# University of Twente Student Theses

## Interface stability in granular open filters in unidirectional flows: investigating the required minimum filter layer thickness for stable geometrical open filters in unidirectional flows.

Joustra, R
(2013)
*Interface stability in granular open filters in
unidirectional flows: investigating the required minimum filter layer thickness for stable geometrical open filters in unidirectional flows.*

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Abstract: | A single-grading stable geometrical open filter is a measure of granular material to protect a bed or construction against scour and erosion. A stable geometrical open filter must have a sufficiently large filter diameter to prevent the shear failure (Chiew, 1995) and a sufficiently large relative layer thickness to prevent transport of bed material through the pores of the filter (winnowing failure (Chiew, 1995)). This thesis focuses on the interface stability in a unidirectional current, e.g. prevention of the transport of bed material through the pores of the filter. The design formula of Hoffmans (2012) can be applied to calculate the minimum required layer thickness to prevent winnowing. This formula is based on the philosophy of simultaneous erosion of the filter material and the bed material, e.g. filter material and bed material are eroded at the same external load conditions. Load damping coefficient αd within the formula of Hoffmans (2012) is the parameter which describes turbulent kinetic energy damping by the filter. The value for αd should be increased when the minimum filter layer thickness is insufficient to prevent winnowing. Van de Sande (2012) recently modified the formula and concluded that the formula is valid for uniform flow conditions. Theoretically the formula is also valid for non-uniform flow (e.g. conditions with additional turbulence) (Hoffmans, 2012), however this has not yet been confirmed with laboratory experiments. Firstly, indicatory results by Van de Sande (2012) show that bed mobility increases for conditions with additional turbulence (non-uniform flow). Secondly, a few results of laboratory tests by Van Velzen (2012) indicated a probable validity of flow with a cylindrical pier (non-uniform flow). Thirdly, Wörman (1989) developed a similar design formula for the layer thickness of geometrical open filters in flows with a cylindrical pier. The similarity of the design formula of Wörman (1989) and Hoffmans (2012) is the design philosophy of simultaneous erosion of filter and bed material. The main difference is that Wörman found a linear relation between the filter and bed grain characteristics and the minimum filter layer thickness, while Hoffmans (2012) described that relation with a logarithmic function. The formula of Wörman is based on experiments of flow at a cylindrical pier and is only tested for small layer thicknesses (Df <0.1 m) and low flow velocities (ū < 0.5 m/s). Recently, a database became available with experiments performed by Joustra (2012) and conducted at the research institute Deltares. The test data give the possibility to test the validity of the by Van de Sande (2012) modified version of the formula of Hoffmans (2012) for (1) uniform flow, and for non-uniform flow conditions in cases of (2) sill-induced additional turbulence and (3) flow with a cylindrical pier. In addition, the database provides the possibility of testing the validity of the formula of Wörman (1989) for flow velocities over 0.5 m/s and layer thicknesses over 0.1 m at the cylindrical pier. The aim of this thesis is: To test the validity of the design formula of Hoffmans (2012) for flows with sill-induced additional turbulence, and flows with a cylindrical pier and to test the validity of the design formula of Wörman (1989) for flow velocities over 0.5 m/s and filter layer thicknesses over 0.1 m at flows with cylindrical piers. First, the tests conducted by Joustra (2012) are redistributed into flow categories (1) uniform flow, (2) flows with sill-induced additional turbulence and (3) flows with a cylindrical pier. Second, the filter material instability and bed material instability are classified separately. This separate classification is determined for each test with visual observation using underwater camera images and processed videos. Third, the separate classification is combined into a general classification. In addition, the bed material instability classification is verified with 3D Stereo photography images for conditions (1) and (2). The simplified (αd = 0.86) and full version (αd = 0.82) of the formula of Hoffmans (2012) are compared with the general classification for respectively flow condition (1), (2) and (3). The formula of Wörman (1989) is compared with the general classification for tests with condition (3). Finally, results based on data of Joustra (2012) are compared with previous validation results (Van de Sande, 2012 and Van Velzen, 2012) based on data of Van Velzen (2012) for flow condition (3). First, the data of Joustra (2012) with condition of uniform flow are in agreement with the formula of Hoffmans (2012) with the load damping coefficient αd = 0.82 for the full version and αd = 0.86 for the simplified version. This result is based on one single test that could be classified and selected for validation and an expected classification of three additional tests when the flow velocity would have been further increased. Second, the data of Joustra (2012) with flows with sill-induced additional turbulence suggest to increase the load damping coefficient αd = 0.82 for the full version and the αd = 0.86 for the simplified version of the formula of Hoffmans (2012). A rough estimate of the new load damping coefficient for both versions of the formula and flows with sill-induced additional turbulence is probably within the range 1.2 < αd < 2.5, but additional research is highly recommended due to the uncertainty in results and scarcity of tests. Third, data of Joustra (2012) for flows with a cylindrical pier suggest to increase the load damping coefficient αd = 0.82 for the full version and the αd = 0.86 for the simplified version of the formula of Hoffmans (2012). A new estimate of αd for flows with a cylindrical pier is probably within the range 2.4 < αd < 3.7. Fourth, data of Joustra (2012) for flows with a cylindrical pier suggest also that the formula of Wörman estimates the minimum required layer thickness reasonably well for average flow velocities over 0.5 m/s and layer thicknesses over 0.1 m, but the gradient (or coefficient 0.16 [-]) of the linear formula of Wörman (1989) should be changed to a gradient probably in the range between 0.22 [-] and 0.33 [-] to be in agreement with test data of Joustra (2012). Fifth, the results for the formula of Hoffmans (2012) and Wörman (1989) based on data of Joustra (2012) are not in agreement with the conclusions from the previous validation by Van Velzen (2012) and Van de Sande (2012), which are based on data of Van Velzen (2012). A probable cause is that the classification of combined filter and bed instability as applied by Van Velzen (2012) and Van de Sande (2012) is not in agreement with the design philosophy (simultaneous erosion) of both design formulas, i.e. filter, bed or both should be instable to compare a test with the both design formulas. Finally, it is recommended to optimize the load damping coefficient for uniform flows, highly recommended to test the validity of the roughly estimated range of the load damping coefficient αd for flows with sill-induced additional turbulence and recommended to determine the characteristic load damping in the filter for flows with a cylindrical pier. For design practice it is recommended to apply the safe upper limit of the load damping coefficient αd = 0.86 as proposed by Van de Sande (2012) for uniform flow, an αd = 2.5 for flows with sill-induced turbulence could be applied after additional validation and for flows with a cylindrical pier the formula of Wörman (1989) with a gradient of 0.33 should be preferred above the formula of Hoffmans (2012) with αd = 3.7. |

Item Type: | Essay (Master) |

Clients: | Deltares |

Faculty: | ET: Engineering Technology |

Subject: | 56 civil engineering |

Programme: | Civil Engineering and Management MSc (60026) |

Link to this item: | https://purl.utwente.nl/essays/63002 |

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