The response of headwater stream channels to urbanization in the humid tropics     

Fola S. Ebisemiju 《水文研究》1989,3(3):237-253

Analysis of the bankfull cross-sections of headwater streams in Ado-Ekiti region of Southwestern Nigeria and their comparison with data from other tropical environments and temperate latitudes reveal that the channel capacities of streams in the humid tropics are relatively smaller than those of temperate regions, averaging 1.51 m² with a coefficient of variation of 87 per cent. This is attributed to the small stream discharge, the predominantly low and highly seasonal flows of the streams, the low shear stress of stream load, and the stabilizing and protective influence of riparian vegetation and surface incrustations. The chanel capacities of the urban streams (mean = 1.13m²) are about 47 per cent smaller than those of the natural streams (mean = 2.12 m²) in the same ecological zone. In terms of hydraulic efficiency, the urban streams also have relatively inefficient cross-sections and larger width/depth ratios than their rural or natural counterparts. Resurveys of seventeen monumented cross-sections reveal that while channel shoulder width increased by only 6 per cent over a one-year period, channel depth and capacity decreased by 16 per cent and 4 per cent respectively; the observed decrease in channel size occurs entirely in the channel depth dimension. Thus the response of stream channels to the urbanization of small headwater catchments in the humid tropics is probably more of vertical accretion of channel bed and reduction in channel capacity rather than the widely-reported anomalous enlargement of urban streams through channel widening. The rapid rate of channel aggradation is attributed to excessive rates of sediment production and delivery to streams in urbanized catchments in the humid tropics, rapid deposition of sediments during small runoff events and on the falling stage of storm hydrographs, and the inability of the streams to evacuate the sediments delivered to them despite the increased discharge and peak flow associated with urbanization. The low competence of the urban streams is attributed to the predominance of low flows, very gentle bed slopes, and most importantly the widespread dumping of refuse into the channels thereby reducing flow velocity and promoting backwater flooding, ponding, and sedimentation. The correlations between drainage basin area, a surrogate for stream discharge, and channel capacity are very strong for the rural watersheds, and the regression analysis indicates a tendency towards a steady-state isometric relationship. Urban channels are, to a large extent, in disequilibrium with the urban hydrological state. However, spatial variations in the degree of urbanization of the catchments, and, therefore in runoff volume and velocity, exercise strong control on channel width, depth, and size. A model of the sequence of stream channel adjustment to the urbanization of small headwater catchments in the humid tropics is presented.  相似文献   

青藏高原东部暴雨云团局地强降水响应特征   总被引：2，自引：0，他引：2  

朱平  李生辰  王振会  肖建设 《遥感学报》2014,18(2):405-431

为研究青藏高原(简称"高原")东部暴雨云团的局地强降水响应特征,使用FY-2卫星红外通道数据,选择降水开始前4小时直到降水结束后2小时的时段,对高原东部19次典型局地暴雨过程分两种方式进行云团分析,第一种方式为针对测站上空7×7像元范围的云团进行云顶温度变化等相关分析,第二种方式为对关键四省范围内的云团进行识别和追踪,并计算对流云团参数。对暴雨云团的雷达回波特征也进行了分析。结果表明:(1)7×7像元范围红外各通道的云顶温度变化趋势一致,降水阶段云顶温度先降后升,云顶温度梯度先升后降。云顶温度梯度极大值Gmax的峰值和半小时内Gmax的最大上升变化值ΔGmax均出现在强降水前(0—11 h),Gmax峰值次数为1—2次,云顶温度极小值Tmin的谷值多出现在强降水之前,Tmin曲线斜率大值阶段对应Gmax的大值阶段,ΔGmax,Gmax和Tmin的极值分别可达到22.3℃,48.3℃和-90.3℃。(2)用7×7范围云顶温度及温度梯度建立的降水量级预报方程能较好地模拟小时降水量随时间的变化趋势且有一定的预报提前时间,预报误差在1个降水量级内。在考虑了Gmax峰值对强降水的贡献后R2由0.23提高到0.54,模拟的降水量峰值与真值峰值明显接近。(3)对流云团的识别追踪方法更简单有效,对形变较小的云团(相关系数≥0.5)的准确率为100%,对发生了合并或分裂等严重形变的云团(一般相关系数0.5)的识别结果正确而追踪结果无效;(4)高原东部暴雨云团均为中-β—中-α尺度,水汽柱深厚但强度比低海拔地区更弱,测站暴雨开始之前多数有对流云团覆盖,若对流云团空间参数位置靠近测站,当空间距离至少小于或等于15个像距时降水或强降水将在几小时内产生。(5)暴雨云团在雷达回波图上表现为强降水超级单体风暴特征,且暴雨测站7×7范围Gmax峰值时刻对应有回波顶高度(18 dBz)的梯度极大值,红外1、红外2和红外3通道的Tmin谷值时刻分别对应回波顶高度极大值和垂直累积液态水含量的极大值。本研究结果对高原强对流云团的识别、跟踪及短时降水预报等具有重要参考价值。  相似文献   

CSI feedback-based CS for underwater acoustic adaptive modulation OFDM system with channel prediction     

蒯小燕  孙海信  齐洁  程恩  许小卡  郭瑜辉  陈友淦 《中国海洋工程》2014,28(3):391-400

In this paper, we investigate the performance of adaptive modulation （AM） orthogonal frequency division multiplexing （OFDM） system in underwater acoustic （UWA） communications. The aim is to solve the problem of large feedback overhead for channel state information （CSI） in every subcarrier. A novel CSI feedback scheme is proposed based on the theory of compressed sensing （CS）. We propose a feedback from the receiver that only feedback the sparse channel parameters. Additionally, prediction of the channel state is proposed every several symbols to realize the AM in practice. We describe a linear channel prediction algorithm which is used in adaptive transmission. This system has been tested in the real underwater acoustic channel. The linear channel prediction makes the AM transmission techniques more feasible for acoustic channel communications. The simulation and experiment show that significant improvements can be obtained both in bit error rate （BER） and throughput in the AM scheme compared with the fixed Quadrature Phase Shift Keying （QPSK） modulation scheme. Moreover, the performance with standard CS outperforms the Discrete Cosine Transform （DCT） method.  相似文献   

Les chenaux de marée autour des îles de Kneiss,Tunisie: sédimentologie et évolution     

M. Bali  M. Gueddari 《水文科学杂志》2013,58(3):498-506

Résumé

L'Archipel de Kneiss, dans le Golfe de Gabès (sud-est Tunisien), est un milieu à originalités multiples. Il est unique en son genre en Tunisie en raison de la variété de son paysage morphologique (marais maritimes, falaises, plages sableuses et îlots qui émergent des hauts fonds). Il est caractérisé par la présence de chenaux de marée submergés. L'objectif principal de ce présent travail est de caractériser les principales composantes qui interviennent dans la dynamique sédimentaire au niveau de ces chenaux. Les résultats des analyses granulométriques et minéralogiques des sédiments de surface prélevés au niveau des chenaux de marée ont permis d'identifier l'origine de ces sédiments et de préciser les facteurs et les phénomènes qui interviennent dans leur transport et leur dépôt. Ces résultats montrent que les sables des chenaux de marée présentent un granoclassement décroissant de l'aval vers l'amont, ce qui prouve que les courants de marée ont une action plus importante lors du flot que lors du jusant. Ces résultats montrent aussi que le mode de transport est exclusivement le roulement par les courants de marée. Les minéraux argileux et non argileux des sédiments de surface auraient pour origine essentiellement les formations géologiques qui affleurent dans l'arrière-pays. L'analyse des photographies aériennes montre que les chenaux de marée présentent une évolution morphologique.

Citation Bali, M. & Gueddari, M. (2011) Les chenaux de marée autour des îles de Kneiss, Tunisie: sédimentologie et évolution. Hydrol. Sci. J. 56(3), 498–506.  相似文献   

Modeling depth-averaged velocity and bed shear stress in compound channels with emergent and submerged vegetation     

《Advances in water resources》2013

This paper presents an approach to modeling the depth-averaged velocity and bed shear stress in compound channels with emergent and submerged vegetation. The depth-averaged equation of vegetated compound channel flow is given by considering the drag force and the blockage effect of vegetation, based on the Shiono and Knight method (1991) [40]. The analytical solution to the transverse variation of depth-averaged velocity is presented, including the effects of bed friction, lateral momentum transfer, secondary flows and drag force due to vegetation. The model is then applied to compound channels with completely vegetated floodplains and with one-line vegetation along the floodplain edge. The modeled results agree well with the available experimental data, indicating that the proposed model is capable of accurately predicting the lateral distributions of depth-averaged velocity and bed shear stress in vegetated compound channels with secondary flows. The secondary flow parameter and dimensionless eddy viscosity are also discussed and analyzed. The study shows that the sign of the secondary flow parameter is determined by the rotational direction of secondary current cells and its value is dependent on the flow depth. In the application of the model, ignoring the secondary flow leads to a large computational error, especially in the non-vegetated main channel.  相似文献   

An Automated GIS Procedure for Delineating River and Lake Boundaries     

Venkatesh M Merwade 《Transactions in GIS》2007,11(2):213-231

Digitization of river and lake boundaries is an essential task in several hydrologic studies, which, in most cases, is performed manually by using aerial photographs. This paper presents an automatic geographic information system (GIS) procedure for delineating river and lake boundaries using aerial photographs. The GIS procedure is based on supervised classification of aerial photographs by using the National Hydrography Dataset (NHD) for creating training areas. Four study areas – Lake Palestine in Texas, Brazos River in Texas, Neuse River in North Carolina, and Snake River in Idaho – are used to demonstrate the approach. The results from the automatic boundary delineation process are evaluated by comparing them to manually digitized boundaries. The boundaries delineated by the automatic procedure matched the manually digitized boundaries with an overall difference of less than five percent in terms of area measurement. Besides saving about 85 percent of manual labor, the GIS procedure provides a consistent way of delineating water bodies, and provides estimation of river channel widths, which, in conjunction with existing NHD data, can be used to estimate average flow depths. Supplementing the regional scale NHD data with local scale attributes such as channel width and depth may lead to increased use of NHD in local scale studies.  相似文献   

The form and function of headwater streams based on field and modeling investigations in the southern Appalachian Mountains     

Rebecca Kavage Adams  James A. Spotila 《地球表面变化过程与地形》2005,30(12):1521-1546

Headwater streams drain the majority of most landscapes, yet less is known about their morphology and sediment transport processes than for lowland rivers. We have studied headwater channel form, discharge and erosive power in the humid, moderate‐relief Valley and Ridge and Blue Ridge provinces of the Appalachian Mountains. Field observations from nine headwater (<2 km² drainage area), mixed bedrock–alluvial channels in a variety of boundary conditions demonstrate variation with respect to slope‐area channel initiation, basic morphology, slope distribution, hydraulic geometry, substrate grain size and role of woody debris. These channels display only some of the typical downstream trends expected of larger, lowland rivers. Variations are controlled mainly by differences in bedrock resistance, from the formation level down to short‐wavelength, outcrop‐scale variations. Hydrologic modeling on these ungauged channels estimates the recurrence of channel‐filling discharge and its ability to erode the channel bed. Two‐year recurrence discharge is generally larger and closer to bankfull height in the Valley and Ridge, due to low soil infiltration capacity. Discharge that fills the channel to its surveyed bankfull form is variable, generally exceeding two‐year flows at small drainage areas (<0·5 km²) and being exceeded by them at greater drainage areas. This suggests bankfull is not controlled by the same recurrence storm throughout a channel or physiographic region. Stream power and relative competence are also variable. These heterogeneities contrast relations observed in larger streams and illustrate the sensitivity of headwater channels to local knickpoints of resistant bedrock and armoring of channels by influx of coarse debris from hillslopes. The general lack of predictable trends or functional relationships among hydraulic variables and the close coupling of channel form and function with local boundary conditions indicate that headwater streams pose a significant challenge to landscape evolution modeling. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

分流河道特征及其识别方法：以东营凹陷东部地区为例   总被引：9，自引：0，他引：9  

于建国  林春明  杨云岭  朱应科  王金铎  赵彦彦 《高校地质学报》2002,8(2):152-159

东营凹陷东部地区分流河道沉积物以砂岩，粉砂岩为主，夹泥岩薄层，分选性较好，砂岩层理发育，横剖面上分流河道顶平底凸的透镜状，在平面上呈树枝形的扇状分布，分流河道的储集体主要有心滩和边滩交错叠置构成，厚度一般为2－10m，分流河道储集体的测井相特征在自然电位曲线上反映明显，可以划分为光滑柱状，齿形柱状，齿形漏斗状－齿形钟状和光滑卵状，分别代表河道中心，中心偏边部，接近边部及河道边部，分流河道储集体在地震反射外形呈透镜状，地震反射连续性差，中振幅，上超充填特征明显。  相似文献   

Submarine lobes and feeder channels of redeposited, temperate carbonate and mixed siliciclastic-carbonate platform deposits (Vera Basin, Almería, southern Spain)     

Juan C. Braga  Jose M. Martin  & Jason L. Wood 《Sedimentology》2001,48(1):99-116

Temperate carbonates and mixed siliciclastics-carbonates of Upper Tortonian age were deposited on a narrow platform along the southeastern margin of the Sierra de los Filabres on the western side of the Vera Basin. The temperate carbonates were unlithified or were only weakly lithified on the seafloor and so were easily prone to synsedimentary removal. Part of the shelf sediments were eroded, reworked and redeposited in submarine lobes, up to 40 m thick and 1 km wide. The lobes consist of turbiditic carbonates (calcarenites and calcirudites) and mixed siliciclastics-carbonates, which contain up to 30% siliciclasts, derived from the Sierra de los Filabres to the northwest, and abundant bioclasts of coralline algae, bivalves and bryozoans. In the inner platform, the feeder channels of the lobes cross-cut beach and shoal deposits, and are filled by strings of debris flow conglomerates (up to 3 m thick and a few metres wide). These channels presumably developed as the continuation of river courses entering the sea. Further towards the outer platform, they pass into large channels (up to several hundred metres wide and 20 m deep) steeply cutting into the horizontally bedded strata of the platform. Significant quantities of platform sediment were removed by erosion during their excavation. Once abandoned, they were filled by new platform sediments. Further towards the basin, the channels associated with the lobes exhibit lateral accretion and internal cut-and-fill structures, and are intercalated between hemipelagic deposits. The channel-filling sediments are in this latter case coarse-grained carbonates and mixed siliciclastics-carbonates. Lobe development concentrated first at Cortijo Grande on the western side of the study area, and then to the east at Mojácar. This migration may relate to the uplift of the Sierra Cabrera, a major high occurring immediately to the south of the channel and lobe outcrops.  相似文献   

An Initial Guess for the Levenberg–Marquardt Algorithm for Conditioning a Stochastic Channel to Pressure Data     

Fengjun Zhang  Albert C. Reynolds  Dean S. Oliver 《Mathematical Geology》2003,35(1):67-88

A standard procedure for conditioning a stochastic channel to well-test pressure data requires the minimization of an objective function. The Levenberg–Marquardt algorithm is a natural choice for minimization, but may suffer from slow convergence or converge to a local minimum which gives an unacceptable match of observed pressure data if a poor initial guess is used. In this work, we present a procedure to generate a good initial guess when the Levenberg–Marquardt algorithm is used to condition a stochastic channel to pressure data and well observations of channel facies, channel thickness, and channel top depth. This technique yields improved computational efficiency when the Levenberg–Marquardt method is used as the optimization procedure for generating realizations of the model by the randomized maximum likelihood method.  相似文献