Department Urban Water Management

Sewer and drainage infrastructure are often not as well catalogued as they should be,considering the immense investment they represent. In this work, we present a fully automaticframework for localizing sewer inlets from image clouds captured from an unmanned aerial vehicle(UAV). The framework exploits the high image overlap of UAV imaging surveys with a multiviewapproach to improve detection performance. The framework uses a Viola–Jones classifier trained todetect sewer inlets in aerial images with a ground sampling distance of 3–3.5 cm/pixel. The detectionsare then projected into three-dimensional space where they are clustered and reclassified to discardfalse positives. The method is evaluated by cross-validating results from an image cloud of 252 UAVimages captured over a 0.57-km² study area with 228 sewer inlets. Compared to an equivalentsingle-view detector, the multiview approach improves both recall and precision, increasing averageprecision from 0.65 to 0.73. The source code and case study data are publicly available for reuse.

Increasing urbanization makes it more and more important to have accurate stormwater runoff predictions, especially with potentially severe weather and climatic changes on the horizon. Such stormwater predictions in turn require reliable rainfall information. Especially for urban centres, the problem is that the spatial and temporal resolution of rainfall observations should be substantially higher than commonly provided by weather services with their standard rainfall monitoring networks. Commercial microwave links (CMLs) are non-traditional sensors, which have been proposed about a decade ago as a promising solution. CMLs are line-of-sight radio connections widely used by operators of mobile telecommunication networks. They are typically very dense in urban areas and can provide path-integrated rainfall observations at sub-minute resolution. Unfortunately, quantitative precipitation estimates (QPEs) from CMLs are often highly biased due to several epistemic uncertainties, which significantly limit their usability. In this manuscript we therefore suggest a novel method to reduce this bias by adjusting QPEs to existing rain gauges. The method has been specifically designed to produce reliable results even with comparably distant rain gauges or cumulative observations. This eliminates the need to install reference gauges and makes it possible to work with existing information. First, the method is tested on data from a dedicated experiment, where a CML has been specifically set up for rainfall monitoring experiments, as well as operational CMLs from an existing cellular network. Second, we assess the performance for several experimental layouts of "ground truth" from rain gauges (RGs) with different spatial and temporal resolutions. The results suggest that CMLs adjusted by RGs with a temporal aggregation of up to 1 h (i) provide precise high-resolution QPEs (relative error <7%, Nash–Sutcliffe efficiency coefficient >0.75) and (ii) that the combination of both sensor types clearly outperforms each individual monitoring system. Unfortunately, adjusting CML observations to RGs with longer aggregation intervals of up to 24 h has drawbacks. Although it substantially reduces bias, it unfavourably smoothes out rainfall peaks of high intensities, which is undesirable for stormwater management. A similar, but less severe, effect occurs due to spatial averaging when CMLs are adjusted to remote RGs. Nevertheless, even here, adjusted CMLs perform better than RGs alone. Furthermore, we provide first evidence that the joint use of multiple CMLs together with RGs also reduces bias in their QPEs. In summary, we believe that our adjustment method has great potential to improve the space–time resolution of current urban rainfall monitoring networks. Nevertheless, future work should aim to better understand the reason for the observed systematic error in QPEs from CMLs.

Seit ca. 10 Jahren folgt die Planung von Anlagen zur Mischwasserbehandlung in der Schweiz den imissionsorientierten Vorgaben des Gewässerschutzgesetzes. Positiv wird beurteilt, dass Handlungsbedarf auf Basis von Gewässeruntersuchungen identifiziert wird und eine breite Evaulation von Massnahmen nicht nur zum besseren Verständnis der Abwassersysteme, sondern oft zu kostengünstigen Lösungen führt. Aufgrund von sich überlappenden Regelwerken und neuem Wissen, u.a. zu Spurenstoffen, wird zurzeit die neue Richtlinie zur „Abwasserbewirtschaftung bei Regenwetter“ erarbeitet, die vor allem auch den Vollzug erleichtern soll. Dazu stellt sie 4 neue Elemente vor: i) die vereinfachte Zulässigkeitsprüfung, ii) Bewertung von Einleitungen in stehende Gewässer, iii) schmutzfrachtbasierte Mindestanforderungen und iv) die Möglichkeit eines Probebetriebs mit Überwachung. Die Erfahrung in der Schweiz zeigt, dass die immissionsbasierte Bewirtschaftung von Abwassersystemen anspruchsvoll ist, aber teilweise zu innovativen Lösungen führt. Der „Preis“ solcher komplexer Planungsinstrumente sind hohe fachliche Anforderungen bei Betreibern, Planern und Regulatoren, um inadequate Massnahmen zu verhindern. Ein guter Wissensstand in der Praxis kann neben Weiterbildungen gegebenenfalls über fachspezifische „Nachbarschaften“ nach dem Modell von Baden-Württemberg erreicht werden. Offene Fragen sind, wie kleine Gewässerschutz-Behörden am besten bei der Bewilligung von Massnahmen unterstützt werden können und wie zukünftige Entwicklungen zum vermehrten Einsatz von Messtechnik den Gewässerschutz bei Regenwetter verbessern können.