In this article we will discuss about relationship between soil erosion and sediment delivery.
In a stream either that is river or any fluvial system, the main source of sediments is the soil erosion from the catchment area of concerned stream/river. Erosion rates can be quite different within the boundary of an individual field, which may be due to variations in soil types, slope, texture and permeability. The sediment load in river/channel/stream can be controlled by checking the soil erosion at u/s area.
However, if the soil erosion control is not practically feasible then there are several alternatives that can be used individually or in combination for cost-effective control of sediment yield. There are several factors, which affect the soil erosion and sediment yield, such as vegetative potential, topography, soil features and climate.
From a large area, the contribution of sediment in stream flow is influenced by the total amount of soil loss produced from the cropland (gross erosion), the density of crops and the amount of eroded soil that is actually reaching into the stream. The ratio of amount of sediment delivered into the stream and gross erosion produced from the watershed is called Sediment Delivery Ratio (SDR).
The amount of delivered sediment into the stream (sediment yield) can be estimated by multiplying the gross erosion and sediment delivery ratio, together. The value of SDR is being less when eroding point is at greater distance from the watercourse; or when eroding area is separated from the watercourse by sediment-trapping zone such as woodlands, or other vegetated areas, or sediment basins.
It also depends on a host of factors, such as size of drainage area, soil texture; topography and predominance of sheet and rill erosion. The value of SDR varies from about 100% to less than 1%, depending on the field slope and all other factors related to erosion. The sediment delivery ratio varies widely for any drainage area. The values of SDR based on the size of watershed are given in Table 3.14.
The relationship between soil erosion at upstream point and sediment delivered to the downstream point is very important for design of hydrological structures, planning of water resource systems and determining the water pollution status. The main sources of sediment yield from watershed are the sheet or inter-rill and rill erosions. The other sources are the upland gullies, valleys, stream channels, roads and highway ditches, various construction sites and surface mined areas.
The erosion process begins when soil particles or aggregates are detached from the soil mass. The raindrop splash, sheet flow/runoff, seepage flow, and the gravitational effect may supply the energy for detachment of soil particles. The meteorological factors also affect the erosion and sediment yield in several ways. There is significant variation in meteorological characteristics from year-to-year and storm-to-storm.
The form and intensity of precipitation in different seasons along with antecedent soil moisture conditions also affect the soil loss/sediment yield. The rainfall distribution in catchment plays an important role on soil loss and sediment delivery. A high intensity storm near the basin’s outlet could cause more sediment yield than the same storm if occurs in the upper part of the basin.
The sediment yield of major rivers of the world has been reported by Holeman (1968). The sediment yield varies from more than 7000 t/km2/year of tributaries of Yellow River in China to 420 to 490 t/km2/year of Yangtze, Indus and Mekong; and less than 100 of the Mississippi, Amazon and Nile.
In Europe the sediment yield of Dnepre (USSR), Rhine (Holland), Loire (France); and Oder (Poland) is less than 3.5 t/km2/year. The sediment yield of river Seine in Paris has been reported to the tune of 25 t/km2/year; and in the Arno and Po (Italy), it is about 280 t/km2/year. Holeman (1968) also reported the world sediment yield to the oceans, presented in Table 3.15.
For USA the sediment yields and their delivery values have been evaluated, and are reported by Robinson (1973).
The report reveals that the:
1. Sediment reduces the storage capacity of reservoirs to the tune of about 1.2 billion m3/year.
2. The Mississippi River delivers the sediment at the rate of 180 to 450 million t/year to the Gulf of Mexico.
3. In Missouri basin if proper management and erosion control measures are not followed then the soil loss from croplands is expected to get exceed 220 t/ha/year, which is equivalent to removal of about 1.8 cm soil depth per annum.
4. The soil loss from croplands along the Lower Mississippi varies from 1 to 29 t/ha/year.
5. Approximately 40% of the sediment produced by erosion in the USA is from the agricultural lands.
6. The stream bank erosion is the next largest contributor (26%) of sediment yield.
Table 3.16 presents the sediment sources and their total contribution of sediment yield in different streams of USA.
There is significant variation in sediment delivery with the variations in basin size. Table 3.17 presents the variation in sediment delivery ratio as per variation in the size of drainage basin. The size of drainage basin, topography, soil, cover conditions and drainage density are the main factors affecting the sediment yield from a given basin.
The larger the drainage area in a given physiographic location, greater will be the total sediment yield; and vice-versa. The sediment delivery ratio gets decrease with increase in the size of drainage area. The variations in SDR with increase in drainage area are shown in Table 3.17.
In large basins the distance of sediment movement downstream is more, as result there is greater opportunity to get deposition of the eroded soil mass over the path. The factors like rainfall, plant cover, sediment texture and land use complicate the relationship between basin size and SDR. Therefore, all these factors are critically evaluated for estimating the quantity of sediments from erosion source, are going to get deposit in the reservoir, or sediment yield at the downstream locations.
The sheet erosion is the largest sediment yield producing process from tilled lands, and few from the vegetated non-tilled lands, also. The sediment producing tendency of sheet erosion can be checked by using the conservation practices. However, sometimes the runoff from conservation treated areas is also found to get increase, which cause greater channel erosion. The rates of normal/geological erosion have been approximated for host of the soils by measuring the soil loss on plot basis under natural vegetations.
That has been reported to the tune of less than 0.7t/ha/year from vegetated areas (A.S.C.E., 1975). The rate of soil erosion gets vary under natural vegetation with variations in amount and intensity of annual rainfall. Langbein and Schumm (1958) reported that when average rainfall is between 250 and 350 mm/year, then vegetative cover is usually sparse, and causing maximum soil erosion. The sediment yield gets decrease with decrease in rainfall amount, because of reduction in runoff. In addition to rainfall variability the soil and vegetation also influence the sediment yield.
The tillage practices and residue management practices affect the upland soil erosion. The residues and mulches reduce the runoff and sediment yield by creating surface cover and reducing the surface sealing and crusting. The erosion from different soils and crops at different growth stages and different levels of residue management can be predicted by using the USLE. The predicted gross erosion is multiplied by the sediment delivery ratio for determining the amount of sediment delivered to the downstream point in the watershed.
Gullies are very prominent source of sediment yield from the watershed. Gully density refers to the total length of gullies per unit area. It affects directly the sediment delivery from watershed. Its consideration for predicting the relationship between soil erosion and sediment delivery is very important. The concentration of runoff encourages the gully formation. The growth pattern of gully may be the cyclic, steady or spasmodic. Gullies are very significant source to transfer a huge quantity of sediments produced from the u/s areas to the d/s.
Although, the total amount of sediment outflow from the gullies is large, but in comparison to sheet erosion it is very less. The gully erosion causes economic losses due to dissection of uplands, damaging the roads & drainage structures and deposition of infertile soil over the fertile lands. In Mississippi, the measured sediment production from active gullies with small drainage area varies from 5 to 18 cm/year.
The stream bank erosion is due to direct or indirect actions of flowing water through the stream.
The actions may be the:
1. Undercutting and collapsing of bank soil mass by gravity effect.
2. Impinging of flow directly on the banks, and
3. Saturation of bank soil either by stream flow or seepage.
The wave action, ice flow, freezing action, wet-dry cycles, rapid changes in flow stage, debris and sediment load are the additional causes of stream bank erosion. The supply of sediments from upland areas to the stream flow depends on the conservation and erosion control measures used there.
A flowing stream has the energy to carry the sediment load; and also if flowing over an erodible bank then picking and carrying of extra sediment loads. The stream bank erosion is very significant source of sediment yield from the basin area. In USA about 480 000 km of stream bank is under erosion, from which about 450 million tones of sediments each year is delivered to the d/s areas.