Garnet Street RFI_20150513_ITC (Part 1)

A risk assessment has been completed on the travel times within the culverts in accordance with Chapter
12 of the Queensland Urban Drainage Manual (QUDM). The risk assessments are attached with this
document.

Assessment for the Requirement of Inlet Grates
Following a risk assessment of the pipe and culvert structures, it was determined that further evaluation
was required to determine whether inlet grates are required to be installed.
Reference is made to Section 12.04.3 of QUDM – Site conditions where barrier fencing or inlet/outlet
screens may not be appropriate. This section highlights that examples where inlet/outlet screens may not
be appropriate include:
   i.   Where the overall risk to human life as a result of the installation is judged to be greater than if the
        device was not installed.
  ii.   Where there is an unacceptable risk of trapping wildlife (aquatic or terrestrial) or causing
        disruption to an essential wildlife corridor.
 iii.   Where debris blockage of the fence or screen will cause or increase floor level flooding. The
        degree of blockage must be comparable with the existing and likely future catchment conditions.
The installation of the grates has the potential to impact on all of the abovementioned items.
It can be argued that the installation of grates can result in a greater overall risk to human life whereby
someone could be pinned against the inlet grates during a storm event. Without the installation of inlet
grates, a person would be swept through the pipes/culverts and exit from the outlet. This is just one
example where the overall risk to human life is increased due to the inclusion of inlet grates.
The inclusion of inlet grates creates a serious potential for wildlife to be trapped. Similar to the example
offered above, it could be conceived that wildlife has the potential to be pinned or trapped against the
grates during storm events. Without the installation of inlet grates, this risk of being trapped is removed as
the wildlife would be swept through the pipes/culverts.
Inlet grates also have the potential of disrupting the wildlife corridor through issues associated with
blockage and the potential for debris to accumulate effectively isolating the southern and northern sides of
Garnet Street. The exclusion of inlet grates reduce the potential for this to occur as the size of the pipes
and culverts are large enough to not collect debris and maintain connectivity between the southern and
northern sides of Garnet Street.
The most important argument for excluding inlet grates is associated with the balance between achieving
non-worsening of flood inundation levels upstream and downstream of the Garnet Street crossing. Inlet
grates will produce increased blockages at the inlet to the pipes and culverts due to the collection of debris.
This will ultimately result in increased flood inundation levels to properties upstream of the culvert
crossing.
The location of the pipes and culverts has been identified as a low risk area due to it being located in a
semi-rural area with very little pedestrian traffic along Garnet Street. The footpaths are located on the
northern side of Garnet Street, reducing the volume of pedestrian traffic directly adjacent to the inlet of the
pipes and culverts. Guardrail has been included on the southern side of Garnet Street to further prevent
and deter entry to the inlet of the pipes and culverts.
Apart from the safety and performance issues identified above, maintenance requirements should also be
considered by Council. It is not unreasonable to suggest that the large size of the proposed culverts will be
primarily self-cleansing and installation of excessively large grates will require more regular maintenance
and potentially impact on performance. During a recent storm event onsite, a representative from SMEC
witnessed the event that involved the existing culverts flowing at full capacity. It was verbally reported that
the pipes did not suffer from blockage due to debris and it could be assumed that the inclusion of larger
pipes would also not impact from the collection of debris.