I RUNOFF Definition of Runoff Runoff can be defined as the portion of the precipitation that makes it’s way towards rivers or oceans etc, as surface or subsurface flow. Portion which is not absorbed by the deep strata. Runoff occurs only when the rate of precipitation exceeds the rate at which water may infiltrate into the soil. Portions of Runoff 1. Surface runoff 2. Groundwater flow 3. Direct precipitation over the river stream. Runoff Process When rainfall occurs: A part of rainfall/precipitation is intercepted by vegetation. Some part is stored in depressions on the ground surface known as depression storage (S d ), which later infilterates or evaporates. Some part of rainfall is absorbed by the soil, the amount of which depends upon the soil moisture condition at the time of percolation. Now if the rain continues further; The water starts infiltrating/percolating to the water table and if the rate of rainfall or t he rate at which the water is reaching the ground exceeds the infiltration rate (f), resulting the surface detention (D). This water flow overland and joins the rivers, lakes, streams, oceans, etc, and is known as surface runoff . One can say runoff as surface runoff. 1) The surface runoff is important for maximum flow. 2) Where as the ground water flow is important for minimum flow. 3) Direct Precipitation over the river or stream is negligible. So for peak flow we are generally concerned with surface runoff and therefore we can say runoff as surface runoff.
II Channel Runoff Streamflow, or channel runoff , is the flow of water in streams, rivers, and other channels, and is a major element of the water cycle. It is one component of the runoff of water from the land to waterbodies, the other component being surface runoff . Water flowing in channels comes from surface runoff from adjacent hillslopes, from groundwater flow out of the ground, and from water discharged from pipes. The discharge of water flowing in a channel is measured using stream gauges or can be estimated by the Manning equation. The record of flow over time is called ahydrograph. Flooding occurs when the volume of water exceeds the capacity of the channel. Factors Affecting Runoff 1. Precipitation characteristics, It is the most important factor for runoff Runoff depends on the type of the storm and it’s duration, which causes precipitation. Runoff depends on the intensity of r ainfall. More the rainfall, more will be runoff. If the rainfall intensity is very less and it rains as light showers then much of the water will be lost in infiltration & evaporation resulting less runoff. If precipitation is in case of snow then less runoff. 2. Shape and size of the catchment, Runoff depends upon size, shape and location of the catchment. Generally more rainfall on smaller area r esulting in greater runoff. Less runoff in larger catchments because due to uniform rainfall over the entire area, thus only few tributaries of the stream feed water to a main stream during a particular storm. Types of catchment Fan shaped catchment All the tributaries are approximately of the same size. Gives greater runoff because the peak flood from the tributaries is likely to reach the main stream approximately at the same time. Fern leaf catchment The tributaries are generally of different lengths and meet the main stream at the regular intervals. In such a narrow catchments the peak flood intensity is reduced since discharges are likely to be distributed over a long period of time.
III 3. Topography, Runoff depends on surface smoothness and slope. If slope is steep, flow will be quick and less evaporation and absorption, resulting greater runoff. If the catchment is in mountainous area and on the windward side of the mountain, then more rain fall resulting more runoff. 4. Geological characteristics,
It is one of the important factor. It includes the type of surface soil, subsoil, type of rock and their permeability characteristics. If soil and subsoil is porous, seepage will be more resulting reduction of the peak flood. If the surface is rocky, then absorption will be nil resulting more runoff. If rocks have fissures, are porous in nature, have lava tunnels water will be lost resulting less runoff.
5. Meteorological characteristics,
Runoff may also be affected by temperature, wind and humidity. If temperature is low and ground is saturated then runoff will be greater. If temperature is high and greater wind velocity give rise to greater evaporation loss and resulting in less runoff.
6. Character of the catchment surface,
Runoff depends upon the surface conditions like drained, undrained, natural or cultivated. If the surface has no natural drainage then absorption loss will be more. If more area of a catchment is cultivated resulting less runoff. Vegetal cover reduces the runoff in smaller storms. No vegetal cover reduction in bigger storm.
7. Storage characteristics.
The artificial storage such as dams, weirs, etc and natural storage such as lakes, ponds, etc tend to reduce the peak flow. They also give rise to greater evaporation losses.
IV Runoff Cycle It is a part of hydrological cycle The part between the precipitation from the atmosphere over land areas and it’s subsequent discharge through streams channels. Conditions of Runoff Cycle 1. End of dry period
At the end of dry period and just beginning of heavy rainfall all the surface and channel storage gets depleted (dried) expect from lakes, reservoirs and ponds resulted from the previous rains. The only source of stream flow is the ground water flow entering the river channel.
2. Shortly after beginning of rainfall
Shortly after beginning of rainfall and before interception, depression storage have been satisfied. The stream flow intercepted by vegetation and buildings can ’t contribute to runoff. This intercepted water is eventually returned to the atmosphere through evaporation. At this stage, a part of precipitation falls directly on the stream which gives an immediate increment to stream flow.
3. Near the end of isolated heavy rainfall
After many hours of heavy rainfall virtually all depression storage and interception requirements gets filled up. The soil moisture deficiency is also satisfied to a considerable extent. Infiltration rate is near the minimum. Similarly the flow into the filled depression is essentially balanced by over land flow and infiltration. Thus at this stage over land flow mainly contribute to stream flow. Sub surface flow also contributes to stream flow.
4. After the end of rainfall
When rain and overland flow ceases (stops), the stream flow consists of only base flow and channel storage. Evaporation takes place quite from soil moisture. Transpiration also takes place from vegetative cover. Water from depression storages also continues to infiltrate. Also the gravity water still not drained up to the water table continues its downward journey to join water table.
V Summary of Rainfall-Runoff Process When a rain starts falling, it is first o f all intercepted by buildings, trees, and other objects, which prevent it from reaching the ground. This quantity is known as rainfall interception. Since this quantity is generally very small, it is not of much importance for intense rains; but many a times, large portions of lighter rains are disposed of in this manner. The difference between the total rainfall and that which is intercepted is called ground rainfall . when the rainfall rate exceeds the interception rate, water starts reaching the ground and infiltration into the sub soil starts . The maximum rate at which the soil in a given condition can absorb water is known as its infiltration capacity . The excess rainwater gets collected into the innumerable small and large depressions existing in the basin, filling them to their overflow levels. This quantity is known as depression storage. All this storage is either evaporated or used by vegetation, or later infiltrates into the soil. None of it appears as surface runoff. If after the depression storage is filled, the rain intensity ( p) continues to exceed the infiltration capacity of the soil ( f ), the difference appears as rainfall excess, which initially accumulates on the ground as surface detention (D), and then flows as overland flow on the basin surface before entering a stream channel. The water that reaches the stream channel of a basin in this manner is called surface runoff (SRO) or direct runoff (DRO). The surface runoff can, therefore, occur only from those storms, which can contribute to excess rainfall, and are simply not dissipated in fulfilling the interception, depression storage, and infiltration needs of the basin. Hence, Excess Rainfall = Rainfall – Interception - Depression storage – Infiltration The sum total of interception and depression storage for a basin is usually called the initial loss or basin loss or initial basin recharge. Excess rainfall can therefore be represented as: Excess rainfall = Rainfall – Initial Basin loss – Infiltration The sum total of initial basin loss and infiltration, is called potential infiltration. Since for intense rains, the initial loss is very small as compared to excess rain, it is usually ignored in hydrological analysis of such rainfalls; or is considered to be included in the infiltration itself. The excess rainfall is, thus, represented as: Excess Rainfall = Rainfall – Potential Infiltration The rain that falls in the beginning of a storm before the depression storage is completely filled is called the initial rain, and the rain that falls near the end of the storm at a rate less than the infiltration capacity is called the residual rain. The intervening period is the net supply interval. The infiltration occ urring after the net supply interval is called the residual infiltration. Runoff and surface runoff are two different terms and should not be confused. Runoff or Discharge or the Stream flow includes all the water flowing in the stream channel at any given section. While Surface Runoff or Direct Runoff includes only the water that reaches t he stream channel without first percolating down to the water table.
Yield of a drainage basin is same as runoff, with the only difference that it is expressed over long 3 periods (M.m /year), while 3 3 Runoff is expressed for short periods (m /sec or m /hr). Runoff =Surface runoff + Ground water inflow (i.e. Base flow). 700.0000
700.0000
600.0000
600.0000
500.0000
Surface Response
Total Hydrograph
500.0000
Surface Response
400.0000 400.0000
300.0000
Baseflow 300.0000
200.0000
Baseflow 200.0000
100.0000 100.0000
0.0000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 3 2 4 8 0 9 6 1 2 2 8 0 7 6 9 2 0 8 0 5 6 7 2 8 8 0 3 6 5 2 6 8 0 1 6 4 8 4 4 6 2 4 4 0 4 2 0 . 0 . 0 . 0 . 0 . 0 . 0 . 1 . 1 . 1 . 1 . 1 . 1 . 2 . 2 . 2 . 2 . 2 . 2 . 3 . 3 . 3 . 3 . 3 .
0.0000 0.00 00
0.50 00
1.00 00
1.50 00
2.0 000
2.5 000
3.0 000
3 .5 000
4 .0 000
VI Computation of runoff METHODS
Use of runoff
Use of infiltration
coefficient
curves
Use of infiltration indices
1. Use of runoff coefficient The volume of runoff can be directly computed approximately by using an equation of the form Q=k.P Where Q= runoff P= precipitation k= constant depending upon imperviousness of the drainage area Truly speaking this equation is not rational because: “runoff not only depends upon the pr ecipitation but also depends upon the recharge of the basin”. but the equation gives more and more reliable results as the imperviousness of the area increases and the value of k tends to approach unity.
Application of method Where USED? Design of storm water drains. Small water control projects especially for urban areas where the percentage of impervious area is quite high. Where should be AVOIDED?
This method of computing runoff should be avoided for rural areas and for the
analysis of major storms. 2. USE OF INFILTRATION CAPACITY CURVE The infiltration capacity (IC) curve is a plot of the infiltration capacity against time. If the IC curve is super imposed on the rainfall hyetograph, the resultant amount will represent nothing but the runoff. Natural rains of varying intensities, sometimes below and some time above the prevailing infiltration capacity, results in a distortion of a capacity time curve. It is generally assumed that the infiltration capacity at any time is determined by the mass infiltration, which has occurred up to that time.
VII 3. USE OF INFILTRATION INDICES
The infiltration capacity curve already determined on test plot cannot be applied
to large basins or heterogeneous areas. At any instant, in a large area, the IC as well as the rainfall rate will vary from point to point. Moreover subsurface flow or interflow will also be substantial. Since this water flow is part of infiltration, it will not normally be included in the runoff computed by IC curve determined on a test plot. Hence runoff volumes are generally computed by using infiltration indices. W-index and Φ-index are the two indices which are commonly used.
W-index is the average infiltration rate or the infiltration capacity averaged over the whole storm period, and is given by Windex = F / tr = (P – Q) / tr
where F = total infiltration including initial basin recharge P = total precipitation Q = total runoff tr = duration of rainfall in hour Φ-index is defined as, “the average rate of loss such that the volume of rainfall in excess of that rate will be equal to the volume of direct runoff”. It can be defined the other way round as, “the rate of rainfall above wh ich the rainfall volume equals to the runoff volume”. Φ-index can be represented graphically as shown.
Φ-index
and W-index will be equal for a uniform rain, but they may not be equal for a non-uniform rainfall. However for rains which are reasonably uniform or for heavy rains these two indices are found to be nearly equal.
In a usual case of moderate rain of non-uniform intensities the Φ-index will be somewhat higher than W-index. These indices will change with a change in interception, depression storage and initial soil moisture. They will also change with the amount of precipitation. These indices are not the actual infiltration rate but the measure of potential basin recharge. The runoff coefficient k can be determined if the W index is known by using the equation k = (P – Windex) / P where, P = rainfall rate
VIII Example # 1 Following are the rates of rainfall for successive 20 minutes period of a 140 minutes storm: 2.5, 2.5, 10.0, 7.5, 1.25, 1.25, 5.0 cm/hr. Taking the value of Φindex as 3.2 cm/hr, find out the runoff in cm, the total rainfall and the value of Windex. Solution: From the given rainfall rates rainfall hyetograh is plotted, to see rain intensity pattern, as shown in below Figure. •
• •
line at a height of 3.2 cm/hr is superimposed. The hatched area is calculated, so as to obtain the value of runoff: Φindex
W index
Example # 2
P
t r
Q
An isolated 3 hour storm occurred over an area of 120 ha as below: (i) What is the total rainfall on the catchment in this storm? (ii) Estimate the runoff from the catchment. (iii) If the Φindex were to remain at the same value, what runoff would be produced by uniform rainfall of 3.3 cm in 3 hours uniformly spread all over the catchment? Partial area of catchment (ha)
Rainfall (cm) Φindex
(cm/hr)
st
1 hour
2
nd
hour
rd
3 hour
36
0.9
0.6
2.4
1.3
18
1.1
0.9
2.1
1.5
66
0.5
1.0
2.0
0.9
