TULLAMORE FLOOD RISK
ASSESSMENT AND
MANAGEMENT STUDY
FINAL HYDROLOGY REPORT
1794/RP/003/D
June 2008
FRAM Section
Engineering Services
Office of Public Works
PROJECT
TULLAMORE FLOOD RISK ASSESSMENT AND
MANAGEMENT STUDY
PROJECT No.
1794
PROJECT ENGINEER:
GREG RYAN, ENGINEER GRADE II
CLIENT:
ENGINEERING SERVICES
OFFICE OF PUBLIC WORKS
DOCUMENT TITLE:
TULLAMORE FRAM STUDY
FINAL HYDOLOGY REPORT
DOCUMENT No.:
F:1794 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
1794/RP/003/D
June 2008
INDEX OF CONTENTS
Section_
Page
1.
Introduction _________________________________________________________ 1
2.
Background _________________________________________________________ 1
3.
Study area___________________________________________________________ 2
4.
Objective of hydrological investigation____________________________________ 3
5.
Data collection _______________________________________________________ 3
5.1.
Historical flood events ________________________________________________ 3
5.2.
Catchment hydrological characteristics ___________________________________ 5
5.3.
Rainfall ___________________________________________________________ 7
5.4.
River level and flow __________________________________________________ 9
5.5.
Long-term catchment changes _________________________________________ 13
6.
Derivation of design peak flood flows ____________________________________ 15
6.1.
Estimation of design flood flows using the available flow dataset ______________ 15
6.2.
Estimation of design flood flows using alternative flow datasets _______________ 16
6.3.
Adoption of design flood flows ________________________________________ 19
7.
Inclusion of allowances for long-term catchment changes ___________________ 20
8.
Grand Canal________________________________________________________ 21
9.
Conclusion _________________________________________________________ 24
10. Recommendations ___________________________________________________ 24
11. References__________________________________________________________ 25
Appendices _____________________________________________________________ 26
Appendix A _____________________________________________________________ 27
Appendix B _____________________________________________________________ 29
Appendix C _____________________________________________________________ 31
Appendix D _____________________________________________________________ 33
Appendix E _____________________________________________________________ 35
Appendix F _____________________________________________________________ 37
D1794 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
iii
June 2008
INDEX OF TABLES
Table
Page
Table 1: Relevant rain gauges. ________________________________________________ 7
Table 2: Summary of analysis using Thiessen Polygons. ____________________________ 8
Table 3: Relevant river level recorders. _________________________________________ 9
Table 4: Rating Equation 1 and 2. ____________________________________________ 11
Table 5: Rating Equation 3. _________________________________________________ 12
Table 6: Smoothed ratings. _________________________________________________ 12
Table 7: Estimation of mean annual flow using continuous flow record. _______________ 15
Table 8: Estimation of design flood flows using the regional growth curve _____________ 16
Table 9: Summary of mean annual flows alternative water level recorders._____________ 17
Table 10: Mean annual flood flow factors.______________________________________ 17
Table 11: Summary of statistical analysis and resulting peak flood flow estimations. _____ 18
Table 12: Design flood flow estimates at the Tullamore water level recorder. ___________ 18
Table 13: Design flood flow estimates at the Tullamore water level recorder. ___________ 19
Table 14: Summary of growth factors _________________________________________ 20
Table 15: Adopted design flood flows at the Tullamore water level recorder. ___________ 20
Table 16: Future design flood flows at the Tullamore water level recorder _____________ 21
Table 17: Dimensions of canal overflow weir ___________________________________ 22
Table 18: Grand Canal overflows relative to adopted design flood flows. ______________ 23
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page iv of 59
June 2008
INDEX OF FIGURES
Figure
Page
Figure 1: Aerial photo of the Tullamore urban area.................................................................. 1
Figure 2: The Tullamore urban area and the Tullamore River floodplain................................. 2
Figure 3: The Tullamore River during a moderate flood event (William Waller Weir) ........... 3
Figure 4: The Tullamore River floodplain (upstream of the Tullamore urban area)................. 4
Figure 5: Site of the Tullamore water level recorder................................................................. 9
Figure 6: Vegetation downstream of the Tullamore water level recorder............................... 13
Figure 7: Overflow weir from the Grand Canal. ..................................................................... 21
INDEX OF EQUATIONS
Equation
Page
Equation 1: Calculation of FSR stream frequency parameter. ________________________ 6
Equation 2: Calculation of FSR SOIL parameter. _________________________________ 6
Equation 3: Calculation of FSR Rsmd parameter. __________________________________ 7
Equation 4: General rating equation in the Republic of Ireland.______________________ 11
Equation 5: FSR six variable equation using Rsmd. ________________________________ 16
Equation 6: FSR six variable equation using SAAR. ______________________________ 16
Equation 7: Flow across a broad crested weir. ___________________________________ 22
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page v of 59
June 2008
1.
Introduction
The Office of Public Works (the ‘OPW’) is undertaking a flood risk assessment and
mitigation study (FRAM Study) in the vicinity of Tullamore, Co. Offaly. The primary output
from this study is a flood risk management plan (FRM Plan) that presents a recommended
strategy to reduce the flood risk within the Tullamore urban area.
The FRAM Study consists of the following stages:
1. Hydrological investigation.
2. Hydraulic investigation.
3. Strategic Environmental Assessment (SEA).
4. Flood damage assessment.
5. Flood risk mitigation option appraisal and recommendation.
6. Flood risk management plan.
This report details the hydrological investigation completed as part of the FRAM study, and
is focused on the derivation of design flood flows relevant to the Tullamore River within the
Tullamore urban area.
2.
Background
Tullamore is located within the Tullamore River catchment, approximately 6 km upstream
of the confluence with the Clodiagh River. The Tullamore urban area has a population of
approximately 10,000 and includes residential, commercial and industrial land uses.
Figure 1: Aerial photo of the Tullamore urban area
D1794 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 1 of 59
June 2008
Three significant waterways pass through the Tullamore urban area:
-
The Tullamore River is a tributary to the Clodiagh River, which is in the Brosna River
catchment. It has a catchment area of 114.8 km2 at the Tullamore water level recorder
(refer to Appendix D for location), which consists primarily of farmland, but also
includes part of the Tullamore urban area.
-
The Grand Canal is a transport link between the east coast (Dublin) and west coast
(Shannon River) of Ireland that is retained for recreational users. The water level in the
Grand Canal is artificially controlled by the series of locks that are operated and
maintained by Waterways Ireland (refer to Section 8 for further comment on the impact
that the Grand Canal has on flood flows in the Tullamore River catchment).
-
The Barony River is a tributary to the Tullamore River. It has a catchment area of 16.9
km2 above the Grand Canal, all of which is within the Tullamore River catchment
referred to above.
Part of the Tullamore urban area is located on the Tullamore River floodplain, and as a result
it has been historically subject to inundation. This has resulted in residential, commercial and
industrial properties being subject to an ongoing flood risk, hence the need for this FRAM
Study.
The need for this FRAM Study is further justified by urban development pressure on the
Tullamore River floodplain upstream and downstream of the existing urban area.
Figure 2: The Tullamore urban area and the Tullamore River floodplain
3.
Study area
The study area for the Tullamore FRAM study was generally described as the existing
Tullamore urban area, including areas that could be subject to future development (refer to
Appendix A). The study area for this hydrological investigation was the Tullamore River
catchment upstream of the OPW Tullamore river level recorder (refer to Appendix B).
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 2 of 59
June 2008
4.
Objective of hydrological investigation
The objective of this hydrological investigation is to estimate design peak flood flows
relevant to the Tullamore River within the Tullamore urban area. These estimates are
required for inclusion in the one and two dimensional numerical hydraulic models that are to
be constructed as part of the FRAM Study.
5.
Data collection
The objective of this data collection exercise is to obtain sufficiently detailed and accurate
data sets that describe:
5.1.
-
Historical flood events in the Tullamore River catchment.
-
The hydrological characteristics of the Tullamore River catchment.
-
Rainfall over the Tullamore River catchment.
-
Flow in the Tullamore River.
Historical flood events
Based on reports gathered from the Design Section Press Archive and local authorities, there
have been five significant flood events in the Tullamore River catchment over the past
century:
-
Late 1844.
-
Late 1924.
-
Early 1946.
-
Late 1954.
-
Early 1990.
-
Early 1995.
-
Early 2001.
Figure 3: The Tullamore River during a moderate flood event (William Waller Weir)
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 3 of 59
June 2008
As the Tullamore water level recorder was commissioned after the early 2001 flood event,
the information available on these flood events is limited to:
-
Press reports: The OPW Design Section Press Archive identified eleven articles
covering the 1924, 1954 and 1995 flood events. These press reports focused on the
damage to rural land, rather than damage sustained within the Tullamore urban area.
-
Data collected by local councils: The Offaly County Council and Tullamore Town
Council were asked to provide any data that was available describing historical flood
events. At the time of preparing this report, the data available included a long section of
the flood level through the Tullamore urban area during the winter 1844 flood event, a
plan of the flood extent between Charleville Demesne and Springfield Bridge during the
February 1946 flood event, a long section of the flood level through the Tullamore
urban area during the February 1990 flood event and a plan of flood levels in the vicinity
of the Whitehall Estate taken during the February 1990 flood event.
-
Data collected by the OPW: The OPW East Region provided flood levels surveyed
during the February 2001 flood event.
Figure 4: The Tullamore River floodplain (upstream of the Tullamore urban area)
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 4 of 59
June 2008
5.2.
Catchment hydrological characteristics
The FSR outlines a number of methods that are suitable for assessing flood flows in Irish
rivers. These methods require a number of inputs that represent various hydrological
characteristics of a catchment. These inputs were abstracted for the Tullamore River
catchment, as outlined in the following sections.
5.2.1 Catchment area
The catchment area for the Tullamore River at the OPW water level recorder was measured
as 114.8 km2. This value was directly adopted by the hydrological methods used in this
investigation.
This value was measured using MapInfo GIS and the contours presented on the OS 1:50,000
map of the area. A diagram of this catchment area is presented in Appendix B of this report.
5.2.2 Urban development of catchment
The area of existing urban development within the Tullamore River catchment was
measured as 6.918 km2. The corresponding catchment characteristic adopted by the
hydrological methods used in this investigation, URBAN, was therefore assessed as 6 %.
These values were measured using MapInfo GIS assuming that the area of existing urban
development within the Tullamore River catchment was equal to the Tullamore urban area
within the Tullamore River catchment as defined by the Offaly County Council
Development Plan 2003 – 2009. A diagram of these urban areas is presented in Appendix C
of this report.
5.2.3 Attenuating effect of lakes
Based on the OS 1:50,000 map of the area, there are no significant lakes within the
Tullamore River catchment. The corresponding catchment characteristic adopted by the
hydrological methods used in this investigation, LAKE, was therefore assessed as 0 %.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 5 of 59
June 2008
5.2.4 Main channel length
The length of the main channel within the Tullamore River catchment was measured as 16.1
km. This value was directly adopted by the hydrological methods used in this investigation.
This value was measured using MapInfo GIS and the OS 1:50,000 map of the area.
5.2.5 Main channel slope
The main channel slope (i.e. S1085) of the Tullamore River catchment was measured as 1.656
%. This value was directly adopted by the hydrological methods used in this investigation.
This value was measured by using MapInfo GIS and the OS 1:50,000 map of the area to
determine the position and elevation of two points on the main channel within the Tullamore
River catchment. These points were located at 10% and 85% of the main channel length.
5.2.6 Stream frequency
The stream frequency (i.e. Fs) within the Tullamore River catchment (i.e. a measure of the
number of channel junctions relative to the catchment area) was measured as 0.319. This
value was directly adopted by the hydrological methods used in this investigation.
This value was measured using MapInfo GIS and the OS 1:50,000 map of the area to count
the number of channel junctions shown. This measured value was then corrected to account
for the lower resolution of channels shown on the 1:50,000 map when compared with a
1:25,000 map, as required by the FSR (refer to Equation 1).
Equation 1: Calculation of FSR stream frequency parameter.
Fs = 2.1318F”s – 0.1081
Where
Fs
F”s
Parameter adopted by the FSR
Stream frequency measured off 1:50,000 scale plan
5.2.7 Catchment soil characteristics
The proportion of Soil Classes within the Tullamore River catchment was measured as 73 %
Class 2 and 27 % Class 4. The corresponding catchment characteristic adopted by the
hydrological methods used in this investigation, SOIL, was therefore assessed as 0.341.
This value was measured using Figure I 4.18.1 and calculated using Equation 2, both of
which are presented in the FSR.
Equation 2: Calculation of FSR SOIL parameter.
SOIL = 0.15 SOIL1 + 0.30 SOIL2 + 0.40 SOIL3 + 0.45 SOIL4 + 0.50 SOIL5
Where
SOIL
SOIL1
SOIL2
SOIL3
SOIL4
SOIL5
Parameter adopted by the FSR
Soil class 1 (very high infiltration potential)
Soil class 2 (high infiltration potential)
Soil class 3 (moderate infiltration potential)
Soil class 4 (low infiltration potential)
Soil class 5 (very low infiltration potential)
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 6 of 59
June 2008
5.2.8 Empirical catchment wetness index
The empirical estimate of the catchment wetness index (CWI) for the Tullamore River
catchment was assessed as 121.9. This value was directly adopted by the hydrological
methods used in this investigation.
This value was taken from the FSR.
5.2.9 Standard annual average rainfall
The standard annual average rainfall (SAAR) for the Tullamore River catchment was
measured as 900 mm. This value was adopted directly by the hydrological methods used in
this investigation.
This value was measured using the Meteorological Service Ireland Mean Annual Rainfall
Map (1951 - 1980).
5.2.10 Jenkinson’s Ratio
Jenkinson’s Ratio for the Tullamore River catchment was assessed as 0.3. This value was
directly adopted by the hydrological methods used in this investigation.
This value was assessed using Figure II 3.5.1, which is presented in the FSR.
5.2.11 Measure of excess rainfall
Excess rainfall (i.e. Rsmd) for the Tullamore River catchment was assessed as 34.4. This
value was directly adopted by the hydrological methods used in this investigation.
This value was measured using Equation 3, which is presented in the FSR.
Equation 3: Calculation of FSR Rsmd parameter.
Rsmd = (2.48 x SAAR0.5) – 40
Where
5.3.
Rsmd
Parameter adopted by the FSR
SAAR Standard average annual rainfall
Rainfall
5.3.1 Identification of potential data sources
The rain gauges considered for inclusion in this investigation were limited to those managed
and maintained by Met Éireann for reasons of reliability, accuracy and length of record.
The Met Éireann rain gauges identified as being potentially relevant to this investigation are
summarised in Table 1. The locations of these rain gauges are presented in Appendix D of
this report.
Table 1: Relevant rain gauges.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 7 of 59
June 2008
Name
Data Frequency
Approximate Location*
Birr
Hourly
38 km southwest
Mullingar
Hourly
29 km north
Tyrrellspass
Daily
14 km north
Geashill
Daily
Within catchment
Clonaslee
Daily
15 km south-southwest
*Relative to the centre of the Tullamore River catchment
5.3.2 Interrogation of available data sources
The rainfall data sources identified as being potentially relevant to this investigation were
interrogated by Met Éireann on behalf of the OPW over a period that is consistent with the
water level and flow data that is available from the OPW Tullamore water level recorder (i.e.
February 2001 onwards). The data sets obtained were forwarded electronically by Met
Éireann to the OPW.
5.3.3 Manipulation of available data sets
The data sets obtained from Met Éireann were manipulated to form one dataset that is
representative of rainfall over the Tullamore River catchment using the following process:
1. With regard to the two hourly rain gauges, it was observed that both were located
outside the Tullamore River catchment, with the actual catchment area located
approximately half way between the two. It was therefore decided that the hourly
rainfall data provided by these rain gauges would be combined using the method of
Thiessen Polygons to give one hourly data set. The result of this analysis is summarised
in Table 2.
Table 2: Summary of analysis using Thiessen Polygons.
Name
Birr
Mullingar
Coverage area*
Thiessen Coefficient
12.87 km2
0.11
2
101.93 km
0.89
* Area of Thiessen Polygon within the Tullamore River catchment
2. With regard to the three daily rain gauges, considered that the daily rainfall data
provided by these three rain gauges could be combined using the method of Thiessen
Polygons to give one daily data set. However, as the Tullamore River catchment
boundary was located approximately half way between the daily rain gauge pairs, it was
decided that it would not be necessary to combine the available daily rainfall data sets.
Instead, the daily rainfall data set for the Tullamore River catchment was assumed equal
to the data provided by the Geashill daily rain gauge.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 8 of 59
June 2008
3. In order to obtain a data set that was representative of rainfall in the Tullamore River
catchment, it was considered essential that the daily rainfall totals of that data set should
match those recorded by the Gleashill rain gauge, which was assumed to provided the
best estimate of daily rainfall within the Tullamore River catchment given that it was the
only rain gauge located within the Tullamore River catchment. To achieve this, the
hourly dataset was first converted to a daily rainfall set, taking into account that the daily
rainfall data provided by Met Éireann represented rainfall between 0900 on the day
identified and 0900 the following day. The resulting two daily rainfall data sets were
then compared, and the percentage difference between the two was applied as a
correction to the hourly rainfall data set.
The resulting hourly rainfall data set was considered representative of rainfall in the
Tullamore River catchment, given that:
5.4.
-
The temporal pattern of the adopted rainfall record is consistent with the data collected
by the Birr and Mullingar rain gauges, which were the closest rain gauges to the
Tullamore River catchment that provided sufficient temporal resolution.
-
The adopted hourly rainfall record provided daily rainfall totals that were consistent with
the daily rainfall total recorded by the Gleashill daily rain gauge, which was assumed to
provide the best estimate of daily rainfall given that it was the only rain gauge located
within the Tullamore River catchment.
River level and flow
5.4.1 Identification of potential data sources
Five river level recorders were identified as being potentially relevant to this investigation.
The details of these are summarised in Table 3 and the location of each is presented in
Appendix D of this report.
Table 3: Relevant river level recorders.
Name
Owner
Reference
River
Tullamore
OPW
90501
Tullamore
Ferbane
OPW
25006
Brosna
Gorteen
OPW
25007
Clodiagh
Millbrook Bridge
OPW
25014
Silver
Rahan
OPW
25016
Clodiagh
Figure 5: Site of the Tullamore water level recorder
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 9 of 59
June 2008
5.4.2 Interrogation of available data sources
The river level and flow data sources identified as being potentially relevant were
investigated by the OPW, as follows:
-
The Tullamore water level recorder was used to obtain an hourly water level data set
over the available data period (i.e. February 2001 onwards).
-
The Ferbane water level recorder was used to obtain an annual maximum water level
and flow data set over the available data period (i.e. 1953 to 2004), along with an hourly
water level and flow data set over a period that is consistent with the Tullamore water
level recorder (i.e. February 2001 onwards).
-
The Gorteen water level recorder was not used to obtain any data given that there was
no flow data available (due to the lack of any rating) and the station was
decommissioned in 2003.
-
The Millbrook Bridge water level recorder was used to obtain an annual maximum
water level and flow data set over the available data period (i.e. 1951 – 2004) , along
with an hourly water level and flow data set over a period that is consistent with the
Tullamore water level recorder (i.e. February 2001 onwards).
-
The Rahan water level recorder was used to obtain an annual maximum water level and
flow data set over the available data period (i.e. 1958 – 2004) , along with an hourly
water level and flow data set over a period that is consistent with the Tullamore water
level recorder (i.e. February 2001 onwards).
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 10 of 59
June 2008
5.4.3 Manipulation of available data sets
In this instance, “manipulation” refers to converting the available water level data sets to
flow data sets using rating equations. The general form of rating equation used in the
Republic of Ireland is presented in Equation 4.
Equation 4: General rating equation in the Republic of Ireland.
Q = k(G + DG)α
Where:
Flow (m3/s) that corresponds to the gauged water level (G).
Site specific constant determined by fitting gauged data points to a
straight line on a log-log plot.
The gauged water level (m) that corresponds to the flow (Q).
Site specific correction to G, determined by fitting gauged data points to a
straight line on a log-log plot.
Site specific constant determined by fitting gauged data points to a
straight line on a log-log plot.
Q
k
G
DG
α
The hourly water level data set obtained from the Tullamore water level recorder was
converted to an hourly flow data set using this general equation and the following process:
1. Two rating equations were previously developed for the Tullamore water level recorder:
-
Rating Equation 1 was developed using fifteen gauged points between February
2001 and July 2001.
-
Rating Equation 2 was developed using three gauged points between February 2002
and November 2002.
These existing rating equations were checked against the relevant gauged points to
ensure that the estimates made for the variables outlined above were reasonable. As a
result it was found that Rating Equation 1 was reasonable and Rating Equation 2
required a slight adjustment to the upper limits due to the change in the required DG.
The details of the corrected and adopted Rating Equations 1 and 2 are summarised in
Table 4. The plot of the applicable gauged points is presented in Appendix E.
Table 4: Rating Equation 1 and 2.
Rating Equation
k
DG
α
Upper Lim
Finish Date
Quality
1
A
21.0 -1.040 2.5
1.190
24/07/2001
56
1
B
21.0 -1.040 2.5
1.497
24/07/2001
16
1
C
9.6 -1.040 1.5
2.000
24/07/2001
16
1
D
9.6 -1.040 1.5
24/07/2001
56
2
I
21.0 -1.139 2.5
1.289
25/11/2002
56
2
J
21.0 -1.139 2.5
1.596
25/11/2002
16
2
K
9.60 -1.139 1.5
2.099
25/11/2002
26
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 11 of 59
June 2008
2
L
9.60 -1.139 1.5
25/11/2002
56
2. The last gauged point used to derive the most recent rating equation (i.e. Rating
Equation 2) was collected during November 2002. To ensure that this rating equation
was still valid for the conversion of more recent water level data, additional gauged
points were requested. As a result, four measurements were carried out during a flood
event in October/November 2005. These additional gauged points were found to require
a “DG” value that was not consistent Rating Equation 2. It was also found that the
required “DG” value was not consistent between the four new gauged points. It was
therefore determined that Rating Equation 2 was not valid for more recently collected
water level data and that a new rating equation would be required, hence the derivation
of Rating Equation 3 using a new “DG” value and the existing k and α values.
The details of the newly derived Rating Equation 3 are summarised in Table 5. The plot
of the applicable gauged points is presented in Appendix E.
Table 5: Rating Equation 3.
Rating
Equation
k
DG
a
Upper Lim
3
Q
21.0
-1.290
2.5
1.440
3
R
21.0
-1.290
2.5
1.747
3
S
9.60
-1.290
1.5
2.250
3
T
9.60
-1.290
1.5
3. There is a significant shift in the relevant “DG” value between Rating Equation 1 and 2,
and Rating Equation 2 and 3. To avoid a step in the derived hourly flow data set due to
this shift, it was decided that smoothed ratings would be required, which involved
steadily changing the relevant “DG” value (and upper limits) over the period that the
change was observed. The details of the smoothed ratings are summarised in Table 6.
Note: Refer to discussion below regarding the observed instability of the rating equation
for the Tullamore water level recorder.
Table 6: Smoothed ratings.
Smoothed Rating
DGstart
DGfinish
Datestart
Datefinish
∆DG/∆T
A (1 to 2)
-1.040
-1.139
27/07/2001
14/02/2002
-2 x 10-5
B (2 to 3)
-1.139
-1.290
25/11/2002
27/10/2005
-6 x 10-6
4. The significant shift in “DG” noted above caused some concern with regard to the
reliability of the rating equations, particularly given that it was consistently increasing
over a relatively short period of time (i.e. 4 years). There are a number of potential
causes of this shift, including:
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 12 of 59
June 2008
-
The vegetation in the Tullamore River channel is becoming more and more
overgrown.
Figure 6: Vegetation downstream of the Tullamore water level recorder.
-
The consistent increase in “DG” is coincidence and the variable rating is due to
debris blockage behind the road bridge immediately downstream of the Tullamore
River recorder.
-
There is a hydraulic control downstream of the recorder that only dominates during
certain flood conditions (e.g. during recession).
To address this issue in the short term, an alternative flow dataset was created by
applying Rating Equation 1 (refer to Table 4) to the entire water level dataset. Given that
Rating Equation 1 represents the Tullamore River channel at the highest recorded
efficiency, the resulting flow estimates would be the highest and therefore most
conservative out of those available from Rating Equations 1, 2 and 3.
Note: The hourly and annual maximum water level data sets obtained from the Ferbane,
Millbrook Bridge and Rahan water level recorders were converted to flow using the
applicable rating equations without the above checks. This is because these water level
recorders have relatively stable ratings.
5.5.
Long-term catchment changes
The Tullamore FRAM Study looks at the long-term management of flood risk. It is therefore
important that this hydrological investigation gives consideration to how the hydrological
characteristics of the Tullamore River catchment may change in 50 to 100 years.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 13 of 59
June 2008
The two most significant (and foreseeable) changes to the hydrological characteristics of the
Tullamore River catchment are a change in the characteristics of rainfall due to climate
change and a change in the catchment land use due to increased urbanisation.
5.5.1 Climate change
The OPW has produced and implemented an internal policy to ensure that the predicted
effects of future climate change are taken into account when completing hydrological and
hydraulic investigations. The technical basis for this policy is contained in the report written
by Michael Bruen (University College of Dublin) entitiled “Climate Change Impacts on
Flooding in Ireland – A Desk Study/Literature Review (September 2003)”.
1. Sea level rise
It is expected that future climate change will result in a general increase in global sea
levels. To account for such an increase (but excluding the impact of changes in storm
surge characteristics), the OPW internal policy recommends that the sea level should be
increased by 300 to 350 mm, depending on geographic location.
Given the inland location of the Tullamore River catchment and the resulting lack of any
tidal influence, it was not necessary to incorporate this increase into this investigation.
2. Change in rainfall
It is expected that future climate change will result in a change in rainfall intensity across
the Republic of Ireland. The magnitude of this increase will depend on geographic
location.
Based on the location of the Tullamore Rive catchment (i.e. Midlands), and the data
provided by the Tullamore water level recorder since February 2001 indicating that most
significant flood events occur during winter season, the assessment of the impact of
climate change on flood flows and extents was completed by increasing design flood
flows by 20 %.
5.5.2 Land use change
It is likely that the land use within the Tullamore River catchment will change, primarily due
to urbanisation of the land surrounding the existing Tullamore urban area. Urbanisation of a
catchment involves the covering of normally permeable surfaces with impermeable surfaces
such as roofs, roads and footpaths. This is likely to result in an increase in direct runoff
reaching the Tullamore River, which is also likely to increase peak flood flows in the
Tullamore River.
This change was quantified by considering the increase in the URBAN hydrological
parameter. The existing URBAN estimation is 6 %, which is based on the existing
Tullamore urban zone. It is likely that this will increase to 8 % based on the “future
development” and “future development (long term)” zones that are defined by the Offaly
County Council Development Plan 2003 – 2009. The resulting increase in the URBAN
hydrological parameter is not considered significant given it’s magnitude and the fact that it
doesn’t take into account the positive impact of sustainable urban drainage practises that
reduce the impact of urbanisation on runoff. The impact of increased urbanisation within the
Tullamore River catchment has therefore not been included in the future flood flows that are
adopted by this hydrological investigation.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 14 of 59
June 2008
6.
Derivation of design peak flood flows
The FSR recommends a number of hydrological methods as being suitable for the derivation
of design flood flows, including (in order of preference):
1. Annual maximum analysis (peak flood flows only).
2. Six variable empirical analysis and regional growth curves (peak flood flows only).
3. Unit hydrograph analysis (peak flood flows and hydrographs).
The adoption of the above method depends on the availability of sufficiently accurate and
detailed data describing rainfall, flow and catchment hydrological characteristics, with the
preference given to methods that include data collected during observed flood events.
6.1.
Estimation of design flood flows using the available flow dataset
Given the preference for observed data, the mean annual flow at the Tullamore water level
recorder was first estimated using the available 4 year record coupled with rating equation 1
(i.e. the most conservative conversion of water level data to flow data). The results of this
analysis are summarised in Table 7.
Table 7: Estimation of mean annual flow using continuous flow record.
Hydrometric Year* Maximum Water Level (m) Maximum Flow (m3/s)
2002
1.92
7.86
2003
2.02
9.33
2004
1.82
6.55
2005
2.02
9.33
8.27 (average)
*The hydrometric year runs from 1st October to 30 September
The FSR Regional Growth Curve was then applied to this estimate of the mean annual flood
flow to derive design flood flows with a 20%, 10%, 5%, 2%, 1% and 0.1% annual
exceedance probability (AEP). The results of this application are summarised in Table 8.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 15 of 59
June 2008
Table 8: Estimation of design flood flows using the regional growth curve
T (Years) AEP (%) Growth factor Design flow (m3/s)
6.2.
2.3
Mean Annual
1.00
8.27
5
20
1.20
9.92
50
10
1.37
11.33
20
5
1.55
12.82
50
2
1.77
14.64
100
1
1.96
16.21
1000
0.1
2.55
21.07
Estimation of design flood flows using alternative flow datasets
Given the short flow dataset available at the Tullamore water level recorder, along with the
low reliability of the applicable set of rating equations (refer to discussion in Section 5.4.3),
an alternative set of design flood flows was derived using the longer flow datasets available
from three alternative water level recorders on the Clodiah River system (i.e. Ferbane,
Millbrook Bridge and Rahan). The steps followed to derive this alternative set of design
flood flows are as follows:
1. The mean annual flow at the Tullamore water level recorder was estimated using the
FSR six variable empirical equations (refer to Equation 5 and Equation 6) and the
catchment characteristics described in Section 5.2.
Equation 5: FSR six variable equation using Rsmd.
Qbar = 0.0172 (AREA)0.94 (STMFRQ)0.27 (SOIL)1.23 (Rsmd)1.03 (1 + LAKE)-0.85 (S1085)0.16
Equation 6: FSR six variable equation using SAAR.
Qbar = 0.00042 (AREA)0.95 (STMFRQ)0.22 (SOIL)1.18 (SAAR)1.05 (1 + LAKE)-0.93 (S1085)0.19
These equations provided an estimate for the mean annual flow at the Tullamore water
level recorder of 12.03 m3/s using Equation 5 and 11.55 m3/s using Equation 6.
2. The mean annual flows at alternative water level recorders were estimated using
the same FSR six variable empirical equations (refer to Equation 5 and Equation 6)
and the catchment characteristics described in Carroll (1981). These estimations are
summarised in Table 9.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 16 of 59
June 2008
Table 9: Summary of mean annual flows.
Variable
Ferbane
Millbrook Br.
Rahan
Tullamore
Area (km2)
1207
165
274
114.8
STMFRQ
0.85
1.92
0.84
0.319
SOIL
0.382
0.300
0.304
0.341
Rsmd
30.2
34.2
29.1
34.4
SAAR
919
941
987
900
LAKE
0.169
0.000
0.080
0.000
S1085
0.87
5.55
0.87
1.656
Mean Annual Flow (Rsmd)
113.84
28.34
21.89
12.03
Mean Annual Flow (SAAR)
119.98
27.47
25.92
11.55
3. Factors relating the estimate of the mean annual flow at the Tullamore water level
recorder to the estimate of the mean annual flood flows at the alternative water level
recorders were produced using the data presented above. These factors are summarised
in Table 10.
Table 10: Mean annual flood flow factors.
Water level recorder
Factor (Rsmd)
Factor (SAAR)
Tullamore
1.000
1.000
Ferbane
0.1057
0.0963
Millbrook Bridge
0.4244
0.4204
Rahan
0.5496
0.4455
The factors derived using Rsmd were adopted as they produce the highest flow estimates
and are therefore most conservative.
4. The design flood flows at the alternative water level recorders were estimated by
completing a statistical analysis of annual maximum flow data assuming the EV1
statistical distribution. This statistical analysis and the resulting design flood flow
estimates are summarised in Table 11. Charts demonstrating the appropriateness of the
EV1 statistical distribution are presented in Appendix F.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 17 of 59
June 2008
Table 11: Summary of statistical analysis and resulting peak flood flow
estimations.
Peak Flood Flow at the Tullamore Water Level Recorder
(m3/s)
T (Years) AEP (%)
Ferbane
Millbrook Bridge
Rahan
-
u
77.10
15.88
3.10
-
α
16.75
3.10
4.02
2.3
Mean Annual
86.79
17.67
23.49
5
20
102.22
20.53
27.19
50
10
114.79
22.85
30.21
20
5
126.84
25.08
33.10
50
2
142.45
27.97
36.85
100
1
154.14
30.13
39.66
1000
0.1
192.77
37.28
48.93
5. The peak flood flows for the alternative water level recorders (refer to Table 11) were
translated to the Tullamore water level recorder using the factors presented in Table 10.
The resulting peak flood flow estimates for the Tullamore water level recorder are
presented in Table 12.
Table 12: Design flood flow estimates at the Tullamore water level recorder.
Peak Flood Flow at the Tullamore Water Level Recorder
(m3/s)
T (Years) AEP (%)
Ferbane
Millbrook Bridge
Rahan
2.3
Mean Annual
9.17
7.50
12.91
5
20
10.80
8.71
14.94
50
10
12.13
9.70
16.60
20
5
13.40
10.65
18.19
50
2
15.05
11.87
20.25
100
1
16.29
12.79
21.79
1000
0.1
20.37
15.82
26.89
Table 12 shows a variation in design flood flow estimates for the Tullamore water
level recorder depending on the alternative water level recorder. To overcome this
variation, an average of the three estimates could be adopted. Instead however, the
design flow estimates using the Rahan water level recorder have been directly
adopted, given that:
-
The Ferbane catchment is more than eighty times the Tullamore River catchment.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 18 of 59
June 2008
6.3.
-
The hydrological characteristics of the Millbrook Bridge catchment are
significantly different from the Tullamore River catchment (refer to Table 9).
-
The Rahan water level recorder provides the largest and therefore most
conservative estimate for design flood flows at the Tullamore River recorder.
Adoption of design flood flows
The two methods used to estimate design flood flows at the Tullamore water level recorder
produced different estimates (refer to Table 13).
Table 13: Design flood flow estimates at the Tullamore water level recorder.
Design flood flows (m3/s)
T (Years) AEP (%)
Method A*
Method B**
2.3
Mean Annual
8.27
12.91
5
20
9.92
14.94
50
10
11.33
16.60
20
5
12.82
18.19
50
2
14.64
20.25
100
1
16.21
21.79
1000
0.1
21.07
26.89
*
Derived using the available flow dataset at the Tullamore River water level recorder.
**
Derived using longer-term and more reliable flow datasets at Ferbane, Millbrook Bridge and Rahan water level recorders
It was decided that the design flood flows adopted for the Tullamore FRAM Study would be
those derived using Method B (refer to Section 6.2). The justification for this decision is:
-
It avoids the need to use the relatively unreliable flow dataset available from the
Tullamore River water level recorder (refer to discussion in Section 5.4.3).
-
It utilises long and reliable flow datasets (i.e. around 50 years).
-
It utilises observed flow datasets that produce mean annual flood flow estimates that, as
is the case with the Tullamore water level recorder, are consistently larger than estimates
produced using the FSR six variable equations.
-
It places significant weight on the flow dataset available from the Rahan water level
recorder (i.e. of the three alternative water level recorders, Rahan has a catchment that
has the most similar hydrological characteristics when compared with the Tullamore
River water level recorder).
-
It takes into account the consistent difference between the observed and regional growth
factors (refer to Table 14).
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 19 of 59
June 2008
Table 14: Summary of growth factors
Growth factor (Qbar indexed)
T (Years) AEP (%)
Ferbane
Millbrook Bridge
Rahan
Regional
2.3
Mean Annual
1.00
1.00
1.00
1.00
5
20
1.18
1.16
1.16
1.20
50
10
1.32
1.29
1.29
1.37
20
5
1.46
1.42
1.41
1.55
50
2
1.64
1.58
1.57
1.77
100
1
1000
0.1
1.78
2.22
1.71
2.11
1.69
2.08
1.96
2.55
Therefore, to confirm, the adopted design flood flows at the Tullamore water level recorder
are shown in Table 15.
Table 15: Adopted design flood flows at the Tullamore water level recorder.
T (Years) AEP (%) Design flow (m3/s)
7.
2.3
Mean Annual
12.91
5
20
14.94
50
10
16.60
20
5
18.19
50
2
20.25
100
1
21.79
1000
0.1
26.89
Inclusion of allowances for long-term catchment changes
As noted in Section 5.5, the Tullamore FRAM Study looks at the long-term management of
flood risk. It is therefore important that this hydrological investigation gives consideration to
how the hydrological characteristics of the Tullamore River catchment will change in the 50
to 100 years.
The foreseeable changes considered by this investigation are discussed in Section 5.5. The
resulting future flood flows are presented in Table 16.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 20 of 59
June 2008
Table 16: Future design flood flows at the Tullamore water level recorder
Flood flow (m3/s)
T (Years) AEP (%)
8.
Design (present)
Future
2.3
Mean Annual
12.91
15.49
5
20
14.94
17.93
50
10
16.60
19.92
20
5
18.19
21.83
50
2
20.25
24.30
100
1
21.79
26.15
1000
0.1
26.89
32.27
Grand Canal
The Grand Canal, which is operated by Waterways Ireland, runs through the Tullamore
River catchment. Under normal operating conditions the Grand Canal would not be expected
to significantly interact with the hydrology of the Tullamore River catchment. However, if
the canal water level cannot be maintained during normal operation of the canal
infrastructure (e.g. during significant rainfall), an overflow into the Tullamore River
catchment is utilised to remove excess flow. This overflow consists of an arch culvert that
discharges into a side channel controlled by a weir (See Figure 7).
Figure 7: Overflow weir from the Grand Canal.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 21 of 59
June 2008
The overflow is located upstream of Tullamore between the 21st and 22nd lock of the Grand
Canal. The channel that the Grand Canal overflows to flows south and discharges to the
Tullamore River 2.5km downstream of the overflow. This channel discharges to the
Tullamore River 5.5km upstream of the Tullamore Urban Area.
The significance of this overflow into the Tullamore River catchment was determined by
assessing the maximum flow that would be expected from the canal overflow. The steps
followed were:
1. The overflow was modelled as a broad crested weir that was assumed to perform based
on Equation 7.
Equation 7: Flow across a broad crested weir.
Q = 2/3 H ( 2/3 g H )½ L
Where:
Q
Flow across the weir (m3/s)
H
Average depth of flow across the weir (m)
L
Length of the weir (m)
2. The dimensions of the weir were measured on-site by OPW staff, while the likely
maximum flow depth across the weir was estimated by Waterways Ireland staff (refer to
Table 17).
Table 17: Dimensions of canal overflow weir
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 22 of 59
June 2008
Dimension
Value
Source
Weir length
24 m
Visual inspection
Maximum water depth observed 0.1 m Waterways Ireland
Note: Due to overgrown vegetation, the observed weir length was 10 m. However, given
that Waterways Ireland may clear this vegetation, the weir length was retained at 24 m.
3. The expected maximum flow was calculated as 1.3 m3/s based on Equation 7 and the
dimensions contained in Table 17.
4. The significance of this overflow from the Grand Canal relative to the adopted design
flood flows at the Tullamore River water level recorder is summarised in Table 18.
5. The expected overflow from the Grand Canal was not included in the design flood flows
adopted by this investigation given that:
-
Given that the overflow is located around the top of the Tullamore River catchment,
the peak overflow is unlikely to coincide with the peak flood flow within the
Tullamore urban area.
-
There is some uncertainty as to when the overflow will operate.
Table 18: Grand Canal overflows relative to adopted design flood flows.
T (Years) AEP (%) Adopted design flow (m3/s) Grand Canal overflow (%)
2.3
Mean Annual
12.91
10.0
5
20
14.94
8.7
50
10
16.60
7.8
20
5
18.19
7.1
50
2
20.25
6.4
100
1
21.79
5.9
1000
0.1
26.89
4.8
-
6. The impact of the overflow from the Grand Canal on flows in Tullamore requires
further investigation and should be examined prior to the detailed design stage of the
Tullamore FRAM, see Recommendations.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 23 of 59
June 2008
9.
Conclusion
The adopted design flows from this report are suitable for the purpose of the FRAM study;
however for detailed design of flood relief measures further hydrological examination is
required to increase confidence and accuracy, see Section 10 Recommendations.
10.
Recommendations
Before a detailed design can be produced three issues need to be addressed:
1. The ability to establish accurate design flows based on observed data.
2. The instability of the rating equation for the Tullamore water level recorder, see
Section 5.4.3 Manipulation of available data sets.
3. Include the overflow from the Grand Canal in the upstream flow estimate..
In order to address these issues, the following steps should be taken as soon as possible:
1. At least one additional gauging station is required in the catchment. It is
recommended that this station be located in the vicinity of Springfield Bridge and
should be capable of collecting data suitable for project analysis, i.e. mid to high
flows. A second station (should it be agreed) would be usefully employed at
Riverside Bridge.
2. Additional flow measurements are required at the Tullamore Hydrometric Station to
address the issues raised in Section 5.4.3 (Manipulation of available data sets).
Should this data prove the lack of suitability of this station then it would be
necessary to locate a new Hydrometric Station within the town.
3. A staff gauge should be placed on the canal overflow weir and readings taken
during times of overflow and, if possible, flow measurements. These readings
coupled with readings at the gauged station can then be used to assess the impact of
the canal overflow on flows in Tullamore.
D1791 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 24 of 59
June 2008
11.
References
Bruen, Michael (2003), “Climate Change Impacts on Flooding in Ireland – A Desk Study”.
Carroll, D (1981), “Estimates of the Values of Catchment Characteristics for OPW Gauging
Stations”.
Hayes, Mark (2002). Unpublished research into the hydrology of the Tullamore River
catchment.
Houghton-Carr, Helen (1999), “Flood Estimation Handbook 4: Restatement and application
of the Flood Studies Report rainfall-runoff method”.
Meteorological Service Ireland (1980). “Mean Annual Rainfall Map (1951 - 1980)”.
Natural Environment Research Council (1975). “Flood Studies Report”.
Offaly County Council, “Offaly County Development Plan 2003 – 2009”.
Office of Public Works Design Section Press Archive.
Office of Public Works (2005). “Interim Report on Hydrology and Hydraulics, Tullow
Flood Relief Feasibility Study”.
D1794 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 25 of 59
June 2008
Appendices
A. Plan of principle study area for the Tullamore FRAM Study
B. Plan of the Tullamore River catchment
C. Plan of the Tullamore urban area within the Tullamore River catchment
D. Plan of relevant rain gauges and water level recorders
E. Gauged points for the Tullamore water level recorder
F. EV1 plots for Ferbane, Millbrook Bridge and Rahan
D1794 – Tullamore FRAM Study
Doc. No. 1794/RP/003/D
Page 26 of 59
June 2008