2.1 Typical Application Environment of LID Facilities in Loess Areas
Bioretentions are usually located in the separation zone between motorized and non-motorized lanes. The typical application environment of bioretention facilities in a loess area is shown in Fig. 1. The site under analysis here is a Class I non-gravity collapsible loess site.
[Insert Figure 1]
Bioretentions in loess areas are mainly transport type and retention type. The primary function of the transfer type bioretention is to transfer runoff rainwater from the upper part of the facility. The retention type bioretention may be a retaining wall with L-shaped concrete retaining wall poured on both sides (referred to from here on as “Biorentinion I”) or a slope on the side of the structure (“Biorention II”), both of which have the same filling materials. The bottom is paved with 35 cm thick gravel (particle size 30-50 mm), and then with 5 cm thick gravel (particle size 5-15 mm), then the top is filled with replacement fill to the proper proportions. The specific structure is shown in Fig. 2. The main function of a retention type bioretention is to absorb and transport rainwater, so the retention bioretention has a more significant impact on the foundation field than the transfer type bioretention. Thus, the influence of rainwater infiltration through retention type bioretentions was the focus of this study.
[Insert Figure 2]
The Xixian New Area in Shaanxi was taken here as an example. It has the highest intensity of LID facility construction in collapsible loess areas in China. The average annual precipitation is 485.22 mm. Within the last 25 years, the maximum annual rainfall was 897.80 mm, in 2003, and the annual minimum rainfall was 302.20 mm, in 1997. The annual difference is 595.60 mm. The precipitation distribution in a given year is uneven and the monthly average evaporation is greater than the rainfall. Cloudy conditions and rainstorms prevail in summer and autumn months. The average monthly rainfall and evaporation in 2018 are shown in Fig. 3.
[Insert Figure 3]