Relevance

Type of model

Data required

Case study: data to underpin hydrological modelling

The purpose of studying the hydrological regime is to determine the movement and fate of water quality parameters, e.g. nutrients, within the site and therefore to better understand the interaction between these and features of conservation interest, to help target monitoring or further survey, and to inform the development of management measures by providing a predictive tool.

The hydrographic regime has a profound influence on the conservation interest of most lagoons, and the factors affecting such interests. Therefore, investigating the hydrographic regime of any lagoon will be crucial to many management measures intended to maintain its conservation interest. Models of annual nutrient budgets are useful at a coarse level to help determine the importance of different nutrient sources but are not sufficient to properly target nutrient sources for further study or reduction in flows or concentrations and predict the outcome of changing inputs through management measures. Deterministic models allow predictions of effects of changing concentrations or flows of all the sources included in them. It is therefore possible to target sources which will make a difference, and avoid ineffective action and use of funds. Use of such models is, however, unlikely to be able to incorporate the effects of extreme events such as storms etc. which may well be important in lagoons.

Models which take into account the hydrographic conditions within the lagoon and incorporate seasonal variations (dependent on rainfall, inflow and flushing characteristics) have been shown by the Fleet study to be essential. The hydrographic conditions within many lagoons will be complex and difficult to ascertain, and will vary between different lagoon sites. Within a single, larger lagoon (such as the Fleet) the lagoon may be divided into different parts with very different hydrographic conditions. In terms of seasonal variation, in the Fleet, for example, seasonal growth of seagrass affects flows because the lagoon is so shallow.

Many saline lagoons, at least of the inlet type, will be similar to the Fleet in being characterised by unusual hydrodynamic properties. The case study has demonstrated the need to develop numerical hydraulic models more suitable to shallow, relatively low flow systems than many current models developed, for example, for estuaries. Therefore, use of appropriate models is recommended. Such models will need to be adapted to each individual lagoon site, and should be capable of predicting the effects of changes in flows and nutrient inputs over a range of conditions.

Knowledge of tidal regimes and freshwater flows into and out of the lagoon (including via groundwater and percolation) will be required. Where separate lagoons are close together, their hydrological regimes may be interconnected (eg. by channels, percolation, or groundwater transfer), and knowledge of this will be required in order to determine sources of nutrients and possible remediation strategies if necessary.

At a coarse level, some of the above parameters can be estimated for inclusion in a crude model. However, in order to estimate better the importance of different sources of water and nutrients, further, more refined data and studies will usually be necessary. These may include studies of the local tidal regime, local bathymetry (especially if it is complex), freshwater, discharge and diffuse input flows (and their variation), percolation and groundwater quality and flows. Building of a more complex model may be expensive, but once constructed it may prove invaluable for future management of the site. Such models can be used to run scenarios and predict the effects of suggested changes to any of the parameters included in them.

For the Fleet study, good hydrographical data were available from earlier studies (Robinson 1983 and Robinson et al 1983) which could be used for both crude and more detailed modelling. This is unlikely to be the case for other lagoon sites. Even with this existing data, due to the shallow nature of the Fleet lagoon and its complex shape, further bathymetric information was required in order to construct a reasonably accurate model of its flow regime. Problems were encountered with use of the new bathymetric data (location and depth of small channels were not fully represented), which could be avoided given better communication between the modellers and the surveyors or commissioning agency on what data were required and from where.

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