Value for Money Examination 16; Arterial Drainage on the Boyle and Bonnet Rivers Summary

Arterial drainage work carried out on a river involves the artificial widening and deepening of the river channels to enable them to carry away a greater volume of water more quickly. The principal objective of arterial drainage schemes carried out in Ireland by the Office of Public Works (OPW) was to bring about a long term improvement in agricultural incomes in river catchments. The work carried out on schemes was designed to allow landholders to install field drainage which reduces waterlogging of land and enables it to carry more livestock or produce higher crop yields. Schemes also have the effect of reducing both the incidence and duration of flooding.

An extensive programme of arterial drainage of Irish rivers has been carried out since the 1950s. The Boyle and Bonet drainage schemes commenced together in July 1982. It was planned that the schemes would be completed in five and three years, respectively, at a total cost of ?17 million (constant 1982 prices).

An examination of the Boyle and Bonet schemes was carried out to establish

  • the outturn on each of the schemes in terms of producing the specified outputs within time and budgetary targets
  • whether programmes of maintenance for the schemes have been developed and are being implemented
  • the adequacy of planning, specification and management of the schemes
  • the economic and efficient use of resources in carrying out the schemes
  • whether the effectiveness of the schemes has been adequately evaluated.

Scheme Outturn

Both schemes continued over much longer periods than originally envisaged. The Bonet scheme was carried out between July 1982 and January 1992 i.e. more than nine years, compared to the scheduled three years. The Boyle scheme continued until December 1995 i.e. more than 13 years, compared to the scheduled five years.

Progress on the schemes was slower than planned because annual funding for the schemes was much less than had been envisaged at the planning stage. At the time the schemes were hahed, about 85% of the work originally planned had been carried out. The schemes were stopped because it was considered that the cost of the remaining scheduled work was greater than the potential economic benefit. In addition, there were concerns that some of the remaining works on the Bonet would have encroached on environmentally sensitive fishing waters.

Quality of Work

Excavation and construction work on the two schemes was carried out to a high standard by OPW.

Flood Protection

Arterial drainage schemes cannot eliminate the risk of flooding. Most of the schemes carried out by the OPW were intended to ensure that flooding of agricultural land from the main river channels is not likely to occur, on average, more than once in a three year period. (This is called a `three-year return period flood'.) The OPW estimated that this level of flood protection could not be achieved in the Boyle and Bonet catchments at a reasonable cost and opted for lower levels of protection. The target adopted for the Boyle scheme was a two-year return period flood. A one-year return period flood was adopted as the target for the Bonet.

Flooding problems in both catchments were reduced as a result of carrying out the arterial drainage schemes. While the drained channels of the Boyle system meet the target two-year return period criterion, the drained channels of the Bonet system do not meet its target Qne-year return period.

Field Drainage

Depending on soil type and the nature of the flooding problems, some land may improve directly as a result of carrying out arterial drainage work. In most cases, however, waterlogged land will only improve if field drainage is installed. As a result, the extent to which field drainage is carried out by landholders is critical in determining the effectiveness of arterial drainage schemes.

Projections of increased farm incomes as a result of carrying out the Boyle and Bonet schemes were based on the assumption that, soon after the arterial drainage work was completed, around 88% of the target damaged land would be improved to the extent that it could carry extra livestock. Based on a sample survey of the target damaged land, it is estimated that less than 25% of the target damaged land shows evidence of improvement. This suggests that there was a very low incidence of installation of field drainage.


Without adequate maintenance, there is a considerable risk that drained channels may revert to their pre-drainage condition. The OPW is required by law to maintain the schemes it carries out in proper repair and effective condition and spent an estimated ?7.9 million on the maintenance of arterial drainage schemes in 1996.

A decision was taken during the Boyle and Bonet schemes to postpone routine maintenance work during construction. Given the long delays on the schemes, this has meant that channels drained early on were not maintained for many years and reverted some way towards their original state.

The four-year maintenance programme adopted for the Bonet scheme in 1994 should be adequate to restore the channels to their post-scheme condition. A five-year programme for maintenance of the Boyle started in 1996 but, given the progress to date, it appears that work on the main channels will need to be speeded up to ensure that the planned maintenance work can be completed within the target period.

Expenditure on Schemes

Budgets for the Boyle and Bonet schemes prepared in March 1982 envisaged total expenditure of ?14.5 million (in constant 1982 prices) on the work which was actually carried out. Actual expenditure up to December 1995 was ?31.6 million (in current prices).

About 30% of the expenditure overrun is related to inflation. The remainder relates mainly to cost overruns on overheads and, to a lesser extent, on overruns on construction of structures, spoil rehabilitation and compensation payments.


The OPW was clearly aware that inflation would result in actual expenditure exceeding the budgeted amount but did not project what the impact might be. Had work proceeded according to the original schedule, actual expenditure would have been an estimated ?1.6 million higher than budgeted due to the inflation factor. Because work was extended over a much longer time period, considerable further inflation (estimated at ?3.5 million) was also incurred.


The OPW planned the Boyle and Bonet schemes on the basis of rates of work progress which would optimise the use of overheads e.g. transport, machinery workshops, stores and supervisory and support staff. Overheads were budgeted to be 54% of the cost of planned direct works but turned out to be 92% of the actual cost. Extra spending on overheads accounted for more than half the entire cost overrun on the schemes mainly because the time required to carry out the schemes was extended so much.

Before the Boyle and Bonet schemes started, the OPW pointed out to the Department of Finance that the Department's policy of restricting funds each year for on-going drainage schemes was impeding work progress and that this had resulted in extensions of the work schedules, thereby leading to extra overheads and higher final costs for the schemes.

Construction of Structures

Structural work on the schemes mainly related to underpinning or replacement of bridges. Actual expenditure on structures was considerably higher than budgeted mainly because the estimate of expenditure was based on the OPW's experience of the scheme carried out earlier on the Boyne river, where conditions were different, and because standards of construction higher than those planned were found to be necessary in many cases.

Cost per Hectare of Target Land

The cost per hectare of agricultural land targeted for improvement on the Boyle scheme was not significantly different from the cost on other major schemes carried out in the 1970s and 1980s. The cost per hectare on the Bonet scheme was much higher than the cost of other schemes carried out, with the exception of the Monaghan Blackwater where special circumstances applied. It must, however, be borne in mind that the level of flood protection provided is lower for the Boyle and Bonet schemes than for all other schemes carried out.

Project Management and Planning

The OPW's project management and planning procedures were appropriate for the scale and complexity of the schemes although some shortcomings were evident in budget preparation and monitoring of overall efficiency levels.

Evaluation of Effectiveness

The OPW had a comprehensive methodology for evaluating the effectiveness of arterial drainage schemes. This was used in appraising the Boyle and Bonet schemes before they commenced. The appraisal indicated that the benefits likely to be derived from carrying out the Bonet scheme were, at best, likely to be only marginally greater than the costs.

The methodology used for appraising the schemes could have been used to reassess the argument for continuing with the schemes, particularly when it became clear that costs were likely to be higher than budgeted and that agricultural policies were moving away from schemes designed to increase agricultural produaion. However, no re-appraisals of the Boyle or Bonet schemes were carried out during the course of their construction. Despite the significant cost overruns which occurred and the low incidence of installation of field drainage, the OPW has not carried out post-scheme evaluations of the effectiveness of the Boyle and Bonet schemes.

The OPW does not propose to carry out any further arterial drainage schemes in the foreseeable future.