hi there today I'm here to talk to you about River discharge and in particular the flood hydrograph or the storm hydrograph exactly the same graph but we can call it either of the two names so it's good to start off with by getting an understanding of exactly what River discharges what do we mean by River discharge or River discharge is essentially just the volume of water that flows in a river and we measure this in something called Q mix which is the geographical word for cubic meters per second we're basically measuring how many cubic meters of water is flowing past a given point per second obviously if you've got a lot of cubic meters or a lot of cubic meters of water that means that there's a lot of water in the river channel where if you've got it as a small number of cubic meters flow and Pacha per second then they may well be not as much water within the river channel at a given time but this all depends actually on the river add the river channel in particular that you that you actually assessing go looking at what we can do is I can show you two images just to give you a visual representation of what we mean by low and high discharge so residents in kind of a low discharge would be something on the left hand side so you can clearly see that the river bed is this river channel is dried up we can see the river bed there isn't as much water within the river channel so we can clearly say that that's got a low discharge there isn't going to be many cubic meters of water flowing faster per second in this channel whereas on the right hand side we can clearly say this river is actually in flood we can see that there's a very high discharge there's a lot of water flowing through this river so therefore you would expect that to be a high number of cubic meters flowing past the given points every second so in order to assess a flood risk actually and to assess essentially how quickly rainfall enter the river channel we use something called a storm hydrograph so when I talk you through the very basics on the toei you through from start to finish of how you would interpret one of these graphs so let's look at the a please to start off with and we'll start off with the y axes that vertical axis now there are two things that we can read from this first precipitation which we're measuring in millimeters and you can see that at the bottom of the AB C's precipitation from 0 to 50 millimeters that corresponds to the bar chart that light green and that very like green bar charts and that shows us how much rainfall we've had and when we've actually had the rainfall itself so you can measure that so for example that first bar looks to me something like around about 21 21 22 sorry 21 to 22 millimeters of precipitation and we can see that the peak rainfall by the maximum amount of rainfall itself was on that third bar and that was around about forty five millimeters of rainfall so we can see how much rainfall cell and when itself which is very important for our flood risk second thing we can measure on the y-axis is the runoff / river discharge which we measure in qmix and you can see that going from 0 to 10 20 30 40 50 using the full extent of the y-axis and that corresponds to the line graph that you can see and the line graph you can see is in dark blue so if we run through this we can see that to start off with a twelve O'Clock along the bottom will work in time sorry on the x-axis we can see that we get a slight decrease and then we get an increase in River discharge up to the peak a decrease in River discharge and then a flattening edge of the curve back to normal levels so let's talk through and run through each of these stages so we get a rainfall events in light green using the bars and if we move to the line graph we can see that that corresponds with an increase a couple of hours later in River discharge within the river system the river channel itself now this increase we call the rising limb so the rising limb is the word we use to describe the increase in River discharge you stay on the storm hydrograph and this continues to increase up until we get to something called peak discharge I use the maximum amount of water in the river channel at a given time the peak discharge the maximum amount of discharge we get so we get peak discharge and then we start to get a decrease and as that decreases we call that the falling limb the opposite to the rising limb it's falling the discharge is falling okay so we're going to fall England decreasing River discharge and we get back to the point where we are kind of normal conditions so as you say that dark blue line is the discharge line this is just showing us how much water is actually in the river at any given time and increases called the rising limb to pick discharge to the falling discharge which we then called the falling limb now if we look at the peak discharge and the peak rainfall we can calculate the time difference between those so looking at this graph the peak rainfall look to have occurred at maybe around about 7 or 8 o'clock something like that and peak discharge looks to have occurred at around about ten o'clock something like that so we can calculate the difference in time or the amount of time taken for the peak rainfall to occur and the peak discharge to occur and that essentially tells us how long it takes for the rainfall to for IES rainfall of precipitation and for it to then move through the grain system and flow and arrive into the river channel so that's the lag time that's probably the most important measurement that we get from the storm hydrograph so how long does it take for the rainfall to enter the river system now well the times that we've got on here that I haven't described yet we've got the antecedent or base flow which is on the bottom at purple color now antecedent again is a plus scientific word for the conditions that occurred before so the normal conditions are the base flow the normal conditions you would expect to be in this river channel is this dark purple arrow the shaded area here this is how I've set up a bit base flow which we get down at this point that's pretty much back to kind of normal flow level and we've also got something called the bankfull discharge and the bank for discharge is that straight line when I cross the hydrograph and the bank for discharge represents the height at which a flood could occur so with dark blue line our discharge line goes above the bank full discharge we might expect the river to potentially flood which could obviously be a risk to anyone in the local area and he houses any businesses etc that are situated within and on the floodplain or near to the flood and sorry near to the river channel itself this obviously changes from river to river dependent on our many different factors now what we also need to know is what controls the time that it takes for that pig landfill to occur and that peak discharge to what controls the lag time and I'm going to go into that in my next video and you'll find that link down in the description of this video thanks for watching see you soon