Developing Sediment Yield Prediction Equations for Small Catchments in Tanzania - Advances in Data, Methods, Models and Their Applications in Geoscience - page 244

prior to reaching stream channels. Six major factors which influence the long-term sediment
yields/delivery from a catchment based on Renfro (1975) as reported in Morris and Fan
(1998) and critically reviewed by Ndomba (2007) are: i) Erosion process - the sediment
delivered to the catchment outlet will generally be higher for sediment derived from
channel-type erosion which immediately places sediment into the main channels of the
transport system, as compared to sheet erosion; ii) Proximity to catchment outlet - sediment
delivery will be influenced by the geographic distribution of sediment sources within the
catchment and their relationship to depositional areas. Sediment is more likely to be
exported from a source area near the catchment outlet as compared to a distant sediment
source, since sediment from the distant sources will typically encounter more opportunities
for re-deposition before reaching the catchment outlet; iii) Drainage efficiency- hydraulically
efficient channels networks with a high drainage density will be more efficient in exporting
sediment as compared to catchments having low channel density; iv) Soil and land cover
characteristics - finer particles tend to be transported with greater facility than coarse
particles. Because of the formation of particle aggregates by clays, silts tend to be more
erosive and produce higher delivery ratios than clay soils; v) Depositional features - the
presence of depositional areas, including vegetation, ponds, wetlands, reservoirs and
floodplains, will decrease the sediment yields at the catchment outlet. Most eroded
sediments from large catchments may be re-deposited at the base of slopes, as outwash fans
below gullies, in channels or on floodplains; vi) catchment size and slope - a large, gently
sloping catchment will characteristically have a lower delivery ratio than a smaller and
steeper catchment.
This chapter is therefore reporting the developed sediment yield-fill equations as
categorized as regional regression relationships for various climatic regions in Tanzania as
simple and efficient planning tools of water supply schemes small reservoirs/pond with
limited data. In this study additional new data on dams is used. It should be noted that
requirements for data and computational modelling skills rule out the use of more
sophisticated methods to predict sediment yields, and as a result a simple regional sediment
yield predictor was chosen for this application. The equations are developed from readily
available data on catchment area and reservoir sediment fill from Ministries and
Government Agencies. The size of the area is very important factor in respect to the total
yield of sediment from a catchment. However, it should be noted that its relative importance
to the influence of the sediment delivery ratio and sediment production rate is subject to
questioning. It is suggested in the literature that sediment production rates declined with
increasing catchment area (Morris and Fan, 1998). This theory is supported by the fact that
the probability of entrapment and lodgment of a particle being transported downstream
increases as the drainage area increases. Besides, for the same length of a river network, the
smaller the catchment area the higher the drainage density as well the sediment yields. The
proximity to the catchment outlet may also be indirectly related to catchment size. The
preceding discussions suggest that catchment area size may sometimes be directly or
indirectly linked to various factors controlling sediment delivery to the outlet of the
catchment. However, catchment area size could not be directly related to erosion process,
soil type and land cover. Such limitations would render the general relationships between
catchment area size as independent variable and the sediment yield-fill be used only for
preliminary planning purposes or as a rough check. It is anticipated that small dam
designers/planners would be able to use these tools /methods; they typically need to carry
out assessments rapidly using limited local data, and may not have software skills or access
to computers.