Segment delineation

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Section of river subject to similar valley-scale influences and energy conditions.

Delineation criteria

Delineation is based on major changes in valley gradient, major tributary confluences, and valley confinement.

The boundaries of landscape units form the first delineation of segments of the river valley network, but subdivision of these large segments is likely to be necessary. The aim is to delimit major segments of the river network (at least 10 km in length but often much longer) that are subject to similar valley-scale influences and energy conditions. Therefore, as with the delineation of landscape units, excessive numbers of segments should be avoided, with typically between one and three segments delineated along a river valley within a single landscape unit. Segments most closely correspond to the scale of WFD water bodies, and so it is recommended that the boundaries of these units are delineated to match one another wherever possible, even if there is some nesting of one within the other.

To achieve any necessary subdivision of the initial segments based on landscape units, three main factors are taken into account:

  1. the degree to which the fluvial system is laterally confined (limited in its lateral mobility) by its valley;
  2. major discontinuities in valley gradient, including large dams;
  3. major changes in catchment area (which takes account of major tributary junctions).
  4. In addition, in steep mountainous areas, major lateral inputs of sediment from, for example, major debris flows and torrents may form additional points for segment delineation. These deliver massive amounts of sediment to the valley floor, and the largest of these will also cause discontinuities in valley gradient, which is already identified under factor (2).

All of the segment properties are investigated using topographic data, with (ii) major discontinuities in valley gradient and (iii) major changes in catchment area readily assessed using GIS tools. Automated methods are being developed to achieve (i) and (iv) although visual checking is strongly recommended. Automated delineation of segments and reaches is investigated in Thematic Annex A to this report and is also discussed below at the reach scale. EU-DEM, ASTER GDEM, NASA SRTM3 DEM, and the CCM2 data base are all useful data sets for this purpose, but additional useful information with regard to valley confinement can be drawn from Google Earth imagery, air photographs or, when available, LiDAR data.

Thus, large dams form an initial basis for defining segments and then (i) an overlay of the river network on a DEM, allows abrupt changes in valley gradient to be recognised; (ii) it also allows the upstream catchment area to be calculated to regularly spaced points along the river network, thus capturing large, abrupt changes in catchment area. Boundaries based on (i) and (ii) often occur at the same location. Finally (iii) inspection of DEM and other data sources, allows the presence of a floodplain to be recognised within the river valley with the aim of distinguishing river segments that abut directly onto the valley edges or ancient terraces (confined), from segments where discontinuous floodplains exist (partly-confined), and segments that possess a continuous floodplain along both sides of the river (unconfined).

Based on Brierley and Fryirs (2005) and Rinaldi et al. (2012, 2013), the following approach to defining segment ‘valley’ confinement is recommended.

Confined: more than 90% of the river banks are directly in contact with hillslopes or ancient terraces. The alluvial plain is limited to some isolated pockets (< 10% bank length).

Partly-confined: river banks are in contact with the alluvial plain for between 10 and 90% of their total length.

Unconfined channels: less than 10% of the river bank length is in contact with hillslopes or ancient terraces - the alluvial plain is virtually continuous, and the river has no lateral constraints to its mobility.