construction and refurbishment of earthen irrigation channel banks

21.8.6 Surface interception drainage21.8.6.1 Surface drains at toe of bankOpen surface drains along a channel will not generally intercept seepage as theywill be too high above the free surface. To make open drains deep enough to beeffective, the high construction cost, extensive land requirements and highmaintenance costs make them impractical. (Refer Figure 21-6). However theycan be useful to intercept leakage from deteriorated channels banks.Drains require effective disposal which may be achieved by pumping back intothe channel.21.8.6.2 Regional Drainage SystemsMost irrigation areas in Victoria and New South Wales are in very flat areas. Forinstance the natural grade of land in Central Goulburn area of Goulburn-MurrayWater is 1 in 10,000. The natural drainage lines tend to be depressions that fill inwet years rather than perennial streams. Construction of regional drains alongthese depressions has intercepted seepage due to the high watertables (2 metres orless from the surface) experienced over much of the irrigation areas.21.8.7 Interception DrainageThis seepage control method can be more accurately described as a countermeasureagainst seepage water reaching the watertable. Essentially, the concept aims toretrieve the water that has already leaked out of the channel and to return it to thechannel.The concept consists of a horizontal perforated pipe set at 2 to 3 metre depthrunning along each side of the channel and between 5 to 10 metres away from thechannel, with collection sumps and pumps, to pump the water back into thechannel.This concept is only effective where the watertable is already shallow. Wherewatertables are deeper than the intercepting pipeline, seepage water tends tomigrate vertically downwards from the channel and will not be picked up by thepipe. Refer to Figure 21-4. Also the soil in the vicinity needs to be sufficientlypermeable to enable seepage water to be efficiently drawn to the pipeline.There are on-going costs associated with the pumping and pipeline maintenance(root clearing) and the recurrent costs can be relatively high.21.8.8 Groundwater interceptionAn alternative to preventing channel seepage is the construction of groundwaterinterception schemes involving groundwater pumping for shallow bores or tiledrainage located adjacent to the channel. Groundwater pumping may be the mostsuitable option where a pumpable aquifer is present. Where hydraulicconductivities are lower, tile drainage may be a more practical and cost-effectiveoption.Construction and Refurbishment of Earthen Channel Banks August 2002 – Edition 1.0 21-38