Limits of Applicability

Catchments for which RFFE Model cannot be applied:

• Urban catchments (more than 10% of the catchment affected by residential or urban development)
• Catchments where the construction of a dam or weir has significantly altered the natural rainfall-runoff behaviour
• Catchments where large scale land clearing has taken place
• Catchments which have been significantly affected by agricultural activities, construction of drainage or irrigation infrastructure, soil conservation works or mining activities

Catchments where RFFE Model estimates have lower accuracy:

• Catchments with an area less than 0.5 sq km or greater than 1,000 sq km
• Catchments located further away than 300 km from the nearest gauged catchment location used to develop the RFFE technique
• Catchments in the arid areas since the RFFE technique for the arid areas is based on a very small number of gauged catchments spanning a vast area of Australia

Catchments with atypical characteristics:

It should be recognised that flood estimates generated by the RFFE Model for a catchment with flood characteristics that are distinctly different from typical gauged catchments in the region may not only be associated with larger error margins but also significant bias. In such situations hydrological judgment must be exercised to assess if any adjustment of the regional flood frequency estimate is required (based on comparison of relevant catchment characteristics). To support such an assessment, the RFFE Model output describes the set of gauged catchments used in developing the RFFE Model, which are located closest to the ungauged catchment of interest. The following additional catchment attributes may need to be considered as a basis for adjustments to the flood estimates obtained directly from RFFE Model:

• Natural flood storage: large flood storage areas in catchments with extensive floodplains or swamps have the effect of attenuating flood peaks; flood estimates from RFFE Model would thus tend to overestimate peak flows and they could be regarded as upper bound flood estimates for these catchments.
• Drainage efficiency: steep catchments, streams with little vegetation along banks, catchments affected by large scale drainage or flood protection works can be expected to produce faster flood flows, less attenuation and thus higher peak flows; flood estimates from RFFE Model would thus tend to underestimate peak flows and they could be regarded as lower bound flood estimates for these catchments.

For flood estimation in catchments with atypical catchment characteristics it is desirable to examine the flood records of a gauged catchment with similar catchment attributes as a basis for adjustments to the flood estimates produced by the RFFE Model.