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An evaluation of urban flood estimation methodologies in Ireland

This paper summarises the findings of a study to evaluate the use in Ireland of the different flood estimation methodologies that can be applied to urban or urbanising catchments. The study, undertaken as part of the Office of Public Works' (OPW) Flood Studies Update (FSU) programme, comprised both quantitative (posted questionnaires) and qualitative research (focus groups) targeting a range of organisations involved with planning and design issues pertaining to urban flooding. A total of 291 questionnaires were circulated. Of these, 100 were returned, yielding a response rate of 34%. Results indicate that formulae and methods for determining runoff rates in urban and urbanising catchments are being applied very generally without due consideration being given to the statistical foundation underlying the methods. The resulting inconsistencies in estimated runoff rates highlight the need for national guidance for flow estimation in urban and urbanising catchments in Ireland.


Introduction
Fuelled by the 'Celtic Tiger', Ireland has recently seen an upsurge in the development of residential, commercial and industrial properties, which has resulted in a rapid increase in the areas of impervious surfaces. As a result, a greater proportion of incident rainfall on these catchments appears as direct runoff and this, combined with the sewers, gulleys and culverting of natural streams that accompany development, causes a more rapid conveyance of stormwater through the drainage network. As levels of imperviousness increase, the runoff response from increasingly smaller rainfall events becomes more pronounced (Sheeder et al. 2002). Failures of the existing drainage network in these areas have become more frequent, requiring both significant improvements of the existing infrastructure and also completely new systems.
In light of this increasing urbanisation, together with the influences of climate change and changing land use on the natural hydrology of catchments, over-reliance on the 1975 Flood Studies Report (FSR) [Natural Environment Research Council (NERC) 1975], the FSR Supplementary Reports (FSSR) and Institute of Hydrology (IoH) methodologies, currently used in Ireland, is not recommended. The Flood Estimation Handbook, which superseded the FSR in the United Kingdom and Northern Ireland, did not study catchments in the Republic of Ireland and thus does not apply there. Recognising the deficit in accurate methodologies and the need to utilise the additional years of hydrometric data that have become available since 1969, together with the advances in computer and digital technologies, the Irish Office of Public Works (OPW) initiated a research project to update the FSR. This project, termed the Flood Studies Update (FSU), comprises Research and Development in six specific work-groups, one of which deals with Urban Catchment Flood Analysis (Reed & Martin 2005). Work-groups are further divided into a number of complementary work-packages. This paper presents the findings of the initial work-package of the Urban Catchment Flood Analysis Work-Group that involved a scoping study focused on identifying and evaluating current urban flooding issues in Ireland. The main objective of the study was to establish the issues with current design practice as they apply to practitioners on a day-to-day basis. Particular reference was made to: reviewing the methods of flood estimation in urbanised catchments currently in use in Ireland and assessing the deficiencies inherent in these methods. The results of this study will be used to inform the Technical Steering Group of the FSU on Research and Development needs for improving methodologies for Urban Catchment Flood Analysis in Ireland. It is envisaged that this work will be completed through additional work-packages in the work-group. These have yet to be defined.

Background
The current approach to stormwater management in Ireland promotes sustainable development by restricting the peak outflow from new developments to the predevelopment greenfield peak flows (Doyle et al. 2003). In practice, this approach involves determining runoff rates for specified design storms from urban portions of development areas using deterministic or fixed percentage runoff formulae and controlling or attenuating the difference between these and the greenfield runoff before these flood flows are allowed entry into drainage networks or water courses. The rational-type methods used for urban catchments are generally based on assigning a fixed-percentage runoff from each type of surface. While simple rationaltype formulae do not properly account for variations in rainfall intensity, flow velocity or temporal storage in a sewer system, they generate acceptable results for small urban catchments. Methods of estimating runoff rates from catchments to be urbanised (small greenfield) are governed by individual Local Authorities but generally follow those outlined in the FSR, the FSSR and reports published by the IoH. As an alternative to determining runoff rates from formulae, some Local Authorities stipulate maximum permissible outflows from development areas (in l/s/ha for example). However, recent times have seen many Local Authorities across the country adopting the criteria for greenfield catchments outlined in Dublin City Council Guidelines (1998). These guidelines recommend that the 3-variable equation presented in the IoH Report No. 124 (Marshall & Bayliss 1994) be used for development areas of up to 24 ha and the three-parameter equation contained in the FSSR No. 6 (IoH 1978) be used for areas in excess of 24 ha. As a consequence, the use of the two methods, particularly the IoH equation that is recommended for an area that accounts for most developments, is well established in Ireland. The 3-variable equation in the FSSR No. 6 evolved following analysis of the FSR methodologies that showed that floods on small catchments were less well predicted than on larger ones.
where Q is the mean annual flood in m 3 /s, AREA the catchment area in km 2 , SAAR the standard annual average rainfall in mm and SOIL a dimensionless number depending on soil class.
It is important that inconsistencies in the generalised application of these (and other) statistical formulations are not overlooked. Peak flows for specified return periods are determined by multiplying Q values [as determined by Eqs (1) and (2)] by dimensionless regional growth curve constants derived in the FSR. These constants are based on average responses to rainfall for 1700 record years from 112 gauged sites across Ireland. The concept of a unique growth factor for a specified design storm that is applicable to all areas of the country is open to question, and Bruen et al. (2005) have illustrated that these growth curve factors for catchments around Dublin are lower than those in the FSR methodology and can result in underestimation of peak flow values.
The FSSR No. 6 3-variable equation is based on a regression analysis of 53 small catchments (o20 km 2 ) of the FSR data set, and the IoH Report No. 124 equation is based on regression studies of 87 catchments of areas o25 km 2 , 71 of which were completely rural (rural fraction of o0.025). While catchment areas and average annual rainfall values (SAAR) were reasonably well distributed over the catchment sample, SOIL values were not. Of the 53 catchments on which the FSSR No. 6 equation is based, 41 have soil classes 4 and 5. Consequently, the equation is likely to perform well for high runoff soils (SOIL indices 4 0.45 and representing soil classes 4 and 5) but less well for soil classes 1, 2 and 3. Similarly, analysis of the SOIL parameter for the IoH Report No. 124 equation indicates that only 16 of the 71 rural catchments were represented by soil classes 1, 2 and 3, but 39 of the catchments were represented by soil class 5, where runoff would be very high (Cawley & Cunnane 2003). It was further shown that for the 17 catchments of the sample having areas of o5 km 2 , soil classes 1 and 2 are not represented at all, soil classes 3 and 4 are represented in four catchments and soil class 5 has nine catchments. Consequently, as with the case of the FSSR No. 6 3variable equation, the IoH equation is significantly biased.
In Ireland, the scale of catchments for urban development is often o1 km 2 , significantly less than the catchment sizes on which the FSSR No. 6 and IoH Report No. 124 equations are based, and this calls into question the 'one size fits all' approach often applied to the analysis of urban catchments in Ireland. This difference, combined with the fact that significant areas of Ireland are represented by soil classes 1, 2 and 3 (low to medium runoff), which are not well represented in either the FSSR No. 6 or IoH equations, raises further uncertainties in the predicted flows. The inaccuracy in flood flow prediction by these equations is further exacerbated by the fact that the SOIL parameter included in the calibration for Ireland is generally based on poor-resolution FSR mapping. Predicted peak flows are very sensitive to SOIL parameters. This sensitivity was demonstrated by Cawley & Cunnane (2003), who, by adding three catchments with soil classes of 1, 2 and 3, respectively, to the catchment sample on which the IoH Report No. 124 equation is based, showed that Eq. (1) would become: Climate change predictions of increased intensity and frequency of winter rainfall can also be expected to impact on urban flood issues into the future (Dublin Drainage Consultancy 2005). Currently, the majority of rainstorms that pass over Ireland are frontal, but it is expected that more convective-type rainfall will occur in the future. Higher intensity storms will further stress drainage networks and this, combined with the 'locking' of outfalls that could potentially occur, will, in some cases, surcharge sewer systems. These factors, combined with changes in catchment land use, have the potential to exacerbate current urban flooding problems, and highlight the need for a more radical and holistic approach to urban runoff control in Ireland.

Methodology
Both quantitative and qualitative research methods were used in the study described below. The quantitative element of the study consisted of circulating a self-completion postal questionnaire to target sectors in all 26 counties of the Irish Republic. Target sectors included organisations in Ireland currently involved with planning and design issues pertaining to urban drainage and flooding. To ensure this was the case, an initial verification process was undertaken by a telephone survey to establish the appropriateness of every potential organisation within these target sectors. Organisations with no history of work in drainage or flooding were excluded at this point. The questionnaire focused on the following issues: (1) Peak flow estimation in greenfield catchments.
(3) An assessment of data available for flow estimation.
(4) Guidelines for peak flow estimation either in green field or in urban catchments. A total of 291 questionnaires were circulated and, of these, 100 were returned, representing a response rate of 34%. A total of 83 of the 100 respondents completed the questionnaire in full. Notwithstanding the verification process, 17 incomplete returns were received from organisations stating they were not involved in urban catchment flood analysis or drainage system design. The majority of the questionnaires (82 in total) were returned from engineering consultancies and public bodies (Local Authorities, County Councils, City Corporations, etc.), who tend to lead in the design, planning and implementation of all aspects that relate to urban flooding. Questionnaires tended to be completed by senior staff members in these organisations. The breakdown of complete and incomplete survey responses for target sectors is summarised in Table 1.
The qualitative work in this study comprised four focus group meetings in major conurbations in Ireland. Focus group participants were recruited from the questionnaire respondents in the main target sectors. A focus group topic guide was developed through consultation with the Technical Steering Group of the FSU and, in the interest of independent and unbiased reporting, external consultants were appointed to facilitate the focus groups.

Experience with peak flow estimation
The majority of survey respondents had experience in calculating or using peak flow runoffs from either greenfield or urban catchments. While some respondents were familiar with only urban or greenfield methodologies, Fig.  1 indicates that approximately 61 and 53% of the respondents had experience with methodologies for both greenfield and urban catchments, respectively.

Equations/methods used for peak flow estimation from greenfield sites
Respondents expanded on this part of the questionnaire by stating the methods with which they were most familiar for calculating peak flows from greenfield sites. A list of commonly used methods was provided in the questionnaire. The responses for this question are shown in Fig. 2. Survey responses contained 200 citations of the equations/methods that were listed in the questionnaire. As illustrated in Fig. 2, the IoH Report No. 124 equation and the rational method are the most commonly used flow estimation methods. Figure 2 also shows the wide variety of other estimation methods used, which reflects the nonuniformity of approach to flow estimation from greenfield sites. The variety of estimation methods available is encapsulated in the following comment of one of the attendees of a focus group: Our main problem with the methodologies is that there's so many of them. If there was a unified methodology for the whole country, it would be easier.
Respondents to the questionnaire were asked to identify the main difficulties/limitations that were encountered when using these methods. A total of 48 of the 83 respondents (58%) who returned the questionnaires provided information for this section of the survey. The most common responses were grouped, and are summarised in Fig. 3. These responses tended to be general and rather sweeping in their nature. It is clear that respondents are aware that uncertainties exist in the application of the various peak flow estimation methods to greenfield catchments but evidence of a deeper understanding of where these uncertainties may originate is not so apparent. As an example, the IoH Report No. 124 equation is commonly used across Irish catchments and a perceived difficulty with the method, identified in questionnaires and discussed in focus groups, lies in extracting catchment descriptors from poor-resolution FSR mapping. While this is without doubt the case, a source of uncertainty may also be present from limitations in the regression study on which it is based and particularly the range of soil classes in the catchment sample (refer to 'Background'), a point not raised by any respondent.
Two main issues are reflected in Fig. 3. The first of these, identified by over 31% of respondents, was the inconsistency in the values of the peak flow determined from the various methods and how these varied over catchments of different sizes. It was suggested that some of the commonly applied methods overestimate design flows. The manner in which the behaviour of ground conditions is oversimplified by the assumptions inherent in many of the methodologies was also perceived as a problem. This lack of confidence in the various approaches is reflected in the following comment that was made at a focus group: The quality of the result is related to the relevance of the formulae used and the catchment size.

25%
36% 28% 11% Both Greenfield Urban catchment Neither  The second main issue, identified by almost 23% of respondents, was the accuracy, resolution and validity of the data on which the methodologies are based. Some of the required variables (SOIL, SAAR, runoff coefficients and concentration times) are not site specific and can vary widely. Where required, the interpolation of these hydrological parameters is often subjective and open to question. In addition, respondents identified that SOIL parameters do not account for site topography or slope. Issues relating to data quality, particularly the SOIL parameter, are reinforced by the following comments that were made in focus group meetings: . . .. the resolution of the maps is so poor. In many cases, all you get is something the size of a postage stamp and the methodology is very sensitive to soil around the catchment. Soils can vary over a catchment, even if you have a map. The hydrologist should understand if he/she has impervious clay or a free-draining material and be able to have some guidance on how to classify that.
The issue of poor-quality data, particularly relating to SOIL, was probed at focus group meetings. The idea of imposing on developers the requirement for site-specific infiltration/percolation tests where planning permissions are being sought was discussed. It was generally agreed that this would be a positive initiative that would facilitate a more accurate assessment of site-specific hydrological characteristics and 'should be part of' all developments. It was accepted that 'it would be very good to get site specific data' and over time this could provide 'very useful information. . . . for the country'.

Equations/methods used for flow estimation from urban sites
Respondents also stated the methods for calculating peak flows from urban catchments with which they were most familiar. Again, a list of commonly used methods was provided in the questionnaire and, from these, 123 citations of these methods were received. These are summarised in Fig. 4. Figure 4 shows that rational-type formulae represented by the rational and modified rational methods and the Wallingford Procedure are the most commonly used formulae for peak flow estimation in urban catchments. Respondents also have familiarity with the rainfall-runoff methods in the FSR and FSSR No. 5 and FSSR No. 16 where 7.3, 4.9 and 5.7% of the respondents, respectively, stated they had either direct experience with these methods or had experience using flows calculated from these methods. The Transport and Road Research Laboratory Method (TRRL) was familiar to 7% of the respondents, with the 'other' grouping comprising respondents who mentioned having experience with methods such as HEC-1 (Hydrological Engineering Centre 1998), TR-20 [United States Department of Agriculture (USDA) 1982] and TR-55 (USDA 1986). HEC-1 refers to a flood hydrograph software package produced in the Hydrologic Engineering Centre of the United States Army Corp of Engineers. TR-20 and TR-55 refer to Technical Releases from the Natural Resources Conservation Service in the USDA in which simplified procedures applicable to urbanising watersheds are presented to calculate runoff volumes, peak flows and hydrographs.
The perceived difficulties and limitations in applying these methods and equations to urban catchments are presented in Fig. 5.
As with greenfield catchments, confidence in the results obtained from the various methods is of most concern to respondents. The problem of poor confidence is inherent in the differences in calculated flows that can result from the various methods, the dependence of the methods on poor-quality and poor-resolution information inputs and on the various assumptions that are made. Estimating SOIL, catchment delineation, coarse map scale, etc.

Methods too simplistic (for large catchments and scales)
Accuracy of data (too general, e.g. SAAR, SOIL,tc,Cv) Confidence of model (too many assumptions leads to inconsistent results) % respondents This inconsistency is reflected in the following focus group comment: . . . different engineers trying to calculate flow for the same catchment would arrive at different results. Difficulties in estimating routing coefficients, runoff coefficients, entry times and delineating catchment areas were identified by 24% of the respondents as limitations of the urban catchment methodologies. Difficulties of this type for methods that require catchment descriptors are that 'you don't have a stream frequency or an S1085 value that you can use. It's basically a small stream going through the site. It doesn't appear on mapping either'.

Availability of data
It was generally recognised in the questionnaire responses and focus group comments that accurate flow estimation from greenfield and urban catchments was reliant on good-quality data. However, only 41% of the respon-dents were satisfied with the quality and availability of these data.
Respondents expanded on the reasons for their dissatisfaction with the currently available data. While many reasons were mentioned, the main factors are represented in the three groupings in Fig. 6. This shows that an overall lack of, or access to, various data for accurate flow estimation is the most significant source of dissatisfaction, with over 40% of the respondents identifying this as a problem. However, the accuracy and the resolution of the data that are available is also a significant problem and is reflected in the 37% of the respondents who mentioned that existing data are more readily applied to large catchments. In this regard, it was specifically noted that the 'data available is not specific to a particular region' and that 'values read from FSR maps are too general and are not site specific'. Characteristics specific to some Irish catchments were also raised by one focus group participant, who recognised the 'problems with karst and the indeterminate Lack of guidance or national structured approach Access to/lack of data (e.g. rainfall) Estimation of Cv, tc; Catchment identification Confidence of methods, inconsistent results, over/underestimates % respondents nature of underground flow as well as runoff '. Regarding these conditions, it was noted that 'they're characteristic of Irish catchments, but we often have to rely on English data (for their analysis)'. A single respondent also noted that there is 'no site specific rainfall data available'. The absence of national guidance also features, with approximately 22% of the respondents citing this as a problem.

Availability of guidance
While many sources provided information, the primary references used by practitioners for flood estimation in Ireland were the FSR and Local Authority Guidelines. The diversity of the reference sources accessed by respondents serves to highlight the perceived inadequacy (Fig. 7) of current guidance for peak flow estimation and, as shown in Fig. 8, where limitations of this guidance are presented in four groupings, the absence of a national policy document continues to be problematic. Issues relating to inconsistencies and lack of confidence in peak flow values determined from various methodologies continue to permeate through the identified deficiencies and limitations with the available guidance as shown, and the need to update some of the commonly used methodologies (FSR methodologies, IoH Report No. 124 equation and the TRRL methods) with respect to Irish catchments was identified by over 37% of respondents.

Discussion
A number of issues relating to urban catchment flood estimation methodologies currently being used in Ireland have been raised in this study. It is accepted that the variability across Irish catchments makes a single approach to flood estimation both unsuitable and unsustainable. However, questionnaire responses and focus group discussions indicate that significant levels of uncertainty exist within the range of methodologies being used for urban and small greenfield sites. Results show that rational and modified rational methods are extensively used in estimating peak flows in urban catchments and while these also feature prominently in methodologies being used for small greenfield sites, the IoH No. 124 equation and to a lesser extent, the 3-variable equation in FSSR No.6, are also well used.
The success of rational-type methods tends to be based on a well-developed understanding of catchment behaviour and runoff processes for different regions. However, focus groups suggest that catchment responses to rainfall events are not particularly well understood in Ireland, thus limiting the confidence in the method when applied to catchments other than those that are hard paved. To overcome this, it was suggested that over time, flow estimation for small urbanising catchments could be developed within the framework of a rational method approach. This would require calibration to Irish conditions and as such, particular attention, in terms of supporting such an approach, would need to be focused on accurate estimations of rainfall-runoff coefficients and concentration times for a full range of catchments. The suggestion was well supported at focus groups. Accuracy of data/too generalbetter for large catchments/based on old maps Access to data/lack of data % respondents  The advantage of the statistical formulae for greenfield sites is their ease of use but in effect they provide an 'envelope' of estimates of indeterminate safety margins that are based on a limited number of recorded occurrences. As such, the results of such methods should be treated with caution. The focus groups, however, indicated that these methods are being 'blindly' applied in a generalised manner without due consideration being given to the underlying statistical foundations on which they are based. The over-reliance on spreadsheet calculations and software packages identified in this study does not help in this regard.
Urban or urbanising catchments for development are typically o1 km 2 but the FSSR No.6 3-variable equation and the IoH Report No. 124 equation are based on runoff responses for catchments with larger areas of up to 20 and 25 km 2 , respectively. Furthermore, Irish catchments are well represented by soil classes 1, 2 and 3, which are not well represented in either the FSSR No. 6 or the IoH equations, which are strongly biased towards high runoff soils in classes 4 and 5. This is particularly relevant when it is considered that these methods are particularly sensitive to the soil index value chosen for a catchment. The prescriptive-based application of these methods is likely to result in peak flow estimates that may be higher than those that will be realised. In terms of designing flood defences in receiving watercourses, a conservative flow estimate may be acceptable. However, in the design of attenuation systems required to restrict peak outflow from developments to predevelopment greenfield values, conservative estimates of flow can result in systems with insufficient capacity. It is clear from this study that a deficit exists between the actual and what would be considered to be an acceptable level of understanding with regard to the implications of the uncertainties in flow estimation methodologies when applied to Irish catchments. This deficit reinforces the need for Engineers Ireland (the Irish Professional Engineering body) to assist in the educational needs of the engineering community. The consensus from focus groups was that this should be provided through a Continuing Professional Development (CPD) framework and sustained through support from engineering organisations.
Furthermore, catchment characteristic equations are based on descriptors, which, for Ireland, are generally determined from the original FSR maps. A major issue with these maps is their poor resolution and for small areas where development is proposed, delineating the catchment boundary is a further problem. It was suggested at focus groups that the availability of site-specific data would be hugely beneficial for flood estimation.
The study shows that the lack of clear guidance on peak flow estimation methodologies for a range of catchment conditions and sizes that represent both the geographical and the geological variations in Irish catchments is also a concern. It is clear from questionnaire responses and focus groups that practitioners require a nationally produced and supported document to provide guidance in this matter. This would aid in eliminating the confusion that currently exists for the different design approaches. Such guidance should be supported by worked examples and design approaches. The idea of developing software to accompany any guidance document was suggested at one focus group, and the need for this software to have the capability to model the full range of predicted climate change scenarios was mentioned at another.

Conclusions
The main findings arising from the questionnaire responses and focus groups that formed the basis of this study are as follows: (1) A wide range of methodologies for peak flow estimation from small greenfield and urban catchments are   applied in Ireland in a general way without due consideration to the underlying statistical foundations on which they are based.
(2) The large number of methods that can be legitimately applied to a given catchment results in significant variations in peak flow estimation. This can create uncertainty in the flow being carried into the design process.
(3) The uncertainties that are inherent in statistical formulae can be compounded by the need to extract numerical values for catchment descriptors from poor-resolution FSR maps that are still being used in Ireland.
(4) The uncertainty in design peak flow estimation could be reduced by the provision of a national policy document or a handbook that would provide guidance on methodologies that are applicable for a range of catchment conditions, for a variety of catchment sizes and represent both the geographical and the geological variations in Irish catchments.