The DWMP blog – Episode 6. Future uncertainty – is it what? or when?
Martin Osborne
Water industry strategic advisor, asset planner and drainage expert Winner of the 2023 WaPUG Prize for contributions to the development of urban drainage practice
If you haven’t already seen the earlier episodes in this series, I suggest that you start from Episode 1 (https://tinyurl.com/DWMP-blog).
DWMPs are long term plans considering the conditions that will occur 25 years or more into the future.?There is therefore considerable uncertainty in predicting those conditions.?So how do we deal with that uncertainty?
Two useful guidance documents are the DWM guidance itself and BS 8631:2021 Adaptation to climate change — Using adaptation pathways for decision making — Guide.
These each focus on a different single aspect of future uncertainty and consider the other aspects as secondary.?The DWMP guidance recommends ignoring uncertainty for a standard catchment assessment but, as part of the Extended BRAVA, recommends consideration of a ±30% uncertainty on predictions of future population growth.?The BS recommends consideration of uncertainty around climate change as the key aspect with other uncertainties almost as an afterthought.
In practice all of the future trends will have uncertainty of at least ±30%. ?These trends include:
Almost all catchments therefore need consideration of future uncertainty, so how do we do this in a simple and efficient way.
Define scenarios
DWMPs are generally assessed for two future planning horizons: 10 years and 25 years.?
For each of these horizons we can have low, medium and high estimates of change.?Even if we consider only all trends as low, all trends as medium or all trends as high, this gives 6 scenarios.?However, some of those scenarios will be very similar to each other.?For example, the 10-year high change scenario might be similar to the 25-year medium change scenario.?This gives us a situation something like the diagram below.
BS 8631:2021 recommends to change from uncertainty of how much change will happen at fixed planning horizons to uncertainty of when defined levels of change will happen.?This allows us to simplify the number of scenarios.
The 10-year Low scenario is likely to occur well within 10 years and so can be ignored for long term planning.?
The two scenarios shown in green are at the extremes of the range and so are taken to define scenarios A and C to assess risks and options.?For catchments with little expected future change and low uncertainty then these two scenarios will be adequate.?For most catchments, another scenario B should be defined with input data for each future change at the mid-point between A and C.?For example, if A has 10% increase in rainfall intensity and C has 30% increase then B should be defined with 20% increase.?
For particularly complex situations we could define additional intermediate scenarios with different balances of the future changes.?For example, one without population increase but with climate change.
The options for Scenario A will be needed in about 10 years, the options for Scenario C will not be needed for 25 years at the earliest and Scenario B will be required somewhere in between.?We produce plans for each scenario but then monitor when they need to be implemented depending on the rate of change.
We have therefore reduced to at most three scenarios that are able to represent the range considered in the Extended or Complex assessments of the DWM guidance, but with only the level of effort of the Standard assessment.?
Summary
Almost all catchments will have significant uncertainty of future conditions.?By borrowing ideas from BS 8631 on climate change adaptation we can consider this without increasing the effort that is required to produce the DWMP.
Questions
Which do you think are the most significant future uncertainties?
How do you think we should monitor when to implement the next scenario?