Sensitivity analysis

Since my last message I have completed a number of steps in the modelling process.

I have been carrying out model sensitivity analysis for Overflow. Basically this process allows me to identify how the model behaves in terms of how important each variable or parameter is to the model behaviour. So far this process has been relatively exploratory.

The model contains a number of parameters such as;
• Channel, floodplain and catchment roughness values – set as Manning’s n values. Roughness affects conveyance
• Rain time maps
• Rainfall input
• Channel depth and width equations.

Each of these parameters has a default value. For example the channel network has a default Manning’s n value of 0.035. (For reference a Catchment Riparian Intervention Measure (CRIM), consisting of a debris dam and buffer strip, is likely to be given Manning’s n values of around 0.16 and 0.14 for the floodplain and channel respectively.)

My sensitivity analysis so far has involved varying the values of each key parameter one-at-a-time (OAT), whilst keeping the values of the other parameters fixed. A model simulation is run and the results recorded. Each simulation takes just under 10 minutes at the moment. The effect of varying each parameter on the model output (discharge) can then be analysed. The simulated discharge is seen to vary to a relatively high degree as a result of varying time maps and Manning’s n values; however it’s important to note that the parameters were varied over a large range.

The next step involves running simulations where every key parameter is varied at the same time, with values selected randomly. The results will show how the influence of each parameter relies on the interactions of the model parameters.

The ultimate aim of the above process is to narrow the range of potential values each parameter can take on, to identify and quantify the uncertainty in the model and its parameters.

Secondly I have run simulations where a CRIM is added to a single reach (234 separate reaches have been identified in the catchment). This is done by changing the Manning's n values of the reach to 0.16 for the floodplain and 0.14 for the channel. 234 simulations have to been run, with a CRIM added to a different reach each time. Whilst the sensitivity analysis is being carried out I will be able to have a first look at which locations it would be beneficial to add a CRIM, in terms of a reduction is peak flood discharge.

Again please feel free to email me with any questions you have at: edward.byers@durham.ac.uk. I hope to update the blog more frequently in order to allow readers a more detailed and better understanding of the development of my study.

Thanks, Ed