HEC-RAS 2D Initial Conditions Time (Warm-up) and Ramp Up Fraction

you have to warm up your model hydraulically there's like three different sediment warm-ups you can do they don't matter if you're not warming up your hydraulic model the problem is is that we don't warm up the hydraulic model by default now if you're used to 1d 1d defines the initial conditions of your unsteady flow model by just running a flow hydraulics so time zero is just a steady flow profile and then we transition into an unsteady flow and that can go poorly but it often doesn't because they're pretty close there is no steady flow 2d that we can transition into um so the model starts dry actually has to like warm up into a steady state and so you have to give it time to fill and kind of achieve a steady state if your hydrograph looks like this this is not my best tito graph but if your hydrograph looks like this if you were to start the model here well you're just going to start putting water into a dry mesh so and then if your sediment also starts here well you're gonna like just erode the channel because you're just sending water through again this is something you can often get away with in hydraulics not in sediment you need to have time zero come after a period of steady flow um and so that's what this is it's called initial conditions it used to be warm up now it's the initial conditions time because we want you to know that this isn't like an optional warm-up if you're like one of my 11 year old boys and thinks they don't need to warm up before they play this is mandatory this is what you need to do to get your initial conditions and so you give it enough hours to not only fill the channel but reach a steady state now this gets really complicated with levies and gates and stuff like that but in most you know 2d sediment models you can have a general sense of how long this will take now what if it's unstable here first of all you probably don't want to start it here right you don't want to warm it up here because that's going to be that's going to be a shock to your system you want to start it kind of at a low flow or a moderate flow but what if it's unstable here you're still hitting it with a kind of a shock of flow well it would be nice to ramp that up to start with a low flow and kind of edge towards this actual initial initial conditions flow well that is what the ramp up fraction is and so the ramp up fraction says we're going to take some percentage or fraction um of your initial conditions time and we're going to use that to ramp up so in this case we're going to take the first 10 of your initial conditions which in this case is 1.2 hours and we're going to use that to ramp up into your initial conditions and so point one point two you can hit point five if you wanna you know kind of get there slowly if your model is unstable in the warm-up time this is a good way to kind of sneak up on it and then reach a steady state uh but you know i feel like i'm getting a lot we're getting a lot of 2d sediment models coming in right now this is the number one error people are used to you know well i'll just run my hydraulic model until it reaches steady state um but if you're doing sediment from the beginning it's going to be a catastrophe so make sure that you're warming up ramping up your model so it has good initial conditions your your model will run twice um and the first time it'll run with negative times in fact if you've got like a 24-hour model in your initial conditions they're like oh look my my run's going to be done in an hour look how fast that's going and then you realize oh those are negative numbers uh we're still in the initial conditions and then it gets to that and then it starts your run in positive time sediment comes on at time zero there are three different sediment warm-ups alex will talk about warming up your sediment model which is a totally different thing you want to warm up your hydraulic model first and then sediment comes on here with all three sediment warm-ups you

Channel: Stanford Gibson

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Id: THJSsT8cwj0

Length: 3min 48sec (228 seconds)

Published: Wed Jun 22 2022