Net Sediment Deposition Research Articles

Sediment transport and channel adjustments associated with dam removal: Field observations

This study documents changes in channel geometry, bed level profile, and bed grain size distribution and their relations with the sediment transport at the reach scale, following the removal of a low‐head dam. After the removal, net sediment deposition occurred downstream of the dam, and net erosion occurred in the reservoir, but approximately less than 1% of the sediment stored in the reservoir was transported downstream. No bank erosion was evident either upstream or downstream of the dam. Bed deposition and scouring in the reservoir accounted for a decrease in the bed slope of 30%. The stations downstream of the dam had surface bed material sizes at least 40% finer than preremoval conditions. However, the sediment transport rates downstream of the dam were not significantly different from predam to postdam removal or from an upstream control. Overall, the removal of the dam had only minor effects on the channel adjustment downstream of the dam. A simple analysis linking transport to channel geometry explains this effect.

Response of a river sediment budget after historical gravel mining (the lower Tordera, NE Spain)

AbstractThe channel of the lower Tordera River (Catalan Coastal Ranges) was intensively mined between 1956 and 1987, at an extraction rate 14 times higher than the replenishment rates. Mining caused incision of up to 2 m over the whole reach, leading to damage of infrastructures and affecting the groundwater system. In this paper we analyse the response of the river sediment budget by means of sediment transport measurements (both in suspension and as bedload) and topographic surveys. Historical data on mining activities was used to verify current values of sediment transport and to locate the main extraction areas in the long profile of the river. This has led to an integrated methodology that can be useful for assessing and managing current and future sediment deficits in heavily mined rivers. In the lower Tordera, a minimum of 3 million cubic metres of sand and gravel were extracted, especially from two gravel pits still visible as concavities in the river's longitudinal profile. Nowadays, 15 yr after mining ceased, the river shows during dry periods a general tendency towards aggradation (3.6 mm yr−1 on average and up to 12 mm yr−1 in central sections where mining took place), but a slight long‐term tendency to erode the riverbed. Sediment yield at the entrance of the study reach attains 552 000 t yr−1, while net sediment deposition reaches 16 400 t yr−1 (78% as bedload), mostly during small floods in dry periods. Estimated time for the river to recover pre‐extraction bed‐level is of the order of 420 yr. Copyright © 2005 John Wiley & Sons, Ltd.

The influence of rainfall on sediment transport by overland flow over areas of net deposition

Overland flow is often only a few millimetres deep. Consequently, the potential for raindrop impact to affect flow hydraulics and sediment transport is high. Furthermore, the relative importance of rainfall impact is highest for shallow low-energy flows on low slopes. In such flows net sediment deposition may occur. Therefore, laboratory experiments were conducted to study sediment deposition in the presence of rainfall over a range of hydraulic conditions. In order to investigate the impact of raindrops on sediment deposition by overland flow, these experimental data were compared to the experimental data collected in the absence of raindrop impact. Comparison of the experimental data shows that raindrop impact retards the flow velocity and has a clear positive effect on sediment delivery. Under rainfall significantly more coarse sediment is transported over areas of net sediment deposition. Subsequently, the experimental results are used to evaluate a multi-class net deposition theory, describing sediment transport and sediment sorting over areas of net deposition in the presence of both raindrop impact and flow-driven processes. The multi-class theory is calibrated using part of the experimental data. Evaluation of the model predictions using the other part of the data shows that the optimised model is able to accurately predict sediment delivery and sediment sorting over areas of net deposition.

Hydro-ecological analysis of a sacred lake watershed system in relation to land-use/cover change from Sikkim Himalaya

The hydrology and ecological linkages between Khecheopalri lake and its surrounding watershed (area: 12 km 2) in Sikkim were investigated to assess the long-term impacts of land-use/cover change on the hydrology of the lake ecosystem and bog formation around the glaciated lake. Significant land-use/cover change occurred in the past 4 decades. The bog area expanded by 67%, while the area under agriculture land in the lake watershed grew by 63% between 1988 and 1997. Overland flow was highest on the bare land (4.77% of the precipitation) and lowest in areas of cardamom-based agroforestry (1.79%). Soil and nutrient losses were highest in the cultivated area and least in the cardamom agroforestry system. Sediment loads of 345 Mg year −1 were recorded at the lake inlet and of 316 Mg year −1 at the outlet. Annual soil loss from the lake watershed was 502 Mg km −2 and a net sediment deposition in the lake was 141 Mg year −1. The lake received high nutrient loads (organic carbon of 10.2 Mg year −1, total nitrogen of 1.01 Mg year −1 and total phosphorus of 0.51 Mg year −1) from soil erosion and overland flow. The pH, total phosphorus and bulk density of the peat increased from the lake towards the bog-forest edge. This reflects the trapping of sediments and nutrients around the bog forest margin, although their retention is limited. Agricultural practices should be minimized in the upper part of the watershed and agroforestry practices should be encouraged to maintain the health and longevity of the lake.

Estimation of erosion, deposition, and net volumetric change caused by the 1996 Skeiðarársandur jökulhlaup, Iceland, from Synthetic Aperture Radar Interferometry

Using repeat‐pass satellite synthetic aperture radar interferometry, we develop a methodology to measure flood‐induced erosion and deposition and apply it to a record 1996 glacier outburst flood (jökulhlaup) on Skeiðarársandur, Iceland. The procedures include (1) coregistration of backscatter intensity images to observe morphological differences; (2) mapping of interferometric phase correlation to identify preserved and modified surfaces; and (3) construction, correction, and differencing of pre‐jökulhlaup and post‐jökulhlaup topography. Procedures 1 and 2 are robust and should be widely applicable to other fluvial environments, while procedure 3 is complicated by uncertainties in phase measurement, baseline estimate, and atmospheric effects. After a correction procedure involving interpolation of digital elevation model elevation differences across low‐correlation areas, we find ∼4 m of elevation change are required to calculate volumes of erosion or deposition. This condition was satisfied for the 40 km2 proglacial zone of Skeiðarársandur, where we estimate +38×106 m3 of net sediment deposition along the ice margin, −2 ×106 m3 of net erosion in channels downstream, and a total net balance of +13 × 106. These estimates are supported by field observations and survey data collected in 1997.

Quantitative model of the growth of floodplains by vertical accretion

A simple one-dimensional model is developed to quantitatively predict the change in elevation, over a period of decades, for vertically accreting floodplains. This unsteady model approximates the monotonic growth of a floodplain as an incremental but constant increase of net sediment deposition per flood for those floods of a partial duration series that exceed a threshold discharge corresponding to the elevation of the floodplain. Sediment deposition from each flood increases the elevation of the floodplain and consequently the magnitude of the threshold discharge resulting in a decrease in the number of floods and growth rate of the floodplain. Floodplain growth curves predicted by this model are compared to empirical growth curves based on dendrochronology and to direct field measurements at five floodplain sites. The model was used to predict the value of net sediment deposition per flood which best fits (in a least squares sense) the empirical and field measurements; these values fall within the range of independent estimates of the net sediment deposition per flood based on empirical equations. These empirical equations permit the application of the model to estimate of floodplain growth for other floodplains throughout the world which do not have detailed data of sediment deposition during individual floods. Copyright © 2000 John Wiley & Sons, Ltd.

An Arctic Kelp Community in the Alaskan Beaufort Sea

The discovery of the "Boulder Patch", an area of cobbles and boulders with attached kelp and invertebrate life, is reported from Stefansson Sound, near Prudhoe Bay, Alaska. Geophysical surveys using side-scan sonar and low-frequency recording fathometers reveal that cobbles and boulders occur in patches of various sizes and densities. Despite a seasonal influx of sediments, the Boulder Patch is a nondepositional environment. Physical disruption of cobbles and boulders by deep draft ice is minimal due to offshore islands and shoals which restrict the passage of large ice floes into Stefansson Sound. The apparent absence of similar concentrations of rocks with attached biota along the Alaskan Beaufort Sea coast is explained by the scarcity of rocks in areas protected from ice abrasion and with no net sediment deposition. In Stefansson Sound, the rocks provide a substratum for a diverse assortment of invertebrates and several species of algae. Recolonization by the biota was minimal on twelve boulders denuded and then left undisturbed for a three-year period. Sedimentation and grazing activity appear to be the major factors inhibiting recolonization. Linear growth in the kelp, Laminaria solidungula, is greatest in winter and early spring when nutrients are available for new tissue growth. The plant draws on stored food reserves to complete over 90% of its annual linear growth during the nine months of darkness under a turbid ice canopy. These reserves are accumulated by photosynthetic activity during the preceding summer. The total carbon contribution made by kelp in Steffansson Sound under these conditions is about 146 million g/yr or 7 g/m²/yr. A small percentage of this carbon is consumed directly by herbivores, but its importance to other organisms in not known and is under investigation.Key words: kelp, Laminaria solidungula, Flaxman boulders, Beaufort Sea, Boulder Patch, productivity, recolonization, geophysical surveys, side-scan sonar, lag deposits

Determination of Some Paleohydraulic Parameters: ABSTRACT

New river measurements combined with existing laboratory evidence show that, in a river reach where traction motion as well as net sediment deposition take place, the mean applied bed shear stress is equal to the critical shear stress required to move particles of median bed material size. Consequently, application of Shields' relation between shear stress and material size permits the determination of mean applied shear stress at an ancient depositional site from the grain-size distribution of the sediment. Furthermore, by applying laboratory values for the friction factor, determined for the appropriate bed configuration, we can calculate the paleoflow velocity by the Darcy-Weisbach relation. This method for the determination of paleoflow velocity is generally applicabl to any sedimentary deposit if a reasonable estimate of the friction factor can be made. For rock units containing climbing ripple cross-lamination, we can go one step farther and determine the rate at which the sediments were accumulated. The rate of bedload transport has a known functional relation to the mean velocity of the overlying column of water as determined in the first part of this study. Very high rates of sedimentation are found to exist in nature. Late Pleistocene deltaic sediments in the Truckee Canyon of Nevada, locally contain climbing ripple cross-laminated units indicating a local rate of aggradation of 0.1 mm/sec or 36 cm/hr. End_of_Article - Last_Page 797------------