Relationship between erosion and sediment yield in drainage basins of loess gully-hilly areas

From a long-term point of view, the balance between erosion and sediment yield in a drainage system can basically realize, i.e., the delivery ratio can be close to 1. However, substantial variations among individual rainfall events or between annual delivery ratio exist, causing frequent sediment retaining or re-erosion and re-delivery of the retained sediments in a short period of time. Thus the delivery ratio will be < 1 or > 1. The sediment delivery ratio is closely related to the spatial distribution of rainfall and magnitude of rise and fall of peak flood and that of runoff depth in the drainage system. Delivery ratio of single event in a drainage system and changes of delivery capacity of silt-laden runoff in various classes of gullies can be expressed by transformation mechanism of shear force of a single rainstorm event with flood resulting from increase and decrease of peak flood per unit area.     

岔巴沟流域次暴雨产沙统计模型

流域综合治理规划、防治土壤侵蚀、合理利用水沙资源 ,无不需要掌握流域产沙情况。流域产沙统计模型结构简单 ,计算方便 ,是现有产沙预报的强有力工具。本文以陕西省岔巴沟流域及其支流实测降雨水文资料为基础 ,系统地分析了流域产沙的降雨、径流、地貌因子在流域产沙中的作用 ,进而将影响产沙的因素概括为径流深、洪峰流量、流域面积、流域沟道密度 ,并作为产沙预报的指标 ,建立了岔巴沟流域次暴雨产沙的统计模型。经检验 ,该预报公式具有一定的精度。     

黄土丘陵沟壑区人类活动对流域系统[2]侵蚀、输移和沉积的影响

从长期来看 ,在治理度达到 70 %条件下与治理前相比泥沙输移比减小 50 %左右 ,治理的效益是十分显著的。在治理的过程中 ,与治理前相比泥沙输移比变幅明显减小 ,但在短期内由于暴雨洪水侵蚀力大于工程设计标准而发生毁、垮坝或淤平后 ,仍可以将前期滞留的泥沙重新搬运而出现泥沙输移比大于 1的情况。在黄土高原地区治理前后流域系统次暴雨泥沙输移比均可用径流深度比来定量计算获得 ,泥沙输移比变化的动力机制可用径流深度增与减时水流剪切力的变化来解释     

黄土丘陵沟壑区人类活动对流域系统侵蚀、输移和沉积的影响

从长期来看,在治理度达到70%条件下与治理前相比泥沙输移比减小50%左右,治理的效益是十分显著的。在治理的过程中,与治理前相比泥沙输移比变幅明显减小,但在短期内由于暴雨洪水侵蚀力大于工程设计标准而发生毁、垮坝或淤平后,仍可以将前期滞留的泥沙重新搬运而出现泥沙输移比大于1的情况。在黄土高原地区治理前后流域系统次暴雨泥沙输移比均可用径流深度比来定量计算获得,泥沙输移比变化的动力机制可用径流深度增与减时水流剪切力的变化来解释     

A geographical approach to post-flood analysis: The extreme flood event of 12 October 2007 in Calpe (Spain)

The study demonstrates the importance, in general, of adopting a geographical (integral) approach and local analysis to determine flood risk in small Mediterranean basins, where normally there is no surface runoff. At the same time, it highlights the possibilities afforded by a post-flood study. More specifically, the study seeks to identify the factors that aggravated the impact of the extreme flood event in the tourist town of Calpe (Alicante, Spain) on 12 October 2007.Our results stress the high volume of discharge that can be generated by small basins in the Mediterranean following intense rainfall events that may exceed 200 mm/day. For the 4.6 km² drainage network studied here, we calculate a maximum discharge of 143 m³/s, for a return period of 500 years. This natural risk (high potential discharge in what are usually dry basins) is exacerbated by the socio-economic transformations that have taken place along the Mediterranean coast. The expansion of urban land use has increased vulnerability to flooding in highly attractive tourist zones that are now at high risk of flooding. The municipal area recognised as presenting a class “2” qualitative vulnerability (on a scale from 1 to 6, where 1 is the most vulnerable) rose from 1% to 34% between 1956 and 2002. Changes to the basins and the traditional drainage systems in endorheic zones have also increased vulnerability to flood events. For example, the area occupied by the mouth of the main basin analysed here has fallen from 2.4 ha in 1956 to just 0.7 ha in 2002.The study’s main conclusions point to the need to introduce hydrological monitoring networks in small drainage systems in ephemeral river basins. It also stresses the importance of implementing structural actions to facilitate surface runoff discharge. In the mid-to-long term we conclude that there should be a declassification of urban land use zones at high risk of flooding. Moreover, detailed (local) studies which adopt an integral approach in their post-flood analyses should be undertaken to identify levels of flood risk and the appropriate land use regulations required.     

泥沙输移比影响因子及其关系模型 研究现状与评述

泥沙输移比是目前预测流域产沙量的一种较实用的方法,本文介绍了国内外泥沙输移比研究的主要成果。分析并总结了泥沙输移比与流域面积、降雨／径流、地质地貌及泥沙颗粒因子之间的影响关系及其关系模型。最后,结合我国泥沙输移比研究的实际情况,提出了我国今后一定时期内在泥沙输移比研究方面的发展设想。     

HYDROLOGY AND GEOMORPHIC EFFECTS OF A HIGH-MAGNITUDE FLOOD IN AN ALPINE RIVER

The catchment of the River Partnach, a torrent situated in a glacial valley in the Northern Calcareous Alps of Bavaria/Germany, was affected by a high‐magnitude flood on 22/23 August 2005 with a peak discharge of more than 16 m³s^‐1 at the spring and about 50 m³s^‐1 at the catchment outlet. This flood was caused by a long period of intense rainfall with a maximum intensity of 230 mm per day. During this event, a landslide dam, which previously held a small lake, failed. The flood wave originating from the dam breach transported a large volume of sediment (more than 50 000 m³) derived from bank erosion and the massive undercutting of a talus cone. This caused a fundamental transformation of the downstream channel system including the redistribution of large woody debris and channel switching. Using terrestrial survey and aerial photography, erosional and depositional consequences of the event were mapped, pre‐ and post‐event surfaces were compared and the sediment budget of the event calculated for ten consecutive channel reaches downstream of the former lake. According to the calculations more than 100 000 tonnes of sediment were eroded, 75% of which was redeposited within the channel and the proximal floodplain. A previous large flood which occurred a few weeks prior to the August 2005 event had a significant effect on controlling the impact of this event.     

城市不透水面与降雨径流关系研究

城市不透水面增加对降雨径流关系有着重要的影响,本研究选择位于福州城区内的厦坊溪和解放溪2个流域,在流域尺度上研究现状和城区未来扩展过程中对应的不透水率及其与降雨一径流关系．研究结果表明不透水率与径流系数在不同降雨情况下都成正相关的线性关系,尺。达到0．9以上;随着降雨量的增大并伴随着前期湿润度由干到湿变化,虽然不透水率对地表径流的影响有所弱化,但整体而言,城市不透水面的增加导致相同降雨情况下径流系数增加,并增大城市排水系统负担．利用研究结果,论文最后从保护河网水系、合理规划城区发展、完善排水系统和加强绿地系统建设等方面对城市规划提出建议．     

森林植被变化对洪水的影响分析——以长江上游典型小流域和洪水事件为例

通过长江上游地区典型小流域和重大洪水事件有关森林植被变化与洪水方面资料的分析，揭示小流域森林面积的增加，可减少洪峰流量模数，而在森林－土壤系统被前期降雨充分润湿的情况下，紧接着的暴雨可引起洪峰流量的快速增加；指出开展长江上游地区森林－土壤系统截留蓄水容量本底调查及其分类与分区，对指导当前森林植被恢复重建与保护工作的健康发展及搞好防洪工作有重要的意义。     

Initiation conditions for debris flows generated by runoff at Chalk Cliffs, central Colorado

We have monitored initiation conditions for six debris flows between May 2004 and July 2006 in a 0.3 km² drainage basin at Chalk Cliffs; a band of hydrothermally-altered quartz monzonite in central Colorado. Debris flows were initiated by water runoff from colluvium and bedrock that entrained sediment from rills and channels with slopes ranging from about 14° to 45°. The availability of channel material is essentially unlimited because of thick channel fill and refilling following debris flows by rock fall and dry ravel processes. Rainfall exceeding I = 6.61(D)^− 0.77, where I is rainfall intensity (mm/h), and D is duration (h), was required for the initiation of debris flows in the drainage basin. The approximate minimum runoff discharge from the surface of bedrock required to initiate debris flows in the channels was 0.15 m³/s. Colluvium in the basin was unsaturated immediately prior to (antecedent) and during debris flows. Antecedent, volumetric moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 4–9%, and 4–7%, respectively. During debris flows, peak moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 10–20%, and 4–12%, respectively. Channel sediment at a depth of 45 cm was unsaturated before and during debris flows; antecedent moisture ranged from 20–22%, and peak moisture ranged from 24–38%. Although we have no measurements from shallow rill or channel sediment, we infer that it was unsaturated before debris flows, and saturated by surface-water runoff during debris flows.Our results allow us to make the following general statements with regard to debris flows generated by runoff in semi-arid to arid mountainous regions: 1) high antecedent moisture levels in hillslope and channel sediment are not required for the initiation of debris flows by runoff, 2) locations of entrainment of sediment by successive runoff events can vary within a basin as a function of variations in the thickness of existing channel fill and the rate of replenishment of channel fill by rock fall and dry ravel processes following debris flows, and 3) rainfall and simulated surface-water discharge thresholds can be useful in understanding and predicting debris flows generated by runoff and sediment entrainment.     

黄河中游马莲河历史与现代洪痕沉积与水文学研究

通过对马莲河流域深入调查研究,在马莲河下游峡谷峭壁发现了一组4个显著的洪痕。洪痕沉积物分析看出,马莲河洪水悬移质泥沙以粉沙为主,粘粒和沙粒含量都较低,磁化率值也很低。将这些数据与流域内马家原剖面黄土土壤的的分析结果对比,可知马莲河现代洪痕沉积物是暴雨洪水对于流域内黄土和土壤侵蚀、搬运沉积形成。采用面积－比降法对该组洪痕所记录的4次洪水事件进行了洪峰流量恢复计算。表明高于常水位3.3 m、5.5 m、6.7 m洪痕洪峰流量分别为1 170m³/s、4 060 m³/s、5 690 m³/s。它们与雨落坪水文站观测2005、2003、1977年的洪水洪峰流量值相当,误差小于5 %。最高洪痕高出常水位10～12 m,计算恢复其洪峰流量为13 980 m³/s,洪水发生年代为1841年（清代道光21年）。这些研究结果对于马莲河流域的防洪减灾、水利水电工程设计和水土流失防治具有重要科学意义。     

基于小波分析的江河源区汛期和枯水期径流特征(英文)

By decomposing and reconstructing the runoff information from 1965 to 2007 of the hydrologic stations of Tuotuo River and Zhimenda in the source region of the Yangtze River, and Jimai and Tangnaihai in the source region of the Yellow River with db3 wavelet, runoff of different hydrologic stations tends to be declining in the seasons of spring flood, summer flood and dry ones except for that in Tuotuo River. The declining flood/dry seasons series was summer > spring > dry; while runoff of Tuotuo River was always increasing in different stages from 1965 to 2007 with a higher increase rate in summer flood seasons than that in spring ones. Complex Morlet wavelet was selected to detect runoff periodicity of the four hydrologic stations mentioned above. Over all seasons the periodicity was 11-12 years in the source region of the Yellow River. For the source region of the Yangtze River the periodicity was 4-6 years in the spring flood seasons and 13-14 years in the summer flood seasons. The differences of variations of flow periodicity between the upper catchment areas of the Yellow River and the Yangtze River and between seasons were considered in relation to glacial melt and annual snowfall and rainfall as providers of water for runoff.     

黄河内蒙古段淤积泥沙洪水冲刷效应

为了探讨黄河内蒙古段淤积泥沙的洪水冲刷效应,于2012年对其三湖河口水文站河道监测断面汛期（7-10月）流量、悬移质泥沙含量以及洪水期间（2012年8月20日-2012年10月1日）悬移质泥沙含量、粒度百分含量的垂直变化特征与流量的关系进行了统计和分析。结果表明：（1）该次洪水具有峰高量大、洪峰过程在河段内持续时间长、洪水起涨和消退缓慢、峰形矮胖的特点;（2）洪水过程中,小于0.05 mm的细颗粒泥沙在2 000 m³·s^-1左右的流量下就能输移通过,在2 000～2 400 m³·s^-1时输沙强度最大,对河道淤积泥沙可以达到输沙最优的效果;而粒径大于0.05 mm的泥沙输移的效果不好。     

Effects of high-magnitude/low-frequency fluvial events generated by intense snowmelt or heavy rainfall in arctic periglacial environments in northern Swedish Lapland and northern Siberia

Abstract In the Latnjavagge drainage basin (68°21′N, 18°29′E), an arctic‐oceanic periglacial environment in northernmost Swedish Lapland, the fluvial sediment transport and the characteristics and importance of high‐magnitude/low‐frequency fluvial events generated by intense snowmelt or heavy rainfall have been investigated and compared with snowmelt‐ and rainfall‐induced discharge peaks in the Levinson‐Lessing Lake basin (Krasnaya river system) on the Taimyr Peninsula, an arctic periglacial environment in northern Siberia (74°32′N, 98°35′E). In Latnjavagge (9 km²) the intensity of fluvial sediment transport is very low. Most of the total annual sediment load is transported in a few days during snowmelt generated runoff peaks. Due to the continuous and very stable vegetation covering most areas below 1300 m a.s.l. in the Latnjavagge catchment, larger rainfall events are of limited importance for sediment transport in this environment. Compared to that, in the c. 40 times larger Krasnaya riversystem rainfall‐generated runoff peaks cause significant sediment transport. The main sediment sources in the Latnjavagge drainage basin are permanent ice patches, channel debris pavements mobilized during peak discharges and exposing fines, and material mobilized by slush‐flows. In the Krasnaya river system river bank erosion is the main sediment source. In both periglacial environments more than 90% of the annual sediment yield is transported during runoff peaks. The results from both arctic periglacial environments underline the high importance of high‐magnitude/low‐frequency fluvial events for the total fluvial sediment budgets of periglacial fluvial systems. Restricted sediment availability is in both arctic environments the major controlling factor for this behaviour.     

Differences in hillslope runoff and sediment transport rates within two semi-arid catchments in southeast Spain

This paper focuses on hillslope runoff and sediment transport within two catchments in southeast Spain. Five monitoring sites were established on hillslope concavities throughout the two catchments. The techniques used were mini-crest stage recorders, spray-painted lines, sediment traps and tipping bucket rain gauges (established during previous research). Results show that a storm event in the Rambla Nogalte on 30 June 2002 of 83.0 mm was responsible for a maximum runoff depth of 12 cm and a maximum hillslope sediment transport of 1886 cm³ m⁻¹. The same storm in the Rambla de Torrealvilla produced 53.4 mm of rainfall on the 1 July 2002, had a maximum runoff depth of 26 cm and resulted in 2311 cm³ m⁻¹ of sediment transport. There is evidence to suggest that measured sediment transport is related to runoff and a qualitative estimate of Morphological Runoff Zones (MRZ). Sediment transport and depth of runoff varied dramatically with lithology; marl sites produced most runoff and sediment transport, followed by the sites of mixed red and blue schist, then blue schist. These results are important for understanding the behaviour of slopes and show that for short lived storms with high, but not rare, rainfall intensities and total rainfall amounts, runoff can cause significant hillslope sediment transport in semi-arid areas.     

Controls on overbank deposition in the Upper Mississippi River

Floodplains contain valuable stratigraphic records of past floods, but these records do not always represent flood magnitudes in a straightforward manner. The depositional record generally reflects the magnitude, frequency, and duration of floods, but is also subject to storm-scale hysteresis effects, flood sequencing effects, and decade-scale trends in sediment load. Many of these effects are evident in the recent stratigraphic record of overbank floods along the Upper Mississippi River (UMR), where the floodplain has been aggrading for several thousand years. On low-lying floodplain surfaces in Iowa and Wisconsin, ¹³⁷Cs profiles suggest average vertical accretion rates of about 10 mm/year since 1954. These rates are slightly less than rates that prevailed earlier in the 20th Century, when agricultural land disturbance was at a maximum, but they are still an order of magnitude greater than long-term average rates for the Holocene. As a result of soil conservation practices, accretion rates have decreased in recent decades despite an increase in the frequency of large floods.The stratigraphic record of the Upper Mississippi River floodplain is dominated by spring snowmelt events, because they are twice as frequent as rainfall floods, last almost twice as long, and are sometimes associated with very high sediment concentrations. The availability of sediment during floods is also influenced by a strong hysteresis effect. Peak sediment concentrations generally precede the peak discharges by 1–4 weeks, and concentrations are usually low (<50 mg/l) during the peak stages of most floods. The lag between peak concentration and peak discharge is especially large during spring floods, when much of the runoff is contributed by snowmelt in the far northern reaches of the valley.The great flood of 1993 on the Mississippi River focused attention on the geomorphic effectiveness and stratigraphic signature of large floods. At McGregor, where the peak discharge had a recurrence interval of 14 years, the flood was most notable for its long duration (168 days above 1600 m³s⁻¹), high sediment concentrations (three episodes >180 mg/l), and large suspended load (1.71 Mt). The flood of 2001, despite its greater magnitude (recurrence interval 70 years), was associated with relatively low sediment concentrations (<60 mg/l). The 1993 and 2001 floods each left 30–80 mm of silty fine sand on most low-lying floodplain surfaces, but the 2001 flood produced sandy levees near the channel while the 1993 flood did not. The stratigraphic signature of these recent floods is more closely related to the duration and total suspended load of the event than to the magnitude of the peak discharge.     

Late Pleistocene outburst flooding from pluvial Lake Alvord into the Owyhee River, Oregon

At least one large, late Pleistocene flood traveled into the Owyhee River as a result of a rise and subsequent outburst from pluvial Lake Alvord in southeastern Oregon. Lake Alvord breached Big Sand Gap in its eastern rim after reaching an elevation of 1292 m, releasing 11.3 km³ of water into the adjacent Coyote Basin as it eroded the Big Sand Gap outlet channel to an elevation of about 1280 m. The outflow filled and then spilled out of Coyote Basin through two outlets at 1278 m and into Crooked Creek drainage, ultimately flowing into the Owyhee and Snake Rivers. Along Crooked Creek, the resulting flood eroded canyons, stripped bedrock surfaces, and deposited numerous boulder bars containing imbricated clasts up to 4.1 m in diameter, some of which are located over 30 m above the present-day channel.Critical depth calculations at Big Sand Gap show that maximum outflow from a 1292- to 1280-m drop in Lake Alvord was  10,000 m³ s^− 1. Flooding became confined to a single channel approximately 40 km downstream of Big Sand Gap, where step-backwater calculations show that a much larger peak discharge of 40,000 m³ s^− 1 is required to match the highest geologic evidence of the flood in this channel. This inconsistency can be explained by (1) a single 10,000 m³ s^− 1 flood that caused at least 13 m of vertical incision in the channel (hence enlarging the channel cross-section); (2) multiple floods of 10,000 m³ s^− 1 or less, each producing some incision of the channel; or (3) an earlier flood of 40,000 m³ s^− 1 creating the highest flood deposits and crossed drainage divides observed along Crooked Creek drainage, followed by a later 10,000 m³ s^− 1 flood associated with the most recent shorelines in Alvord and Coyote Basins.Well-developed shorelines of Lake Alvord at 1280 m and in Coyote Basin at 1278 m suggest that after the initial flood, postflood overflow persisted for an extended period, connecting Alvord and Coyote Basins with the Owyhee River of the Columbia River drainage. Surficial weathering characteristics and planktonic freshwater diatoms in Lake Alvord sediment stratigraphically below Mt. St. Helens set Sg tephra, suggest deep open-basin conditions at  13–14 ka (¹⁴C yr) and that the flood and prominent shorelines date to about this time. But geomorphic and sedimentological evidence also show that Alvord and Coyote Basins held older, higher-elevation lakes that may have released earlier floods down Crooked Creek.     

Geomorphological and sedimentological analysis of a catastrophic flash flood in the Arás drainage basin (Central Pyrenees, Spain)

On August 7th, 1996, an intense and short-duration convective storm occurred over the 18.6-km² Arás drainage basin (Central Pyrenees, Spain). This high relief basin is composed of three subbasins, Aso, Betés and La Selva, and feeds the Arás alluvial fan, in the Gállego river valley. This alluvial fan had been drained by an artificial channel (about 125 m³/s at bank-full capacity). More than 30 check dams in its feeder channel, the Arás barranco, had been previously filled by earlier sediments. The heaviest rain was over the Betés subbasin (total rainfall 178.4 mm; maximum rainfall intensity of 153 mm/h for a 10-min time interval was estimated). Most of the rainfall fell in a 70-min period. This storm resulted in high runoff, causing catastrophic damage and significant geomorphic changes in the drainage basin, especially in the Betés subbasin. The high discharge, concentrated in the Arás barranco, destroyed most of the check dams, flushing out a great amount of debris. Major channel trenching and widening occurred in this barranco. When the confined sediment-laden flash flood reached the basin mouth, it sheet-flooded the southern sector of the Arás fan depositing a massive amount of debris. On this fan 87 people lost their lives and the direct physical damage has been estimated at 55 million dollars. Two stages in the development of the flood have been differentiated from the sedimentological and morphological analysis of the flooded fan lobe. A first stage (peak discharge) of sheet-flooding deposited a coarse boulder lobe, burying the artificial channel at the fan head and causing a darnming effect on the water flood. During the second stage (discharge decline) the flood made its way through the fan head, incising the previous debris accumulation and splitting into two main flow paths.     

输沙量 、侵蚀量与泥沙输移比的流域尺度关系——以赣江流域为例

输沙量、侵蚀量与泥沙输移比的流域尺度转换研究是当前流域侵蚀产沙研究领域的前沿课题,旨在通过尺度转换理论将坡面小区试验研究成果转换到流域的更大范围。以赣江流域实测输沙量和计算侵蚀量与泥沙输移比数据为基础,探讨了该流域3个变量的流域尺度关系,进而研究分析了3个变量尺度转换的可能性。3个变量与流域面积的关系散点图和相关方程都反映了这3者与流域面积不存在明显的相关关系,相悖于前人反比关系的结论。文章还阐述了流域面积的内涵及输沙量、侵蚀量和输移比的影响因素与流域面积的关系,发现3个变量的影响因素与流域面积不存在尺度效应。由此推断在赣江流域输沙量、侵蚀量和泥沙输移比实现尺度转换存在的可能性不大。这一研究结论是否成立或是否具有普遍性意义还有待于更多流域的研究成果来进一步证实。     

Medium-term evolution of a gully developed in a loess-derived soil

Field surveys in the Belgian loess belt revealed the presence in many forested areas of large, permanent gully systems, most of which are currently inactive. In cultivated areas, such gullies can only be observed in cross-sectional soil profiles through hollows, as virtually all such large gullies are currently infilled with colluvium. Little is known about the spatial distribution, initiation and temporal evolution of these large, permanent gully systems on loess-derived soils. Therefore, the medium-term evolution of a gully initiated in a cultivated area on loess-derived soils southwest of Leuven (Belgium) in May–June 1986, was studied over 13 years. Two intense rainfall events created this (ephemeral) gully, which was not erased by subsequent tillage. Between June 1986 and the December 1999, eight field surveys were conducted to measure gully dimensions. During two surveys, topographic indices (e.g., slope and drainage area) were also measured. Daily rainfall for the measuring period were obtained from a rainfall station located some 10 km southwest of the gully. Analysis of rainfall data showed that no extreme rainfall event was required to initiate such large (permanent) gullies, as observed in forested areas and through cross-sectional profiles in cultivated fields in the Belgian loess belt. Return periods of the event that caused the gully varied between <1 year and 25 years, depending on the assumptions used for defining event rain intensity. Once established, length, surface area and volume of the studied gully evolved with time, cumulative rainfall or cumulative runoff, following a negative exponential relation. This accords with observations reported for gullies in Australia and the USA. This study shows that a degressive increase of gully extension, can be largely explained by the evolution of a “slope–drainage area” factor (S×A, which is proportional to stream power) with time. While gully length and gully surface area asymptotically evolve towards a final value, gully volume decreased at a given point in time. From this, it is inferred that sediment deposition will potentially infill the gully to such an extent that the farmer can drive across it. From this moment on, the combined effect of water and tillage erosion in the gully drainage area, will lead towards rapid infilling. This expected evolution of a gully in cultivated fields accords with observations of large infilled gully systems in cultivated areas in eastern Belgium. The permanent gullies observed under forest are attributed to the fact that after severe gully erosion, this area was reforested or abandoned. Therefore, the sediment source was cut off and the gully was not filled in by sediment deposition.