Dunboyne Combined Residents Association
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Comments on Interim Report on Flooding
Submitted to Meath County Council by Consultants and received by the Council on 30.01.03.
Date of this DCRA Submission to Meath County Council: 27.02.03.
Contents
Flood Event 2002
Flood Impact and Alleviation Measures:-
1. Initial Comments
2. River Basin Modelling
3. The Geul River Study
Comments on the Interim Report
________________________
Further to DCRA's Submission on 31.01.03 to Meath County Council on flooding in Dunboyne we are now pleased, having had sight of the Interim Report from Consultants M C O'Sullivan, to submit additional comments.
Flood Event of 2002
DCRA does not agree with the conclusion:- "At this stage, it can
be stated that the November 2002 flood is likely to have a risk of recurrence
of less than 1% in any years on the basis that it is the largest flood in over
100 years of record." for the following reasons:-
1. Because of the evidence of climate change, the usual practice of estimating
return periods on the basis of peak annual discharges must now be called into
question. It can no longer be assumed that the climate of the past 100/120 years
provides a reliable indication of the weather even in
the immediate future. It is believed that storms are likely to be both more
intense and more frequent.
2. We are more concerned with the probability of events that give rise to
serious flooding rather than the probability of a peak flood of a given size
in any year. The inter-arrival period of the eight serious floods, exceeding
54 m³/s, since 1880 is 16 years. The probability of a serious flood in
any year is therefore about 6.5%. Generally, reliable information is usually
available regarding the precipitation sequence of a storm. However the runoff
peak and volume are highly dependent on antecedent moisture conditions and infiltration
in the catchment. The destruction of the floodplains and reduction of green
belts due to extensive development have drastically altered the nature of the
catchment. It cannot therefore be assumed that the particular storms that occurred
in the past would necessarily result in the same peak flows under present conditions.
3. It now seems likely that a similar misconception occurred in the case
of the 2000 event. The peak flow in 2000 was the second highest in the past
120 years indicating a return period of 60 years. Consequently the need for
the sort of remedial action which would have greatly alleviated matters in 2002
did not receive the urgent attention it deserved.
4. Trapped water: We have, in previous submissions, drawn attention to
the fact that it is reasonable to suppose that the recorded peak discharge of
98 m³/s at Glasnevin is significantly lower than the flow that would have
been recorded if the enormous volume of water, trapped along the upstream valley,
had not been restrained. It is not inconceivable that in the absence of any
impedance the peak discharge could have reached 120 m³/s. Some evidence
is given below that could support this estimate, but the volume of trapped water
can readily be calculated from the DEM and a reasonable adjustment made to the
peak flow to take account of its affect.
5. Peak flow estimates: The Clonee catchment is only 70 km2 and is half
that at Glasnevin. The first peak discharge at Glasnevin occurred 22 hours after
the levels began to rise. It is not unreasonable, therefore, to base the estimate
for the peak flow at Clonee on the 12 hour duration. The rainfall records for
the three stations Casement, Glasnevin and Phoenix Park indicated that the average
intensities for the 12 hour duration were 3.04 mm/hr, 3.08 mm/hr and 4.16 mm/hr.
Based on the lowest of these three intensities (3.04 mm/hr), the rational method
indicates a peak flow of 59.1m³/s. This is some 34% higher than the 44
m³/s stated in the report. It may also be noted that the 24 hour duration
at Phoenix Park gives an intensity of 3.17mm/hr although this was the highest
of the three stations.
6. The only rainfall data for the event of 13th to 15th November 2002 that provides
any indication of the precipitation within the Tolka catchment area are obtained
from the four stations in the Dublin area at Phoenix Park, Dublin Airport, Glasnevin
and Casement. The average rainfall of the four stations was about 80mm giving
an average intensity of not more than 2.50mm/hr. For a catchment area of 137.8km2
and assuming coefficients of say 90% for run-off and say 85% for the volume
of floodwater trapped along the river valley, the rational equation gives a
value of 73.7m3/s for the peak flow. This is well below the recorded value of
98.7m3/s. It follows that the average rainfall intensity in the catchment area
during the 2002 storm must have been a good deal higher than 2.5mm/hr.
Note: It is very difficult to know what allowance should be made for climate change. On the other hand, it is obvious that design standards for the river and river structures will now have to be revised on the basis of the 2002 flood experience. This requires reliable estimates of the rainfall pattern so as to determine the appropriate flows into the river. The difficulty now is that it does not seem possible to do so without taking the value of the peak flow at Glasnevin into consideration. This means that the water levels at Glasnevin which would have been ideal candidates to provide boundary values for the mathematical model cannot now be used for this purpose. Moreover, assuming that tide levels are used as boundary values, it is doubtful if it would be permissible to use the Glasnevin water levels in order to validate model.
Flood Impact and Alleviation Measures (Interim Assessment)
1. Initial Comments
To begin we must say that we take the view expressed by the Minister of State
at the Department of Finance (Mr.Parlon ) in Dail Eireann on the 19th November:-
" 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."
However on the 12 December The Minister added:-
"Following the severe flooding of the Tolka on 14 and 15 November
2002, the three local authorities involved - Dublin City Council, and Meath
and Fingal County Councils - have each requested the consultants undertaking
the flood study to produce interim reports for each local authority functional
area on the immediate measures that can be taken to alleviate flooding in the
worst affected areas."
In reply to Dail questions the Minister stated on Tuesday 19th November 2002:
"As the lower and middle catchments of the river, in particular,
and certain concentrated areas within the upper catchment have witnessed significant
increase in land development, the risk of flood damage to property and infrastructure
has also increased."
Before dealing with some of the proposals outlined in this report we wish
to comment on what we believe are serious omissions:-
1. The sort of action that is becoming increasingly urgent is the prevention
of any further significant development in the River Tolka catchment until a
coherent policy has been established that gives priority to flood risk
control. This topic is not even given recognition by the consultants at this
stage. There is no reason why a flood risk management scheme at catchment scale
that is almost universally recommended in similar instances should not be installed
without delay. This should not present any great difficulty as embryo structures
are already in existence
2. We also note that the urgent need for flood warning has not been addressed.
Forecasting or what is called 'now-casting' will almost certainly form part
of an integrated flood management system. Failure to recognise the need for
an early warning system after the 2000 event, resulted in the complete absence
of any effective measures to deal with the event of 2002. The interval between
the peak rainfall and the peak discharge at Glasnevin is about five hours. This
information supported by normal weather forecasts would have been sufficient
for an organised response that would have provided considerable alleviation
assistance in many cases where none at all was available.
3. The Interim Report does not address the significance of developments in the
river catchment areas or the extent to which they contributed to flooding. This
is promised in the final report. We are told that the information will become
available as a result of modelling that is to be undertaken. The reference for
the model must be what is there at present prior to any work being undertaken
because the model must be validated. Indeed without the full results of modelling
of the rivers and their catchment areas as they are at present then the proposed
work could in fact prove wasteful and not contribute to the final resolution
of the flooding problem at Dunboyne and downstream.
2. River Basin Modelling
To provide a backdrop for our comments we outline below a number of important conclusions reached as a result of the RIBAMOD Concerted Action project initiated and funded by the European Commission D.G. Research and Development and co-ordinated by H.R.Wallingford.
1. Integrated Catchment Modelling (ICM) has been recognised as an important
area for research and development in the coming years.
There is need to develop integrated catchment modelling, based on an "open
system" philosophy to combine existing process models, tailored to the
local needs and preferences.
2. The exposure of a community or enterprise in a particular area to flood
risk is a combination of two factors, the probability of flood hazard in the
area and the vulnerability of the area to undesirable consequences and economic
loss should flooding occur:
There is need for a catchment view of flood risk management, fully integrated
with environmental effects, rather than a collection of unconnected, individual
measures
3.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.
The prominence of non-structural measures for flood defence will increase
as part of the sustainable management of rivers.
Non-structural measures mainly control the "vulnerability" component
of flood risk, they include:
· spatial planning policy with a presumption against development or
encroachment of economic activities onto flood plains
· building regulations to control the additional runoff from any green-field
development in the catchment outside the flood plain
· regulation of increases in vulnerability to flooding and of flood plain
use
4. The objectives of river restoration are normally to create a wider diversity
of eco-systems and improve biodiversity, by bringing the river into a closer
contact with its flood plain. The visual amenity of the watercourse may be improved
and its natural function for flood storage and conveyance regained.
The restoration of flood plains to their natural function should be encouraged (where socially and politically acceptable).
5. Environmentally sound strategies to minimise flood damage:
"All significant infrastructure works should be integrated into a
strategic plan for the whole basin and should be preceded by an impact study
including hydraulic and sedimentological effects, environmental impact, and
cost effectiveness; communication with the public before and after reaching
any decision is essential".
There is a need to broaden economic evaluations to include "intangible" costs and benefits to assess the non-engineering aspects of flood defence activities within a common methodology for the assessment of flood damages.
Note: In order to emphasise the pertinence of the above conclusions, it is worth mentioning that at the close of the Second RIBAMOD Workshop, Professor Jim Dooge, reminded the participants of the purpose and importance of the Concerted Action (Dooge & Samuels, 1998)
"In the midst of all the exciting technical and scientific issues raised during the workshop, it is important for us not to lose sight of why the European Commission has funded the RIBAMOD Concerted Action. These workshops and expert meetings have been sponsored because we, in the scientific community, have been set the task of responding to a real social problem which affects the quality of life of many European citizens. Indeed flooding from all causes is the most significant natural disaster world-wide with over 200,000 human lives being lost in floods in the decade between 1986 and 1995 (Munch Re, 1997) and over 10,000 in 1997 (Munich Re, 1998). Each one of these deaths has been a tragedy for the family involved. More than this, those who survive the flood may suffer prolonged health problems or face financial ruin through the loss of home, possessions and livelihood."
3. The Geul River Study
It is now 29 months since the flooding event in Oct 2000 and some 21 months
since the start of the present study of the Tolka River and the Castle Stream.
There is now evidence to show that there is no justification for the undue delay
in the completion of this study. Moreover had the study been completed in a
reasonable time then it may well have been possible to introduce measures before
the last flooding in 2002 to alleviate the damage.
In May 2000 the final report of a blue print study Pilot Project (The Geul River
Study) was published on the Internet. The pilot study was undertaken by two
Consultantcy companies specialising in environmental management and research
in co-operation with four university scientific departments. The Geul catchment
area is more than twice the size of the Tolka catchment and the rivers flow
across three national boundaries and have far more structures requiring modelling.
Despite the obvious problems due to differences in data formatting, language,
and complex legal framework this study was completed within 18 calendar months.
We draw attention to this project because of both the uncompromising underlying
philosophy and the clearly expressed objectives it set:
"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."
"Objectives: The Geul project intends to propose an effective policy to tackle the current trans-boundary water management problems in the Geul River catchment, particularly concerning flash flood events. The present policy only solves or reduces flooding problems at local scale, while being inappropriate to avoid severe flooding problems at catchment scale. The project envisages an integrated policy, aimed to prevent or reduce accumulated peaks in water supply to flood-prone areas along the Geul River, by increasing the water retention in the upstream parts of the river system and/or in the surrounding uplands."
Geul Pilot Project URLs
http://europa.eu.int/comm/environment/civil/pdfdocs/report-final-i.pdf
http://europa.eu.int/comm/environment/civil/pdfdocs/report-final-ii.pdf
Comments on the Interim Report
In the light of the foregoing we have the following comments to make regarding the proposed immediate measures.
1. River Basin Modelling: All five points mentioned above under this heading
should be evidently addressed in the plan.
2. There is no statement as to a clearly defined overall catchment scale objective.
Thus some measures are designed to retain some floodwater in an upstream location
such as Loughsallagh. Apart from the unrealistic proposal for future consideration
of diverting the flood water from the proposed holding basin at Loughsallagh
into the Liffey, there is too much emphasis on flushing bearing in mind that
only 50% of flood water arises upstream of Clonee where some control can be
exercised over it.
3. The interim measures are determined on a purely localised basis rather than
on a catchment scale. This approach results in a bewildering number of independent
measures whose interaction cannot be assessed even with the aid of subsequent
modelling. Consequently, neither can the need for nor the scale of any particular
one be properly established in advance.
4. Recommendations are premature because neither the need for, nor the effectiveness
of, a given measure has been properly tested by simulation. The final design
is therefore compromised by having to incorporate elements that might otherwise
be found unnecessary. This could well result in considerable waste of expenditure.
5. Indeed, assuming a similar approach to the problems all the way downstream,
our research indicates that given an alternative strategy based on proven catchment
scale objectives many of the proposed measures could well be found to be unnecessary
. Furthermore, if implemented there is a danger that some of these proposed
measures could easily exacerbate problems both upstream and downstream.
6. In the absence of a unifying objective, the potential effectiveness of any
given measure cannot be properly evaluated because it is not possible to assess
the contribution made towards overall risk reduction in isolation. Nor is it
possible to gauge the adverse affect downstream independently of the other proposed
measures. It follows that it is clearly impossible to perform any reliable cost
benefit analysis.
7. Rather than the recommended practice of giving prominence to non-structural
measures, there is over-reliance on structurally oriented measures.
8. The affect downstream of the more significant of the proposed measures cannot
at this point in time be estimated. One cannot guaranteed that 'maintaining
the constraint on flows upstream of Clonee should ensure that these relief measures
at Dunboyne would not have significant adverse impact on downstream areas at
risk'. In one way or another, a policy whose objective is to 'flush' the
floodwaters downstream cannot do so without having a significant adverse impact
on downstream areas at risk. There is no indication in the report apart from
the suggested by-pass north of Clonee of what is proposed in order to 'maintain
constraint on flows upstream of Clonee.' The by-pass, however, is also designed
to 'flush' the floodwaters downstream and this will clearly have an adverse
impact downstream.
9. Neither the need for nor the scope of the measures proposed for Dunboyne
can be determined ahead of the modelling. The immediate and remedial work proposed
will not ensure a flood free Dunboune or Clonee. The communities who live in
these villages are sceptical that the proposed intermediate initiatives will
be used as evidence that planning applications and developments will get approval
and support the County Council's policy of the urbanisation of the catchment
areas of these rivers and the conversion of flood plains into prime building
land.
10. There is a need for a single River Authority to manage the catchment and
river basin.
11. The minimum cost solution is likely to be that which constrains the greatest
amount of water in the minimum number of locations.
12. It would have been helpful to us had the maps indicated the levels of the
contours.
13 With a view to having an interdisciplinary approach to the solutions on flooding
it would have been helpful to have had the involvement of scientific experts
from our universities.
14. At this stage before any the hydrodynamic modelling there is no rational
basis for the proposed recommendations nor are the consequences of proposed
immediate works predictable.
15. The problem is caused significantly by interference and misuse due to inappropriate
development and change of land use in the catchment area.
16. It is regrettable that politicians have zoned floodplains and lands that
have been subjected to flooding for development, that the county Council has
breached the Strategic Planning Guidelines for the Greater Dublin Area and the
Minister for the Environment does not enforce the compliance required.
17. Flooding in "Walsh's Land", in Dunboyne Castle grounds and environs,
on the Summerhill Road and at Bennets Bridge have not been addressed. No report
can be complete until these floodings are acknowledged and solutions incorporated
into the flood alleviation plan for Dunboyne.
18. We are hopeful that the final report will adequately address the environmental
and amenity opportunities that could form part of flood alleviation solutions
such as water based environmental and recreational facilities, especially east
of the old railway line, forestation, hedgerow retention and development, ecological
restoration and other such land uses to increase the water absorption and detention
in the catchment area lands.
19. Meath County Council should now make no changes to catchment area land uses
in Dunboyne that would reduce soakage and retention of water and permit developments
that add to the risk of flooding because of run-off.
20. In our previous submission to the County Council we made 23 Recommendations.
We ask that these be included for consideration with this submission.
_____________________________
Contact: dcramail@netscape.net