Levee Slope Research Articles

Development of Mowing Technologies for Large Levee Slopes

Development of Mowing Technologies for Large Levee Slopes

Submarine channel levee shape and sediment waves from physical experiments

Abstract Submarine channel levees aggrade through repeated overspill events from the channel axis. The shape of the levees may therefore reflect some characteristic(s) of the overspilling flow. It has been noted that basin floor levees typically have a relatively low-relief and taper exponentially to their termination; in contrast slope channel levees may be much steeper close to the channel. A simple physical experiment was performed where a surge-like sediment-laden current flowed through a curved channel. Significant overspill occurred and generated a deposit flanking the channel on either side. The experiment was repeated 25 times to build up low-relief channel-levees. It was found that in proximal areas, levees were steep and characterised by power-law decays, a transitional zone of logarithmically thinning levee was found a little further down-channel, followed by exponential decays in medial to distal areas. The style of levee decay is a function of spatial variation in overbank sedimentation rates. Where flows rapidly lose momentum and deposit across the grain-size spectrum, i.e., in proximal areas, levees tend to be steep; farther down the channel, the steep levee slope gives way to a more gradually tapering deposit. In more distal parts of the channel, deposition is directly related to sediment settling velocity (rather than the suspended load exceeding flow transport capacity as is the case in proximal areas), the deposit reflects this with relatively simple exponential thickness decays. Additionally, small-scale sediment waves developed under lee wave conditions on the inner-bend overbank. The waves initially migrated slightly towards the channel, but as the style of overspill evolved due to intra-channel deposition, flows moved out of the lee wave window and sedimentation became out of phase with the wavelength of the features and the topography was healed.

Morphodynamic evolution of experimental cohesive deltas

Here we describe new techniques for creating river‐dominated (birds foot) deltas with strong channelization in the laboratory. The key to achieving strong self‐channelization is the addition of a commercially available polymer to the sediment mixture. This polymer enhances the substrate strength increasing the critical erosion stress, an important geomorphic threshold. More importantly it increases the rate of cohesion onset to account for increased rates of morphodynamic evolution in small‐scale experiments. A cyclic pattern of delta evolution is observed. The delta “avulsion cycle” begins with channel avulsion, erosion, and channel elongation and ends with channel backfilling and abandonment. This cycle appears to be universal but is subject to a range of controls, including sediment size distribution, sediment concentration, substrate cohesiveness, and Froude number. We propose that the observed depositional cycle is characteristic of an avulsion mechanism that is more complex than current models of fluvial systems, which generally explain avulsion probability as an upstream effect dependent on channel superelevation or levee slope. The experiments suggest that in many distributary channel systems, including deltas, alluvial, and deep water fans, downstream mediated topographic effects or “morphodynamic backwater effects” may dominate over upstream avulsion processes and control the surface mechanics and stratigraphy. The experimental observations are synthesized into a new depositional model for river‐dominated deltas which emphasizes the importance of self‐organization and feedback in delta surface evolution and stratigraphy.

有機物マルチが畦畔法面のグラウンドカバープランツの生育および雑草発生に及ぼす影響

有機物マルチが畦畔法面のグラウンドカバープランツの生育および雑草発生に及ぼす影響

Relationship between period after abandonment and species richness or species composition of semi-natural grasslands at paddy levee slopes

Relationship between period after abandonment and species richness or species composition of semi-natural grasslands at paddy levee slopes

畦畔法面に植栽した被覆植物の生育に及ぼす有機物マルチの影響

畦畔法面に植栽した被覆植物の生育に及ぼす有機物マルチの影響

The width of a bankfull channel; Lacey's formula explained

According to Lacey's formula, the width of a natural channel at bankfull flow is proportional to the root of the discharge. It is a very simple formula that has been confirmed by many authors and which, until now, has had no physical explanation. It appears that Lacey's equation is composed of physical and measurable parameters which agree with field observations. The equation hinges on the fact that the velocity at bankfull discharge is a sole function of the bed material. At bankfull discharge the average velocity is no longer a function of the discharge, as is assumed in regime theory. At discharges below bankfull level the stream velocity is a function of the discharge to the power 1/6. However at bankfull discharge a singularity occurs where the water slope is forced on the slope of the natural levees. The velocity of flow associated with this slope is fully determined by the bed material, and is independent of the discharge. An analytical expression for Lacey's coefficient of proportionality is presented.

Effects of Covering of the Cultivars Originated from Native Species Introduced into Levee Slope around Paddy

基盤整備後の棚田畦畔を自然に近いかたちで緑化する目的で, 我国の在来種に由来する園芸植物を畦畔法面に定植し, 1年間にわたりそれらの法面被覆過程を調査した.シロバナサギゴケ区では成長の開始が早く, 5月下旬には全面が被覆された. しかし秋季にはほとんどの親株が枯死した. キチジョウソウやコグマザサは初期成長が遅く, 処理区内に多くの雑草が発生した. しかしながら何度か草刈りを行えば, 秋季までには活着し法面を十分被覆した. 土壌の水分状態に対応し, 法面下部ではシロバナサギゴケの生育がよく, 逆にコグマザサは生育が抑制された.

Architecture Formation of Shrub on Levee Slope around Paddy Field

水田畦畔の緑化に木本植物を導入することを前提に実験を行い, 畦畔草地の草刈り管理における刈取の高さ及び畦畔斜面の上・中・下部の違いによる樹形形成の変化を観察した.試験地の畦畔は北向きの斜度45度の斜面, 斜面の長さは7m, 土壌水分量は斜面上部ではpf値約2.4, 中部約2.1, 下部約1.8である. クサボケは, 斜面中部において草地の刈取の高さがOcmの区で253%, 20cmの区で120%の高い成長率を示した. ヤマツツジは, 斜面中部で刈取の高さ10cm. 20cmの区でそれぞれ118%と123%, 斜面下部の0cm, 10cmの区で各々104%と108%の高い成長率を示した.

Overbank sand deposition in relation to transport volumes during large-magnitude floods in the Dutch sand-bed Rhine river system

Using aerial photographs and field measurements, sandy overbank deposits formed by the large-magnitude floods of 1993/94 and 1995 were quantified along two branches of the Dutch Rhine river system: the Waal (1993/94 and 1995) and the IJssel (1995). These deposits were laid down intermittently all along the length of these rivers on the top and landward slope of the natural levees, and covered about 4 per cent of the embanked floodplain on the Waal and about 1 per cent on the IJssel. The overbanks and transport mechanism is basically convective by nature. The spatial variability of overbank sedimentation points to the important role played by helicoidal currents in determining overbank deposition. The presence of embankments and training works appears to influence the sand transport to and morphological development of the floodplains along the Dutch Rhine river system. Overbank deposition volumes about equal present estimates of sand transport during a large-magnitude flood. It appears that studies on sand transport in the Dutch Rhine carried out so far underestimate sand transport during floods. © 1998 John Wiley & Sons, Ltd.

Estimation of Erosion Resistance of the Levee Slopes Covered by Vegetation Which is Managed by Two Cuttings per Year.

Levee slope protection by use of vegetation is more economical and suitable for river enviroment than ordinary protection constructed by concrete blocks. It is important for practical use of protection by use of vegetation to clarify the relation between erosion resistance and vegetation management. In this study, erosion resistance of the levee slopes covered by vegetation which is carried out most popular vegetation management, two cuttings per year, was estimated on the basis of results of levee survey which was conducted at six different rivers. Futhermore estimation method of frequency of cuttings per year was proposed to maintain higher resistance to erosion.

Effects of Agricultural Land Consolidation on Erosion Processes in Semi-Mountainous Paddy Fields of Japan

In semi-mountainous paddy fields of Japan, severe erosion occurs as a result of heavy rainfall or storm. In view of the considerable significance of agricultural land conservation for sustainable agriculture in semi-mountainous paddy fields, this study was conducted to show erosion phenomena and to consider the conservation effects of agricultural land consolidation which is carried out to make field sizes larger, mechanization easier and productivity per unit area higher. As the paddy field surface is flooded, levelling of adjacent surfaces and construction of levee slopes are essential. Also, soils of subsurface layers in paddy fields and levees are compacted to decrease percolation. Erosion in paddy fields occurs mostly on levee slopes, so soil structures in levee slopes were examined to explain erosion processes. The number of erosion events per hectare in consolidated paddy fields was smaller than that in non-consolidated paddy fields. Additionally, soil permeabilty at the toe of non-consolidated levee slopes was lower than that at the top. It was judged that soil structural changes were brought about by percolating water in non-consolidated levee slope. During heavy rainfall, water percolates through levee slopes and increases soil water pressure and degree of saturation in the upper zone of clay deposition. Consequently, these conditions cause erosion phenomena such as slope failures or gully erosions. Therefore, agricultural land consolidation appears to be of importance in modifying soil structural changes and in reducing erosion events.

Vegetation dynamics of abandoned paddy fields and their levee slopes in mountainous regions of central Japan.

Vegetation dynamics of abandoned paddy fields and their levee slopes in mountainous regions of central Japan.

畦畔除草作業を効率化する排水路の形態:畦畔除草に適した圃場整備技術の開発 (V)

畦畔除草作業を効率化する排水路の形態:畦畔除草に適した圃場整備技術の開発 (V)

急傾斜地水田における除草作業の安全性・効率と畦畔法面形状:畦畔除草に適した圃場整備技術の開発 (III)

急傾斜地水田における除草作業の安全性・効率と畦畔法面形状:畦畔除草に適した圃場整備技術の開発 (III)

Minimizing contamination hazards to waterbirds using agricultural drainage evaporation ponds

In much of the San Joaquin Valley, California, USA, inadequate drainage of applied irrigation water and accumulating salts in the soil have necessitated the installation of subsurface tile drainage systems to preserve crop productivity. At present, these subsurface drainage waters are disposed of by means of evaporation ponds or discharges into the San Joaquin River. Unfortunately, most of these agricultural drainage waters contain high concentrations of salts and naturally occurring trace elements, such as selenium, and recent evidence indicates that substantial numbers of waterbirds are exposed to contamination by selenium in the evaporation ponds. In order to avoid, minimize, or mitigate the adverse impacts on wildlife using the ponds, alternative pond management methods must be identified and evaluated for implementation. A number of methods have the potential to be cost-effective in significantly reducing the contamination hazard to birds using agricultural evaporation ponds. Twenty general methods were evaluated in this study, and four methods are recommended for implementation: remove levee vegetation, remove windbreaks, deepen the ponds, and haze birds. A number of other methods are recommended for further consideration because they appear to have good prospects for reducing the contamination hazard: steepen interior levee slopes, apply herbicides and insecticides, place netting on pond shorelines, and provide freshwater habitat adjacent to evaporation ponds. It may be necessary to use a combination of methods to effectively control selenium contamination of aquatic birds because it is unlikely that a single affordable pond management method will be able to entirely eliminate the contamination hazard.

Geotechnical characteristics and slope stability on the Ebro margin, western Mediterranean

Sedimentological and geotechnical analyses of core samples from the Ebro continental slope define two distinct areas on the basis of sediment type, physical properties and geotechnical behavior. The first area is the upper slope area (water depths of 200–500 m), which consists of upper Pleistocene prodeltaic silty clay with a low water content (34% dry weight average), low plasticity, and high overconsolidation near the seafloor. The second area, the middle and lower slope (water depths greater than 500 m), contains clay- and silt-size hemipelagic deposits with a high water content (90% average), high plasticity, and a low to moderate degree of overconsolidation near the sediment surface. Results from geotechnical tests show that the upper slope has a relatively high degree of stability under relatively rapid (undrained) static loading conditions, compared with the middle and lower slopes, which have a higher degree of stability under long-term (drained) static loading conditions. Under cyclic loading, which occurs during earthquakes, the upper slope has a higher degree of stability than the middle and lower slopes. For the surface of the seafloor, calculated critical earthquake accelerations that can trigger slope failures range from 0.73 g on the upper slope to 0.23 g on the lower slope. Sediment buried well below the seafloor may have a critical acceleration as low as 0.09 g on the upper slope and 0.17 g on the lower slope. Seismically induced instability of most of the Ebro slope seems unlikely given that an earthquake shaking of at least intensity VI would be needed, and such strong intensities have never been recorded in the last 70 years. Other cyclic loading events, such as storms or internal waves, do not appear to be direct causes of instability at present. Infrequent, particularly strong earthquakes could cause landslides on the Ebro margin slope. The Columbretes slide on the southwestern Ebro margin may have been caused by intense earthquake shaking associated with volcanic emplacement of the Columbretes Islands. Localized sediment slides on steep canyon and levee slopes could have been caused by less intense shaking. In general, the slope is stable under present environmental loading conditions and is fundamentally constructional. Nevertheless, rapid progradation caused by high sedimentation rates and other processes acting during low sea-level periods, such as more intense wave loading near the shelfbreak, may have caused major instability in the past.

Erosion Limit of Sod on the Slope of Levees

Many failures of levees due to running water are started with the erosion of the surface of levees by running water in an early stage of the failure. From the results of analysis of disaster cases and the results of experiments, it was clarified that sodded levees having vegetation on the surface of levees have much higher strengths against erosion due to running water compared to earth levees having no vegetation. Therefore, one of the important factors governing the safety of levees against failures by running water is the erosion resistance of vegetation on the surface of earth levee. However, previous studies were little made on the erosion chracteristics of the earth levees having vegetation both domestically and overseas nevertheless the importance of this theme.This paper will present a method of on-site erosion test using a new device for checking the erosion mechanism and characteristics of earth having vegetation, the erosion characteristics of sod on the slope of levee determined by said method, and a method of predicting the erosion-resisting strength based on the results of the above. Finally, the applicability of the prediction method will be investigated on the basis of survey of sodded levee erosion by a large flood.

Snake River Bank Stabilization

The Snake River in Idaho and Wyoming has annual floods that approximate 20,000 cfs. Stream beds have steep gradients, are composed of clean, sandy gravels and there is heavy bedload movement during floods. Channels are braided, subject to extensive changes during floods, and threaten adjacent farmland with erosion and evulsion. Experience with Corps of Engineers levees and bank protection along approximately 37 miles of the stream during the last 20 yr shows that failures of levees and riprap are generally caused by undermining, but such failures can be avoided by extending riprap below the active depth of bedload movement and scour. Stone size of riprap, as specified in engineering manuals, is sufficiently large but small stone and dirt that prevent interlock must be avoided. Riprap slopes should not be steeper than 1 on 2. Riprap can be overtopped several feet without damage, provided levee slopes are relatively flat and are protected with coarse gravel.