Palaeoflood records to assist in design of civil infrastructure in ephemeral rivers with scarce hydrological data: Ugab River, Namibia

Study region: A flood study was performed in the ephemeral Ugab River in the arid north-western part of Namibia. The headwaters of the river lie on the higher Namibian plateau, where most floods are generated and propagate west, through the Namib desert, to the Atlantic Ocean. Study focus: Estimation of a 1 % Annual Exceedance Probability (AEP) flood is required for upgrading of road infrastructure. In compliance with revised road standards, the existing Ugab River bridge needs to accommodate the 1 % flood or be replaced at a high cost. From hydraulic modelling it was determined that the bridge can accommodate a similar to 2000 m(3)s(-1) flood. The Regional Maximum Flood model estimates the 1 % flood at similar to 3600 m(3)s(-1) while a Flood Frequency Analysis (FFA), based on short term systematic data, predicts the flood at -550 m(3)s(-1). This inconsistency needed to be resolved. Incorporation of palaeoflood discharges (14 floods over the last 1260 years) into the FFA produced a 1 % flood of similar to 820 m(3)s(-1), indicating that the existing bridge has sufficient flood capacity. New hydrological insights for the region: This study shows that palaeoflood hydrology improves conventional hydrological methods in dryland regions, providing long-term registers for reliable estimation of FFA. This is especially applicable to regions with complex hydrology or with scarce gauged discharges, or where data is simply unavailable.