Unit - 2

Runoff and Hydrographs

• Runoff may be referred to as stream flow, river discharge or catchment yield. It is normally expressed as volume per unit time.

• Water which infiltrates the soil surface and then moves laterally through the upper soil horizons towards the stream channels above the main groundwater table is known as the interflow. It is also known as subsurface runoff, subsurface storm flow, storm seepage and secondary base flow.
• If the lateral hydraulic conductivity of the surface layers is substantially greater than the overall vertical hydraulic conductivity, it is a favorable condition for the generation of interflow. Generally, inter flow moves more slowly than surface runoff
• Depending upon the time delay between infiltration and its outflow from the upper crusts of the soil, the interflow is sometimes classified into prompt. Interflow and delayed interflow

• Direct precipitation into the water surface and into the stream channels will normally represent only a small percentage of total volume of water flowing in the streams. This component is usually ignored in runoff calculations.

• The delayed flow that reaches a stream essentially as groundwater called base flow. Many times, delayed interflow is also included under this category.
• The infiltrated water which percolates delay becomes ground and when groundwater table rises and intersects the stream channels of the basin it discharges into as the groundwater runoff
• Ground water flow is sometimes referred to as base flow, draw down, effluent seepage. For the practical purpose of analysis total runoff in stream channel is generally classified as direct runoff and base flow.

• A perennial stream is any which always carries some flow throughout the year. Even during dry seasons, the water table will be above the bed of the stream. Thus, considerable amount of ground water flow occurs during non precipitation period.
• In perennial streams, 100% dependable flow has a finite value
• Time Vs discharge graph

• Intermittent stream has limited contribution from the groundwater.
• Stream remains dry for most part of dry months
• Base flow (ground water flow) occurs significantly during wet season.
• During dry seasons the water table drops to a level lower than that of the stream bed.

• An ephemeral stream is one which does not have any base-low contribution
• Annual hydrograph shows series of short-duration spikes marking flash flow in response to storms
• The stream becomes dry soon after the end of the storm flow.
• An ephemeral stream does not have any well-defined channel
• Most rivers in arid Zones are of the ephemeral kind.
• In Intermittent ephemeral streams Q100 (100% dependable flow) zero (0)

• A graph of the cumulative value of any hydrologic quantity, generally as the ordinate, plotted against the time as abscissa is known as a mass curve. When cumulative volume of runoff taken as the hydrologic quantity, the mass curve may be called a flow mass curve or simply a mass curve .it is the integration of the hydrograph and therefore represents the area under the hydrograph from one time to another. Mathematically, the mass curve, is expressed by

• When it is desired to use water at uniform or nearly. Uniform rate greater than the minimum discharge in the stream, it is necessary to provide storage where water is impounded during periods. Of high flow for use during periods of low flows, Mass curve serves as a very useful tool to determine the required storage capacity for any uniform rate of demand.

• The difference in the ordinates of any two points on the curve is the summation of the flow during the intervening period of time. Thus, if two points such as Panda Qin figure are joined by a straight line, the slope of the line PQ would give the average rate of flow during the period t5 to t7.
• The rate of flow at any time is indicated by the slope of the tangent drawn to the mass curve at the corresponding time.
• If the mass curve is horizontal i.e., having zero slopes, for a particular period of time it means that there is no discharge in the stream during that period.
• The rate of demand is the rate at which water is required for any use. A straight line having the slope equal to this rate is called a demand line or draft line or use line.
• Where the mass curve is steeper than the draft line, the flow in the streaming at a higher rate than the rate of draft and consequently some of the stream flow is available for, storage or it will spill if the reservoir is already full.
• Where the mass curve has flatter slope than the draft line. Water will be drawn from storage in order to maintain the required rate of draft.

• Let the line OJ represent the cumulative demand the slope of which is equal to the rate of draft. Let us. Assume that the reservoir is full at the beginning.
• From O to the mass curve is steeper than OJ, indicating that the flow rate into the reservoir is more than the draft rate.
• Since the reservoir is already, full, no water can be 'stored though there is surplus water. Hence, the reservoir will be overflowing during the period from 0 to t.
• The ordinate AA' indicates the total volume of the. Spill in this period.
• From A onwards the mass curve is flatter than OJ. So, withdrawals from the reservoir start from the time t1.
• As the amount of water indicated by AA is already wasted, it cannot be included further in the analysis for the purpose of cumulative inflow. So, all the inflows into the reservoir and the withdrawals from it are to be reckoned from A in other words. The origin will be shifted from O to A.
• So, a line drawn parallel to OJ and tangential to mass curve at A that is AC, will now become the Cumulative demand.
• From A to B the mass curve is flatter than OJ. The reservoir is continuously depleted from t1 to t2 and S1 the vertical intercept between the line AC and B would indicate the maximum withdrawal from it. Thus S, determines the storage requirement for the dry period, to t
• From B to D the mass curve is, again steeper than OJ and the reservoir starts filling from t2 and it will be full at t3, S1 may be more easily measured as the vertical intercept between the two tangent lines AC and BM. Which are parallel to OJ and drawn to the mass curve at peak (or the ridge) point A and the trough (or the valley) point B
• Again, the reservoir will be overflowing from t3 to t4 and DD' indicates the amount of spill during this period.
• At D, the situation is identical to what it was at A.
• Therefore, the vertical intercept S2 between the two tangents DF and EN gives the storage requirement for the dry period t4 to t6.
• If the flow data for a large time period is available, the tangent lines are drawn at other ridge points and the storages are determined. The largest of these storages is then taken to be the actual storage requirement. That is,

• Mass curve can also be used to solve the reverse problem of determining the maximum demand rate that can be maintained by a given storage volume. However, it is a trial-and-error procedure,
• Straight lines tangential to the first peak point of the mass curve are drawn with different trial slopes such that all of them intersect the mass curve at or below the next higher peak point.
• The slope of the line for which the vertical intercept between itself and the touch point is equal to given storage defines the possible maximum draft rate that can be maintained.
• Similar straight lines are drawn at other ridge points across the subsequent valleys and the possible demand rates are obtained.
• The smallest of the various demand rates thus found gives the maximum demand rate that can be maintained by a given storage.

• In the residual mass curve, cumulative net inflow into the Reservoir i.e., the difference between the cumulative runoff and the emulative demand, is plot against time as shown in figure. In such a curve the inclined draft line O of previous figure is transformed into the horizontal line OJ of as shown in figure below and so are the tangent lines AC, BM and DP and EN. The storage required for the first dry period is now given as the difference between the ordinates of the residual mass gory at A and B and for the next dry period as that between the ordinates of D and E.

• It occurs either as rainfall or as snowfall. Precipitation falling as rain contributes directly to runoff. Hail and sleet which melt rapidly after contact with the ground may also contribute immediately. Precipitation falling as snow, in winter months, will not contribute to runoff until melting occurs.

• Heavy rain falling in excess of the infiltration capacity of the soil will largely contribute to surface runoff and will therefore tend to reach the stream channels very rapidly while the rain falling at lower intensities will cause delayed runoff.

• If the rainfall duration is equal to or greater than time of concentration (Tc i.e., the time in which the entire catchment starts contributing to runoff) the potential runoff is at a maximum. On the other hand, if the duration of rainfall is less than T c then the potential runoff will be lower than the maximum because only part of the catchment will be contributing to runoff before the rainfall ceases
• Also, since infiltration capacity is maximum at the beginning of rainfall and tends to decrease through the period of rainfall until it attains a constant value, the longer the rain continues, the smaller will the infiltration capacity become, and the greater the amount of surface runoff is likely to be.

• The time relationship between rainfall and runoff may be great affect by the distribution of rainfall over the catchment area a uniformly distributed rainfall volume may lead to delay runoff then the same volume falling over a localized part of catchment.
• The first type of rainfall distribution will tend to result in an increased groundwater flow and consequently a long-term increase in stream flow, while the latter type of distribution will tend to give large volumes of surface runoff and thus, a more sudden, sort-lived increase in stream flow.

• Factors such as shape, topography and soil type remain constant over long periods while others such as land use etc. May change radically and so profoundly influenced runoff.
• Shape of the catchment area influences the time of concentration of the catchment
• Catchment shape is quantitatively expressed as form factor and compactness factor.
• The form factor is derived by dividing the average width of the catchment by its axial length measured from the outlet to the most remote point
• A long, narrow catchment will yield a low value of the form factor, and generally a lower-peak runoff total than a similar sized area with a high form factor.
• The compactness coefficient is derived by dividing the periphery of the watershed by the circumference of the form circle whose area equals that of the catchment, and thus has a theoretical minimum value of unity for a completely circular catchment. The lower the value of the coefficient, the more rapidly is water likely to be discharged from the catchment area via the main streams.

• Factors such as shape, topography and soil type remain constant over long periods while others such as land use etc. May change radically and so profoundly influenced runoff.
• Shape of the catchment area influences the time of concentration of the catchment
• Catchment shape is quantitatively expressed as form factor and compactness factor.
• The form factor is derived by dividing the average width of the catchment by its axial length measured from the outlet to the most remote point
• A long, narrow catchment will yield a low value of the form factor, and generally a lower-peak runoff total than a similar sized area with a high form factor.
• The compactness coefficient is derived by dividing the periphery of the watershed by the circumference of the form circle whose area equals that of the catchment, and thus has a theoretical minimum value of unity for a completely circular catchment. The lower the value of the coefficient, the more rapidly is water likely to be discharged from the catchment area via the main streams.

• If slope of catchment area is large, water moves faster over the surface and channel towards the stream channel. Hence runoff peak comes early.

• Coarse textured, sandy soils will tend to give rise to little runoff. On the other hand, fine grained closely compacted clay soil will tend to yield more surface runoff. Moisture content of soil also affects runoff through its effect on infiltration capacity

• The most important effect of the vegetal cover is to slow down the movement of water over the surface after rainfall, thus, allowing more time for infiltration to take place. The timing of runoff after rainfall may be considerably modified and. Furthermore, peak stream flows a will tend to be much lower.

• Closely spaced streams allow efficient drainage of precipitated water, and in such cane overland flow will be short and surface runoff will rapidly reach the mains streams.

• The runoff from a basin is influenced by the direction of storm movement. If a storm striking a long and narrow basin is moving in an upstream direction the runoff contributed by the lower tributaries would have been already drained out by the time the runoff from the middies and upper tributaries reaches the basin outlet and therefore less peak discharge would be observed in this case.
• When, on the other hand, the storm moves downstream the runoff peaks from the individual tributaries are more likely to arrive at the basin outlet at approximately the same time with the result the runoff peak will be many times more than that in the case of storm moving
• A storm crossing the basin in the transverse direction would produce peak discharge which would be somewhere between the peaks produced by the two cases described above.

• The role of unit hydrograph in hydrology is to provide an estimate of direct runoff hydrograph resulting from given excess rainfall hyetographs
• In order to use unit hydrograph as a tool for predicting direct runoff hydrograph, we first need to derive a unit hydrograph for a given watershed.
• This method was first suggested by Sherman in 1932 and has undergone many refinements since then.
• A unit hydrograph is defined as they hydrograph of direct runoff resulting from one unit depth (1 cm) of rainfall excess occurring uniformly over the basin and at a uniform rate for a specified duration (D hours). OR
• A unit hydrograph (UH) is the hypothetical unit response of a watershed (in terms of runoff volume and timing) to a unit input of rainfall. It can be defined as the direct runoff hydrograph (DRH) resulting from one unit (e.g.,1 cm) of effective rainfall occurring uniformly over that watershed at a uniform rate over a unit period of time. As a UH is applicable only to the direct runoff component of a hydrograph (i.e., surface runoff), a separate determination of the base flow component is required. OR
• It can be defined as the direct runoff hydrograph (DRH) resulting from one unit (e.g., 1 cm) of effective rainfall occurring uniformly over that watershed at a uniform rate over a unit period of time.
• 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 24-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

• Separate the base flow and obtain the direct runoff hydrograph (DRH).
• Compute the total volume of direct runoff and convert this volume into the equivalent depth of effective rainfall (in 1 cm) over the entire basin.
• Normalize the direct runoff hydrograph by dividing each ordinate by the equivalent volume (cm) of direct runoff (or effective rainfall)
• Determine the effective duration of excess rainfall. To do this, obtain the effective rainfall hyetograph (e.g., use the φ-index, the Horton) and its associated duration. This duration is the duration associated with the unit hydrograph
• Unit hydrographs are fundamentally linked to the duration of the effective rainfall event producing them. They can only be used to predict direct runoff from storms of the same duration as that associated with the UH, or from storms which can be described as a sequence of pulses, each of the same duration as that associated with the UH.