Old Guildford Overland Flood Study

Study Objectives

Key objectives of this study are to:
<ul>
<li>Identify the major overland flow paths within the Old Guildford catchment study area.</li>
<li>Determine the nature and extent of overland flooding and flood risk in the study area.</li>
<li>Identify properties at risk of local overland flooding and quantify the risk of flooding to these
properties.</li>
<li>Produce flood model results (flood level, velocity and flow) for the 20, 100, 200, 500, 2,000
and 10,000 year ARI and PMF storm events</li>
<li>Prepare flood extent (depth and velocity) maps and flood risk precinct maps for the study area
for the 100 year ARI and PMF events.</li>
<li>Assess the sensitivity of flood behaviour to changes in hydrologic and hydraulic characteristics
in the catchment.</li>
</ul>

Originally, it was intended to also identify “Zones of Significant Flow” to determine those sections
of overland flow paths through properties which would need to be kept clear in order to reduce
flood risk. Due to time constraints, these zones will be identified in the floodplain risk
management study and plan.

Conclusions

The Old Guildford Overland Flood Study has been successful in achieving its objectives, being:
<ul>
<li>Define flood behaviour and identify the major overland flow paths within the Old Guildford
catchment; and</li>
<li>Identify properties at risk of local overland flooding and to prepare flood risk precinct maps.</li>
</ul>

The study’s modelling approach consisted of a XP-STORM model that dynamically linked the 2D
floodplain and 1D stormwater drainage network to assess flood behaviour and determine flood risk
to properties. The model allows flows to be transferred in and out of the drainage network
depending on the hydraulic conditions. This approach is considered to be able to efficiently
produce a reliable representation of overland flood behaviour compared to those used by Council
previously.

The amount and quality of the data available to define physical features in the study area, including
the ground surface, open channels, pits and pipes and building footprints, was adequate for the
development of the study models, though information on historical flood events in the study area
was lacking. Council should, if practical, collect flood marks in overland flood areas following
flood events to permit a more thorough model calibration and validation process for future overland
flood studies.

Sensitivity analysis indicates that the overland flood behaviour is typically not sensitive to variation
in floodplain roughness or increased rainfall intensity. Hence, overland flood depth estimates are
not expected to be significantly impacted by uncertainties in these parameters.
Overland flood depths are likely to increase if a high degree of pit blockage occurs during a flood
event. This should be taken into consideration during the development of overland flood risk
management strategies during the floodplain risk management study phase.
The flood extent and risk precinct mapping has been prepared to present only the areas which are
affected by significant levels of overland flooding. This has been achieved by removing “nuisance”
flood areas, typically of depths less than 150mm from the mapping. The merit of this approach is
that properties which are within or adjacent to these areas are not unduly coded with a flood risk.

The overland flood risk precinct delineation process itself has been developed over a number of
years in consultation with FCC. It clearly and objectively defines the level of flood affectation of
each part of the study area. Consideration of the flood event ARI in determining the flood risk, in
addition to the hydraulic hazard posed by flood events to life and property, is particularly
appropriate for the urban setting of the study area. By definition it provides an indication of the
probability of a property being flood affected during a given time frame, in addition to the degree
of hazard that it would experience.

The study has ultimately provided a good foundation from which to prepare the floodplain risk
management study and plan as the next step in the floodplain risk management process.

Tags: Old Guildford, hydraulic model, hydrologic model
Organization: Fairfield City Council