Modeling of sediment transport and self-cleansing of sea outfalls

Abstract

I dette afgangsprojekt beskrives transportfænomenet for ikke-kohæsivt sediment i marine udløbsledninger set fra en eksperimentel og numerisk synsvinkel. Det ikke-kohæsive sediment anvendt i dette studie var fint sand (Sand No. 0000) fra en grusgrav nær Horsens i Danmark. Såvel udløbsledninger som kloakker har traditionelt været designet efter konceptet om selvrensning, hvor sedimenter forventes at flytte sig kontinuert uden deposition. Ikke destromindre opstår der deposition af materiale i disse systemer, specielt under perioder med lav vandføring. De to centrale elementer i projektet er udviklingen af en numerisk model og en tilsvarende fysisk model i laboratoriet. Den numeriske model dækker sedimenttransport over såvel akkumuleret sediment som sedimenttransport over sidementfri bund. Eksperimenter med sedimenttransport (specielt bundtransport) af ikke-kohæsivt sediment der følger kriteriet for selvrensning blev udført i såvel et 240mm fuldtløbende rør som i en 300mm bred og 300mm dyb strømrende. En éndimensionel numerisk model blev udviklet til beskrivelse af sedimentfordelingen røret såvel som forudsigelse af transporttiden og hastigheden der skulle til for at borterodere allerede opbygget sidement og holde rører rent. Således blev den numeriske model udviklet med det formål at beskrive et selvrensningskriterie der stemmer overens med resultaterne af den fysiske model. Transporten af ikke-kohæsivt sediment blev modelleret ved løsning af kontinuitetsligningen for bundsediment. Ackers-White sedimenttransportteori blev anvendt i beskrivelsen af den ikke-kohæsive sedimenttransport. Modelen blev kalibreret med data far den fysiske model. Som en del af projektet blev der udført en artikel omhandlende modellering af sedimenttransport i udløbsledninger med fokus på selvrensningsproblematikken på grund af de daglige vandføringsvariationer der ses i disse systemer. Artiklen indeholder resultater fra både den fysiske og den numeriske model.

The present thesis describes the transport phenomena of non-cohesive sediment in sea outfalls from an experimental and numerical point of view. The non-cohesive sediment used during the course of the whole study was fine and artificial sand from a gravel pit near Horsens in Denmark denoted as Sand N0. 0000. Outfalls as well as sewers have been designed on the concept of the self-cleansing criteria, where sediments are expected to move continuously without deposition. Nevertheless, due to erratic nature of flow deposition of solids in these systems occurs, particularly at low flows periods. The two central elements of the project is the development of the numerical model and a matching physical model in the laboratory. The numerical model covers both sediment transport over bed accumulations as well as transport over clean bottom. The physical modeling emphasizes on measurement of the non-steady removal and transport of well-defined and limited accumulations along the pipe. Experiments on sediment transport (especially bed load) of non-cohesive sediments following “without deposition” criteria were carried out in both a pipe of 240 mm dia. and a rectangular flume of 300 mm in width and 300 mm in depth, both running full. A one dimensional numerical model was developed describing the sediment distribution along the pipe as well as predicting the transport time and velocity needed to erode and clean the pipe. Hence, numerical modeling was developed with the purpose to find an approach to formulate the self-cleansing criteria with a formula that included in the model, will give relatively similar results as experimental data. The transport of non-cohesive sediment is modeled by solution of the continuity equation for bed sediments. Ackers – White sediment transport theory was implemented in the description of the non-cohesive sediment transport. The model was calibrated using the laboratory experiments. As part of the project, a paper was written in describing modeling of sediment transport in outfalls, with focus on the self-cleansing problem occurring due to the daily flow variation seen in these water structures. The paper contains results from both the physical and the numerical modeling.

A master thesis from Aalborg University

Modeling of sediment transport and self-cleansing of sea outfalls

[Modellering af sedimenttransport og selvrensning af havet udløb]