construction and refurbishment of earthen irrigation channel banks

In the case where the groundwater is high, it may even cause a gain to the channel.21.3.4 Water salinityThe higher the salinity, the higher the seepage.The presence of electrolytes (ie salt) can arrest the tendency of clay particles todisperse and promote aggregation of soil particles which in turn increases theporosity of the soil medium and hydraulic conductivity. This is reaction isdependent on the soil type.When the electrolyte concentration and composition of channel water exceeds 500-800 EC, flocculation of clay particles will occur, leading to increased hydraulicconductivity of clay material lining the channel.Saline water is not generally used in irrigation channels but drainage orgroundwater may be discharged into irrigation channels. It is important that suchwater is diluted with fresh water to minimise increased seepage.21.3.5 Variation over YearSeepage rates from earthen channels at a single point can vary over a year and it isunusual to measure a consistent seepage value for a given reach of channel.In seasonally used channels, the seepage rate will be higher at the beginning of theseason when water is first turned into a dry channel, and will decrease as the banksand subgrade approach saturation. Cracks in the surface of the channel swell andseal and more steady state conditions are established.This behaviour is illustrated in Goulburn-Murray Water tests on aged silt-sealedchannels through fine sandy loam where the initial seepage rates were 0.005 to 0.02m 3 /m 2 /day for the first 5 to 10 days after filling, and then an average seepage rate of0.002 m 3 /m 2 /day for the balance of the season.Seepage losses during a year can also be influenced by variations in turbidity, watertemperature, weed growth, microbiological activity, hydraulic head andevaporation. As evaporation increases during the summer, watertables can belowered and the increased hydraulic head leads to higher rates of seepage.Investigations in northern Victoria found that the seasonal variation in channelseepage was mainly due to fluctuations in the shallow ground water levelsunderlying the channel, changing the available head driving seepage.In a channel on a fine sandy loam soil, measured seepage rates varied between0.014 m 3 /m 2 /day and 0.034 m 3 /m 2 /day over a three year period. At another site onloam soil, the measured seepage losses varied from 0.005 m 3 /m 2 /day to 0.009m 3 /m 2 /day.Measurement of seepage provides only a snapshot at a given point in time for agiven section of channel, and the extrapolation of measurements over an entirechannel system should be treated with caution.Construction and Refurbishment of Earthen Channel Banks August 2002 – Edition 1.0 21-9