Medowie Drainage and Flood Study

The flood study carried out for Medowie identifies flood liable land for a range of floods and finds
that many properties and approximately 20 residences, are likely to be flood liable during the 1%
AEP event. Recommendations going forward are as follows:
• In some locations existing drains are incapable of carrying 20 – 10% AEP (and in some
cases even lower) flows. An example of this is the open drain which passes through
Ballet Close from north to south. Enhancing the capacity of these trunk drainage
systems should be examined as this may achieve a degree of flood relief for those
properties which are exposed to flooding on a regular basis. Such an examination should
however ensure that flooding issues are not simply transferred downstream;
• Houses in County Close which remain subject to inundation should be closely monitored.
It is likely that local flows cause inundation of these low lying properties and minor works
by Council may alleviate this circumstance for impacted residents;
• More generally there a number of locations where, due to the limited range of design
flood behaviour (flood depths do not change markedly for variable AEP), minor works
such as bunding could provide good levels of flood protection for individual properties. A
good example of this is on Kirrang Drive (western side) where houses are currently
impacted by flows from the swale at the front of their properties;
• Generally Council needs to recognise the main overland flow paths identified in the study
and move to restrict development in these “corridors” in the future;
• In order to assess the issue in the CDIA with regard to upstream development potentially
increasing flooding downstream of Ferodale Road, satisfactorily answer questions in the
CDIA in regard to the impact of proposed development on the wet sequences which
often deny property owners of access/enjoyment of their land, Council need to have a
water balance type of exercise carried out. This will address the kinds of events which
impact on inundation within the CDIA but which do not have any relevance to the
definition of peak flood levels. It is noteworthy that during the Management Study
process it will also be necessary to ensure mitigation works designed to alleviate flooding
liability do not negatively impact on long term inundation for water sequences in the
CDIA. The Management Study should consider a range of solutions but ensure that
water quality is either maintained or improved;
• Council should take care not to raise those sections of roads currently inundated as
doing so will directly impact on peak flood levels. This is due to the fact that much of the
1% AEP flow moves over roads rather than through corresponding culverts; and
• During the Management Study it is recommended that runs are carried out which
examine the impact of removing the pinch which currently exacerbates the duration,
extent and magnitude of inundation further upstream in the CDIA; and
• Pinch plus pump capacity enhancement runs be carried out. Note unless pinch works
are modelled, nothing will be achieved by looking at pumping rate enhancement alone.

Tags: Campvale catchment, Medowie, Moffats Swamp catchment, hydraulic model, hydrologic model
Organization: Port Stephens Council