A sediment yield index as a criterion for choosing priority basins

Transcription

1 A sediment yield index as a criterion for choosing priority basins Y. P. Bali and R. L. Karale Abstract. In the absence of measured sediment data, a sediment yield index expressing the relative sediment yield from different basins formed the basis for grading the basins in order of priority for soil and water conservation measures. The methodology involves erosion intensity mapping on a base material on which natural units of sub-basins and basins are delineated and codified. The erosion intensity mapping units, based on significant parameters of sediment detachment, reflect the relative magnitude of sediment yield expressed as a weighting value. A basic factor K = 10 signifies a delicate balance between the different processes of sediment detachment and deposition in a particular unit. Addition of (x) and K is indicative of effective sediment detachment somewhat proportional to the value of x. Subtraction from K suggests deposition. The transport of sediment is estimated using a delivery ratio which is based on the intricate interrelationship of site and soil factors. Following this methodology, demarcation of priority basins has been accomplished for the whole or part of the twelve River Valley Projects covering a total area of about 5.5 million ha. Comparisons with the sediment yield predictive equation show fairly dependable correlations with the weighting values of erosion intensity units. L'indice d'apport de sédiments en tant que critère de choix des bassins versants prioritaires Résumé. En l'absence de données exactes sur la sédimentation, on a utilisé l'indice d'apport de sédiments, qui caractérise l'apport relatif de sédiments de plusieurs bassins versants, comme base du classement de ces bassins par ordre de priorité vis-à-vis des mesures à prendre en matière de conservation du sol et des eaux. La méthodologie utilisée comporte une cartographie de l'intensité de l'érosion à partir d'un document de base sur lequel ont été délimités les sous-bassins et les bassins. Les unités cartographiques utilisées pour représenter l'intensité de l'érosion prennent pour base certains paramètres significatifs de l'entrainement des particules et elles traduisent les quantités relatives de sédiments entraînés exprimées sous forme de pourcentage pondéral. Un facteur de base K = 10 traduit en équilibre fragile entre les différents paramètres de l'entrainement du sédiment et les dépôts de ce sédiment dans une unité déterminée. Un complément (x) au facteur K traduit en entrainement effectif de sédiments, proportionnel dans une certaine mesure à la valeur de x. Une soustraction par rapport à K traduit un phénomène de dépôt. Le transport de sédiments est apprécié à partir du débit solide, qui dépend d'inter-relations complexes entre le site et le facteur sol. Grâce à cette méthodologie, on a pu déterminer quels étaients les bassins versants prioritaires dans tout ou partie des douze Projets d'aménagement de Bassins, couvrant au total 5.5 millions d'hectares environ. Les comparaisons qui ont été établies avec l'équation de prévision du transport solide ont mis en évidence des corrélations assez étroites avec les valeurs pondérées des unités d'intensité de l'érosion. INTRODUCTION Sedimentation of reservoirs built for purposes including hydroelectric generation, irrigation, water supply for domestic and industrial needs has been a topic of concern all over the world. Normally the reservoirs are designed to accommodate the estimated rate of sedimentation. However, it has been observed in many cases that the actual rate of sedimentation has exceeded the estimate used in the project design. The few examples from India in Table 1 are illustrative. The wide disparity between the estimated and actual rates of siltation calls for proper conservation management of the basin to reduce the rate of sedimentation in order to prolong the useful life of the reservoirs. With this objective, the Government 180

2 A sediment yield index as a criterion for choosing priority basins 181 TABLE 1. Rate of siltation [ha M/100 km ] Presumed Name of basin area in the Obserproject [km ] design ved Hirakud Maithon Panchet Tungabhadra Ramganga Nizamsagar , of India initiated a scheme of soil conservation in 13 selected River Valley Projects (RVPs) during Since then the number of projects under this scheme has been raised to 30 involving a total catchment area of million ha. The basin areas of individual RVPs vary from 0.08 to 8.30 million ha. NEED FOR SELECTING PRIORITY BASINS The financial resources and trained technical expertise available are too small to undertake conservation programmes in an area as vast as million ha. Sediment contribution is also not of the same rate from each and every unit of the basins. For maximum efficiency of the programme, it is necessary that areas, preferably small sized hydrological units ( /ha) that are actively eroding and contributing maximum sediment yield, should be located for immediate conservation treatment on a priority basis. Identification of priority hydrological units needs to be accomplished. Conventional methods of sediment measurement for streams and silt detention ponds are time consuming and cannot be applied for individual small-sized basins. Several other methods have been proposed to estimate erosion or sediment yield in quantitative terms. In most of such methods, including the Musgrave equation, the universal soil loss equation and sediment yield prediction equations, constants have either been worked out for different factors of erosion for a specific set of conditions, thereby restricting their applicability, or they require such data that may not be readily available. A sediment yield index method of universal applicability based on actual mapping has, therefore, been developed. BRIEF DESCRIPTION OF THE METHODOLOGY Mapping of the erosion intensity units Sediment yield from any area is a resultant of the interaction of meteorological factors with the land surface. Rainfall is the most important single meteorological factor although in arid areas wind power assumes equal significance. The land factors include physiography, slope, soil, land use, vegetation and present status of erosion. Soil factors comprise the broad nature of the soil, effective depth, texture of the surface soil, soil reaction and stone content. It is the combined and reciprocal influence of these factors that determine the magnitude of the sediment yield from any locality. The first step of estimating sediment detachment from an area, therefore, requires evaluation of the area in terms of these factors and their orderly mapping. The mapping is preferably carried out on air photographs for higher efficiency and greater accuracy. By following a sample area method (Buringh, 1960) with limited field checks, most of the relevant factors can be accurately identified and evaluated for precise mapping. Panchromatic aerial photographs on 1:60000 scale are ideal for such mapping. Alternately, if photographs are not available, interpretation of reliable topographical maps

3 18 Y. P. Bali and R. L. Karaie and field traverses are utilized for the mapping. The composition of the mapping units is exemplified in the appendix. The mapping units are developed, primarily, around four parameters, namely, physiography and slope gradient; soil characteristics like surface texture, depth, and colour; land use and surface condition; and present erosion. Weighting values Each of the mapping units, designated as 'erosion intensity units', are assigned a weighting value that represents the collective effect of the different factors of sediment detachment in terms of sediment yield. A basic factor of 10 is assumed to indicate a static condition. At this value the interaction of the factors is such that no sediment is produced from the land surface. Specific combinations of the different factors and their intensities determine the weighting value. Two striking examples of first, steeply sloping land without a soil mantle, and second, nearly level land with very deep porous soils and luxuriant vegetation, may be quoted in which the resultant sediment yield is almost nil despite large variations in the causative variables. Values higher than 10 imply erosion, whereas values less than 10 suggest deposition in proportion to the additive or subtractive figures. For example, weighting values of 10, and 1 indicate that sediment yields from the units to which these values are assigned are expected to be approximately in that proportion, whatever the absolute figures. Weighting values are, therefore, careful judgements of the relative erosion in each mapping unit. Delivery ratios The sediment detached from an area may get deposited in an adjacent area or may be transported for long distances. In studies concerning the sedimentation of reservoirs, the amount of sediment reaching the reservoir or any point of measurement is of more significance than the amount of sediment detached at a particular location. The proportion of the detached sediment reaching the reservoir is termed the delivery ratio. The basin characteristics that govern the delivery ratio include grain-size distribution of the detached material, shape and size of the basin, relief length ratio, density and kind of drainage system, slope, sediment detention opportunities, proximity of an active stream, and finally the distance from the reservoir. In practice, each of the mapping units is assigned a maximum delivery ratio depending upon the composition of the unit and assuming a location very near the reservoir. The exact value of the delivery ratio for each mapping unit occurring in a particular basin is obtained by considering the likely collective effect of the various factors listed above that govern the delivery ratio. The range of delivery ratios for the different mapping units used in one case are listed in the appendix. It must be made amply clear that delivery ratios cannot be decided without reference to the position of the unit on the map. Publication map The erosion intensity units mapped either on photographs or on topographical maps are transferred on to a 1: line map showing the drainage pattern. On this map the natural hydrological units of sub-basins and basins are systematically delineated working upstream and properly codified. First, the big tributary subcatchments are demarcated and codified in order of their position on the major stream as a, b, c, etc. At this stage, the code comprises a capital alphabet for the major basins (D Dantiwada, B - Betwa, etc.) and lower-case letter for its major tributaries such as Da, Db, Be, Bd, etc. Subcatchments are divided into basins following the same procedure and are designated by arabic numerals, as Dal, Da, Dbl, etc. Each of these basins is further subdivided and these subdivisions are denoted by the addition of a lower-case letter

4 A sediment yield index as a criterion for choosing priority basins 183 FIGURE 1. Delineation and codification of subcatchments, basins and sub-basins in part of the Dantiwada basin. such as Dal a, Dad, etc. The final map as illustrated in Fig. 1 (part of the Dantiwada RVP basin map) shows the different erosion intensity mapping units distributed in the different sub-basins. Computation of the sediment yield index and ranking of the basins From the map, the different mapping units in each of the sub-basins are planimetrically

5 184 Y. P. Bali and R. L. Karale TABLE. Dantiwada RVP - Rajasthan: computation of the sediment yield index and ranking of the basins into priority classes Sr. no. Subbasin code Erosion intensity units Area under erosion intensity unit [ha] Weighting Delivery ratio [%] sediment yield potential Sediment yield index Relative priority 1. Da3c Gl G PI P V V Dbla P P4 S S3 VI V V Dblf SI S S4 V measured and the sediment yield index is computed as follows: SI=X (EiAieD) AW 100 where SI Ei Aie D AW sediment yield index, weighting value of erosion intensity unit, area of the erosion intensity unit in a basin, delivery ratio, total area of the basin. The computations involved are listed in Table. Finally, the different basins are arranged in decreasing order of their SI values implying decreasing order of priority for treatment. Generally, the basins are categorized into very high, high, medium, low and very low priority classes. The range in sediment yield index values is governed by the specific characteristics of the individual basin. limits of the sediment yield index values for each priority class are assigned in accordance with the frequency distribution of sub-basins within the total range of sediment yield index values. COMPARABILITY OF THE RESULTS It has been proposed that the sediment yield prediction equation gives reliable results in arid, semiarid and sub-humid areas. Results have, therefore, been computed using

6 A sediment yield index as a criterion for choosing priority basins 1 TABLE 3. Comparison of the values obtained from the sediment yield prediction equation and the proposed method Sediment yield prediction equation The proposed method Sr. No. XI X X3 X4 Final value [ham/ 100 km ] Mapping unit symbol Weighting value A B C D E H J N P A- Very gently to gently sloping, broad intermesital valleys and plateaus; greyish brown, very deep, fine textured, cracking soils; mostly cultivated; moderate sheet and rill erosion and a few gullies. B Gently sloping stream banks; greyish brown, deep, fine textured cracking soils; mostly cultivated; moderate and severe sheet, rill and gully erosion. C - Very gently to moderately sloping, slightly undulating uplands; greyish brown, very shallow and shallow, fine textured, stony soils; shrubs and grasses; moderate erosion. D - Gently to strongly sloping isolated subdued hillocks with lateritic cappings; reddish brown, moderately fine textured, gravelly soils; shrubs and grasses; moderate erosion. E - Very gently to moderately sloping, undulating uplands and moderately to strongly sloping, subdued hillocks; reddish brown, shallow, moderately coarse textured soils; grasses and shrubs; moderate erosion. H - Gently sloping pediment back slope; reddish brown, shallow, bouldery and stony soils; shrubs and bushes, slight erosion. J - Nearly level toe slopes; greyish brown, very deep fine textured, cracking soils showing hydromorphic influence in the lower layers; under cultivation; moderate erosion. N - Very gently to gently sloping upland structural terraces; reddish brown, shallow, moderately coarse textured soils; shrubs and grasses; slight erosion. P - Level to gently sloping valley lands; greyish brown, very deep, fine textured cracking soils; cultivated; moderate erosion with occasional gullies. the sediment yield prediction equation for an area in Matatilla RVP basin and compared with the weighting values used under the proposed method. The results are indicated in Table 3. It may be seen that whereas the general trend of the values in both the cases is comparable, there lacks a proportionality or direct relationship between the results obtained from the sediment yield prediction equation and the proposed method. The special feature of the proposed method is the adjustment of the sediment yield for its expected delivery in relative terms at the point of measurement. DETERMINATION OF PRIORITY AREAS IN BASINS The results of the determination of priority basins carried out in 10 RVP basins involving a total area of 5.3 million ha are given in Table 4. The data show that the proportion of the total area falling under very high and high categories varies from basin to basin depending upon its specific characteristics. However, for the same catchment it gives the total area that has to be treated on a priority basis to retard siltation rates in the reservoirs. It is also observed from the work done so far on priority identification that nearly per cent of the area qualifies for very

7 186 Y. P. Bali and R. L. Karale TABLE 4. Distribution of areas under priority categories in a few selected basins Priority category 1 Sediment yield index No. of basins 3 Area [ha] 4 %of the total area 5 X.Mahi > < Dantiwada > < Pochampad > < er Bhawani Very > < Nizamsagar > < è.mayurakshi > < Matatilla > <

8 A sediment yield index as a criterion for choosing priority basins 187 TABLE 4 - continued Seas > < Ramganga > < Machkund > < high and high priority categories, 8.43 per cent for medium priority and for low and very low priorities. Such information is considered to be of immense importance for policy decisions in programme planning, fund allocations and implementation of conservation measures on a basin basis. REFERENCES Buringh, P. (1960) The application of aerial photographs in soil surveys. In Manual Photogr. Interpret: Amer. Soc. Photogr., Washington, DC. Soil Conservation Division (197) Handbook of Sedimentation: Central Unit for Soil Conservation (Hydrology and Sedimentation) Govt, of India, Ministry of Agriculture, New Delhi. Appendix EROSION INTENSITY MAPPING LEGEND, DANTIWADA RVP BASIN - RAJASTHAN Map symbol Description of mapping units Weighting Delivery ratio [%] G - Gl G Hilly landscape of granite and granitic gneiss complex Strongly to very steeply sloping hillocks; shallow to moderately deep, dark brown, medium textured gravelly soils; rock outcrops (10- per cent); dense forest vegetation; slight sheet and rill erosion. Strongly to very steeply sloping hills; very shallow to shallow, dark brown to dark reddish brown, medium to moderately coarse textured gravelly soils; rock outcrops (30 40 per cent); sparse forest vegetation, moderate sheet and rill erosion. Hilly landscape of quartzite, limestone, sandstone and gneissic complex

9 188 Y. P. Bali and R. L. Karale Appendix continued Map Delivery symbol Description of mapping units Weighting ratio [%] 51 Strongly to very steeply sloping hills; very shallow to moderately deep, dark brown and dark greyish brown, moderately coarse to medium textured gravelly soils; medium to dense forest vegetation; slight erosion. 5 Strongly to very steeply sloping hills; very shallow to shallow, 45- brown to reddish brown, moderately coarse textured gravelly soils; rock outcrops (approx. per cent) sparse forest vegetation, moderate to severe erosion. 53 Moderately to steeply sloping hillocks giving rugged topo- 45- graphic appearance; very shallow to shallow, brown and reddish brown, moderately coarse textured gravelly soils; rock outcrops (-30 per cent); almost devoid of vegetation; severe sheet and rill erosion. 54 Gently to moderately sloping cultivated lands along channels within hills and on small hillocks; dark brown, moderately deep to deep, moderately coarse to medium textured soils; severe sheet and rill erosion. P Pediplain PI Very gently to gently sloping plain with inselbergs; dark 60- brown, very shallow to shallow, moderately coarse textured soils; rock outcrops (60 70 per cent); almost devoid of vegetation, slight sheet erosion. P Very gently to gently sloping lands and interfluves; dark reddish brown, shallow to moderately deep, moderately coarse textured soils; rock outcrops (60-70 per cent); shrubs and grasses, moderate to severe sheet and rill erosion. P3 Very gently to gently sloping lands; dark reddish brown and dark brown, shallow to moderately deep; moderately coarse textured soils; occasional rock outcrops; shrubs and well maintained grasses; slight to moderate sheet erosion. P4 Very gently to gently sloping lower buried pediment slopes; dark brown, moderately deep to deep, medium textured soils; poorly managed cultivation, moderate sheet and rill erosion. F - Piedmont plain F Very gently to gently sloping plain lands at the foothills; reddish brown, moderately deep to deep, moderately coarse to medium textured soils; mostly cultivated; moderate sheet and rill erosion. V - Valley lands subject to periodical sedimentation VI Nearly level to very gently sloping point bar complex and levees; dark brown, very deep, moderately coarse to coarse textured soils; mostly cultivated; slight to moderate erosion. V Nearly level to very gently sloping valley lands and basins, dark brown to very dark greyish brown, deep to very deep, medium to fine textured soils; mostly cultivated; moderate sheet and rill erosion. V3 Nearly level to very gently sloping basins and plain; dark brown, medium to moderately fine textured soils; well managed cultivation; slight erosion. V4 Very gently to moderately sloping, undulating lands along 45 the streams; dark brown, deep to very deep, medium textured soils; moderate to severe sheet, rill and gully erosion.