cRHS Metrics

Habitat Modification Score (HMS) & Habitat Quality Assessment (HQA)

Recording different engineering structures during a cRHS, such as culverts, bridges and weirs, feed into the HMS, helping quantify the occurrence, extent and impact of engineering structures on river channels and banks (Walker, 2005). It is categorised into five classes (Habitat Modification Class, HMC) representing increasing levels of engineering impact, from 'semi-natural' to 'severely modified'.

The HQA scores the occurrence and diversity of natural habitat features and land uses in the cRHS site (Raven et al., 1998). This The score provides an overall site assessment of habitat diversity and conservation value.

To be meaningful, HQA scores need to be interpreted within the context of sites of similar type. HQA scores are classified into five quality bands following a ‘context analysis’. This consists of comparing the HQA of one site, to the distribution of HQA scores for sites of similar type using a nearest neighbour approach.

This method, approved by the Environment Agency, uses a statistical recombination of map-derived attributes representing known drivers of geomorphological change (i.e. specific stream power and shear stress; Jeffers, 1998), to select 150 sites of similar type. A site is then assessed according to its position within the distribution of HQA scores for those 150 sites. The bottom quintile of the distribution represents very low habitat quality and the top quintile very high quality (all relative).

The River Habitat Quality (RHQ) index combines HQA and HMS classes into one index representing the overall quality and integrity of river habitats. RHQ of sites can be compared spatially and temporally to determine varying habitat quality.

Find out more on the River Habitat Survey website