走廊南山河流纵剖面高海拔裂点的成因

河流水力侵蚀物理模型表明基岩河道纵剖面在均衡状态时表现为平滑上凹的形态,其特征反映了构造、基岩抗侵蚀能力和气候的作用;然而自然界河道纵剖面多呈现以裂点为特征的不均衡形态,不均衡的剖面形态以及裂点的研究同样可以对外力作用的变化起到很好的指示作用。位于北祁连的走廊南山高海拔河道纵剖面普遍呈现不均衡形式且发育海拔较高的裂点。通过对裂点成因分析发现,这些裂点并不主要受控于岩性、气候、构造等因素,而反映了冰川作用遗留地形与河流地形的分界。这一结果说明在对河道纵剖面高海拔裂点进行分析时要考虑到古冰川遗留地形也会对现代河道纵剖面产生重要影响,为进一步认识和理解造山带地貌演化以及控制因素提供了思路。     

The spatial and temporal patterns of aggradation in a temperate,upland, gravel‐bed river

Intensive field monitoring of a reach of upland gravel‐bed river illustrates the temporal and spatial variability of in‐channel sedimentation. Over the six‐year monitoring period, the mean bed level in the channel has risen by 0·17 m with a maximum bed level rise of 0·5 m noted at one location over a five month winter period. These rapid levels of aggradation have a profound impact on the number and duration of overbank flows with flood frequency increasing on average 2·6 times and overbank flow time increasing by 12·8 hours. This work raises the profile of coarse sediment transfer in the design and operation of river management, specifically engineering schemes. It emphasizes the need for the implementation of strategic monitoring programmes before engineering work occurs to identify zones where aggradation is likely to be problematic. Exploration of the sediment supply and transfer system can explain patterns of channel sedimentation. The complex spatial, seasonal and annual variability in sediment supply and transfer raise uncertainties into the system's response to potential changes in climate and land‐use. Thus, there is a demand for schemes that monitor coarse sediment transfer and channel response. Copyright © 2009 John Wiley & Sons, Ltd.     

Epigenetic gorges in fluvial landscapes

Epigenetic gorges form when channels that have been laterally displaced during episodes of river blockage or aggradation incise down into bedrock spurs or side‐walls of the former valley rather than excavating unconsolidated fills and reinhabiting the buried paleovalley. Valley‐filling events that promote epigenetic gorges can be localized, such as a landslide dam or an alluvial/debris flow fan deposit at a tributary junction, or widespread, such as fluvial aggradation in response to climate change or fluctuating base‐level. The formation of epigenetic gorges depends upon the competition between the resistance to transport, strength and roughness of valley‐filling sediments and a river's ability to sculpt and incise bedrock. The former affects the location and lateral mobility of a channel incising into valley‐filling deposits; the latter determines rates of bedrock incision should the path of the incising channel intersect with bedrock that is not the paleovalley bottom. Epigenetic gorge incision, by definition, post‐dates the incision that originally cut the valley. Strath terraces and sculpted bedrock walls that form in relation to epigenetic gorges should not be used to directly infer river incision induced by tectonic activity or climate variability. Rather, they are indicative of the variability of short‐term bedrock river incision and autogenic dynamics of actively incising fluvial landscapes. The rate of bedrock incision associated with an epigenetic gorge can be very high (>1 cm/yr), typically orders of magnitude higher than both short‐ and long‐term landscape denudation rates. In the context of bedrock river incision and landscape evolution, epigenetic gorges force rivers to incise more bedrock, slowing long‐term incision and delaying the adjustment of rivers to regional tectonic and climatic forcing. Copyright © 2008 John Wiley & Sons, Ltd.     

Development of a river delta: a case study of Cimanuk river mouth,Indonesia

This paper describes delta development processes with particular reference to Cimanuk Delta in Indonesia. Cimanuk river delta, the most rapidly growing river delta in Indonesia, is located on the northern coast of Java Island. The delta is subject to ocean waves of less than 1 m height due to its position in the semi‐enclosed Java Sea in the Indonesian archipelago. The study has been carried out using a hydrodynamic model that accounts for sediment movement through the rivers and estuaries. As an advanced approach to management of river deltas, a numerical model, namely MIKE‐21, is used as a tool in the management of Cimanuk river delta. From calibration and verification of hydrodynamic model, it was found that the best value of bed roughness was 0·1 m. For the sediment‐transport model, the calibration parameters were adjusted to obtain the most satisfactory results of suspended sediment concentration and volume of deposition. By comparing the computed and observed data in the calibration, the best values of critical bed shear stress for deposition, critical bed shear stress for erosion and erosion coefficient were 0·05 N m^?2, 0·15 N m^?2, and 0·00001 kg m^?2 s^?1, respectively. The calibrated model was then used to analyse sensitivity of model parameters and to simulate delta development during the periods 1945–1963 and 1981–1997. It was found that the sensitive model parameters were bed shear stresses for deposition and erosion, while the important model inputs were river suspended sediment concentration, sediment characteristics and hydrodynamic. The model result showed reasonable agreement with the observed data. As evidenced by field data, the mathematical model proves that the Cimanuk river delta is a river‐dominated delta because of its protrusion pattern and very high sediment loads from the Cimanuk river. It was concluded that 86% of sediment load from the Cimanuk river was deposited in the Cimanuk delta. Copyright © 2008 John Wiley & Sons, Ltd.     

Regression‐based approach to low flow prediction in the Maryland Piedmont region under joint climate and land use change

We examine the low flow records for six urbanized watersheds in the Maryland Piedmont region and develop regression equations to predict annual minimum low flow events. The effects of both future climate (based on precipitation and temperature projections from two climate models: Hadley and the Canadian Climate Centre (CCC)) and land use change are incorporated to illustrate possible future trends in low flows. A regression modelling approach is pursued to predict the minimum annual 7‐day low flow estimates for the proposed future scenarios. A regional regression model was calibrated with between 10 and 50 years of daily precipitation, daily average temperature, annual imperviousness, and the daily observed flow time‐series across six watersheds. Future simulations based on a 55 km² urbanizing watershed just north of Washington, DC, were performed. When land use and climate change were employed singly, the former predicted no trends in low flows and the latter predicted significant increasing trends under Hadley and no trends under CCC. When employed jointly, however, low flows were predicted to decrease significantly under CCC, whereas Hadley predicted no significant trends in low flows. Antecedent precipitation was the most influential predictor on low flows, followed by urbanization. Copyright © 2006 John Wiley & Sons, Ltd.     

The linkage between velocity patterns and sediment entrainment in a forced‐pool and riffle unit

A field‐based project was initiated in order to characterize velocities and sediment entrainment in a forced‐pool and riffle sequence. Three‐dimensional velocities and turbulence intensities were measured with an acoustic Doppler velocimeter at 222 different points at three similar flows that averaged approximately 4·35 m³ s⁻¹ within a large pool–riffle unit on North Saint Vrain Creek, Colorado. Sediment‐sorting patterns were observed with the introduction of 500 tracer particles painted according to initial seeding location. Tracer particles moved sporadically during a 113 day period in response to the annual snowmelt peak flow, which reached a maximum level of 14·8 m³ s⁻¹. Velocity data indicate high instantaneous velocities and turbulence levels in the centre of pools. Patterns of sediment deposition support the notion that stream competence is higher in the pool than the downstream riffle. Flow convergence around a large channel constriction appears to play a major role in multiple processes that include helical flow development and sediment routing, and backwater development with low velocities and turbulence levels above the constriction that may locally limit sediment supply. Jet flow, flow separation, vortex scour and turbulence generation enhance scour in the centre of pools. Ultimately, multiple processes appear to play some role in maintenance of this forced pool and the associated riffle. Copyright © 2008 John Wiley & Sons, Ltd.     

Estimating evapotranspiration from small watersheds using a water and energy balance approach

Meteorological and environmental data measured in semiarid watersheds during the summer monsoon and winter periods were used to study the interrelationships among flux, meteorological and soil water variables, and to evaluate the effects of these variables on the daily estimation of actual evapotranspiration (AET). The relationship between AET and potential evapotranspiration (PET) as a function of soil water content, as suggested by Thornthwaite–Mather and by Morton, was studied to determine its applicability to the study area. Furthermore, multiple linear regression (MLR) analysis was employed to evaluate the order of importance of the meteorological and soil water factors involved. The results of MLR analysis showed that the combined effects of available energy, soil water content and wind speed were responsible for more than 70% of the observed variations in AET during the summer monsoon period. The analyses also indicate that the combined effects of available energy, vapour pressure deficit and wind speed were responsible for more than 70% of the observed variations in AET during the winter period. However, the test results of two different approaches, using the relationships between AET and PET as a function of soil water content, indicated some inadequacy. Copyright © 2007 John Wiley & Sons, Ltd.     

Changes in stream water quality due to logging of the boreal forest in the Montmorency Forest,Québec

Summer stream water quality was monitored before and following the logging of 50% of the boreal forest within three small watersheds (<50 ha) nested in the ‘Ruisseau des Eaux‐Volées’ Experimental Watershed, Montmorency Forest (Québec, Canada). Logging was conducted in winter, on snow cover according to recommended best management practices (BMPs) to minimize soil disturbance and protect advance growth. A 20‐m forest buffer was maintained along perennial streams. In watershed 7·2, cut‐blocks were located near the stream network and logging was partially allowed within the riparian buffer zone. In watersheds 7·5 and 7·7, logging occurred farther away from the stream network. Observations were also made for watershed 7·3 that collected the runoff from watersheds 7·2 and 7·5, and watershed 7·6, the uproad portion of watershed 7·7. The control watershed 0·2 was contiguous to the impacted watersheds and remained undisturbed. Following clearcutting, changes in summer daily maximum and minimum stream temperatures remained within ± 1 °C while changes in diurnal variation did not decrease by more than 0·5 °C. Concentrations of NO₃^? greatly increased by up to 6000% and concentrations of K⁺ increased by up to 300% during the second summer after logging. Smaller increases were observed for Fe_total (up to 71%), specific conductance (up to 26%), and Mg²⁺ (up to 19%). Post‐logging pH decreased slightly by no more than 7% while PO₄^3? concentration remained relatively constant. Suspended sediment concentrations appeared to increase during post‐logging, but there was not enough pre‐logging data to statistically confirm this result. Logging of moderate intensity and respecting established BMPs may account for the limited changes of water quality parameters and the low exceedances of the criteria for the protection of aquatic life. The proximity of the cutover to the stream network and logging within the riparian zone did not appear to affect water quality. Copyright © 2008 John Wiley & Sons, Ltd.