An overview of hydro-sedimentological characteristics of intermittent rivers in Kabul region of Kabul river basin

摘要

Intermittent rivers are classified as rivers with a high seasonal variability of flow. Such rivers may partially dry up during the year (usually from weeks to a few months), resulting in water as well as habitat discontinuity. However, due to the strong correlation to the hydrological characteristics of the catchment this occurs on a more or less predictable basis. The majority of Afghanistan's basins are drained by intermittent river systems and are the main water sources for irrigation and energy production. The river discharges and catchment precipitation comparison indicate that the high flow conditions originate from snowmelt and seasonal rain events, which result in slightly delayed discharge peaks in the rivers. The delayed discharge is considered very important for water management and especially for irrigation because the demand for water use increases when the temperature rises. The objective of this study is to give an overview on the intermittent rivers of Afghanistan and to out-line the major hydrosedimentologic characteristics of these special river systems. Intermittent rivers undergo a strong seepage loss, where the downward component of flow velocity carries fine sediment in-to the coarse gravel bed and causes external as well as internal colmation of the river bed. This phenomenon can be seen from the grain size analysis of the armour layer of the riverbed, which contains a considerable amount of fine sediments. As a result of river bed colmation, the groundwater recharge from the surface water is reduced until deposited sediments are remobilized by higher discharge events. In this study, the seepage loss through the river bed is analyzed by comparing the measured groundwater levels in wells located in the vicinity of rivers, together with the measured river discharge rates. Field observations of the river bed, surface and sub-surface's bed material through grain size analysis and suspended sediment data are used to analyze external colmation effects. This is shown on the example of Maidan River, a tributary of Kabul River in figure 1.