The Estuary Guide

Cause Consequence Model

Perhaps the most difficult aspect facing any approach to understanding estuarine morphology is the lack of any clear cause-effect hierarchies. Within an estuary, form and process are inextricably linked and there are no obvious dependent and independent variables or clear cause-effect hierarchies. This results in the potential for small changes to have far reaching effects. Although the size and shape of an estuarine channel is a response to tidal processes, the tidal discharge is itself dependent on the morphology of the estuarine channel, since this determines the overall tidal prism. One possible constraint on the estuarine system is the tidal length of the estuary, which is dependent on the macro-scale slope of the coastal plain, the fluvial discharge, and the tidal range in the nearshore zone. Additionally, in some specific cases, further constraints to the closed cause-effect relationship may include geological or, in some cases, anthropogenic controls on estuary width or depth, such as existing urban areas or harbour facilities.

As a consequence, when developing plans for estuary management, it is important to retain a clear estuary wide perspective, even when addressing seemingly local issues. This is of course made more complex by the superposition of external influences, such as sea level rise and changes in storminess, which impose changes on a variety of spatial and temporal scales.

The purpose of the cause-consequence model is to map the possible routes from a particular causal action and the resultant changes to the system (primarily in terms of changes in form/morphology). Both actions and induced changes can take place on one or more spatial and temporal scales. As a consequence, it is difficult to map all the possible routes in a simple flow diagram. A matrix approach has therefore been adopted, where a number of matrices form several layers to the model with defined links between layers. This is summarised in Figure 1, where there are five layers:

(i) the first matrix defines the spatial scale of action;
(ii) the second is for the temporal scale of action;
(iii) the various response models that could be used to compute the changes forms the third layer;
(iv) possible changes to different attributes makes up the fourth layer; and
(v) the time scale of any response is the final layer.

The first two combine to define the inputs to the response model, in conjunction with relevant boundary conditions and constraints on the system. It is not the case, however, that every spatial scale of a particular causal action combines with every temporal scale of the action. In most cases a sub-set is all that needs to be considered. Typically actions that are spatially on a large scale (estuary/external), also have long-term time scales of action.

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