Medium Gravel Research Articles

Spatial and Temporal Distribution of Spawning Events and Habitat Characteristics of Sacramento River Green Sturgeon

AbstractSpawning of the Southern Distinct Population Segment of Green Sturgeon Acipenser medirostris occurs annually within the Sacramento River in California. Artificial substrate samplers were used to collect Green Sturgeon eggs between 2008 and 2012 and in a reach of the river 94 river kilometers (rkm) long (rkm 426–332). A total of 268 eggs and 5 posthatch larvae were sampled from seven identified spawning sites between April 2 and July 7, primarily from medium gravel substrates. At these sites the mean water column velocities were 0.8 m/s at depths ranging from 0.6 to 11.3 m (6.4 ± 2.3 m, mean ± SD). We noted an average discharge of 314 m3/s and a median turbidity value of 3.9 NTU during estimated spawning events. Spawning at all sites occurred when average water temperatures were 13.5 ± 1.0°C and during water year types ranging from critically dry to wet. Green Sturgeon eggs averaged 4.11 ± 0.20 mm in diameter (n = 207), were very adhesive, and were between developmental stages 2 (just fertilized) and 44 (posthatch larva). We estimated that eggs were collected from a minimum of 54 different spawning events, based on sample date and location, egg developmental stage at capture, and water temperatures. Green Sturgeon spawning data indicates there is spatial separation from sympatric White Sturgeon A. transmontanus, but some temporal overlap exists. The thermally and hydrologically managed Sacramento River with its numerous diversions and competing water demands appears to have an approximate reach of 120 rkm in the 405‐km‐long river that is favorable for Green Sturgeon spawning in most years. Management decisions need to assess and incorporate the spawning habitat requirements of Green Sturgeon and coordinate this information with that of endangered winter‐run Chinook Salmon Oncorhynchus tshawytscha while attempting to meet the diverse demands of the limited Sacramento River water resources.

Hydrologic and geomorphic flow thresholds in the Lower Brazos River, Texas, USA

River science and management often require a design or reference discharge. The common (and sometimes unavoidable) use of such discharges may, however, obscure the fact that the magnitude and frequency of critical flows can differ due to various hydrological, geomorphological, and ecological criteria. Threshold stages and discharges were identified for six lower Brazos River, Texas gaging stations corresponding to thalweg connectivity, bed inundation, high sub-banktop flows, channel–floodplain connectivity (CFC), and overbank flooding. Critical flows were also identified for estimated thresholds for sandy bedform and medium gravel mobility, critical specific stream power for potential channel modifications, and cohesive-bank channel erosion. These thresholds have variable relationships to mean, median, and maximum flows. For four of the six stations, daily recurrence probabilities for all but flood flows are at least 1%, and as high as 11%. All stations achieve channel–floodplain connectivity at stages less than banktop. Estimated threshold flows for sediment mobility and channel erosion occur relatively frequently, with daily probabilities of 2–77%. Critical flows for bank erosion occur least often, and for sandy bedform and gravel mobility most often. Thalweg connectivity is always maintained at all sites, while bed inundation flows have a daily probability of about 80% or more. Overall, results suggest that no single flow level is dominant in hydrological or geomorphic dynamics, and that the frequency of a given threshold varies considerably even along a single river. The results support the idea that multiple flow levels and ranges are necessary to create and maintain the hydrological, geomorphological, and ecological characteristics of rivers, and that no single flow level is a reliable determinant of fluvial state.Editor Z.W. Kundzewicz; Associate editor Q. Zhang

Mix Optimization for Precast Concrete Members in Building Industrialization

The traditional mix proportion for concrete used in building field could not meet the requirements for industrialization production any more, and the optimization needs developing to obtain an ideal mix proportion for precast concrete with high early strength and good workability, as well as economy. To achieve the high early strength, Low-Alkalinity Sulphoaluminate Cement was selected, accompanied by Ordinary Portland Cement. The properties of other materials, namely aggregates (medium sand and gravel) and admixtures (polycarboxylate-based superplasticizer and sodium sulfate), were determined through literature review. The parameters of materials, such as the fineness modulus, mud-content of aggregates and cement strength, were determined. Besides, the range of water-cement ratio, admixture content and the ratio of two kinds of cements etc. in a total of 13 preliminary concrete mixes are analyzed. With further analysis, the four dominant factors were chosen, that is, water-cement ratio, cement content, superplasticizer content and sand ratio, leading to a series of orthogonal experiments with 3 levels of each factor. By analysis of test result data and consideration of cost, the optimized mix for precast concrete in building industrialization was carried out, of which the compressive strength in 10 hours reached the required strength and other properties reasonable.

Influence of large wood on channel morphology and sediment storage in headwater mountain streams, Fraser Experimental Forest, Colorado

Large fallen wood can have a significant impact on channel form and process in forested mountain streams. In this study, four small channels on the Fraser Experimental Forest near Fraser, Colorado, USA, were surveyed for channel geometries and large wood loading, including the size, source, and characteristics of individual pieces. The study is part of a larger effort to understand the impact of mountain pine beetle infestation on a suite of watershed properties. Here, we present baseline data collected at the onset of widespread tree mortality. Channels range from 1.5 to 2m in width, with slopes ranging from 3 to >10%. Median (D50) streambed particle sizes range from gravel to very coarse gravel. Channels are characterized as cascade, step-pool, and plane bed over varying scales. Large wood loads ranged from about 0.4 to 1.0 piece per meter length of channel, which is comparable to values reported for other Colorado sites. Much of the wood showed indications of being in place for long periods of time (decayed/rotten, broken into ramps, and partially buried in beds and banks). Nearly all surveyed reaches contained steps formed from small boulders and/or logs. Significant portions of the elevation drop in some of the reaches were made up by log steps, though the percentages varied (0 to 60%). Individual log steps trap a portion of the coarse sediment moved as bedload, forming wedge-shaped accumulations upstream of the logs. The particle size distributions for measured bedload and step accumulations largely overlapped, but more so for the coarse ends of the distributions, suggesting a trapping inefficiency for the finer component of bedload. Estimates of the total volume of sediment stored behind log steps were approximately an order of magnitude greater than the mean sediment volume exported on an annual basis, as determined from measured accumulations in weir ponds. The particle size distribution of sediment in the ponds – ranging from sand to medium gravel – is considerably finer than sediment stored in steps. The series of comparisons between storage volumes, particle size distributions, and sediment export suggests that log steps effectively trap coarse sediment moved in these small streams and act as a series of check dams that inhibit channel erosion, but may be less effective at trapping finer sediment (sand and small gravel).

Bed Forms and Sediment Characteristics along the Thalweg on the Tanana River near Nenana, Alaska, USA

Sediment sampling and longitudinal river-bottom surveys were conducted along the thalweg on the Tanana River near the city of Nenana, Alaska, USA, to provide basic information for the engineering design requirements of hydrokinetic devices to be deployed in the area. The study reach was located at approximately 64°33'50N and 149°04'W. The Tanana is a large glacier-fed river, with open-water flow conditions from May to October. The river presents a single channel in the study area. Granulometric analyses of sediment moving near the riverbed reveals the coexistence of three distinctive types of sediment along the study reach: 1) nearly uniform fine sand; 2) bimodal distributions containing fine sand and medium gravel; and 3) medium gravel. Preliminary relationships between sediment loads and discharge were developed. Dunes with small superimposed dunes were found along the reach. The basic geometric parameters (i.e., wavelength and height) of dunes were measured, and steepness was calculated. In general, dune wavelength increased with increasing discharge. Dune wavelengths ranged from 41 to 67 m, while small-dune wavelengths ranged from 13 to 16 m. Steepness increased slightly with increasing discharge.

교과서에서 사용하는 퇴적물의 입도에 대한 한국어 용어의 검토

The Korean terms of sediment grain size demonstrated in the 30 textbooks of elementary, middle, and high schools and university levels are compared and reviewed, and the problems of its use and alternative terms are proposed. The Korean terms of sediment grain size shown in the most textbooks are the translated terms of the Udden-Wentworth grade scale, and the different terms have been used in these textbooks. In the case of gravels, granule, cobble, and boulder have commonly been translated as wangmorae (king sand), janjagal (fine gravel), wangjagal (king gravel), and pyoryeog (drift gravel) or georyeog (large gravel), respectively. However, it is regarded to be reasonable that they are termed as janjagal, jungjagal (medium gravel), keunjagal (large gravel), and wangjagal, respectively. Adjectives such as 'maeu goun' (very fine), 'goun' (fine), 'junggan' (medium), 'gulgeun' (coarse), and 'maeu gulgeun' (very coarse), attached with each sediment name seem to be suitable to terms for sediments smaller than gravels. Silt has been commonly termed as misa (fine sand) in many textbooks, but it may be appropriate that silt is expressed as silt. Finally, mud, which is a mixture of silt and clay, should be named ito (mud) as shown in several textbooks including Dictionary of Korean Earth Science, though mud has been frequently termed as jinheug (slush or watery soil) in the most of textbooks for elementary and middle school students, and some high school and university textbooks.

Evaluation of evapotranspiration in small on-site HSF constructed wetlands

Experimental results on evapotranspiration (ET), relevant to small on-site facilities are presented, derived from one-year controlled experiments in five pilot-scale horizontal subsurface flow (HSF) constructed wetlands (CW) used as lysimeters. The CW units operated in Northern Greece. They were rectangular tanks made of steel, with dimensions 3m long, 0.75m wide and 1m deep. Three different porous media were used, i.e., medium gravel, fine gravel and cobbles. Two plants were used, namely common reed (R, Phragmites australis) and cattails (C, Typha latifolia). One unit was unplanted. ET was estimated based on the water budget method. Conclusions were drawn on its relation to season and vegetation density. Furthermore, Pearson correlation coefficient analysis identified the main factors affecting wetland plant ET. Seven well-known ET empirical methods were applied to estimate ET using the measured meteorological and wetland data. ET estimated by the empirical methods were multiplied with appropriate correction coefficients to match measured ET, providing this way appropriate plant coefficient (Kc) values, and equations for predicting HSF CW evapotranspiration. The suitability of these methods for the particular constructed wetland type is discussed through comparison with the measured data. The Blaney-Criddle method was found as best. Furthermore, stepwise multiple linear regression analysis was used with the measured ET and meteorological data to produce simple empirical equations to predict ET rates according to meteorological factors, plant and substrate material.

Effects of gravel on grassland soil carbon and nitrogen in the arid regions of the Tibetan Plateau

Many previous studies have focused on soil gravel concentrations and their effect on crop yields in agricultural systems. The extent of carbon and nitrogen sequestration in soils under steppe systems in relation to surface gravel mulch remains largely unexplored. This study investigated the effects of gravel mulches on soil organic carbon and total nitrogen stocks in the arid and windy regions of the Tibetan Plateau. Surface gravel mulches provide a more favorable environment for soil carbon and nitrogen stocks than do non-mulched sites. Soil organic carbon and total nitrogen stocks were highest (46.9 Mg ha − 1 SOC and 2.8 Mg ha − 1 TN) in the medium gravel mulch sites with ~ 40–50% gravel, and lowest (29.5 Mg ha − 1 SOC and 1.4 Mg ha − 1 TN) in no gravel mulch sites. Analysis of aggregate size fractions indicated that the vast majority of SOC was present in microaggregate fractions throughout the top 30 cm of soil. Considering the low level of soil disturbance in the study area, the carbon contained in the macroaggregate fraction might become stabilized in the soil. Gravel mulches above the soil surface have an important bearing on soil carbon sequestration as they control wind erosion, decrease soil surface evaporation and change soil physical behavior in the arid and semiarid regions.

Effects of substrate condition on habitat use and survival by white sturgeon (Acipenser transmontanus) larvae and potential implications for recruitment

To understand links between substrate and recruitment of white sturgeon ( Acipenser transmontanus ), I evaluated the effects of substrate condition on larval drift, hiding, and predation between hatch and 15 days posthatch (dph). Over porous substrates (small gravel = 1.2–1.9 cm; medium gravel = 2.5–5.0 cm; cobble = 10–15 cm), rapid interstitial hiding was observed from 0 to 6 dph at low water velocity (4 cm·s–1), whereas larvae drifted in response to nonporous substrates (sand < 0.2 cm; embedded cobble). Velocities of 20 cm·s–1 led to significantly lower drift only at 1 dph over small gravel. Hiding occurred an average of 2.0–13.3 s after release at 0–6 dph. Predation by sculpins ( Cottus spp.) on larval sturgeon also decreased significantly in response to porous substrates at 1 dph. The strongest expression of increased hiding and decreased predation when small gravel was available suggests that yolksac larvae prefer small interstitial spaces created by that substrate. Considering behavioural responses in preferred natural spawning habitat suggests yolksac larvae predominantly hide in the vicinity of spawning locations. Identification of strong effects of substrate condition on age-specific drift and survival suggests that substrate degradation may contribute to recruitment limitations for sturgeon.

Laboratory study of maximum scour depth downstream of sills

Bed sills have been widely used as a sequence of grade-control structures in mountain streams where incision has to be prevented. Major local scouring downstream of these structures is, however, often observed. In order to investigate the characteristics of local scouring, a number of experiments have been carried out with 105 cases in different situations. The initial bed slopes were 0·176 and 0·268; sediments were coarse sand, fine gravel and medium gravel. The sill spacings were 0·4, 0·6, 0·8 and 0·9 m, and the sill heights above initial stream bed level were 0, 0·02, 0·04 and 0·06 m, respectively. Three stages of scour evolution could be classified. The size of scour hole depends on the bed slope, sediment diameter, sill height and sill spacing. The slope of the stream bed was found to be the most important parameter affecting the maximum scour depth. Using dimensional analysis, the relative maximum scour depth at the equilibrium stage was formulated as functions of the relative morphological jump to mobility of sediment and the relative energy loss to flow energy. Compared with the measured data at the Maso di Spinelle River in the Italian Alps, the proposed formulae showed satisfactory results.

Bed-material mobility in a large river below dams

The mobility of river-bed surface particles is examined in the lower Ebro River downstream from dams that were closed several decades ago. Mobility is analysed by comparing photographs of painted bed areas taken before and after the 2003-2004 flood season, and also by comparing lines of painted particles after the floods. The peak of the maximum flood had a recurrence interval of 2 years. Medium and coarse gravels were widely exported from all the study reach, reflecting a disequilibrium between the competence of the flow to entrain and transport them and the rate of supply from upstream. By contrast, very coarse gravels and larger particles remained immobile, since floods were not competent enough to mobilise them. The partial entrainment of surface particles caused a coarsening of the river-bed surface, even in areas where sediments are loose and the river-bed is highly active during small floods.

Effect of temperature, HRT, vegetation and porous media on removal efficiency of pilot-scale horizontal subsurface flow constructed wetlands

In order to investigate the effect of temperature, hydraulic residence time (HRT), vegetation type and porous media material and grain size on the performance of horizontal subsurface flow (HSF) constructed wetlands treating wastewater, five pilot-scale units of dimensions 3 m in length and 0.75 m in width were operated continuously from January 2004 until January 2006 in parallel experiments. Three units contained medium gravel obtained from a quarry. The other two contained one fine gravel and one cobbles, both obtained from a river bed. The three units with medium gravel were planted one with common reeds and one with cattails, and one was kept unplanted. The other two units were planted with common reeds. Planting and porous media combinations were appropriate for comparison of the effect of vegetation and media type on the function of the system. Synthetic wastewater was introduced in the units. During the operation period, four HRTs (i.e., 6, 8, 14 and 20 days) were used, while wastewater temperatures varied from about 2.0 to 26.0 °C. The removal performance of the constructed wetland units was very good, since it reached on an average 89, 65 and 60% for BOD, TKN and ortho-phosphate (P-PO 4 3−), respectively. All pollutant removal efficiencies showed dependence on temperature. It seems that the 8-day HRT was adequate for acceptable removal of organic matter, TKN and P-PO 4 3− for temperatures above 15 °C. Furthermore, based on statistical testing, cattails, finer media and media obtained from a river showed higher removal efficiencies of TKN and P-PO 4 3−.

Reply to comment by F. Molz et al. on “Investigating the Macrodispersion Experiment (MADE) site in Columbus, Mississippi, using a three‐dimensional inverse flow and transport model”

Reply to comment by F. Molz et al. on “Investigating the Macrodispersion Experiment (MADE) site in Columbus, Mississippi, using a three‐dimensional inverse flow and transport model”

Evaluation of the sand-trap structures of the Wonji-Shoa sugar estate irrigation scheme, Ethiopia

A study was undertaken to evaluate the performance of sand-traps at Wonji-Shoa Sugar Estate, Ethiopia. Inflow and outflow samples from the sand-traps and deposited sediment samples from the sand-traps, main canal, and reservoirs were taken. The samples were analyzed for sediment concentration and particle size distribution. During the study period, the sand-traps were performing satisfactorily in removing a good portion of suspended sediment (as high as 63% in case of sand-trap A and 54% in case of sand-trap B) in the week after flushing and they were performing poor (as low as −40% in case of sand-trap A and 6% in case of sand-trap B) when the sand-trap was running without flushing for about two months. Sediment particle size analyses of the samples indicated that the sand-traps retained almost all the sand fractions (90–95%) greater than 0.15 mm. Fifty to sixty percent of the sediment particles passing the sand-traps into the main canal was smaller than 0.002 mm. Sieve analyses of the bed materials taken from the sand-traps showed that particles up to medium gravel size (6.3–13.2mm) were entering the sand-traps and almost all were trapped. This result shows that performance of the sand-traps under the existing sediment load of Awash River was found to be satisfactory. However, it was observed that performance level of the sand-traps was very much dependent on the flushing interval of deposited sediment.

Clear-water scour at bridge piers in fine and medium gravel beds

An experimental investigation on scour at circular and square piers in uniform and non-uniform gravels (fine and medium sizes) under clear-water scour at limiting stability of gravels is presented. From the experimental results, it is observed that the equilibrium scour depth increases with decrease in gravel size. The variation of equilibrium scour depth with gravel sizes departures considerably from that with sand sizes. Consequently, the resulting sediment size factors for gravels, obtained from envelope curve fitting, are significantly different from the existing sediment size factor for sands. The influence of gravel gradation on scour depth is also prominent in non-uniform gravels. The time scales to represent the time variation of scour depth in uniform and non-uniform gravels are determined. For uniform gravels, the non-dimensional time scale increases with increase in pier Froude number and gravel size, whereas for non-uniform gravels, it decreases with increase in geometric standard deviation of particle size distribution of gravels.Key words: bridge pier, gravel beds, scour, erosion, sediment transport, open channel flow, hydraulic engineering.

Transport of Gellan Gum Microbeads in Soil Columns of Various Grain Size Distributions

AbstractThe transport of gellan gum microbeads as potential cell carriers was investigated in horizontal columns packed with different grain size classes of gravel (2–16 mm) and sand (0.25–2 mm). A suspension of microbeads was pulsed into each column for 6 h, followed by injection of water for 42 h. In general, the total amount of microbeads travelling across a given section of the column increased with injection time but decreased towards the column outlet, varying as a direct function of grain size. The results of this study demonstrate the feasibility of the transport of gellan gum microbeads through medium sand to medium gravel across distances up to 110 cm.

Effects of surface rock fragment characteristics on interrill runoff and erosion of a silty loam soil

The role played by rock fragments in water erosion has received much attention in recent years. Knowledge of the effects of rock fragment characteristics on interrill erosion is incomplete. Hence, in order to investigate these effects on a small scale, a simulation experiment was conducted in Bragança, Northeast Portugal. The experimental setup consisted of 48 bottom perforated rectangular metal boxes (612 cm 2 ), placed at a 10% slope, filled with 3.5 cm of a sieved silty loam soil over 2 cm of sand, covered by simulated rock fragments and maintained at near saturation. Twelve treatments, four replicates each, were exposed to 240 mm natural rainfall, comprising selected combinations of rock fragments size (small, medium—gravel range, and large—stone range), shape (rectangular and circular), position (surface, half-embedded and embedded) and cover percentage (17, 30 and 66%), and bare soil. Infiltration depth, runoff depth, washed and splashed sediment were repeatedly measured in appropriate collection devices. For bare soil, total wash and total splash were equivalent to 42.2 g m −2 and 70.6 g m −2 , respectively. Infiltration and runoff represented 52% and 13% of total rainfall, respectively. Wash has a negative exponential relationship with rock fragment cover (RC). The regression coefficient varies negatively with cumulative precipitation, decreasing significantly after a surface seal is formed (at about 80 mm cumulative precipitation). The relationship between splash and RC, linear and negative, varies with time, too. Correlation with RC is positive for infiltration depth and negative for runoff depth, both reflecting the seal development with time. The effects of rock fragments size, position and form were tested for 30% RC. Size has a positive effect on runoff depth, wash and splash, and a negative effect on infiltration depth. The effect of rock fragment size on infiltration, runoff and erosion is more pronounced than that of position. The effect of shape was less significant than that of size and position.

Effects of surface rock fragment characteristics on interrill runoff and erosion of a silty loam soil

The role played by rock fragments in water erosion has received much attention in recent years. Knowledge of the effects of rock fragment characteristics on interrill erosion is incomplete. Hence, in order to investigate these effects on a small scale, a simulation experiment was conducted in Bragança, Northeast Portugal. The experimental setup consisted of 48 bottom perforated rectangular metal boxes (612 cm 2), placed at a 10% slope, filled with 3.5 cm of a sieved silty loam soil over 2 cm of sand, covered by simulated rock fragments and maintained at near saturation. Twelve treatments, four replicates each, were exposed to 240 mm natural rainfall, comprising selected combinations of rock fragments size (small, medium—gravel range, and large—stone range), shape (rectangular and circular), position (surface, half-embedded and embedded) and cover percentage (17, 30 and 66%), and bare soil. Infiltration depth, runoff depth, washed and splashed sediment were repeatedly measured in appropriate collection devices. For bare soil, total wash and total splash were equivalent to 42.2 g m −2 and 70.6 g m −2, respectively. Infiltration and runoff represented 52% and 13% of total rainfall, respectively. Wash has a negative exponential relationship with rock fragment cover (RC). The regression coefficient varies negatively with cumulative precipitation, decreasing significantly after a surface seal is formed (at about 80 mm cumulative precipitation). The relationship between splash and RC, linear and negative, varies with time, too. Correlation with RC is positive for infiltration depth and negative for runoff depth, both reflecting the seal development with time. The effects of rock fragments size, position and form were tested for 30% RC. Size has a positive effect on runoff depth, wash and splash, and a negative effect on infiltration depth. The effect of rock fragment size on infiltration, runoff and erosion is more pronounced than that of position. The effect of shape was less significant than that of size and position.

An experimental investigation of air flow patterns in saturated soils during air sparging

Air sparging is an emerging method used to remediate saturated soils and groundwater that have been contaminated with volatile organic compounds (VOCs). During air sparging, air is injected into the subsurface below the lowest known depth of contamination. Due to buoyancy, the injected air will rise through the zone of contamination. Through a variety of mechanisms, including volatilization and biodegradation, the air will serve to remove or help degrade the contaminants. The contaminant-laden air will continue to rise towards the ground surface, eventually reaching the vadose zone, where the vapours are collected and treated using a soil vapour extraction (SVE) system. Air sparging performance and ultimately contaminant removal efficiency is highly dependent on the pattern and type of subsurface air flow. This paper presents the results of a laboratory experimental study which investigated the injected air flow pattern development within an aquifer simulation apparatus. The test apparatus consisted of a tank measuring 61 cm long by 25.4 cm wide by 38.1 cm high. The apparatus was equipped with one air injection well and two vapour extracton wells. Three different soils were used to simulate different aquifer conditions, including a sand, a fine gravel and a medium gravel. Experiments were performed with different injected air pressures combined with different vacuum and groundwater flow conditions. Experiments were also conducted by injecting air into simulated shallow aquifers with different thicknesses. The air flow patterns observed were found to depend significantly on the soil type, groundwater flow conditions and system controls, including injected air pressure, flow rate and applied vacuum. © Rapid Science Ltd. 1998

Surface sealing as affected by various rock fragment covers in West Africa

Field studies on the influence of rock fragments on surface sealing, hence infiltration, remain scarce and contradictory. To document this issue, rainfall simulation experiments were carried out on 37 undisturbed 1-m 2 plots located along a pedo-climatic transect across West Africa. An important part of the variability of infiltration coefficient ( R 2 = 0.71) could be explained by a simple model based on the position of rock fragments in the top layer and accounting for the areal percentages of three types of soil surface: (1) bare surface, (2) soil surface covered with rock fragments embedded in the soil surface, and (3) surface with rock fragments resting on top of the soil surface. An even higher determination coefficient ( R 2 = 0.76) was gained when rock fragment size were accounted for in combination with vesicular porosity. Furthermore, a positive linear relationship was obtained between infiltration coefficient and mean annual rainfall suggesting that additional factors related to climate might be involved, including organic matter content and clay mineralogy. Two main regions could thus be differentiated. In the arid and semi-arid zones, coarse gravel and cobbles embedded in a seal are predominant and generate high runoff. Conversely, fine and medium gravel, mainly free at soil surface, are dominant in the wetter zone, favouring therefore higher infiltration rate.