Mio Kasai

Estimation of temporally averaged sediment delivery ratio using aggradational terraces in headwater catchments of the Waipaoa River, North Island, New Zealand

The sediment delivery ratio was estimated for two periods (28 years and eight years) following reforestation of seven tributary catchments (0·33 to 0·49 km2) in the headwaters of the Waipaoa River basin, North Island, New Zealand. In these catchments, gully erosion, which largely resulted from clearance of the natural forest between 1880 and 1920, is the main source of sediment to streams. Reforestation commenced in the early 1960s in an attempt to stabilize hillslopes and reduce sediment supply. Efforts have been partially successful and channels are now degrading, though gully erosion continues to supply sediment at accelerated rates in parts of the catchment. Data from the area indicate that the sediment delivery ratio (SDR) can be estimated as a function of two variables, ψ (the product of catchment area and channel slope) and Ag (the temporally averaged gully area for the period). Sediment input from gullies was determined from a well defined relationship between sediment yield and gully area. Sediment scoured from channels was estimated from dated terrace remnants and the current channel bed. Terrace remnants represent aggradation during major floods. This technique provides estimates of SDR averaged over periods between large magnitude terrace-forming events and with the present channel bed. The technique averages out short-term variability in sediment flux. Comparison of gully area and sediment transport between two periods (1960–1988 and 1988–1996) indicates that the annual rate of sediment yield from gullies for the later period has decreased by 77 per cent, sediment scouring in channels has increased by 124 per cent, and sediment delivered from catchments has decreased by 78 per cent. However, average SDR for the tributaries was found to be not significantly different between these periods. This may reflect the small number of catchments examined. It is also due to the fact that the volume of sediment scoured from channels was very small relative to that produced by gullies. According to the equation for SDR determined for the Waipaoa headwaters, SDR increases with increasing catchment area in the case where Ag and channel slope are fixed. This is because the amount of sediment produced from a channel by scouring increases with increasing catchment area. However, this relationship does not hold for the main stem of the study catchments, because sediment delivered from its tributaries still continues to accumulate in the channel. Higher order channels are, in effect, at a different stage in the aggradation/degradation cycle and it will take some time until a main channel reflects the effects of reforestation and its bed adjusts to net degradation. Results demonstrate significant differences among even low order catchments, and such differences will need to be taken into consideration when using SDR to estimate sediment yields. Copyright

13 Changes in basin-scale sediment supply and transfer in a rapidly transformed New Zealand landscape

Abstract Society has an ever-increasing need to manage landscapes. To do this effectively requires improved understanding of the way landscapes behave, and the controls on that behaviour. This is certainly the case where sustainable resource use and hazard mitigation involve the management of the generation, transport, and storage of sediment. Landscapes are complex systems, consisting of a mosaic of landforms. At the broad regional level these landforms are arranged in characteristic patterns, reflecting environmental conditions and associated processes. At the catchment level, assemblages of landforms have a unique configuration, forming an interactive functioning system through which water and sediment are passed. It is this unique, catchment-based assemblage that we seek to manage. Controls on the way landscapes behave are numerous and operate at a variety of scales both spatial and temporal. The way these controls interact on a complex and unique arrangement of landforms is difficult to predict. The East Coast of the North Island of New Zealand is a dynamic landscape. High natural erosion rates have been augmented by recent and rapid anthropogenic activity. Several studies in the Waiapu catchment, involving a range of spatial and temporal scales, are used here to illustrate the impact of natural and anthropogenic controls on basin-scale sediment supply and transfer. In this landscape, the use of vegetation, specifically targeted reforestation, is the most effective method of sediment management. This will be enhanced by improved understanding of stability thresholds and hill slope–channel connectivity.

A quantitative analysis of emergency survey techniques for large-scale sediment disasters

近年,平成23年(2011年)3月の東日本大震災や同年 9月の紀伊半島大水害等の激甚な地震・豪雨により,複 数の都道府県に被害が及ぶような,被害が広範囲に及ぶ 大規模な土砂災害(以下,広域大規模土砂災害)が発生 している。広域大規模土砂災害に対する危機管理におけ る初動時の対応の代表的なものとして,国土交通省が実 施する土砂災害防止法に基づく緊急調査(以下,緊急調 査,全国治水砂防協会,2016),TEC-FORCE(緊急災害 対策派遣隊,国土交通省水管理・国土保全局,2016)に よる被災自治体支援等の制度・体制の整備・運用が行わ れている。 上記のような初動時の対応を円滑に実施するためには, 例えば,緊急調査では河道閉塞(天然ダム)の形成の有 無,TEC-FORCEの活動では土砂災害が集中して発生し ている地域といった被害状況を初動時に迅速に調査し, 把握することが,危機管理に関する人的資源・資機材の 配分を適切に行うために重要である。広域大規模土砂災 害発生時に,どのような種類の調査技術が用いられたか, 及び,調査結果については個別に多く記録がなされてい るものの(例えば,国土技術政策総合研究所・土木研究 所,2011;国土技術政策総合研究所・土木研究所,2013), 調査技術の調査能力を定量的に把握・比較し,評価を 行った事例はほとんどない。また,被害の及んでいる可 能性のある範囲・天候等の災害の発生状況,調査技術の 特徴に基づいて,調査技術の組み合わせについて検討し た事例もほとんどない。このため,より迅速な被害状況 把握ができるような,調査技術のさらに適切な活用方法 について検討ができていない。 本研究では,近年の広域大規模土砂災害の事例として, 平成23年に発生した東日本大震災(地震)と紀伊半島大 水害(豪雨)の2つの誘因の異なる災害を取り上げ,そ の初動時にどのような調査がどのような量で行われたか 評価し,その結果に基づいて,初動時にどのように調査 技術を組み合わせると効率的であるかについて考察を行 う。