A12. Morphological Impact Assessment System (MImAS)

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1. Introduction[edit]

MImAS is an assessment system developed by Greig et al. (2006) which is based around the assumption that rivers have the capacity to resist and absorb a certain level of morphological pressures, and that this can be quantified and assessed. Beyond a defined threshold, the hydromorphological condition and ensuing ecological status of the river is at risk of being degraded. It is used by the Scottish Environment Protection Agency (SEPA) to assess activities (i.e. management actions or structures such as dredging or embankments) that could impact the morphological and ecological condition of rivers. The assessment involves the collection of pressure data and river type information which is then used to quantify the level of morphological impact.

2. Method summary[edit]

MImAS is a modular system which combines different levels of assessment to identify the risk of degradation for a given activity in a particular setting. The five modules are listed in Table 1.

Attribute Module A list of indicators of geomorphic and ecological condition such as channel substrate size and bar character grouped into six groups and two zones (channel and bank/riparian). These attributes have been chosen for their role in supporting ecological communities.
Typology Module The typology module groups rivers using morpho-types that represent different levels of energy, sediment supply, and transport capacity at the reference condition. Although the typology is based on the recognition of forms, it is aimed at the identification of dominant processes which provide an insight into the river ability to resist and recover (i.e. resilience) from modification impact as assessed in the sensitivity module.
Sensitivity Module This module provides an assessment of geomorphic and ecological sensitivity. Geomorphic sensitivity is based on river type resistance (ability to absorb) and resilience (ability to recover) to a change and is measured at the level of individual eco-geomorphic attributes such as substrate and vegetation structure with regards to specific activities such as dredging or bank management. Ecological sensitivity measures the potential impact of pressures on groups of species at class level (i.e. fish, invertebrates and macrophytes) using broad preference relationships with eco-geomorphic attributes.
Pressure Module This module calculates the likelihood of impact of a range of generic activities on eco-geomorphic attributes. This includes an assessment of the zones of change (whether an impact will be local or have a wider influence upstream or downstream).
Scoring Module This final module combines the outputs of the above modules to calculate the capacity used by each pressure as well as the overall capacity used.

Table 1: MImAS Modules (adapted from Greig et al., 2006)

The modules were created and populated with data by a panel of experts during the creation of MImAS (Greig et al., 2006) with simple scoring methods. For example the sensitivity module is scored as follows: Insensitive (score 0), Sensitive (score 0.5), Highly sensitive (1).

3. Data collection[edit]

MImAS requires accurate field survey data to feed into its scoring module, including:

  • The reference typology of the site.
  • Pressure types and the length of channel they cover.

The survey is a simple mapping method which records pressures and river types along a reach. MImAS was initially designed for 500m reaches, but it can be applied to any river length. It can be completed on a paper map or with a tablet and GIS software. Recording morphological pressures is a relatively simple and quick task for a surveyor, but assigning a reference typology requires hydromorphological expertise.

4. Assessment[edit]

The MImAS scoring system combines the modules (Table 1) to calculate the percentage of channel and bank capacity used by single or combined activities. Scores are compiled into an Excel spreadsheet which contains the relationships between river types, geomorphic attributes and morphological pressures. These relationships were derived by experts using a simple ‘low-medium-high’ system. During the creation of MImAS, an expert panel defined limits on morphological capacity for 500m reaches of 5% and 15% for ‘High’ and ‘Good’ ecological status.

5. Advantages[edit]

  • Useful data on morphological pressures is collected and mapped during a MImAS assessment.
  • Highlights potential impacts as a result of morphological pressures.

6. Disadvantages[edit]

  • Requires geomorphological expertise to identify river type.
  • Requires expert interpretation of the results.

7. References[edit]

  1. Greig S.M., Richardson R. and Gibson J. (2006). A new impact assessment tool to support river engineering regulatory decisions: SNIFFER Technical Report. Project No. WFD49.