Coastal Levees Research Articles

Breaking process and mechanism of coastal levees on Sendai Bay Coast hit by the 2011 mega tsunami

Mano, A., IIda, T., Udo, K., and Tanaka. H., 2013. Breaking process and mechanism of coastal levees on Sendai Bay Coast hit by 2011 mega tsunamiThe 2011 mega tsunami induced by the East Japan Earthquake, Mw 9.0 hit the Sendai Bay Coast, overtopped the coastal levees and intruded deeply onto the Sendai and Ishinomaki plains by flushing people, houses, trees, etc. The levees with 6 to 7 m height had been constructed along 65 km-long sandy beach to protect the hinterland from the storm surges and wind waves. Eighty percent of the levees were broken by the tsunami in various degrees ranging from scatter of the surface blocks to the complete disappearance. The government decided to rebuild the levees with the target of the local major hazards as the storm surges but in a durable way even for the tsunami overtopping. Based on this background, this study aims to find the breaking process and mechanism of the coastal levees especially for the destructive cases. We conducted field survey, and collected aerial photos, tsunami records and videos, blue prints of the levees, etc. We integrated information of the tsunami records and videos, and matched the spatial information of the various data by GIS. These analyses enabled well understanding on the breaking process and mechanisms composed from two steps: (1) The impact of the leading bore with the maximum amplitude of 6 to 7m broke weaker parts like structural joints, parapets, landward soil slopes. (2) Return flow concentration to the broken parts in the first step or to the channels and swamps expanded erosion to make tsunami channels and bays and to lead levee breach.

DESTRUCTION MECHANISM OF COASTAL LEVEES ON THE SENDAI BAY COAST HIT BY THE 2011 TSUNAMI

A mega tsunami hit the Sendai Bay Coast on March 11, 2011, overtopped coastal levees and intruded into far inland while sweeping houses, people and others away. Eighty percent of the levees which rimmed the coast to protect the land from storm surges together with the wind waves were broken in various degrees of damage by the tsunami. The national and local governments decided to rebuild the levees to be durable even for mega tsunamis. This requirement motivates us to find the destruction mechanism of the coastal levees. We conducted field investigations and collected the tsunami records, aerial photos and tsunami videos. Especially, the video taken from the helicopter "Michinokugo" which flew along the Sendai Coast to the south during the attack of the tsunami's leading wave enables us to see the breaking process. Integrated analysis leads to two step mechanisms of the destruction: the first step of breaking the upper structure of the levees by the surging bore of the leading wave and the second step of expanding erosion by the return flow concentration.

Field Verification on Calculation Methods of Wave Runup Height

Wave runup is an important factor for coastal levee design and flood defence activities along the coast. A calculation method of wave runup height often used for facility design is based on model experiments with relatively-steep slope (steeper than 1/30). We proposed new calculation equations of wave runup height for very gentle slope (milder than 1/30), and verified the equations and other methods by field observations in three coasts. The field verification results that our equation can precisely calculate wave runup height in a gentle-slope coast, and that calculation method of wave runup height should be selected based on sea bottom slope and wave steepness.

Hamburg University Radiocarbon Dates III

The following list consists of dates of soil samples, partly produced in the former Bonn laboratory and bearing the code designations, BONN, and partly in the present dating lab at Hamburg University. The list comprises Mollisols and Inceptisols from Germany, sampled by layer and dated as whole soil, hydrolysis residue, and hydrolysate. Other profiles represent selected Australian Vertisols and Krasnozems, sampled by layer as well. Dates derived from marshes of the Elbe River as well as from paleosols buried by coastal levees are also included in the list. Pretreatment of soil samples is described in Scharpenseel and Pietig (1969) and Scharpenseel (1972; 1977).

Holocene sedimentation at Cape Sable, south Florida

A regionally distinct mosaic of sedimentary environments including beaches and beach ridges which formed under relatively high-energy conditions through low energy, tide-dominated environments to quiescent inland lakes and ponds, exists at Cape Sable, south Florida. Environments of deposition from the sandy capes inland are: (1) shell beaches and beach ridges; (2) black mangrove mudflats; (3) ponded mudflats; (4) exposed mudflats; (5) Lake Ingraham; (6) coastal levees and supratidal plain; and (7) ephemeral ponds. With exception of the beaches and beach ridges, sediments of the other environments are dominantly calcareous muds and silts and represent a depositional history related to frequency of tidal inundation and storms. Of the three capes exposed roughly from northwest to southeast at Cape Sable, radiocarbon dating shows that Northwest Cape formed in its present position or possibly migrated to it 1980 ± 100 years B.P., while Middle Cape and East Cape data from 1610 ± 100 years B.P. and 1230 ± 95 years B.P. Relict shoreline features, coastal levees, on the supratidal plain indicate an episode of coastal progradation which started 2280 ± 100 years B.P. and apparently ended about 1560 ± 80 years B.P. The entire column of Holocene carbonate sediments in the cape area rests on a thin, intermittent basal peat which dates 4950 ± 120 years B.P. Cores through the supratidal plain reveal a basal carbonate-mud sequence much like subtidal sediments of modern Florida Bay. This unit has a typical marine carbonate-mineral suite, numerous shallow-marine molluscs, and an abundance of Thalassia root burrows. An intermediate intertidal unit characterized by marine brackish carbonate sediments with algal laminations is overlain by massive aragonitic supratidal silts and silty clays. The entire sequence is capped by a dark, highly organic zone representative of the coastal levee and intervening algal-flat sediments. Trace quantities of dolomite and substantial amounts of low-Mg calcite found throughout the cores are considered to be of detrital origin. Localized surface concentrations of dolomite associated with the coastal levees may have a mixed in-situ and detrital origin. Investigation of the extractable organic matter in the intertidal facies of cores through the three prominent coastal levees indicates that diagenesis of organic matter has systematically occurred from the youngest to the oldest levee. Fatty alcohols and total hydrocarbons increase with time while fatty acids decrease. Partial decarboxylation and chemical reduction of fatty acids can occur within the time interval of 750 years between the oldest and youngest coastal levees. Geochemistry of interstitial water from the supratidal-plain cores indicated a general increase in chlorinity from the youngest to the oldest coastal levee. Chlorinity levels drop at the base of all three cores analyzed which may be related to fresher groundwater influence from the Miami Limestone. Ratios of calcium, magnesium, and strontium to chloride are remarkably constant throughout the cores. Higher relative strontium concentrations in the core from the intermediate coastal levee may indicate a subtle dissolution of aragonite. In general, evidence of diagenesis in these carbonate sediments is not readily apparent.