Mudgee Local Creeks Flood Study 2008

Background

Mid-Western Regional Council commissioned Lyall & Associates to prepare a Floodplain Risk Management Study and Plan for the floodplains of the Mudgee Local Creeks. This work was completed in March 2008. A number of documents were produced that are available below:

  • Floodplain Risk Management - Study And Plan    (6mb) 
  • Flood Behaviours Studies (Volume 2)    (7mb),    map (5mb)  
  •  - Catchment A     (5mb) 
  •  - Catchment B     (4mb)  
  •  - Catchment C     (5mb)  
  •  - Catchment D     (527 kb)  
  •  - Catchment E      (1mb)  
  •  - Catchment F     (3mb)  
  •  - Catchment G - Sawpit Gully    (3mb) 

 

Mudgee Catchments

Mudgee Catchments Flood

Study Objectives

The overall objectives of the investigation were to assess the impacts of flooding, review policies and options for management of flood affected land and to develop a draft Floodplain Risk Management Plan (FRMP) which:

  • Proposes modifications to existing Council policies to ensure that the development of flood affected land adjacent to the creeks is undertaken so as to be compatible with the flood risk. 
  • Proposes Flood Planning Levels for various Land Uses in the floodplains.
  • Sets out the recommended program of works and measures aimed at reducing over time, the social, environmental and economic impacts of flooding.
  • Provides a program for implementation of the proposed works and measures.

Study Activities

The activities comprising this study included:

  • Review of flooding patterns in the floodplains of the Local Creeks at Mudgee.
    Assessment of the impacts of flooding on urban development bordering the creeks.
  • Review of potential floodplain management measures aimed at reducing flood damages. 
  • Ranking of measures using a multi-objective scoring system which took into account economic, financial, environmental and planning considerations. 
  • Review of the current Development Control Plan (DCP) “Managing Our Flood Risks”, which currently provides flood related controls over development of the floodplains within the Mid-Western Shire Council LGA, and recommendations as to its suitability for adoption in the Mudgee Local Creeks floodplains.
  • Preparation of a draft FRMP for the Mudgee Local Creeks.

Summary of Flood Impacts

The Study deals with the floodplains of the Mudgee Local Creek system which comprises seven creeks on the southern bank of the Cudgegong River draining the urban areas of Mudgee and having a total catchment area of about 20 km2. Flooding on the streams is “flash flooding” in nature with flood levels peaking less than 30 minutes after the commencement of heavy rainfall.

The creek channels in general have low hydraulic capacity and would be surcharged by comparatively minor storms of around the 5 year ARI. Apart from the channels of Catchment A on the western side of town, Sawpit Gully on the eastern side and Catchment D which drains the Mudgee CBD, significant lengths of the creeks traverse residential allotments without the benefit of drainage easements.

Residences have been constructed adjacent to the creeks and in many cases, flows are obstructed by inter-allotment fences and gratings placed across the channels to restrict the movement of domestic animals.

A major storm occurred in February 2003, which, on the basis of recorded rainfalls may have caused flood peaks greater than the design 100 year ARI values. Although numerous residential allotments were flooded, little above floor inundation appears to have been experienced. The main areas of flooding appear to have been at Gladstone Street in Catchment A, Cox Street in Catchment B, the lower reaches of Catchment C downstream of Market Street and at Industrial Road in Sawpit Gully.

The detention basin constructed by Council near the Showground on Catchment C reduced downstream flood peaks as far as Douro Street and improvements to the piped stormwater system in Catchment D appear to have prevented flooding in the Mudgee CBD.

Table S.1 (below) shows predicted damages to residential property in the event of a 100 year ARI flood. Total damages in the creek system are predicted at $930,000, with most of the damages being experienced in the residential area bordering Cox Street in Catchment B.

TABLE : S.1 - RESIDENTIAL PROPERTIES FLOODED BY 100 YEAR ARI FLOOD
Catchment Number of Flooded Allotments Number of Residences Flooded (above Floor level) Flood Damage $ x 103
A 14 3 225
B 31 8 575
C 13 0 90
E 3 0 25
F 2 0 15
Sawpit Gully 0 0 Neg.
TOTAL 63 11 925

The Floodplain Risk Management Plan

The draft Floodplain Risk Management Plan showing recommended flood management measures is presented in Table S.2 and shown on Figure S1 (in the attached document). The measures comprise:

  • Refining the broad scale analysis undertaken in Volume 2 of this investigation of detention basins, channel improvements and enlargement of waterways at road crossings. An Investigation/Concept Design Study is proposed to provide more detailed costs and confirm priorities for implementation. The Study would be carried out with the benefit of more detailed survey of the areas bordering the creeks. 
  • Undertake works on the catchments according to the results of the above Study. A preliminary assessment of the works and their priorities is given in Table 3.2, with a more detailed consideration presented in Volume 2.
  • The application of the graded set of planning controls included in the flood policy document “Managing Our Flood Risks” for future development on the creek floodplains.  This policy was formulated in the Mudgee Floodplain Management Study and Plan, 2002 for controlling development in the LGA, including land subject to flooding from the Cudgegong River and is also suitable for application in the floodplains of the creeks. Application of these controls by Council will ensure that future development in the floodplains is compatible with the flood risk.
  • Improvements in the SES’s emergency management planning for Mudgee, including incorporation of the information in this study in the Local Flood Plan for Mudgee. 
  • A feasibility study and development (subject to the results of the feasibility study), of a Flash Flood Warning System for the catchments.
  • The encouragement of flood proofing measures and preparation of individual Business Flood safe Plans for industrial properties bordering Sawpit Gully subject to flooding.

The timing of the proposed works will depend on Council’s overall budgetary commitments and the availability of funds. It is envisaged that the works and measures nominated in the Plan could take 5 to 10 years to complete. Four high priority drainage improvements on Catchments A and B have been identified which would mitigate flooding and associated damages in residential areas of those catchments for floods up to the 100 year ARI magnitude (see Table S.1 above). These improvements comprise channel improvement Options A.1 and A.2 on Catchment A and detention basin and drainage pit Options B.1 and B.2 on Catchment B. The indicative cost of these four works is $795,000.

The Need for Better Survey Information

Improved survey of the areas bordering the creeks is a pre-requisite for the recommended Investigation/Concept Design Study of drainage improvements. The survey would also assist Council with the application of the Flood Policy DCP and implementation of a Flash Flood Warning System, as the extent and depths of inundation along the creeks could be identified with greater accuracy than is presently possible with the available 2 m contour mapping.

The survey could be provided at comparatively modest cost by undertaking an Airborne Laser Survey of the study area, which would achieve accuracies in defining natural surface levels in the range 150 - 200 mm. This would be a vast improvement on the accuracy of existing mapping sources and would also assist Council in the planning and design of other engineering and town planning disciplines (roads, stormwater management, strategy studies and the like). However, the cost of the survey would be outside the scope of the NSW Government’s floodplain program and would therefore need to be borne by Council.

Note that since this report was prepared, Council has obtained the ALS referred to above