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9.2.1 Model size and setting up

The program used a rectangular grid to analyse the flood plain. The flood plain however was not rectangular in shape. The analysis made the calculation domain rectangular by inserting between the edge of the domain and the DTM, areas higher than the flood plain so water could not flow over them into the DTM model area.

Next the modelling assessed the problem to find the most suitable grid size. This showed that, with a 20 m grid, 10 hours of real time simulation for each flood could be run on the Pentium 166 Mhz machine in 25 hrs. In this case the program was run on the UNIX based personal computer language, Linux. A 10 m grid size would have required a running time of 8 x 25 hours or 200 hours. The reason for this was that the 10 m grid had 4 times as many points and the time step for the analysis was half that for the 20 m grid. This time was considered too long to obtain results that may have required to be rerun several times

grid may give a better result therefore it was proposed that the 20 m grid results be checked using the results for a 10 m grid on area S1, the smallest area.

The analysis divided the flood plain into 4 calculation areas using the 20 m grid. Two on the north side (area N1 of 40,500 and area N2 of 45,350 points) and two on the south side (area S1 of 20,500 and area S2 of 51,500 points). This required an increase in the maximum size of the 2de grid. As there were two models on each side of the flood plain the software was modified for this project to allow it to input results from a row or column from an upstream area into a downstream area.

9.2.2 Flood Plain Features

The 20 m grid size was too large to pick up all the features of the flood plain. These features were:

i) the railway line that runs north-south through the flood plain (see Figure 9-3).

ii) a grass waterway channel called Sinclairs Creek on the southern flood plain (see Figure 9-3 and Figure 9-4 ).

iii) several irrigation channels on the southern side of the flood plain (see Figure 9-3 and Figure 9-4 ).

iv) Willowbridge Creek that runs on the northern flood plain (see Figure 9-5)

v) a channel cut through an area of higher ground just north of Lundies Ford (see Figure 9- 5).

9.2.2.1 Railway Line Embankment

Where the 20 m grid did not give the railway line embankment levels, ARC/INFO was used to alter the levels of the closest adjacent points to the railway embankment to its level. The changes were done using a contour map showing the railway line levels and position in the background altering the levels of these points one by one.

not have a significant effect on the results as the volume was a very small percentage of the overall water volume.

At one point there was a small bridge of 10 m width through the railway embankment. The modelling put the invert ground level at this point of the railway embankment. The width of 10 m was modelled in the 20 m grid by increasing the Manning’ s ‘n’ value to twice the normal value for this area. The increase in resistance was important as the flood levels for the large area of water ponded up against the embankment were dependent on the flow characteristics through this bridge. This procedure was correct as in this cell even though the cell did not have the correct volume (it is twice the actual volume) the velocity is half the true velocity. These changes satisfied both the volume conservation and momentum conservation equations. However it was recognised that the program did not have options to allow for inlet and outlet head losses through the bridge.

This however highlighted a potential problem with structures as these can have an effect on the flood levels of quite large areas upstream of them. In addition to bridges, the model also needs to be able to represent structures such as weirs, culverts and allow for bridge piers which are too small to be placed in the grid. These generally provide drainage of an area through an embankment and therefore effect the flood levels in these areas. In some cases the areas affected can be quite large.

9.2.2.2 Sinclairs Creek

The modelling changed the ground levels for Sinclairs Creek similarly to the railway embankment levels. It also ignored the conveyance of the Creek in this case as the Creek was nearly full during the flooding. This was done by taking the invert level of the Creek out of the model before ARC/INFO generated the ‘TIN’ surface. This meant that the Creek was acting in the model as an area of high ground equal to the height of the creek stopbanks. Again in this case there is a small volume conservation error as the creek was less than 20 m in width.

9.2.2.3 Irrigation Canals

The modelling handled the irrigation canals embankments above the flood plain similarly to Sinclairs Creek. However unless they were overtopped these would not be full of water. If they were, then any flows in these channels would have little effect on the flood plain flows as their conveyance was very small and many of them were at right angles to the flood plain slope. The irrigation race embankments also had siphons to keep the natural drainage channels of the flood plain and stop water ponding up behind the races with nowhere to go. These gaps in the irrigation race embankments were less than 20m in width and the Manning’ s ‘n’ values for these points were also increased to give the correct water conveyance through the gaps.

9.2.2.4 Willowbridge Creek and Drainage Channel

Where the 20 m grid did not show Willowbridge Creek on the northern flood plain the modelling inserted actual creek levels into the grid. The changes were done similarly to the railway embankment. A similar exercise was carried out for the drainage channel through the high ground with the Manning’ s ‘n’ value being altered to give the correct conveyance as the channels were not 20 m in width. In both these cases the channel zigzagged through the flood plain where the channel was flowing diagonally over the calculation grid. The zigzag meant that the channel had a small volume conservation error as it had more volume in it as it was longer than the diagonal channel. It also meant that the flood conveyance was less than it should have been as the channel was longer and also flatter in these areas than the actual channel.

9.2.3 Modelling of Breaches and Overtopping

The results of Chapter 8 gave the discharges onto the flood plain from the stopbank breaches and overtopping. The analysis needed to put these into the flood plain at the correct positions.

It modelled the breaches by placing channels from the edge of the calculation area to the DTM, i.e. through the higher areas that were inputted to make the calculation domain rectangular. These channels were the same width as the breach, to the nearest 20 m, and entered the flood plain at the same level as the flood plain. These are shown on the contour plans of the model areas (see Figure 9-2)

The overtopping was modelled similarly (see Figure 9-2 for the diagram that is applicable to the following description). In this case there was a channel one cell wide from the edge of the calculation domain to the centre of the length of the stopbank where the overtopping occurred. At this point the channel turned in both directions parallel to the stopbank going the full length of the overtopping. Between this channel and the flood plain the modelling inserted higher ground one cell wide. This area was uniform in height to allow the uniform flow per unit width over the length of overtopping. This was not exactly correct as the overtopping was not uniform over its length. However this was considered close enough to be insignificant within the expected errors from the other sources of error such the DTM elevations and the flood plain resistances.

9.2.4 Beach Dune

During the flood events the flood water flowed over the flood plain to the beach dune and built up behind the dune, eventually breaching it by piping failure. The DTM did not include the beach dune at the coastline. This meant that an approximate beach dune had to be inserted into the model. The modelling put a dune that had a reduced level at its crest of 5.5 m (about the average level from the beach dune surveys) and part of the Pacific Ocean into the DTM model. The latter allowed the water flow through the breaches to be correctly modelled. Two models were run. The first model had the dune with no breaches in it and was run until the water levels had risen to the correct level. The second model contained the breaches and was run until the peak flows had passed into the Pacific Ocean.

The flood hydrographs calculated in Chapter 8 were inputted into the overtopping and breach channels at the model boundaries. The modelling stored the results of the runs for every real time hour as storage at any closer time would have resulted in very large files. At this time period the results for the four runs for one flood were stored in four files of about 100 Mb. However this did not give the maximum values as the flood wave peaks were not exactly at each hour. This required further software development to allow 2de to give files containing the maximum water levels, velocities and depths in addition to the the files containing the hourly values.

Finally before the models were run the hydrographs were modified to account for the volume of the inflow channels from the edge of the calculation domain to the flood plain. These channels can be seen in Figures 9-2 to 9-7 in the folder of plans. The volumes were important as these channels were quite long, up to 2 km for the S2 area.

9.2.6 Contour Plans of the Individual Models

Figures 9-1 to 9-7 in drawings at the end of the Volume show the flood plain models for both floods. Figure 9-1 shows the area N1 used for both the 1986 and 1994 floods. Figures 9-2 to 9-4 show areas N2, S1 and S2 respectively for the 1986 event and the final three figures show the 1994 flood models for areas N2, S1 and S2.

Figure 9-1 Contour Plan of the Model area N1 for both the 1986 and 1994 Floods Figure 9-2 Contour Plan of the Model area N2 for the 1986 Flood

Figure 9-3 Contour Plan of the Model area S1 for the 1986 Flood Figure 9-4 Contour Plan of the Model area S2 for the 1986 Flood Figure 9-5 Contour Plan of the Model area N2 for the 1994 Flood Figure 9-6 Contour Plan of the Model area S1 for the 1994 Flood Figure 9-7 Contour Plan of the Model area S2 for the 1994 Flood

9.2.7 Flood Plain resistance.

As stated in Chapter 7, a layer of Mannings ‘n’ values was digitised into ARC/INFO. The 20 m grid did not pick up all the areas that were digitised such as hedges. Therefore another plot was done of the Mannings ‘n’ values. Figure 9-8 in the drawings at the end of the Volume shows the values used for the model and this can be compared with Figure 7-1 which uses a 5 m grid and shows almost all the features (it also misses some hedges) of the Mannings ‘n’ layer.

Figure 9-8 Contour Plan of the Mannings ‘n’ values for the 20 m grid (see drawing at the end of the Volume).