Armor Coat Research Articles

Predicting the equilibrium bed slope in natural streams using a stochastic model for incipient sediment motion

ABSTRACTIn this paper, a method to predict the equilibrium bed slope in natural streams based on the incipient motion criterion is proposed. The method is based on the criterion suggested by Gessler to simulate the grain size distribution of the armour coat using the concept of critical shear stress of a sediment mixture. In particular, a different expression of the probability for a single particle size to be part of the armour coat is firstly proposed; then, a simple two‐steps criterion is suggested to estimate the safety factor required by the proposed approach. The method is applied in three different Italian regions (Calabria, Basilicata, and Tuscany) and required several field campaigns involving the survey of 251 stream reaches upstream of grade‐control structures. The area including five Calabrian streams is firstly used to test the suitability of the two‐steps approach. In this area, considering the detailed information about the particle size of bed material, the effect of using a simplified grain size distribution for each torrent is also checked. The method is then applied within the two additional areas (Basilicata and Tuscany) in order to check the geographic influence on the prediction of the stable longitudinal profile. A comparison between measured and estimated values of the equilibrium bed slope showed a good agreement for the Calabrian streams, where the number of the investigated stream reaches is greater, and satisfactory results for the torrents located in Basilicata and Tuscany where the field data set is more limited. The overall results encourage the extension of the proposed method to additional field data and suggest the use of this approach by hydraulic designers in order to stabilize the longitudinal profile of natural streams. Copyright © 2011 John Wiley & Sons, Ltd.

INVESTIGATION OF RECOVERY TECHNIQUES OF ECOLOGY POTENTIAL IN THE DOWNSTREAM OF DAM BY BED FORM IMPROVEMENT

After dam construction, the flow and sediment regime change downstream from the dam. For example, the armor coat progresses because of sediment trapping by the dam. To solve these problems, ’sediment replenishment’ has been executed recently. Nevertheless, it is difficult to maintain effects of sediment replenishment over the long term because the restored sediments are washed away easily in steep rivers, especially in slippery bedrock sections. Therefore, the authors have proposed techniques that promote gravel sedimentation by improving riverbed forms. The effects of proposed techniques such as the river widening and multistep masonry weirs were investigated using a two-dimensional numerical model. Results showed that the proposed techniques are useful to reduce tractive force in steep rivers. They can create a desirable bed state for river ecosystems with various gravel sizes.

Changes in bedload transport rate associated with episodic sediment supply in a Japanese headwater channel

Abstract We conducted field monitoring of bedload transport rate associated with experimental sediment release in a natural channel to clarify behavior of the supplied sediment on mixed size bed. Observation of bedload rate at two sites along the 30 m channel reach revealed that downstream migration of finer particles delay compared with coarser particles. Ratio of the bedload sediment that deposited during the migration was higher for finer sediments. These behaviors of the mixed size particles were clear during passage of the sediment wave without changes in water discharge. Flashing peak of discharge that caused artificially by opening of the dam gate did not destroy channel bed structure including steps and pools formed by coarser sediments, and only small amount of bedload was mobilized. Both reach scale channel features including steps, pools, and riffles as well as fine scale features (i.e., armour coat) likely increased critical shear stress of particles and decreased bedload rate during our experiment. Extreme sediment supply induced two types of sediment deposition; (1) filling the pools in reach sales and (2) the intrusion of fine particles into the coarser sediment that formed an armour layer. The all grain size fractions can deposit as type (1) when shear stress of stream water is not enough to entrain bedload particles, while deposition type (2) occurs when finer sediment pass over channel bed on which armour coat is well-developed. Deposition of finer sediment into coarser sediment that forms armour coat is affected by grain size distribution of bed surface sediment. Thus, impact of the sediment supply on downstream channel depends on both bedforms and grain size distribution of bed surface sediment over which the supplied sediment pass.

A simple mathematical model to predict the particle size distribution of the armour layer

A simplified method to predict the particle size distribution of the armour coat in the case of parallel degradation has been proposed. Using all available data, the particle size distribution of the armour layer was found to follow a normal probability distribution.A mathematical formulation to predict the median size of the armour coat has also been given. Some of the earlier methods for the prediction of the same have been studied and their accuracy has been compared to that of the proposed method. The time evolution of the river bed armouring process has also been investigated and the findings discussed.

NUMERICAL SIMULATION OF EXFOLIATION PROCESS OF SESSILE ALGAE BY MOVABLE BED SIMULATOR

Construction of a dam promotes the decrease of the sediment transport, and the resultant development of the stable armor bed in their downstream river. Under such condition, the river bed tends to be less disturbed and the eutrophication develops. This river bed situation allows the sessile algae growing thickly over the stable armor bed, which has a bad influence on the environment. One of the measures to remove the sessile algae is a sand sprinkling over the stable armor bed. The sprinkled sand, which moves in saltation mode over the stable armor bed with scratching the sessile algae, makes the sessile algae removed from the armor bed. In this paper, the numerical simulation for the exfoliation process of the sessile algae under saltaion mode was simulated by the movable bed simulator based on the three dimensional distinct element method. Furthermore, the exfoliation process due to collapse of the armor coat under a high bottom shear due to a flood is simulated.

CHANGES OF CHANNEL CHARACTERISTICS OF YOSASA-RIVER AFTER DISASTER RESTORATION

Disaster restoration works are commonly preformed based on the technical standard from the reason that there is not sufficient time of investigation and study for emergency construction. Therefore, some rivers that have special characteristics require additional studies on the changes of channel characteristics.In this paper, the result of study of Yosasa-river was described after the disaster restoration works. The channel was considered to have such special characteristics that the natural banks were greatly scoured laterally due to the existence of tight armor coat, during the big flood in 1998 after 60 years. But, the armor coat was found not to be developed by the field investigation, and it was concluded that the degradation was not occurred by the existence of huge amount of sediment transport that was yielded by the bank erosion due to the loose materials of the banks. Now, the channel has protected banks on both sides and the rate of sediment transport is considered to be dramatically decreased. Therefore, additional counter measures are proposed in this case.

SEDIMENT RUNOFF CONTROL BY A SERIES OF SABO DAMS

We have studied the function of a series of sabo dams on sediment runoff control using a numerical simulationm ethod of one dimensionabl ed variation.A closedt ype dam, a verticals lit dam, and a horizontals lit dam are taken up as sabo dams and five dams of each type were placed at an interval of 400m in a river. First of all, the applicabilityo f the simulationm ethod to the sedimentd epositionp rocess and the erosionp rocess in the upstream reach of a dam was verified. Next, the bed variation was calculated for the riverbed of sediment mixture not covered by armor coat. The second to the fifth sabo dam was filled up with the sediment eroded in the reach between the dam and the upper dam while the upper dam was trapping the sediment. Therefore, they could not have a storage capacity enough to reduce the amount of runoff sediment. On the other hand, a series of sabod ams in the riverbedc overedb y armor coat couldc ompletelyt rapr unoff sedimentt o be controlled.

MOVEMENT OF SEDIMENT REMOVED FROM A SABO DAM AND ITS IMPACT ON RIVER ENVIRONMENT

The sediment trapped by a sabo dam during a flood should be back to the downstream reach if the sediment trapping causes the sediment imbalance in the river system. The sediment flushing must, however, give a great impact on river environment. In this study we have investigated sediment movement in mountain rivers after sediment flushing and its impact on the habitats of fishes and aquati insects with some experiments on sediment flushing in a field. In the experiments, the released sediment passed the observation section completely covering the armor coat and filling up a series of pools. Such a sediment movement took the habitats from fishes and aquatic insects. The deposition volume in the pools and the deposition depth on the armor coat were taken as the indexes of the influence on fishes and aquatic insects. A method for estimating the indexes was presented with a numerical bed variation model. The method can be used for establishing the criterion for sediment flushing.

吉野川河道内における樹木および土地利用壁の分布の変化とそれに及ぼす流域の諸環境

The purpose of this paper is to describe distributional changes of trees and land-use types in a river and discuss factors in the changes from several points of view. The study area is the alluvial section of the Yoshino River in Shikoku, Japan, extending for about 75 km from the river mouth. Based on aerial photographs taken in 1964, 1975 and 1990, meshed maps of the distribution of trees and land-use types were made and compared with one another.Distribution of bamboo plantation widely changed between 1964 and 1975. This change was caused mainly by progression of embankment. Distribution of trees expanded remarkably in the period between 1975 and 1995. Factors contributing to this expansion are as follows: 1. Cessation of gravel digging; large scaled gravel digging, which had been continued until ca.1975, destroyed tree habitats and inhibited tree establishment. 2. Regulation of low flows; stable water supply in the seedling season regulated by four major dams since the 1970s have been advantageous for tree colonization. 3. Reduction of sediment supply due to dam constructions and ‘sabo’ works; the reduction of sediment supply developed socalled armor coat, and thus caused fixation and stabilization of bar-beds.

Armor Coat Formation and Fractional Sediment Discharge of Sand Mixtures caused by Rainfall on Bare Slope

The sediment transport process on the slope caused by rainfall impact and overland flow is the fundamental phenomenon, and many sediment discharge equations were proposed. Though most of them are proposed for the uniform sand, they are usually used for the estimation of the total sediment discharge of sand mixtures which is one of characteristics of the natural slopes. In this study, we experimentally dealt with the armor coat and the fractional sediment discharge on the slope caused by rainfall impact. Next, in order to simulate the non-equilibrium process of sediment transport we have applied the stochastic model which includes the effect of the sediment pick-up rate by rainfall impact. Since the simulated results have explained the experimental results, the validity of the proposed model has been clarified.

PARALLEL DEGRADATION OF A STREAM BED

ABSTRACT A stream bed undergoes parallel degradation due to preferential transport of sediments clear water released from a large dam. Progressive coarsening leading to the formation of I static armour coat eventually stops the degradation process for a given flow conditions. Mc elling of this process presents complexity due to nonuniform nature of the bed material and transient nature. A conceptual model for parallel degradation has been developed and tested using laboratory data collected by different investigators and is presented in this paper. 1 model has been tested for a field situation also.

A simplified method to simulate the time evolution of the river bed armoring process

Abstract A simplified procedure for determining the lime evolulion of the armor coat is presented. The method is developed for a parent bed material characterized by a log-normal grain size distribution function. The interaction of the armoring process with flow resistance and sediment transport are also discussed. Experimental data are used for the calibration of the model.

A simplified method to simulate the time evolution of the river bed armoring process

Abstract A simplified procedure for determining the lime evolulion of the armor coat is presented. The method is developed for a parent bed material characterized by a log-normal grain size distribution function. The interaction of the armoring process with flow resistance and sediment transport are also discussed. Experimental data are used for the calibration of the model.

Vertical Sorting and Armoring of Bed Mixtures during River Bed Degradation

Grain size variations of bed materials with time and space during river bed degradation are measured in an experimental flume. It is observed that the bed surface sand becomes much coarser than the original sand mixture during the bed degradation. This coarsening of the bed surface sand is analyzed with a vertical sorting model of sand mixture in the moving layer on the bed surface, while the final armor coat formation during the parallel bed degradation is analyzed by the longitudinal sorting model.

The self armouring process under unsteady flow conditions

AbstractLaboratory experiments were conducted to investigate the effect of the original size distribution of sediment and unsteady flow characteristics on the composition of an armour layer in a straight open flume. Bed sediments having different size distribution were subjected to flow hydrographs that varied in depth (D) and duration (T). The sensitivity of the sequence of changed bed sediment compositions to the variation in hydrograph duration was more than that in hydrograph depth. Such sensitivity increased as the geometric mean diameter of the original sediment decreased and as the geometric standard deviation of the sediment increased. The gradation of the armour coat developed under unsteady flow condition was greater than that developed under equivalent steady flow condition (i.e. having the same mean discharge and friction slope), and was found to depend on the value of the parameter P = (D/T)(1/u*) up to the limit of P = 0·01, where u* is the shear velocity. A model is proposed to predict the final composition of the armour layers developed under the effects of unsteady flow conditions.

Friction Factor of Armored River Beds

Friction factors of armored river beds at the discharge that forms the armor coat are investigated. The analysis shows that the friction factor is rather independent of grain‐size distribution of the material forming the bed and the maximum grain size of this material. The major controlling parameter is the slope of the energy grade line. These unexpected results are explained by the coupling of the friction factor with the incipient motion problem and the rearrangement of the grains of the coarsest fraction in the armor coat. The difference between rounded and angular material is discussed. Field data collected in irrigation canals support the results obtained from laboratory experiments.

Channel pattern and sediment transport in a steep gravel bed river

This study describes the sediment transportation and its relating phenomena typical in mountain streams. In a stream with the step-pool system, the sediment transportation depends greatly on sediment storage in the pools. The destruction of armor coat results in the destroy of step-pool system, which causes a Change of stream morphology. In addition, a meandering channel, the characteristics of which accords with the degree of the flood, is formed.Stream channels do not change so much in the conditions the armor coat can not be destructed even if a flood comes. In such cases, the sediment transportation depend greatly on the sediment storage in the step-pool system. In the case that the destruction of armor coat occurs, stream channels change very much.Debris flow brings a great change to the stream channel in the areas of its running-out; deep erosion in the occurrence areas and rapid aggradation in the ceasing area. In the deposition area, a new stream channel is formed due to the eroding action of water flow after the event.

SEDIMENT DISCHARGE OF NONUNIFORM SAND BED DURING INCREASE AND DECREASE PERIODS OF FLOOD DISCHARGE

Sediment discharge in alluvial channels with coarse and nonuniform bed materials depends strongly on the bed surface grain size condition. Sediment discharge is very small when the bed surface is covered with armor coat, while it increases sharply when the armor coat is destroyed during flood. In this paper sediment discharge characteristics of nonuniform sand bed during the increase and decrease periods of flood discharge are discussed experimentally in connection with the grain size distribution of bed surface materials, through which the sediment discharge formula of nonuniform sand and the process of armor coat formation are also discussed.

Effect of Suction or Injection through Bottom on Armor Coat Formation

Effect of Suction or Injection through Bottom on Armor Coat Formation

Recent Trend of Research of Sediment Transport

The key to the study of an alluvial process is to elucidate interactions between flow properties, sediment movements and bed or channel configurations. For this reason, any physical model of sediment transport is required to describe reasonably the non-equilibrium state of an alluvial process. From this point of view, the sediment model has been recently reinvestigated on the basis of knowledge accumulated on the mechanism of sediment motion. In this paper. the latest research on development of sediment models is reviewed. Particularly, importance is attached to unified treatment of bed material load, taking into account the transition from bed load motion to suspension both through incipient motion and transport process, and a physical model for description of non-equilibrium situation is proposed by applying a stochastic approach. Further in order to predict the scouring process of river banks, formation of armor coat, incipience of debris flow, etc., the effects of channel boundaries such as side slope, sand mixtures and permeable bed on sediment transport process are discussed.