Temporal analysis: Field survey
Field survey approach to the temporal characterisation of hydromorphology
At the most basic level, hydromorphological change can be assessed using a geomorphological field survey. In this approach, contemporary channel and floodplain features are interpreted in the context of the channel type by a trained geomorphologist to identify changes that are ongoing or have occurred at some point in the past.
The field assessment approach offers very limited temporal resolution and is applicable primarily at the reach scale. It provides an indication of channel change that has happened in the past or processes that are currently operating. For example, exposed bridge pier foundations or pipelines suggest channel bed incision (Thorne, 1998; NCRS 2007, Ch. 3, Table 3.2) but without any supporting information, all that can be determined is that the channel has incised at some point in time post construction. Amounts or rates of change can only be estimated if additional historical documentation exists, e.g. bridge surveys, which would then shift the analysis to the historical approach described below. On the other hand, it should be recognized that for many streams some types of data may be not available, in particular historical bed-levels, and thus geomorphological survey can be crucial to gaining information on past changes.
Geomorphological field surveys can generate accurate assessments of the type and magnitude of change that has occurred in a reach over time. This is particularly true if the geomorphologist conducting the survey is familiar with the setting and river type (chapter 7), and can make comparisons with similar rivers in the area that have better historical records. Without any other supporting information, though, the uncertainty in the assessment can be high, particularly when rivers have gone through several types of changes in succession, thereby masking or removing the evidence of the earlier changes.