Pritchard (1952, 1967) and Dyer (1973) used a geomorphic approach to classify estuaries. From a geomorphological standpoint, Pritchard identified four primary subdivisions of estuaries: 1) drowned river valleys (e.g., Chesapeake Bay), 2) fjord-type (e.g., Penobscot Bay), 3) bar-built (e.g., Laguna Madre), and 4) estuaries produced by tectonic processes (e.g., San Francisco Bay). This approach has intuitive appeal, but it generally lacks a quantitative framework allowing further elaboration. Thus, this approach is of limited usefulness in understanding, predicting, and managing estuary response to nutrient loading.
The hydrodynamic approach to estuarine classification focuses on the interaction in narrow estuaries of tidal currents and river flow. Tidal currents provide energy for mixing while river flow is a source of stratification or buoyancy. Stommel and Farmer (1952) divided estuaries into 4 categories based on stratification: 1) well mixed, 2) partially mixed, 3) fjord-like, and 4) salt wedge. This simple classification was made quantitative by defining a stratification number G/J, where G is energy dissipation over a defined channel length and J is the rate of gain of potential energy of water moving through the estuary over the same length (Ippen and Harlemann 1961; Prandle 1986). By incorporating Richardson number and critical depth criteria, Fischer (1976), Simpson and Hunter (1974), and Nunes Vaz and Lennon (1991) have made variations on this classification.
The one-parameter (e.g., G/J) classifications capture one important aspect of estuarine circulation but they have no direct relation to the various estuarine types (Jay et al. 1999). Accordingly, two-parameter classifications were developed that describe the interaction of geomorphology, fresh water, and tides. The most widely known two-parameter scheme (Figure 6-4; Hansen and Rattray 1966) employs two parameters to classify estuarine circulation: 1) a stratification parameter, ∂S/S0 and 2) a circulation parameter, Us/UF. The first parameter describes stratification as the ratio of the top-to-bottom salinity difference to mean salinity over the section. The circulation parameter is a ratio of the net surface current to the mean freshwater velocity through the section. Numerous clarifications, modifications, and additions have been made to this approach, most focused on providing a closer connection between the density field and tidal processes (Fischer 1976; Officer 1976; Oey 1984).
Hansen and Rattray (1966) identified seven types of estuaries, basically following the conventional usage of Stommel and Farmer, but
Rationale’s interface has been designed to provide a path for critical thinking. From gathering research, to weighing up evidence to formulating a judgement, Rationale will assist you.
Take a look at these 6 critical thinking steps with examples to demonstrate the path to better outcomes.
Step 1: ORGANISE INFORMATION
We have no difficulty in locating information. The key is that the information is selected and structured appropriately. With Rationale’s grouping maps you can drag information from the web onto your workspace via the scratchpad and include colour, hyperlinks and images. The structured, pyramid like maps provide a guide for students to structure the information in such a way that reveals the connections between the main topic and its various themes or categories.
Step 2: STRUCTURE REASONING
Many people provide opinions but rarely provide supporting reasons for their view. Rationale’s reasoning maps encourage people to support their responses and to consider different opinions. It uses colour conventions to display reasoning – green for reasons, red for objections and orange for rebuttals. It also includes indicator or connecting words so that the relationship between statements is clearly understood.
Step 3: CONSIDER EVIDENCE
A test of a solid argument is how good the evidence is that underpins the claims. Rationale’s basis boxes provide a means to identify the basis upon which a statement is given. The icons provide a visual guide as to the range of research utilised and the strength of the evidence that is provided.
Step 4: IDENTIFY ASSUMPTIONS
We often talk about analysing arguments. This can mean a few things including looking at the logical structure of the argument to ensure it is valid or well formed and also identifying assumptions or co premises. For those who require higher levels of analysis, Rationale provides the analysis map format to show the relationships between main premises and co premises.
Step 5: EVALUATE ARGUMENTS
Once arguments for and against an issue have been logically structured, they need to be evaluated. Rationale provides a visual guide for the evaluation of claims and evidence – the stronger the colour, the stronger the argument while icons designate acceptable or rejected claims. While learning this process of evaluating arguments, the colour and icons provide immediate undertanding and communication of the conclusion.
Step 6: COMMUNICATE CONCLUSION
Presenting ideas orally or in writing is crucial and is often the distinguishing feature between good results and average ones. Rationale has essay and letter writing templates to build skills and confidence. Templates provide instruction and generation of prose. When exported, there is a structured essay plan with detailed instructions to assist understanding of clear and systematic prose.