Components of hydrograph & Introduction pdf

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rising limb hydrograph
Components of hydrograph & Introduction pdf

HYDROGRAPHS INTRODUCTION

Consider a concentrated storm producing a fairly uniform rainfall of duration, D over a catchment. After the initial losses and infiltration losses are met, the rainfall excess reaches the stream through overland and channel flows. In the process of translation a certain amount of storage is built up in the overland and channel-flow phases. This storage gradually depletes after the cessation of the rainfall. Thus, there is a time lag between the occurrence of rainfall in the basin and the time when that water passes the gauging station at the basin outlet. The runoff measured at the stream-gauging station will give a typical hydrograph as shown below. The duration of the rainfall is also marked in this figure to indicate the time lag in the rainfall and runoff. 

The hydrograph of this kind which results due to an isolated storm is typically single-peaked skew distribution of discharge and is known variously as storm hydrograph, flood hydrograph or simply hydrograph. 

It has three characteristic regions: 

  1. the rising limb AB, joining point A, the starting point of the rising curve and point B, the point of inflection, 
  2. the crest segment BC between the two points of inflection with fa peak P in between, 
  3. the falling limb or depletion curve CD starting from the second point of inflection C.
Components of hydrograph & Introduction pdf
Components of hydrograph & Introduction pdf

The hydrograph is the response of a given catchment to a rainfall input. It consists of flow in all the three phases of runoff, viz. surface runoff, interflow and base flow, and embodies in itself 1 the integrated effects of a wide variety of catchment and rainfall parameters having complex interactions. Thus, two . different storms in a given catchment produce hydrographs differing from each other. Similarly, identical storms in two f catchments produce hydrographs that are different. The interactions of various storms and catchments are in general extremely complex. If one examines the record of a large number of flood hydrographs of a stream, it will be found that many of them will have kinks, multiple peaks, etc. resulting in shapes much different from the simple single-peaked hydrograph. These complex hydrographs are the result of storm and catchment peculiarities and their complex interactions. While it is theoretically possible to resolve a complex hydrograph into a set of simple hydrographs for purposes of hydrograph analysis, the requisite data of acceptable quality are seldom available. Hence, simple hydrographs resulting from isolated storms are preferred for hydrograph studies

Components of a Hydrograph

As indicated earlier, the essential components of a hydrograph are 

  1. the rising limb, 
  2. the crest segment
  3. the recession limb

few salient features of these components are described below. 

Rising Limb hydrograph

The rising limb of a hydrograph, also known as concentration curve, represents the increase in discharge due to the gradual building up of storage in channels and over the catchment surface. The initial losses and high infiltration losses during the early period of a storm cause the discharge to rise rather slowly in the initial periods. As the storm continues, more and more flow from distant parts reach the basin outlet. Simultaneously the infiltration losses also decrease with time. Thus, under a uniform storm over the catchment, the runoff increases rapidly with time. As indicated earlier, the basin and storm characteristics control the shape of the rising limb of a hydrograph.

Also read - Types of infiltrometer - Single & Double ring infiltrometer

Crest Segment

The crest segment is one of the most important parts of a hydrograph as it contains the peak flow. The peak flow occurs when the runoff from various parts of the catchment simultaneously contribute amounts to achieve the maximum amount of flow at the basin outlet. Generally, for large catchments, the peak flow occurs after the cessation of rainfall, the time interval from the centre of mass of rainfall to the peak being essentially controlled by basin and storm characteristics. Multiple-peaked complex hydrographs in a basin can occur when two or more storms occur in succession. Estimation of the peak flow and its occurrence, being important in flood-flow studies are dealt with in detail elsewhere in this book.

Recession Limb

The recession limb, which extends from the point of inflection at the end of the crest segment (point C in Figure above) to the commencement of the natural groundwater flow (point Din Figure above) represents the withdrawal of water from the storage built up in the basin during the earlier phases of the hydrograph. The starting point of the recession limb, i.e. the point of inflection represents the condition of maximum storage. Since the depletion of storage takes place after the cessation of rainfall, the shape of this part of the hydrograph is independent of storm characteristics and depends entirely on the basin characteristics.

The storage of water in the basin exists as 

  1. surface storage, which includes both surface detention and channel storage, 
  2. interflow storage, and 
  3. groundwater storage, i.e. base-flow storage. Barnes (1940) showed that the recession of a storage can be expressed as

Qₜ=Q₀Kᵗᵣ.........   (6.1)

in which Q, is the discharge at a time t and Q is the discharge at t=0; K is a recession constant of value less than unity. Equation (6.1) can also be expressed in an alternative form of the exponential decay as

Qₜ=Q₀e⁻ᵃᵗ................ (6.1a)

Also read - Forms of precipitation in hydrology pdf & Weather system for precipitation 

where a = -In K

The recession constant Kᵣ ,can be considered to be made up of three to account for the three types of storages as

Kᵣ=Kᵣₛ.Kᵣᵢ.Kᵣ꜀

where Kᵣₛ = recession constant for surface storage, Kᵣᵢ recession constant for interflow and Kᵣ꜀ = recession constant for base flow. Typically, the values of these recession constants, when time t is in days, are

Kᵣₛ=0.05 to 0.20

Kᵣᵢ = 0.50 to 0.85

Kᵣ꜀ = 0.85 to 0.99

When the interflow is not significant, Kᵣᵢ can be assumed to be unity.

If suffixes 1 and 2 denote the conditions at two time instances t₁, and t₂

From Eq. (6.1)

Q₁/Q₂=Kᵣ ⁽ᵗ¹⁻ᵗ²⁾      .......     (6.2)

or from Eq. (6.1a)

Q₁/Q₂=e⁻ᵃ⁽ᵗ¹⁻ᵗ²⁾          ..........    (6.2a)

Equation 6.1 (and also 6.1 a) plots as a straight line when plotted on a semi-log paper with discharge on the log-scale. The slope of this line represents the recession constant. Using this property and using Eq. 6.2 (or 6.2a) the value of K, for a basin can be estimated by using observed recession data of a flood hydrograph. Example 6.1 explains the procedure in detail.

The storage S, remaining at any time t is obtained as

Sₜ=Qₜ/a

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