(c) Hydrological conditions
The rivers in Lanzhou City fall into the mainstream system of the Yellow River.
Lanzhou City is located in the upstream of the Yellow River, which is a transitional
section from the mountainous area to the basin. The Yellow River enters the urban
area from Bapanxia and passes through the periphery of the demonstration site on
the east side. The average annual runoff amounts to 345.9 Â 10
8 m
3 and the annual
flow averages 1080 m
3 /s with the minimum of 60.2 m
3 /s. The flow rate ranges from
0.42 to 5.75 m/s and the corresponding flow ranges from 221 to 5660 m
3 /s. The
Liujiaxia Reservoir has, since put into operation, played a significant role in regulating the hydrological dynamics of the Lanzhou section of the Yellow River.
According to the 1971–1983 observation data of Lanzhou Hydrological Station, the
average, maximum and minimum flow were 997, 5600 and 213 m
3 /s respectively;
and the average sediment concentration during the year reached 1.59 kg/m
3 , with
the maximum of 5.02 kg/m
3 (in August) and the minimum of 0.046 kg/m
3 (in late
December and early January).
(d) Topography
The study area is located in the Lanzhou Basin, where the loess landform and the
layered landforms are well developed. The Lanzhou Basin is situated in western
Longxi Loess Plateau and dominated by loess hills with fragmented terrain. There is
strong gully erosion on the loess. The sulcus topography is characterized by loess
ridge and loess hillock and steep slope with 34°–40° gradient, which implies young
landforms in the area. The layered landforms of the Lanzhou Basin include first
erosion surfaces and multi-step river terraces. The first erosion surface is a first
widely distributed mountain erosion surface developed in the Tertiary red layer at
the beginning of the Quaternary period, and it was once called “Gansu quasi-plain”.
At the same time, river terraces are relatively stable and mainly shaped by the
Yellow River that runs from west to east through the basin. Each river terrace is
covered with loess of varying thickness. The latest data shows that there are at least
nine terraces of the Yellow River in the Lanzhou Basin. Loess-covered hills and
basins with an elevation of 1500–2000 m form the most part of the Lanzhou Basin,
followed by rocky mountains with higher elevations. The Yellow River flows from
the southwest to the northeast and crosses the whole territory, creating a beaded
valley between the canyon and the basin.
(e) Lithology
The main strata exposed in the demonstration site and its vicinity are of Quaternary
and Neogene age. The lithology and distribution characteristics are described as
follows:
Neogene (N) stratum: It is comprised of brown-yellow and brown-red mudstone
and sandstone. It is dense and hard, and distributed in the lower part of the loose
5.2 Northwest China
187
The rivers in Lanzhou City fall into the mainstream system of the Yellow River.
Lanzhou City is located in the upstream of the Yellow River, which is a transitional
section from the mountainous area to the basin. The Yellow River enters the urban
area from Bapanxia and passes through the periphery of the demonstration site on
the east side. The average annual runoff amounts to 345.9 Â 10
8 m
3 and the annual
flow averages 1080 m
3 /s with the minimum of 60.2 m
3 /s. The flow rate ranges from
0.42 to 5.75 m/s and the corresponding flow ranges from 221 to 5660 m
3 /s. The
Liujiaxia Reservoir has, since put into operation, played a significant role in regulating the hydrological dynamics of the Lanzhou section of the Yellow River.
According to the 1971–1983 observation data of Lanzhou Hydrological Station, the
average, maximum and minimum flow were 997, 5600 and 213 m
3 /s respectively;
and the average sediment concentration during the year reached 1.59 kg/m
3 , with
the maximum of 5.02 kg/m
3 (in August) and the minimum of 0.046 kg/m
3 (in late
December and early January).
(d) Topography
The study area is located in the Lanzhou Basin, where the loess landform and the
layered landforms are well developed. The Lanzhou Basin is situated in western
Longxi Loess Plateau and dominated by loess hills with fragmented terrain. There is
strong gully erosion on the loess. The sulcus topography is characterized by loess
ridge and loess hillock and steep slope with 34°–40° gradient, which implies young
landforms in the area. The layered landforms of the Lanzhou Basin include first
erosion surfaces and multi-step river terraces. The first erosion surface is a first
widely distributed mountain erosion surface developed in the Tertiary red layer at
the beginning of the Quaternary period, and it was once called “Gansu quasi-plain”.
At the same time, river terraces are relatively stable and mainly shaped by the
Yellow River that runs from west to east through the basin. Each river terrace is
covered with loess of varying thickness. The latest data shows that there are at least
nine terraces of the Yellow River in the Lanzhou Basin. Loess-covered hills and
basins with an elevation of 1500–2000 m form the most part of the Lanzhou Basin,
followed by rocky mountains with higher elevations. The Yellow River flows from
the southwest to the northeast and crosses the whole territory, creating a beaded
valley between the canyon and the basin.
(e) Lithology
The main strata exposed in the demonstration site and its vicinity are of Quaternary
and Neogene age. The lithology and distribution characteristics are described as
follows:
Neogene (N) stratum: It is comprised of brown-yellow and brown-red mudstone
and sandstone. It is dense and hard, and distributed in the lower part of the loose
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