Development of a screening tool for risk assessment of groundwater infiltration into sewer systems - including capillary fringe induced rapid groundwater rise during rain

Abstract

Problemet med uvedkommende vand er kommet i stigende fokus i de seneste år på grund af dets effekt på spildevandsmængder. Derfor blev projektet Ternært Vand startet i 2022, hvor projektet fokuserede på den del af uvedkommende vand, der stammer fra grundvandsinfiltration. Under andet år af Ternært Vand-projektet arbejdede denne kandidatafhandling med hypotesen om, at grundvand infiltrerer ind i afløbssytemet pga. stigninger i grundvandsspejlet. For at hjælpe beslutningstagere med risikovurdering af grundvandsinfiltration udviklede denne kandidatafhandling et screeningsværktøj, hvor den danske by Beder blev brugt som caseområde. Her var hypotesen, at infiltrationen skyldtes hurtigt stigende grundvand som steg op til afløbssystemet under en regnhændelse grundet dynamikker i kapillærzonen. Grundvandsstigningen blev modelleret med en 1D numerisk model for vandføring og grundvandsstigning i umættet jord. Udover at blive brugt i screeningsværktøjet blev 1D modellen også udvidet til en 2D model. Begge modeller blev kalibrerede og validerede mod sandboksforsøg, som undersøgte disproportional grundvandstigning og den resulterende infiltration ind i et nedgravet rør. Ét af de fem eksperimenter viste en tydelig disproportional stigning, hvor de resterende fire også blev betragtet som at have vist disproportionale stigninger, men pga. eksperimentets udformning kunne det ikke definitivt konkluderes. Begge modeller viste et tilstrækkeligt fit til de målte data, hvor både 1D og 2D modellen kunne estimere hurtig og disproportional grundvandsstigning og 2D-modellen kunne yderligere estimere den infiltrerede mængde vand. Alt i alt konkluderede denne kandidatafhandling at screeningsværktøjet ikke kunne valideres for caseområdet, dog viste værktøjet de forventede dynamikker i Beder og det konkluderedes derfor, at det kan bruges i praktiske applikationer som et supplement i dimensioneringsprocessen.

The issue with extraneous water has come into increased focus in recent years due to its effect on wastewater loads. Therefore, the project Ternært Vand was started in 2022 and focused on the part of extraneous water originating from groundwater infiltration. Being part of the second year of the Ternært Vand project, this Master’s thesis worked with the hypothesis of groundwater infiltrating into pipes due to increases in the groundwater table. To aid decision-makers in risk assessment of groundwater infiltration this Master’s thesis developed a screening tool using the Danish city Beder as a case area. Here, the infiltration was hypothesised to originate from a capillary fringe induced rapid water table rise submerging the sewer system during a rain event. The increase was modelled with a 1D numerical model of water flow and groundwater rise in unsaturated soil. In addition to being used for the screening tool the 1D model was also expanded to a 2D model. Both the 1D and 2D model were calibrated and validated against sandbox experiments which investigated disproportionate increases in the groundwater table and a subsequent infiltration into a buried pipe. One out of five of the experiments showed a distinct disproportionate rise. The remaining four experiments were also considered to have shown disproportionate increases but due to the experiment setup it could not be definitively concluded. Both models of the sandbox showed a sufficient fit with the measured data where the 1D and 2D model could estimate rapid and disproportional groundwater rise and the 2D model could also estimate the infiltrated amount of water. Overall, this Master’s thesis concluded that the screening tool could not be validated for Beder, but showed the expected dynamics in the case area and could be used in practical applications as a supplemental approach.

A master thesis from Aalborg University

Development of a screening tool for risk assessment of groundwater infiltration into sewer systems - including capillary fringe induced rapid groundwater rise during rain