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| factors affecting shape of hydrograph pdf |
Factors Affecting Flood Hydrograph
The factors that affect the shape of the hydrograph can be broadly grouped into climatic factors and physiographic factors. Each of these two groups contains a host of factors and the important ones are listed in below. Generally, the climatic factors control the rising limb and the recession limb is independent of storm characteristics, being determined by catchment characteristics only. Many of the factors are interdependent. Further, their effects are very varied and complicated. As such only important effects are listed below in qualitative terms only.
Factors Affecting Hydrograph
Physiographic factors
1. Basin characteristics:
- Shape
- size
- slope
- nature of the valley
- elevation
- drainage density
2.Infiltration characteristics:
- land use and cover
- soil type and geological conditions
- lakes, swamps and other storage
3. Channel characteristics; cross-section, roughness and storage capacity
Climatic factors
1. Storm characteristics: precipitation. intensity, duration, magnitude and movement of storm.
2. Initial loss
3. Evapotranspiration
Shape of the Basin
The shape of the basin influences the time taken for water from the remote parts of the catchment to arrive at the outlet. Thus, the occurrence of the peak and hence the shape of the hydrograph are affected by the basin shape. Fan-shaped, i.e. nearly semicircular shaped catchments give high peak and narrow hydrographs while elongated catchments give broad and low-peaked hydrographs. Figure below shows schematically the hydrographs from three catchments having identical infiltration characteristics due to identical rainfall over the catchment. In catchment A, the hydrograph is skewed to the left, i.e. the peak occurs relatively quickly. In catchment B, the hydrograph is skewed to the right, the peak occurring with a relatively longer lag. Catchment C indicates the complex hydrograph produced by a composite shape.
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| Effect of catchment shape on hydrograph |
Size
Small basins behave different from the large ones in terms of the relative importance of various phases of the runoff phenomenon. In small catchments, the overland flow phase is predominant over the channel flow. Hence the land use and intensity of rainfall have important role on the peak flood. On large basins these effects are suppressed as the channel flow phase is more predominant. The peak discharge is found to vary as A" where A is the catchment area and n is an exponent whose value is less than unity, being about 0.5. The time base of the hydrographs from larger basins will be larger than those of corresponding hydrographs from smaller basins. The duration of the surface runoff from the time of occurrence of the peak is proportional to A", where m is an exponent less than unity and is of the order of magnitude of 0.2.
Slope
The slope of the main stream controls the velocity of flow in the channel. As the recession limb of the hydrograph represents the depletion of storage, the stream channel slope will have a pronounced effect on this part of the hydrograph. Large stream slopes give rise to quicker depletion of storage and hence result in steeper recession limbs of hydrographs. This would obviously result in a smaller time base.
The basin slope is important in small catchments where the overland flow is relatively more important. In such cases the steeper slope of the catchment results in larger peak discharges.
Drainage Density
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| Effect of drainage density - hydrograph |
The drainage density is defined as the ratio of the total channel length to the total drainage area. A large drainage density creates situation conducive for quick disposal of runoff down the channels. This fast response is reflected in a pronounced peaked discharge. In basins with smaller drainage densities, the overland flow is predominant and the resulting hydrograph is squat with a slowly rising limb
Land Use
Vegetation and forests increase the infiltration and storage capacities of the soils. Further, they cause considerable retardance to the overland flow. Thus, the vegetal cover reduces the peak flow. This effect is usually very pronounced in small catchments of area less than 150 km². Further, the effect of the vegetal cover is prominent in small storms. In general, for two catchments of equal area, other factors being identical, the peak discharge is higher for a catchment that has a lower density of forest cover. Of the various factors that control the peak discharge, probably the only factor that can be manipulated is land use and thus it represents the only practical means of exercising long-term natural control over the flood hydrograph of a catchment.
Urbanisation
Urbanisation of a rural watershed results in the following significant changes in the characteristics of the watershed;
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| effect of urbanisation on infiltration |
- Urbanisation increases the amount of impervious surface areas such as roofing on built up areas, concrete and asphalt road surfaces, parking lots. This is a major factor that would inhibit infiltration and surface retention. Further, the amount of rainfall that would be available as surface runoff would be large. These effect of decrease in infiltration and initial losses and consequent increase in the effective (runoff producing) rainfall is shown schematically in Figure above. From this figure, it can clearly be seen as to how the decrease in infiltration due to urbanisation results in increase in runoff producing rain.
- Due to urbanisation the natural drainage of the rural catchment will be removed or improved. Also, there will be a superposition of man-made artificial surface drainage system consisting of impervious conduits. The relatively slow process of interflow to small streamlets which feed the large drainage will not exist any more.
The new drainage in urbanised catchment would start with rapid overland flow over relatively impervious surfaces of roads, pavements and parking lots to the nearest curb or drainage inlet. Then onwards the storm runoff in the drainage channel will move rapidly down the drainage system. In general, a unit of runoff would take the shortest path to the outlet. The net result of the changed drainage pattern is the rapid response of the system that would cause the hydrograph to have higher peak at a shorter time from the start and also rapid receding of the peak.
Thus, while the larger and higher impervious areas in the urban catchment causes more quantity of surface runoff, the drainage system of urban areas cause higher peaks at short time from start and a rapid recession of the peak. The change in the shape of the hydrograph due to urbanisation of a rural catchment is shown in Figure below wherein the above characteristic features are clearly shown.
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| Effect of Urbanisation - hydrograph |
The increase in the volume of direct runoff due to urbanisation of the catchment can be estimated by using SCS method .
Climatic Factors
Among climatic factors the intensity, duration and direction of storm movement are the three important ones affecting the shape of a flood hydrograph. For a given duration, the peak and volume of the surface runoff are essentially proportional to the intensity of rainfall. This aspect is made use of in the unit hydrograph theory of estimating peak-flow hydrographs, as discussed in subsequent sections of this chapter. In very small catchments, the shape of the hydrograph can also be affected by the intensity.
The duration of storm of given intensity also has a direct proportional effect on the volume of runoff. The effect of duration is reflected in the rising limb and peak flow. Ideally, if a rainfall of given intensity i lasts sufficiently long enough, a state of equilibrium discharge proportional to iA is reached.
If the storm moves from upstream of the catchment to the downstream end, there will be a quicker concentration of flow at the basin outlet. This results in a peaked hydrograph. Conversely, if the storm movement is up the catchment, the resulting hydrograph will have a lower peak and longer time base. This effect is further accentuated by the shape of the catchment, with long and narrow catchments having hydrographs most sensitive to the storm-movement direction.
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