Dunboyne Combined Residents Association
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Submission to Meath County Council on the River Tolka Flooding Study - Final Report: 01.03.04
1. DCRA is pleased to receive the River Tolka Flooding Study - Final Report and to acknowledge the progress in implementation of same in the works carried out to date towards the alleviation of flooding in the Dunboyne/Clonee area. DCRA further acknowledges the Meath County Council response both to the urgency to protect the properties of residents vulnerable to flooding and to put in place medium and long-term flood protection measures.
2. After the flooding event in November 2002 tension developed between political urgency and professional procedures. Evidence of this is available from demands for immediate remedial action made by politicians and Dublin City Council against advice of the Dublin Drainage Consultantcy as expressed by the Minister of State at the Department of Finance in Dail Eireann on 19 November 2002. "My officials in the OPW have confirmed that the consultants carrying out the study are not in a position to indicate at this stage what the findings of the study will be or what flood alleviation measures can be recommended. Neither is it possible to undertake interim works at specific locations. In the absence of the full report of the study any interim works could exacerbate flooding problems upstream or downstream.
3. The appointment of M.C.O'Sullivan & Co. Ltd. to do this report "on
behalf of the Dublin Drainage Consultancy" implies a deficit in that it
does not involve the engagement of two internationally renowned and reputable
professional expert groups in this field, namely, Wallingford and Hyder. (Appendix
1) Secondly, "acting on behalf of" without involvement of the other
members of the consultancy group, makes the assertion less than credible. In
view of the Minister's statement, it cannot be said that MCO'S, when proposing
recommendations for immediate interim works that it was acting on behalf of
the consortium, that is to say, with the latter's collective approval.
Arrangements need to be made transparent so that due weight can be given to
the recommendations of the Report and the removal of any lingering political
suspicions.
4. It appears that initial urgent works proposed were not based on proper analysis and prior modelling to reach an adequate definition of the problem. As a consequence the solutions adopted consisted mainly of "flushing options" rather than a "retentive policy" that we advocated in our submission to you last year. Urgency and expediency has taken precedence over professional best judgement and expertise in deciding on a scheme of action. The political intervention in the autumn of 2002 clearly set in motion a policy rejected by European experience as unsustainable, ineffective and costly.
5. The final report shows that 32 items and some 6 kilometres of defences went
well beyond what could be termed immediate emergency action. The risk involved
did not amount to an emergency because the probability of two 1/100 extreme
events occurring in two successive years is 1/10,000. In contrast, the precipitous
action ran the risk of making things worse particularly in the most vulnerable
places, for example at Glasnevin and Drumcondra.
6. There has been a radical change of River Basin Management and Flood Mitigation policy throughout Europe, from the unsustainable traditional one, that results in 'flushing' the flood water down the river, to an alternative one of water retention in the sparsely populated upland parts of a river catchment area. The latter policy also demands a catchment scale approach rather than the traditional localised one. (Appendix 2).
7. The risk of flooding depends on the response of the river system to the inflows from a given rainfall under varying catchment conditions and is clearly stochastic. On the other hand if the recommended policy of retention of the flood water in the upstream part of the catchment had been adopted then the outflow from the former, that is inflow to the parts of the river subject to risk, would have been controlled (as in the case of the discharges from the hydroelectric reservoirs). It follows that, given adequate upstream storage capacity, the risk of flooding could have been effectively eliminated. Instead, we are now faced with a situation where the risk of flooding and its impact cannot be predicted with any known degree of confidence.
8. Furthermore, the policy of accelerating excess rainfall downstream increases
the risk and results in more severe flooding in the catchment area downstream.
(Appendix 3)
9. Reference (Item 11 on page 44): "Upstream of Dunboyne and the Castle
Stream, outside the study area proper…" suggests that the catchment
of the Castle River was deliberately excluded from the study. The option of
constraining the floodwater upstream of Dunboyne was thereby eliminated. This
option seemed to us to be a viable alternative option that would certainly have
been considered by the internationals consultants had they been given the opportunity
to do so. In any event it should have been carefully explored before embarking
on the costly, environmentally destructive and unsustainable alternative in
the Dunboyn/Clonee area. Besides, we cannot understand why this option was not
even considered in view of the fact that plans are approved for a ring road
and motorway that will intersect both the Castle River and the Tolka River upstream
of Dunboyne presents an unique opportunity. This could well have been proved
a wholly cost effective and holistic solution in view of its enormous positive
impact all the way downstream.
10. We must also comment on the fact that all the remedial measures for Dunboyne
were proposed as emergency interim measures prior to proper analysis provided
by mathematical modelling on a catchment scale. The emergency measures involved
massive changes to the river profile over a distance of more than two kilometres.
The impact of these on the critical conditions at Clonee could not have been
properly assessed without the aid of catchment modelling. Furthermore, the interim
measures were implemented without any environmental impact study and as a consequence
there is no reliable information in the public domain as to the impact of these
works.
11. The emergency measures involved massive changes to the river profile over
a distance of more than two kilometres. The downstream impact of these on the
critical conditions at Clonee could not have been properly assessed without
the aid of catchment modelling. Furthermore, the interim measures were implemented
without any environmental impact study and as a consequence there is no reliable
information in the public domain as to the impact of these works.
12. An Environmental Impact Study has not been carried out as required by EU
Directives and the Planning Act 2000. This is required not only to evaluate
the consequences of the scheme of work to be carried out but also to validate
or not the consequences of the implied toleration of huge future developments
in the catchment areas and in the flood plains of the Tolka and Castle Rivers
in Meath and Fingal County Council lands. The Report accommodates and accepts
predicted developments rather than critically appraising and advising on same.
Yet the Report is required to consider the impact of such developments on flood
risk in the catchment. (Ref 2.2)
13. 5.2: According to the Draft Regional Planning Guidelines Dunboyne and Clonee will be incorporated into the Dublin Metropolitan Area and so will come under pressure of high density and intensive development. Therefore the protection of the catchment and flood plains areas should be specifically designated to prevent exploitation.
14. Ref 5.4: New developments: Infiltration and storage techniques are recommended to ensure that run-off will not be increased and will moderate run-off effects but as is acknowledged " the performance of such systems under extreme rainfall conditions is unproven." The use of infiltration and storage techniques in new developments cannot be regarded as providing significant nor adequate protection against flooding nor should they be regarded as adequate compensation for the loss of the absorbent greenfield areas caused by developments.
15. We are pleased that a mathematical model was used to underpin the decision for the proposed scheme of works. We believe, however, that the mathematical model does not validate the scheme for three reasons, namely, that the number of variables required to be taken into consideration would lead to a level of intolerance for the model to cope, secondly that climate change predictions, as you acknowledge, are not reliable and thirdly the history of flooding is not helpful as a predictive tool.
16. Validity of model is questionable:-
· The report discloses a critical discrepancy between the return periods
of the rainfall data and the return periods of the corresponding/ resulting
river discharges. To date the technical reports based on the mathematical model
used - INFOWORKS RS - have not been available to us. We cannot now accept without
evidence that a suitable validated model has been used to determine the details
of the 58 different items proposed in the report and the associated dimensions.
· If the rainfall used as input does not result in the observed water
levels / discharges as output then the model is invalid, and is not only useless
but is in fact dangerous. Moreover we drew attention to this matter last year
and to the difficulty of calibrating and validating the model in view of the
absence of reliable boundary conditions. Besides, it is critical that the data
used for validation and testing is independent in every respect of that used
for calibration. Modification of the rainfall data by adding unreliable radar
estimate in order to ensure that the model generates water levels and river
discharges that agree with predetermined estimates would invalidate the study
completely.
· There appears to be another fundamental flaw in the statistical analysis
of return periods, which would result in serious errors in the estimates of
the risks involved. It arises as follows. The building of the N3 and M50 together
with the associated link roads caused substantial changes to the topography
of the river. The flows during recent flood conditions is now much restricted
and one cannot therefore treat the peak discharges of 75 m3 per second for the
2000 event and the 98 m3 per second of the 2002 event as if occurring under
the same conditions as the data from the period from 1880 and 1985. The 1/100
discharge based on corrected data is likely to be significantly different from
the one published (Appendix 4)
· The reference on page 35 to "the high level of data availability
for the Tolka Catchment" indicates gross underestimate of both the quantity
and the quality of the data essential for developing a properly validated river
model of general use. The advantage provided by the chance acquisition of the
flood levels for the 2002 event should not be over emphasised. Despite being
the maximum levels reached, they are only spot readings in contrast to the continuous
record of the water levels at a number of given locations for the whole time
period of the event that are needed for accurate calibration/validation. Urgently
needed for the proper future management of the upstream part of the Tolka Catchment
(Finglas to Dunboyne) is a number of automatic rain gauges strategically placed.
These are essential in order to enable the correct relationship to be established
between the rainfall in the catchment under varying conditions and the corresponding
flows in the river and its tributaries.
· Linear defences such as embankments are accorded priority and given
undue emphasis in the proposed scheme of options and recommendations. These
imply reduction of flood plains and an increased concentration of flow in the
basic channel, and consequently a 'flushing policy'. This is contrary to the
advice of a number of European experts as being unsustainable, ineffective,
unreliable and costly. It is also contrary to your statement (Ref 8.2) "It
is proposed to retain as far as practicable, existing flood storage and constraints
to flow to exploit attenuation in the existing flood plain."
17. We strongly recommend that the responsibility for the ERBD/Catchment wide
River Basin Management be vested in an independent authority, independent of
Local Authorities, with strong representation from OPW and EPA.
18. We fully agree with the Report that recommendations be implemented immediately
when adopted by local authorities.
19. Section 1.4: River Tolka Catchment Management is a highly desirable requirement. However, the phrase (at final*) "…prevent as far as practicable development within the flood plain…" is permissive, unhelpful and should be removed and in its place should be stated a clear and precise delineation of development prohibited areas and within the brief of the proposed ERBD.
20. The Flood Alleviation Scheme incorporates a mixture of alleviation, defences and storage/retention elements. The storage/retention element located between Dunboyne and Clonee could be greatly enhanced and creatively developed not only to control water flow but also towards environmental and leisure enrichment of this green belt area. Such an initiative should be written into the Report as a future objective.
21. We agree that urbanisation can have a profound effect on catchment hydrology i.e. the conversion of permeable greenfield areas to impermeable surfaces. (Ref 7.2.6) and that Sustainable Drainage Systems should not be considered as a method of river flood protection. Furthermore, SDS should not be overly relied upon to be used in future development as satisfactorily dealing with run-off from such, bearing in mind the loss of absorbent catchment landscape. The Report states that storage/attenuation initiatives offer only "modest benefit" (Ref 1.7)
22. (Ref. 6.0) Flood response = rainfall+antecedent conditions. To imply that development and land use is not part of the equation is incorrect.
23. (Ref. 7.3) "Planning authorities should develop guidelines for permitted development in the different risk category areas and should accommodate these principles in the Development Plans." The ERBD should approve all land use proposals in the catchment of the Tolka and Castle Rivers by Local Authorities. The ERBD should also definite "development exclusion zones".
24. (Ref. 7.2.5) While we strongly support the action list in this section we wish it to be made clear that anything that reduces flood plains should be strongly discouraged with the presumption always in favour of retention.
25. Every application for development should provide a Flood Impact Assessment, which would identify potential loss of flood plain absorption/storage.
26. (Ref. 9.2) "…n order to minimise the down stream impact, it is not proposed to upgrade these culverts." The consequences of not upgrading these culverts should be stated. The N3 road works and the two culverts at Clonee had a major impact on the extent of the flooding at both Clonee and Dunboyne in 2000 and 2002 bearing in mind that the flood water rose to well over one metre above the roof of the upstream culvert. This clearly calls into question the proficiency of the Environmental Impact Assessment assuming that one was actually done The residents of Clonee and those of Dunboyne have a right to demand that this gross error, which puts them at greatly increased risk of flooding, is adequately corrected. The decision not to do so in order to minimise the down stream impact is not acceptable, because this stems entirely from insisting on adopting the rejected policy of "Flushing" rather that the recommended one of "Retention". Moreover there is no valid reason why residents of Clonee and Dunboyne should, deliberately, be put at significantly increased risk in order to reduce the risk of those living downstream.
27. DCRA requested access to the Technical Reports drawn on the mathematical modelling used as a basis of this report but regrets that it was not able to have sight of these prior to making this submission.
28. DCRA is not convinced that the Report sustains the conclusion that the measures proposed will ensure long-term alleviation of flooding in Dunboyne.
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Appendix 1
Hyder Consulting is a planning, engineering, environmental and management consultantcy. Employing over 2,500 people, they advise on, design and manage a huge diversity of projects. The reputation for delivering 'landmark' projects, such as Sydney Harbour Bridge, developed from the company's early origins in the 19th century has continued throughout the years. More recent examples include the Melbourne City Link, the Emirates Twin Towers in Dubai.
HR Wallingford was founded in 1947 as the UK Government's Hydraulics Research Organisation within the Department of Scientific and Industrial Research and was transferred from the Department of the Environment to the private sector in 1982. H.R Wallingford was selected by the EU to co-ordinate the RIBAMOD (River Basin Modelling Management and Flood Mitigation) Consorted Action under the Forth Framework Program. Wallingford Software was founded in 1987 and as recorded on Hyder's website, Wallingford's "InfoWorks software which incorporates the power and functionality of Geographical Information Systems (GIS) has become the industry standard hydraulic modelling software for the UK and Ireland".
Appendix 2
This shift can be clearly seen from the following extract taken from the conclusions
reached by the review group on Integrated Flood Protection at the First RIBAMOD
Workshop
"A major aspect of flood mitigation has been traditionally the provision
structural flood defences (embankments, storage reservoirs, relief channels
etc). These can have substantial impact on the riverine environment and ecology
and the trend of national legislation and Community directives has been to require
detailed impact assessments and environmental statements to support the promotion
of the project. This requirement drives the need for multidisciplinary working
on the design of the flood defences.
However, many major structural flood defence projects have been completed, particularly on lowland rivers and the recognition that future flood defence must be sustainable will influence the choice of measures implemented to further mitigate flood risk. It can be argued that a cycle of raising flood embankments and allowing unrestricted increase in vulnerability to potential flood damage on the flood plain is not sustainable".
Appendix 3
The EU sponsored "Pilot Study for the definition of environmental- friendly measures to reduce the risk of flash floods in the Geul River Catchment", referenced in our previous submission, clearly confirms this philosophy:-
"Until now, the water management policy to prevent local flooding was designed to accelerate excess rainfall water directly into the Geul River and its tributaries. This however, resulted in more severe flooding problems at the catchment scale, particularly in the downstream part of the catchment. The risk of flooding is not expected to decrease in the future if this policy does not change. Furthermore, the International Panel for Climate Change predicts a change in the weather conditions for Western Europe: rainfall will be more erratic, probably resulting in more frequent and sever flash floods."
Appendix 4
Design Floods at Clonee/Dunboyne
It is clear from the maps that it is now accepted that the N3 acted as a dam across the Tolka valley downstream of Clonee. It follows that the failure of the culverts under the N3 (stated as surcharging) to cope with the discharge greatly increased the flood levels at Clonee during the events at 2000 and 2002 and caused far more extensive damage than would have otherwise been the case. Moreover it is also likely that flood levels at Dunboyne were also adversely affected with similar increased damage. The impact of the N3 on flooding at both Clonee and Dunboyne, and indeed for all areas downstream, could easily be determined by modelling. In fact, this task is far simpler than assessing the impact of the 58 remedial measures.
We note that the remedial measures not only fail to eliminate the hazard caused by the N3 but in fact greatly increase the risk by effectively raising the 'dam' by well over a metre. Under the circumstances, the wall that has been build as protection for Clonee is quite inadequate. Firstly, we note that it is intended also to act as a dam to hold back flood-waters. A design flood of 100 years is far from satisfactory. A paper presented to the Civil Division of the Institute of Engineers in 1986 show that the ESB use design floods of up to 10,000 years for its large dams. A design flood of around 1,000 would appear to us to be a reasonable level of protection for both Dunboyne and Clonee.
End