construction and refurbishment of earthen irrigation channel banks

(ii) EM SurveyElectro-magnetic (EM) survey to prioritise sites for installation of piezometers(iii) PiezometersPiezometers to identify if there is a direct hydraulic connection between thewatertable and the channel water.(iv) Pondage TestPondage test to measure actual seepage from a channel(v) Idaho Seepage meterSeepage meter to make point measurements of infiltration rates along the channeland on the bed and banks of the channel.A number of channel seepage studies in Australia have estimated overall seepagelosses by relating a constant seepage rate to each soil type underlying the channels.This may be an appropriate methodology in some situations, but research hasshown that seepage varies according to a range of factors and is not constant duringthe irrigation season. As the assigned seepage rates do not consider factors otherthan soil permeability influence seepage, results of this type of methodology, whilestill useful, need to be applied with some degree of caution.Channel seepage measurements on the same soil type, can show a range of results,and using upper and lower seepage rates for each soil type rather than a singlefigure, may be a more realistic approach, which reflects the level of confidence inthe results.Taking into consideration other factors which influence seepage, such as depth tothe watertable and channel size would further increase confidence in the results.It is important that when methodologies for channel seepage estimation are used:• they are applied correctly having in mind the assumptions associated with themethod• two or more methods are used to obtain reasonable confirmation• adequate description and reporting is included• upper and lower bounds or confidence limits based upon realistic estimates ofthe errors are provided.21.7.2 Measurement techniquesSeepage losses cannot be predicted accurately without extensive on-siteinvestigations. Seepage rates from existing channels can be determined by directmeasurement or estimated by indirect measurement.It is difficult to make any general assumptions regarding seepage from unlined orlined channels or from channels constructed in various types of soil. The mostunambiguous way of determining seepage from a channel is to make direct seepagemeasurements.Construction and Refurbishment of Earthen Channel Banks August 2002 – Edition 1.0 21-24