Modeling of sediment transport and self-cleansing of sea outfalls

Student thesis: Master thesis (including HD thesis)

  • Ina Ibro
4. term, Water and Environment, Master (Master Programme)
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.
LanguageEnglish
Publication date17 Jun 2011
Number of pages53
Publishing institutionAalborg Universitet
ID: 53008275