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Procedia Engineering 116 ( 2015 ) 285 – 292
8th International Conference on Asian and Pacific Coasts (APAC 2015) Department of Ocean Engineering, IIT Madras, India. Study of Beach Erosion and Evolution of Beach Profile Due to Nearshore Bar Sand Dredging
Duc Thang Chua,Gen Himoria, Yuki Saitoa, Trong Vinh Buib, Shin-ichi Aokia,*
aOsaka University,2-1 Yamadaoka, Suita, Osaka 565-0871, Japan bHo Chi Minh City University of Technology, 268, Ly Thuong Kiet, ward 14, Dist. 10, Ho Chi Minh, Vietnam
Abstract
The sand dredging activities may cause various physical and environmental changes such as beach erosion, evolution of beach profile, change in current and wave field. To evaluate the relationship among the volume of sand dredging, method of sand dredging, time of sand dredging, shoreline retreat and evolution of beach profile, some experiments were conducted and a two-dimensional numerical model was used. The effect of sand dredging volume on shoreline retreat was investigated by dredging various amounts of sand at one time under the same regular wave condition. The results showed that the relationship between shoreline retreat and dredging volume is nonlinear. In the other considerations, sand bar was dredged as different methods i.e. periodic sand dredging method under the same regular wave condition as well as one time dredging under different regular wave conditions. The final shoreline retreat in the last two methods was nearly equal to the first method, however, the progress of shoreline retreat was slower. After sand dredging in all cases, the beach profiles were not translated to the same original form. Infill rate of the bar after sand dredging is also discussed in term of average absolute sediment transport rate. © 20152014 TheThe Authors. Authors. Published Published by by Elsevier Elsevier Ltd. B.V. This is an open access article under the CC BY-NC-ND license Peer(http://creativecommons.org/licenses/by-nc-nd/4.0/-review under responsibility of organizing committee). of APAC 2015, Department of Ocean Engineering, IIT Madras. Peer- Review under responsibility of organizing committee , IIT Madras , and International Steering Committee of APAC 2015 Key Words: Nearshore bar; volume of sand dredging; shoreline retreat; beach profile; infill rate.
1. Introduction
Because of the increase of sand resource demand for construction, beach nourishment, navigation, sand is dredged at offshore zone. To be cost effective, in some countries, sand dredging at nearshore zone is allowable. However, dredged area close to the shore can have significant negative impacts, causing erosion of the foreshore (Price et al., 1986). On the western Black Sea coast of Turkey where sand is dredged at the near-shore zone for construction, it can be demonstrated that the project will not results in significant negative impacts or changes in physical or biological processes (Marine Habitat Committee 2000). The direct impacts are the sediment transportation from the foreshore to the dredged hole via maximum of shoreline retreat and infill rate of dredged hole. Evolution of beach profile after sand dredging can cause indirect impacts to the shoreline retreat. To quantify the physical impact such as shoreline retreat, infill rate, and beach profile evolution and suggest some countermeasures for the coastal erosion due to sand dredging in nearshore zone, a combination of theoretical, experimental and numerical analysis is carried out.
* Corresponding author. Tel.: +81-06-6879-7613; fax: +81-06-6879-7613. E-mail address: [email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer- Review under responsibility of organizing committee , IIT Madras , and International Steering Committee of APAC 2015 doi: 10.1016/j.proeng.2015.08.292 286 Duc Thang Chu et al. / Procedia Engineering 116 ( 2015) 285 – 292
Nomenclature
1,2 spatial decay coefficients in zones I and II slope related sand transport rate coefficient proportionality factor y maximum of shoreline retreat a width of dredged hole A scale parameter b length of dredged hole B berm height d depth of dredged hole D wave energy dissipation per unit water volume Deq equilibrium wave energy dissipation per unit water volume d50 median grain size h water depth h* closure depth H wave height Hb breaking wave height K sand transport rate coefficient q net cross-shore sand transport rate T wave period Ve volume of sand erosion (volume per unit length) Vd volume of sand dredging (volume per unit length) W* seaward limit of active profile y cross-shore coordinate directed positive offshore
2. Literature review
The potential negative impacts of sand dredging to the shoreline change and infilling rate of dredged hole have been investigated so far. Kojima et al. (1986) studied the shoreline change and infilling rate in Genkai Sea in Japan. Beach profiles at and around dredged holes situated above the water depth of about 30 meters changed substantially by filing up the holes with sand that was mainly transported from the onshore side in less than 1 year. Moreover, it causes severely shoreline retreat along the coast. The change of beach profiles due to dredged holes situated at 35 to 40 meters depth is insignificant. Van Dolah et al. (1998) investigated the infill rate of six borrow holes at the offshore zone in South Carolina. The dredged holes are filled up within 5-12 years, with this time being proportional to distance from shore. Demir et al. (2004) discussed the impacts due to sand dredging at the nearshore zone via direct impacts and indirect impacts on sediment transport. The direct impact caused by the loss of sediment from the dry beach via infilling of the dredged pit. While the indirect impact is the result of modification of the nearshore wave conditions via the modified bathymetry. Chu et al.(2014) studied relationship between the shoreline retreat and sand dredging volume, sand dredging time and sand dredging methods based on the experimental study.
3. Methodology
3.1. Theoretical analysis
Bruun (1954) suggested the simple relationship for equilibrium beach profile as
2 yh )( Ay 3 (1)
A definition sketch of the profile before and after sand dredging in the nearshore zone is presented in Fig. 1. It is common to assume that the profile retreats uniformly at all active elevations as long as the beach maintain its shape across the profile. In addition, it will be assumed that within the surf zone wave height is proportional to the local water depth with the proportionality factor, , i.e. H= h ( =0.78). At the breaking point,Hb = h*. Referring to Fig.1, sand dredging volume, Vd, is equal to the volume eroded, Ve
VV ed (2) Duc Thang Chu et al. / Procedia Engineering 116 ( 2015) 285 – 292 287
When Eqs. (1) and (2) are combined the following implicit equation for the maximum shoreline change, y, is obtained
5 3 Vd y 3 h* y 3 h* (3) 1 BW* W* 5 B W* 5 B
3 2 Hb (4) W* κA
In this case the shoreline is in recession thus, y < 0. Eq. (3) can be expressed in non-dimensional form as
3 5 '' yhyV '11' 3 (5) d 5 *
In which the non-dimensional variables are
y h* Vd (6) y h* Vd ';';' W* B BW*
Eq. (5) is plotted in Fig.2. This Figure showed the non-linear relationship between shoreline retreat and sand dredging volume. For small values of y’, Eq. (5) can be approximated by
V y d (7) hB *
Fig. 1. Definition sketch for profile response due to nearshore sand dredging.
Fig. 2. Isolines of dimensionless shoreline change, y’, vs dimensionless sand dredging volume in nearshore zone, Vd’, dimensionless breaking depth, h*’. 288 Duc Thang Chu et al. / Procedia Engineering 116 ( 2015) 285 – 292
3.2. Experimental study
The experimental study of the sand dredging in the near shore zone was carried out at the Hydraulic Laboratory of Osaka University. The experimental 2D wave flume is 30 m long, 0.7 m wide, and 0.7 m deep. In order to evaluate the impacts of volume, time and methods of sand dredging to the intensity of the shoreline retreat, 7 cases of the experiment were conducted (Tab. 1). To obtain an equilibrium barred profile from the initial slope, regular wave was generated for 60 minutes with wave height H = 0.14 m and wave period T = 1 s (Sunamura et al., 1974). Then, sand was dredged only once except for periodic dredging method where sand was dredged at the bar every hour during a 5-hour wave generation period. After the sand dredging, the same wave condition was generated for 5 hours in all the cases except for experiment E5D. In experiment E5D, wave condition was shifted to wave height H = 0.05 m and wave period T = 2 s and generated for 5 hours and then shifted back to wave height H = 0.14 m and wave period T = 1sand generated for 5 hours.
Table 1. Experimental conditions (Chu et al., 2014). No. Case No. Dredging method Wave condition (H/T) Dredging volume (m3/m) 1 E1 Without dredging 0.14/1 0 2 E3D1 One-time dredging 0.14/1 0.02 3 E3D2 One-time dredging 0.14/1 0.04 4 E3D3 One-time dredging 0.14/1 0.06 5 E3D4 One-time dredging 0.14/1 0.08 6 E2D Periodic dredging 0.14/1 0.066 7 E5D One-time dredging 0.05/2 - 0.14/1 0.06
3.3. Beach profile change model
SBEACH was developed to simulate storm induced beach change (Larson et al., 1989). SBEACH has been validated and used extensively for erosive conditions. In this study, it was used to simulate the changing of beach profile response of the nearshore bar sand dredging. Model was slightly modified for the experimental approximation. Sediment transport zone is divided in 4 zones as in Fig. 3. As the wave height distribution is calculated across-shore for a given time step, location of the boundaries between the different sand transport zones is determined. Sediment transport is calculated from zone I to zone IV by following equations respectively
λ 1 yy b )( beqq b yy (8)
λ 2 yy p )( (9) peqq p yyy b
ε dh ε dh DD DDK eq K dy (10) q eq K dy ε dh 0 DD eq K dy
yy r qq r yyy z (11) z yy rz
Fig. 3. Definition sketch for four principal zones of cross-shore sand transport (Larson et al., 1989). Duc Thang Chu et al. / Procedia Engineering 116 ( 2015) 285 – 292 289
Subscripts b, p, z, and r stand for quantities evaluated at the break point, plunge point, end of the surf zone and run-up limit, respectively. Equation of mass conservation is written as
q h (12) y t
The undistorted geometric scale used to constructed model is 1:20 with the respect to the experiment. Froude similarity is considered for the simulation of the wave motion, whereas the size of the sediment has been imposed preserving in the model the value of settling velocity parameter (Pugh, 2008). Six cases of sand dredging N2D, N3D1, N3D2, N3D3, N3D4, and N5D were simulated using SBEACH model, corresponding to six cases of the experimental sand dredging.
4. Results and discussion
4.1. Shoreline change due to sand dredging at nearshore zone
Influence of sand dredging volume
In the theoretical analysis, with small of sand dredging volume, the relationship between maximum of shoreline retreat and sand dredging volume is assumed linearity; however, it is not the case in the experimental and numerical analysis. The final shoreline retreat increases with increasing volume of dredged sand; however, the ratio between the maximum of shoreline retreat and the dredged sand volume is not constant and shows a decreasing trend as the volume increases (Fig. 4).
Time of sand dredging
In the calm season, accretion profile is created i.e. the shoreline is advanced. In the cases of E5D and N5D, volume of sand dredging is similar to the cases E3D3 and N3D3, sand bar however, was dredged in calm wave condition. The sand dredging in calm wave condition reduces the speed of shoreline retreat (Chu et al. 2014); however, the final of shoreline retreat is nearly similar to the cases of sand dredging in storm wave condition (Fig.5a).
Method of sand dredging
The shoreline is retreated significantly within 2 hours after sand dredging (Chu et al. 2014). In the periodic sand dredging of experiment E2D Chu et al. (2014) just generated the wave 1 hour after the 5th sand dredging. In addition, although the progress of shoreline retreat is slower however, a large volume loss was observed (Chu et al., 2014). Therefore, the maximum of shoreline retreat in the experiment E2D can be increased if the wave are generated in progress. In the numerical analysis, the wave was generated to get the equilibrium beach profile after sand dredging. Thus, the final of shoreline retreat in case of periodic sand dredging N2D give the high magnitude of shoreline retreat (Fig. 5b).
Fig. 4. Final shoreline retreat after near shore bar sand dredging (h*/B=2.2). 290 Duc Thang Chu et al. / Procedia Engineering 116 ( 2015) 285 – 292
Fig. 5. (a) Shoreline retreat after difference time of sand dredging; (b) Shoreline retreat after difference methods of sand dredging.
4.2. Evolution of the beach profile after sand dredging
Under the conditions where no longshore transport, after sand dredging on-offshore transport causes a redistribution of sand across the profile and the beach profile approaches a new equilibrium. In this study, the same wave condition of bar profile is generated after sand dredging. Therefore, sediment is continuously transported offshore before the new equilibrium beach profile is reached. Looking at distribution of sediment transport in zones I, II of the beach profiles, the sand volume increases in these zones after dredging (Fig. 6). Chu et al. (2014) named this volume of sand is volume loss. Volume loss tends to be decreased to zero, as the sand dredging volume is large enough (Fig. 7). Figure 5 showed that with the large volume of sand dredging, the maximum of shoreline retreat of experimental and numerical analysis tends to follow the theoretical analysis i.e. beach profile is completely translated onshore as the original beach profile.
4.3. Infill rate
Sand bar is located at breaker zone, i.e. in the most activation of seabed zone. Any change of bathymetry in this area can cause significant change of hydrodynamic conditions within the nearshore zone, thereby causing highly transport gradient and profile change. After the sand bar was dredged, under the same wave conditions, a new bar is formed by sediment that transported from foreshore area; i.e. old sand bar is filled up and located at new position. Sawaragi and Deguchi (1981) used an exponential decay to derive a time-dependent transport relationship; however, since the shape of the transport rate distribution also varies with time, a peak transport rate may not be the best indicator (Larson et al., 1989). Therefore, infill rate of dredged hole is discussed in term of average absolute sediment transport rate. Just after dredging, sand bar was filled up within 2 hours in the experiment and 10 days in the numerical analysis (Fig. 8). Dredged hole in the nearshore zone is filled up very quickly after sand dredging.
Fig. 6. Beach profiles after sand dredging (numerical analysis). Duc Thang Chu et al. / Procedia Engineering 116 ( 2015) 285 – 292 291
Fig. 7. Relationship between volume loss and dredged sand volume.
Fig. 8. Average absolute sediment transport rate after sand dredging (numerical analysis).
5. Conclusion
Maximum of shoreline retreat and infill rate of dredged hole are two appropriate indicators to evaluate the impacts of nearshore sand dredging to beach erosion. Nearshore bar should be dredged in the calm season instead of in storm season in order to reduce the speed of shoreline retreat. Periodic sand dredging method causes increment of volume loss and the maximum of shoreline retreat. The volume of sand dredging changes the form beach profile. Beach profile is translated completely onshore as the volume of sand dredging is large enough. The evolution of beach profile may indirectly affect the sediment transport at the foreshore area, hence the maximum of shoreline retreat.
Acknowledgements
The authors would like to thank Emeritus Prof. Ichiro Deguchi, Assoc. Prof. Susumu Araki and the laboratory members for their valuable contribution to this study.
References
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