(II) A trial of interpretation of soft ground by the morphometric analysis of micro-geomorphology.
1) A study based on the methods and techniques of “micro-landform” analysis on aerial photographs mentioned previously is carried out in case of the alluvial lowlands in the lower Tenryu River Region, close to Hamamatsu City, on the Pacific coast of Central Japan, before the discussin on the morphometric analysis of micro-geomorphology. From this study, the methods of the systematic classification of “micro-landform units” are proved to be useful to infer but surface materials also the surface layer type of soft ground.
2) However the surface distribution ranges and types of soft ground are ascertained, the depth or thickness of the soft ground is not determined. As mentioned by Belcher (1960), it is difficult to interpret the thickness or depth of materials from aerial photographs. In order to examine the possibility of interpretation of the thickness of soft muddy layers which compose soft ground from aerial photographs, it is necessary to study the relationship between the thickness of the layers and the micro-geomorphological features seen on the photographs. For this purpose the writer tried to measure the distribution pattern of the “micro-landform units” by a simple morphometric analysis using aerial photographs. The morphometric data are compared with the geologic data obtained from bore hole excavations, in case of the alluvial lowlands formed by the Tenryu and the Ota Rivers.
The distribution pattern of the “micro-landform units” is measured by “Micro-Relief Ratio (MR) ” defined by
MR=Total area of micro-relief in a unit area/Unit area (1 sq. km.) ×100 (%) .
Where, micro-relief means areas of shight elevation such as former channel bars, natural levees, etc., as well as abandoned low water channels.
The value of MR shows the distribution density of coarse granular materials on the surface of alluvial lowlands, and the relative area formed by muddy deposits. From Fig 2-2., the total value of MR in a lowland area is thought to be a approximate index showing the geomorphic development processes, the stability of the depositional environment and the ratio of the volume of fine grained materials to the total volume of materials transported by the rivers in the recent years. Consequently, one can expect that the smaller velues of MR indicate thicker fine grained deposits.
Based on these hypotheses, the relationship between MR and the thickness of muddy deposits including peat layers (UM & LM) is analyzed and discussed. Furthermore, the relationship between the “Damage Percentage (DP) ” of wooden houses caused by To-Nankai Earthquake in 1944 and MR is discussed.
3) The correlation between the thickness of muddy deposits (Tm) and the value of MR is not so distinct as shown in Fig. 2-11 a & b. But, generally speaking, Tm increases exponentially with decreasing MR. The maximum thickness of UM appears at the value of MR between 8% and 32%, and the total thickness of UM and LM exceeds more than 30m. or 40m. at the value of MR of less than 20%. When the mean value of MR was calculated in the 1km. zone crossing the lowlands (MR mean), it is the area where MRmean is less than 20% or 30% that one can expect the coexistence of both surface layer type (UM) and deep layer type (LM) of soft ground. And in such areas, for instance natural levee zone and lagoonal delta of the Ota River, one can also expect that the total thickness of soft muddy deposits is greater than 30m. or 40m (see Fig. 2-10) .
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