Horizontal ridge rupture
Horizontal ridge rupture or breaching is a common concern in Northeast China, as erosive storms can occur in summer with short duration but high intensity; these storms often coincide with snowmelt runoff in spring (Li et al., 2016; Lu et al., 2016; Xu et al., 2018). Contour ridge stability is mainly related to ridge geometry, sloping land microtopography, soil physical properties of the ridge body, and rainfall characteristics (Liu et al., 2014a).
Generally, Hr can increase water infiltration before breaching (Liu et al., 2015; USDA-ARS, 2008, 2013) and lead to abundant sediment storage (Xu et al., 2018). The time of ridge rupture shortened with higher rainfall intensity as previous studies (Liu et al., 2015; Liu et al., 2014a; Liu et al., 2014b; Xu et al., 2018). The result of extremely high runoff and soil loss rates after rupture (Figure 4 and 5) was like the relationship between the peaks of runoff and sediment and ridge failure (Liu et al., 2015; Liu et al., 2014b; Xu et al., 2018). Averaged peak runoff and soil loss rates after ridge failure were 9.33- and 36.67-fold larger than their prior neighboring points, respectively (Figure 6 and 7). The ratio of peak sediment rate to base sediment rate under Hr in this study ranged from 13.84 to 94.70 g L-1. The varied range differed but included previous results reported by Liu et al. (2014b) and Xu et al. (2018). Our study showed that contour ridges ruptured at 50 mm h-1 were not in agreement with the results of Xu et al. (2018), possibly because of the differences in ridge geometry characteristics, such as ridge height. Liu et al. (2014b) suggested that increasing ridge height might be useful to prevent horizontal ridge failure and decrease soil loss hazard risk, according to enhanced water storage capacity.
Our study illustrated that mulching could not always avoid ridge rupture but can significantly postpone the collapse time of ridge failure (Figure 6 and 7), possibly because mulching improves soil properties (Kader et al., 2017; Kurothe et al., 2014; Prosdocimi et al., 2016a, b) and, therefore, alters runoff and soil erosion characteristics (Gholami et al., 2013).