Tools and Methods to inform Planning and Design of Nature Based Stormwater Control Measures

Stormwater sewers are one of several essential infrastructure systems that enable life in modern cities. In recent years, a combination of increased urbanization and changes in precipitation patterns has challenged the capacity of existing sewer systems, and is expected to increasingly do so in the future.

Nature based stormwater control measures (SCMs) have shown potential to help meet these challenges while also improving the sustainability of the stormwater system and liveability in the city. The Three Point Approach (3PA) has proven a useful tool for improving communication among the different types of professionals who plan and design SCMs.

It defines three distinct domains of stormwater management: domain A is the domain of “day-to-day values”, domain B is the domain of “technical optimization”, and domain C is the domain of “extreme events”. The bottom line message of the 3PA is that it is important to address all three domains whenever new projects are considered.

Unfortunately, this is rarely done in practice, and especially domain A seems to be neglected. The overall objective of this PhD project was to develop tools to assist different professionals in planning and designing nature based SCMs. The tools were to be simple and easy to use, provide essential quantitative information about the impact of SCMs on a site’s hydrology, and build upon the proven communication capabilities of the 3PA.

To this end, we investigated which tools are already available and how useful they are; we explored ways to “operationalize” the 3PA; and we developed methods for condensing the most essential information from complex hydrological models to tangible, intuitively logical indicators. Our literature review of existing tools showed that there is a myriad of tools.

We proposed categorizing them according to what type of questions they could assist in answering: “how much” (water, pollution, etc. can be managed via SCMs), “where” (to situate SCMs) and “which” (SCM is the best); or any combination hereof. Other variations among the tools include what degree of complexity they include in their processing, what degree of expertise they require from the user, what type of professional background of the user they are designed for, whether they are most useful for making overall planning strategies or for designing specific solutions, and much more.

We concluded that although there are many tools out there already, the variability in the demands that shape them also entails that there is ample room for more tools. The first tool we developed was a quantitative version of the 3PA. We defined a return period for each domain, and through analysis of historical rainfall records calculated a rainfall depth that can be considered representative for the upper boundary of each domain.

We also added a second vertical axis that notes each domain’s share of the annual rainfall. This shows that in Denmark, although the upper boundary of domain A is only 20 mm, the domain includes 75% of the annual rainfall, which illustrates the large potential that lies in this domain. The second tool, the Characteristic Rain Events (CREs), was developed specifically to draw attention to the aesthetic performance of SCMs during times where there is only little water in them, i.e. in domain A (which is the bulk of the time).

The CREs were carefully chosen among historical rain events to give designers tangible manifestations of frequent rains. These can be used to assess how much rainwater becomes visible in an SCM under frequent events, thus improving the day-to-day aesthetic value of the SCM. For situations where strategic planning requires thorough understanding of possible retrofit options’ impact on complex stormwater systems, we demonstrated how the 3PA can be used to structure state-of-the-art distributed modelling studies and their output.

Our method includes running long term simulations for quantifying water balance impacts as an indicator for performance in domain A, and presenting this alongside single quantitative indicators for domain B (sewer surcharge) and C (surface flooding), calculated from results of traditional short term simulations using the design storm approach.

Results of using this method on a case study show that this may highlight sustainability gains that would otherwise be ignored. The third tool, SCM-potential, suggests a new method for quickly calculating SCM impact on two key hydrological indicators, designed to give professionals interactive feedback on their site design choices.

Here we used the 3PA as a visual framing of the first key indicator, which shows the return period of overflow from the site. The second key indicator shows how the design will impact the annual water balance of the site. Both indicators are quickly returned by the tool thanks to tabulated values of key results from long term hydrological simulation of SCMs.

In this manner we make essential hydrological information more easily accessible to professionals in situations where time or resources do not allow for setting up complex hydrological models, such as the early stages of a design process