Smithfield Overland Flood Study

The 292 ha Smithfield overland flow catchment is located in the north-eastern portion of the
Fairfield LGA, to the south-west of Prospect Creek, and encompasses parts of the suburbs of
Smithfield, Fairfield Heights and Fairfield West. The catchment is roughly bisected lengthways by
the Cumberland Highway. The catchment is highly urbanised and comprises residential, industrial
and commercial development.

The catchment generally drains in a north-easterly direction via a network of stormwater pipes and
flow paths, with the main trunk sections of the network running alongside the Cumberland Highway, before diverging from the Highway before the final 200m of pipe discharges into
Prospect Creek, approximately 300m downstream of the Cumberland Highway bridge.
Because of urban and industrial development in the catchment, parts of the stormwater network
were not designed to cater for the progressive increase in impervious area. Flooding problems
along the main overland flow paths within the catchment are exacerbated by stormwater pipes built
under private property and by development extending to the top of bank of open channels.

The adopted modelling approach used XP-STORM to simulate the urban sub-catchment hydrology,
as well as the hydraulics of the stormwater pit and pipe network. Further, the approach using XPSTORM
allowed modelling of the stormwater drainage system in conjunction with the overland
flow in the two dimensional floodplain, with a dynamic link between the two components. The
dynamic link between the one dimensional pipe network and two dimensional floodplain, provides
the best representation of flood behaviour.

A one dimensional hydrologic and hydraulic model was initially established using topographic
survey, spatial data and rainfall data. Relatively standard values for network and hydrologic
parameters were assigned. A total of 259 pits and 278 pipes were represented in the model.

The floodplain in the XP-STORM model was defined as a two dimensional domain based on a 2m
topographic grid. Open channels were represented in the model but fencelines were excluded.

Buildings were treated as solid objects within the floodplain in which floodwaters could not flow
through. A downstream boundary condition was assigned based on the stage hydrographs
developed in Prospect Creek Flood Study (Bewsher Consulting, 2006).

The XP-STORM model was constructed such that overland flows may enter the next downstream
pit if there is sufficient inlet capacity. Flows in excess of the inlet capacity, or flows that surcharge
from the pipe network, form overland flow which are routed through the two dimensional domain.

Although the model could not be calibrated because of a lack of historical data, model results were
compared and found to agree relatively well with the findings from previous drainage
investigations and Council’s database of known flooding trouble spots.

Sensitivity analyses revealed that the XP-STORM model was not sensitive to changes in
Manning’s n roughness values and only partially sensitive to increases in rainfall intensities.

Increasing blockage factors of pits increased flood depths in some residential areas at the upstream
ends of the drainage network.

The model was run for the 20, 100, 200, 500, 2000, 10,000 year average recurrence interval (ARI)
events and the PMF, for a range of storm durations from 30 minutes to three hours. The peak water
level and velocity for each storm duration, at each 2D grid point, were extracted and used to form a
‘peak of peaks’ grid that was subsequently used a basis for the flood mapping.

Flood model results and the flood mapping for the 100 year ARI event indicate that:
? There are a number of overland flowpaths which originate in the upper catchment and carry
stormwater in a north to north-west direction towards the Cumberland Highway. Stormwater
is carried along Maud Street in the south of the catchment, across Reserve Street onto
Rosemount Avenue. In the east of the catchment, stormwater is carried along Oxford Street,
across Brennan Street and breaks through properties on Oxford Street to join the flow path
along the Cumberland Highway. In the west of the catchment, there is a flowpath along
O’Connell Street which carries stormwater south towards Brenan Street to the Cumberland
Highway.

? Overland flooding is generally deepest in open space areas adjacent to the Highway, where
flood water is ponding, particularly between Brenan Street and the Boulevard. Depths in these
are in the range of 0.6-1.0m. Flood depths in excess of 1m are located in the flowpath between
Rosemount Avenue and the Cumberland Highway. Typical depths of flooding at properties in
the upper parts of the catchment are less than 0.3m. In the middle part of the catchment a
number of properties in the Rosemount Avenue, Alexander Street and Brenan Street area are
affected by overland flood depths between 0.5m and 0.8m. In the lower catchment, north of
Horsley Drive, a small number or properties are affected by overland flood depths of up 1.2m.

? The depth of flooding in road corridors is typically less than 0.3m. Some roads experience
flooding greater than 0.5m deep, and include Percy Street, Beemera Street, Ainslie Street,
Musgrave Crescent, Reserve Street and Oxford Street. There are small sections of road on
Rosemont Avenue and Alexander Street where overland flood depths are greater than 1m.

? Overland flow velocities within properties in the 100 year ARI event across the study area are
typically less than 0.5m/s. There are some isolated areas (for example properties fronting the
Cumberland Highway) where flow velocities are between 1-1.5m/s. Higher velocities, greater
than 1.5m/s are observed on some streets, including the Cumberland Highway and Maud
Street.

Flood risk precinct maps were prepared based on modelling of the 100 year ARI (medium risk) and
PMF (low risk) events and using the flood risk precinct categories outlined in the Fairfield City-
Wide Development Control Plan. The flood risk precinct mapping has identified:
? Approximately 1365 properties are within the floodplain outline defined by the PMF event.

? Areas of high flood risk occur in: the lower catchment south of Kiola Street; in the middle
catchment at the Rosemont Avenue/Alexander Street intersection; and at the corner of Brenan
Street and Cumberland Highway.

? The medium flood risk precinct follows the pattern of the trunk drainage system from the
upper to lower catchment.

Tags: Smithfield, hydraulic model, hydrologic model
Organization: Fairfield City Council