Evolution of Bars in Braided Rivers Controlled by Discharge Variability
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摘要: 流量变化控制辫状河心滩形成演化进而影响心滩内部结构和叠置样式,但目前流量变化控制的辫状河心滩沉积过程与演化特征尚不明确.利用Google Earth软件在全球范围内选取了发育辫状河沉积的13个河段,并在全球径流数据中心(GRDC)中查询整理所选河段的流量数据,通过峰值流量变异系数(CVQp)对心滩演化过程进行研究.结果表明:就流量变化程度而言,(1)低流量变化辫状河(CVQp < 0.4)心滩演化以坝尾沉积和下游迁移为主,高流量变化辫状河(CVQp > 0.4)心滩演化相对快,洪水前后原有心滩易被破坏、改造形成新的心滩,心滩冲裂现象普遍;(2)单一河流内,低流量变化辫状河心滩长宽比分布相对集中,形态较为稳定,易形成相对稳定型辫状河,而高流量变化辫状河心滩长宽比分布离散,形态各异且发育规模变化较大,易形成游荡型辫状河;(3)顺流加积在低流量变化辫状河心滩演化中较常见,因此心滩经过长时间的演化易形成复合心滩,高流量变化辫状河的洪水事件对心滩演化过程影响较强,在时间尺度上心滩演化相对复杂、规律性较差.明确辫状河在不同流量条件下的心滩演化可为辫状河储层的砂体分布预测提供模式指导,也可为古环境重建和古流量恢复提供依据.Abstract: The discharge variability controls the formation and evolution of braid bars and influences the internal structure and superimposition of braid bars. However, the control of discharge variability on the developments and evolution of braid bars are unclear. Google Earth software was used to selected 13 river reaches with braided river deposits worldwide, and the discharge data of the selected river reaches are collected from the Global Runoff Data Center (GRDC). Then, the coefficient of annual peak discharge variation (CVQp) was used to study the process and evolution of the braid bars. The results are as follow. (1) In terms of the degree of discharge variability, the braid bar evolution of braided river with lower discharge variability (CVQp < 0.4) mainly develops bar tail deposition and downstream migration. In braided rivers with higher discharge variability (CVQp > 0.4), the braid bar evolves relatively fast. Before and after flood event, the original braid bar is easy to be destroyed and transformed into a new braid bar, and the braid bar burst is common. (2) In a single river, the braid bar distribution of length to width ratio of the braided river with lower discharge variability is relatively concentrated, and the morphology is relatively stable, which is easy to form a relatively-stable braided river. While braided river with higher discharge variability is easy to form classical braided river because of the scattered braid bar distribution of length to width ratio, different morphology and great changes in development position.(3) Downstream accumulation is more common in the evolution of braid bar with lower discharge variability, so it is easy to form compound bars after long time evolution. The flood events of braided rivers with higher discharge variability have a strong influence on the evolution process of the bars, and the braid bar evolution is relatively complex and has poor regularity in the time scale. Clarifying the evolution of braid bars under different discharge conditions can provide guidance for the prediction of sandstone distribution of braided river reservoirs and may provide a basis for the paleo-environment reconstruction and paleo-discharge restoration.
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Key words:
- braided river /
- discharge variability /
- peak discharge /
- braid bar evolution /
- depositional model /
- petroleum geology
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表 1 流量变化表征参数
Table 1. Characterization parameters for discharge variability
流量变化指标 计算方程 峰值流量变异系数CVQp $ {\sigma }_{{\mathrm{Q}}_{\mathrm{p}}}/{Q}_{{\mathrm{p}}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}} $ 平均流量变异系数DVIa $ ({Q}_{{\mathrm{W}}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}}-{Q}_{{\mathrm{D}}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}})/{Q}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}} $ 累计流量变异系数DVIc $ ({Q}_{{{\mathrm{W}}^{\mathrm{R}}}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}}-{Q}_{{{\mathrm{D}}^{\mathrm{R}}}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}})/{Q}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}} $ 年流量变异系数DVIy $ [\sum\limits_{i=x}^{x+n}({Q}_{{\mathrm{x}}_{\mathrm{D}\mathrm{a}\mathrm{y}\mathrm{m}\mathrm{a}\mathrm{x}}}-{Q}_{{\mathrm{x}}_{\mathrm{D}\mathrm{a}\mathrm{y}\mathrm{m}\mathrm{i}\mathrm{n}}})/{Q}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}]/n $ 洪水强度Qpeakedness $ {Q}_{{\mathrm{W}}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}}/{Q}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}} $ 注:σQp为年洪峰流量标准差,QPmean为年平均洪峰流量;QWmean为平均最湿润月流量;QDmean为平均最干旱月流量;QWmeanR为有记录以来平均最湿润月流量;QDmeanR为有记录以来平均最干旱月流量;QXDaymax为x年最大日流量;QXDaymin为x年最小日流量;Qmean为平均流量. 表 2 研究河段参数统计
Table 2. River parameters in the study river reaches
序号 河流 位置 水文站 控制流域面积(m2) 气候 测量心滩个数 测量心滩区域位置 平均洪峰流量
(m3/s)标准差 CVQp 1 亚马逊河 巴西 Santo Antonio Do Ica 1 134 540 热带雨林 20 3°58′48″S—3°43′25″S
70°59′47″W—64°4′21″W68 192.99 5 928.93 0.09 2 巴拉那河 阿根廷 Corrientes 1 950 000 潮湿亚热带 20 27°56′58″S—31°10′37″S
58°49′29″W-59°59′46″W21 917.69 3 564.69 0.16 3 马更些河 加拿大 Norman Wells 1 570 000 寒冷 20 62°4′7″N-66°48′22″N
122°7′42″W—130°7′40″W18 130.50 3 444.79 0.19 4 雅鲁藏布江 印度 Bahadurabad 636 130 亚热带季风 20 26°41′55″N—26°0′57″N
93°39′34″E—90°1′38″E50 531.08 10 106.22 0.20 5 恒河 印度 Farakka 835 000 热带季风 20 26°34′49″N—25°28′16″N
82°39′54″E—85°56′5″E44 419.30 10 634.65 0.24 6 Rakaia River 新西兰 Fighting Hill 2 560 温带 20 43°34′37″S—43°41′35″S
171°44′38″E—171°55′17″E398.31 125.36 0.31 7 育空河 美国 Ruby, Alas. 670 810 寒冷 20 65°8′50″N—62°42′5″N
152°5′10″W—160°7′10″W12 966.21 4 122.94 0.32 8 维斯瓦河 波兰 Warsaw 84 945 温带 20 52°24′56″N—52°28′35″N
20°37′26″E—19°47′1″E1 151.13 398.48 0.35 9 奈厄布拉勒河 美国 Near Verdel, Nebr. 29 992 半潮湿-
半干旱25 42°46′1″N—42°49′33″N
99°54′10″W—98°45′44″W98.11 43.37 0.44 10 南萨斯喀彻温河 加拿大 Saskatoon 141 000 半潮湿-
半干旱25 51°49′4″N—51°25′40″N
106°44′2″W—107°4′41″W846.79 389.52 0.46 11 松花江 中国 哈尔滨 391 000 季风 27 45°55′9″N—46°17′22″N
128°5′30″E—129°27′55″E3 397.06 1 624.19 0.48 12 普拉特河 美国 Louisville, Nebr. 218 078 干旱 25 40°53′3″N—40°56′41″N
98°16′23″W—98°12′1″W534.69 302.25 0.57 13 HalilRud River 伊朗 Hossein Abad Jiroft 8 420 干旱 20 28°40′32″N—28°39′29″N
58°32′13″E—58°28′55″E36.40 43.58 1.20 -
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