Let me define the river boundary as the line usually along the river course, which has clearly distinguished the river area and the others. I assume that the river area is the non-usable area because of the high flood risk and inappropriate for people’s living. I learned river engineering in Japan. In the case of Japan, the river boundary has been designated by law and managed by the authority since 100 year ago, aiming at promoting the flood control works smoothly resulting into damage reduction. Due to the topographical condition in Japan, the river boundary is almost same as the existing dike alignment. So for Japanese people, the river boundary is quite clear because of the existence of the dike system which have been made mostly empirically in over one hundred years.
When I visit other countries for some flood risk management work, I recognize some of them are facing or in the process of solving the issue how to manage the riverine area which is flood-prone without dike, and how to define the river area as high risk area. Very often they have some laws, originating a kind of riparian law, which defines an easement from the river bank or normal water line as a constant distance such as 20m. But I see an argument if such constant easement is applied to the riverine area land use plan, the flood risk along the river is quite different from local people’s experience.
Japan learned such constant easement several decades ago from Europe, but since then Japan has already constructed dike along the river channel and managed the river area together with such dike, such argument on the constant easement is no longer discussed.
If you know some country / area who has the defined river boundary for the purposes of land use and/or river management, please share any information on the definition and the methodology to define empirically or theoretically.
Thank you very much.
I am assuming that by "river boundary" you mean the flood plain. If so, modeling flood plain is commonly based on the criteria based on some recurrence interval. In the USA, a common recurrence interval used is 100-years flood. You can assess area in the flood plain that inundates by a 100-year storm to delineate a 100-years flood plain. This is used by emergency management agencies and land use planners to develop policies on land use and river management.
Hello,
i hope this example of flood plains delimitation in Australia will be helpful :
Georges river floodplain risk management study & plan (2004).
Dear Kazunori
I will give a partial answer explaining the situation in two Australian States.
Land law in the six Australian States follows a similar pattern but there are unique characteristics in each. From earliest European settlement (began in 1788, but accelerated in the second half of the 1800s) the concern about river boundaries was not so much flooding, but first to protect rivers from pollution and riverbanks from degradation by grazing stock; and second, to allow general public access - for public purposes and also for travelling stock purposes in the sparsely populated inland. (Remember that most of Australia is dry and except in the coastal rivers, water is precious). For these reasons land was often reserved along rivers to ensure that the neighbouring landowners did not exclude the public.
For example, in 1881, the State of Victoria reserved a land frontage of usually 1 chain (about 20m) along most named creeks and rivers from alienation (that is, sale from the State). While frequently leased back to adjoining landowners (to avoid costs of fencing), they remain State land.
In the State of Queensland, with much larger distances and much larger landholdings (commonly thousands or tens of thousands of hectares, reservation of a frontage was less common. However in the early 1900s, legislation was passed to compulsorily acquire (expropriate) the bed and banks of boundary watercourses (forming the boundary between two titles), without compensation. Since then there has been much controversy within the surveying profession about what "bed and banks" means. Some hold that it always meant the "high bank", the top of the topographical feature containing the river; others hold that it meant only the land covered by ordinary winter flow. An aberrant court case (Kedron Brook) complicated matters. The Government delayed for many years the amending legislation to make the situation clear.
The issues are explained in two departmental guidelines, attached. Please note first that although authoritative at the time, these were only ever internal working documents and never officially published by the Department; and second they are now old and superseded and cannot be relied upon as a guide to current legislation.