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]