THE HYDROLOGIC AND GEOMORPHIC OUTCOMES OF A SMALL DAM-BREAK FLOOD, KFAR YEHOSHUA C RESERVOIR, NW JEZREEL VALLEY, ISRAEL

Kfar Yehoshua C Reservoir in NW Jezreel Valley is a small agricultural reservoir capturing the flood waters of Nahal Bethlehem, a small tributary of Nahal Kishon, designed to enhance the dry summer irrigation season of local crops. In April 2019, human-related overfilling of the reservoir created internal piping in the western dike that slowly developed into a full breach that released a total volume of 150,000 m³. The floodwaters flowed into a 30 m long, newly-formed channel cut into the local alluvial soil. A peak discharge of 17 m³/s was reached 2 hours after the initial piping process, with a week-long recession. Daily observations of the breach and channel development was followed by field survey and sampling: 1. Cross-sections to characterize the topography of the new channel. 2. Hydraulic modelling using HEC-RAS program. 3. Grain size textural surveys in the field and laboratory, and 4. Soil samples were taken for laboratory geotechnical tests.

At the cohesive and consolidated breached dike’s toe, only minimal erosion was documented, whereas downstream, where the floodplain sediments of Nahal Kishon were not cohesive, a 2 m high vertical waterfall marked the main knickpoint of the longitudinal profile separating the reach into 2 distinct segments. The outflow was cone-shaped, narrow at the base of the breached dike (3 m width) and expanding to 53 m at the entrance into the Kishon River. Near the confluence, a small fan formed over an area of 15 m³. Most of the fan was made of newly transported fine sediments derived from the local soils with some cobbles and boulders originating from the breached dike’s riprap. The amount of sediments eroded from the breached dike was estimated at 246 m³, compared to only 4 m³ at the alluvial fan (1.6%), suggesting that most of the sediments entered the main Nahal Kishon and transported downstream.

The HEC-RAS modelling showed that near the main waterfall, the maximal water depth was 3 m, but it was usually around 1 m or less. The peak flow velocity was 2 m/s at the dike’s toe but reached 5.7 m/s at the waterfall. The maximal shear stress ranged 100-200 N/m² at the upper segment and 400-500 N/m² at the downstream segment. The soil strength tests showed marked difference between the upper and lower segments.

The results of the small dam-break flood suggest that unusual point hydrological events have limited impact, in terms of the eroded sediment volumes and the resulting morphology. Furthermore, it is highly probable the newly formed morphology will not be preserved for a long time, due to larger floods in the main Kishon channel.