26.03.2021

Category: Components of hydrograph pdf

Components of hydrograph pdf

River discharge is the volume of water flowing through a river channel. This is the total volume of water flowing through a channel at any given point and is measured in cubic metres per second cumecs. The discharge from a drainage basin depends on precipitation, evapotranspiration and storage factors.

Hydrographs can be used to illustrate discharge. These can be used to show annual discharge patters of flow in relation to climate.

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Over the short term, a flood or storm hydrograph figure 1. They cover a relatively short time period, usually hours or days rather than weeks or months. Storm hydrographs allow us to investigate the relationship between a rainfall event and discharge. The starting and finishing level show the base flow of a river.

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The base flow is the water that reaches the channel through slow throughflow and permeable rock below the water table. As storm water enters the drainage basin the discharge rates increase. This is shown in the rising limb. The highest flow in the channel is known as the peak discharge.

The fall in discharge back to base level is shown in the receding limb. The lag time is the delay between the maximum rainfall amount and the peak discharge. The shape of a hydrograph varies in each river basin and each individual storm event. The hydrographs below show two contrasting environments. Rural areas with predominantly permeable rock increases infiltration and decreases surface runoff.

This increases lag time. The peak discharge is also lower as it takes water longer to reach the river channel. Urbanisation is the main human impact on a storm hydrograph. As roads, pavements and buildings are constructed the surface becomes impermeable.

Laying drains leads to the rapid transportation of water to river channels which reduces the lag time. Large drainage basins catch more precipitation so have a higher peak discharge compared to smaller basins. Smaller basins generally have shorter lag times because precipitation does not have as far to travel. The shape of the drainage basin also affects runoff and discharge.

Drainage basins that are more circular in shape lead to shorter lag times and a higher peak discharge than those that are long and thin because water has a shorter distance to travel to reach a river.In hydrologic analysis, many times hydrographs resulting from isolated periods of rainfall are to be studied and analysed to calculate volume of runoff.

Therefore it is essential to understand various components of total runoff and parts of the hydrograph. Any typical simple hydrograph with one peak can be divided into five parts Fig. Such a hydrograph results from an isolated and concentrated storm rainfall.

The approach segment indicates a condition when the stream is fed by groundwater runoff or base flow. As rainfall starts the increasing contribution to the stream occurs by way of surface runoff and prompt sub-surface runoff as shown by the total rising limb 2-P.

The rising limb is the function of the time and the rainfall pattern feeding the basin. The crest segment extends from the point of inflection 3 on the rising side of the hydrograph to the another point of inflection 4 on the recession or falling side P-5 of the hydrograph.

The crest point P indicates the peak of hydrograph and represents the highest instantaneous flow in the stream or the peak flow. The point of inflection on the falling side of the hydrograph is commonly assumed to indicate the time at which the overland flow or surface runoff stops.

From this point onward the falling or recession limb indicates withdrawal of water from the valley storage or channel storage as the case may be. Valley storage or channel storage is that part of the overland flow which does not get dissipated instantaneously. This volume of water temporarily occupies the space in the river valley or within the banks of the river channel till such time that the passage becomes free enough for this flood to get depleted. The lower portion of the falling or recession side shows decreasing rate of ground water runoff.

components of hydrograph pdf

It can be seen from Fig. The time difference between the centeroid of rainfall excess and the peak of the hydrograph is called the lag time. In some situations when after a concentrated heavy rainfall rain further continues with low intensities for extended time, it is possible that the peak flow may occur even before the rainfall has completely ceased.

The shape of the falling or recession limb P-5 is generally not dependent on the rainfall characteristics but mostly depends on the basin characteristics.

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Top 3 Types of Unit Hydrographs.You seem to have disabled JavaScript. You should really enable it for this site but most things should work without it. Discharge is normally measured at gauging stations that are situated at different points along the river. The discharge of a river changes over time depending on a few factors.

The most influential factor is the weather. A storm hydrograph is a specific type of hydrograph that, surprise, surprise, shows precipitation and discharge during and after a storm.

The main difference between a normal hydrograph and a storm hydrograph is that a storm hydrograph is over a much shorter period of time. Below is a storm hydrograph for the fictional River Shui:. First is the lag time. The lag time is the time difference between the peak precipitation and and the peak discharge. Conversely, a short lag time indicates that the precipitation is entering the river fairly quickly. The rising limb is the steep part of the discharge line that has a positive gradient, indicating that the discharge is increasing.

The falling limb is the opposite showing that the discharge is falling. The shape of a hydrograph is altered by a few different things. One factor is the shape of the drainage basin. Drainage basins come in a wide assortment of shapes.

Roughly Circular shapes are common as are more elongated and narrow shapes.

Separation of Runoff Components, Base Flow and Complex Hydrograph

This is because all points in the drainage basin are again, roughly equidistant from the river so all the precipitation reaches the river at the same time. The size of the drainage basin obviously has an impact on the hydrograph. Basins with steep slopes will have a high peak discharge and a short lag time because the water can travel faster downhill.

Storm Hydrograph Overview

Finally the drainage density of a basin will affect the lag time and the steepness of the falling limb. Basins with lots of streams and rivers a high drainage density will have a short lag time and a fairly steep falling limb because water will drain out of them quickly. If a river is surrounded by non-porous and impermeable rocks e.

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This is much faster than groundflow, interflow and throughflow so the lag time is reduced. Unconsolidated soils allow water to infiltrate and so act as a store in a drainage basin.Thanks for sharing this informative information.

For Storm components all over information you may also refer Hydrograph and its components. It is an inverted U shaped diagram. Rising Limb : It is the ascending curved portion of the hydrograph. The rising limb rises slowly in the early stage of the flood but more rapidly toward the end portion.

The shape of rising limb depends on duration and intensity distribution of rainfall. This is because in early stages the losses is more and water reaches to the stream faster. This is the part which is taken as matter of interest by hydrologists. Peak of hydrograph occurs when all parts of basins contribute at the outlet simultaneously at the maximum rate. Depending upon the rainfall-basin characterstics, the peak may be sharp, flat or may have several well defined peaks.

It extends from the point of inflection at the end of the crest to the beginning of the natural groundwater flow. Labels: flood hydrographHydrographwater. Unknown August 20, at PM. Floyd Pokharel November 4, at AM.

Hydrograph

Unknown March 27, at PM. Spark Mallikharjuna April 24, at AM. Unknown December 21, at PM. Older Post Home. Subscribe to: Post Comments Atom.Read this article to learn about the definition, assumptions, limitations and advantages of unit hydrograph theory! It is a typical hydrograph of direct runoff which gets generated from one centimeter of effective rainfall falling at a uniform rate over the entire drainage basin uniformly during a specific duration.

Effective rainfall is that portion of rainfall which fully contributes towards direct runoff. Therefore, unit hydrograph can also be defined as the hydrograph of a drainage basin which gives one centimeter of direct runoff from a rain storm of specific duration.

This requirement calls for selection of storms of so small a duration which would generally produce an intense and nearly uniform effective rainfall and would produce a well defined single peak of hydrograph of short time base. This important assumption is called principle of linearity or proportionality or superposition.

In other words the hydrograph of direct runoff resulting from a given pattern of effective rainfall will remain invariable irrespective of its time of occurrence. This assumption is called principle of time invariance. In practice, however, uniformly distributed effective rainfall rarely occurs on large areas. Also on large areas effective rainfall is very rarely uniform at all locations, within its specified duration. Obviously bigger the area of the drainage basin lesser will be the chances of fulfilling the assumptions enunciated above.

The limiting size of the drainage basin is considered to be km 2. Beyond it the reliability of the unit hydrograph method diminishes. When the area of the drainage basin exceeds a few thousand km 2. The catchment has to be divided into sub-basins and the unit hydrographs developed for each sub-basin.

The flood discharge at the basin outlet can then be estimated by combining the sub- basin floods adopting flood routing procedure. The reason is that the storm rainfall gets mixed up with the snow pack and may produce delayed runoff differently under different conditions of snow pack. Obviously the principle of time invariance is really valid only when the time and condition of the drainage basin are specified.

But it is not practicable to derive separate unit hydrograph for each possible time- intensity pattern. This is so because due to variability in proportion of surface, subsurface and groundwater runoff components during smaller and larger storms of same duration, the maximum ordinate peak of the unit hydrograph derived from smaller storm is smaller than the one derived from larger storm.

Obviously the character and duration of recession limb which is a function of the peak flow will also be different. When appreciable non-linearity is seen to exist it is necessary to use derived unit hydrographs only for reconstructing events of similar magnitude.

Obviously it necessitates construction of several unit hydrographs to cover different durations of storms. Thus a 2 hour unit hydrograph can be applied to storms of 1. The limitation to the theory of unit hydrograph can be overcome to a large extent by remaining within the various ranges and restrictions indicated above.

The unit hydrograph theory has several advantages to its credit which can be summarised as below:. Uses of Rainfall-Runoff Relationship: 2 Uses.A hydrograph is a graph showing the rate of flow discharge versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second cms or cfs. It can also refer to a graph showing the volume of water reaching a particular outfallor location in a sewerage network.

Graphs are commonly used in the design of seweragemore specifically, the design of surface water sewerage systems and combined sewers. A stream hydrograph is commonly determining the influence of different hydrologic processes on discharge from the subject catchment. Because the timing, magnitude, and duration of groundwater return flow differs so greatly from that of direct runoff, separating and understanding the influence of these distinct processes is key to analyzing and simulating the likely hydrologic effects of various land use, water use, weather, and climate conditions and changes.

In part this is because these two concepts are not, themselves, entirely distinct and unrelated. Return flow from groundwater increases along with overland flow from saturated or impermeable areas during and after a storm event; moreover, a particular water molecule can easily move through both pathways en route to the watershed outlet.

Nevertheless, various graphical and empirical techniques have been developed to perform these hydrograph separations. The separation of base flow from direct runoff can be an important first step in developing rainfall-runoff models for a watershed of interest—for example, in developing and applying unit hydrographs as described below. An unit hydrograph UH is the hypothetical unit response of a watershed in terms of runoff volume and timing to a unit input of rainfall.

Hydrograph Components

It can be defined as the direct runoff hydrograph DRH resulting from one unit e. As a UH is applicable only to the direct runoff component of a hydrograph i. A UH is specific to a particular watershed, and specific to a particular length of time corresponding to the duration of the effective rainfall. That is, the UH is specified as being the 1-hour, 6-hour, or hour UH, or any other length of time up to the time of concentration of direct runoff at the watershed outlet.

Thus, for a given watershed, there can be many unit hydrographs, each one corresponding to a different duration of effective rainfall.

The UH technique provides a practical and relatively easy-to-apply tool for quantifying the effect of a unit of rainfall on the corresponding runoff from a particular drainage basin. UH theory assumes that a watershed's runoff response is linear and time-invariant, and that the effective rainfall occurs uniformly over the watershed. In the real world, none of these assumptions are strictly true. Nevertheless, application of UH methods typically yields a reasonable approximation of the flood response of natural watersheds.

The linear assumptions underlying UH theory allows for the variation in storm intensity over time i. This allows for a relatively straightforward calculation of the hydrograph response to any arbitrary rain event. An instantaneous unit hydrograph is a further refinement of the concept; for an IUH, the input rainfall is assumed to all take place at a discrete point in time obviously, this isn't the case for actual rainstorms.

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Making this assumption can greatly simplify the analysis involved in constructing a unit hydrograph, and it is necessary for the creation of a geomorphologic instantaneous unit hydrograph. The creation of a GIUH is possible given nothing more than topologic data for a particular drainage basin. In fact, only the number of streams of a given order, the mean length of streams of a given order, and the mean land area draining directly to streams of a given order are absolutely required and can be estimated rather than explicitly calculated if necessary.

It is therefore possible to calculate a GIUH for a basin without any data about stream height or flow, which may not always be available.

components of hydrograph pdf

In subsurface hydrology hydrogeologya hydrograph is a record of the water level the observed hydraulic head in wells screened across an aquifer. Typically, a hydrograph is recorded for monitoring of heads in aquifers during non-test conditions e. When an aquifer test is being performed, the resulting observations are typically called drawdownsince they are subtracted from pre-test levels and often only the change in water level is dealt with.

Raster hydrographs are pixel-based plots for visualizing and identifying variations and changes in large multidimensional data sets. Originally developed by Keim they were first applied in hydrology by Koehler as a means of highlighting inter-annual and intra-annual changes in streamflow. The raster hydrographs in WaterWatch, like those developed by Koehler, depict years on the y-axis and days along the x-axis. Users can choose to plot streamflow actual values or log valuesstreamflow percentile, or streamflow class from 1, for low flow, to 7 for high flowfor Daily, 7-Day, Day, and Day streamflow.

For a more comprehensive description of raster hydrographs, see Strandhagen et al.In this article we will discuss about:- 1. Meaning of Runoff Hydrograph 2. Components of Hydrograph 3. Factors Affecting the Shape 4. Base Flow Separation. Sometimes, it is also known as storm hydrograph, flood hydrograph or simply hydrograph. It also shows the distribution of total runoff with respect to time at a certain point of measurement.

All hydrographs have three characteristics regions viz. These characteristics regions are shown in the schematic diagram of the hydrograph Fig. The hydrographs are mainly in two types, i. The multi-peaked hydrograph is also known as complex hydrograph. The occurrence of single or multi-peaked hydrograph depends on rainfall characteristics, complexity of watersheds and their peculiar interactions.

components of hydrograph pdf

For example an isolated rainfall yields the single peaked hydrograph, while complex storm varying rainfall intensity yields the complex hydrograph. It is also known as concentration curve, is the ascending portion of hydrograph. Its slope steepness depends on the rise of discharge due to gradual building of storage in drainage channels as well as over the watershed surface.

The shape of rising limb is dependent on the storm and watershed characteristics, both. In general, the shape of rising limb is being concave upwards and rises slowly in the early stage of the flow, but as the storm continues and more and more flow from distant apart reaches to the outlet of watershed, the rising limb rises very rapidly up to the peak point of the hydrograph. The time base of hydrograph is fixed by the duration of outflow. In a simple hydrograph, the extent of rising limb is comparatively shorter than the falling limb, as a result the area below this limb is less to that of the falling limb.

This segment is one of the very important parts of the hydrograph, as it contains the peak flow. It is extended from the point of inflection on the rising limb to a similar inflection point on the falling limb.

The peak flow occurs, when various parts of the watershed yield the runoff simultaneously to the outlet. Generally, in large watersheds the peak flow occurs, when rainfall gets stop. The time interval from centre of mass of rainfall to the peak is controlled by the storm and watershed characteristics. Hydrographs of some watersheds resulted from a single and relatively short duration rainfall, have two or more peaks.

Multi-peak, i. It is the descending portion of hydrograph, is also known as recession limb. The falling limb is extended from the point of inflection at the end of crest segment to the commencement of natural ground water flow.

It represents the withdrawal of water from the storage build up in the watershed during initial phase of hydrograph. The point of inflection on the falling limb of the hydrograph indicates the stage, when rainfall has been stopped and channel flow is due to storage made over the watershed.

The shape of falling limb is dependent only on the physical features of the channel; and is independent of the storm characteristics. Generally, falling limb is in convex shape due to continuous decrease in runoff volume. Variation in areal rainfall distribution minutely affects the shape of recession curve. Unusually high rainfall intensity results the rapid recession, while delayed recession is due to concentration of rainfall in upper portion of the basin.

The shape of hydrograph is dependent on the runoff volume and time to peak of the watershed.

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