Journal of Coastal and Hydraulic Structures

Optimising the wave attenuation of bamboo fences using the numerical wave model SWASH

marijn alferink — 2023-09-25

The coastline of Demak, Indonesia, has been eroding during the
last 15 years. Coastal retreat in Demak is caused by a combination
of mangrove deforestation and local subsidence due to groundwater
extraction in the nearby city of Semarang. To restore the lost mangrove
forest, permeable dams, consisting of bamboo poles with a
brushwood filling, have been built to attenuate the waves, facilitate
sedimentation at their land side, and thus create a suitable habitat
for mangroves. However, existing designs required frequent brushwood
maintenance. Therefore, a new type of design is proposed,
consisting of only vertical bamboo poles without a filling of brushwood.
Nevertheless, the hydrodynamic performance of this type of
structure is not known. This study assesses the wave transformation
through structures formed by bamboo poles for the physical
conditions of Demak, Indonesia, with the numerical wave model
SWASH. Field measurements and WaveWatch III data are analyzed
to obtain the design conditions for the structures in Demak.
SWASH is validated against laboratory experiments, and applied
to investigate different structure designs. The model shows that for
a structure consisting of two rows of bamboo poles, the transmission
rate Et/Ei decreases from 75% to 55% when the row spacing
in the wave direction is increased from sx = 0.42 m to sx =5.8
m. Even larger spacings do not result in less transmission, and
at least three rows are needed to have a transmission rate lower
than 50 % - a common wave reduction target used in restoration
efforts with structures. This study thus identifies potential strategies
to maximize wave attenuation by bamboo structures, which
can be used to reduce wave attack along muddy coasts without
the need of a brushwood filling. Hereby it provides an economically
and user friendly alternative with respect to the previous
brushwood structure designs, as it requires less material costs and
maintenance.

CFD modeling of flow and local scour around submerged bridge decks

Gábor Fleit — 2023-09-21

In this study, the level set method-based, multiphase hydro- and morphodynamic numerical model REEF3D is used for the simulation of the flow conditions and local scouring around submerged bridge decks. Presence of hydraulic jumps, bridge overtopping, pressurized jets and the resulting scouring make the selected test cases especially challenging from the numerical modelling point of view. The models are validated against data from laboratory experiments. The influence of the submergence ratio on the prevailing flow and scouring is investigated. The level set method showed robustness and good accuracy for the treatment of the complex free surface as well as for the tracking of the mobile bed. The submergence ratio showed no clear correlation with the prevailing erosion/deposition patterns. As the simulations offered insights into the prevailing hydrodynamics and thus the relevance of numerical modelling was emphasized for such complex sediment transport problems.

Driftwood Accumulation and Passage at V- and I-Rock Weirs in Mountain Streams

Brian Mark Crookston — 2023-08-18

The transport and accumulation of driftwood, large wood, or large woody debris (LWD) in mountain streams is a natural part of catchment health and river connectivity. At hydraulic structures, the presence of driftwood has impacts on total discharge and upstream energy. Driftwood has been studied at a variety of spillways and weir types; however, little is known about its interaction at rock weirs. This study seeks to determine what factors affect the transport of driftwood and potential upstream impacts of driftwood accumulations at rock weirs through field-informed scaled model testing. Observations of driftwood at rock weirs located on the Blacksmith Fork River, a mountain stream located in Utah, USA, were used to replicate driftwood dynamics in V- and I-shaped rock weirs in a large flume. The river response to rock weirs on the corresponding section of the Blacksmith Fork River was also investigated using historic aerial imagery and field data. Approaches to driftwood management typically prioritize either natural processes or hydraulic structure safety and flow conveyance. A new hybrid approach should consider both aspects for rock weirs in mountain streams.

On the design of bank revetments at inland waterways subjected to ship-induced water level drawdown: A probabilistic infinite slope analysis

Julia Sorgatz — 2023-05-16

To protect embankments along German inland waterways against local slope sliding failure caused by ship-induced water level drawdown, they are mainly secured by bank revetments. Often, large embankment sections are designed on the basis of a limited number of field and laboratory tests. Thus, uncertainties arise with regard to the mechanical and hydraulic ground properties. Current design standards account for these uncertainties by conservative design assumptions and empirical knowledge. This paper investigates the effects of vertically non-homogeneous ground properties on the required armour layer thickness using 1D random fields and an infinite slope model, which was modified to account for ship-induced drawdowns. Within the limitations of the infinite slope assumptions, the effects of a spatially variable friction angle and hydraulic conductivity are investigated and compared to deterministic benchmark cases. The investigations show that the level of safety obtained with the deterministic design depends strongly on the choice of the characteristic values. Particularly, the hydraulic conductivity determines the reliability of the design. In some cases, the 5 % quantile of the hydraulic conductivity does not yield a conservative estimate of the required armour layer thickness. In the case of the effective friction angle, the 5 % quantile may overestimate the required armour layer thickness for permeable soils. For less permeable soils, the 5 % quantile meets the solution of the random field analyses. For the combination of random effective friction angle and random hydraulic conductivity, all investigated benchmark studies seem to ensure engineering safety, but on different reliability levels. Based on these findings, recommendations regarding site exploration and choice of characteristic values of hydraulic conductivity and effective friction angle are provided.

Investigating hydraulic loads on a crossbar block ramp using two different computational fluid dynamics models and a physical validation model

Philipp Helmer — 2023-05-09

Crossbar block ramps provide bed stability and facilitate ecological connectivity in rivers. Two major sources of uncertainty in determining their design loads are their massively turbulent flow and backwater influence.

Here, they were addressed using two computational fluid dynamics (CFD) models (created with OpenFOAM) of a complete crossbar block ramp by recording loads (forces and moments) on single crossbars. To model turbulence, the first model used the Reynolds-averaged Navier-Stokes (RANS) approach, and the second model used Large Eddy Simulation (LES). The flow in both models was transient and the free surface was tracked. Their mesh was identical and consisted of about 30.3 million cells distributed on 480 processor cores. The computed sampling interval was 180 s. The CFD models were tested against measurements of water level and pressure from a physical model. All models were in the same scale, 1: 20/3. Three discharges representing three typical flow regimes were studied in each model.

Characteristics of the flow regimes were reproduced in all models. The RANS model was up to 5.3 times faster, but produced excessive waves, which likely caused over- and underestimation of crossbar loads. The LES model showed good agreement to the physical model and could be used for load predictions. However, a longer sampling interval as well as a larger variety of discharges would be required to obtain stochastically reliable estimations for the maximum loads. Both models indicated that unsteady waves in the wake-interference flow regime must be considered to find critical loads. The results can support decisions on the methods for future investigations of hydraulic loads on crossbar block ramps.

Vulnerability of power distribution utility poles to tsunami bore impacts

Max Stephens — 2023-04-23

Recent events have demonstrated that power distribution networks located in low-lying coastal areas are susceptible to damage from tsunami. Utility poles are a critical component of distribution networks as they support overhead power lines. Damage to the poles could therefore compromise the electricity supply to emergency facilities as well as to homes and businesses over large areas. This work quantifies the component-level tsunami vulnerability of common power distribution line utility poles, considering hydrodynamic wave-impact loading effects but neglecting debris impact and scour effects. First, a series of scaled flume experiments were used to identify the relationship between the tsunami wave properties and hydrodynamic loading histories. Next, nonlinear numerical distribution line utility pole models were validated using the experimental data and extended to account for soil-structure interaction effects. Finally, the loading histories from the flume tests were scaled and used in the numerical models to perform an incremental dynamic tsunami analysis on varying pole geometries and loading orientations at prototype scale. The results from this work provide valuable insight into the response of power distribution poles subjected to tsunami attack. This includes validating idealised approaches to determine the expected failure mode(s) based on pole embedment depths and soil properties and providing probabilistic tools capable of estimating damage based on expected tsunami inundation depths.