Gully Bed Research Articles

Rainstorm erosion difference and topographical changes induced by heavy rainfall between afforestation and grassland restoration catchments on the Chinese Loess Plateau

The frequency and intensity of heavy rainfall on the Chinese Loess Plateau (CLP) are increasing due to global climate change, which has caused severe soil erosion. However, precise evaluation of heavy rainfall-induced soil erosion, deposition, and related topographical changes at the catchment scale is lacking, and there is uncertainty as to how effectively trees can inhibit erosion, especially during heavy rainfall. In this study, we selected a pair of catchments with afforestation and natural grassland restoration on the CLP. Precise evaluation of erosion and topographic changes induced by heavy rainfall was conducted based on airborne light detection and ranging (LiDAR) data. In the grassland catchment, the gully bed area increased by 17.6 %, the gully slope and inter-gully land area decreased by 1.4 %, and the length of the gully shoulder line decreased by 4.4 %. Conversely, the forestland catchment experienced a decrease in gully bed area of 8.8 %, an increase in gully slope area of 5.3 %, a decrease in inter-gully land area of 9.9 %, and a decrease in the length of the gully shoulder line of 11.4 %. Following heavy rainfall, both catchments experienced a substantial increase in terrain relief. Soil erosion in the gully slope accounted for >90 % of the total erosion in both catchments, while deposition in the same area contributed to >75 % of the total deposition. The grassland catchment experienced a unit area erosion of 233,908 m3 km−2, which was 4.7 times greater than that of the forestland catchment (49,768 m3 km−2). In both catchments, erosion predominantly transpired in the steep slope region adjacent to the gully shoulder line, accounting for >15 % of the gully slope erosion area. The grassland catchment exhibited a continuous distribution pattern of erosion and deposition, whereas the forestland catchment exhibited a scattered and intertwined distribution pattern. This study is essential for further research on erosion and topographic changes induced by extreme weather events and to determine appropriate measures.

Dynamics of run‐off hydraulic characteristics during gully headcut erosion driven by head height

AbstractIn the granitic red soil hills region of South China, gully headcut erosion is considered one of the main processes in the development of gullies. However, the dynamics of run‐off hydraulic characteristics such as hydrodynamics and energy consumption during erosion have not been clarified. We investigated the effects of five headcut heights (25 to 125 cm) on hydrodynamic and energy consumption characteristics using field in situ water discharge scouring experiments. The results demonstrated that the flow velocity (V), Froude number (Fr), Reynolds number (Re), run‐off shear (τ) and run‐off power (ω) all indicate the upstream area (UA) > gully bed (GB). Except for V and Fr, several other hydraulic parameters exhibited a trend of sandy soil layer > red soil layer, and a fluctuating increase with increasing headcut height was observed in the GB. The jet velocity at the edge of the headcut (Vb), jet shear stress (τj), potential energy conversion (∆EP) and bottoming instantaneous kinetic energy (EKbottom) all increased exponentially with increasing headcut height by factors of 1.10 to 1.37, 1.16 to 2.00, 1.07 to 2.06 and 1.22 to 1.86, respectively. Except for ∆EP, all others performed a trend of sandy soil layer > red soil layer. The energy consumption at the gully head (∆EH) and its contribution tended to significantly increase with increasing headcut height, and the ∆EH of sandy soil layer was 13.20% to 49.89% higher than that of red soil layer. When the headcut height was greater than 100 cm, the contribution of energy consumption exceeded 50%. In addition, the sediment deposition process throughout the system was significantly influenced by the hydraulic and jet characteristics, and energy consumption was positively correlated with soil loss. The study results contribute to a profound understanding of gully headcut erosion and its hydrodynamic mechanisms.

The study on morphological evolution process of gully headcut erosion in granite red soil hilly area based on an in situ scouring experiment

Gully headcut erosion (GHE) is a key process of gully erosion. However, there is a lack of effective monitoring of the spatiotemporal changes in gully head morphological characteristics that occur during the continuous GHE process. Through a continuous scouring experiment in the field, GHE morphological evolution characteristics were examined for different exposed soil layers (laterite layer and sandy layer), headcut heights (25, 50, 75, 100 and 125 cm) and flow discharge levels (60 and 120 L/min) in typical granite red soil hilly area. The results indicated that the width–depth ratio (WDR) of the erosion gullies decreased in the slope direction and that erosion gullies violently developed on the downslope when stable runoff was generated across the upstream surface. The GHE could be divided into three periods, namely, the gully formation period, active headcut period and stable erosion period, which were mainly reflected in the gully head (GH) and plunge pool erosion (PPE) below the gully bed. The gully headcut retreat (GHR) distance and headwall collapse (HC) range increased with increasing headcut height and flow discharge. PPE exhibited a disordered downward cutting pattern, and the degree of development fluctuated. The GHE degree was jointly affected by GHR, HC and PPE, and these three processes interacted. The gully headcut erosion amount (GHEA) of the laterite layer was smaller than that in the sandy layer, but the average headcut erosion contribution rate (HECR) was 58.51 %, which was higher than that of the sandy layer (48.53 %). The GHE was the main erosion path in the two soil layers, and the HECR was not affected by a single erosion process. The sandy layer was easily eroded, and the GH morphology changed markedly during scouring, while the gully erosion amount (GEA) ranged from 0.02 m3 to 0.21 m3. This was mainly affected by GH and PPE. In this study, niche-like caves may form in the sandy layer to intensify gully erosion. When the headcut height and flow discharge reached a certain degree, the GEA of the laterite layer with strong anti-erodibility will also reach a higher level, and the GEA varied between 0.01 and 0.17 m3. GHR and HC were the dominant gully erosion factors. GHE prevention should focus on the laterite layer, as an effective protective layer in granite red soil areas.

Assessment of check dams and afforestation in mitigating debris flows based on dendrogeomorphic reconstructions, field surveys and semi-empirical models

Check dams and afforestation are two common measures for debris-flow mitigation; however, there is a lack of methods allowing quantitative evaluation of check dam and afforestation effects on debris flow motion and deposition. Laogan Gully is an active debris-flow system with a large landslide on the right hillslope, currently representing the main source of debris flows in the catchment. Several debris flows have occurred since 1985 which led to the construction of check dams and the plantation of silver wattle (Acacia dealbata) trees between 1991 and 1993. These measures significantly reduced the volume of debris flows reaching downstream portions of the system. At the same time, large amounts of sediment were trapped on the gully bed and thereby increased the base level of the torrent, which again tended to stabilize the large landslide on the right hillslope. This paper analyzes sedimentation of debris-flow material on the gully bed and on the fan using 12 trial pits behind the check dams and 6 incised geological sections on the fan. The timing and extent of debris flow motion and deposition was reconstructed using scars and growth suppression events in the annual growth ring series of 306 Acacia trees trapped in the newly formed deposits. The gravitational potential energy of the sediment of the large landslide is calculated according to the change in the erosion base level behind the check dams. On the basis of these results, we constructed a semi-empirical vegetation coverage–landslide energy–debris flow volume model to quantitatively evaluate the effects of the check dams and afforestation on debris-flow mitigation. The model exhibited predictive capabilities for estimating the volume of debris flow generated by the reactivation of landslides. Check dams are found to play a major role in debris-flow mitigation in the early stages, whereas the Acacia trees play an increasingly important role once the check dams are filled. The partial resurgence of the landslide caused a sudden increase in the gravitational potential energy of the sediment source, which in turn affected the effectiveness of the debris-flow mitigation measures.

Gully internal erosion triggered by a prolonged heavy rainfall event in the tableland region of China's Loess Plateau

Gully erosion is a severe form of soil erosion, but gully internal erosion processes are poorly understood, especially at the event scale. To investigate gully internal erosion intensity and understand the related gully development mechanism in an agricultural environment with gully head stabilization and vegetation restoration efforts, two successive field investigations were carried out just before and after a prolonged rainfall event in 2021 in the tableland region of China's Loess Plateau. Thirteen gullies were investigated and all experienced gully internal erosion, while most gully boundaries were stable during the heavy rainfall event based on the comparison of the UAV digital orthograph maps (DOMs acquired with Unmanned Aerial Vehicle) before and after the rainfall event. The proportion of gully internal erosion area to gully internal area of the 13 investigated gullies ranged from 3 to 55%, with average areal erosion proportion of the gully sidewall and gully bed of 21% and 36%, respectively. The erosion area of subdrainage units (SDUs) on the gully sidewall was positively correlated to the SDU area, average SDU slope gradient and vegetation type, while the erosion area on the gully bed was positively correlated to the gully area, gully depth and gully bed slope gradient. Gully internal erosion was not significantly correlated with gully drainage area because the connectivity between the upslope and gully areas was interrupted and the effective drainage area of the gully was obviously reduced by soil erosion conservation measures, including terraces on the upslope drainage area, shrub belts, and water barriers. Thus, gully internal erosion is still active under the heavy rainfall storm against the background of the ‘Grain for Green’ and ‘Gully Stabilization and Tableland Protection’ programs, and integrated measures for preventing both gully expansion and gully internal erosion must be further enhanced in the context of climate change.

The Theoretical Approach to the Modelling of Gully Erosion in Cohesive Soil

The stochastic gully erosion model (STOGEM) is based on a combination of deterministic mechanics and a stochastic description of the erosion control factors. The main proposition in the model is that the depth of the active surface layer of eroded cohesive soil is equal to one particle diameter, and the deposition of eroded particles is negligible. The erosion rate at the gully bed is calculated directly from the equation of the balance between driving and resistance forces acting on soil particles in flowing water using the probability density functions of stochastic variables: flow velocity, soil aggregate size and cohesion. Probability density functions of cohesion in the model vary through time and space during the erosion event due to the changes in soil composition—armoring and loosening. This theory is still far from achieving practical application, but opens up a new way for better understanding the experimental results of gully erosion and shows the direction for future investigations.

계곡형 토석류가 발생한 급경사 신기 계곡의 특성

In mountain gully, channelized debris flow is an important phenomenon in the process of topographical change. Social infrastructure as roads may be damaged by channelized debris flows, but there has been little information about their occurrence and movement to prepare for the risk of the debris flow. Most of the channelized debris flows occur during heavy rains in mountainous valleys that are difficult to access, so there are not many field data. In this study, the topographical characteristics of the catchment, the rainfall and runoff related to the debris flow, the sedimentary pattern and the cross-sectional change of the channel bed, and the underflow velocity of the gravel bed have been investigated and analyzed in the Singi gully where the channelized debris flows occurred. In the catchment, there was almost no sediment runoff because the vegetation combine with the debris landforms and covered the surface. Therefore, the obvious cause of the channelized debris flows is the collapse of the slope and bed of the gully. Even if the gravel, cobbles, and boulders of the channel bed were lost by debris flow, the thalweg change due to debris flow may not be significant because they are supplied from the gully side slope normally. After the gabion structures were installed, the debris flow increased the thalweg change, bed erosion and side slope of the gully. Various sedimentary structures in the gully were classified according to the factors supporting the sedimentation. The hypsometric curve of the gully reflects the debris landforms and vegetation characteristics of the watershed and the sediment runoff due to debris flow, etc. The relationship between the flow velocity and the hydraulic gradient was non-linear under the condition that the porous medium with gully bed gravels is saturated with water. These results may be used as basic data for channelized debris flow research.

Spatiotemporal changes in flow hydraulic characteristics and soil loss during gully headcut erosion under controlled conditions

Abstract. The spatiotemporal changes in flow hydraulics and energy consumption and their associated soil erosion remain unclear during gully headcut retreat. A simulated scouring experiment was conducted on five headcut plots consisting of upstream area (UA), gully headwall (GH), and gully bed (GB) to elucidate the spatiotemporal changes in flow hydraulic, energy consumption, and soil loss during headcut erosion. The flow velocity at the brink of a headcut increased as a power function of time, whereas the jet velocity entry to the plunge pool and jet shear stress either logarithmically or linearly decreased over time. The jet properties were significantly affected by upstream flow discharge. The Reynolds number, runoff shear stress, and stream power of UA and GB increased as logarithmic or power functions of time, but the Froude number decreased logarithmically over time. The Reynolds number, shear stress, and stream power decreased by 56.0 %, 63.8 %, and 55.9 %, respectively, but the Froude number increased by 7.9 % when flow dropped from UA to GB. The accumulated energy consumption of UA, GH, and GB positions linearly increased with time. In total, 91.12 %–99.90 % of total flow energy was consumed during headcut erosion, of which the gully head accounted for 77.7 % of total energy dissipation, followed by UA (18.3 %), and GB (4.0 %). The soil loss rate of the “UA-GH-GB” system initially rose and then gradually declined and levelled off. The soil loss of UA and GH decreased logarithmically over time, whereas the GB was mainly characterized by sediment deposition. The proportion of soil loss at UA and GH is 11.5 % and 88.5 %, respectively, of which the proportion of deposited sediment on GB reached 3.8 %. The change in soil loss of UA, GH, and GB was significantly affected by flow hydraulic and jet properties. The critical energy consumption initiating soil erosion of UA, GH, and GB is 1.62, 5.79, and 1.64 J s−1, respectively. These results are helpful for deepening the understanding of gully erosion process and hydrodynamic mechanisms and can also provide a scientific basis for the construction of gully erosion model and the design of gully erosion prevention measures.

Impacts of native vegetation on the hydraulic properties of the concentrated flows in bank gullies

To quantify the impacts of native vegetation on the spatial and temporal variations in hydraulic properties of bank gully concentrated flows, a series of in situ flume experiments in the bank gully were performed at the Yuanmou Gully Erosion and Collapse Experimental Station in the dry-hot valley region of the Jinsha River, Southwest China. This experiment involved upstream catchment areas with one- and two-year native grass (Heteropogon contortus) and bare land drained to bare gully headcuts, i.e., Gullies 1, 2 and 3. In Gully 4, Heteropogon contortus and Agave sisalana were planted in the upstream catchment area and gully bed, respectively. Among these experiments, the sediment concentration in runoff in Gully 3 was the highest and that in Gully 2 was the lowest, clearly indicating that the sediment concentration in runoff obviously decreased and the deposition of sediment obviously increased as the vegetation cover increased. The concentrated flows were turbulent in response to the flow discharge. The concentrated flows in the gully zones with native grass and bare land were sub- and supercritical, respectively. The flow rate and shear stress in Gully 3 upstream catchment area were highest among the four upstream catchment areas, while the flow rate and shear stress in the gully bed of Gully 4 were lowest among the four gully beds, indicating that native grass notably decreased the bank gully flow rate and shear stress. The Darcy-Weisbach friction factor (resistance f) and flow energy consumption in the gully bed of Gully 4 were notably higher than those in the other three gully beds, clearly indicating that native grass increased the bank gully surface resistance and flow energy consumption. The Reynolds number (Re), flow rate, shear stress, resistance f, and flow energy consumption in the gully beds and upstream areas increased over time, while the sediment concentration in runoff and Froude number (Fr) decreased. Overall, increasing vegetation cover in upstream catchment areas and downstream gully beds of the bank gully is essential for gully erosion mitigation.

Gully controlling practices associated with soil geotechnical properties in the subhumid Ethiopian highlands

Check dams are business as usual practices used to avert gully erosion development and sedimentation in the downstream areas of the humid highlands of Ethiopia. We investigated the status of check dams and their relationship with geotechnical soil properties in the sub-humid Fogera floodplain. The density of gullies in the area was more sever having about 3.6 km km-2 which shows the severity of gully erosion. Thirty-two dams constructed and monitored over one rainy season. In the beginning of the rainy season, all the dams filled up with sediments, and at the end of the monitoring period ninety five percent of them had destroyed and the remainings had partially destroyed and had sediments accumulated on one side of the gully bank. The longitudinal gradient of streams above the dams decreased due to sedimentation. The morphological change of the gully showed an increase in width-depth ratio, gully bank erosion, and sediment aggradation in the gully bed. We found that the amount of sediments deposited behind the dams were higher than the amount of eroded material. The higher clay content and Atterberg limits increased soil erodibility once the gully channel formed. The erodibility and saturation in these soils were highly contributing to gully development and reducing the effectiveness of check dams. In conclusion, it is better to adopt an integrated novel practice to control gullying than solely using check dams.

A geotechnical approach to gully erosion control and management at Inyishi, Ikeduru L.G.A., Imo State, Nigeria

This research is aimed at appraising the root causes of gully erosion at Inyishi in Ikeduru Local Government Area of Imo State, Nigeria, and the possible geotechnical control and management techniques that could be applied in curbing the menace. In pursuance to these objectives, field and laboratory investigations were undertaken. The field works involved reconnaissance and chain surveys, and contouring of the gully catchments. It was observed that gullies were initiated in the research area by flow concentrations induced by environmental factors related to roads and aligned buildings constructed with inadequate drainage facilities upstream of the gully locations. Laboratory investigations carried out involved engineering analyses of soil samples collected at the gully sites and beds for the determination of the soils grain sizes, Atterberg limits, moisture content, specific gravity, permeability, shear strength and bulk density relations. A summary of the engineering properties of the soil revealed its poorly graded and sandy nature, its inadequate fine materials content, low liquid and plastic limits, which ranged between 35.80 to 42.20 % and 28.35 to 35.00 % respectively, as well as its low permeability index of 1 × 10-2cm/sec. These findings, coupled with the fact that the gully area was underlain with friable and cohesionless soil, with small time of concentration, which renders it easily disaggregated by runoff are pointers to the relative ease of water infiltration and seepage at the gully locations. The erosive power of the slope within the gully area was estimated by chain surveying of the entire catchment. The efficacy of natural local materials such as vetiver grass, wood shavings, palm kernel shells, coconut husk, bread fruit husk, dead plant mulch, etc. for the geotechnical works on gully erosion control and management was explored. General specifications on the geotechnics of managing the installed control works for their sustainable services were proffered.

Measurement of rill and ephemeral gully flow velocities and their model expression affected by flow rate and slope gradient

Water flow velocities in rill and ephemeral gully are useful for estimating hydrodynamics in the study on soil erosion. Laboratory experiments were conducted to measure erosive rill and ephemeral gully flow velocities, including nine unit width flow rates of 0.17, 0.33, 0.67, 1.33, 2.67, 5.33, 10.67, 21.33, and 42.67×10-3 m2 s−1, and five slope gradients of 8.7%, 17.6%, 26.8%, 36.4%, and 46.6%, on sandpaper slope glued with silt loam soil materials to imitate fixed rill and ephemeral gully beds, using an electrolyte tracer method. The flow velocities measured in this research ranged from 0.31 to 3.34 m s−1 for sediment-free water flow. The flow velocity fitted well with unit width flow rate and slope gradient by a power function, yielding an improved model with higher coefficient of determination than that related flow velocity to flow rate alone. The improved model is applicable for flow velocity and soil erosion predictions.

The impact of flow discharge on the hydraulic characteristics of headcut erosion processes in the gully region of the Loess Plateau

AbstractHeadcut erosion is associated with major hydraulic changes induced by the gully head of concentrated flow. However, the variation in the hydraulic characteristics of the headcut erosion process is still not clear in the gully region of the Loess Plateau. A series of rainfall combined scouring experiments (flow discharges ranging from 3.6 to 7.2 m3 hr−1, with 0.8 mm min−1 rainfall intensity) were conducted on experimental plots to clarify the variation in the hydraulic parameters induced by gully head and erosion processes under different flow discharges. The results showed that concentrated flows in the catchment area and gully bed were turbulent (Reynolds number ranging from 1,876 to 6,693) and transformed between supercritical and subcritical (Froude number ranging from 0.96 to 3.73). The hydraulic parameters, such as the flow velocity, Reynolds number, shear stress, stream power, Darcy–Weisbach friction factor, and unit stream power in the catchment area were 0.45–0.59 m s−1, 2086–6693, 1.96–5.33 Pa, 0.89–2.86 W m−2, 0.08–0.16, and 0.023–0.031 m s−1, respectively. When the concentrated flows dropped from the gully head, the hydraulic parameters in the gully bed decreased by 3.39–26.07%, 1.49–29.99%, 65.19–67.14%, 67.25–74.96%, 28.53–61.31%, and 67.82–77.14%, respectively, which contributed to the flow energy consumption at the gully head. As flow discharge increased, Reynolds number, shear stress, and stream power increased, while flow velocity, Froude number, unit stream power, and Darcy–Weisbach friction factor did not. The flow energy consumption at the gully head was 9.66–10.13, 13.25–13.74, 15.68–16.41, and 19.28–20.25 J s−1, respectively, under different flow discharges and accounted for 60.58–68.50% of the flow energy consumption of the experimental plots. Generally, the sediment discharges increased rapidly at the initial stage, then increased slowly, and finally reached a steady state condition, which showed a significant declining logarithmic trend with experimental duration (P<.01) and increased with increasing flow discharge. Accordingly, the flow energy consumption was significantly correlated with the sediment yield. These findings could improve our understanding of the hydraulic properties and flow energy characteristics of headcut erosion.

Rainfall-triggered mass movements on steep loess slopes and their entrainment and distribution

Mass movements are predominant geomorphic processes on steep hillslopes. However, the mechanisms governing the erosion and entrainment of mass movements remain poorly understood. In this study, experiments on natural loess slopes were conducted to induce a series of mass movements under simulated rainfalls in the Liudaogou Catchment on the Loess Plateau of China. A novel topography meter was used to observe random mass movements. A total of 499 mass movements in 42 rainfall events and an average of 11 mass movements for each rainfall event were observed. Three mass movement types were detected: landslides (67%) > mudflows (21%) > avalanches (12%). The volume of landslides dramatically increased through the entrainment of a wet gully bed material, and the volume of landslide mass was magnified by 29% on average through material entrainment. Based on the observed data, the probability of mass movement occurrences decreased with the increasing mass movement volume in a power-law relationship. The critical rainfall amount for mass movement failure was approximately 25.6 mm at a rainfall intensity of 50 mm h−1. These results can serve as guides to mitigate geological hazards and assess erosion processes on steep loess slopes of the Loess Plateau.

Geomorphological controls on soil fertility of semi-arid alluvial fans: A case study of the Joghatay Mountains, Northeast Iran

Alluvial fans are ideal locations to examine the effect of geomorphic processes and landforms on the soil characteristics. The rate of soil development and fertility differs between active surfaces, exposed to aggradation, and old or relict surfaces, subject to degradation. The aim of this research is to evaluate the effect of landforms and processes on soil fertility based on the studies on three alluvial fans of different ages (relict, old and young). A total of 36 soil samples were collected from these three alluvial fan surfaces. The samples were gathered from different locations of fans (apex and toe) and landforms (gully beds, interfluves, swales, and bars) and for each sample, nine variables including soil texture, acidity (pH), electrical conductivity (EC), total neutralizing value (TNV), available phosphorus (AP), available potassium (AK), total nitrogen (TN), total organic carbon (TOC), and hydraulic conductivity (K) were measured. The results show that the values of TNV, AP, AK, TN and TOC increase as clay% increases. In the relict and old fans, the values of TNV, TOC, TN, AP, AK, clay%, and silt% are higher in the interfluves than in the gully beds. Also, in the relict and old fans, the rates of clay%, silt%, TOC, TN, and AP are higher in the apexes than in the toes. In the young fan, the values of most variables are higher in the toe compared to the apex. The wide and relatively flat interfluves of the apex of old and relict fans have soils that are more fertile compared to the extensively entrenched surfaces of relict and old fan toes with relatively steep sloped interfluves. The less fertile soils in the apex of young fan can be attributed to its coarse-textured soils and also to repeated flooding and depositional processes. Overall, this study reveals that geomorphological landforms and processes such as aggradation and degradation play an important role in the spatial variation of soil fertility.

Gully erosion as a mechanism for wetland formation: An examination of two contrasting landscapes

AbstractWetlands occupy broad, gently sloping valleys characterised by net deposition. Consequently, gully incision is viewed as an agent of wetland destruction. However, little attention has been given to the role of gullies in natural wetland formation. This paper determines if gully cutting and filling cycles have led to wetland formation in both an extant eroding wetland and a semi‐arid environment that once supported wetlands. At both sites, the valley fill was investigated to identify infilled gullies, and a 14C date was obtained from the gully fill. The longitudinal slopes of the valley floor prior to incision, the valley bedrock floor, and the gully fill were also measured. In the Krom wetland, four infilled gullies (5–8.2 m deep) were found adjacent to alluvial fans and dated to 470–7060 bp. Ten infilled gullies were found at Zuurplaats (Karoo), which were more recent, although one gully started filling 1260 bp. At both sites, large gullies were grounded on bedrock. Alluvial fans deposited by tributaries cause a reduction in mainstem valley width and locally increase longitudinal slope, leading to gully initiation. The slope of the gully beds was less than the surface prior to erosion, and the slope of the gully fill was comparable with the valleys' regional slope. The bedrock on both valley floors had a near‐horizontal cross‐sectional profile, suggesting that repeated gully incisions had led to valley floor planing. A new model of wetland formation is proposed where gully cut‐and‐fill cycles laterally plane valley bottoms and contribute to valley widening and longitudinal slope reduction.

Assessment of Practices for Controlling Shallow Valley-Bottom Gullies in the Sub-Humid Ethiopian Highlands

Rehabilitation of large valley bottom gullies in developing countries is hampered by high cost. Stopping head cuts at the time of initiation will prevent large gullies from forming and is affordable. However, research on practices to control shallow gully heads with local materials is limited. The objective of this research was therefore to identify cost-effective shallow gully head stabilization practices. The four-year study was conducted on 14 shallow gullies (<3 m deep) in the central Ethiopian highlands. Six gullies were used as a control. Heads in the remaining eight gullies were regraded to a 1:1 slope. Additional practices implemented were adding either riprap or vegetation or both on the regraded heads and stabilizing the gully bed downstream. Gully heads were enclosed by fencing to prohibit cattle access to the planted vegetation. The median yearly head retreat of the control gullies was 3.6 m a−1 with a maximum of 23 m a−1. Vegetative treatments without riprap prevented gully incision by trapping sediments but did not stop the upslope retreat. The gully heads protected by riprap did not erode. Regrading the slope and adding riprap was most effective in controlling gully head retreat, and with hay grown on the fenced-in areas around the practice, it was profitable for farmers.

Check dams and afforestation reducing sediment mobilization in active gully systems in the Andean mountains

Gully erosion is an important process of land degradation in mountainous regions, and is known to be one of the major sediment sources in eroded catchments. Recent studies have suggested that living and dead vegetation can be effective for ecosystem restoration, and large-scale restoration projects have been implemented in the tropical Andes in recent decades. However, few quantitative studies exist on the effectiveness of gully restoration to reduce sediment production and mobilization. In this study, sediment mobilization and transport was studied in five micro-catchments (<1 km2) with different soil and water conservation treatments. The techniques that were used for soil and water conservation involve vegetation restoration on the hillslopes and check dams in active gully channels. To characterize the routing of sediment within the micro-catchments, we measured erosion and sediment deposition within the gully channels. Sediment yield was estimated from measurements of sediment accumulation in sediment traps that were constructed at the outlet of the micro-catchments. Flow barriers are shown to be very effective in stabilizing active gully systems in badlands through significant reduction (of >70%) of the amount of sediment exported from the micro-catchments. The construction of wooden barriers (or so-called check dams) in active gully channels enhances sediment deposition in the gully bed. The latter is strongly dependent on the rainfall intensity, as well as gully channel slope and vegetation cover. The experimental data suggest that there exists a threshold value of rainfall intensity (I30max) of about 23 mm h−1, above which all sections of the gully system are actively contributing water and sediment to the river network. Also, forestation of active gully systems with rapidly growing exotic species such as Eucalyptus has a positive effect on the stabilization and restoration of the badlands, and effectively reduces the sediment export.

Effect of grass basal diameter on hydraulic properties and sediment yield processes in gully beds in the dry-hot valley region of Southwest China

Vegetation is an important factor impacting the hydrodynamic processes of gully beds and further affecting the headward erosion of gullies. Gully erosion is one of the major contributors to severe land degradation in the Yuanmou dry-hot valley region of Southwest China where soil erosion rates are estimated ranging from 8000 to 20,000t·km−2·a−1, with a mean gully distribution density ranging from 3 to 5km·km−2. However, few studies have been performed in this area which focused on the influence of the aboveground part of grass on soil erosion under natural conditions in gullies. To quantify the temporal variation of hydraulic properties (i.e., shear stress (τ), Darcy–Weisbach friction factor (resistance f) and Sediment Concentration (SC) and analyse the change trends of hydraulic properties and SC in gully bed along with the downslope direction under different grass basal diameters, a series of in situ scouring experiments were conducted in development areas of gully erosion in the Yuanmou dry-hot valley region. With the grass basal diameter increased from 0 (no grass) to 17 to 43 to 70 to 98mm in gully beds, the mean shear stress of concentrated flow increased slightly (4.03 to 4.49Pa) and then decreased obviously (4.49 to 3.45Pa). On the other hand, increasing trends were observed in temporal variations of shear stress for every grass basal diameter and the increase rate varied from 0.05 to 0.18. Whereas no notable regular changes in shear stress were detected in the downslope direction for grass basal diameters of 0 to 43mm, obviously increasing trends were observed for the grass basal diameters of 70mm and 98mm. The resistance f experienced a notable increase with increasing grass basal diameter in this study. A logarithmic growth of resistance f was observed in the gully bed as the experiment progressed (f=aln(t+b), P<0.01), and increasing trends were detected for the resistance f in the downslope direction of the gully bed for all grass basal diameters although regression equation could only be fitted for grass basal diameter of 98mm (f98=2.438ln(DOH98−2.643), P<0.01). However, the SC showed an exponential decline with the scouring time. And the SC showed an increasing trend along with the downslope direction of the gully bed in all experiments. In addition, a negative correlation could be detected between SC and resistance f in all five grass basal diameter experiments. In this study, only the disposal for grass basal diameter of 98mm was clearly more effective than other disposals in conserving gully bed, which might because grass basal diameter (≥98mm) that could cover a relatively large section of gully bed could exert apparent impact on reducing runoff shear stress, increasing resistance coefficient and then further decline the sediment yield. In contrast, when grass basal diameter was <98mm, the reduction effect for soil erosion was very limited due to overland concentrated flow could detour around the grass base and erode the gully bed.

Geomorphologic analysis and physico-dynamic characteristics of Zhatai-Gully debris flows in SW China

Zhatai gully is a typical debris flow channel in Butuo county of Sichuan province, southwestern China. The geomorphologic features are analyzed and the physical-dynamic characteristics are discussed on the basis of field investigation and laboratory tests. Geomorphologic analysis indicates that Zhatai-gully drainage in relation to debris flow can be divided into source area, transport area, and deposition area. The source area has a steep slope and has very limited vegetation cover, which favors runoff, allowing loose solid materials to be mobilized easily and rapidly. In the transport area, there are many small landslides, lateral lobes, and loose materials distributed on both banks. These landslides are active and constantly providing abundant source of soils for the debris flows. In the deposition area, three old debris-flow deposits of different ages can be observed. The dynamic calculation shows that within the recurrence intervals of 50 and 100 years, debris flow discharges are 155.77m3/s and 1y8.19m3/s and deposition volumes are 16.39 x 104 m3 and 18.14 x 104 m3, respectively. The depositional fan of an old debris flow in the outlet of the gully can be subdivided into six layers. There are three debris flow deposits on left and two on the right side of the gully. Grain-size tests of sediments from the soil, gulley bed deposits, and the fresh and old debris flow deposits showed that high amounts of clay and fine gravel were derived from the soil in the source area whereas much of the gravel fraction were sourced from the gully bed deposits. Comprehensive analysis indicates that Zhatai gully is viscous debris-flow gully with moderate to high frequency and moderate to large magnitude debris flows. The risk of a debris flow disaster in Zhatai-gully is moderate and poses a potential threat to the planned hydroelectric dam. Appropriate engineering measures are suggested in the construction and protection of the planned hydroelectric station.