Degrading river network due to urbanization in Yangtze River Delta

doi:10.1007/s11442-016-1293-0

Abstract

Abstract:

Evolution of river systems under the background of human activities has been a heated topic among geographers and hydrologists. Spatial and temporal variations of river systems during the 1960s-2010s in the Yangtze River Delta (YRD) were investigated based on streams derived from the topographic maps in the 1960s, 1980s and 2010s. A list of indices, drainage density (Dd), water surface ratio (WSR), ratio of area to length of main streams (R), evolution coefficient of tributaries (K) and box dimension (D), were classified into three types (quantitative, structural, and complex indices) and used to quantify the variations of stream structure. Results showed that: (1) quantitative indices (Dd, WSR) presented decreasing trend in the past 50 years, and Dd in Wuchengxiyu, Hangjiahu and Yindongnan have decreased most, about 20%. Structurally, the Qinhuai River basin was characterized by significant upward R, and K value in Hangjiahu went down dramatically by 46.8% during the 1960s-2010s. Decreasing tendency in D was found dominating across the YRD, and decreasing magnitude in Wuchengxiyu and Hangjiahu peaks for 7.8% and 6.5%, respectively in the YRD. (2) Urbanization affected the spatial pattern of river system, and areas with high level of urbanization exhibited least Dd (2.18 km/km²), WSR (6.52%), K (2.64) and D (1.42), compared to moderate and low levels of urbanization. (3) Urbanization also affected the evolution of stream system. In the past 50 years, areas with high level of urbanization showed compelling decreasing tendency in quantitative (27.2% and 19.3%) and complex indices (4.9%) and trend of enlarging of main rivers (4.5% and 7.9% in periods of the 1960s-1980s and the 1980s-2010s). In the recent 30 years, areas with low level of urbanization were detected with significant downward trend in Dd and K. (4) Expanding of urban land, construction of hydraulic engineering and irrigation and water conservancy activities were the main means which degraded the river system in the YRD.

Key words: stream structure, spatial and temporal change, urbanization, Yangtze River Delta

Longfei HAN, Chaogui LEI, Liu YANG, Xiaojun DENG, Chunsheng HU, Guanglai XU. Degrading river network due to urbanization in Yangtze River Delta[J].Journal of Geographical Sciences, 2016, 26(6): 694-706.

Figures/Tables 7

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Indices	Definitions	Formulas	Units	Explanations
Dd	Drainage density	Dd=L/A, L is total stream length in the basin, A is area of the basin	km/km²	Dd was firstly introduced by Horton (1945) to indicate the drainage ability
WSR	Water surface ratio	WSR=(A_w/A)×100%, A_w is total water area, including the water area of streams, ponds and lakes	%	WSR denotes the regulating and storing ability
K	Development coefficient of tributaries	K=L_b/L_m, L_b is length of tributaries, L_m is length of main streams	No dimension	K refers to developing degree of tributaries
R	Ratio of area to length of main streams	R=A_m/L_m, A_m is area of the main streams, L_m is length of main streams	km²/km	R is the average width of main streams, indicating developing degree of main stream
D	Box dimension	Intersecting the river networks with square boxes with side length r, count the number N(r) of boxes with stream segments. N(r) increases with decreasing r of the box, and we can get a serial of r-N(r).	No dimension	Fractal theory was first introduced to geography, and it denotes the complexity of the pattern of river network (Zhang et al., 2015).

Indices	Period	Study area					Other areas
Indices	Period	A	B	C	D	E*	Shanghai* (Yang et al., 2004; Yuan et al., 2005)	Pearl River Delta* (Zhou et al., 2008)
Area (km²)	-	3841.0	4914.0	7621.0	497.1	476.1	4962.5	1991.8
Dd	1960s	3.80	3.54	3.75	1.25	3.40	3.45	0.86
	1980s	3.27	3.87	3.24	1.58	3.06		0.84
	2010s	2.93	3.41	2.93	1.14	2.75		0.65
WSR	1960s	6.10	18.86	10.63	5.53	9.50	5.62
	1980s	5.59	17.47	9.88	5.60	7.60
	2010s	4.66	15.20	8.76	7.52	6.70
K	1960s	4.93	2.54	3.65	2.52	3.39	3.39	4.42
	1980s	4.00	3.07	2.83	2.74	2.96		3.04
	2010s	3.83	2.56	1.94	4.37	2.53		3.50
R	1960s	41.53	43.75	45.54	44.11	37.40
	1980s	42.83	48.22	47.03	39.96	34.10
	2010s	38.85	42.94	45.66	73.23	35.60
D	1960s	1.71	1.68	1.69	1.32	1.62	1.40
	1980s	1.65	1.71	1.64	1.32	1.58
	2010s	1.58	1.65	1.58	1.27	1.54

	Administrative division	Urbanization degree (%)*
High level of urbanization	Municipal district of Changzhou	58.15
	Municipal district of Suzhou	57.76
	Municipal district of Wuxi	49.94
	Municipal district of Hangzhou	46.79
Medium level of urbanization	Kunshan County	39.68
	Zhangjiagang County	38.38
	Taicang County	35.62
	Changsu County	34.64
	Wuxian County	31.93
	Wujiang County	30.18
Low level of urbanization	Haining County	29.82
	Haiyan County	22.20
	Qingpu County	22.05

Indices	Periods	Areas of different urbanization degree
Indices	Periods	High level	Medium level	Low level
Dd	1960s	2.45	3.80	4.47
	1980s	2.32	4.04	3.46
	2010s	1.78	3.64	2.96
WSR	1960s	6.88	14.69	17.23
	1980s	7.14	13.51	16.25
	2010s	5.55	11.85	15.30
K	1960s	2.93	4.90	9.12
	1980s	2.90	4.65	6.19
	2010s	2.10	4.57	2.89
R	1960s	40.91	41.39	41.08
	1980s	42.76	45.72	44.39
	2010s	46.14	42.37	40.68
D	1960s	1.45	1.67	1.69
	1980s	1.43	1.69	1.61
	2010s	1.38	1.66	1.61

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