Agricultural Drainage Practices in Ireland

Agricultural Drainage Practices in Ireland "1". D. Ryan ments by developing main channels, and field drainage of An Foras Taluntais smaller parcels of land using pipes and open trenches. The Creagh, Ballinrobe, Co. Mayo use of cost/benefit analysis on the arterial drainage program Ireland is considered and the inherent errors are discussed, Conser- vation of the environment is described as it applies to land- scaping, fisheries, and wildlife, and the drainage authorities ABSTRACT/Agricultural drainage practices are reviewed are shown to have an enlightened attitude to proper preser- under two main headings: arterial drainage of river catch- vation of the world around us. Introduction vened and much of the work done then and later was Ireland is basically saucer-shaped, with a rim of relief work. The Drainage Act of 1863 transferred re- mountains and a flat central lowland. Rivers rising in- sponsibility for design and execution of schemes to the land must circumvent the mountains to reach the sea. owners of affected lands, the t\mction of the OPW Most of the rivers in Ireland do not flow directly to being to examine the merits of proposals, to consider the sea. For example the Shannon rises near Lough objections to schemes, and to advance the money re- Allen, 30 km from the coast but flows south and west quired. Further drainage acts were passed in 1925, 240 km to reach its tidal estuary. Likewise, the river 1926, and 1927. During all this time, 229 schemes Slaney rises west of Wicklow Mountains 30 km from were completed benefiting 198,000 ha, Most of these the sea, but travels 90 km to its' outfall at Wexford. schemes were heavily subsidized by the state although Such conditions give rise to poor gradients and poor in the period 1863-1925 the state contributed less channel development (Lynn, 1980). Flooding and high than 7% of the cost of arterial drainage. water-table levels result in consequent hardship and fi- In 1938 a drainage commission was set up which nancial loss. According to Gardiner and Radford recommended two fundamental and far-reaching (1980) wet mineral lowland soils cover 21% of the changes as fi~llows: First, the entire river catchment country or 27% of the agricuhural land. This land is should be used as the basis for the design and execu- inhibited by surplus water and requires drainage in tion of schemes; second, the initiative for undertaking one form or another to achieve its full potential. schemes should be vested in a central drainage au- Two organizations in Ireland are principally re- thority, the OPW, which would meet the full cost of sponsible for promoting improvements to drainage design and execution of schemes. The 1945 Arterial conditions. The Office of Public Works carries out im- Drainage Act gave effect to the reconnnendations and provements to arterial drainage systems (rivers and also conferred on t:he OPW responsibility for maintenance of completed schemes, with the cost of mainte- large collection drains), while the Farm Development nance to be borne by tim county councils. The provi- Service, a branch of the Department of Agricuhure, sions of the act meant the end of piecemeal drainage, administers field drainage operations. which often led to the transfer of a flooding problem fl-om one area to another, and protected the capital Arterial Drainage investment in drainage by ensuring that maintenance would be carried Otlt on a regular basis and to ade- History quate standards. Maintenance work is essential if The history of arterial drainage is described by tile channels are not to revert quickly to their original con- Ottice of Public Works (OPW) in their account of the ditkms. Under the 1945 act, 9 major catchments and cost-benefit analysis for the River Maigue Drainage 25 small catchments have been drained. Schemes cur- Scheme. The OPW first became associated with arte- rently in progress include the Boyne, Maigue, Corrib- rial drainage with the Drainage Act of 1842. It was Mask, Br and Boner (Fig. 1). made responsible for the design and execution of Under the early acts completed schemes were con- drainage works. It was the intention of this first act stituted as drainage districts, with responsibility tin that the cost of the works would be borne by the land- maintenance being vested in drainage trustees or holders, but the famine years 1846 and 1847 inter- boards. The cost of maintenance was to be levied on Environ Geoi Water Sci Vol. 9, No. 1,31-40 (c) 1986 Springer-Veria9New York Inc. 32 T.D. Ryan million af 12 I0 H Su r', e) plus ('onstruct ion Figure 1. Expenditure on arterial drainage has fluctuated substantially 6 over the last 30 years when ,neasured in real terms. Following government 4 cutbacks in the late 1950s, the eco- nomic boom of the 1960s caused expenditure to double until the Depart- ment of Finance intervened in 1964. Membership of the EEC made further funds available and activity increased 0 , , , ~ ~ I i I ; i 1 I J I I I J I I I I 1950 1955 1960 1911~ 1970 1975 198(I once more from 1974 onwards. the benefiting owners. This method of organization was for the most part a failure. Under the 1924 act, the OPW was empowered to carry out maintenance works on its own initiative. State grants of up to 50% of the cost were made available, the rest of the cost to be borne by the benetiting owners. The act was inef- tective, however, because many of the drainage districts had fallen into such a state of disrepair that com- plete redrainage rather than maintenance was re- quired. In subsequent drainage acts responsibility for financing maintenace work was transferred progres- sively to the county councils. Design and Construction During famine times, arterial drainage work was done by hand and it employed about 40,000 people at peak (Bruton and Convery, 1982). Now the process has become highly merchanized using dragline exca- vators and floating dredgers fi)r excavation, and spe- cialized equipment fi)r drilling and blasting rock (Fig. 2). Less than 1,000 people are currently employed on the schemes. Ahnost all of the arterial work has consisted of deepening and widening river channels to accommo- date existing river flows (Fig. 3). Other forms of im- provement, such as channel diversions and provision of storage reservoirs have been rarely used. The preliminary stage to design of ()pen channels Figure 2. Excavator. constituting a scheme is to survey the levels and the discharges in the catchment. Bed levels of the channels structures, sucb as flumes, weirs, and orifices, or and the levels of the benefiting lands are determined without fixed structures and using a calibrated section to provide a basis for the design levels of the tinished of streambed. For the latter system, a calibration curve scheme (Figs. 4, 5, 6). relating water level to flowrate must be built up by Flow measurement may be performed by tixed measuring flowrate for different water levels in the Agricultural Drainage Practices in Ireland 33 Figure 3. Design for flashfloods. A larger cross-section allows a greater increase in flow tot a given increase in bead. Figure 4. River before drainage. Figure 5. River after drainage, showing stripping of vegetation and canalization of the stream. Figure 6. Where the channel tmderneath a bridge or other structure is deepened, the slructtn'e is sometimes strengthened by underpinning to m,ercome damage to the fouudation. In this view, new concrete is visible as the base of two old sluice gates where the river Robe has been deepened by about 1.0 m. river. There are several methods for measuring water full development of agricultural potential of the im- flow in this situation. The OPW use a currentmeter proved lands and reduce overall flood damage to rela- placed at certain points in the water, e.g. at 0.2 and 0.8 tively small proportions (OPW, 1984). of the depth below the surface. Flow is measured like Bed levels in streams and rivers are determined by this at several points across a section. The w~lues are taking into account both dry weather flow and peak used to calculate a weighted average which when mul- discharge (3-year flood), and must permit drawdown tiplied by the cross-sectional area of the river yields the of the water table to at least 0.75 m below the surface flow through that section. This process is repeated on in benetiting lands (Howard, 1980). The procedure several different occasions with several different flow starts with the smallest drain and levels, and cross-sec- rates until a full staging curve can be drawn. Water tions are computed downstream to suit soil conditions level at this point is measured regularly, enabling a at as many places as is economically viable, Bed levels good record of flows to be built up. The OPW have allow for a gradient of 1:300 in incoming field drains 200 such gauging stations around the country, over at peak discharge in the stream. Channel cross-sections half of which have records extending over 30 years. for a given flow and gradient are generally obtained "/'he intbrmation on flow tlms gathered is used to pre- from the formula of Manning: dict the magnitude and fl'equency of floods. The information on water flows is used to deter- mine the 3-year flood, and this is normally used as the where Q = Discharge rate design criterion. This standard is considered capable R = Hydraulic radius of reducing the incidence of flooding during the 1 = Hydraulic gradient cropping period considerably (probably up to once in A = Wet cross section area 15 years) and would make possible the more or less Km = Roughness coefticient 34 T.D.

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