## COMPONENT PARTS OF A STORM HYDROGRAPH

COMPONENT PARTS OF A STORM HYDROGRAPH shows a hydrograph for any isolated duration of rainfall. A is the point from which hydrograph starts rising. The hydrograph continues to rise at a very steep rate till peak point B is reached. After this flood discharge starts receding. AB limb of the hydrograph is called rising limb and BD limbs as the receding limb. On limb BD, there is a point C known as point of inflection. It has already been stated in this article, that the hydrographs have three types of flows, over land flow (surface runoff), Interflow (influent streams or subsurface flow) and ground water flow. Overland flow and interflow are generally grouped together and this combined flow is known as Direct run-off. During floods the streams contribute ground water to the soil but during low water flows, streams derive most of its water from ground water. See Figs 6.14,…

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## Run-off by using unit hydrograph

RUN-OFF BY USING UNIT HYDROGRAPH Before we explain the method of use of unit hydrograph to estimate the run-off from a basin let us first of all learn some important terms  follow: 1. Hydrograph. It is graphical relation between discharge or flow, against time at a particular point of a stream or river. Hydrograph represents the time distribution of total run-off at the point of measurement. As volume of run-off, discharge or flow, is obtained by multiplying discharge with time, the area under the hydrographs gives the volume of flow during that period. The hydrographs have three types of flows: (i) Surface run-off or water flowing in the stream or river. (ii) Sub-surface storm flow i.e. infiltrated water in the top layers of soil. This water reaches the streams within short time. It is also known as inter-flow or influent stream. (iii) Ground water flow or water contributed as underground…

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USE OF INFILTRATION INDICES

## Use of Infiltration indices

USE OF INFILTRATION INDICES The infiltration capacity curve as shown in Fig. 6.11 cannot be used for computing run-off. run-off from large basins. It is, because, in large basins the infiltration capacity as well as rainfall rate vary from point to point. Moreover sub-surface flow (interflow) will also be substantial. Since this water-flow is a part of infiltration, it will not normally be included in the run-off compute by using infiltration capacity curve determined on a small test plot. Run-off volumes for large areas are computed using infiltration indices. W and φ are the two commonly used indices. W-index in the average infiltration rate or the infiltration capacity averaged over the whole storm period and is given as follows: $W-index= \frac{P-R}{Tr}$ where P = Total precipitation or rainfall. R = Total run-off. Tr = Duration of rainfall in hours. φ-index may be defined as the average rate of loss of…

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RATIONAL METHOD OF ESTIMATING RUN-OFF

## Rational method of estimating run-off

RATIONAL METHOD OF ESTIMATING RUN-OFF This method is a very useful method for evaluating the peak rate of run-off. This method is based on the fact that if a rainfall is appli to an impervious surface at a constant rate, The resulting run-off from the surface would finally reach a rate equal to the rate of rainfall. In the beginning only a certain amount of water will reach the outlet, but after sometime, the water Will start reaching the out let from the entire area and in this case the run-off rate would become equal to the rainfall rate. The time require to reach this equilibrium condition is know time of concentration and the peak rate of run-off would be equal to the rate of rainfall. This is the basis of the rational method. The peak rate of run-off can be estimated using the following formula: $_{Rp}= \frac{1}{36}kpA$ where Rp…

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Run-off by using infiltration characteristics

## Run-off by using infiltration characteristics

RUN-OFF BY USING INFILTRATION CHARACTERISTICS The process, whereby water enters the surface strata of the soil and thus moves downward towards the water-table is know infiltration. In fact when water falls on the soil, a small part of it is first of all absorb by the top thin layer of soil so as to replenish the soil moisture deficiency. After this any excess water moves downward where it is trapped in the voids and becomes ground water. The amount of stored ground water mainly depends upon the number of voids present in the soil. The number of voids further depend upon the size, shape, arrangement, and degree of compaction of the soil. Hence different soils will have different number of voids and hence different capacities to absorb water. The maximum rate at which a soil in any given condition is capable of absorb water. It is evident that rain water…

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FACTORS AFFECTING THE RUN-OFF

## Factors affecting the run-off

FACTORS AFFECTING THE RUN-OFF The characteristics of the rainfall play an important part in determining the amount of consequent run-off’. The various factors that affect the run-off can be summarise under two heads. Characteristics of precipitation, Characteristics of drainage basin. 1. Characteristics of Precipitation (a) Type of precipitation. Precipitation may be in the form of rain or drizzle. Run-off pattern or the hydrograph of run off is considerably governed by this factor. If precipitation occurs in the form of heavy rain,it will immediately produce bulk of run off (Peak flow of short duration). If precipitation is in the form of a drizzle it will produce run off at a slow and steady rate. (b) Rain intensity. Rain intensity has a lot of effect on the run off. If the intensity of rain increases, the run off increases rapidly. For example, if the intensity is increase four times, the run off…

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MEASUREMENT OF RAINFALL

## MEASUREMENT OF RAINFALL

MEASUREMENT OF RAINFALL Rainfall is the principal source of all waters. It is expresse as the depth of water in centimetres which falls on a pucca, impermeable levelled surface. The rainfall is measured with help of rain-gauges. Rain-gauges may be automatic or nonautomatic. Govt of India has approved use of non-automatic rain gauges at all the rain-gauge stations. Following are different types of rain-gauges. Simon’s rain-gauge. Weighing bucket rain-gauge. Float type rain-gauge. Tipping bucket rain-gauge. 1. Simon’s Rain-gauge. A typical Simon’s rain gauge is shown in Fig.  It is also known as non-recording type of rain-gauge, as it does not record the rate of rainfall at any moment but only collects rain water. It consists of a funnel fixed at the top of a receiving bottle. The receiving bottle is about 8 to 10 cm in diameter and is encased in the metal casing. This bottle is fixed in the…

Computation of run-off

## Computation of run-off

COMPUTATION OF RUN-OFF The Run-off available from a basin can be compute daily, weekly, monthly, or yearly. The following are the methods which can be used for finding out the run-off Using empirical formulae and tables Infiltration characteristics method A rational method, and Unit hydrograph method. All these methods have been explaine separately.

Average annual rainfall & index of wetness

## Average annual rainfall & index of damp

AVERAGE ANNUAL RAINFALL AND INDEX OF WETNESS The amount of rain collect by a rain gauge in 24 hours is know daily rainfall and the amount collect in one year is know annual rainfall. This annual rainfall at a given station should be record over a number of years say 35 to 40 years or so. In India this rainfall cycle period is take about 35 years. When we talk of the rainfall of a given place we generally refer to the average annual rainfall of that place. Thus when we speak of rainfall figures of a particular place, it means that this figure has been averaged over a long period of about 35 years. This is know as normal rainfall. But in any given year the rain may not be equal to this amount. It may be less than this average value or may exceed it. The ratio of…

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Some terms in most common use in regard to run-off

## Some terms in most common use in regard to run-off

SOME TERMS IN MOST COMMON USE IN REGARD TO RUN-OFF 1. Hydrograph. It is a curve or plot of discharge versus time at any section of a river. It represents the flow characteristic of the river. 2. Period of Surface Run-off. It is the time taken by the surface run off to pass the given section of the river, after the surface run-off makes its first appearance at the section. 3. Period of Rise. It is the time take by the surface run-off to reach its maximum value from the time of its beginning. 4. Time of Concentration. It is the time require by the water to reach the outlet point from the most remote point of the drainage area. When a storm has been in progress for a time equal to the time of concentration. it is assume that all the part of the catchment start contributing to the…

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