4. existing drainage practice...4. existing drainage practice this chapter examines the traditional...

Greater Dublin Strategic Drainage Study Regional Drainage Policies - Volume 3 Environmental Management

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4. EXISTING DRAINAGE PRACTICE

This chapter examines the traditional drainage practices within the Greater Dublin area, from the earliest provision of combined sewerage pipes up to the operation of the present day network. The overall historical development of Dublin’s drainage network is firstly discussed in terms of the significant drivers of the day. The more specific philosophies and procedures for designing stormwater drainage networks are then introduced, leading to a brief look at the consequences of such with respect to flooding. Existing drainage policies of the individual Local Authorities are then documented with particular reference to sustainable drainage components. Finally, the recent work undertaken to address drainage issues is broadly discussed.

4.1 History of Network Development

The provision of piped drainage networks within the Greater Dublin area can be traced to the early development of present-day Dublin City, when stormwater and foul drainage was closely tied to the interrelated issues of public health and sanitation. Prior to the 17th Century, Dublin City was enclosed within medieval walls and suffered from overcrowding and poor sanitation. Domestic sewage was discharged directly to crude, open channel stormwater drains in the streets (refer Figure 19 below) and as a consequence epidemics were frequent, especially when waste and sewage became widely spread following heavy rainfall.

Figure 19: Representation of open street drainage in 17th -18th Century Dublin

(Source: Sweeney 1991) This situation worsened throughout the 17th and 18th Centuries as Dublin expanded outside the city walls. The industrial revolution of the 18th and 19th Centuries and the increase in concentrations of population and industry around Dublin resulted in even larger quantities of untreated domestic sewage and trade waste entering the street drains and ultimately the city rivers. Open street drains were gradually deepened and improved from the mid 18th Century and by 1825 drains were being slabbed or bricked over (refer Figure 20 below) to create a recognisable concept of a modern sewer.


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Figure 20: Typical Drainage Construction Details

(Source: Sweeney 1991) Throughout the early to mid 19th Century the Dublin City authorities, with the support of more progressive public health infrastructure legislation, undertook the first systematic construction of piped drainage. These early pipes ranged in size from 99 x 68 mm up to the man-entry sewers that were ovoid in shape, built of brick and designed to carry combined flows at three times dry weather foul flow. Larger natural watercourses were also arched over or culverted for health and aesthetic reasons, with many to the west of the city centre being heavily modified by the construction of overflows, weirs and diversions. This early drainage programme was successful in improving city sanitation but a consequence was that sewage was transferred to rivers like the Camac, Poddle and Liffey, which in turn became seriously polluted. Several proposals were put forward in the mid to late 19th Century to mitigate this problem, but it was only in 1886 that the Main Drainage Scheme for Dublin City commenced, involving the construction of the North and South Quay interceptor sewers and the Ringsend treatment plant, the latter being completed in 1906. Independent drainage schemes for the Rathmines and Pembroke township and other smaller townships were also constructed in the 1870s and 1880s. The early 20th Century saw an expansion of the Main Drainage Scheme network into the townships and rural districts surrounding the City, so that by the 1940s, Ringsend was the primary treatment plant and outfall servicing the then Greater Dublin area. Combined sewer overflows (CSOs) were also constructed to relieve overloading on the system infrastructure, such as the trunk pipes, pumping stations and treatment plants, during storm events.


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In the 1930s the partially separate principle was introduced to new developments, whereby surface water from the rear of premises was allowed into the foul sewer, while front roofs, gardens and roads drained to a newly constructed, dedicated stormwater network. This principle was subsequently incorporated into the design of the North Dublin Drainage Scheme (NDDS), built between 1944 and 1958, in an effort to help relieve overloading on the existing combined network and to cater for increasing development in North Dublin. Various network plans from the 1930s onwards are still retained by Dublin City Council (DCC) and show that stormwater is positively drained via small road gullies and closed pipes directly to the nearest receiving watercourse, whether a stream, river or Dublin Bay. Despite the partial separation, CSOs frequently continued to operate, resulting in untreated sewage being discharged directly to the stormwater pipe network and open watercourses. The situation was further exacerbated by the direct discharge of industrial and trade waste from the numerous factories built close to rivers, such as the Camac, Tolka and Liffey. Complete separation of foul and stormwater was consequently made a planning condition by the Local Authorities in the early to mid 1960s, though it was not till the 1970s that the practice of total separation became more accepted. Despite this, recent surveys and other investigations suggest that the planning condition was not being fully complied with, and that it is still common to find small, private sewage drains misconnected to stormwater pipes. Maintenance of the stormwater drainage network and rivers was not seen as a critical function, and was usually undertaken on an as-needs basis by a combination of Council’s drainage, roads and parks departments. As a consequence, records of stormwater networks were not as reliable as those for foul networks, with a lot of knowledge of the drainage network residing with the local overseers and Area Engineers.

4.2 Past Drainage Philosophy and Design

Stormwater networks were designed with a standard engineering approach focussed on conveying surface runoff to underground pipes and to a receiving watercourse as quickly as possible, the aim being to minimise the impact of surface flooding as a public hazard. Various building standards and codes of practice for constructing stormwater networks have been written over the years, although it was not until recently that guidelines were issued to assist developers in relation to stormwater policy.

4.2.1 Minor Drainage Developments, such as housing estates, are serviced by both private drains and public sewers, which are both legal terms defined under the Public Health Act, 1878. Briefly, private drains are usually 150mm diameter or less and can be either storm drains or combined drains within the boundaries of a private property and are the responsibility of the landowner. Private drains will then connect to a public sewer, which is normally greater than or equal to 225 mm diameter (Ø), often located under a road, and usually taken in charge by the local authority. The following discussion relates only to public sewers. A review of available drainage records over the last 60 years show that the older housing estates have a greater proportion of smaller diameter pipes than similarly sized but newer estates. The difference is most noticeable over the last 20 years and is particularly evident when comparing outfall pipe diameters. This upsizing of pipes could be due to: • an increase in the average rainfall intensity as more data is collected;

• increasing impervious areas concurrent with an increase in housing densities;

• a raising of design standards (say from 2 to 5 year Average Recurrance Interval (ARI)) imposed by Local Authorities;

• setting Ø225 mm by the DoEHLG as a minimum pipe diameter for public areas; and/or

• an allowance for areas zoned for future development upstream to connect through the site under investigation.


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Street drainage for small developments was generally designed using the Rational Method and Manning’s formula, and pipes sized to accept minor storm events, such as the 2 to 5 year ARI events, thereby alleviating nuisance flooding. Normal practice for small developments in recent times was to apply a 50 mm/hr rainfall intensity and assume 100% runoff from impervious areas, while larger developments were designed using the Bilham or MOT formulas, usually with a reduced rainfall intensity. Pipes also had to be laid at gradients sufficient to achieve self-cleaning velocities. Piped street drainage would then feed into the larger trunk drains or nearby open channel watercourses. French drains and soakaways were sometimes used however in flatter areas where ground conditions permitted infiltration, or where there was no existing pipe network or watercourse to connect to. In recognition of flooding problems in Dublin City and the growing international acceptance of best management practice, Dublin Corporation started to promote a rudimentary method of attenuation for new developments sometime prior to 1998. This consisted of attenuating 85% of a 30 minute, 30 mm/hr storm on site, but it is not known how widely adopted this policy was. This first policy was superseded in 1998/99 by Dublin City Councils Stormwater Management Policy (refer Section 4.4.2 below).

4.2.2 Trunk Drainage The construction of trunk drains and watercourse culverts was often undertaken by the Local Authority and were sized to accept higher return period storms, such as the 10 to 20 year ARI event. DCC Drainage Division records shows that for design purposes on some watercourses like the Kilbarrack Stream they assumed a fully developed catchment, an overall run-off of 40%, all streams and ditches culverted and a pipe flow velocity of 1.5 m/s (refer Figure 21 below).

Figure 21: Kilbarrack Stream Catchment: Runoff Calculations

(Source: DCC Drainage Division, undated)


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Past performance of these trunk drains suggests that they were adequately sized, however as development extended further up the catchment, hydraulic capacity was gradually reduced. Furthermore, a review of DCC drainage records revealed numerous instances of pipe sizes actually decreasing in a downstream direction, possibly as a result of the staged construction of the pipe line to cater for expanding development, revised hydraulic calculations or changes in slope. Some of the outer local authorities today have left open trunk pipes at the edge of urban development and at the base of undeveloped, rural catchments, in readiness to accept future development, although it remains to be determined whether these trunk drains have sufficient capacity to cater for the increased runoff and to meet today’s design standards. To increase hydraulic capacity, an emphasis was originally placed on diverting stormwater from critical to non-critical trunk drains. This could either be undertaken at the catchment watershed where stormwater from a new development within a highly urbanised catchment could be diverted across to a less developed catchment, or within a single catchment where diversions between trunk drains were common. The long term potential for cumulative flooding impacts or the provision of overland flow paths to cater for higher ARI events when siting the line of a trunk drain was not of primary concern as evidenced by the numerous developments built on such paths today. Pipe diversions, particularly in the older areas of Dublin City, are also not well defined in terms of their connectivity and flow direction.

4.2.3 Open Channel Watercourses As the Greater Dublin area became more urbanised, natural, open channel watercourses came under increasing pressure, and were at worst seen as hazardous features that should be isolated from the public. For example, the deterioration of water quality in many urban watercourses due to discharges of sewage, industrial waste and litter, led to some watercourses being seen as a possible source of infection. Open channels that were naturally either steep sided, deep or exhibited fast flowing flood waters also raised fears over public safety. Finally, open channel watercourses often had reasonably wide meanders and riparian vegetation that occupied developable, and hence valuable land. To therefore maximise developable area, increase the flow capacity of the river and address concerns over safety, long sections of these watercourses were straightened, channelised and in many cases eventually culverted to the extent that once significant named streams now run entirely underground. This was common drainage practice in most cities throughout the world at the time, and Sweeney (p.114, 1991) estimates that some 139 km of Dublin’s original open channel watercourses are hidden or forgotten. Nearly all the large city rivers that remain in open channel have been modified in some way, with the inclusion of walls, bridges, weirs, culverts and screens. Many of the newer structures have been sized to accept large flood flows up to the 100 year ARI event, although their actual performance has yet to be quantitatively assessed. Rivers such as the Tolka, Dodder and Liffey have had substantial sections of their length reserved over the years as linear riparian parks available for community use and to preserve floodplain functions. Many of the smaller open channel watercourses remaining though are often cordoned off for safety reasons and for the most part are not integrated as a natural feature of a developed site or of the urban landscape generally. The original mill races on the larger rivers around the city also acted as stormwater diversions but their exact operation is difficult to determine because of complex take-off arrangements and the unknown number of stormwater connections made to them. Nearly all the mill races today have however been filled in and are non-operational.


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4.3 History of Flooding

4.3.1 Network Flooding The Greater Dublin area is fortunate in that it generally does not suffer from frequent or large scale flooding of the pipe network when compared to some other cities within Ireland and overseas. This is due to several reasons: • Dublin City was not established on marshy or extremely low-lying ground that was subject to

periodic inundation;

• The Greater Dublin area has the lowest annual average rainfall within Ireland;

• There are adequate slopes within the majority of stormwater catchments draining to the main rivers and Dublin Bay, which act to minimise pipe sedimentation and blockages and allow stormwater to drain relatively quickly; and

• The stormwater pipe network is reasonably well designed and constructed.

Naturally there are exceptions, most notably where development has occurred in flat, low lying, flood prone areas or on reclaimed land. For example, the stormwater network in Baldoyle is very low-lying and tidally influenced, and hence suffers from frequent stormwater flooding. The towns of Kildare and Ashbourne are relatively flat and have significant problems caused by an undersized trunk drainage network. The combined network in both Dublin City Centre and the reclaimed Dublin Docklands is also very low-lying and often becomes tide locked during high tides in Dublin Harbour. Recent tidal flooding in Clontarf, East Wall, North Wall, Ringsend and Sandymount has indeed highlighted the vulnerability of these areas to extreme tidal action. Local Authorities have also reported some localised network flooding usually caused by blockages from gross pollutants, structural defects and inadequate hydraulic capacity caused by increasing runoff from upstream developments. An instructive example is the township of Bray where all three types of problems are encountered, although solutions applied here are more likely to be small scale and specifically targeted. There are actually very few cases of properties within Greater Dublin being affected by flooding from the pipe network.

4.3.2 Main River Flooding Significant flooding has occurred on several of Dublin’s main rivers, such as the Tolka, Dodder and Camac Rivers, particularly in the flatter, low-lying areas of the inner City. Parts of some towns, such as Dunboyne, Clonee, Ratoath and Ashbourne that are located directly adjacent to a river are also subject to flooding. The impacts of flooding are compounded due to the historical construction of: • developments within the natural floodplain;

• weirs for water impoundment, diversions and mill races; and

• undersized bridges and culverts.

Conversely, many rivers have been channelised and straightened to maximise developable area, which reduces roughness factors, contains flood flows and ultimately helps to reduce flood impacts at that location. The River Liffey for example, although the largest to enter Dublin City, is a controlled river which has been dammed, channelised and walled, thereby mitigating all but the most extreme fluvial floods. Another example is the River Tolka, where Figure 22 below shows the works proposed under the 1955 Improvement Scheme. The maintenance of linear parks and the exclusion of development from these parks also assist in curtailing flood damages if rivers overtop their banks.


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Figure 22: Plan from 1955 River Tolka Improvement Scheme

(Source: DC Drainage Division, undated) Some of the more important flood alleviation schemes that have been proposed and/or undertaken in response to extreme river flooding include the: • 1955 River Tolka Improvement Scheme;

• 1961 Broadmeadow and Ward Drainage Scheme;

• 1965 and 1987 River Dargle Flood Protection Scheme; and

• 1995 and 1997 River Camac Improvement Schemes Phase 1 and Phase 2.

Smaller schemes would involve the upgrading of undersized culverts or the installation of flap gates into stormwater outfalls to prevent floodwaters or high tides from backing up the pipe network. A variety of methods were employed to assess flood behaviour pre- and post-scheme, encompassing unit hydrographs, UK Flood Studies Report (FSR) procedures, and various propriety hydraulic modelling packages. Recent flood relief schemes have also recognised the increase in stormwater runoff from urban development in the upper catchment as a significant contributor to downstream flooding, but up until the last 3-4 years few recommendations were made to limit runoff rates from new developments. The Tolka, Carrickmines, Deansgrange, Poddle, Camac, Finglas, Santry and Mayne Rivers are now all being numerically modelled and their floodplains defined under the GDSDS. Studies have also been undertaken of the River Dodder but as yet no detailed hydraulic modelling has been proposed.


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4.4 Existing Drainage Policies

4.4.1 Development Plan Policies and Objectives Local Authorities within the Greater Dublin area have all made a commitment to the principles of sustainable development, which will ultimately encompass stormwater drainage. However, the Local Authorities have varying approaches to stormwater management, reflecting both their differing priorities and unique local conditions. From a review of each Development Plan, which establishes Council’s medium-term policies and objectives for their administrative area, it can be seen that the focus on stormwater management generally increases with the degree of urbanisation i.e. moving from the rural areas of Greater Dublin to Dublin City. Councils like Meath and Kildare have not included specific policies on stormwater management in their development plan primarily because there is no dedicated department responsible for managing the stormwater network. Moreover, the main priorities for these Councils have traditionally been focussed on supplying and maintaining basic infrastructure services such as water supply and foul sewerage to a fast growing population. Both network and fluvial flooding was traditionally not seen as a significant issue, however recent flooding of the River Tolka and the trunk drainage networks in Ashbourne and Kildare has focussed attention on the need for a guiding policy and improved management. Councils with larger urban areas share the same priorities on service provision but simultaneously recognise the deleterious impacts of past drainage practices on water quality and flooding. With concerns about the potential impacts of developing large Greenfield sites at the edge of existing urban areas, the Councils of Dublin City, South Dublin and Dun Laoghaire Rathdown have codified specific stormwater policies within their respective Development Plans. These state that some type of management policy or strategy should be prepared and implemented for the management of watercourses. Dublin City Council and Fingal County Council further recognise the need to reduce fluvial flood impacts.

4.4.2 Specific Drainage Policies and Codes of Practice Dublin City Council (1998 & 1999) and Dun Laoghaire Rathdown County Council have published documents on stormwater management for use by urban planners, drainage engineers and developers. These deal specifically with the application of Best Management Practices (BMP) to control and treat stormwater runoff, and draw heavily from the UK experience, with particular reference to CIRIA, SEPA and EA publications. Dublin City Council - Stormwater Management Policy Dublin City Council has produced two policy documents that together: • Explain the current situation and introduce the concept of BMP;

• Recommend methods for calculating permissible stormwater outflows from new developments;

• Examine the issues of floodplain management and outline policy statements for the planning and development of floodplains; and

• Discuss the characteristics and concept designs of various BMP structures.

The policy is applied in practice by Dublin City Council’s Drainage Division as follows: • All new developments that have > 1,500m2 impervious area or discharge to overloaded sewers

must have attenuation measures;

• Developer must prove that development is not at risk of, or will not cause additional flooding;

• Maximum permissible outflow from site of 3.1 L/s/ha for a 0.5 ha site and 2 L/s/ha in general;

• Attenuate the 30 year ARI event e.g. using ponds, oversized pipes, etc; and

• Contain the 100 year ARI event on site e.g. by carpark flooding, bunding, etc.


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The technical guidelines were written by consultants in 1998 when SuDS, and especially the water quality component, was still in its infancy in the UK. As such, the measures considered relate more towards source control via hard structures rather than water quality functions. This may be understandable considering that there is very little land available in Dublin City for wetlands and other water quality control structures. This emphasis is also reflected in the fact that most designs presently submitted to DCC do not incorporate water quality functions. In assessing designs, DCC are more accepting of ponds and oversized pipes rather than swales, infiltration devices and underground tanks because of issues with taking in charge and subsequent maintenance. The long term efficiency of SuDS structures, the potential extra funding and resourcing required to maintain them, and health & safety aspects are all important concerns held by DCC that need to be addressed. The policy does represent an important first step in promoting an awareness of BMP. The policy documents are made readily available to developers and consultants, who mostly understand the need for BMP and submit appropriate designs, despite a large difference in quality. DCC are now integrating sections on permissible outflow and floodplain management into an updated drainage code of practice. Application of DCCs Stormwater Policy in other Councils South Dublin County, Fingal County, Kildare County and Bray Town Councils all accept the principles of BMP/SuDS and are all using the DCC Stormwater Policy in various ways, albeit the policy has not been formally adopted nor have there been many sites on which the policy has so far been applied. These councils believe that SuDS must be flexible and site specific, but are most effective if applied to large Greenfield sites, rather than small individual or infill sites. Indeed the current requirement for high housing densities in Greater Dublin and concerns over public safety have combined to produce designs focussed primarily on ponds or underground structures. Again, small SuDS schemes and infiltration devices are not yet seen as practical because of the difficulty in maintaining them either by Council or the local landowner. The floodplain management policy is being applied by Councils by attempting to limit development in the floodplain and to preserve a riparian corridor. Application of the policy is based on flooding history and local knowledge. The ultimate merit of the floodplain policy will more likely be observed in the upper reaches of rivers (i.e. outside Dublin City) like the Tolka, Camac and Mayne, where there is major development planned. Dun Laoghaire Rathdown County Council - Drainage Planning Policy Dun Laoghaire Rathdown County Council uses an 11 page document that: • Explains the issues associated with current drainage practice and the need for BMP in light of

EU and National Policy; and

• Discusses the characteristics and proposes concept designs (with simple calculations) for various types of source control.

The policy places comparatively more emphasis both on water quality and BMP in relation to overall sustainable development principles. It suggests that its recommendations be adopted and implemented as interim measures pending any adoption of this Environmental Policy recommended under the GDSDS. DLRCC are therefore well versed in the principles of SuDS but, like most Councils, are very new to their application. Very few developers have submitted SuDS designs as yet, and it appears that landtake and maintenance will again be the dominating issues. Attenuation ponds are favoured over infiltration devices, especially if those devices are on private land where DLRCC believe it unlikely that they will be maintained. Resources and responsibilities for maintenance will also need to be defined for Council departments, preferably prior to SuDS being taken in charge.


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DLRCC has also produced a guidance document (DLRCC 1998) that advocates leaving streams in a natural state and incorporating them into public open space. These guidelines represent an evolution from previous standards by suggesting that the need for culverting of streams should be minimised and that erosion protection measures be included on all surface water outfalls to open watercourses.

4.5 Surveys, Drainage Studies and other Projects

Significant steps have been taken in recent years to quantify the difficulties and deficiencies within the drainage system, including: • Infrastructure surveys and digital recording of drainage assets;

• Hydraulic modelling, and verification with short duration flow / rainfall surveys;

• CCTV inspection, sewer cleaning and remediation programmes;

• Environmental monitoring of receiving waters, based on standards established in Water Quality Management Plans, e.g. River Liffey, Dublin Bay, River Tolka (draft);

• Assessment of environmental impacts of urban drainage in The Three Rivers Project (Camac sub-catchment) and the SEM motorway project (Carrickmines – Shanganagh, Slang and Crinken Rivers); and

• Preparation of Drainage Area Plans e.g. the City Centre / Docklands catchment.

These studies have given Councils a greater understanding of the operation of their drainage networks and have often included recommendations for improving network performance. The majority of these studies have concentrated on the foul / combined system, primarily because it is seen as more overloaded than the storm system. Councils in the Greater Dublin area recognise that with continuing economic growth and high demand for serviced land, a comprehensive drainage strategy for the region is required. The GDSDS is therefore examining these past studies and undertaking new ones in order to assess regional drainage issues; primarily flow capacity and water quality. An ultimate goal for all Councils is to establish a system whereby all drainage assets can be sustainably managed so as to not to impede development or create unacceptable environmental impacts.


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4.6 References

Anonymous (2001), Dublin Drainage – An Outline History, Dublin Corporation. Bray UDC (1999), Bray Development Plan 1999. SDCC with Conway, E., Kennedy, T.K., O’Byrne, N., Trevis, D., Carroll, G. & McEvoy, L. (1998), South Dublin County Development Plan 1998 – Written Statement. Dublin City Council (1999), Dublin City Development Plan 1999 (written statement), Dublin Corporation. DC Drainage Division (undated), Draft Report – Cataloguing Drawings in the Drainage Division, Dublin Corporation. DC Drainage Division (1996), Code of Practice for Development Works - Drainage, Dublin Corporation. DC Drainage Division & DSA Consulting Engineers Ltd (1998), Dublin Corporation Stormwater Management Policy – Technical Guidelines, Dublin Corporation. DC Drainage Division (1999), Dublin Corporation Stormwater Management Policy for Developers, Dublin Corporation. DLRCC with Casey, P.F. (1998), Development Works in Residential and Industrial Areas (Guidance Document), Dun Laoghaire – Rathdown County Council. Dun Laoghaire – Rathdown County Council (1998), Dun Laoghaire – Rathdown County Development Plan – Written Statement. ESB International Consultants (2000), Surface Water Attenuation – Derivation of allowable run-off in the South Dublin area. FCC with Soffe, W.M., Walsh, M., Hyde, D. & Simpson, K. (1999), Planning for the New Millennium – Fingal County Council Development Plan 1999 – 2004. KCC Planning Department (1999), Kildare County Development Plan 1999. MCC (2001), Meath County Council – County Development Plan 2001, Volume One – Objectives for the County at Large. M.C. O’Sullivan & Co. Ltd (2001), Sustainable Urban Drainage in Dun Laoghaire-Rathdown County, Drainage Planning Policy – June, 2001. M.C. O’Sullivan & Co. Ltd (2002), Sustainable Urban Stormwater Drainage Policy Guidelines, Report No. 088-006-1-1Rp004A01 for Dun Laoghaire Rathdown County Council. Sweeney, C.L. (1991), The Rivers of Dublin, Dublin Corporation. Sweeney, C.L. (undated), Dublin Main Drainage System, Old and New. WCC Planning Section (1999), Wicklow County Development Plan 1999, Part 1 – Written Statement, Wicklow County Council.

4. existing drainage practice...4. existing drainage practice this chapter examines the traditional...

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