Takayuki Kumada

75. THEORY AND FIELD TEST OF BEACH NOURISHMENT USING COARSE SAND AND GRAVEL

On the Akiya coast a breakwater of Kuruwa fishing port was extended, inducing longshore sand transport from outside to inside the wave-shelter zone. Sand accumulated inside the wave-shelter zone, whereas beach erosion occurred on the nearby coast. A tracer test using sand of d50=0.5, 1.5 and 15 mm showed that coarse material was deposited near the shoreline and slowly moved alongshore, whereas fine material was carried offshore. The contour-line-change model considering changes in grain size was applied to the coast to predict the effect of beach nourishment using material with different grain sizes. Gravel nourishment was confirmed to be effective for recovering a stable beach without damage to the offshore fishing ground. Beach nourishment using 7,600 m of gravel with d50=15 mm was carried out in 2007 on the Akiya coast. The nourishment gravel was deposited near the shoreline forming a steep slope of 1/5, indicating that the nourished beach is very stable.

Seasonal Beach Changes on Hokota Coast in Ibaraki Prefecture and Effect of Submerged Groin

計算対象は,図-1に示すように鹿島灘に面した鉾田 (地先)海岸である.鉾田海岸の北15kmには大洗港が, 南25kmには鹿島港が立地し,これらの港に挟まれた区 域の中央やや北側に鉾田海岸は位置している.侵食対策 の必要区間は,鉾田海岸の23~29号HL間であるが,隣 接海岸への影響も考慮し,31号HL~21号HL間の10km 区間を計算対象とした. 鹿島灘海岸では,鹿島港と大洗港での防波堤建設に伴 う波の遮蔽効果により防波堤近傍での堆積と周辺海岸で の侵食が起こると同時に,波向変動に起因する沿岸漂砂 の方向の変化に伴う地形変動がこれに重なっている(今 井ら,2010).すなわち,冬季には海岸線への法線に対し て反時計回りから,一方,夏季には時計回りからの入射 となるが,全体的には海岸線形状はほぼ平衡状態にある. これは,宇多ら(2008)が指摘したように,冬季には高 波高(エネルギー平均波高1.85m)の波がN89°Eを中心 とする方向から入射するのに対し,夏季には冬季と比較 して相対的に波高の低い波(エネルギー平均波高1.49m) がN59°Eを中心に入射することにより,平均的には汀線 付近では北向きの漂砂フラックスが卓越するものの,沖 合では南向きの漂砂フラックスとなって全体として漂砂 循環が起こり,防波堤による波の遮蔽効果や浚渫などの 影響がない場合,海浜が動的平衡を保つことによる.本 計算ではこの考え方に基づいて波浪条件を設定した. 茨城県鉾田海岸の季節的海浜地形変動と潜突堤の効果検討 Seasonal Beach Changes on Hokota Coast in Ibaraki Prefecture and Effect of Submerged Groin

Experimental Study on Improvement of Artificial Reef with Crests

実験は固定床と移動床模型実験からなる.まず,固定 床模型実験により人工リーフの形状を様々変化させた場 合の消波効果や海浜流の変化などを調べ,一連の実験結 果から相対的に効果が高い3タイプを選択し,比較のため の離岸堤の実験と合わせて4タイプを対象として移動床模 型実験を行った.いずれの実験も,国総研の幅30 mの平 面水槽を用い,固定床実験では図-1の左端に示すように 幅2 mに仕切った水槽を用いた.また,移動床実験では, 幅16 mに仕切った平面水槽を用いた(図-1).両者とも 実験縮尺は宇多ら(2010)と同様1/50とし,海浜縦断形 は皆生海岸の平均的な縦断形を与え,前浜勾配は1/15,0 ~-7.5 mの海底勾配は1/30,-7.5 m以深は1/100とした. 固定床実験では,人工リーフの堤長120 mと開口幅80 mの1/2を水路内に製作した.入射波は規則波とし,皆生 海岸の静穏波,代表波,暴浪波,設計波の4種を対象とし 水理模型実験によるクレスト型人工リーフの改良検討 Experimental Study on Improvement of Artificial Reef with Crests

Field Experiment on Change in Thickness of Sand Mixing Layer on Jinkoji Coast

Field experiment on changes in thickness of the sand mixing layer was investigated on the Jinkoji coast, where beach nourishment using coarse material has been carried out. Four trenches were excavated on the shore face, andcoarse material, fine sand and their mixture were filled in each trench, and subsequent mixing due to waves were investigated. Comparing the mobility of sandy beach composed of various materials, minimum mobility was obtained at gravel, and the mobility of the beach composed of the mixture of sand and gravel was the largest, implying that the effect of the beach nourishment using the mixture of sand and gravel is inferior to that using gravel only.

BEACH EROSION ON NARUSAWA, TAGA AND KAWARAGO COASTS IN IBARAKI PREFECTURE

Beach changes on the Narusawa, Taga and Kawarago coasts in Hitachi City in Ibaraki Prefecture were investigated on the basis of the aerial photographs and field observation. The shoreline changes between 1984 and 2006 were studied. Sea cliff extends along these coasts, and narrow sandy beaches have been formed mainly by the sand supply from small rivers. The breakwaters of Kawarago port and Ohse fishing port were constructed, resulting in the formation of the wave shelter zone. Longshore sand transport from outside to inside the wave shelter zone was induced, causing erosion outside the wave shelter zone and accretion inside it. In the eroded zone, sea cliff is exposed to waves and abrasion by gravels is underway. To protect these beaches, comprehensive management such as the sand recycling is required.

RESTORATION METHOD OF SANDY BEACH AT TENNOZAKI IN LAKE KASUMIGAURA

A restoration plan recovering sandy beaches at Tennozaki in Lake Kasumigaura was investigated. Past lakeshore changes were first investigated through the comparison of old map and aerial photographs. A stable shoreline of a pocket beach was predicted by applying a modified Hsu model (Serizawa et al., 1996). Numerical simulation using a contour-line-change model considering the change in grain size proposed by Kumada et al. (2003) was also carried out. Three-dimensional beach changes when beach nourishment was carried out were predicted by this model.

Beach Erosion of Kashimanada Coast and Prediction of Beach Changes

On the Kashimanada coast, fine sand was selectively transported and deposited in the wave-shelter zone behind the offshore breakwater built on both ends of the coastline, whereas beach was eroded in the central part of the coastline. As a measure, artificial headlands have been constructed on this coast, but beach erosion is still severe. The contour-line-change model considering the effect of changes in grain size was applied to this coast, and the effect of the measure was evaluated. The differences between the total accreted and eroded volumes of sand in the condition with/without headlands became 7.4×106 and 8.1×106 m3. Thus, it was found that the movement of approximately 8.0×106 m3 of sand was controlled by headlands.

A model for Predicting Distribution of Habitat of Japanese hard clam Meretrix lamarckii

A model for predicting the distribution of the habitat of Japanese hard clam Meretrix lamarckii was developed, taking the life history between a larva and an adult clam into account. The coupling was made with the contour-line-change model by proposed Kumada et al.(2005), which considers the sorting effect due to change in grain size. The habitat change corresponding to the development of the clam as well as the spatial change in habitat can be predicted by this model. The model was applied to the Hazaki coast. Predicted habitat was in good agreement with the measured habitat. This model can be used for predicting the effect of release of this clam to increase the clam resources.

BEACH EROSION CAUSED BY OBSTRUCTION OF CONTINUOUS LONGSHORE SAND TRANSPORT

1正 会 員 工 博(財)土 木 研 究 セ ンタ ー 理 事 な ぎ さ総 合 研 究 室 長(〒110 -0016台 東 区台 東1-6-4タ カ ラ ビル) 2海 岸 研 究 室(有)(〒160 -0011東 京 都 新 宿 区 若 葉1-22ロ ー ヤル 若 葉301) 3正 会 員 国 際 航 業(株)河 川 砂 防 ・海 岸 部(〒191 -0065東 京 都 日野 市 旭 が 丘3-6-1) 4正 会 員 海 岸研 究 室(有)(〒160 -0011東 京 都 新 宿 区 若 葉1-22ロ ー ヤ ル 若 葉301) 5正 会員 工博(株)水 圏 科 学 コ ンサ ル タ ン ト技術 部(〒145 -0064東 京 都 大 田 区上 池 台1-14-1明 伸 ビル) 6正 会 員(株)水 域 ネ ッ トワー ク(〒134 -0088東 京 都 江戸 川 区 西 葛西6-16-7第2白 子 ビル302)

EROSION MECHANISM OF AKIYA COAST LOCATED IN WESTERN PART OF MIURA PENINSULA

Beach changes of the Akiya coast located in the western part of Miura Peninsula was investigated through field observations, analysis of aerial photographs and numerical simulation using the contour line change model. One of the causes of beach erosion is due to the longshore sand movement associated with the formation ofwave shadow zone of the breakwater of Kuruwa fishing port. The other main cause is the loss of sand due to the northward longshore sand transport, overflowing the foot of Chojagasaki Point located at the north end of the coastlineduring rough wave conditions.