Risk and vulnerability analysis, including an update of the restoration plan for the Hegga river

Sammendrag

NIVA, in collaboration with CBEC, was commissioned by the Hurdalsvassdraget/Vorma River Basin Sub-district to assess restoration options for the lower River Hegga in Nannestad, Hurdal, and Gran municipalities, Norway. The river has Moderate Ecological Status under the EU Water Framework Directive due to limited trout spawning and juvenile habitats caused by channel modifications.The project aimed to (1) develop restoration plans and (2) model hydraulic impacts. Work included data review, LiDAR drone survey, and field surveys. Findings show the river, once a multi-thread system, is now a straightened single channel with embankments, disconnected from floodplains. This has increased transport capacity, caused bed armouring, and eliminated key habitats. The most effective measure is selective embankment breaching along the south bank to reconnect floodplain and side channels. Modelling confirmed that targeted breaches can redistribute flows, reduce shear stress, stabilize spawning substrates, and create low-velocity habitats. Larger boulders (>0.5 m) should be reintroduced to enhance roughness and habitat complexity. While beneficial, these measures will not fully resolve habitat or flood risk issues. The report recommends reconsidering reactivation of the historic southern channel, which could reduce flood risk and restore wet woodland, subject to landowner consent and detailed design.