Gravel is an example of a rigid-porous granular material. It is available with several mean stone sizes and hence with a range of flow resistivity. The flow resistivity of gravel varies significantly with flow velocity. Data for low- and high-amplitude impedance are presented and compared with predictions of linear and nonlinear theories based on the Johnson-Allard model for rigid-porous media. Comparisons are made also between data and predictions for shock wave reflection and transmission at single- and multiple-layer gravel surfaces including recent data obtained with laser-generated acoustic shocks. Tolerable agreement is obtained between data and predictions. It is found that a low flow resistivity gravel layer has a reflection coefficient that has a minimum as the incident pressure is increased. A layered system that offers the lowest reflection coefficient at linear sound pressures does not continue to do so as the incident pressure is increased. [Work supported by USACE ERDC BT25 program.]