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Physical shape and structure: morphological conditions
A decision-supporting framework is proposed for use in rivers, based on expert opinion of the impact of morphological changes on river ecology, and supported by numerical thresholds. The proposed framework considers the impact of new applications for engineering or construction works as a percentage change from natural conditions. The framework applies to all rivers, as it includes a module for river type.
This is the first holistic framework to be used for regulation in the UK and should ensure consistent initial assessment of new applications for engineering and construction works that would impact on river morphology. Such works in freshwater have only been regulated in Scotland since April 2006, with the introduction of CAR. In the past, SEPA has used River Habitat Surveys to describe the existing morphological condition of rivers. SEPA also used a type-specific framework for assessing the environmental risk associated with engineering works, to inform responses to consultations on development proposals.
The risk assessment criteria included in the proposed framework were used for the 'Article 5' characterisation analyses carried out in 2004 to identify water bodies at risk of failing to achieve 'good' status.
1. River morphology framework
The approach to assessing the appropriate level of morphological change that can be accommodated in a river is to:
1. Set numerical limits for the maximum acceptable percentage use of a river's capacity that support 'high' and 'good' ecological status: see table below.
Table A20: Morphology - maximum reduction in capacity (%) - rivers |
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Zone | Maximum permitted reduction in capacity (%) |
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High | Good |
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Channel † | 5 | 15 |
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Bank and riparian ‡ | 5 | 15 |
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† 'channel' includes 'river depth and width variation' and changes in the 'structure and substrate of the river bed', as defined by the WFD. ‡ 'bank and riparian' includes changes in the 'structure of the riparian zone', as defined in the WFD. |
2. Establish a decision-support framework to identify whether a proposal is either:
- likely to be sustainable by the river in question, without causing damage to ecology; or
- likely to threaten the ecological balance of the watercourse.
The proposed framework has five modules, as set out in Table A21. Users select from a list of options to create a picture of the specific river conditions and the proposed activities in each case. The user enters the appropriate information for the stretch of river in question for each of the modules (attributes, type, sensitivity, pressure) and the model then calculates the capacity used.
Table A21: Decision support framework: morphology change - rivers |
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Module | Description |
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1 | Attributes: descriptions of the watercourse's existing condition: effectively a picture of 'what needs to be protected'. | A list of attributes has been produced, which includes features that either - directly support aquatic biology, such as the structure and extent of riparian (riverside) vegetation; or
- control the physical environment on which ecological communities depend, such as the slope of the channel, or rate of channel migration.
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2 | Channel type: a classification of the physical shape and structure of the section of the river in question. | Reaches have been classified into a series of six morphological types (A to F) that each respond in a particular way to an external pressures. These types incorporate the ability of a watercourse to accommodate change and also to recover from the external change. Only six types have been defined at present, so this is a simplification of the possible responses. |
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3 | Ecological sensitivity: a classification of how sensitive the ecology in that part of the river is to an external pressure. | Sensitivity is classified in three levels, which are considered in terms both of resistance to change (the ability to accommodate it) and resilience (the ability to recover from change). The model considers the probability of a pressure causing a change in the morphology of the watercourse, and then what impact these changes in attribute would have on the biology. |
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4 | External pressure: a classification of the external pressure that is proposed. | A list of twenty potential pressures has been compiled, including activities such as 'hard' bank protection (such as strengthening river banks with concrete), building bridge piers and dredging. The pressures are considered in terms of the probability of them affecting the river morphology and the scale of that impact. Activities such as protecting banks cause local effects, but dredging a bed has a far more extensive impact. |
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5 | Scoring system: this incorporates the information from the first four modules to calculate a numerical score for the proposed work. | An Activity Impact Score (0 to 2) is calculated for each attribute in turn, using the information about the pressure, channel type and ecological sensitivity of the reach in question. 
Scores are averaged for each attribute within zones. These scores are then used to calculate the amount of capacity used by a pressure in a specific river type, using the equation: 
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This process is carried out twice:
1. to the existing watercourse, to calculate the impact of any existing structures in the river, and therefore the available capacity for more work; and then
2. for the proposed work, to calculate the additional impact of the proposals.
The capacity used is compared with the percentage thresholds for 'high' and 'good' ecological status (5% and 15%: see Table A20), to assess whether the watercourse has spare capacity to accommodate morphological change, and then whether the proposed works are feasible, or are likely to risk a deterioration in status, and a risk to the ecology.
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