Comparison of Geomorphological Land Classification Maps with Flood Hazard Maps at Hachinohe Plain, Aomori Prefecture

Abstract

 To present meaningful information about the progress of flood hazard prediction at the alluvial plain, first, the relationships between geomorphological land classifications of the Hachinohe plain, Aomori Prefecture, conducted for three different periods, are considered; then, the relationships between landforms and flood hazards are discussed through a comparison of geomorphological land classification maps with flood hazard maps of the plain. All three landform classification maps show the distribution of lowlands, which consist of floodplain, natural levee, present and abandoned channel, and beach ridge, and surrounding terrace surfaces. However, the geomorphological land classification in the older two maps (published in 1977 and 2015), conducted mainly by interpreting aerial photographs, provides a different interpretation; part of floodplain in the 2015 map is shown as a terrace surface in the 1977 map. The newest map, dated 2021, which incorporates sedimentary facies and formation age of the geomorphic surface, indicates that the difference in interpretation between the older two maps can be explained by the lowland of the Hachinohe plain being divided into Middle to Late Holocene fluvial terrace surface (alluvial surface I) and Late Holocene to modern lowermost floodplain (alluvial surface II). A comparison of the geomorphological land classification map with the flood hazard map shows that both alluvial surfaces I and II are correlated with the conceivably flooded area at the assumed highest precipitation, and that these surfaces are divided into areas where rapid evacuation is necessary (alluvial surface II) and unnecessary (alluvial surface I). These investigations suggest that the lowermost terrace can be flooded at the assumed highest precipitation and that identifying the lowermost terrace can contribute to predicting the spatial distribution of degree of flooding. To understand general features of flooding at the lowermost terrace, further examinations are necessary at other alluvial plains on the relationship between landform and flooding.