Physical biotopes in representative river channels :identification, hydraulic characterisation and application

Abstract

There is a need to predict the river flow conditions necessary to support instream (and river corridor) biota. Many rivers have their flow regulated in some way and the field of 'habitat hydraulics' (or 'ecohydraulics') has grown up to encourage classification and characterisation of lotic physical environments. This thesis, based on field measurements at eleven sites in Northeast England (which are nationally representative), presents the data nonnally collected for the calibration of the PHABSIM model in alternative ways, to identify and characterise 'physical biotopcs'. These are segregated on the basis of subjectively (visually)-defined flow types. Statistical validation of biotopes as hydraulically discrete units shows them to be defined by a characteristic range of hydraulic variables, especially the Froude number. An investigation of the spatial and temporal variability of biotopes in different channel types and at different flows was carried out (1993-1995), which included the most extreme floods and droughts on record at some sites. The 'biotope approach' is shown to have potential as a reconnaissance approach to the assessment of habitat quality. Despite the success of the biotope approach e.g. in River Habitat Surveys, methodological improvement is still required, notably in the area of hydraulic characterisation by velocity measurements. Hydraulically rough channels and those with substantial macrophyte growth present problems; a logarithmic velocity profile cannot be assumed. A relationship between biotope diversity and stream ecosystem health is suggested; biological surveys are required to determine the direct relationship. This thesis represents a shift from the geomorphological riffle-pool theory to a broader classification of instream hydraulic units. It provides a framework for testing the debate in ecological theory regarding the role of patch dynamics within the continuum of the river environment. The study indicates that an 'environmentally acceptable flow regime' is one which creates a range of biotopes and maintains 'critical biotopes' at periods coinciding with key lifestage events. Flood events are beneficial for their role in 'resetting' the system.