 Hello, I am Dr. Satish Kumar Skashid, Professor in Civil Engineering at Balchan Institute of Technology, Sulakur, presenting a topic, Fundamentals of Unit Hydrograph. Learning outcomes of this session, the students will be able to define the concept of unit hydrograph. The students will be able to state assumptions in unit hydrograph theory with the logic behind it. The students will be able to state significance of unit hydrograph for computing ordinates of stream flow hydrograph. Coverage in this presentation, just to have the idea of floods, flood pictorial views, the rainfall and off process we will discuss in brief. Typical stream flow hydrograph we will discuss and then we will come to unit hydrograph and its significance in predicting hydrograph. Now this is a typical picture which shows us a flood. So this can tell us how important it is to predict stream flow. The stream flow comes from a catchment. So how the catchment characteristics influence the hydrograph that we should know and how a particular rainfall event can be used to define the unit hydrograph that we learn. This is another picture of flooding on the Picos River, New Mexico. We see the spread of water on both the banks and hence the importance of stream flow prediction one can understand. This is an aerial view of the flooded Ganges in Alaba city. This particular figure shows you the process of precipitation, surface runoff and groundwater flow to which we are quite familiar. So when it rains some flow infiltrates some water comes out as overland flow some water again penetrates into soil and again comes out as interflow and ultimately we find a contribution of the surface water to the channel. This is typical runoff hydrograph. So this is say day 1, day 2, day 3, day 4 and on this bi-axis we are showing two things. One is rainfall in millimetres. So the variation of rainfall in millimetres typically it is shown in this way and another is the runoff in cubic meter per second. So the blue part gives us the storm flow, surface water flow and this yellow part shows us the base flow or the groundwater flow contribution in a hydrograph. Now let us come to a definition of unit hydrograph. A conceptual direct runoff hydrograph resulting from rainfall excess of unit depth and constant intensity for a particular watershed is called as unit hydrograph. The unit hydrograph method is employed to calculate the direct runoff hydrograph at the watershed outlet given the rainfall excess produced by the storm event. We will discuss the things in details. In unit hydrograph method the method is categorized as a lumped model in which the hydrology characteristics of entire watershed are combined and typified by one or more parameters. A simple mathematical expression or graphs. The unit hydrograph is a useful tool in the process of predicting the impact of precipitation on stream flow. The unit depth is 1 centipede in SI units or it can be 1 inch in US system. It is usually abbreviated as UHC. The subscript C indicates the duration of rainfall excess. Rainfall excess means it is over the percolation and evaporation which is ultimately converted into surface flow. Let us take one example. For instance the direct runoff hydrograph produced by rainfall excess that is a duration of 3 hours. What I say duration of 3 hours and constant intensity of one third centimeter per hour is denoted by UH3 that is unit hydrographs of a 3 hour rainfall event or 3 hour unit hydrograph. So here one can understand that 1 centipede rainfall excess has taken place in 3 hours. That is why it is called as 3 hour unit hydrograph means unit means 1 centipede rainfall excess has taken place in 3 hours time. You can develop a unit hydrograph for a gaged watershed by analyzing the simultaneous rainfall and runoff records. Unfortunately most small urban or rural watershed are ungaged. It means we do not have the rain gage stations there. We do not have the stream gaging there. So there what happens we do not have the recordings. In such cases we can also go for a synthetic unit hydrograph method to tackle such problems. Now why to have unit hydrograph? So it simplifies our task or procedures and it gives a baseline for a specific watershed. It standardizes the hydrograph for different watersheds and it gives us the information that how the flow of a stream will be affected over time by the addition of one unit of runoff. The role of unit hydrograph theory in the flood prediction process is to provide an estimate of stream flow given an amount of precipitation. It means if we have a watershed, its area and its rainfall records spread over time and we should be able to convert that particular information in a probable stream flow varying with a time. For that purpose we need a hydrograph. Once we know how much rainfall or snow melt has occurred or is likely to occur and we have an idea how much of this will turn into runoff, we still need to know how the flow of stream will be affected over time by the runoff. The unit hydrograph provides us a way to estimate this and is an integral part of many hydrological and modeling systems. Now here we will understand the flood prediction process and it will tell us how unit hydrograph helps us there. So once we know the watershed conditions, how will the water move once it reaches the ground surface? Then once we have the rainfall information, how much rainfall and with what intensity it has rained, we have a snow melt information, how much snow melt is there and how it is going to melt with the period of time. Then the next step is to understand runoff processes, how much of the rain water and snow melt will turn into runoff. So here we give the input from a unit hydrograph, how will the runoff affect the flow over time, also we give the information of flood frequency, how often and how big are the floods over the long term. Almost here also we have the role of river ice, how does the river ice affect the moment of water through the stream channel. And then we will come to flash flood processes, how do rainfall and land use combine to produce a flash flood? How stream flow routing has to be done? It is how certain volume of water flows through a channel over a period of time. Then we also give the stage discharge relationships to this particular information is given here. So ultimately we get a hydrograph of a particular river or particular stream which will give us the variation of stream flow over a period of time. So this is the flood prediction process. Now there are certain assumptions in unit hydrograph theory, why because rainfall is a natural process and we need to approximate certain things when we are going for a unit hydrograph. What are those assumptions? The primary assumption of unit hydrograph theory is that the rainfall has uniform distribution both in space with minimal variations across the basin. And also unit hydrograph theory says that it has uniform distribution in time also. In other words, the rainfall rate did not vary much during the event. Secondly one should understand in reality precipitation events are rarely uniform in space and they are rarely uniform in time also. One portion of the basin experiences higher intensity of rainfall and other experiences small but still we need to assume this so that we get a unit hydrograph which has to be useful for prediction of further stream flow. The next assumption is that the base duration of direct turn of hydrograph due to an effective rainfall of unit duration is constant. The ordinates of direct run of hydrograph are directly proportional to the total amount of direct turn of of each hydrograph. It means here we are taking the principle of linearity, superposition and proportionality so that the computations can be easy. For a given basin the run of hydrograph due to given period of rainfall reflects the combined physical characteristics of a basin which are time invariant. So this picture shows you that though we say that the precipitation will be uniform all over the basin still we will find that the precipitation varies over the basin. So some approximation we need to do that you can understand. So here on this basin we find the intensity of rainfall is changing, here intensity of rainfall is smaller. So we need to do averaging over. Let us take some questions. A unit hydrograph has one unit of rainfall duration, rainfall excess, time base of direct run of discharge. One more question. Excess or effective rainfall may be defined as the amount of rainfall which produces equal run of total rainfall minus infiltration minus losses, water depth collected during the net supply interval for all the above. And these are the answers. A unit hydrograph has one unit of rainfall excess and excess or effective rainfall is going by all these 1, 2, 3. These are the references. Thank you.