Growing demand for sustainable urban drainage systems (SUDS) to limit run-off has opened new areas of research into the application of plastic materials to form underground voids that are used either to attenuate (built in layers of boxes as tanks), or to convey storm water (laid down in rows as panels). This solution replaces the more expensive use of holes filled with gravel and crushed stones. Although a limited number of flow experiments had previously been carried out on thin geocomposite materials, there was no prior research into the panel of boxes used in SUDS. This thesis presents results of a programme of laboratory tests performed to investigate the hydraulic capacities of a panel of boxes for lateral conveyance of stormwater. The results highlighted two design cases, *box' structures where voids in the boxes are unfilled and blocks' structures where the voids are filled with granular material. A physical based mathematical model to simulate the flow through either type of structure is presented in the thesis. The computer model developed from the laboratory studies is a 'compartment model' comprising two main components. In zone 1 calculations are performed for gradual losses in hydraulic conditions that range from open channel flow to the flow through porous media. The modified Manning's equation, or Izbach's exponential equation are both used depending upon the flow type. In zone 2 the orifice equation is used to calculate local losses at the end of each box. A set of three unique calibration parameters is required for each type of material. Close correlation between measured and calculated data confirmed that the theory was the appropriate one to describe the flow through different types of geosynthetic panel of materials with varying number of boxes in height, width and length and ranging from highly voided boxes, to more dense grids filled with different types of aggregates.