but the weirs have to be set low enough that if you have a large inundaition, the drainage will happen in a manner that won't cause a back-up in the system and flood properties and streets like you saw in the previous slide. we have ways of prevents the back flow and that's what we really have to look at in the next little while. in fact, we have a 30-year master plan we're working on now that's looking at this problem of inflow into the system from high tides in a very serious way. we believe we probably have to start implementing some back flow prevention measures, i'd say, within the next several years, probably 3 to 5 years. it's already happening now and it's just going to get worse. so some of the things we could probably do would be, for example, these -- we call them duck bill valves that we could install on some of our weirs. sounds simple enough, but if you remember the configuration of the weir i just showed you, they are quite long and we don't have too many sort of pipes, small pipes, like that that just end and you can put a duck bill on the end to prevent the flow from c

as the top right-hand picture shows, it might be due to coastal inundaition, or it might be due to river flooding. most likely is that it will be due to all 3 concurrently. how we address that from an engineering perspective is very difficult. despite all our advance modeling tools, we still have some way to go firstly to properly explain all those interactions and secondly to be able to develop a proper engineering integrated solution to those problems. and then, thirdly, one of the problems we have with flood protection is that drainage engineers and flood protection engineers, for many, many years in the developed world have been working in their own compartment. as the picture shows, this is always the way we did our urban drainage planning. but what we're beginning it realize is that we have to think about this in the much wider context of urban areas and recognize that there are other agencies involved, not just the drainage agencies. there's a transportation agency, the planning agencies, health and social services and so on. when we start to get these agencies around the table al