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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 ~2000 m3s–1 flood. The Regional Maximum Flood model estimates the 1 % flood at ~3600 m3s–1 while a Flood Frequency Analysis (FFA), based on short term systematic data, predicts the flood at ~550 m3s–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 ~820 m3s–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.

Ephemeral rivers in dryland regions exhibit a high interannual variability of streamflow regime, mainly dominated by floods. In these environments, floods are a water resource and a potential hazard with important socioeconomic implications. The Fish River (86,600 km2) is the largest ephemeral stream in Namibia and, recently, also the focus of new development plans, including construction of the largest dam in Namibia. The hydrological analysis to support decisions implies large uncertainties owing to spatial-limited and short hydrological records (since 1962). Here we investigate the current and past patterns of extreme floods combining instrumental, historical, and palaeoflood records. Palaeoflood studies were performed at two reaches with preserved sedimentary evidence: at the upper sector (Vogelkranz) upstream of Hardap Dam (~13% catchment area) and in the lower part of the river, in the Fish River Canyon National Park (70% catchment area). In the Hardap reach, the palaeoflood record identified at least eight large floods during the last 350 years, with the largest flood reaching a minimum discharge of 4800 m3 s−1 (150-year return period). In the Fish River Canyon reach, the sedimentary record shows at least 12 large floods over the last 400 years, the largest with an estimated minimum discharge of 8700 m3 s−1. The elevation of alluvial surfaces without flood evidence provided an upper bound for flood stages, associated with discharges of 6400 m3 s−1 and 16,140 m3 s−1 for the upper and mid-lower Fish River respectively. In the upper reach, the flood frequency analysis (FFA) combining systematic and palaeoflood data provided lower discharges (~25%) of the flood quantiles than the ones using only systematic data sets. In the Fish River Canyon reach, incorporation of the palaeoflood data into the FFA results in slightly higher values in the magnitude of the higher flood quantiles (~4–7%). The FFA analysis using an upper limited lognormal distribution function (LN4)

This paper summarises innovative research into the assessment of longterm groundwater recharge from flood events in dryland environments of the Kuiseb (Namibia) and the Buffels (South Africa) rivers. The integrated water resource management (IWRM) policies and institutions affecting the exploitation of groundwater resources in each of these developing countries are compared. The relatively large alluvial aquifer of the Kuiseb River (∼240 Mm3) is recharged from irregular floods originating in the upper catchment. Reported abstraction of 4.6 Mm3 per year is primarily consumed in the town of Walvis Bay, although the groundwater decay (pumping and natural losses along the period 1983–2005) was estimated in 14.8 Mm3 per year. Recharge is variable, occurring in 11 out of 13 years in the middle Kuiseb River, but only in 11 out of 28 years in the middle-lower reaches. In contrast, the Buffels River has relatively minor alluvial aquifers (∼11 Mm3) and recharge sources derive from both lateral subsurface flow and floodwater infiltration, the latter limited to a recharge maximum of 1.3 Mm3 during floods occurring once every four years. Current abstractions to supply the adjacent rural population and a few small-scale, irrigated commercial farms are 0.15 Mm3 yr−1, well within the long-term sustainable yield estimated to be 0.7 Mm3 yr−1. Since independence in 1990, Namibia’s water resource management approach has focussed on ephemeral river basin management of which the Kuiseb BasinManagement Committee (KBMC) is a model. Here, some water points are managed independently by rural communities through committees while the national bulk water supplier provides for Walvis BayMunicipality from the lower aquifers. This provides a sense of local ownership through local participation between government, NGOs and CBOs (community-based organisations) in the planning and implementation of IWRM. Despite the potential for water resource development in the lower Buffels River, the scope for im

Flood water infiltrates ephemeral channels, recharging local and regional aquifers, and it is the main water source in hyperarid regions. Quantitative estimations of these resources are limited by the scarcity of data from such regions. The floods of the Kuiseb River in the Namib Desert have been monitored for 46 years, providing a unique data set of flow hydrographs from one of the world’s hyperarid regions. The study objectives were to: (1) subject the records to quality control; (2) model flood routing and transmission losses; and (3) study the relationships between flood characteristics, river characteristics and recharge into the aquifers. After rigorous quality-testing of the original gauge-station data, a flood-routing model based on kinematic flow with components accounting for channel-bed infiltration was constructed and applied to the data. A simplified module added to this routing model estimates aquifer recharge from the infiltrating flood water. Most of the model parameters were obtained from field surveys and GIS analyses. Two of the model parameters—Manning’s roughness coefficient and the constant infiltration rate—were calibrated based on the high-quality measured flow data set, providing values of 0.025 and 8.5 mm/h, respectively. This infiltration rate is in agreement with that estimated from extensive direct TDR-based moisture measurements in the vadose zone under the Kuiseb River channel, and is low relative to those reported for other sites. The model was later verified with additional flood data and observed groundwater levels in boreholes. Sensitivity analysis showed the important role of large and medium floods in aquifer recharge. To generalize from the studied river to other streams with diverse conditions, we demonstrate that with increasing in infiltration rate, channel length or active channel width, the relative contribution of high-magnitude floods to recharge also increases, whereas medium and small floods contribute less, often not r

A paleoflood study in the Gardon River gorge in southern France identified extreme floods larger than any modern or historically gauged flood. During the course of our study, an extreme flood on the 8–9th of September 2002 claimed the lives of 21 people and caused millions of Euros worth of damage to the towns and villages along the river. The magnitude of this flood was larger than any known historical flood on record,. However, there is evidence of greater magnitude flood events in the form of slack-water flood deposits preserved in caves elevated 17–19 m above the normal base flow, and up to 3 m above the level reached by the 2002 floodwaters. The stratigraphy and radiocarbon dating show that at least five extreme events occurred during the past 500 years. The discharge estimation associated with these flood units, achieved using the HEC-RAS one dimensional model, indicates that at least three floods were bracketed by discharges between 6850 and 7100 m3 s− 1, and at least two floods reached a magnitude above 8000 m3 s− 1. Therefore, the extraordinary flood of 2002 was not the largest in the basin.