The mathematics of filling and draining a permeable geotextile bag were developed in a previous investigation of how large geotextile containers can be used to dewater dredged material and mine tailings. The resulting outflow from the draining bag looks remarkably like an overland flow hydrograph. An analog model for overland flow is developed from the draining bag and compared with experimental runoff data obtained in the laboratory for a number of rolled erosion control blankets. The analog model is defined by a dimensionless parameter that includes a permittivity term that characterizes the rate of runoff. A comparison of the analog model with measured runoff is excellent across the entire range of natural and geosynthetic erosion control products investigated.