Siltation

About: Siltation is a research topic. Over the lifetime, 1420 publications have been published within this topic receiving 20983 citations.

Wetland Vegetation Colonization and Expansion in Small Impoundments in Yaounde, Cameroon, West Africa

Abstract: Temporal changes in the pattern and distribution of tropical wetland vegetation in four small impoundments over a 28-year period were documented for a wetland ecosystem in the Olezoa drainage basin in Yaounde, Cameroon, West Africa. These impoundments were constructed along the Olezoa stream for the purpose of aquaculture and ranged in size from 1.4 to 3.0 ha, had depths between 200 to 400 cm, and were devoid of vegetation. Evaluation of the impoundments and the surrounding catchment was accomplished using low altitude aerial photographs for the years 1964, 1974, and 1986 and field surveys for the periods 1985 through 1992. The results reveal a progressive decline in open water surface area of the impoundments ranging from 70 to 100% due to plant colonization and expansion. Measured impoundment depths along profiles show more than 50% reduction from original depths of 300–400 cm to less than 150–200 cm in 1992. We suggest that vegetation colonization and expansion in the impoundments is a function of the reduction of water depth due to siltation accompanying increasing urbanization. During the period of evaluation, urbanization in the surrounding catchment tripled. Siltation in the impoundments resulted mainly from the accumulation of soil eroded from the upland accompanying devegetation and construction activities. The shallowing of the impoundments by siltation provided a favorable water-depth regime and substrate for colonization by aquatic and emergent wetland vegetation. Continued shallowing resulted in development of vegetation zones along a water-depth gradient, and the migration of these zones toward the interior of the impoundments. The history of degradation of the catchment area and siltation of the impoundments is thus recorded by temporal changes in vegetation.

Developing Sediment Yield Prediction Equations for Small Catchments in Tanzania

Abstract: Sediment yield refers to the amount of sediment exported by a basin over a period of time, which is also the amount which will enter a reservoir or pond located at the downstream limit of the basin (Morris and Fan, 1998). Estimate of long-term sediment yield have been used for many decades to size the sediment storage pool and estimate reservoir life. However, these estimates are often inaccurate especially for small catchments. Besides, it is known from literature that long term period sampling programmes are required to capture the high variability of sediment fluxes in these catchments (Horowitz, 2004; Thodsen et al., 2004). The correlation of sediment yields to erosion is complicated by problem of determining the sediment delivery ratio, which makes it difficult to estimate the sediment load entering a reservoir/pond on the basis of erosion rate within the catchment (Morris and Fan, 1998). Sediment yield from the dam catchment is one of the parameters controlling sedimentation of small dams. This has to be estimated if future sedimentation rates in a dam are to be predicted. Non Governmental Organizations (NGOs) and Government Agencies (GAs) have constructed thousands of small dams in semi-arid regions of East and Southern Africa including Tanzania to provide water for livestock and small-scale irrigation (Lawrence et al, 2004; Faraji, 1995). In Tanzania, in particular, at present it is not known whether the original storage capacities of these dams still exist as a result of many years of operations. Besides, irrigation/water supply schemes ponds/reservoirs are normally draining small catchments. Most of the small catchments are characterized as ungauged. The effective life of many of these dams is reduced by excessive siltation – some small dams silt up after only 2 years. This issue is poorly covered in the many small dam design manuals that are available, which mostly focus on civil engineering design and construction aspects. While a capability to estimate future siltation is needed to ensure that dams are sized correctly, and are not constructed in catchments with very high sediment yields, little guidance is available to small dam planners and designers (Lawrence et al, 2004). Therefore, prediction of sediment yields from catchments is very important where water resources sedimentation is a serious problem like Tanzania and construction of dams is needed (Mulengera, 2008). This chapter discusses also the findings of a few previous representative and related research works in Tanzania and the region at large as this study is a follow up research. These studies were selected in order to cover a wide range of study methods.

Earthquakes, catastrophic sediment additions and the response of urban stream communities

Abstract: Disturbance events can regularly impact stream ecosystems; however, large-scale catastrophic disturbances are rare. From September 2010 to September 2011 Christchurch City experienced over 8500 earthquakes including a magnitude 7.1. One consequence was catastrophic additions of silt and sand into waterways throughout the city. Of 161 km of permanent waterways, 102 km (63%) were affected by earthquake siltation. Benthic invertebrates and fish communities were compared across 16 streams with differing siltation. Invertebrate taxonomic richness decreased significantly (mean 17 taxa reduced to 10 taxa) and EPT taxa (Ephemeroptera, Plecoptera and Trichoptera) were removed entirely from streams receiving heavy siltation. Fish richness and density decreased significantly, with fish absent from some heavily silted streams. Many of these urban streams are sourced from springs and their stable flows and low gradient limit their ability to flush sediment. We predict that without human intervention there will be a lo...

Geomorphic approach for mitigating impact of harbor construction on sandy beaches

Abstract: Some harbors constructed on sandy shore have interrupted littoral drift, resulting in updrift beach accretion, siltation in navigation channel, or downdrift erosion. Conventional harbor design proc...

Sediment Morphology, Distribution, and Recent Transport Dynamics in a Reforested Fragment

Abstract: Erosion generates large amounts of sediment, which may be deposited at the site of origin, on the slope of a hill, or along waterways. The transportability of each type of sediment by runoff and its potential for subsequent deposition largely depends on its morphological features and particle size distribution. The aim of this study is to describe and characterize the morphology and micromorphology of sediments deposited in a reforested area and to understand the dynamics of the transport process. In order to understand the sedimentation processes, the following analyses were performed: particle size distribution, with and without the use of a dispersant solution; statistical analysis of the sand fraction using Folk and Ward parameters; sediment and soil micromorphology; and two-dimensional distribution and geometry of soil horizons and sediments. The sedimentation occurred in a localized and discontinuous manner and in the lowest parts of the landscape. The sediments were generated by low intensity erosive events producing well-sorted sediments and the clay in these deposits was transported mainly in the form of silt-sized clay aggregates. Microscopic analysis of the sediments was important for clarifying the genesis and transport dynamics of the sedimentary deposits.