Performance of Beach Nourishment with Detached Submerged Breakwater

PERFORMANCE OF BEACH NOURISHMENT WITH DETACHED SUBMERGED BREAKWATER

Kyungmo Ahn1*, Se-Hyeon Cheon2 and Jeho Chun2

1School of Spatial Environment System Engineering 2Institute of Construction and Environmental Research Handong Global University Pohang City, Kyungbuk, KOREA *[email protected]

ABSTRACT

This paper presents beach restoration project proposed for Songdo beach which is located in Pohang city, Korea. Songdo beach has been suffered from gradual erosion for last 20 years. Aerial photos and water depth changes from series of nautical charts were utilized for the investigation of the main causes of beach erosion. For the optimum design of beach nourishment with protective structures, numerical as well as physical modeling tests had been conducted. For the final decision of optimum design from two different configurations of submerged detached breakwater, curvilinear shoreline change model was used. Beach nourishment with three 300m submerged detached breakwaters was recommended for the restoration of Song-do beach, which does not require periodic re-nourishment.

EROSION AT SONG-DO BEACH

Aerial photograph

Song-so beach is located in Pohang city, Korea. As is shown in Figure 1, Song-do beach resides deep inside of Young-il bay which is located in the south-eastern edge of Korean peninsula. The Korea mouth of Young-il bay is open to East sea in the

Japan north-east direction, thereby Song-do beach is subject to and largely influenced by the north- east wind seas and swells whose fetches ranges up to 1,500 kilometers. However, waves coming into the mouth of bay travels about 7km to reach Song-do beach lost most of their energy.

Wave measurements during a severe storm showed that significant wave heights from the mouth of the bay with water depth of 30m to the nearshore of the beach with a water depth of Figure 1. Location of Song-do beach in 12m changes from 5m to 2m, which indicates Pohang city that the wave energy is dissipated up to 80%

212 through shoaling, refraction, and diffraction of waves. Most of the waves propagating into Song- do beach are near perpendicular to the shoreline due to the nearshore bottom topography. Song- do beach was used to be one of the best beaches in Korea for it’s mild beach slope, wide beach face, fine-to-medium quarts sand, and forest of sea-pine trees at the dune.

Industrialization of Korea was initiated from the construction of heavy industries such as steel and petroleum refinery factories in early 1970s. POSCO (POhang Steel COmpany) was built adjacent to Song-do beach. In order to understand the cause of beach erosion at Song-do beach, series of aerial photographs were utilized. Aerial photographs dated back in 1967 and 1980 shown in Figures 2 and 3 give us ideas why beach erosion has been accelerated during last two decades.

Figure 2. Aerial photograph of Song-do Figure 3. Aerial photograph of Song-do Beach beach in 1977 during the extension of POSCO

As is shown in Figure 3, the Korean government allowed POSCO to expand its factory site by land reclamation using sands dredged right in front of Song-do beach. During the winter from late 1978 to early 1979, Song-do beach had been completely eroded by winter storms (Fig. 4). Figure 5 shows the photos of restored beach in 1981. Restoration of the beach was done by beach nourishment and placement of three groins. The sand borrower site was right next to Song-do beach which is the estuary of Hyeong-San river.

We can estimate the volume of dredged sand during the land reclamation of POSCO in 1977 and 1978 by comparing water depth changes from series of old Nautical charts as well as surveyed water depth data. From the nautical charts dated in 1970, 1984, 1990, 1994 and together with water depth data surveyed in 2006. The volumetric change of sand in the rectangular region of 5km by 5km in front of Song-do beach was estimated to be of about 10 million cubic meters. We were also able to estimate the recharging of sands from Hyung-San River to be of 600,000m3/yr, which is the sole source of sand into this beach sediment system.

213 Figure 4. The severe erosion from Song-do beach took place in 1978 and 1979. The erosion is caused by the dredging of sands used for the land reclamation of POSCO.

Figure 5. The restoration of Song-do beach was done by beach nourishment and construction of groins in 1981.

Figure 6 shows the shoreline changes obtained from the series of aerial photos from 1967 to 2004. As is shown in the figure, sands were deposited in the northern part of the beach and severely eroded in the upper middle part of the beach. This is mainly due to the extension of the breakwater located at the entrance of Dongbin port.

Figure 6. Shoreline changes obtained from the series of aerial photographs from 1967 to 2004

214 RESTORATION OF THE BEACH

Design processes

As is mentioned in the previous section, erosion of Song-do beach is mainly due to dredging of sands for the extension of POSCO and extension of breakwater in the entrance of Dongbin port. Based on the preliminary study on the causes of erosion at Song-do beach, Pohang city decided to initiate the restoration of the beach by seeking funding from central government through National Coastal Preservation Program as well as a funding from POSCO.

It is required by Pohang city that the restoration of the beach should be completed by one time funding. Therefore, if beach is restored through beach nourishment, additional re-nourishment should be avoided.

Estimation of design waves

One of the most important input parameters in the design of beach restoration is design wave information such as wave height, period, and direction. Estimation of design waves has been carried out from the measured waves using wave-rider buoy at the mouth of Young-il bay. The wave-rider buoy had been deployed in the water depth of 30m from 1999 to 2004. Partial duration series analysis of extreme waves was done by collecting significant wave heights greater than 1m for more than 24 hours for wave directions of NE and NNE. Figure 7 shows the cumulative distributions of Type I, Type II, Type III extreme wave heights in the wave direction of NNE. As is shown in the figures, extreme wave distribution of Type I with k-s test of 0.123 is shown to be the best fit. Type I extreme wave height distribution which is also called Fisher- Tippet Type I is

fy=−−−−−ααexp⎡⎤ yu exp⎡ exp α yu⎤ Ynn ( ) n⎣⎦ nnn( ) ( nnn( )) ⎣ ⎦ where the parameters obtained from wave data are αn = 1.6520, un = 2.4584.

EXTREME TYPE 1 DISTRIBUTION (NNE) EXTREME TYPE 3 DISTRIBUTION (NNE) EXTREME TYPE 2 DISTRIBUTION (NNE)

6 7 14

5.5 6 12 5

10 5 4.5

4 8 Hs [m] Hs 4 [m] Hs [m] Hs

3.5 6 3 3 4 2 2.5

2 2 1 2 3 5 10 30 50 100 3 10 50 500 3 10 50 100 500 1000 Return Period [year] Return Period [year] Return Period [year] Figure 7. Type I, Type II, and Type III distributions from left to right with k-s test values of 0.123, 0.130, and 0.190, respectively.

Figure 8 shows the cumulative distributions of Type II, Type I, Type III extreme wave heights in the wave direction of NE. As is shown in the figures, extreme wave distribution of Type II with

215 k-s test of 0.063 is shown to be the best fit. Type II extreme wave height distribution which is also called Fisher-Tippet Type II is

kk+1 ⎡⎤ kv⎛⎞nn ⎛⎞ v fyYn()=−≥>⎜⎟exp⎢⎥ ⎜⎟ , y n 0, k 2 n vy y nn⎝⎠⎣⎦⎢⎥ ⎝⎠ n where the parameters obtained from wave data are vn = 2.4259, k = 5.7969.

EXTREME TYPE 2 DISTRIBUTION (NE) EXTREME TYPE 1 DISTRIBUTION (NE) EXTREME TYPE 3 DISTRIBUTION (NE) 12 8

11 5.5 7 10 5 9 6 4.5 8 5 7 4 Hs [m] Hs Hs [m] Hs 6 [m] Hs 4 3.5 5 3 4 3

3 2.5 2 2 2 3 10 50 100 500 1000 3 10 50 100 5001000 3 10 50 1000 Return Period [year] Return Period [year] Return Period [year] Figure 8. Type II, Type I, and Type III distributions from left to right with k-s test values of 0.063, 0.129, and 0.207, respectively.

Based on the extreme wave height analysis, design wave heights with return periods of 85 years, 50 years, 1 year, and 1 month were estimated for the wave directions of NE and NNE, respectively.

Joint probability distribution of significant wave heights and significant wave periods were obtained from the data as is shown in Figure 9 for wave directions of NNE and NE, respectively. Nonlinear regression analysis between the significant wave heights and significant wave periods was carried out to estimate the design wave period. Design codes require 50 years return period for the input wave data of shore structures such as submerged detached breakwater. However, no specific guidelines for the design of soft shore protection measures such as beach nourishment. Henceforth we utilized the return periods of 1 year and 1 month of wave information for the design of beach nourishment. Table 1 shows the design wave information obtained from in-situ wave measurements data.

216

Figure 9. The joint probability distribution of significant wave height and wave period obtained from wave data measured from year 1999 to 2004 for wave direction of NNE and NE, respectively.

Table 1. Design wave conditions obtained from the analysis of the in-situ wave measurement data with tidal elevations. Return Location C1 (10m depth) Waves Period T.E. (m DL)

(years) H S (m) TsS ( ) Dir W1a 50 +1.05 2.5 12.4 66

W1b 50 0.00 2.5 12.4 66 W1c 85 +1.05 2.7 14.0 66 W2a 1 +0.40 1.3 9.7 66 W2b 1 +0.40 0.7 12.5 66 W2c 1 0.00 0.7 12.5 66 W3 1/12 +0.12 0.9 10.8 66

Beach nourishment with submerged detached breakwater

Pohang city required that the beach restoration should be done with one-time funding. It is a rather strict constraint on the choice of design alternatives. Many different design schemes of beach restorations were considered.

First, we considered the beach nourishment only scheme. Many different configurations of beach profiles including effects of the slope of beach profile and the width of beaches had been tested

217 through analytic, numerical, as well as physical model tests. And we concluded that it requires at least 10,000,000m3 of sands for the beach nourishment without periodic re-nourishment plan. It is not feasible because no sand borrower site is available for providing 10,000,000m3 of sands.

Second, we had considered the beach nourishment with protective structures. Many different protective structures such as detached breakwater, artificial reef, head-land had been tested through numerical and physical modeling. It narrowed down to schemes of beach nourishment with submerged detached breakwater. After considering many different configurations of submerged detached breakwater, final two configurations of submerged detached breakwater were chosen as shown in Figure 10. Figure 11 shows the plan and side views of the beach nourishment with submerged detached breakwater.

2000

1800 POSCO

1600

1600 Cross- Cross- section section 1400 1400

1200 1200

1000 y (m) 1000 100m 100m y (m)

800 300m 300m 300m 800 900m

길이:900m 600 600

400 River 400 River

200 200

0 0 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 x (m) x (m) Figure 10. The plan view of 900m single and 3x300m with 100 gaps submerged detached breakwater, respectively. The top width of breakwater is 40m and the top elevation is 0.5m below the free surface.

B=100m

300 100 300 100 300

900

100m S = 1 : 50 20 1

40 mil $

Figure 11. The plan and side view of the beach nourishment with of the submerged detached breakwaters.

218 In order to decide the final layout of the breakwaters, numerical and physical modeling tests had been conducted for various design waves conditions. Figure 12 shows the result of numerical wave modeling tests with wave condition of 50 years return period. Figure 13 shows the result of numerical wave-induced current modeling tests with wave condition of 1-year return period. There seems to be no anomaly behavior of waves and wave-induced current patterns around breakwaters as shown in Figures 12 and 13. Therefore, it is not easy to choose the optimum configuration of breakwater from numerical modeling test of waves and wave-induced currents.

Figure 12. The result of the numerical wave modeling tests of three 300m and single 900m submerged detached breakwater with design waves of 50 years return periods, respectively. The width of beach nourishment is 100m.

Figure 13. The result of the numerical wave-induced current modeling tests of three 300m and single 900m submerged detached breakwater with design waves of 1-year return periods, respectively.

Curvilinear shoreline change modeling

In order to choose an optimum configuration of breakwaters, numerical shoreline change model developed by Suh and Hardway (1994) is used. Suh and Hardaway developed a curvilinear shoreline change model for prediction of shoreline change in the vicinity of offshore breakwater. The model uses curvilinear coordinates that follow the shoreline and is capable of handling the formation of tombolos and the growth of salients behind offshore breakwaters.

The curvilinear shoreline change model is capable of predicting shoreline changes accurately and computationally very efficient. The use of a curvilinear coordinate system may be more advantageous than the cartesian coordinate system for the situation in which the shoreline orientation is deviated substantially from the straight shoreline as on a tombolo behind a detached breakwater (Suh and Hardway, 1994).

219 The curvilinear coordinate system is shown in Figure 14. The continuity equation is

∂zs 1 ∂⎡Q ⎛⎞π ⎤ =−exp ⎢⎥i ⎜⎟θ + (1) ∂∂tDs⎣⎦⎝⎠2 r where zxiys =+ss is a complex variable of position of shoreline. D is the depth of profile closure at which no measurable change in bottom elevation occurs. Q is the volumetric longshore sediment transport rate written as ⎡⎤∂H 5/2 b QH=Γbb⎢⎥ K12sin() 2δ − K cotβδ cos b (2) ⎣⎦∂s where g Γ= 16()()Sps −− 1 1 κ

δbb=−a θ

is the breaking wave angle relative to the shoreline, g = gravitational acceleration, Ss = specific gravity of sediment relative to fluid, p = porosity, κ = ratio of wave height to water depth at breaking, Hb = breaking wave height, tan β = beach slope, KK12, = empirical longshore sediment transport coefficients.

WAVE ab

nr Q

mr s θ

y (xs,ys)

x Figure 14. The curvilinear coordinate system

For the input waves of the curvilinear shoreline change model, measured wave data obtained by using p-u-v wave gage located at C2 (water depth of 12m) shown in Figure 15 were used. The result of numerical shoreline change modeling is shown in Figure 16. The overall performance of beach protection (30% less erosion) using three 300m submerged detached breakwater is found to be better than that of the single 900m breakwater.

220 3990

3989.5

3989 + C1

3988.5 + C 2 Northing (km) 3988

3987.5

3987 532.5 533 533.5 534 534.5 535 535.5 536 536.5 Easting (km)

Figure 15. P-u-v wave gage was deployed at C2 (water depth of 12m) for wave measurement in 2006.

Initial Shoreline Initial Shoreline 1000 1000 Final Shoreline Final Shoreline

800 800

600 600

400 400 Distance [m] Distance 200 [m] Distance 200

0 0

-200 -200

0 200 400 600 800 1000 1200 1400 1600 1800 0 200 400 600 800 1000 1200 1400 1600 1800 Distance [m] Distance [m] Figure 16. Result of curvilinear shoreline change modeling

CONCLUSIONS

Aerial photos and water depth changes from series of nautical charts were found to be instrumental for the investigation of the causes of beach erosion. From the series of old nautical chats, amount of sands dredged as well as recharged from adjacent river could be estimated, which is by far most important information for the design process. For the optimum design of beach nourishment with protective structures, numerical as well as physical modeling tests were utilized to understand the specific design aspect. The final design configuration of beach nourishment with three 300 m long submerged detached breakwater for Song-do beach is found to be optimum subject to constraint without periodic re-nourishment.

221 ACKNOWLEDGEMENT

This research was supported by a grant (C105E1020001-06E020200310) from the Regionally Characterized Construction Technology Program funded by the Ministry of Construction & Transportation of Korean government.

REFERENCE

Suh, K.D. and Hardaway, C.S. 1994. Numerical modeling of Tombolo formation, International Conference on Coastal Engineering.

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