This paper examines the intentions, successes and failures of remarkable Romano-Libyan floodwater farmers who settled the arid Libyan pre-desert and used complexes of walls to manage the occasional floodwater as well as the desert's soils, sediments and biota. The paper presents and analyses a near ‘total’ data-set on the determinable locational properties of these ancient walls within one wadi — the 10 km long Wadi Umm el-Kharab. The organisation of these wall systems is shown to alter along the wadi according to changes in topographic, geomorphic and hydrological factors, as well as in relation to what are regarded in this paper as design and settlement factors. The organisation of the walls changes at confluences and distinctive topographic features. An analysis of the shape of enclosed areas indicates that the wadi can be divided into three broad sectors, each of which can be identified by differences in the shapes of the areas enclosed by walls. The enclosed shapes, as well as the general properties of the walls themselves do not vary in any simple linear manner along the wadi. Precise field mapping, as well as statistical displays of wall-related data, indicate the presence of similar ‘gaps’ in both the frequencies of settlement and of walls along the wadi. These gaps are, in part, explicable as the secondary consequences of hypothesised past flood-management problems associated with the confluences within the wadi system. They are also interpreted in this paper as some type of land use or other territorial ‘break’ created between the ancient community groups who occupied the wadi. This study has demonstrated the existence of distinct ‘preferences’ in the distances between walls, the areas enclosed between walls, and the thicknesses of walls. Overall, the types of organisation, and presumed functions of walls in this wadi are similar to those established from elsewhere in the Libyan pre-desert, but the balance is different in the Wadi Umm el-Kharab. The emphasis of wall-design in the Wadi Umm el-Kharab appeared to be upon the containment and management of floodwater, rather than the maximisation of the input of waters to the wadi floor, as deduced elsewhere. The long-term robustness and the environmental sophistication of these wall-systems are evident.