Flood Surveys


What is AHD?
Surveyors have expertise in determining levels on land. These levels are referred to as "Reduced Levels" (R.L.) which means a height above (or below) a datum. In Australia, this datum is called the "Australian Height Datum" (A.H.D.) and is calculated from the average of many tide gauges on the East Coast of Australia. Prior to A.H.D. there were numerous isolated datums in use. The establishment of A.H.D. has provided a common datum throughout the area which refers to the height above mean sea level.

What does 1:5, 1:100 flood mean?

Flood levels and the extent of flooding have been recorded for over 120 years in NSW. This data is the basis for determining the areas of land that are affected by flooding and the expected height of flood waters. The frequency at which various heights of flooding can be expected is referred to as once in five years (1:5), once in twenty (1:20), up to a maximum of once in one hundred years (1:100).

Your Local Surveyor can Help

Before buying property it is wise to check for flood risk. Unless your property is obviously in a high position, it can be difficult for the inexperienced to determine whether or not the property is flood prone. Surveyors are experts. They can investigate records and enquire from authorities as to the flood risk for a particular property. Observations can be made on the ground to determine the specific extent of the risk. Advice can be obtained on precautions required for new building or the likely depth of flooding which can be expected on existing buildings or land.

Building in a Flood area

In flood-prone areas it is possible that the local Council will issue a building approval with the specific condition that the proposed floor level be not less than a particular reduced level usually 0.5 Metres above the 1:100 flood. Surveyors can provide you with a reference point adjacent to your building site so that council's conditions can be met. When the building has been erected he can measure the reduced level of the floor "as built" and provide you with a report or certificate. Advice should be sought from a Surveyor with regard to determining potential problems which may result from floods. Surveyors are called upon to assess the seriousness of flooding to permit approvals for proposed development in flood-prone areas. Whatever the flood problems may be, a Surveyor can help you build a bridge to cross most of them.

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