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Cardno Forbes Rigby (CFR) Report: Flood Study and Stormwater Management Plan on Collector Village Dec 2007

Review of Report Regarding its Potential to Limit Upper Lachlan Shire Council's Legal Liability if the Proposed Culvert is Built at Collector

Introduction

This Report was commissioned by Upper Lachlan Shire Council and is apparently in response to the flooding of the new housing area of lower Bourke Street in prior years. It focuses on documenting the characteristics of major flood events (mostly 1 in 100 years) in and around Collector. The report proper consists of 26 pages with a framework of Introduction, Available Data , Hydrology, Flood Levels and Flood Planning, Stormwater Master Planning. In addition there is a Conclusion, References, and 7 Appendices.

On 20 May 2010, Council discussed its plans to replace the Collector Bridge with a low level culvert and to dig up and lower the road for 100 metres either side of it. In response to issues of safety, damage to property, the legal liability of Council and unnecessary damage to the beautiful and historic entrance to the town raised by some residents, Council referred the matter to CFR for advice on whether a low level culvert or high level one is appropriate. This seemed a curious step by Council since:

  • the original Report had a very different purpose
  • clearly identified that there were a number of assumptions and limitations which limited its ability to make specific findings
    • and
  • Found that the height of the current bridge successfully deals with floods

The details are outlined below.

Assumptions and limitations

The Report makes clear there are many assumptions and limitations in its study. These include the following:

  • "...limited rainfall data is available that would enable a reasonable representation of past events. For example, daily rainfall values are not accurate enough to use got calibration of a hydrologic model - pluvio-graph style data would be required for this purpose."(p6)
  • "Combined with the unknown dates of particular floods and their corresponding levels, we can only ascertain the flood level of the 1952 flood...(ie, from all of the flood events that have occurred in Collector since 1892 - when rainfall was first recorded.) (p6)
  • "Andrew Brooks of DECC CFR advised in November 2007 that no water quality data is available for the area....it recognises [sic DEC]...the effect of structures on water quality. These will have an impact on any future development int eh catchment."(p7)
  • "The NSW State Government Flood Plain Development Manual...The document is ... adhered to quite rigidly [ie, by this Report](although lack of clarity has clouded its application in some areas, as for example in the application of blockage criteria on structures)"... .(p7)

In relation to their modelling approach CFR in listing its assumptions, states that it:

  • allows 0 mm for initial loss (pervious surface) reflecting prior rainfall not accounted for in storm burst
  • 0 mm initial loss for impervious surface
  • continuing loss pervious surface of 2.5 mm/hr
  • C lag parameter of 1.7 which is the Regional default for NSW
  • Stream routing factor 1.00 model default for naturals streams (p9)

Since the above are general guides and not based on empirical data relating to the Collector Catchment, they are of limited use in a situation where there is a specific legal liability issue for Council.

One of the above assumptions is further explained as follows: "We believe an initial loss of 0mm is appropriate as it indicates that there is no storage available within pervious surfaces (ie soils) which would be fully saturated by rainfall prior to the critical storm 'burst' ". Thus the model assumes in all cases there will be a period of prior rainfall which will lead to saturated soils and therefore regular flow characteristics. While this may be necessary for developing a standard flow model it is clearly not what happens in real rainfall events.

In addition the above assumption, this study does not deal with the effect of acquifers and other subsoil structures on the surface waterflow and flooding characteristics at various points of the catchment. For example, there is considerable anecdotal evidence that after prolonged rain water actually wells up out of the ground around the former Lynwood Cafe. In this situation, the proposed low level culvert in a flood may well fill with debris, act like a mini dam, raising ground water and thereby exacerbating the flooding of this property. (p10) This analysis would seem plausible as any given that on page 12 of the Report it notes that the Collector catchment waterflow has a considerable lag factor.

In using their standardised modelling techniques, the CFR consultants are careful to not lock their findings into any definitive statement. For example, in relation to their use of the Water Bounded Network Model (WBNM), they note that: "the lower peak flows estimated...can be readily explained by the Collector Creek catchment's unique connectivity, catchment shape and drainage pattern and its inherent lag." (p12) As with the many other qualification in the Report, if Council wishes to make a definitive assertion based on such matters, CFR won't be responsible.

In the Hydraulic Model Section, the CFR consultants further qualify their findings. For example, in relation to their HECRAS model they state that: "This model is well suited to the task of determining flood levels within normally expected modelling tolerances..." .(p13). Again, it is another caveat emptor situation for Council, CFR cannot be held responsible if there may be other factors outside "normally expected modelling tolerances..." .

CFR continues to qualify their data in the Modelling Approach Section on page 13 the Report. They state that, due to lack of data, estimates were required "for the southern part of Collector Creek Reach. To overcome this problem a channel was introduced into our model, based on the ortho-rectified aerial imagery, interpreted stream slope, interpreted invert levels of the structures and field measurements..." . Further in this section (p16) they refer to their use of another standard model "Mannings n values for Existing Conditions".

On page 17 limitations of data available for hydraulic model construction are discussed. The key statement here is that "Because...survey data showing creek morphology was not available downstream of the Federal Highway we selected the Federal Highway as the downstream limit of the hydraulic model." By their own omission the picture they are presenting is therefore incomplete.

On page 19 the Report discusses results and comparisons with historic floods. It notes that "because pluviograph data is not available for the catchment, it is difficult to determine the exact size of corresponding storms." The only "major" flood event this section deals with is one which apparently occurred on 17 June 1952 - where the information is based on rainfall data and the recollections of one resident (!). This would hardly seem to be a reasonable basis for making projections. The rest of the information in this section is based largely on anecdotal information. This is hardly the kind of "evidence" which could be relied on for legal liability purposes of Council.

The Section on Stormwater Master Planning (pp23-27) does not deal with creek flood dynamics and therefore is not relevant to the discussion here.

The recommendations section (p28) consists of four items. These are, in summary:

  • Additional flooding analysis needs to be undertaken to assess the opportunities to minimise flood impacts on low lying areas around Collector
  • A more detailed hydraulic model be constructed to determine flood hazard
  • Additional downstream and upstream survey and whole survey of floodplains is undertaken and incorporated into future hydraulic models
  • A pluvimeter is installed to accurately capture rainfall data pertaining to the intensity of rainfall within the various parts of the catchment

These recommendations thus make plain that there is not sufficient data for Council to be definitive about its claims that the low level culvert will be safe in high rainfall/flood circumstances.

Conclusion:

The referral of this matter by Council on 20 May to CFR for advice on whether a low level culvert or high level one is appropriate has already been answered by this Report. Essentially it says that, while there is insufficient data available to model floods in any detail, there is sufficient evidence that the existing bridge structure has been "sized large enough by the RTA (during road construction) to fully convey the peak of the 100 year ARI flood." (p10).

The proposal by the Works and Operations Branch of Council for a low level culvert in Collector is therefore contradicted by the CFR Report. This demonstrates that the low level culvert is a cost cutting exercise at the expense of the safety of Collector residents, visitors who use the road and an unnecessary destruction of the most significant heritage area of the town. It is a shameful situation that the residents may be forced to take this matter to the Land and Environment Court to obtain a decent, safe bridge with a height which has been proven as correct by 130 years of experience and the Council's own consultants.


Frank Ross
6 June 2010