construction and refurbishment of earthen irrigation channel banks

Since many of these factors are closely related, act simultaneously and some of them tendto counteract each other, so it is difficult or practically impossible to determine therelative contributions of each to seepage. Therefore, the factors thought to be leastimportant to seepage losses, such as soil temperature, velocity of flow and ground slopeat right angles to the flow, generally are not taken into account.The main factors known to have a definite effect on seepage rate can be grouped asfollows:1. Characteristics of the channel surface and subsoil.2. Depth of water in the channel, wetter perimeter of the channel and the depth togroundwater.3. Amount of sediments in the water, velocity in the channel and length of time thechannel has been in operation.Group 1Permeability is the soil property that indicates the relative ease with which water willflow through the soil. Permeability is the ability of a soil to conduct or dischargewater under a hydraulic gradient. This property is mainly dependent on the porosity ofthe soil, shape and arrangement of the soil particles, soil density and degree ofsaturation.Coarse-grained soils are highly pervious and have high permeability co-efficients;fine-grained soils are much less pervious and have low co-efficients.The co-efficient of permeability K, cm per sec, is defined by Darcy’s law:Q = KiAWhere; Q = rate of flow of water through soil mass, cm 3 per seci = hydraulic gradient or total head lost per unit of flow distance, cm per cmA = total cross-sectional area of soil through which flow takes place, cm 2A survey of the soil profile along a proposed channel alignment is the most importantsingle step in a pre-construction seepage investigation. This survey should determinethe location, extent and physical characteristics of the various underlying soil layers.The sequence of permeable and impermeable strata and the capability of these strata totransmit water will largely determine the amount of water lost by seepage.Group 2The following relationships have been found to exist between seepage and water depthin the channel, depth to groundwater and wetter perimeter of channel:1. Seepage losses generally increase with increase of water depth in the channel,although the two factors are not directly proportional. Depth of water has beenfound to have more influence on seepage rate when the rate is high than when therate is low.Construction and Refurbishment of Earthen Channel Banks August 2002 – Edition 1.0 21-5