Gulgong Stormwater Drainage Study June 2009

Following public exhibition of the Study in August 2009, Council at its Strategic Committee Meeting held on 7 October 2009 resolved to adopt the Gulgong Stormwater Drainage Study June 2009.

To download a copy of the study, please click here  (2mb)

SUMMARY

The urban area of Gulgong comprises two stormwater catchments which have been denoted Catchments X and Y for the purposes of the study. The catchments are separated by a north-south ridge of high ground and are drained by a system comprising kerb and gutters, and sections of pipeline. In general the system is capable of controlling minor storm events up to the 5 year ARI magnitude, in conformance with accepted engineering practice.

During major storm events which surcharge the capacity of the piped drainage system, or in areas where there is no existing piped drainage, several of the streets which are aligned with the direction of flow act as overland flow paths. There are also areas in Catchment X where residential development is located on the low side of the street (Wynella, Young and Caledonian Streets) and where overland flow occurs through allotments.

Site inspection of the drainage networks in Catchments X and Y was undertaken during the course of the study to identify directions of gutter flow and clarify details shown on drawings of the existing drainage system supplied by Council. Locations where problems are experienced during major storm events due to surcharges of the stormwater system were identified by Council and from information provided by residents resulting from the issue of a Community Questionnaire at the commencement of the study.

Hydrologic models of the catchments were prepared using the DRAINS rainfall runoff program. The results of the model were used to determine the behaviour of the systems for storms ranging between 5 and 100 years ARI.

Hydraulic modelling was undertaken to assess depths and velocities of flow in several streets during major storm events. Those parameters were related to the provisional flood hazard (which is dependent on the product of velocity and depth of flow) using principles set out in Australian Rainfall and Runoff, 1998 and the Floodplain Development Manual, 2005.

Street and gutter flow in the street systems of Catchments X and Y may lead to unsafe situations along some roads during major storms. At those locations new piped drains have been sized which have a capacity sufficient to convey 20 years ARI flows. The purpose of the improvements is to reduce overland flows in the streets in the event of a 100 year ARI storm. A preliminary assessment of pipeline requirements for the two catchments, together with indicative costs and priorities for implementation are summarised in Table S.1. Locations of potential schemes are shown on Figure S1.

The investigation has been undertaken using existing sources of data mainly comprising 1m contour plans of the catchments. Consequently, refining cost estimates and the preparation of concept designs for the schemes will require additional survey information.

It is not feasible to implement large community owned retarding basins to reduce downstream flood peaks to the capacity of the exisiting system due to the absence of suitable sites. Consequently, stormwater management in Gulgong will need to rely on augmentation of the existing pipes stormwater system in the streets, in conjunction with drainage swales to convey overland flows through several residential allotments in Catchment X. The co-operation of the owners will be required to enable construction of the swales to proceed, and easements will need to be created for the maintenance of these flow paths.