Like many chalk streams, The River Nar suffers from excessive sediment input which cloaks the riverbed in a homogeneous layer of silt.

At Castle Acre, The Norfolk Rivers Trust commissioned Cain Bio-engineering to restore several kilometres of stream channel using large woody debris (LWD). The idea was to recreate a large natural disturbance similar to the “Great Storm” of 1987 which would have been a key driver of the LWD cycle in rivers prior to deforestation and management practices which sought to remove fallen trees from streams. Behind LWD structures ponded sections with trapped sediment develop, whereas adjacent to the LWD the channel is ‘pinched’, increasing water velocity. The ponded areas allow plants and slow water dwelling organisms (e.g. juvenile lamprey and midge larvae) to colonise, while increasing water velocity in certain areas helps to scour gravel which rheophilic (riffle dwelling) species favour, including brown trout and flat bodied mayfly species.

Objectives:

The aim of the restoration was to increase species populations and biodiversity by reinstating natural processes which act to diversify habitat conditions. The Norfolk Rivers Trust has commissioned ENSIS to undertake an extensive survey in order to better understand ecological response to habitat restoration.

Methods:

Invertebrate samples were collected alongside geomorphological measures from control and restored sites both before and after restoration (i.e. BACI). This monitoring design enables the estimation of natural variation so that the effect of the restoration can be quantified: using data from the pre and post-control site the amount of natural variation in invertebrate biomass or species diversity, for instance, can be estimated; ecological restoration is then assessed by the difference between the pre-restored site compared to the post-restored site.

Results:

The project is still ongoing

Links:

Norfolk Rivers Trust

LWD 1
LWD used to narrow over-widened channel, trapping sediment at the stream edge which is then colonised by plants and slow water dwelling organisms; at mid-stream water increased flow velocity scours gravels favouring riffle dwelling species including brown trout © Norfolk Rivers Trust
Nar and Castle
The R. Nar with Castle Acre in the background. The key to success of LWD is the complex 3D habitat created by smaller twigs that extends from the river bed, throughout the water column, to above water. This “high-rise living” within the debris jam provides considerable feeding opportunities and also predation refuges for many organisms © Norfolk Rivers Trust