in the flood-plain was limited (Figure 3.9). A small-scale,

Figure 3.9

25

single-family scheme was chosen for visual con-tinuity with earlier buildings (Figure 3.10). Allhouses, a small amount of private space, and allutilities are located on the common filled area.Low intensity land uses such as parking, road anddriveways, playgrounds, etc., are located on thelower surrounding areas. Ramps and steps are usedto accommodate the height differences fromparking to the finished first floor.

Figure 3.10

26

Newport

Development in the wharf area in Newport, RhodeIsland, is structured by a combination of naturaland cultural conditions. Although separated fromthe older historic areas of Newport by a highway,its proximity to them requires special considera-tion of height, materials, and size. It is in a specialflood hazard zone, yet its water's edge locationmakes it visually attractive. Changes in the use ofthe wharf area and its new relationship with neigh-boring areas have resulted in an expansion of com-mercial and residential development. The lowheight above sea level means that new structureswould have to be raised approximately to the levelof the highway to comply with local flood regu-lations. For the restoration of historic buildings,however, there is no need to elevate the first flooras long as a variance is obtained.

Analysis indicated that the optimal solution wouldbe a combination of elevation techniques, becausedifferent zones in the wharf area are suited to dif-ferent elevation strategies (Figures 3.11 to 3.13).

Base Flood

F igu re 3.1 1

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Base Flood ' MSL

Figure 3.12

27

Figure 3.13

28

t

In the area farthest from the water, earth fill offersflood protection and a gradual level change fromthat of the highway. A transitional middle sectioncould combine berming with raised structures.Level changes can be integrated by linkingextended decks with ramps and stairs. In the areaclosest to the water, raised structures would notalter the water-to-land relationship or block views.Commercial uses are most likely to locate in thefilled area, where first floor spaces are usable.Residential, restaurant, and small office uses aremore suitable to the raised structures, which affordincreased privacy and better views.

Spaces under and between the new buildingscan be used for pedestrian malls and thus rein-force the tourist and commercial uses of thearea. Decks, balconies and trellises can connectdifferent building levels. Utilities for the raisedstructures could be run beneath these raiseddecks and trellises and then into the fill, beingprotected from flood damage. This manipulationof the spaces and level changes created by floodprotection enhances the visual intricacy and humanscale of the wharf.

29

SAN FRANCISCO, CALIFORNIA

Pacific coast flooding is generally associated withhigh seas and rains. Ocean storms accompanied byhigh winds have caused considerable erosion anddamage to beach and coastal floodplain property.Inland rain storms, on the other hand, falling onthe mountainous terrain cause major canyon andvalley flooding. Both coastal and canyon floodingare dangerous high-velocity situations. Slow-risingand lower-velocity conditions occur on coastalmarshes and low-lying riverbeds.

The architect has developed several very interestingand distinctive residential concepts for single- andmulti-family housing. The use of landscaping,fences, and exterior decks minimizes the elevatedappearance of the structures while providing func-tional visual highlights. Structurally the twoconcepts are quite different. Although bothconcepts use wood posts, the single-familyresidence uses a two-way structural grid supportingprefabricated housing units, while the multi-familystructure is conventional wood frame constructionbuilt upon a wood-post-supported platform.

Parking for both residential concepts is under thestructure.

Figure 3.14

30

Single-Family Residential Concept

A two-way wood post structural grid supports theliving units at levels above the base flood and servesto organize and unify the various units withminimal impact on the ecology of the area (Figures3.14 to 3.16). A seven-foot clearance beneath thehorizontal structural members allows for parking,storage, and sheltered recreation space separatedfrom and below the living units. The reduced landcoverage of this design is in keeping with thearchitect's concern for efficient land use. Sharedfacilities, clustering buildings, etc., further givethese houses a unique identity and sense ofcommunity. Within the prescribed vernacular ofpoles, decks, railings, and fences, architecturalvariety with continuity is achieved. The fences arestrapped together to prevent pieces from floatingaway if damaged during a flood. Water heater andfurnace and air conditioning equipment are located18 inches above base flood level with all ductworkin second floor or attic space.

Figure 3.15

31

Base Flood Level

Figure 3.16

32

Multi-Family Residential Concept

To reduce costs, the architects have designed aconventional wood frame structure built upon awood post platform (Figures 3.17 and 3.18).Raising the first floor to at least eight feet abovegrade provides an opportunity to put parkingunder the building. This reduces the area of thesite that has to be built upon and places cars closerto apartments. However, parking under thestructure requires fire separation. Exposedentrance stairs and fencing minimize the elevatedappearance of the structure while providing visualvariety and privacy.

1

Figure 3.18

33

Figure 3.17

CHICAGO, ILLINOIS

Flooding in the Midwest is of two types: riverineand lake flooding. The characteristics of both areusually slow rise and low velocity. However, flashflooding and lake shore scouring can and do occur.The Great Lakes area, more specifically, theWisconsin, New York, Ohio, and Michigan lakeshores, have experienced growing problems of lakeflooding and slow erosion caused by the increasingoccurrence of high waters and high winds.

Garden Apartment Concept

Although elevated eight feet and constructed ofreinforced concrete block, this rowhouse does notappear to be designed for a potential flood condi-tion (Figures 3.19 and 3.20). Tlhe covered parkingand entrance level is handsomely integrated withthe above living levels by reinforced concrete blockwalls that organize the entire structure. The wallsare constructed parallel to the direction of possiblewater flow. Unfortunately, the architect enclosedthe stairway-entranceway, with a potentiallyserious effect on flood insurance rates.

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Figure 3.20

34

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Aesthetic ConsiderationsThere is a common misconception than an elevatedresidential structure will be inherently unattrac-tive-a box on stilts (Figure 3.21). This is not true.Elevated structures offer challenging design oppor-tunities to be aesthetically appealing as well asfunctionally sound.

Residential development requires a significantfinancial investment, and if it is aestheticallyappealing it contributes to the economic value ofthe area, both for the owner and for the com-munity as a whole. All communities have bothpositive and negative examples of this. Goodquality tends to foster better quality, and poorconditions lead to even poorer conditions.Appealing design can thus be an important elementof making the most of our limited developmentresources.

Figure 3.21

35

SITE DESIGN

Integration of development and site should bedone so that the two complement each other. Acareful site analysis can give many clues to the bestdesign of the building for its relation to topo-graphy, location and orientation, and location offenestration (views, etc.), entrys, and parking.

Landscaping-creative use of trees, shrubs, fences,walls, etc.-serves two purposes. It integrates theelevated portion of the development with itssurroundings and, at the same time, helps controlerosion and protect the dwelling from the impactof debris and fast-moving water (Figures 3.22 and3.23).

The relationship and compatibility of developmentwith the surrounding neighborhood and corn-inunity should be considered in order to give asense of continuity with the surrounding areas,rather than an unattractive "hodge-podge" of un-related development.

Terracing and level changes can be used to give asense of variety and to identify different uses, aswell as to integrate building with site.

gA( H MYOUMP P fMOVING16 9 -St AW PftioOW~fM -- -- tWOOPWAWrV~

qlqt6-1jo

Figure 3.23

36

Figure 3.22

BUILDING DESIGN

The integration of the foundation with the site andthe building is perhaps the most important aesthe-tic challenge when designing elevated structures.Many elevated structures give the impression thatthe support foundations are treated separatelyfrom the building and the site, giving the impres-sion of a building set on spindly legs (figure 3.24).It is essential to recognize that the foundation is anintegral part of a building, rather than only "some-thing to set the building on." A well-designedelevated residence should provide a smooth transi-tion from ground to dwelling, with the foundationintegrated with and complementary to the buildingitself.

Other special considerations when designingelevated residences include the design of anyneeded stairs and the use of the areas under thestructure. More general considerations include theshape and form of the building (configuration,shape of roof, etc.), textures and color of buildingmaterials, the use and treatment of balconies,terraces, railings, windows, shutters, screens, andentries, and the arrangement of interior spaces.

I~~~~~~~~~ E

Figure 3.2437

Figure 3.25. This wood structure successfully usesthe same material throughout the building- Figure 3.26. This is an example of integrating thefoundation, structure, treatment of railings, wall, site, the building, and the foundation so they relateand roof material, as well as connection and well to each other. This foundation appears to beanchorage details. The design honestly expresses part of the building rather than stilts holding it up.the structure, foundations and other building It shows how a modest, simply designed buildingelements. While it is obvious this is an elevated can also be very aesthetically appealing through thestructure, it still feels very much a part of the site. use of natural materials and interesting treat-The foundation members are also integrated well ment of fenestration and lighting fixtures. Simplewith the building itself (see also Figures 3.54 to but well-thought-out landscaping ties the building3.57). effectively to the site.

Figure 3.27. This is a good example of how the configuration of a cluster layout can contribute tofunctional advantage as well as visual appeal. The sawtooth arrangement allows for two sides of eachunit to have access/view to the ocean. This form also breaks up the long, continuous (and often mono-tonous) wall approach, thus adding variety and interest. With this configuration the materials, treatmentand form of the units can be simple but still attractive.

38

Figures 3.28 to 3.29. This is an excellent example of cluster-type elevated residential development.The development is well-integrated with the site; the various levels seem to roll over and blend with thedune. The vegetation and simple fencing add much to this marriage. The individual units also relate verywell to each other, providing a good example of an overall development's being "more than the sum ofits parts." The individual units provide the individual amenities-privacy, plan layout, etc.-while stillbeing a part of a comprehensive whole with a strong sense of community. The form, scale and characterof the development are also excellent. The sloped roofs, the balcony treatment, use of levels, and thearticulation of the other elements add variety and a character that complements the site and overalldevelopment. The use of materials-color, texture, scale-also contributes to the design's appeal (seealso Figures 3.63 to 3.70).

Figure 3.30. The exterior treatment of this devel-opment adds visual appeal to a development thatcould otherwise be quite monotonous. Theexterior colored panels with white structure andcoordinated interior panels provide interest,as does the simple treatment of balconies with avariety of planes, panels, railing and roof trellismembers.

39

Figure 3.31. This is a good example of how a simple structural grid infrastructure can be used as a basisfor a relatively modest, well-designed and visually appealing residence. The plan is simple, developedaround the columns, but provides a very livable, interesting and functional space. The cantileveredbalconies also add interest as well as defined exterior areas. The roof shape contributes to a spaciousinterior that makes the house feel larger than it really is, allows in natural light through the transomwindows, and through its form adds much to the overall aesthetic appeal of the design (see also Fig-ures 3.40 through 3.45).

40

Figure 3.32. The diagonal battens used to enclosethe stairwells for protection provide an aestheti-cally appealing screen-textural affect. The coloredawnings also add a necessary highlight to anotherwise colorless exterior. Notice also the polelight fixture.

Figure 3.33. Passersby have to look very carefullyto see that this development is actually elevated.Good use of landscaping and building formincludes attached and detached units.

:p:'S:

A:U D XA E :0 ; : :A

Figure 3.34. This structure uses a mixture of materials, texture and color very successfully and providesa variety of form for visual appeal. The space under the building remains open and light through acombination of white unobstructed walls and piers, landscaping, and layout relative to other buildings.A human scale is accomplished by breaking the building up into different heights and sections, ratherthan an imposing three-story box, as is often done (see also Figures 3.58 through 3.62).

41

Figures 3.35 and 3.36. This is a good example ofusing a variety of shapes and forms (wall surfaces,planes, balconies, etc.) as well as wall treatments(materials, texture, color) to create a sense ofvariety essential for an aesthetically pleasingdevelopment.

42

Figure 3.37. In the interior, color, scale, texture, and floor arrangement must be given careful attention(see also Figures 3.40 through 3.45).

43

Figures 3.38 and 3.39. Well-designed elevated residential structures can take many forms and styles. Theprinciples in this manual are applicable to any style.

44

Recent Design ExamplesThe projects in this section are some of the bestdesign examples discovered in a state-of-the-artsurvey conducted as part of the development ofthis manual. While these examples range from asingle-family detached unit to a multi-family highrise, there appears to be a clear trend toward higherdensity, cluster-type development. This isprobably due to higher land values and the experi-ence gained from major floods over the last coupleof decades. This is a promising trend that encour-ages professional design involvement in residentialstructures and leads to a more comprehensiveapproach to elevated residential and other develop-ment in flood-prone areas.

Virtually all the recent design examples that weresubmitted in response to our survey were coastal,as opposed to riverine, projects. This suggests thatthe state of the art is being set for the most partin coastal areas, especially in the higher-use resortareas. It should be noted, however, that what isbeing done in coastal areas can often be appliedsuccessfully in riverine, lake, and other flood-prone areas as well.

45

THE LOGAN HOUSETampa, FloridaArchitects: Rowe Holmes Barnett

Architects, Inc., Tampa, Florida

The Logan House (Figures 3.40 to 3.45), locatedadjacent to a federally protected tidal estuary nearTampa, Florida, exemplifies a skillful blend offlood protection and energy conservation. Thenatural site of the house, only four feet above sealevel, suggested the possibility of flooding. Floodregulations required Rowe Holmes Associates toelevate the structure an additional six feet. Theychose, however, to raise the house almost eight feetto be able to use the first level as both a carportand protected outdoor living area.

The 2,000-square-foot structure is designed inwhat is known in Southern vernacular as the"dog trot" style, incorporating a long breeze-way/ventilating device covered with the sameroof as the house but open on the sides. Thewood frame house is supported on 10-inch-squarepressure-treated pine poles augered deep into thesoil to withstand hurricane forces common to thisarea of the country. The floor serves as a horizon-tal diaphragm to provide the pole structure addi-tional rigidity.

Several of the features that protect the LoganHouse from flood damage also promote energyconservation. For example, elevating the structure,the major flood protection strategy, helps drawcool (lower) air up and through the house.

A central utility core-unfortunately located onthe lower level where it is vulnerable to stormforces-is serviced by a stairway, allowing pro-tected access to the carport and outdoor space.

Figure 3.40

46

be- m b o om l

bedroom bedrooT I ro w mo

living level0 5 S1

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sectionO . .,~~~~~~1

south elevationO , .,~~~~~1

47

Figure 3.41

Figure 3.42

Figure 3.43

I

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Figure 3.44

Figure 3.45

48

SUMMERWOOD ON THE SOUNDOld Saybrook, ConnecticutArchitects: Zane Yost & Associates, Inc.,

Bridgeport, Connecticut

Summerwood on the Sound (Figures 3.46 to 3.50),a 76-unit cluster development, won a 1979 designaward for architects Zane Yost & Associates, Inc.The development is built on a peninsula tidalestuary protected by a barrier beach.

Equal in importance to protecting the buildingsfrom flooding was the preservation of the saltmarsh ecological environment. For this reason,the architects chose to locate the units only alongthe natural contours of the 30-acre site. Forfurther protection of land as well as buildings,the structures are elevated above flood level,topping crawl spaces with internal drains to permitflood water to pass in and out. The wood framestructures are covered with horizontal siding anduse picket fences to soften the effect of the raisedstructures. Redwood stairs and decks adorn thewater side of the units.

Figure 3.46

Although the overall density on the site is low(2.5 units/acre), the clustering of the units makesfor a comfortable neighborhood scale.

0 E i; iii i ; & . .u? *w ,,jZ.

SITE PLAN0 25 50 100

Figure 3.48

_

Front Elevation

I Ll

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Rear Elevation

Figure 3.49

50

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