Reach delineation

Reaches

Definition

Section of river along which boundary conditions are sufficiently uniform that the river maintains a near consistent internal set of process-form interactions. (A river segment can contain one to several reaches). As a general rule, the length of a reach should not be smaller than 20 times the mean channel width, although shorter reaches can be defined where local circumstances are particularly complex.

Delineation

The boundaries of river segments form the first delineation of river reaches. However, subdivision may be necessary, since the aim is to define reaches of similar channel and floodplain morphology, which are likely to reflect local changes in bed slope that were too small to demarcate a segment, and changes in sediment calibre, discharge and sediment supply associated with smaller tributary confluences or artificial discontinuities such as dams, major weirs / check dams that disrupt water and sediment transfer. Changes in river confinement as indicated by the ratio of channel width to alluvial plain width within a segment can also affect channel and floodplain characteristics and so a river confinement index (Rinaldi et al., 2012, 2013), defined as the ratio between the alluvial plain width (including the channel) and the channel width (or the reciprocal, defined as ‘entrenchment’, e.g. Polvi et al., 2010), can help in delineating reaches.

Automated methods are becoming increasingly available that delineate or aggregate homogenous reaches using topographic and other data (e.g. Alber and Piégay, 2011, Bizzi and Lerner, 2012, Notebaert and Piégay, 2013). The application of such automated procedures to a Spanish river is presented in Thematic Annex A. However, it may be necessary to refine the outputs of automated delineations for the present purposes or to base the entire delineation on a visual analysis of imagery and map data.

At this scale, the controlling factors are mainly reflected in the planform characteristics of the river channel and floodplain, including the geomorphic units that are present, which can be viewed on aerial imagery. The following provides a simple working definition and classification, based on Rinaldi et al. (2012) and summarised in Table 4.3 and Figure 4.1.

Confined reaches

In the case of valley confined reaches, streams are first divided into three broad categories based on the number of threads, i.e. single-thread; transitional (wandering); multi-thread.

Type 1: Single-thread confined reaches

In the case of single-thread, confined reaches, sinuosity is not meaningful as it is determined by the valley rather than the channel planform. Therefore, single-thread confined channels are not further sub-divided at this stage, because it is not possible to make accurate distinctions based on other characteristics, particularly the bed configuration, from remotely sensed sources.

Transitional and multi-thread confined reaches are identified using the same criteria as for unconfined and partly-confined transitional and multi-thread channels (see below). These confined channel types are usually sufficiently large to be discriminated by remote sensing. It is also possible that some small transitional or multi-thread streams can only be confirmed following field survey. In that case they are classified as Type 1 reaches during the delineation phase.

Unconfined and partly-confined reaches

Six broad types (2. Single-thread: Straight; 3. Single-thread: Sinuous; 4. Single-thread: Meandering; 5. Transitional: Wandering; 6. Multi-thread: Braided; 7. Multi-thread: Anabranching) are distinguished, based on a planform assessment (from aerial imagery) of three indices:

• The sinuosity index (Si) is the ratio between the distance measured along the (main) channel and the distance measured following the direction of the overall planimetric course (or ‘meander belt axis’ for single thread rivers).
• The braiding index (Bi) is the number of active channels separated by bars at baseflow. (Recommended method for estimating Bi is the average count of wetted channels in each of at least 10 cross sections spaced no more than one braid plain width apart - Egozi and Ashmore (2008) suggest that this is the least sensitive to flow stage, channel sinuosity and channel orientation).
• The anabranching index (Ai) is the number of active channels at baseflow separated by vegetated islands (Ai). (Recommended method for estimating Ai is the average count of wetted channels separated by vegetated islands in each of at least 10 cross sections spaced no more than the maximum width of the outer wetted channels apart)

Single-thread

Bi and Ai equal or very close to 1 (i.e. only local braiding or anastomosing is possible).

Type 2: Single thread: Straight (Si<1.05)

Type 3: Single thread: Sinuous (1.5<Si<1.05)

Type 4: Single thread: Meandering (Si>1.5)

Transitional

Transitional channels exhibit intermediate characteristics in terms of braiding or anabranching between single-thread and multi-thread channel types. As a consequence, Ai and Bi indices are between 1 and 1.5.

Type 5: Transitional: Wandering

A distinctive characteristics of many wandering rivers is the presence of a relatively wide channel (high width / depth ratio) occupied by active bars, similarly to braided rivers. Therefore, 1 < Bi < 1.5, but bars are continuously present, occupying most of the channel bed. This morphology is close to multi-thread, with a relatively wider channel than single-thread rivers and a significant presence of braiding or anabranching phenomena. Rivers with a relatively high value of Ai (but <1.5) and no braiding phenomena can also be classified as wandering. The latter type could be described as ‘wandering anabranching’ whereas the former could be described as ‘wandering braiding’.

Multi-thread

Multi thread (channel) planforms have Bi > 1.5 or Ai > 1.5. Two types are distinguished: braided systems have individual threads (low-flow channels) that are highly unstable within the ‘bankfull’ channel bed, while anabranching/anastomosing systems have relatively stable low-flow channels.

Type 6: Multi-thread: Braided (Bi>1.5 and Ai<1.5).

Type 7: Multi-thread: Anabranching (Ai>1.5 and Bi<1.5 or Bi>1.5)

Highly altered reaches

Type 0: It is important to identify reaches of sufficient length with highly modified characteristics (e.g. urban and other highly channelised / reinforced reaches) as a separate category, since their lateral stability and geomorphic units cannot reflect ‘natural’ boundary conditions.