CONTROL OF SEDIMENT TRANSPORT BY BANK BLOCKS WITH DIRECTIONAL RESISTANCE PROPERTIES

Toshimitsu Komatsu¹, Tomonori Saita², Naoko Kohashi²

Takahiro Adachi¹ and Takuya Shibata²

¹ Professor, Dept. of Civil Engineering, Kyushu University, Fukuoka 812-8581, Japan,

tel/fax: +81-92-642-3279, E-mail: komatsu@civil.kyushu-u.ac.jp ;

² Postgraduate, Graduate school of Kyushu University, Fukuoka, 812-8581, Japan,

tel/fax: +81-92-642-3281, E-mail: kohashi@civil.kyushu-u.ac.jp ;

Abstract: Beach erosion in Japan is becoming more and more of an issue, and countermeasures against this problem must be implemented without delay. In this study, we develop a new and effective method, using blocks called "BaNK" blocks, to control sediment transport in order to deal with this problem. We examine the fundamental properties of this method by conducting laboratory experiments and field tests on the Nogita Coast. We initially investigated the conditions of flow around BaNK blocks installed in an experimental wave field, confirming that this technique allows bottom flow to be effectively controlled and providing a basis for appropriate BaNK block layout. We then installed two block arrangements at separated location on the Nogita Coast and observed sediment transport under these conditions. These field tests confirmed that sediment transport can be controlled in practice through the use of BaNK blocks in the appropriate arrangement. The results of this study indicate the possibility of beach nourishment using BaNK blocks.

Keywords: sediment transport, BaNK block, beach nourishment method, wave energy

1    INTRODUCTION

The shapes of coastlines are molded by the flow of water, which is complicatedly affected by wave motion and tide. When averaged over a fixed period, the installation of small-scale structures with directional flow and resistance characteristics in the flow field produces a net resistance in a particular direction, resulting in net flow in that direction (see Fig. 1). The net flow i.e. the residual flow component in the wave field is referred to as the wave-induced residual flow. In the same context, the net resistance over a fixed period is referred to as the wave-induced residual resistance.

Fig. 1    Generation mechanism of wave residual flow

Sediment transport is closely related to the flow conditions in the bottom layers, as induced by wave action. If the bottom flow can be controlled, it is possible to control sediment movement too. One such use of asymmetrical structures to control sediment movement is referred to as the BaNK (Beach and Navigation Keeper) system, and the basic structures used to realize this control are referred to as BaNK blocks.

Many of the details of sediment transport remain unclear, and as each coastal area has unique characteristics, it is almost impossible to develop perfect solutions to the problems of sediment transport in advance. BaNK blocks are easily installed and removed, allowing fine adjustment of layout, spacing, and direction to accommodate unexpected reactions. Therefore, this system provides significant flexibility and allows optimization while observing the results, granting an understanding of the unique characteristics of individual coastal areas.

2    LABORATORY EXPERIMENTS

Experiment

Previous research¹⁾ has dealt with the technology for controlling flow in tidal fields using structures commonly referred to as undersea blocks, involving investigations on the effects of generating tidal residual flows. The current series of tank tests focused on the question of whether it is possible to use small structures with directional resistance characteristics to generate average flows over a fixed period only in the bottom layers in wave fields, which typically have much shorter cycle periods than tidal fields. The tests also investigated the optimum spacing and layout of BaNK blocks when installed on the seabed.

Fig. 2    Schematic diagram of an experimental setup

        A two-dimensional wave tank (see Fig. 2), 15 m long and 0.25 m wide, was employed for the tests. Regular waves were generated, at an average water depth of h =  30 cm, and with amplitude a = 2.5 cm, period T = 1.0 s, and wavelength L = 1.3 m. Flow velocity was measured with equipment ADV . Blocks were positioned in both grid and zigzag patterns, and in both cases, the blocks used were 1/4 spheres¹⁾ (height 2.0  cm), placed facing the beach to generate residual flow (see Fig. 3).

Fig. 3    Arrangement of Bank block Arrangement of Bank block

Blocks were positioned at the same spatial density in both grid and zigzag cases, and tests were conducted for arrangements of blocks at intervals of between 1/8 (approximately 18cm) and twice (approximately 260 cm) the wavelength of the waves. To obtain the width-average of residual flow velocity, the flow velocity was measured at five different points along the central axis of the layout area (see Fig. 3). A weighted width-average of the period average of the flow velocity obtained at the same level at each measurement point was used to determine the vertical profile of residual flow velocity. For the purposes of comparison, the same measurements were taken with no blocks in place.

Results and discussion

The vertical profile of the horizontal component U_x of the wave-induced residual flow velocity  are shown in Fig. 4 as a representative example of the difference that the presence of the blocks makes. The wave-induced  residual flow velocity U is expressed as a non-dimensional value in terms of wave amplitude a, and angular frequency s. In all cases with blocks present, new flows were generated towards the beach under non-dimensional water depths of 0.25 ~ 0.30, and away from the beach on that non-dimensional water depths. This implies that the resistance characteristics of the blocks had created a strong wave-induced residual flow towards the beach in the bottom layer, with a complimentary flow away from the beach in the top layer.

Fig. 4    Vertical Profile of nondimentional wave residual velocity

As shown in Fig. 5, the maximum residual flow velocity increases with installation length, although the rate of increase gradually tapers off. This indicates that the maximum residual flow velocity in the bottom layer reaches 15% to 40% of the maximum of unsteady wave-induced flow verocity, and that sufficient control of bottom flow is possible. This was also confirmed by the fact that neutrally buoyant particles introduced near the seabed were observed to be transported towards the beach.

A comparison of the generation of residual flows for various blocks layouts reveals that, in all cases, a grid array produced a stronger residual flow than zigzag patterns. The difference in residual flow between the two layouts is thought to be due to the fact that the offset pattern creates an area of dead water zone in the bottom layer rather than flow field, thus reducing the net flow velocity for the same wave conditions. The grid array produced distinct channels of flow between rows of blocks, and a region of residual resistance at the blocks. This is considered to be the reason for the efficient generation of residual flow.²⁾

To investigate flow conditions in the vicinity of the blocks, and to investigate the extent of the affected area, the horizontal component U_x and vertical component U_z were measured for a number of cross-sections. The vertical two-dimensional pattern of residual flow averaged over a width is shown in Fig. 6 (black represents block area). These results show that a very strong vertical circulating flow was created over a wide area encompassing the block area. Residual flow extended to an area 1.5 times that of the block area.

Fig. 5    Relation between installation length of BaNK blocks and nondimensional maximum wave residual flow

Fig. 6    Vertical circulation of wave residual flow

3    FIELD TESTING

Test procedure

Under real conditions, a number of complex and variable factors are at work, and it is therefore quite possible that the effects observed in the test tank may not be observed under actual conditions.

In order to verify the validity of the BaNK system, blocks were installed offshore from a beach near Nogita Port in Itoshima-gun, Fukuoka Prefecture, Japan. The effects of the installation were evaluated by measuring actual sediment transport (see Fig. 7).

The BaNK blocks and sand collectors used in this field test were installed in an area having an average depth of 6.5 m, at a distance of approximately 350 m from the beach, on November 5th, 1999. The six 1/4 sphere BaNK blocks were 30 cm in height, and were placed on a concrete base 2.2 m long, 3.4 m wide, and 25 cm thick to form the basic unit (see Fig. 8). For the purposes of comparison, an arrangement of 1/2 spheres were also installed (see Fig. 9). The two types of blocks were arranged so as to provide equal area projected away from the beach, ensuring that the blocking effects were equal, and also permitting the isolation of the effects of sediment control due to differences in resistance characteristics.

The sand collectors used were tubular with an internal diameter of 85 mm, a height of 270 mm, and a rectangular sand collection aperture of 10 mm ´ 50 mm. The directional nature of the sand collection entrance allows the collection of sand that is moved in the direction controlled by the blocks only.

Fig. 7    Ouline map of Nogita Coast

Fig. 8    Block unit and sand collector used in field test

Fig. 9    Condition around the blocks at the test field

The sand collectors were placed at four points out from blocks and at four points in from the blocks, and were buried to ensure that the base of the blocks and the height of the sand collection apertures were the same. The placement of the sand collectors is detailed in Fig. 9 (A-H).

Results and discussion

The accumulated sand was recorded on November 20th and December 16th, 1999. The results are shown in Fig. 10. For the 16 day interval between November 5th and November 20^th, 1999, the 1/2 spheres produced a greater accumulation on the outward side (C and D) than on the inward side (A and B). It is thought that the offshore bottom flow during this period may have been. On the other hand the sand accumulated on the outward side of the 1/4  sphere array (G and H) was extremely small in comparison to that on the inward side (E and F), indicating that the residual flow generated by the BaNK blocks resulted in the transport of sediment toward the beach.

Fig. 11    Temporal variation of accumulated depth

In addition, comparing the sand accumulation in each collector over time (see Fig.11), from the beginning of the experiment on November 5th, 1999, the accumulation for the 1/4 sphere array on the outward side (G and H) was approximately half that for the inward direction, indicating control of bottom flow in the desired direction (toward shore).

4    CONCLUSIONS

This study provided the following results.

(1) BaNK blocks may be used to generate residual flow at the sea bottom. The effects of this flow extend for approximately four times the block height.

(2) Residual flow is generated by the BaNK blocks up to a velocity of 15% to 40% of the maximum of wave-induced flow velocity.

(3) For the same installed length, the grid array generates a stronger residual flow than an offset pattern.

(4) A strong vertical circulating flow is generated over an area of 1.5 times the block area. It is possible to generate a continuous residual flow even when the blocks are installed in groups (bands).

(5) Sediment was confirmed to be successfully controlled using BaNK block arrays in a complex inshore environment.

References

[1]    Komatsu,T., Yano,S., Gug,S., And Kohashi,N.(1997), ¡°On Creation And Control Of Tidal Residual Current By Bottom Roughness With Directional Characteristic¡±, Annual Journal Of Hydraulic Engineering,Jsce,1997,Vol.41,Pp323-328 (In Japanese). 

[2]    Fukuoka,S., Uchida,T., Fukushima,T. And Mizuguchi,M.(2000), ¡°One Dimensional And Two Dimensional Calculation Of Shallow Water Flow Over Submerged Large Roughness¡±, Annual Journal Of Hydraulic Engineering,Jsce,2000,Vol.44,Pp.533-538(In Japanese).