Abstract
Check dams are widely distributed and abundant on the Chinese Loess Plateau and play an important role both in reducing damage from flash floods and in retaining sediment. Some check dams are destroyed either partially or fully during rainstorms. The investigation of the actual damage caused to check dams as a result of specific rainstorm floods is of great value for analyzing their effects and failure modes, and is also helpful for proposing a reasonable approach for their improvement. The “7.26” extreme rainstorm event on July 25 and 26, 2017, in Yulin in northern Shaanxi Province, China, resulted in serious infrastructural damage and economic losses. This study reports on a detailed investigation undertaken between August 27 and October 28, 2017, of the causes and severity of the damage to the check dams in the Chabagou watershed. The results showed that out of a total of 572 check dams surveyed, 63.6% were destroyed and 9.3% were damaged. The mean dam opening was 4.8 m wide and 3.4 m deep, while the mean water culvert was 1.7 m wide and 2 m deep. The main causes of the damage were as follows: (1) the flood return period exceeded the dam design standards, (2) 94.1% of the check dams lacked a spillway, and (3) the main land use types on the check dams were farmland and bare soil. Based on the findings of this study, the following recommendations are made: (1) improve the existing design standards for check dams, (2) build additional spillways and maintain existing spillways in a timely fashion, and (3) encourage the development of natural grass and shrub land as the best land use type on check dams.
A check dam is a barrier constructed of logs, gabions, dry stones, masonry, and reinforced concrete or other material that are placed across a river or channel (Nyssen et al. 2004; Piton et al. 2016). It is an important engineering measure that serves to reduce the water velocity, retain floodwater, intercept sediment, improve gully slope stability, reduce downstream sediment transport, limit channel erosion, and increase farmland for agricultural production (Abedini et al. 2012; Borja et al. 2018; Kang et al. 2013; Li et al. 2016; Tian et al. 2013).
As important hydrological structures, check dams have been implemented globally for watershed management, especially for silt-up dam land, runoff retention, and soil erosion control (Boix-Fayos et al. 2008; Liu et al. 2017; Martín-Rosales et al. 2007; Mishra et al. 2007; Nyssen et al. 2004; Quiñonero-Rubio et al. 2016; Ran et al. 2008). The check dam system is highly effective at runoff retention and sediment reduction and plays a critical role in soil and water conservation as well as the protection of downstream areas from flood calamity (An 2011). However, some check dams are damaged or destroyed due to a low design standard and/or lack of uncorrected maintenance, which can be a potential source of additional hazard (Mazzorana et al. 2014). Investigating the actual damage caused to check dams after a specific rainstorm flood is of great value for analyzing their effects and failure modes, and is also helpful for proposing a reasonable approach for their improvement (Piton et al. 2016).
On August 25, 1973, a 200-year return-period rainstorm in Yanchuan County in Shaanxi Province, China, led to 3,570 check dams (43.6% of the total) being damaged to varying degrees. During August of 1975, two heavy rainfall events with a 100-year return-period resulted in the destruction of 1,830 check dams (30.5% of the total) in Yanchang County in Shaanxi Province, China (Li et al. 2003). Chen and Liu (1984) reported that 50% of check dams and 20% to 30% of silt-up dam land were ruined by the rainstorm floods during 1977 and 1978 in Zizhou, Suide, and Qingjian in northern Shaanxi Province. They also indicated that low engineering standards and poor maintenance were primarily responsible for the severe losses. On July 22, 1989, a heavy rainfall event in Xing County in Shanxi Province destroyed 91 dams (205 in total), which resulted in an economic loss of 1.438 million yuan. An area of 1.67 km2 of silt-up dam land was destroyed, which accounted for 16.5% of the county’s total silt-up dam land, and 0.84 km2 of crops were flooded (Ma et al. 1990). Gao et al. (2017) indicated that approximately 53% of check dams were damaged in the Jiuyuangou watershed in northern Shaanxi Province following an excessive rainstorm in 2012. From an analysis of the failure modes, the authors suggested that the engineering standards should be improved, and that the dams should be repaired and heightened to maintain dam efficiency. Wei et al. (2015) reported that a rainstorm in the Yanhe River basin during July of 2013 destroyed 516 check dams, 21 reservoirs, and 8,100 km2 of farmland. Forty-two people were killed and 133 were injured due to the disaster, and the cumulative economic loss was 7.35 billion yuan. The investigation revealed that the extreme rainstorm, low flood-prevention standard, and lack of maintenance were responsible for the damage caused to the check dams.
The Chinese Loess Plateau is located in the middle reaches of the Yellow River and is one of the world’s most eroded regions due to high intensity rainstorms, eroded quaternary loess, gully landforms, and steep cropland (Tilman Rost 2000; Xu et al. 2004). Prior to 2009, approximately 910,000 check dams had been built on the Chinese Loess Plateau, which efficiently retained runoff and sediment (Wang et al. 2011; Xu and Wang 2000; Zeng et al. 1999). However, many of these dams were ruined during rainstorm floods. Between 16:00 on July 25 and 8:00 on July 26, 2017, an extreme rainstorm occurred in Yulin City in northern Shaanxi Province. This was the heaviest rainfall event recorded in this area since the rain gauges were installed in 1949. The rainstorm resulted in a record-breaking flood in Wuding River and Dali River. This extreme flooding caused serious economic losses and infrastructure damage, including that to check dams. The objective of this study is therefore to investigate the damage caused to check dams in one of the watersheds affected by this extreme rainfall event. Furthermore, suggestions for the design, management, and protection of check dams are also provided.
Materials and Methods
Study Area. This investigation was conducted in Chabagou, which is a small watershed in Yulin City in northern Shannxi Province (109.5° to 110.1° E, 37.5° to 37.8° N) (figure 1). The watershed belongs to the main tributary of the Dali River basin and covers an area of 205 km2 (mean width of 7.8 km and main channel length of 26.5 km) (Liao et al. 2009; Xie et al. 2007; Yang et al. 2005). The outlet hydrological station, Caoping, controls a drainage area of 187 km2. This area is characterized by a dry continental climate with relatively low rainfall (annual mean precipitation of ~480 mm). The annual distribution of precipitation is quite uneven, with 81% concentrated in the period from June to September, which falls mainly in short, intensive rainstorms (Cai et al. 2004; Zhang et al. 2010; Zhou et al. 2010).
Study area.
The Chabagou watershed belongs to an area with a high and coarse sediment yield on the Chinese Loess Plateau. Intensive rain, loose soil, and destroyed vegetation result in serious soil erosion (Lu et al. 2012). To control this erosion, soil and water conservation measures, such as terraced fields, grain for green, and check dams (figure 2) have been implemented since the 1950s (Zhang et al. 2010). Prior to 1960, there were only 25 check dams in the watershed. This increased dramatically during the 1960s, reaching 124 in late 1969 and then 444 by late 1977. Since then, the construction of new check dams has slowed markedly (Hu 2013; Qi et al. 2011, 2010).
Check dams in study area. (a) A dam built with soil, and (b) silt-up dam land.
During the “7.26” rainstorm on July 25 and 26, 2017, the rainfall measurements at Xinyaotai station, Caoping station, Zhujiayangwan station, and Jijiajian station in the Chabagou watershed were 214.2, 212.2, 201.2, and 200.6 mm, respectively. The peak flow was 299 m3 s–1, and the maximum sediment content was 272 kg m–3.
Field Investigation Method. The investigation was carried out from August 27 to September 8 and from October 15 to 28, 2017. Figure 1 shows the spatial distribution of 572 check dams that were investigated. The size and constitution of each check dam, including spillway, drainage channel, vertical shaft, and horizontal tube, were all recorded. The majority of check dams in the Chabagou watershed are located in the upstream and midstream areas (figure 1). Their drainage areas, lengths, upstream slope heights, downstream slope heights, and top widths ranged from 0.51 to 127 km2, 2.8 to 88 m, 0 to 12.9 m, 0.48 to 30 m, and 0.15 to 23.6 m, respectively. The dams were categorized by drainage area (table 1). The mean dam length, mean upstream slope height, mean downstream slope height, and mean top width per category ranged from 25.5 to 49.2, 0.7 to 1.7, 4.3 to 9.1, and 1.5 to 4.3 m, respectively (table 1). Ninety percent of the dams were less than 2 m tall, and only seven dams were higher than 4 m. Thirty-four check dams had a spillway, only three of which were higher than 2 m (table 2). The land use type on the upstream and downstream slopes was characterized as grass, shrub, farmland, or bare land, while those of the dam crests were characterized as grass, shrub, farmland, bare land, and road (table 1).
Assessed check dams categorized by drainage area, showing dam body parameters and land use type.
Parameters of the check dams categorized by dam height.
A great effort was made to investigate the damage sustained by the dams, and was divided into three categories: intact, damaged, and destroyed. “Destroyed” indicates that the flood waters created an opening or culvert in the dam body, which allowed water to move from the upstream to the downstream of the dam (figure 3). “Damaged” indicates that either (1) the dam body was intact but that rills or cave-in developed on the upstream slope, downstream slope, or top of the dam (figure 4); or (2) an attachment of the dam (spillway or drainage channel) was damaged. The types and sizes of the openings were recorded for the damaged and destroyed dams, and the land use types associated with each dam were also recorded.
Destroyed check dams during the “7.26” rainstorm. A destroyed dam indicates that the flood waters created an opening or culvert in the dam body, which allowed water to move from the upstream to the downstream of the dam. (a) An opening on a dam body across a stream, and (b) an opening on a dam body across silt-up dam land.
Damaged check dams during the “7.26” rainstorm. A damaged dam indicates that the dam body was intact, but that rills developed on the upstream slope, downstream slope, or top of the dam. (a) Minor damage on the upstream slope of a check dam; (b) severe erosion on the downstream slope of a check dam; (c) cave-in on the downstream slope of a check dam; (d) minor damage on the downstream slope of a check dam.
Results and Discussion
Damage to the Check Dams. As shown in figure 5, the numbers of intact, damaged, and destroyed dams were 155, 53, and 364, respectively, thus accounting for 27.1%, 9.3%, and 63.6% of the total. Check dams with heights of <1 m were the most likely to be damaged or destroyed, and accounted for 67.9% of damaged dams and 51.9% of destroyed dams. Moreover, 86.8% and 89.0% of damaged and destroyed dams, respectively, were <2 m in height. These results indicate that the majority of check dams were severely damaged by the “7.26” extreme rainstorm on July 25 and 26, 2017.
(a) Number of check dams and (b) proportion of check dams in intact, damaged, and destroyed situation as classified by dam height.
The check dams were destroyed in the manner of openings or culverts, as shown in table 3. The openings in the dam bodies ranged from 0.6 m to 23.3 m wide (mean of 4.8 m) and from 0.4 m to 14.8 m in depth (mean of 3.4 m). The widths of the culverts were between 0.3 m and 7.2 m (mean of 1.7 m), while their depths ranged between 0.4 m and 5.5 m (mean of 2 m). The majority of the dams that were destroyed by openings were <4 m in height, while the majority dams that were destroyed by culverts were <3 m in height.
Opening size of the destroyed check dams.
Causes of Dam Destruction: Low Design Standard. The “7.26” rainstorm was characterized by short duration, high intensity rainfall, which was centered in the Dali River basin. The maximum precipitation reached up to 252.3 mm in 12 hours (Zhaojiabian station in Suide County). The areas covered by rainfall over 50 mm and 100 mm accounted for 97% and 66%, respectively, of the total area of the Dali River basin. The total rainfall that fell during the storm was close to 50% of the local average annual rainfall (Zhang et al. 2017). The area in the upper reach of the Caoping station of the Chabagou watershed received the highest average rainfall (177.8 mm). The return period of the rainstorm in the upstream and downstream regions of the Dali River basin exceeded 50 years and 100 years, respectively (Dang et al. 2019).
The investigation results revealed that most of the check dams in the Chabagou watershed were productive dams of a small scale aimed at forming farmland. These types of dams were traditionally designed for a flood return-period of once in 10 to 20 years, calibration recurrence return-period of once in 30 years, and the designed sediment deposition period was 5 years (Cao et al. 2012). Furthermore, the flood control capacity of a check dam was dynamic because of the sediment deposition. As mentioned in the “Field Investigation Method” section, most of the check dams in the Chabagou watershed were constructed during the 1960s and 1970s. After 40 to 50 years of operation, many were almost full of sediment; hence, the actual flood control capacity was considerably smaller than the design standard. It is obvious that the recurrence of rainstorms exceeded the design standard and actual flood control capacity of the check dams, thus resulting in serious destruction.
Poor Drainage Capacity. A spillway is a flood control accessory that is designed to drain flooded to downstream areas when the water level exceeds a dam’s safety limit, thus protecting the dam from damage that could be caused by overflow. In this study, only 34 check dams had spillways (table 1), 75% of which remained intact, while only 15% were damage and 10% were destroyed. However, for the check dams without spillways, 23.5% remained intact, 8.8% were damaged, and 67.7% were destroyed (figure 6). Hence, dams with spillways largely remained intact and suffered considerably less destruction during the rainstorm in comparison to those without spillways.
Spillway and security of check dams.
The investigation also revealed that 76.5% of spillways were man-made road. A small percentage included were openings in the dam bodies that had developed during previous floods and served as spillways in the “7.26” rainstorm. In some cases, the bottom of the spillway had become higher than the dam crest due to inadequate maintenance, and the spillway lost its ability to function as a means of flood discharge. Thus, the lack of a spillway and inadequate maintenance meant that flood waters were not discharged in time, which may have also been responsible for the damage caused to dams during the extreme rainstorm.
Inappropriate Land Use on Check Dams. Most of the check dams in the Chabagou watershed are earth-filled dams. Such dams involve a small investment; they are not very secure and can be easily destroyed by floods. Moreover, land use type can also influence dam security. Among the check dams that were destroyed, 38.7% were characterized by grass and shrubs on their upstream slopes, while the other 61.3% were either characterized by farmland or bare land. Similarly, 39% of destroyed dams had grass and shrubs on their crests, while 58% had farmland or bare land. Among the damaged check dams, 41.5% had grass and shrubs on their upstream slopes, while the other 58.5% had farmland or bare land. Furthermore, 45.3% of damaged dams were characterized by grass and shrubs on their crests, while 47.2% were characterized by farmland or bare land (figure 7). Hence, the percentages of farmland and bare land were larger than that of grass and shrub for both destroyed and damaged dams. The anti-scour ability of farmland and bare land is generally relatively poor, thus resulting in rill erosion, shallow gully erosion, and poor stability of the check dams. These aspects mean that the dams in the study area would have been more easily destroyed under extreme rainfall. Conversely, grass and shrub on the upstream slopes and dam crest may have effectively protected the dams from being washed away.
Land use and security of check dams on the (a) upstream slope and (b) crest.
Effects of the Flood and Dam Failure. The failure of the check dams in the study area during the “7.26” rainstorm caused a serious loss of life and property, and also resulted in serious economic, ecological, and environmental damage to the Chabagou watershed. This investigation found that 417 check dams were damaged or destroyed, and that repairs or rebuilding would require a massive investment of manpower as well as physical and financial resources. According to our findings, the damage caused to check dams resulted in the severe destruction of a large area of silt-up dam land and hence crop losses. The watershed has a total silt-up dam land of 2.56 km2, all of which was planted with corn (Zea mays L.). It was estimated that a 1.87 km2 area of cornfields was flooded and ruined during the “7.26” rainstorm (i.e., ~73% of the total silt-up dam land). Besides, an aquafarm in the upstream of the watershed, which had a significant economic value, was destroyed due to dam failure in 2017.
As mentioned previously, many roads in the watershed served as drainage channel, which led to serious road erosion. Some damaged roads were impassable and impeded traffic for several months following the rainstorm. In addition, the water and electricity supplies were interrupted, roads were obstructed, infrastructure was seriously damaged, and stores were closed due to flooding in the lower reaches of the catchment (Dang et al. 2019; Wang et al. 2017). The infrastructure destruction resulted in a great economic loss and also had a social impact, both of which significantly affected the livelihoods of the local population. Check dams have considerable ecological and environment benefits. Dam failure can cause a large ecological loss by reducing (1) water resource conservation, (2) the usable land resources, (3) land productivity, and (4) biodiversity. Besides, the destroyed check dams suffered reduced function in retaining the sediment, which can result in serious downstream sedimentation.
Suggestions for Maintaining Check Dam. The effectiveness of check dams decreases over time and gradually loses their storage capacity due to sediment infilling, which ultimately influences the lifespan of check dams (Poff and Hart 2002). Taye et al. (2015) reported that this effectiveness reduced to about one-third of the initial effectiveness by the end of the third rainy season in the semiarid Ethiopian highlands. Quiñonero-Rubio et al. (2016) and Romero-Díaz et al. (2007) obtained similar conclusions, whereby the life expectancies of most of the check dams in their studies (in southeast Spain) were less than 30 years. These studies suggest that the lifespan of a check dam is affected by sediment deposition. Consequently, it is believed that the control of slope erosion (to reduce flooding and sediment supply through the establishment of vegetation and careful land use planning) offers a more sustainable management strategy for long-term sediment control, which would reduce the risks posed to dams (Quiñonero-Rubio et al. 2016).
As mentioned, the designed flood return-period for most small-sized check dams is once in 10 to 20 years. Practices have proven that this design standard is too low and should be enhanced to resist massive flooding. The standard should be planned, designed, and integrated reasonably by considering the budget, efficiency, safety, and in situ situation of a given watershed. It was suggested that the lowest design flooding frequency for check dams should be increased to 30 to 50 years (Wei et al. 2004, 2015).
A lack of correct maintenance of check dams is a potential source of additional hazards because this kind of dam can easily collapse as a result of extreme rainfall, which can therefore cause damage to downstream areas and put human lives at risk (Boardman and Foster 2011; Mazzorana et al. 2014). The present investigation found that the availability and maintenance of spillways had a significant effect on the lifespan and efficiency of the surveyed check dams. It can therefore be concluded that there is a need to increase the dam construction standard and build spillways. The design of spillways should take into account the proper type, size, and location of the cross-section as a means of ensuring that dams are not overtopped and do not collapse as a result of extreme rainstorms and floods. The silted sediment on spillways should be frequently cleared, especially before the rainy season, in order to ensure that flood water can be discharged in time to protect the dam body. In addition, drainage systems should be designed and built to form artificial flow paths as a means of enhancing the drainage function and protecting the silt-up dam land.
Many dams become structurally unstable and dangerous after several years of operation and improper use. Dangerous check dams are becoming one of the main problems to be solved with respect to current and future soil erosion control on the Chinese Loess Plateau, and there is an urgent need to remove and strengthen dangerous check dams. Increased investment and a greater emphasis on assessing, maintaining, strengthening, and repairing dangerous dams are required—for example, by increasing the height of a dam body for those filled with sediment, repairing damage of the dam surface, and strengthening thin dam bodies. Moreover, this study found that the land use type on dam body had a significant effect on the dam stability. Crops were planted in some dam bodies, which led to poor stability of the check dams and meant they could be more easily destroyed by extreme rainstorms. It was not advisable to plant crops on the dams; the growth of natural grass and shrub should be encouraged to protect the dam.
Summary and Conclusions
The check dams in the study area experienced serious damage during the “7.26” rainstorm on July 25 and 26, 2017, whereby 9.3% were damaged and 63.6% were destroyed. This above normal rainstorm along with the low design standard of the check dams, a lack of spillways, poor drainage capabilities, and inappropriate land use on dam body were collectively responsible for the destruction of the surveyed dams. Dam failures resulted in the destruction of cornfields, roads, an aquafarm, and heavy financial losses, which had a significant impact on the livelihoods of the local population.
The findings of this investigation suggest the need to improve the design standard to resist large floods. Spillways or drainage channels should be incorporated into the design and construction of check dams as a means of increasing the flood release capacity and reducing damage caused to dams. In addition, spillways and drainage channels should be regularly maintained and cleaned to ensure their flood release ability. Finally, dam body maintenance should be strengthened. Farmland and bare land on the dam body could reduce the stability and anti-scour ability, while natural grass and shrubbery is a much better management approach for protecting the stability and flood retention capacity of a dam.
Acknowledgements
This work was supported by the National Key Research and Development Program (2017YFD0800502) and the West Light Talent Program of the Chinese Academy of Science.
- Received August 7, 2019.
- Revision received January 4, 2020.
- Accepted February 29, 2020.
- © 2020 by the Soil and Water Conservation Society
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