Delineation of Historical Fluvial Territories and the Implications for Flood Mitigation, with Reference to Four Selected Reaches in Taiwan

Levee construction in support of land reclamation policies has been implemented on almost all well-developed braided channels in Taiwan during the twentieth century. Increasing record-breaking high-intensity rainfall events and consequent severe flood disasters since the mid-1990s, however, have raised great concerns about reclaimed active floodplains. In light of the non-hard engineering flood mitigation approach being adopted, a geomorphic perspective of land management seems to be absent in Taiwan. Thus, with reference to four braided channel reaches, this chapter aims to demonstrate the delineation of the reclaimed active floodplains, so-called historical fluvial territories (HFTs), and to examine the landcover/landuse type change in the HFTs over the twentieth century. The reason why a geomorphologically-informed active floodplain is essential and an urgent need on flood disaster mitigation in Taiwan is also justified. A historical approach, employing sequential historical map overlays, aerial photographs and orthogonal maps with GIS analysis, was adopted. Various official archives were also consulted. It is established that HFTs identified from historical maps and aerial photographs published in the first half of the twentieth century can be regarded as active floodplains in the near-natural condition. Although most lands in the HFTs remain in agricultural use, as originally planned, land use intensity has been gradually increased especially in those close to the economic cores. It is argued strongly that land development should be strictly monitored, not only in those areas at present officially defined as flood-prone but across the entire active floodplain in near-natural conditions, such as the HFT proposed by the current work.