Lower Main Channel Research Articles

Boundary shear stress and apparent shear forces in compound channels with different floodplain widths

ABSTRACT The present study has studied a model to predict depth-averaged velocity distribution and boundary shear stress in compound channels with different floodplain widths. The model was verified against three different data sets of the present experiments and two data used in the literature. The results indicate that the present model can predict the depth-averaged velocity and shear stress distributions, promising to simulate the flow field and related characteristics better. The results show that the non-dimensional coefficient plays a dominant role in portraying the variation of depth-averaged velocity and shear stress distributions in compound channels with different floodplain widths. In addition, the apparent shear forces were examined by dividing the channel cross-section into seven parts, and each element carried shear forces and acted on the horizontal and vertical interfaces for three different width-to-depth ratios were discussed and analyzed. In all configurations, the horizontal interface was negative, indicating that the upper region of flow accelerates the flow in the lower main channel. In contrast, the vertical interfaces were positive, suggesting that the slower floodplain flows retards the faster main channel flows.

Flow distribution in diverging compound channels using improved independent subsection method

Experiments have been conducted in three diverging compound channels for different flow conditions to study the flow distribution in floodplain, upper and lower main channel. In a compound channel, vertical apparent shear exists on the interface between the upper main channel and the floodplain, which generally accelerates the flow on the floodplain and resists the flow in the upper main channel. In addition, a horizontal apparent shear stress also occurs on the interface between the upper and lower main channels, which generally accelerates the flow in the lower one and resists the flow in the upper one. Therefore, it is essential to consider the exchanges of momentum at both vertical and horizontal shear layer regions. In this paper, an attempt is made to improve the classical independent subsection method (ISM) to determine the magnitudes of flow and velocities in both upper and lower main channels. Four subsections are created in improved ISM according to the vertical and horizontal division lines that correspond to the vertical interface between the main channel and floodplain and the horizontal interface between upper and lower main channels respectively. The improved ISM consists in a set of four coupled 1D momentum equations (instead three equations of classical ISM) for subsections and a mass conservation equation for the total cross-section. The computed results show that the method is well capable of predicting the discharge distributions in the floodplain and main channel (both at upper and lower main channel).

An analytical solution for flow estimation of a meandering compound channel

An analytical model is proposed to determine the discharge capacity in a meandering compound channel. The channel cross-section is divided into four sub-sections, such as the lower main channel, the floodplain within the meander belt and the two outer floodplains. Momentum transfer in-between these four subsections is taken into consideration in the analytical model. The model basically determines the force balance of each individual subsection to predict its mean velocity and thereby the sub-sectional discharge. The paper suggests a non-dimensional parameter, αT, which is the momentum transfer coefficient, that is determined to be unique for each individual channel. The paper deals with the calibration of this parameter for both largescale and small-scale data sets.

ADJUSTMENT OF PRAIRIE POTHOLE STREAMS TO LAND‐USE, DRAINAGE AND CLIMATE CHANGES AND CONSEQUENCES FOR TURBIDITY IMPAIRMENT

ABSTRACTChanges in land use and drainage have contributed to channel adjustment in small‐order to medium‐order streams in the prairie pothole region of south‐west Minnesota. Although conversion from prairie to agriculture occurred a century ago, recent decades have seen increased subsurface tile drainage, annual row crop coverage and channel modifications, particularly at road crossings such that channel adjustment is ongoing. Channel evolution in Elm and Center Creeks, two fourth‐order streams in the Blue Earth River basin, was studied to understand relationships between changes in channel morphology and suspended sediment concentrations. The construction of drainage ditches and expanded subsurface tiling has connected isolated basins to stream channels, effectively increasing drainage areas of Elm and Center Creeks by 15–20%. Sinuosity has been reduced by grading and drainage of first‐order sloughs, channel straightening at road crossings and natural cut‐offs and agricultural ditching that have shortened Elm Creek by 15% between 1938 and 2003. Stream cross‐sectional area was enlarged in response to the land‐use and drainage changes. In the headwaters, public ditches are wider than historic channels and entrenched by berms. Unchannelized headwater and upper mainstem portions of Elm Creek are also highly entrenched (up to 1.07 meters below the pre‐channelization bed elevation with a bank height ratio > 1.5) but have not widened substantially. In contrast, the lower main channel has widened by an average of 68%. These channel adjustments contribute to the suspended sediment load and violations of Minnesota's turbidity and Index of Biotic Integrity standards. The watershed has a low sediment delivery ratio because it is a flat, poorly connected landscape and likely delivers less sediment to the Minnesota River than steeper rivers downstream, such as the Blue Earth River. Entrenchment and increased sediment transport capacity in the lower reaches of the river have lead to increased sediment delivery to the downstream Blue Earth and Minnesota rivers. Understanding geomorphic changes will be important for addressing water‐quality impairments in the region. Copyright © 2011 John Wiley & Sons, Ltd.

4D Monitoring: Example of 4D Interpretation in Lower flanks systems, Dália - Angola

Some field of Lower Congo Basin deep offshore are composed of heterogeneous turbidites channels of Miocene age located in Angola at water depth that range between 700 – 1500 meters. The Dalia field, located in Block 17, is characterized by four main sedimentary complexes, called Upper Main Channel, Lower Main Channel, Lower Flanks and Camelia. This area is characterized by high resolution seismic data (10 m) that is used for fine reservoir characterization and monitoring. After one year and half of production, a first 4D seismic survey was acquired. This 4D seismic monitor bring more accurately interpretation on facies distribution and a better understanding of reservoir communications. The 4D effects seen on Dalia are large since time shifts associated to production were used to interpret 4D signals and upscale them inside the reservoir model.

Major and trace element provenance signatures in stream sediments from the Kando River, San’in district, southwest Japan

Abstract Basement rocks in the catchment of the Kando River in southwest Japan can be divided into two main groups. Paleogene to Cretaceous felsic granitoids and volcanic rocks dominate in the upstream section, and more mafic, mostly Miocene volcanic and volcaniclastic rocks occur in the downstream reaches. Geochemically distinctive Mount Sambe adakitic volcanic products also crop out in the west. X‐ray fluorescence analyses of major elements and 14 trace elements were made of two size fractions (<180 and 180–2000 µm) from 86 stream sediments collected within the catchment, to examine contrasts in composition between the fractions as a result of sorting and varying source lithotype. The <180 µm fractions are depleted in SiO2 and enriched in most other major and trace elements relative to the 180–2000 µm fractions. Na2O, K2O, Ba, Rb and Sr are either depleted relative to the 180–2000 µm fractions, or show little contrast in abundance. Sediments from granitoid‐dominated catchments are distinguished by greater K2O, Th, Rb, Ba and Nb than those derived from the Miocene volcanic rocks. Granitoid‐derived <180 µm fractions are also enriched in Zr, Ce and Y. Sediments derived from the Miocene volcanic rocks generally contain greater TiO2, Fe2O3*, Sc, V, MgO and P2O5, reflecting their more mafic source. Sediments containing Sambe volcanic rocks in their source are marked by higher Sr, CaO, Na2O and lower Y, reflecting an adakitic signature that persists into the lower main channel, where compositions become less variable as the bedload is homogenized. Normalization against source averages shows that compositions of the 180–2000 µm fractions are less fractionated from their parents than are the <180 µm fractions, which are enriched for some elements. Contrast between the size fractions is greatest for the granitoid‐derived sediments. Weathering indices of the sediments are relatively low, indicating source weathering is moderate, and typical of temperate climates. Some zircon concentration has occurred in granitoid‐derived <180 µm fractions relative to 180–2000 µm counterparts, but Th/Sc and Zr/Sc ratios overall closely reflect both provenance and homogenization in the lower reaches.

Drift benthos relationships in the seasonal colonization dynamics of alpine streams

We examined seasonal patterns in macroinvertebrate benthic abundance and drift in relation to their colonization dynamics in five contrasting streams. The streams, within an alpine glacial flood plain, differed over an annual cycle in flow permanence and surface hydrologic connectivity with the main floodplain channel. Study streams included an upper and lower main channel, a side-channel seasonally connected upstream to the main channel, and an intermittent side-slope tributary and permanent springbrook having no upstream surface connection to the main channel. Flow in the main and side-channels was influenced seasonally by upstream glaciers, whereas the tributary and springbrook had no direct glacial input. Macroinvertebrate assemblages differed markedly among streams. Density, biomass, and taxonomic richness typically were lowest in the upper main channel and highest in the springbrook. Seasonal changes in assemblage composition were more pronounced in the main and side-channels than in the tributary and springbrook. In the upper main channel, larvae colonizing cages filled with natural substrata reflected benthic assemblage structure regardless of season and colonizing taxa exhibited a high propensity to drift. In contrast, larval colonists differed significantly from benthic assemblages in the tributary and springbrook, and colonists were mostly from drift. The lower main and side-channels displayed high seasonality in colonization dynamics, being related to their hydrologic connectivity to other streams that may have provided larval colonists. Benthos to drift abundance ratios suggested that the upper main channel represented a relatively closed system with little interaction among other streams in the flood plain, whereas the springbrook and tributary seasonally subsidized larval colonists to downstream channels.

Seasonal patterns in macroinvertebrate drift and seston transport in streams of an alpine glacial flood plain

1. We measured drift of macroinvertebrates and organic matter in five different channel types (upper and lower main channel, intermittent side‐channel, side‐slope tributary, and a groundwater channel) draining a glacial flood plain in the Swiss Alps. Samples were collected seasonally over 3 years (May 1997 to November 1999) to encompass major periods of floodplain expansion and contraction.2. Total wetted channel length ranged from ≤ 5 km during maximum floodplain contraction to ≥ 24 km under system expansion. Principal components analysis (PCA) of key physical and chemical measures collected over an annual cycle showed that the main and side channel sites differed spatially and seasonally from the tributary and groundwater sites, reflecting the glacial influence on the former.3. Two‐way ANOVA indicated a significant Site × Date interaction for all measured variables. Transported organic matter, caused by blooms of Hydrurus foetidus, was typically five to 10 times higher in the lower main channel and side‐channel than the other channel types, and was least variable among dates in the groundwater and tributary channels.4. The number of drifting macroinvertebrates was usually highest in the groundwater channel, although highly variable among years. Other channel types had highest drift rates in November and lowest rates in May, reflecting seasonal changes in the life cycles of particular taxa, such as Baetis alpinus and the Simuliidae. The lower main channel exhibited the greatest, and the groundwater the lowest, seasonality in drift compared with the other channels.5. The Chironomidae displayed the highest drift rates in most seasons, especially in summer when other taxa were relatively low in the drift. The number of taxa in the drift also varied among channel types and season, ranging from less than two in early spring to four to six in other seasons.6. The observed differences in drift rate and transported organic matter among channel types and seasons have important implications for ecosystem dynamics, through its effect, for instance, on dispersal among channel types or on organic matter exchange.

Paleoseismicity of the Cascadia Subduction Zone: Evidence from turbidites off the Oregon‐Washington Margin

Cores from Cascadia deep‐sea channel contain sequences of turbidites that can be correlated and dated by the first occurrence of volcanic glass from the Mount Mazama eruption (6845±50 radiocarbon yrBP). Turbidity currents from the tributaries appear to have occurred synchronously to form single deposits in the main channel, there being only 13 turbidite deposits in the lower main channel since the Mazama eruption, instead of the twice as many expected if the tributaries had behaved independently. In addition to the Cascadia Channel, 13 post‐Mazama turbidites have been deposited in the Astoria Canyon and at two sites off Cape Blanco, sample locations that span 580 km of the Oregon‐Washington margin. Pelagic intervals deposited between the turbidites suggest that in each place the turbidity currents occurred fairly regularly, every 590±170 years on average. The best explanation of the spatial and temporal extent of the data is that the turbidity currents were triggered by 13 great earthquakes on the Cascadia subduction zone. The variability of turbidite timing is similar to that for great earthquake cycles. The thickness of the topmost pelagic layer suggests the last event was 300±60 years ago (from three places along the margin), but this number may be a biased underestimate. It is, however, consistent with the youngest sudden‐subsidence event on the Washington coast. The turbidite data demonstrate that the near‐term hazard of a great earthquake on the Cascadia subduction zone is of the order of 2–10% in the next 50 years.