An Experimental Study of Beach Evolution with an Artificial Seepage
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An Experimental Study of Beach Evolution with an Artificial Seepage Paper: An Experimental Study of Beach Evolution with an Artificial Seepage Changbo Jiang∗,∗∗, Yizhuang Liu∗,BinDeng∗,∗∗,†,YuYao∗,∗∗, and Qiong Huang∗∗∗ ∗School of Hydraulic Engineering, Changsha University of Science and Technology Changsha 410114, P. R. China †Corresponding author, E-mail: [email protected] ∗∗Key Laboratory of Water-Sediment Sciences and Water Disaster Prevention of Hunan Province, Changsha 410114, P. R. China ∗∗∗Guangzhou Zhengjian Construction Engineering Design Co., Ltd., Guangzhou 510220, P. R. China [Received April 10, 2016; accepted September 8, 2016] Beach erosion caused by extreme wave events (storm face, which has significantly influenced coastal sediment surges) is reported to occur in many coastal areas. transport [4]. Many researchers have suggested that such Artificially lowering the groundwater table effectively erosion could be induced by exfiltration or higher ground- stabilizes sand beaches in an environmentally friendly water table, while infiltration or lower groundwater table way. Mechanisms affecting beach stabilization remain contributes to onshore sediment transport [5–11]. Other unclear, however, due to the complex interaction be- studies suggest that the groundwater table does not sig- tween waves and coastal seepage. This study discusses nificantly change the beach profile [12–13] possibly be- the effects of coastal seepage on beach profile evolu- cause either infiltration increases the effective bed sedi- tion and bed materials sorting based on laboratory ex- ment weight and thus impedes sediment mobility or infil- periments in which seepage is induced artificially by tration tends to increase bed stress, thus enhancing sedi- a drain pipe at three cross-shore locations on a 1:10 ment mobility. The two types of mechanics mutually con- beach. Morphodynamic beach responses with and flict in infiltration and vice versa for the exfiltration [14]. without seepage under a typical cnoidal wave condi- The so-called beach drainage system (BDS) is based tion are reported. Results show that artificial seep- on the concept of lowering the groundwater table to re- age impacts only insignificantly on total upper-beach duce erosion. The BDS consists of a drain pipe parallel to deposition volume but could increase accretion on the the shoreline connected through a blind pipe to a pump- berm’s leeside by reducing seaside sand accumulation. ing station that transports drained water back to the sea or It also induces a steeper berm slope and shoreline re- to another destination [15]. Despite the above two oppo- cession. A drain pipe near the shoreline generated the site effects, many investigators report its positive effect on greatest accretion height on the upper beach. Seep- beach stabilization. Results of the first field survey, con- age location had no significance effect on bed material ducted by Chappell et al. [16] in Australia, showed that sorting, however. sediment accreted on the beach face by artificially lower- ing the groundwater level. Goler [17], in reviewing the Keywords: beach evolution, coastal seepage, bed sorting, BDS from 1981 to 2004, claimed that the BDS affected cnoidal wave shoreline accretion positively. Law et al. [18] demon- strated that the BDS affects the shore profile little over the long term period, but induced seepage retards erosion 1. Introduction in the first few hours of their laboratory experiment. A full-scale laboratory investigation series on a BDS in the Storm surges are natural phenomena of water rising large wave flume Grosser WellenKanal of the Coastal Re- commonly associated with strong atmospheric distur- search Centre in Hannover, Germany [19–22], indicated bances such as tropical cyclones and strong extratropical that the BDS decreases undertow current and water table cyclones. A large sum of field studies has shown that dev- rise due to wave motion while it increases the reflection astating storm waves mobilize a multitude of coastal sand, coefficient. An increase in bed stabilization is observed significantly eroding the coast in affected areas [1–3]. under low wave energy. Although the BDS is considered Many active engineering solutions to stabilize the shore- a possible “soft” method for reducing beach erosion and line have been attempted, such as spur dikes, breakwa- enhancing sand accretion, however, questions remain con- ters, and seawalls, but these solutions, considered “hard” cerning, e.g., the location effect of the drain pipe on beach engineering, modify original coastal morphology – some- evolution and bed materials sorting. We investigated the times harming near-shore ecology and environment. Sev- response of beach evolution and bed material sorting in eral recent environmentally friendly methods have been the location of seepage under a typical cnoidal wave be- proposed to stabilize beaches by controlling the ground- cause the cnoidal wave most closely resembles the wave water table through infiltration or exfiltration on the beach form seen during a storm or hurricane in rather shallow Journal of Disaster Research Vol.11 No.5, 2016 973 Jiang, C. et al. Fig. 1. Experiment setup. water [23]. flume’s water level, drained water was pumped to the end We describe the beach evolution laboratory setup with of the flume through a discharging pipe and returned to artificial seepage in Section 2. Section 3 presents results the front of the flume via a pipe connecting both ends of and discussion of beach profile variation, berm slope, de- the flume. The infiltration rate was measured by a flow position volume on the upper beach and bed sorting with meter installed near the discharging pipe outlet (Fig. 2c). and without seepage. Section 4 summarizes our study’s We tested the case first without artificial seepage, ob- main conclusions. taining the profile shown in Fig. 3a. A drain pipe was then introduced to induce seepage in each test, with a to- tal of three pipe locations (S1 – S3) selected based on the 2. Experimental Setup obtained profile: S1 was located at the point of maximum erosion depth, S2 was near the shoreline and S3 was at Experiments were conducted at Changsha University the maximum accretion point as detailed in Table 1.The of Science and Technology, P.R., China, in a wave flume pipe was kept 10 cm beneath the beach surface during 0.8 m deep, 0.5 m wide and 40 m long. A flap wave- all tests. To ensure that measured results were replicable, maker was installed at one end of the flume and a sand three runs were conducted for each test. Taking the beach beach with an initial 1:10 slope was constructed in the with drain pipe S1 as an example, we compared beach wave flume. The beach was well-sorted quartz sand with a profile changes after 2-hour wave action among the three characteristic 0.438 mm diameter measured by a Malvern runs as shown in Fig. 4. Mastersizer 2000, D50, as shown in Fig. 1. The typical cnoidal wave condition had an incident Six resistance wave gauges (RBR Co., Ltd., Canada) wave height of 0.09 m, a wave period of 2 s, and a water (WG1∼WG6) and three ultrasonic water gauges (Sin- depth of 40 cm. The wavelength for our wave condition fotek Co., Ltd., China) (UWG1∼UWG3) were used to was 3.69 m, or less than 10% of the total flume length measure wave surface evolution at selected sites. WG1 of 40 m. Before each run, the beach was reestablished was set 14 m away from the beach toe to measure incident with a 1:10 slope and the pump was opened for half an waves. WG2 was placed at the beach toe itself. Shoaling hour to obtain a stable groundwater level. The tested in- waves were monitored by WG3-WG6. Wave breaking, filtration rate from S1 to S3 was 28.15 l/s, 23.00 l/s, and runup and rundown were monitored by UWG1-UWG3. 6.64 l/s. The wavemaker was run for 2 hours in each test Morphological beach measurements were made using and beach profiles were recorded at 0, 0.5, 1 and 2 hours. a URI-III topographic surveying meter system (Electronic We assumed that the wave reflection effect in this study Information Institute, Wuhan University, China), with an was insignificant for three reasons: accuracy of ±1 mm. The URI-III consisted of a URI-III profiler, a measuring bridge controller, a measuring bridge (1) We have divided the 2-h measurement for each case and a PC-based acquisition system. We used the system’s into 40 stages and run the wavemaker only for 3 min- ultrasonic measurement in this study. To reduce measure- utes in each stage, then stopped it until water in the ment error, three cross-shore beach profile sections were flume became calm, thus minimizing multiple reflec- measured along the flume width, each twice. We aver- tion. aged the six measurements and treated the result as the (2) The wavemaker had an active absorbing function final beach profile. found to work well for short waves. To lower the groundwater level, we arranged a PVC drain pipe 25 mm in diameter parallel to the shoreline (3) The sand beach itself was porous thus further absorb with its length equating the flume width (Fig. 2). The reflected wave energy. drain pipe collected infiltrated waters induced by a pump- ing system during experiments. A series of holes 2 mm in To analyze sediment sorting on the beach under the diameter were drilled along the pipe at 10 mm intervals. seepage effect, sand samples were collected at four cross- To avoid sand blockage during tests, the pipe was covered shore regions in each test: the erosion zone; the near with a geotextile filter sheet (Fig. 2b). To maintain the shoreline; around the berm crest, i.e., the maximum ver- tical distance between the bed profile after 2-hour wave 974 Journal of Disaster Research Vol.11 No.5, 2016 An Experimental Study of Beach Evolution with an Artificial Seepage Fig.