EXECUTIVE SUMMARY

The Griffith CBD catchment forms part of the overall Main Drain “J” catchment. A key component of the drainage system are the inverted siphons or subways that convey CBD catchment flow to the south under the Main Canal.

Flooding of sections of the Griffith CBD has occurred in the past. This has been attributed to the insufficient capacity of the existing pipe drainage system, the flat topography across much of the area and the impediment to overland flow paths posed by the Main Branch Canal and the Temora-Roto Railway Line that passes through the middle of the catchment. Previous investigations have generally covered the broader areas of the Main Drain “J” system whilst detailed drainage studies were carried out only for the sub-surface drainage systems at the commercial areas, i.e. Yambil Street.

In order to define the overland flow flood behaviour and assess the flood liability of Griffith, WMAwater have been given the task of carrying out an overland flow flood study for the Griffith CBD catchment. As part of this work various data sets have been collected, previous reports have been reviewed, a community survey has been carried out, calibration data has been collected and also models have been built. A key step in the model build process is model calibration and validation and this work has been carried out successfully. Model results generally match observed flood depths as well as indicative flood behaviour observed from historic photographs provided by Council and local residents.

Design flood modelling indicates that, with respect to inundation of properties, flood liability within the study area is widespread. Of the 4,000 properties in the study area, it appears that 168 are flood affected (though not necessarily flooded over floor level) during the 1% AEP event, 52 of which are residential and 116 are commercial/industrial/special use. A further key feature of the flooding is that flood liability of property is high for the relatively flat CBD area where the commercial and industrial buildings are concentrated at, with floodwaters “build up” behind the Main Canal as well as areas upstream of the railway embankments.

Another feature of the flooding levels or depths is that the flood levels or depths do not change significantly from event to event. This has specific implications for the establishment of the Flood Planning Level (to be carried out in the subsequent Floodplain Risk Management Study and Plan) since the 1% AEP + 0.5 m FPL will in all but very few cases exceed the PMF level. For the same reason, the Floodplain Risk Management Study and Plan will also have to carefully consider the approach to take when defining the Flood Planning Area (FPA).

Results of property inundation indicate that within the subsequent Floodplain Risk Management Study, certain areas currently flood liable Study, certain areas currently flood liable in even small events should be focused on in regards to mitigation efforts. These areas include Yambil St, Wakaden St, Noorla St, Cutler Ave and Lenehan Rd. In regards to flood liability within the study area, one of the key issues is that the capacity of the drainage systems to discharge flow is limited in most areas.

Solutions to the Griffith CBD flooding problems are likely to include: (a) detention of runoff (preventing water from travelling downstream); and (b) enhancing flow conveyance downstream. The issue with the latter however will be the possibility of simply transferring the existing flood problem downstream. For example, commercial and residential properties downstream of the Main Canal are currently protected from floods due to the restricted conveyance from upstream of the Main Canal to the downstream areas.

Study, certain areas currently flood liable in even small events should be focused on in regards to mitigation efforts. These areas include Yambil St, Wakaden St, Noorla St, Cutler Ave and Lenehan Rd. In regards to flood liability within the study area, one of the key issues is that the capacity of the drainage systems to discharge flow is limited in most areas.

Solutions to the Griffith CBD flooding problems are likely to include: (a) detention of runoff (preventing water from travelling downstream); and (b) enhancing flow conveyance downstream. The issue with the latter however will be the possibility of simply transferring the existing flood problem downstream. For example, commercial and residential properties downstream of the Main Canal are currently protected from floods due to the restricted conveyance from upstream of the Main Canal to the downstream areas.