Saltmarsh Management Manual

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Causes of Change

Identify the Possible Causes of Change

In order to design an effective maintenance or enhancement scheme, it is essential to determine, firstly, whether a problem actually exists and, secondly the cause of the problem (that is, the installation of a wave break will not stop erosion due to plant death as a result of oil pollution). Moreover, if the saltmarsh does not face an imminent threat (i.e. it falls within Categories A or B), then there should be time to monitor and fully define the causes of erosion, and thereby the most appropriate management action, before intervention needs to occur.

It is normally possible to identify the cause of a localised erosion problem. For example, erosion adjacent to a sluice could be due to increasing drainage flows or contamination; erosion in close proximity to dredging (e.g. to create new berths) may be due to slumping or a change in wave/flow conditions; erosion on one side of a channel, coupled with accretion on the other, may indicate channel migration; while plant death adjacent to an outfall may be due to pollution or changes in freshwater discharge causing variable stress on plants. In cases where such localised erosion threatens coastal defences or causes the degradation of an important site for wildlife, it may be appropriate to consider correspondingly local schemes to stabilise or re-establish the saltmarsh.

In contrast, the cause of widespread erosion is often not obvious and there is likely to be more than one causal factor. The box below identifies types of saltmarsh change and possible causes.

However in order to inform the process of determining the nature of your problem, it is important to have a good understanding of how your saltmarsh is formed and how it functions.

Types of saltmarsh change and possible causes

Reduction in saltmarsh width

Change in the shape of the tidal curve reducing the period available for sediment accretion to occur

Increase in wave energy / increase in current speeds causing direct erosion through greater shear stress

Boat/ship wash causing direct erosion

Berth and channel deepening, leading to lowering of the intertidal area due to slumping (an increase in inshore water depth can also increase wave height in the intertidal zone)

Channel meandering in the estuary as a result of changes to tidal or fluvial flow

Change in the position of deep water channels or banks offshore, altering the pattern of waves and tidal currents

Creek lengthening and widening

Relative sea level rise (increasing the tidal prism)

Change in the shape of the tidal curve

Change in the floral community

Relative sea level rise leading to a change in inundation patterns

Grazing, potentially reducing diversity through alteration of conditions (including compaction and selective grazing)

Change in freshwater/seawater input

Loss of plant vigour

Increase in wave/tidal energy causing rotation of plants, loosening roots and general demise

Undesirable sediment chemistry (lack of nutrients, highly reduced environment etc.)

Pollution (as a result of spillage or from terrestrial runoff including herbicides and pesticides)

Disease (some species may be prone to diseases such as Wheat Rust)

Based on an assessment of saltmarsh changes and its possible causes, possible management solutions can be sought. However, the selection of the most appropriate management option should be informed by a robust appraisal of the available options.