Supervisor:

M. Green

Editor: 

Assoc. Prof. J. M. Rüeger


Safe subdivision design starts with identification of all possible hazards by consultation of hazard maps and other hazard information and a weighted multi-hazard analysis. Location of the subdivision in conjunction with safe subdivision design can mitigate the effects of natural hazards and reduce or eliminate artificial hazards. Buffer zones, doubling as public space, can be used to move habitation from areas of greatest risk. Natural hazards cannot be controlled, but their impact can be minimised through good design and ensuring adequate routes for safe evacuation and emergency access. Stormwater is both a natural and artificial phenomenon and safe subdivision design needs to consider surrounding areas as well as providing appropriate retention areas and overland flow paths. The artificial hazard of road accident can be reduced by designing a road layout which encourages a low 'feel safe' speed, minimises points of conflict, and provides good visibility. Footpaths and cycleways that are attractive and convenient to use encourage people to be out and about in the subdivision and increase a sense of community responsibility and casual surveillance. This is turn, along with lot orientation, discourages the artificial hazard of criminal activity. As well as making the subdivision attractive, lighting and landscaping when used appropriately can also reduce accidents and opportunity for criminal activity. Lot size should be large enough to allow for adequate off-street parking and easements. Safe subdivision design can be achieved by considering the stormwater system, road layout, lighting, landscaping, and parking design features proposed.

Previously discussed issues are summarised by subdivision design element. By following these guidelines and the Local Council's requirements, the subdivision design reduces all hazards that threaten the subdivision. Natural hazards are identified by State and/or Local Government and hazard maps are created. The knowledge of all possible hazards (and their attendant hazards, which may not be mapped) and their affects should be known or become known through a site analysis before the subdivision design has begun. It is important that the subdivision will not impact the surrounding neighbourhood by increasing their vulnerability to the hazards that affect them. Government guidelines and policies restrict or prevent development in identified hazard prone areas and require professional reports if development proceeds in these areas; however, if a subdivision is being developed in an adjoining area, then similar precautions should be taken. Subdivision design is controlled by DCPs and construction by the BCA and covenants.

 Element

Hazard

 Recommended Guidelines

 Slope

 Bushfire

 Landslip
 Earthquake
 Roads

  • Less than 10 degrees 
  • Avoid gullies, ridges and creating wind tunnels 
  • Less than 3 degrees 
  • Less than 3 degrees 
  • Less than 20 degrees 

 Land

 General

 Flood

 Landslip

  • Avoid weak rocks, alluvial plains, old sedimentary   lakebeds, flood plains, and soft sediment soil 
  • Develop only above 1% AEP 
  • Avoid landfill below 1% AEP 
  • Minimise cut and fill 

 Aspect

 Bushfire

  • Southerly or easterly land 
  • Town water or a static water supply 

 Buffer Zones /  Easements


 Coastal
Bushfire

 Flood

  • Use nature strips and public areas 
    • along the coast and wave run-up area 
    • as a firebreak between the subdivision and bushland /  Asset Protection Zone (APZ) 
    • as flood overflow, below 1% AEP 

 Lot Size

 General

  • Large enough to extend the buffer zone into 


    individual lots / accommodate easements 

 Shape and  Layout

 Bushfire

  • Circular or square for maximum area and minimum direct exposure to fire 

 Major Roads

 Access / Egress
 

 Location

  • Egress from at least two points 
  • One access route for every 100 - 200 houses 
  • Access to the main road as direct as possible 
  • In a low risk area 

 Emergency  Service

 Location

 Access

  • Placed in low risk areas 
  • Direct roads to locations 
  • At least two access points 

 Public Areas

 Criminal Activity

  • Maximum opportunity for casual surveillance by passers-by and overlooking houses 
  • Provide facilities people want 
  • Easily accessible 
  • Encourage use to enhance the community spirit 
  • Create a focus for the community by grouping areas 

 Stormwater

 System
 
 
 
 
 
 
 
 
 
 

Landslip

  • Design for subdivision considering upstream flow and downstream system capacity 
  • Do not increase the chance of flooding elsewhere 
  • Adequate permeable surfaces with public areas such as parks 
  • Use retention areas if too many hard surfaces 
  • Place retention areas in naturally low areas 
  • Make overland flow paths visually obvious 
  • Do not alter land and restrict use 
  • Secure outlet, use warning signs and fences 
  • Maximum depth should allow a child to stand 
  • Use sediment pits to avoid blockage and flooding 

 Lots

 General
 

 Criminal Activity
 
 
 

 Road Accidents

  •  Council sets minimum lot sizes 
  • Provide convenient off-street parking for saturation car ownership 
  • Orient houses towards the street and public areas 
  • Minimise side and rear access to minimise opportunity for criminal behaviour 
  • Make boundaries between public and private areas obvious 
  • Angle driveways to ensure clear vision lines (horizontally and vertically) for drivers 

 Road Design

 Bushfire
 

Storm Surge
 General
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Access
 Criminal Activity

  •  Cul-de-sacs should be less than 200m long and have adequate turning circles 
  • Provide a perimeter road / fire trail 
  • Place utilities underground 
  • Reduce 'feel safe' speed: 
    • use small radius 90 degree bends 
    • use 'T' intersections 
    • make roads minimum width 
    • discourage through traffic 
  • Reduce complexity of intersections: 
    • at least 25m between intersections 
    • no more than 3 per 100m 
    • only 1 decision need be made at any time 
    • use roundabouts if more than 3 streets converge 
  • Predict people's movements so that roads: 
    • are convenient 
    • are direct 
    • provide multiple ways of getting somewhere 
    • direct local traffic to focal point/community facilities 
  • provide parking at community facilities 
  • Allow drivers to see what is happening around them and to have sufficient time and distance to react to a potential accident 
  • Place road furniture, such as power poles, signs, safety barriers and light poles, sufficiently away from the kerb to avoid collisions. 
  • Open carparks should be small and visible 

 Footpaths and  Cycleways

 Safety
 
 
 
 
 
 

 Criminal Activity

  • Design for pedestrians first, then the cyclists and then the motorist 
  • Provide direct or meandering routes 
  • Identify bicycle lane by road surface / white line 
  • Reduces opportunity for criminal activity by increasing numbers of people walking and cycling (increases casual surveillance and the feeling of safety) 
  • Provide open, straight and visible routes 
  • Avoid underpases 

 Landscaping

 Flood
 
 
 

 Bushfire
 
 
 
 
 
 
 

 Road Layout

  • Use vegetation that will bend with the water flow along waterways and not obstruct / alter the flow 
  • Specify fencing which allows water to pass through rather than act as a dam 
  • Use vegetation wind breaks· 
  • Specify vegetation: 
    • in isolated stands 
    • with no solid canopy 
    • with smooth barked, evergreen species with dense foliage 
    • with the maximum distance between the ground and the canopy and minimum undergrowth 
  • Select and locate vegetation based on mature size and shape 
  • Use groundcovers to protect pedestrians 
  • Avoid vegetation 0.6-1.2 m high 
  • Avoid dense vegetation in corners and behind high walls for visibility and personal safety 
  • Select vegetation for pathways that is not dense or creates shadows 

 Lighting

 Road Accidents

 Criminal Activity

  • Avoid harsh shadows and sharp contrast especially around potential accident sites 
  • Illuminate vegetation alongside pathways 
  • Light popular routes 

Council should create a weighted multi-hazard map or zone map, which reflects a weighted multi-hazard analysis, which is permanently displayed for residents and designers to use. The zone map should provide guidelines for subdivision and construction design, for example, lot size, buffer zones, ratio of hard and soft surfaces, suitable vegetation, and minimum floor level. The designer needs to know the area he is working with, the whole area not just the development so that if the area is prone to natural hazards, then this can be taken into account. Every identified hazard has a professional who can give advice such as geotechnician or coastal engineer, who should be used even if the Council does not require it. Council requirements are the minimum.

Safe subdivision design can mitigate effects of natural hazards and reduce the risk of artificial hazards. An awareness of what natural hazards may occur should inform the choice of location and site of the subdivision and alteration of the land. The stormwater system should remove water from and prevent flooding of the subdivision and surrounding areas. The artificial hazard of road accident can be reduced by providing a road layout that encourages low speed, minimising points of conflict, and providing good visibility. The orientation of lots and provision of footpaths and cycleways, provides opportunity for casual surveillance and discourages criminal activity. Landscaping and lighting can also contribute to a feeling of safety and community spirit, and reduce opportunity for criminal activity and accidents. The end result is a subdivision where people are safer due to safe subdivision design.

 

Further Information

For more information contact:
 
Mr. M. Green (supervisor)
Email: Michael.Green@unsw.edu.au
 
Mail:
School of Surveying and Spatial Information Systems
University of New South Wales
UNSW SYDNEY NSW 2052
Australia
 
Phone: +61-2-9385-4193

Fax: +61-2-9313-7493