Logjams With a Lower Gap: Backwater Rise and Flow Distribution Beneath and Through Logjam Predicted by Two?Box Momentum Balance

Logjams with a gap at the bed form naturally in small channels and are used engineering practice to maintain river connectivity base flow. Limited understanding of jam's effect on backwater rise flow velocity limits assessment geomorphic ecological impacts natural logjams, design restoration interventions, representation flood models. The distribution through beneath jam satisfies two-box, momentum-based model constrained by drag generated jam, momentum loss lower gap, net pressure force. was validated 68 flume experiments. Backwater is predicted from discharge following established models for solid sluice gates. As result, increases resistance, which forces greater jam. Below-jam Shields parameter increased ratio friction coefficient slope decreasing height.