Formation and evacuation of a large gravel-bar deposited during a major flood in a Mediterranean ephemeral stream, Nahal Me'arot, NW Israel

An unusual 240 mm rainstorm and a consequent major flood (peak discharge 110 m³ s^{- 1}) with an estimated recurrence interval of about 50 years, occurred on Dec. 31, 1998 in Nahal Me'arot, an ungauged 25 km² coastal ephemeral stream in Mt. Carmel, NW Israel. The flood caused morphological modifications along the channel, including intensive stripping of the channel bed and floodplain along several segments and deposition of large gravel bars downstream of these segments. One of such segments served as a study reach; composed of a massive (1270 m³), coarse, 200 m long gravel-bar formed downstream of a segment (150 m long) that was deeply scoured. The study reach was carefully mapped; cross-sections and driftwood lines allowed the calculation of the hydraulic parameters and peak flow using the slope-area method. The bar's sedimentological characteristics, such as surface and substrate texture and stratigraphy, were studied in detail using a survey of 15 pits. The initial recovery of the channel started when dense low shrub vegetation re-stabilized along the affected reaches a year later. Two smaller floods, 3 and 4 years later, evacuated and reworked the sediment until it lost most of its volume. The study suggests that the flood's large impact and disturbance was local, short-lived and the channel almost returned to its pre-flood state within about 4 years.