Hey Sealtheseal!
I had a very similar assignment in Unit 1 QCE methods last year. We had to design three waterslides with piece-wise functions and then prove their reasonability.
I would recommend looking at international amusement park standards for waterslides (set by ASMT International). The guidelines put in place ensure that all slides:
- Don't exceed acceleration limits,
- Have appropriate emergency safety devices in place,
- Have suitable hydraulic systems,
- Are able to bear the load of passengers, water, etc.
You could make it an assumption that your new design abides by these standards.
(on the topic of observations and assumptions: don't forget to mention the scale of the graph (e.g. one unit on the graph = 1 meter of displacement), ensure that the graph doesn't contain any negative x-values (as a slide in real life can't have negative length) or y-values (unless your slide goes underground)).
Does your task sheet specifically state that you have to explore the physics behind water slides? If not, you can just state that as a limitation in your design (the fact that physics hasn't been taken into account).
As for designing a safer version and justify how it is safe, I would recommend just simply making the decline more gradual.
You could try graphing something like this:


But adding an extra drop (thats where you can bring in a polynomial function) so it more closely resembles the Verruckt Waterslide.
To justify how safe it would be all I can think of doing is making sure all the parts of you slide (the different functions) meet up. This can be achieved via the use of simultaneous equations. For example:

(the functions in this image are not the same ones in the last - but I used the same process for all three slides)
If you can show that the two functions meet up then you have proven the reasonableness of the design - if the points meet up, the slide is probably safe. I wouldn't want to go on a slide if all of the parts didn't meet up
Hope this somewhat helps, good luck!