Appendices

The appendices collate information on RHS indices developed so far to help with diagnosing issues.  The existing indices are summarised below and more information on their calculations and interpretation can be found in the appendices and online on the RHS website resource section: http://www.riverhabitatsurvey.org/manual/rhs-indices/

RHS indices:

• The Habitat Modification Score (HMS) quantifies the extent, potential impact and persistence of engineering structures on river channels, banks and riparian zones (Walker 2005). It is categorised into five classes (HMC) representing increasing levels of engineering impact, from semi-natural to severely modified. The HMS is split into sub-scores describing different engineering structures: culverts; bridges; outfalls and deflectors; bank and bed reinforcement; bank and bed resectioning; berms and embankments; weirs, dams and sluices; fords; poaching.
• The Habitat Quality Assessment (HQA) scores the occurrence and diversity of natural habitat features, land uses and floodplain features, such as channel substrate types, riffles, pools, woodland and wetland (Raven et al. 1998). 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’. A context analysis consists of comparing a site HQA to the distribution of HQA scores for sites of similar type using a nearest neighbour approach. The 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 taken from one of the baseline surveys (1994-6 and 2007-8). A site is then assessed according to its position within the distribution of HQA scores for the 150 sites. The bottom quintile of the distribution represents very low habitat quality and the top quintile very high quality (all relative). In general, we recommend assessing the scores within the context of the 2007-8 England and Wales baseline survey as they have the highest compatibility in terms of survey format and standards of surveying.
• The River Habitat Quality (RHQ) index combines HQA and HMS classes into one index representing the overall quality and integrity of river habitats (Walker 2005, Walker et al 2002).
• The Channel Substrate Index (CSI), Flow Regime Index (FRI), Channel Vegetation Index (CVI) and Geomorphic Activity Index (GAI) (Appendix 2) are derived using RHS spot-check data on channel substrate, flow-types, channel vegetation structure, erosion and deposition features, riffles and pools (Naura et al. 2016). The indices represent natural hydromorphological dimensions in British rivers.  The four indices were mapped across the entire river network at 500m interval using geostatisical modelling techniques (Naura et al, 2016). Each index was mapped twice to represent current observable (including any impacts) and semi-natural conditions. The maps of observable and semi-natural condition form the basis for the calculation of impact ratios (HIR and HIA).

Map of CSI using geostatistical techniques at every 500m across the 1/50,000 river network on a gradient from bedrock/boulder (blue) to gravel-pebble (green) and silt-sand-clay (brown).

• Hydromorphological Impact Ratios (HIR) are indices describing departure from natural condition and impact for four hydromorphological indices (CSI, FRI, GAI and CVI) .
• The composite Hydromorphological Impact Ratio (HIR) combines individual HIRs for four hydromorphological indices/dimensions (CSI, FRI, GAI and CVI) to assess departure from natural condition and impact. The composite HIR varies from 1 (no impact/semi-natural condition) to 5 (very high impact).
• The Riparian Quality Index (RQI) combines information on bankface and banktop vegetation structure, bank material and modification in an assessment of riparian vegetation complexity, naturalness and continuity).
• The Channel Resectioning Index (CRI) quantifies the number of occurrences of bank and channel modification by reprofiling or dredging along a 500m stretch. Resectioning is the most common modification type and the most prevalent in the UK.