Veranstaltungsprogramm

The relevance of non-uniform rainfall for the results of urban drainage design is often insufficiently discussed and underestimated. Using the example of the combined sewer system model of the city of Basel, the influence of non-uniform rainfall on the discharge behaviour is investigated on the basis of a high-quality radar long-term event time series. The results are compared with a uniform rainfall from a rain gauge observation. The radar rainfall analysis shows a clear pattern in the spatial rainfall distribution with lower rainfall heights in the heavily paved city centre compared to the neighbouring settlement areas in the transition area to natural, unsealed surfaces. The differences for the total rainfall height are about 10%; for statistical heavy rainfall heights of convective events, the differences are more pronounced and amount to 30% - 40%. The small scale spatial rainfall distribution has also an effect on the results of the hydrodynamic pollution discharge modelling. In the overall view of the drainage system, the pollution discharge for Ammonium (NH₄ kg /a) is reduced by 15% compared to the uniform rainfall from the rain gauge; the number of discharge days is reduced by 11%.

Urban drainage practitioners and academics increasingly recognise the value of using continuous rainfall/outflow data to derive a comprehensive understanding of how vegetated Sustainable Drainage Systems (SuDS) respond to long time-series rainfall inputs. For event-based analysis, an agreed definition of ‘an event’ is crucial. Typically events are defined based on a Minimum Inter-Event Time (MIT). The objective of this research was to understand how the specific MIT impacts upon event metrics and on the selection of suitable initial conditions for design storm simulations. This was achieved using modelled outflow from a representative green roof (GR) and bioretention cell (BIO) combined with MITs of 6, 12, 24 and 48 hours. The choice of MIT used to separate events in rainfall/outflow analysis has an impact on the derived performance metrics, particularly for devices offering significant detention. The analysis has also highlighted that – for SuDS that do not include infiltration – the most probable retention storage is small, around 5 mm for typical green roofs and 0 mm for bioretention cells in a temperate climate.

Recently, outputs from convection permitting climate models started to be available for some regions. Unlike traditional climate models, they solve deep convection which make them relevant to provide the high spatial and temporal resolutions needed in urban hydrology. In this study, we compare the ability of such models to replicate hourly rainfall patterns for a few European locations with diverse climates, and we compare future rainfall produced by these models to outputs from a statistical downscaling model to quantify their added value for urban drainage studies.

Commercial microwave links (CMLs) have recently shown great potential in urban drainage modelling due to their ability to provide rainfall-runoff dynamics. Previous studies typically used mechanistic hydrodynamic models driven by quantitative precipitation estimates (QPEs) derived from CML attenuation data. Naturally, some errors related to CML rainfall retrieval are introduced, including uncertainties in wet antenna attention correction and path-averaged QPEs, which result in a loss of valuable information in raw data. To address these issues, we employ raw CML attenuation data without QPE derivation into a Random Forest (RF) model to estimate urban runoff. We find that: (1) RF integrates mean values from CMLs and rain gauges achieved an average NSE of 0.80, PCC of 0.90, RMSE of 38 L/s, and RE of 0.01 across all cross-validation iterations. (2) RF model using raw CML data (RF-CML) yields results comparable to the SWMM model benchmark, especially in high-value discharges. (3) RF-CML enables runoff prediction beyond the catchment's rainfall-runoff concentration time, maintaining PCC > 0.82 for forecasts up to 30 minutes. These results demonstrate that CML raw data can accurately yield runoff dynamics and volumes, and it can be used for short-term runoff predictions.

Traditional urban drainage system design typically relies on a single design storm, which may not adequately capture the range of potential rainfall patterns. This study builds upon previous research investigating the effects of hyetograph shape variability on system performance by introducing a multi-storm design approach. We examined the robustness of urban drainage system designs by comparing systems developed using single and multiple design storms. Our methodology involved investigating model responses to an ensemble of 1000 hyetographs with varying temporal patterns but consistent duration and total precipitation. Students designed systems using a computer model, optimizing pipe dimensions, retention basin volume, and weir dimensions while minimizing costs and preventing flooding. We compared designs created with one versus three design storms. Results revealed that systems designed with three design storms showed lower flooding and overflow volumes, with average reductions of 75% and 35% respectively compared to single-storm designs. However, the overall system robustness was only marginally enhanced. This research highlights the limitations of current urban drainage system design approaches and suggests the need for more comprehensive methodologies to address climate variability and design uncertainty.

Lead local flood authority Central Bedfordshire Council (CBC) faced an overwhelming emergency response following unprecedented rainfall (up to 148mm/day) in September 2024. In a changed climate, we can read CBC as “any lead local flood authority” and the lessons drawn from two weeks of 24-hour emergency response as widely applicable.

In this paper we evaluate the response to the back-to-back storm events which occurred between 21-29 September 2024 and delivered rainfall totalling 396% of the monthly average, overwhelming watercourses and infrastructure and flooding over 200 homes. We present the key findings from interviews with emergency responders from a range of partner organisations conducted within weeks of the emergency occurring.

CBC was in the unusual position of having one highly sensored catchment, as a result of its participation in a government-funded flood innovation pilot programme. Flood risk managers’ experience of having access to this data was positive, highlighting the benefits of sensor-driven flood monitoring and prediction systems for emergency responders.

Key results highlight challenges in handling public enquiries, limitations in current flood warning systems, and the potential of sensors and a streamlined flood reporting platform to enhance response efficiency. Recommendations include expanding sensor coverage and integrating a centralised public reporting framework.