North Wagga Submission for BMT Group Peer Review

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NORTH WAGGA RESIDENTS’ ASSOCIATION MARCH 2021 | Fi Ziffwhich could be quite easily addressed if Council were genuinely interested adequately protecting NorthWagga.(vi) North Wagga will become a swamp for longerThe notion that North Wagga should not be protected by a 1 in 100 levee because it will become a swampfor longer, mentioned several times in the plan if the levee is raised, is flawed. Once again, Council did nothave this discussion with residents. Residents believe that having a little longer inundation if an upgradedlevy is overtopped is better than more frequent flooding (and more frequent damage events, displacementetc) by having a lower levee.The 2018 FRMP states “In flood events that overtop the North Wagga levee, floodwaters can be impoundedby the ring levee bank for many days or weeks. Accounts from residents of North Wagga have noted thevillage becoming like a swamp following the 2012 event, as the contaminated floodwaters could not drainuntil the river flood level had dropped”. The context of feedback on the issue of North Wagga being aswamp is omitted. Residents in North Wagga have raised this in relation to council’s lack of vegetationmanagement impeding flow and causing extended periods of flooding, to make the point that vegetationmanagement needs to be addressed (to reduce flood risk and duration) rather than as an argument againstan increase in the levy.Notwithstanding the above, in Section 7.2 of the 2009 FRMP, WMA confirms that geographically, there isno drainage issue in North Wagga when it states “a profile of the surveyed levee heights indicates that thedownstream side of the levee is sufficiently low to allow most floodwaters to escape should the levee beovertopped from the upstream side. The downstream side of the levee may require breaching to fully drainthe suburb and to assist in the reduction of ponding depth”. Once again, the two flood study documentscontradict each other.(4) The cumulative effects on flood levels from the thickenedvegetation and the Main City Levee upgrade, which expose NorthWagga to the increased risk of a Council-made flood, are notconsidered in the 2018 FRMP.Council undertook a revegetation project in 2002, planting thousands of trees and shrubs in the 34hareserve situated between North Wagga and the river, even though the effects that vegetation has on floodlevels is well-known.According to Peter Morris (former resident and former DMR employee with 30 years’ experience infloodplain road and bridge design) the location of this revegetation reserve is a “hydraulically criticallocation” between the two levees, known as a pinch point, where flood flow is most adversely affected dueto a narrowing in the floodplain.Page 20 of 25
NORTH WAGGA RESIDENTS’ ASSOCIATION MARCH 2021 | Fi ZiffFollowing a study of the 2012 flood event it was established that the river conveyance had significantlydeclined because far less water flowed in 2012 (3600 m3/s) compared to 1974 (5200 m3/s). WMAwater’s2014 Flood Model Revision Report concluded that “the change in stage/discharge relationship at Wagga issubstantially due to vegetation changes on the floodplain that have occurred over time. More specifically“a change in effective roughness of approximately 20% has led to the stage/discharge relationship changingsuch that a given flow now produces relatively higher flood levels. A flow of 3,000 m3/s previouslyproduced a height of 9.9 m, the same flow is now estimated to produce a height of 10.1 m, an extra200mm”.The reserve is protected by the biodiversity protection legislation and cannot be touched. It has had eightyears to thicken since it caused the last flood and will likely have an even greater impact on flood levels inthe future. It is therefore a critical factor to take into consideration when exploring flood risk mitigationoptions for North Wagga. It is not however a consideration in the 2018 FRMP.Residents’ do wonder how it is possible, that Council was able to proceed with this revegetation project inWagga’s most vulnerable floodplain location and then ignore its obvious impact on our community. It is inour view sheer negligence.The Main City Levee also has an impact on flood levels of some 0.2m, according to WMAwater, which whenadded to the thickened vegetation impact, will likely cause flood levels to rise by some 500mm or more.In our view the impacts need to be considered if there is to be a genuine investigation of adequate leveeoptions for North Wagga. Residents believe that under the circumstances, the 1 in 20 levee is not adequatehowever the impacts are excluded from the 2018 FRMP.(5) The 2018 FRMP lacks assessment of intermediate Levee options.The logic seems to be that while the 1% AEP levee gives a high BC ratio, indicating that it is economicallyfeasible despite the costs, there are “other concerns” which outweigh this, such that it is discounted fromfurther consideration. Notwithstanding the concerns highlighted in this paper in relation to the analysison the effects of a 1% AEP levee, this approach is flawed.A standard approach in modelling any stepwise change is to model intermediate levels. So, to model only1% and 5% AEP (i.e., a 5-fold range) is absurd. Surely an intermediate AEP should be modelled? A 5% AEPresults in a reduction in AAD of around $650k while a 1% AEP results in a reduction in AAD of around $1.3m,so presumably an intermediate AEP will give an intermediate reduction in AAD but at a lower risk of otherissues such as loss of life.Section 7.2 of the 2009 FPRM Study confirms that a 1 in 60 levee was in fact modelled. It is stated thatwhile the 1 in 100 levee showed only “small” increases upstream, the 1 in 60 levee showed less impact.Wouldn’t it be logical for the Committee to seriously consider the 1 in 60 levee as a potential alternative?Instead, without any further investigation and in the absence of any pertinent information, the FloodplainCommittee apparently discusses the issue of the 1 in 60 levee and was apparently in a position to consider“issues of equity, risk and impact, cost benefit, the history of the current design level and what benefit ahigher levee would actually achieve” and conclude that the 1 in 60 levee was not appropriate.Page 21 of 25
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North Wagga Submission for BMT Group Peer Review

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