6.1 Introduction
New tools and functionality have been incorporated into the IWMS to develop its data analysis and decision support role. These give a wave modeller the opportunity to generate results and perform analysis in a way that would not be possible if they were using SEAWORKS or any other similar model independently of the GIS. This chapter describes how these developments in the IWMS allow more results to be generated from the coastal process model, and how these data can provide useful management information from its subsequent analysis.
The Nash Bank and Scarborough data sets were again used to ensure that a range of system responses were investigated and to reduce the possibility of any ‘area specific’ findings being misinterpreted as generally applicable.
The topics under investigation are:
• Inshore wave height generation and analysis. • Morphology analysis tools.
The following sections look at each of these in turn, detailing the rationale behind the testing procedures and presenting some results and conclusions. The underlying premise for the development and design of the tools is addressed in Chapter 3.
6.2 Inshore Wave Height Analysis
When using SEAWORKS or similar backtracking wave refraction models, the normal output is a text file of information detailing the offshore wave condition modelled and the resulting inshore wave climate at a single inshore location. A typical SEAWORKS short
output file (Figure 6.1) lists the offshore wave condition used in the modelling and the resultant inshore wave condition.
OFFSHORE INSH ORE
D IR N HS TZ D IR N HS TZ
(DEG N ) (M) ( S ) (DEG N) (M) ( S )
3 4 7 . 8 4 0 . 6 5 2 . 5 6 3 2 6 . 8 8 0 . 3 9 2 . 8 3
3 5 9 . 8 3 0 . 6 0 2 . 4 5 3 2 8 . 3 9 0 . 3 3 2 . 7 8
Figure 6.1. SEAWORKS short results file
In this example, two different offshore wind directions have been modelled with the same strength and duration but with different initial directions. SEAWORKS can generate this information for any number of offshore wind directions, strengths and durations. The limiting factor is that only one inshore location can be processed at any one time.
In many cases, coastal management decisions are based on information generated at a single position. Generally there is no information to inform a coastal manager how far along the coastline this inshore wave condition remains valid.
There are many situations when knowledge of the spatial variation in inshore wave climate along a coastline would be of great help to coastal managers. Examples of some of these are shown below.
• Are some parts of the coast more prone to erosion or sedimentation, and if so, why? • Do some parts of the coast require stronger coastal defences than others?
• Is one stretch of the coast more suitable for the siting of a harbour entrance? • Where is the best place for the designation of a swimming or sailing area?
W ave height is only one o f m any factors under consideration by the coastal manager when addressing such issues, but im proved information about the w ave clim ate w ill be o f benefit to those making the final d ecision s. It is this issue o f spatial variation in inshore w ave clim ate that the new functionality in the IW MS has been d evelop ed to address.
The procedure in the IWMS for the generation o f inshore w ave data is as follow s:
i) An inshore contour is selected. It should be in deep enough water to ensure that
w a ves w ould not normally have broken before reaching this point.
ii) An inshore point spacing is selected and points are generated at this nominated
spacing along the contour.
iii) Batch processing procedures are used to calculate results at all the points through SEA W O R K S.
iv) The results, stored in the sam e text files as show n in Figure 6 .1, are re-imported into the IWMS for graphic visualisation on the bathymetry. An exam ple o f the graphical representation is show n in Figure 6.2.
ID 14 xcoord 283953.821 ycoord 173466.432 Odirn(deg) 202.342 y Ohs(m) 5.16 y Otz(s) 7.82 Idim(deg) 234.76 Ihs(m) 5.01 Itz(s) 6.87 1 Approximately I km
Figure 6.2 SEA W O R K S results displayed on the N ash Bank data.
The triangular graphics are oriented to sh ow w ave direction, sized to show w ave period and coloured to sh ow w ave height. The m odel results stored in the table can be queried on the v iew with the standard information tool. In this w ay all the data relating to each inshore point can be accessed im m ediately. In Figure 6.2, O dim , Ohs and Otz refer to offsh ore w ave direction, offshore w ave height and offshore w ave period respectively and Idim , Ihs and Itz are the equivalent inshore values.
A ny o f the parameters stored in the table can be view ed with standard GIS chart tools. Larger data sets can be exported to M icrosoft E xcel or a similar package for chart visualisation. A chart representing the variation in inshore w ave height in Figure 6.2, can be seen in Figure 6.3. The inshore points are separated by 500m .
0 5 Q) X 03 6 I D = 14 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 In sh ore Po int N u m b e r
Figure 6.3 N ash Bank: Variation in inshore w ave height along the 6.5m contour
(W ind 30m /s for 12 hours from 2 3 5 °)
The inshore points are generated from south to north in this data set. Increasing distance along the x axis in Figure 6.3 therefore represents a m ovem ent northward from the sm all green inshore w ave height graphic in the south o f the bathymetry show n in Figure 6.2. Such analysis identifies areas along the coast where the local bathymetry generates w ave heights that do not fo llo w general trends.
The ability to generate this level of detailed information about the spatial variation in the inshore wave climate for any bathymetry is a great improvement over normal model outputs. Analysis tools are provided which allow the model results to be queried to allow a better understanding of trends within the data.
The tools implemented to date in the IWMS are designed to query the absolute wave height generated from the coastal process model. In some cases this may not be the most important parameter and there may be more interest in the percentage change in wave height, or changes in wave direction for example. The simple design of the tools enables them to be easily amended to investigate any chosen parameter.
The currently available analysis tools address two main issues: i) How much variation is there in the inshore Wave climate?
ii) What is the optimum number of inshore points required to model all the necessary inshore wave detail for any particular study?
By answering these questions, the wave modelling procedures can be used to identify the full range of spatial variation in the inshore wave climate required for the study of each particular management scenario.
6.2.1 Inshore Wave Height Variation
The variation in the inshore wave climate along a contour for one wind direction and strength can be generated with a single batch processing procedure in the IWMS as described above and shown in Figure 6.3. Often, more than one offshore wind condition is needed to satisfy the modelling requirements. In these situations the batch processing procedures are run as many times as required to generate results at all inshore points for all required wind conditions. This results in a set of Arc View tables with the results stored for all points, for each wind condition.
Chapter 3 details the different types o f analysis tools that are available for use in these