CONSTRUCTION OF UNIT HYDROGRAPH The unit hydrograph method is use for determination of the maximum flood discharge of a stream and also for developing a flood hydrograph corresponding to any anticipated rainfall. For this a unit hydrograph of suitable unit duration is derive from an observe hydrograph of the drainage basis under consideration. Using the unit hydrograph, the flood hydrograph corresponding to any rainfall of the same unit duration can be obtained. Use following steps to derive a unit hydrograph. Choose a hydrograph resulting from an isolated, intense, short-duration rainfall of nearly uniform distribution over the entire drainage basis as well as within its duration. To obtain direct run-off deduct the base flow from the total run off represent by the hydrograph. For base flow separation any of the methods explain earlier may be use. Draw a series of ordinates at some uniform time interval for the entire hydrograph. From…

ASSUMPTIONS IN THE THEORY OF UNIT HYDROGRAPH It is essential to learn about the assumptions considered in the theory of unit hydrographs: The rainfall is uniformly distribute within the specified duration. The rainfall is uniformly distribut in the whole of the catchment area. The base or time duration of the hydrograph of direct run-off due to an effective rainfall of unit duration is constant. For a given basis the hydrograph of run-off due to a given period of rainfall reflects all the combint physical characteristics for the catchment.

UNIT HYDROGRAPH The basis of unit hydrograph is that if two identical rainfalls occur on a drainage basin having identical conditions prior to the rains, the hydrograph of run-off from the two storms would be expect to be same. In actual conditions two identical storms are rare to occur. Most of the practical storms vary in duration, amount, and distribution of rainfall. A unit hydrograph is a hydrograph of a rainfall of a specified duration and areal pattern, resulting in a run-off of 1 cm. In other words, the total volume of water contained in this run-off hydrograph will be equal to 1 cm depth of water on the catchment. Various storms of varying intensities and having different volumes of run-off, but a particular unit duration, will have the same shape of their run-off hydrographs. Different unit hydrographs for different unit duration can be prepare. A particular unit hydrograph when…

TO DEVELOP STORM HYDROGRAPH FROM UNIT HYDROGRAPH The unit hydrograph of a specifie unit duration can be easily use to develop the hydrograph of storms of the same duration. The total volume of run-off (in cm) resulting from a storm when divid by 1 cm will give the multiplying factor i.e. the factor by which the ordinates of the unit hydrograph must be multipli in order to obtain the ordinates of storm hydrograph of the same duration. The following example illustrates the method by which storm hydrograph is develop from unit hydrograph of the same duration. Example 6.2. Date, time, ordinates of unit hydrograph and base flow are given as follows. Determine the ordinates of direct run-off and total discharge ordinates. Rainfall excess is 10 cm. Time of duration is 6 hrs.

CALCULATION OF DIRECT RUN-OFF FROM STORM HYDROGRAPH From storm hydrograph measure ordinates at fixed time intervals. These ordinates represent discharge or flow (Q). Also measure the ordinates of base flow, at the same time interval as adopted earlier. By subtracting ordinates of base flow from corresponding ordinates of total discharge Find out the net ordinates which represent direct run-off in that time interval. Add up all the ordinates of direct run-off which is nothing but total volume of direct run-off. By dividing total volume of direct run-off by area of the basin, direct run-off available from the catchment is obtained. The method can very well be understod from the following illustrative example.   Direct run-off in depth of water = Total \[= \frac{Total volume of direct .run-off }{Area of the basin}\] \[= \left [ \frac{\Sigma E\times t\times 60\times 60}{A\times ^{}10} \right ]\times 100\] \[= \frac{0.36 \Sigma R\times t}{A}cm\] where t…

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…

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…

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…

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…