What is 1D and 2D flood modelling

hello I'm John wicks and in this webinar I'm going to provide an introduction to the world blood modeling I want to start by explaining some of the reasons why that morning he said modeling is important are going to introduce how you imitate both 1d and 2d modeling and then talk about where and when 1d and 2d methods are appropriate I'll then talk a little bit about linking 1d and 2d modeling together say something about the data requirements and then finish looking at some of the alternatives to one in 2d flood modeling through the presentation I'm going to use flood modeler to help provide examples of some of the information and on the screen at the moment there's a screen grab of led modeler with in this case a 1d flood model shown so why do we want to undertake third modeling it's because third risk management activities involve a range of interacting activities now the diagram on the right and the screen here shows some of these interactions between these third management responses so what we're talking about now it's focusing on those colored squares on here the assets for instant management property levels spatial community engagement in natural federal management and let's talk a little bit about how that modeling is required as part of the processes of implementing some of those types of responses so start at the top with the flood incident management so flood modeling informs the derivation of the flood warning areas and also is a key part of the process of flood forecasting predicting water levels to issue warnings moving on to assets these may be active asset such as barriers and sluices are passive asset such as flood defense walls and embankments our third modeling is very important here and through many phases of the process of appraisal and designers such schemes and for example could be used to help set the crest levels for flood defense walls moving over to property level responses now again further modeling can be useful here in a signing which areas it's appropriate to put property level responses in and also help defining the types of levels you may need to put these in it moving on to spatial planning and development management fled modeling and rapping is really important for zoning the floodplain to help guide appropriate development into the right areas of the floodplain to raise awareness and an improved Community Engagement flood mapping is also very important particularly on the community awareness raising to help people understand whether their whether their properties or their routes to work or whatever it is are living areas at risk of flooding and then down at the bottom in natural flood management third modeling can also be used to help assess the benefits and identify the most appropriate types of flooded natural flood management options so as you can see third modeling informs lots of these options that make up some of the portfolios of measures to try and manage risk the general third modeling process is shown on the screen here where we start by thinking about what is the most appropriate approach for a particular issue we're dealing with we go ahead and obtain data and build our initial model we test calibrate and validate it and we go into the production runs and quality control our outputs now let's yes this presentation is really focusing on that item number one they're selecting the best modeling approach and that is you know it's quite a complicated part of the process really there's lots of things that help you decide what is the best approach to use and some of those are shown on the right-hand side here internal in terms of looking at the objectives of this of the work you trying to do whether it's for flood warning or for calculating defense levels or something like that you also need to understand what are the flood characteristics you know it's a steep channel or a flat channel whether it's do we breaching and what type of routes the water will be taking once it gets onto the floodplain then the other thing that's helping you decide leave us off type of things that helping you decide the most approach best best available approach is the skills of the team you've got working on it the data you've got available what software you've got or can purchase the budget you've got to undertake the work and then the timescales you've got available to undertake the work and all of that can help guide what is the best modeling approach for a particular scheme now a key part of our designing on the approach is deciding whether you do one or two D modeling so let's start by just explaining a little bit about what 1d modeling is well 1d modeling generally refers to solving what's called the one-dimensional equations of flows and channels and these equations are things that look at the conservation of mass and conservation of momentum and if when you're looking at these 1d equations essentially you've got a single water level velocity and flow rate calculated for each cross section or node in the model that's for the river parts and the channel parts of it then there's also a great range of other features on the in the river and perhaps on the potentially on the floodplain that you may want to include and you can model these using 1d equations from this point features now there's a great heap a great potentially a large number of these features that could be included in a particular scheme or a location trying to do the work on the screen at the moment you'll see a massive and many differences or building blocks one 1d representations of features such as we as bridges and sluices the ones on the screen here are the ones that come with the flood modeling software other software also has these sort of building blocks in their model has got particularly wide range of building blocks it's easier to represent to find a one-to-one match between the tools in the software and the things you're trying to represent in the real world now let's try and show this this 1d approach graphically so on the screen let's we've got a meandering river going across the page here now 1d modeling is where we for the river part of it where we survey probably a cross section of the embanked annal and that's shown by these brown lines on the screen here they're to the planned position of a cross section of the channel and then we wrap it and then we sort of identify that as a know that's what those circles filled in circles on the screen or the nose that are those that have got data on the cross section and there's a little picture at the bottom left there showing what a cross section of a river would look like so that would be an in bank 1d model where the only information it's got on the river are the cross sections at those points and the distance between those cross sections that's a standard traditional 1d model of a river now what we can do again but still using 1d approaches is to extend those cross sections onto floodplain and have a singing effect it's like an extended cross-section across the in channel and the left and right Bank floodplains these cross-section lines need to preferably be perpendicular to the main flow directions that's why some of these we've got kinks doglegs in them and you see the picture at the bottom of the cross section it's now going across the floodplain so when we do the calculations we care what level that goes across the river and on the floodplains so that's an extended cross section approach another 1d essentially 1d approach for modeling the floodplain is to not extend the cross sections but to treat the different areas on the floodplain these four gray areas on the screen at the moment using what a cold flood cells or in inferred model the cold reservoir units and they're representing so that bucket storage so its water will flow into those gray areas and gradually fill up and then may may reach the same level as the engine is the embankment level on or may not and you represent these flood cells using a volume against elevation table and then on here we've got these red lines representing the top of bankers these of crest levels of the river are of the defense and they we saw in again in flood model they're called spill units and we spill water out of the river into the flood cells or vice versa it can come back into the river across those red lines so again this is a 1d representation one day in Bank River section and then one day flood cells will spill the units and linking those fro now 1d modeling is fast to run really fast to run it's good at representing the in channel water levels and flows and it's good at representing a lot of these point features like bridges and sluices and Weir's and culvert entrances and trash screens and this sort of thing and for many situations it actually produces good enough estimates of that extent old and you can calculate depth groups from it as well to understand for the plain flow and to be used for flood mapping however it when you have a complicated flow routes on the floodplain it's harder to work with you've got to try and predefined those routes before you set up the model and it can take time to do that in particularly complicated areas around in urban areas around roads and around buildings and things it can produce a very poor representation of those flow rates also there's no velocity distribution on the floodplain so if you turn into a hazard calculation where hazard means a function of both velocity and depth it can't really do that very well you can do it with zoning but you can't do it them explicitly from from this from the approach let's move over to 2d modeling so 2d modeling now it's solving what's called the 2d equations of flow sees the same characteristics those equations are representing so it's conservation of momentum and conservation of mass but this time it's solving them in two down two dimensions that's all back an x and y or an easting and northing direction if you like there's a variety of ways of solving these equations with inferred model for example isn't called the ADI in the TVD scheme these different solvers are better for different types of things so adi is better on this sort of lower slope lower velocity areas and the TVD is aim to capture shock so it can do a very sharp front to a flood wave it's 2d not 3d so that means it usually means that it's doing the depth average calculation of velocity on a grid working on a grid or a mesh the key data item for a 2d model is a digital terrain model for the floodplain and if you're also modeling the channel that you need to have for both imagery of the channel now what this looked like if we take our example bit of channel again so here we've got the in battle in the floodplain and here if we're doing it all in 2d modeling we would cut that up into our a mesh or a grid and represent the whole thing in 2d so it made the calculation domain may look something like we've got on the screen at the moment so you know what the pros and cons a 2d modeling well it that some of the good things about it is you don't need to pre define the flow routes it will calculate the flow routes across the domain using the information within the terrain model and the momentum of flow it can be very easy to set up a purely 2d model it can be very accurate you do get a velocity distribution across the floodplain and on the right hand side of the screen here we've got a again results from fed modeler in this case you little little line three tragedy velocity arrows and it's showing you the velocities at a particular time stamp I think in this case as it as the water moves its way in this case from the south of the from the bottom of the picture up towards the north which is the downstream end and it directly the reference directly produced the flood maps and the flood depth grids as direct output you don't need to know any post-processing of them problems with 2d modeling is it can be very slow it can take hours and days to run it's poor at representing the point features such as bridges which isn't bridges Weir's and sluices and if you're modeling the channel in 2d you generally need a relatively really fine mesh particularly small rivers to capture the you know what's happening in the channel and this is I guess it's those dealing with those strengths and weaknesses where we've led to a lot of work being now being done in what's called links 1d 2d modeling and this takes the best bits from 1d and 2d so the 1d channel I thought rather the channel will generally be represented in 1d using cross sections and nodes it could be the way you're going into a wide s tree or something like that you also you want to model the that party into day but generally the the river will be modeled in one day and then the floodplain where we want the detail we model that in 2d and then we link the to 1d and 2d domains dynamically together so that both calculated at the same time what this would look like in our example bitter river here is shown on the screen where we've got the channel the in bank flows and water levels modeled using 1d with those cross sections where the orange lines are on the orange dots are and then the floodplain in this case we've got all of the floodplain modeling 2d so we've got in this case a regular grid across the whole of the floodplain and then we need to tell it where to link the one D into D flows and that's what the red line on here is showing that will be the linked lines linking the 1d and the 2d or that the linking the 1d in the 2d model together and maybe things set up that tells it to take the water levels calculated the 1d nodes and look at the water levels in the floodplain and decide how much water flows across the the we're taught using we're type equations perhaps along those red lines that's one way of doing it the other way of doing it is to mix and match some of these 1d and 2d approaches together so what we've got here is the same channel the bottom left there we once perhaps his nerve an area where we want to use a 2d method to simulate flows across that bit of the floodplain then north of the channel perhaps its rural areas with relatively simple flow routes and lo receptors we're just going to use extended cross sections in that area because that's nice and easy appropriate to use in that area and then bottom right we've got a 1d flood cell and perhaps that's a little bit of washed lands with a bank around it and it's appropriate to set that up using this 1d flood cell or reservoir unit in flood maudlin so that's explaining some of the an overview of the background to 1 and TD modeling so where would you use 1d and where would you use 2d weather there's no hard and fast rules in this but in general here is some advice on that we're doing the 3rd forecasting of a third forecasting it's run time is really important in general in the moment the focus is on calculating water levels at some forecasting points 1d approaches are very much the approach that the standard way of doing it and I think will continue to be the standard way of doing it for many K for many situations because it's very robust and very quick and gives you those water levels to help with warning for flood mapping there's a over recent years has been a big move towards 1d and 2d linked models so the channel itself is modeled in one day and then the foot plane generally modeled in 2d although the actual grid sizes or the mesh sizes can vary depending important that particular areas are the competitive complexities of flows so I've heard mapping generally linked 1d 2d models the operational studies where you're trying to work our operational routines versus their movement at barriers or sluices and things and you want to try many scenarios and generally you're controlling things in - in bank in 1d very much the approach to use there when you're designing petunia big interventions then it's probably a combination of 1 D and 1 D 2 D linked and it maybe that's in effort for setting levels way keeping all the water level in Bangka the 1 D approach is appropriate but when you're looking at appraisal and looking at residual flooding and this type of thing it's likely the 1 D 2 D linked approach is most appropriate then if we're looking at breach analysis where we got holes in defences and things and water flowing out into the floodplain usually you 2d modeling of that breach flow is what's going to be the inundation from that breach is what's going to be most appropriate so what are the data requirements it does vary a bit between whether you're doing one day in 2d but here's some general statements here for the river itself if you're doing 1d you just need cross sections which maybe every I don't know 20 50 100 200 meters apart whereas if you're doing full 2d for 2d of the channel then you need a full briefing eatery and that's not just at the cross sections you need everything in-between that as well for things for these our point features like bridges and Weir's and sluices you need the dimensions you need some coasts and coefficients for those you need operating rules of sluices and things like that for the floodplain generally now both both wand Bundy extended foot cells and 2d modeling on the floodplain is driven by gridded data be it lidar or other lower original resolution gridded data you'd for linear features which could be raised roads or embankments railway lines going through through and that sort of things you need perhaps you need to capture those separately if they may not be kept may not be identified well enough in the gridded data you need to have information on roughness again on the floodplain you could use land land use or land cover information things like master map to work out different zones of roughness and then in the channel it's usually based on I'm looking at seeing see what the thing looks like comparing to standard values and then calibrate it the models now need boundary conditions both 1d and 2d usually the upstream boundary conditions are in flows that's flow against time if you do an unsteady model and usually the downstream boundary conditions are water levels water levels against time or rating curves generated and things like normal depth calculations you need to start all of these models offer and steady running to start them off with initial conditions or initial States and for the floodplain you can usually assume that's dry whereas in the river you need to have an initial water level and flow in that river then when you come to do the simulations you need to tell it what time to start what time to finish time steps another set of optional data to control the run that may be required by the software using and then as you go for a process of calibrating and validating your model you need some observational data probably to drive that and there's lots of types of observational data it can be just observed water levels from those flows or it could be flood maps or wrack marks and that type of thing so I focused on a 1d and 2d modeling so that's the most used in the within flood risk management but there are other types in the in the past very much so physical models we use to try and predict what would happen with yeah so at different flows in different conditions different scenarios they still have a very very good point place in third management for the design of pickney and some one-off structures and for getting into the detail flow flow routes around the round the size of things that are in the river then physical models may be appropriate tend not to be used a lot really then on to the 3d on to 3d approach assumptions where you've got like a layers of water in that but a particular point in the and the domain you want to know they've lost is at different elevations within that flooded column so this is also computational fluid dynamics CFD these are used a little bit in flood management but it's of a similar area to where the physical models are used we've got one-off features in the channel particularly where you want to understand sub complex flows round piers or our other features than you may need to do some CFD modeling not used a lot to be honest then the other extreme is going to much simpler approaches where we're reducing the representation of physics so it may be ignoring the momentum effects and there's a range of simplified methods that are available that had tend to be quicker although they don't always have to be likely to produce in some conditions which is good enough result servers that you know they're gonna be lower lower accuracy methods but are good for covering large areas offer in the initial parts of a project an example of one of these reduced physics methods is the fast solver there's inferred modeler and the image at the bottom here of the Greenwich Peninsula is for direct rainfall in that area and we've been able to do near to simulate direct rainfall and identify areas susceptible to surface water flooding based on fast runs very very quickly it won't give you information on velocities and won't do with the momentum effects but may be useful for particular types of studies so to sum up I've been talking about one two and three D approaches and they can all be appropriate and so can physical models and so and and the reduced physics models when you undertake in the study are using the results you need to select wisely and they can be depending the range of things and particularly the objectives of the study and the third and flow characteristics of the location you turn to model at the moment linked 1d 2d models are the most used and the most useful but for many studies you actually need more than one approach it's beneficial to have more than one approach applied and you could have simplified methods at the beginning 1d 2d and then as you go into operational control or forecasting you should shift back to just using a 1d only approach and it's much more efficient for the modeling process if you can do all of this using the same data sets in the same user interface so it's good to have a modeling system that supports and multiple approaches and examples of a system that does that so the flood model Pro and flood model of free software because that includes 1 D 2 D 1 D 2 D linked and the simpler first approaches and with that I'd like to end my presentation thank you

Channel: Flood Modeller

Views: 33,650

Keywords: Flood (Disaster Type), flood risk modelling, Simulation Software (Software Genre), flood risk modeling, 1D modelling, 2D modelling, Flood Modeller, Flood Modelling, Flood Modeling, Flood Simulation

Id: GIAqQDs5Gjc

Length: 22min 23sec (1343 seconds)

Published: Thu Aug 27 2015