Walcha Floodplain Risk Management Study by lindahy


Walcha Floodplain Risk Management Study

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									Walcha Floodplain
Risk Management Study
Volume 1 of 2
April 2009
                                            Walcha Floodplain Risk
                                             Management Study

       Prepared For:               Walcha Council                                              Morwell
       Prepared By:                BMT WBM Pty Ltd   (Member of the BMT group of companies)    Sydney

                                                 DOCUMENT CONTROL SHEET

 BMT WBM Pty Ltd
 BMT WBM Pty Ltd
 Level 11, 490 Upper Edward Street                             Document :                   R.B14260.010.02.FPMS.doc
 Brisbane 4000
 Queensland Australia
 PO Box 203 Spring Hill 4004                                   Project Manager :            Cathie Barton / Fanny Houdré

 Tel: +61 7 3831 6744
 Fax: + 61 7 3832 3627

 ABN 54 010 830 421
                                                               Client :                     Walcha Council

                                                               Client Contact:              Steve Roberts

                                                               Client Reference

 Title :                        Walcha Floodplain Risk Management Study
 Author :                       Cathie Barton, Fanny Houdré
 Synopsis :                     This document details the Walcha Floodplain Risk Management Study. On the basis
                                of this Study, the Floodplain Risk Management Committee recommended a scheme of
                                measures presented here for inclusion into the Floodplain Risk Management Plan.
                                The Plan exists as a separate document.


         REVISION                  DATE OF ISSUE                          CHECKED BY                             ISSUED BY
               0                        4 Jul 2008                  GJR                                    CLB
               1                       12 Mar 2009                  SAW                                    FMH
               2                       29 Apr 2009                  SAW                                    FMH


            DESTINATION                                                               REVISION
                                                       0                         1                 2                         3

 Walcha Council                                      3 + PDF                3 + PDF              3 + PDF
 BMT WBM File                                          1                         1                 1
 BMT WBM Library                                      PDF                    PDF                  PDF

FOREWORD                                                                                                                I

             The NSW State Government’s Flood Policy is directed towards providing solutions to existing flooding
             problems in developed areas and ensuring that new development is compatible with the flood hazard
             and does not create additional flooding problems in other areas. Policy and practice are defined in
             the NSW State Government’s Floodplain Development Manual (NSWG, 2005).

             Under the Policy the management of flood prone land remains the responsibility of Local
             Government. The State Government subsidises flood mitigation works to alleviate existing problems
             and provides specialist technical advice to assist Councils in the discharge of their floodplain
             management responsibilities.

             The Policy provides for technical and financial support by the State Government through the following
             four sequential stages:

                                            Stages of the Floodplain Risk Management Process

                            Stage                              Description
                   1        Flood Study                        Determines the nature and extent of the flood problem.
                   2        Floodplain Risk Management         Evaluates management options for the floodplain in
                            Study                              respect of both existing and proposed developments.
                   3        Floodplain Risk Management         Involves formal adoption by Council of a plan of
                            Plan                               management for the floodplain.
                   4        Implementation of the Plan         Implementation of management options to protect
                                                               existing development. Use of environmental planning
                                                               instruments to ensure new development is compatible
                                                               with the flood hazard.

             This study represents the second of the four stages for Walcha. It has been prepared for Walcha
             Council to describe each of the management options considered by the Floodplain Risk Management
             Committee. Not all options presented in this document will be accepted by the Committee for
             inclusion into the Plan. The Floodplain Risk Management Plan contains a summary of the selected
             options. Council will use the Plan as a resource in implementing the selected options.

             The NSW State Government has provided financial assistance towards the cost of the study under its
             Floodplain Management Program.

EXECUTIVE SUMMARY                                                                                                   II

             A Floodplain Risk Management Study has been undertaken as part of the Floodplain Risk
             Management Process (as outlined in the Foreword) for Walcha township. The Study evaluated
             potential measures to manage existing, future and continuing floodplain risks. Based on the findings
             of the Study, the Floodplain Risk Management Committee then recommended measures for inclusion
             in the Walcha Floodplain Risk Management Scheme. This Scheme was then outlined in the Walcha
             Floodplain Risk Management Plan to guide implementation of the measures.

             The township of Walcha is located on the Apsley River in the Northern Tablelands of New South
             Wales. Walcha has a long history of flooding, including severe flooding in the 1860’s and more
             recently in 1962 that led to mitigation works including channel straightening and levee construction in
             the late 1960s. Since then, the township has not been flooded (as of 2008) leading to a general
             feeling of immunity within the community.

             A preliminary review of the levee system and local drainage issues in 2001 identified that the level of
             flood immunity is not as high as perceived in the community and that Council’s development controls
             provided no guidance with respect to flooding risks. An audit of the levees by the Department of
             Commerce in 2002 reported that, subject to implementation of remedial works, the levees are in a
             sound condition. In view of the recommendations of these reports Walcha Council established its
             Floodplain Risk Management Committee in 2002. The Committee’s role is to oversee the studies, the
             preparation of a Floodplain Risk Management Plan and to make recommendations to Council.

             Existing flood behaviour in Walcha due to Apsley River flood events was defined in the Walcha Flood
             Study (WBM, 2005). In particular, flooding of the Walcha township is caused by two mechanisms:

             •     Flooding along the Apsley River, generated by intense rainfall upstream of Walcha; and/or.

             •     Localised flooding due to failure of the stormwater drainage system (especially in the Blairs Gully
                   catchment area behind the western levee)..

             Predicted numbers of affected properties for flooding along the Apsley River were derived for a range
             of events from the 20 year ARI to the PMF (probable maximum flood) as part of the Walcha Flood
             Study (WBM, 2005) and are summarised in the table below:

                                                               Number of      Number of
                                                   ARI         Residential   Commercial
                                                  Event AEP    Properties     Properties
                                                 (years)        Damaged       Damaged
                                                 PMF     0%        92            78
                                                 500y   0.2%       43            55
                                                 100y   1.0%       14            41
                                                  50y   2.0%        3            39
                                                  20y    5%         0            28

             Current development controls do not reflect this level of risk for the township of Walcha. Moreover,
             no adequate flood warning system is currently in place in the area, with no operational ALERT rain
             gauge or water level ALERT gauge upstream of Walcha.

EXECUTIVE SUMMARY                                                                                                 III

             During the process of this Floodplain Risk Management Study, many options for reducing the flood
             risk to Walcha were considered and some were investigated in detail. Reduction of existing, future
             and continuing flood risk was considered via three broad types of options: flood modification, property
             modification and response modification.

             Flood modification options are those options that alter the physical behaviour of the flood. For
             Walcha, these included levee modification and augmentation, river bed modification and changes to
             the Blairs Gully drainage system. Property modification measures are those measures that seek to
             alter existing and/or future property to reduce the number of buildings that are inundated and thus
             damaged. For Walcha, the investigation focussed on development controls aimed at controlling
             future development on the floodplain. Response modification measures aim to increase the ability of
             people to respond appropriately in times of flood and/or enhancing the flood warning and evacuation
             procedures in an area. For Walcha, these include community flood awareness and preparedness,
             flood predictions and warnings, and emergency response planning for floods.

             Of the measures considered and investigated during the study, the Floodplain Risk Management
             Committee recommended that the following 8 measures be included in the Floodplain Risk
             Management Scheme used to develop the Floodplain Risk Management Plan..

EXECUTIVE SUMMARY                                                                                                            IV

                      Measure                               Cost                          Benefit                BC Ratio

                Minor Levee Bank
                                                          $211,440                       $104,900                   0.5

                                                                                  Reduction of future and
                  Levee System                                                                                 Not possible to
                                                           $40,200               continuing damages – not
                 Remedial Works                                                                                  determine
                                                                                    possible to quantify.

                  Levee System                    Council’s Normal Operating    Reduction of future damages    Not possible to
                  Maintenance                              Budget                – not possible to quantify.     determine

                                                                                 Reduction of levee erosion
                                                                                                               Not possible to
              Levee Bank Capping                          $194,000              problems and social benefits
                                                                                 – not possible to quantify

             West Levee Drainage                                                                               Not possible to
                                                          $186,000               To be further investigated
               Augmentation                                                                                      determine

                                                                                Aesthetic and environmental
                 Minor River Bed                   Dependent upon scale of
                                                                                  benefits– not possible to    Not Applicable
                  Modification                     modification and planting

                                                  Council’s Normal Operating    Reduction of future damages    Not possible to
                                                           Budget                – not possible to quantify.     determine

                                                                                  Reduction of future and
                  Walcha Flood                   $229,800 + Council, BoM and                                   Not possible to
                                                                                 continuing damages – not
                 Warning System                  SES normal operating budgets                                    determine
                                                                                    possible to quantify.

                                                                                 $104,900 + aesthetic and
                                                  $861,440 + Council, BoM &
                        TOTAL                                                   environmental benefits and          0.51
                                                 SES Normal Operating Budget
                                                                                future reduction in damages

    Based on that measure with a calculable BC Ratio only


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