CRIM uncertainty analysis

The results of investigation into combinations of Catchment Riparian Intervention Measures (CRIMs) have been previously presented. The figure below shows the location of the CRIMs throughout the Uck catchment. Those in red have a positive effect in terms of peak flow reduction. Those in yellow have a negative effect in combination!



Uncertainty analysis was carried out. The 52 positive sites (see previous post and results table) were the focus of this analysis. Essentially the roughness of the channel and floodplain along these reaches was varied in order to show how different levels of roughness effect the peak flow at Uckfield during the (simulated) 2000 flood event.




The key finding from the CRIM Uncertainty Analysis was that similarly fine results in terms of peak flow reduction at Uckfield can be achieved when channel Manning's n is ~0.10, as opposed to the default value for a CRIM of 0.14. This suggests a lower level of channel blockage is required for equally positive results.

HOWEVER, IMPORTANTLY, if channel n is only increased to ~0.10, floodplain roughness MUST be increased to a high level (Manning's n > ~0.16) in order to achieve the best results in relation to downstream peak flow.

Similarly, floodplain roughness can be much lower than 0.16, and providing channel n is high (> ~0.14), similarly fine results can be achieved.

it is important to note that this uncertainty analysis is only carried out for the 52 reaches identified from previous work, and will need to be repeated if different combinations of reach are selected.

Table 1: Results from exploration of effects of simulating CRIMs in combination on downstream peak flow (just upstream of Uckfield; based on 2000 flood event). Initial peak flow simulated for the 2000 event (no interventions) was 124.68 cumecs (2 d. p.). The ‘individual effect on peak flow’ is the effect on downstream peak flow of increasing roughness in that single reach. The ‘effect in combination’ is the simulated peak flow downstream as a result of the number of CRIMs added (shown in the final column).

Table 1

The effects of CRIMs applied in combination throughout the Uck catchment - uncertainty analysis to follow

As discussed previously, from the screening runs there are 127 reaches where increasing channel and floodplain roughness to simulate the adding on a Catchment Riparian Intervention Measure (CRIM) decreased downstream peak flow. It was decided that the 100 reaches with the greatest individual impact would be explored in combination.

Brief method:

Essentially for looking at CRIMs in combination I ranked the 100 reaches in order of how effective increasing channel and floodplain roughness (to simulate a CRIM e.g. debris dam and riparian vegetation planting) of that single reach in question, is on reducing downstream peak flow. So, for example the best reach reduces peak flow by ~3.5 cumecs when roughness is increased in just that reach.

A CRIM was simulated along the reach with the largest individual effect. Then a CRIM was also added to the reach with the second largest effect on downstream peak flow - a CRIM was therefore simulated in both of these reaches. If simulating a CRIM in these 2 reaches reduced downstream peak flow by more than simulating a CRIM in one reach both were retained in combination. I then simulated a CRIM in the third reach and so on. Each time a CRIM was simulated the cumulative effect on downstream peak flow was recorded. If simulating a CRIM in a new reach did not increase the positive impact (cumulative peak flow reduction was reduced) this reach was rejected and excluded from future combinations.

The results can be seen in the table accompanying this blog. Please give feedback on the quality of the table.


Issues

You can see from the table that the title 'reach applied (no.)' is highlighted yellow. These numbers are used in the model OVERFLOW to identify each reach and change, amongst other characteristics, roughness values. However they will have no meaning to those not using the model. Producing a figure with each reach numbered is not especially practical considering there are 234 reaches. This creates a bit of a problem for me in communicating the results, especially in terms of where I'm simulating an intervention.

There is also somewhat of a challenge in explaining the methods I have used; the process is likely to be very confusing to others. Hopefully the table will help you see the process I went through.

To think about/uncertainty:

As previously discussed, these results must be viewed with an appreciation of the uncertainty associated with the model and its inputs.

For looking at the combination of CRIMs I have ranked reaches primarily order of effect on peak flow. Whilst a consideration was made of reaches with a positive impact on the flood volume above approximate channel capacity this was not the focus. There may be reaches where increasing roughness reduces excess flood volume by a large amount, whilst having a slightly negative effect on peak flow; I will try and look at such reaches in some form.

The results presented here are for one combination of interventions. A different combination could suggest a different pattern of effects and different key reaches to focus on - this would reduce uncertainty of results/recommendations.

Is simulated roughness higher than might be expected from CRIMs in reality? I hope to get results from simulating the effects of increasing channel roughness by a smaller amount.

Again feedback is much appreciated, especially regarding any problems understanding the method or results, or issues with the presentation. I would like the blog to be as easy as possible to follow so results and the uncertainty can be understood.

New update on results soon...

New update on results soon...

Initial screening results from new calibrated model

As discussed the simulated hydrograph produced by the calibrated model shows a much better fit with the observed hydrograph from the 2000 flood event. In a few weeks (thousands of simulations takes quite a while) I'll be able to show more results on the uncertainty associated with the new calibrated model.

For now here are the initial screening results, similar to those presented a while ago for the old model.

A key aspect is the effect of placing a CRIM at one reach is greatly reduced. No reaches individually reduce flow peak by over 4 cumecs, as opposed to around 10 (depending on the time map used) which reduced peak flow by >5 cumecs with the old model.

One explanation for this is that the simulated peak discharge is more accurate and also lower. Therefore even if the CRIMs were still having the same effect in terms of % reduction of the initial flow peak, the reduction would be less.



Importantly, from initial work with combinations of reaches, it appears that in combination the reaches have less of a reducing effect on peak flow than the sum of their parts. For example, as a simplification, two CRIM sites which on their own reduce simulated peak flow by 3 cumecs each may in combination only reduce peak flow by around 4 cumecs.

These results will be discussed in more depth soon

Background to my project

Please see my first post for a brief context on my project, which will hopefully make the subsequent posts easier to follow for those not familiar with the work

Thanks, Ed

New model and new uncertainty

Following on from my results of the GLUE uncertainty analysis (see previous updates) the decision has been made to use a calibrated version of the Overflow model, with the aim of producing flood hydrographs which more closely match the observed hydrgoraph

Nick has been working hard producing a calibrated version of the Overflow model. The key different is a different rain time map (rainfall rate) is used throughout the storm event.

The result is a much closer match between the observed and simulated hydrograph at Isfield.

However there are are few issues with the new model;

The key issue is that when a hydrograph is simulated for the meadows area upstream of Uckfield, the largest flood peak now has a double peak. This is not expected and is slightly problomatic; the simulated peak discharge upstream of Uckfield now occurs only very slightly before the peak discharge at Isfield downstream, when perhaps a slightly bigger lag would be expected. If the peak were smoother, this would most likely bring the peak discharge just upstream of Uckfield forward a couple of hours.



On the positive side, the peak discharge is much more realistic.

As seems to be the way with modelling, as one aspect gets better, this can create a whole new set of problems. Just like a more comlex model can bring with it just as much uncertainty and a simple model.

Ill discuss the issue of the troublesome flood peak with Stuart Lane and I'm hoping that it wont be too much of a problem. The simulated hydrograph is now certainty much a closer fit to the previous uncalibrated model. Im looking forward to (hopefully) getting some final results I can properly work as time is running short in terms of my role in this study - I think we're all hopeful of some interesting results.

Whilst slightly frustrating to work on the previous model and then have to move on to a slightly different version, my previous work has allowed me to explore the behaviour of the model and get a better understanding of the uncertainties.


I have been now working on getting results of the screening runs (see previous updates) with the calibrated model, looking at the effects of increasing channel and floodplain roughness one reach at a time to represent the adding of a CRIM. Initial results to come...