Effect of flood asynchronicity on the adjustments of longitudinal profiles for tributaries of the Moulouya river, Morocco
This paper looks at the effect of dam impoundments on the flood synchronicity of the Moulouya River (Morocco) and its consequences on the morphological adjustments of its ephemeral tributaries. The comparative analysis of monthly discharges measured upstream and downstream of the dams shows a diminution of 52% of mean discharges and a diminution of 33% of annual maximum discharges. The frequencies of discharges reaching 50, 75 and 100 m3•s-1 values are respectively reduced by 55, 47 and 22%. Monthly variations are levelled and this leads to a homogeneous discharge regime (coefficient of immoderation for median values decreases from 8.99 to 1.23). Flood coincidence in time is assessed and shows that downstream discharges are independent of upstream discharges. The description of longitudinal profiles of tributaries downstream of the dams reveals morphological singularities such as convex profiles, perched accumulation forms on the alluvial plain and knickpoints. Three models of morphological adjustments (convex, mixed and by flooding) based on the synchronicity of floods between the main channel and its tributaries are proposed to explain these morphological observations. The convex adjustment model occurs when floods are synchronous and explains the observation of convex profiles. The mixed adjustment model occurs when tributaries are flooded and the main channel remains to its base level and explains morphological forms such as scouring of tributaries or perched accumulation forms on the alluvial plain. Finally, the model of adjustment by flooding occurs when the main channel is activated and the tributaries remain inactive and explains knickpoints and the lack of integration observed at the mouths of the tributaries. Asynchronous flooding induced by dam impoundments is proposed as the main factor affecting tributary adjustments in semi-arid environments.