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Western Dam Engineering Technical Note May 2017 6 Figure 4a. Conceptual Design of an Inverted Filter (Concentrated Seepage). Figure 4b. Conceptual Design of an Inverted Filter (Disperse Seepage). An alternative approach to consider at a concentrated seep is the use of geotextile as the filter material in place of sand (see Lake Needwood Dam case history below). With this intervention, the geotextile material would be deployed either directly over the area of concentrated seepage or on an initial layer of coarse (preferably not too angular) gravel. Deployment would ideally begin well upslope of the area of active seepage as protection against enlargement of the area of seepage over time. Also, deployment would be easier and safer using gravity where possible to roll out the material. As the geotextile is deployed it should be initially ballasted with sand bags at close enough spacing to counteract any tendency for billowing and lifting due either to the concentrated flow or wind. The placement of the sand bags would follow immediately along with the deployment. If ground conditions and accessibility allow, the sand bags may be deployed manually; otherwise a hydraulic excavator or other equipment with suitable reach may be used. Immediately following or together with the deployment of the geotextile and sand bags, a layer of gravel should be placed over the fabric to more uniformly ballast the geotextile, while not impeding free drainage from the geotextile. One potential concern with the use of geotextile as the filter is that it tends to clog more readily than a properly designed earthen (i.e., sand) filter. If possible, a non-woven geotextile with the greatest apparent opening size (AOS) should be used to enhance permeability of the filter. This clogging factor will have