What is a floodline?
A floodline is an imaginary line on the ground that denotes the edge of the water during a flood. A floodline delineation includes the analysis of the rainfall, the soils, the vegetation, the river conditions and the river flows. Hydraulic and hydrological modelling is then used to determine that line.
In some countries, the word floodplain is used, which is related to floodline but not the same. The floodplain is the area that will flood. The boundary of this area coincides with the floodline.
There is not just one “floodline” for a river but several – each relating to a different size rainfall event. The size of the flood is described by how often it is likely to occur: 1 in 50 years or 1 in 100 years. This can also be thought of as the probability in any one year. For example, if there is a stream on your property, there is a 1% chance that a 1 in 100-year flood will occur in that stream next year and a 2% chance that a 1 in 50-year flood will occur. People will often merely say “floodline” and, if so, they nearly always mean the 1 in 100-year floodline because that’s the most used floodline and it’s mentioned in South Africa’s National Water Act.
When are floodlines required?
For those of you reading this article who don’t already know for certain that they require a floodline, the below might help.
Floodlines are required in several situations. For example, any development in South Africa requires a floodline delineation to show that all buildings are outside the 1 in 100 year floodline. Another example is for mining companies, where activities might encroach on rivers or streams. In that case, regulators may insist on a floodline. Water Use Licenses are also sometimes tied to floodlines – for example, a license might be required if you plan to work near a river thus potentially disturbing the area within the floodline. Still another example is if urban areas are rezoned, especially to residential. In that case, a floodline is also required. The floodlines are also used by insurance agencies to assist with determining premiums for developments. A development that is within, say the 1:100 year floodline, will have a higher premium as compared to a similar development that is outside of the 1:100 year floodliine.
The various regulations and regulatory bodies can become quite confusing and you might still be wondering whether you need a floodline, even after reading through the examples. The simple answer is that if you or your organisation are planning to build on a property and a river or stream runs across that property, you’ll almost certainly require a floodline. If a river is within a few hundred meters of your property then you might need a floodline. In that case, it’s a good idea to contact a floodline specialist to confirm the relevant regulations for your area.
Other reasons to suspect that you might need a floodline are as follows. If you require an EIA or basic assessment (Can be thought of as an EIA “lite” ). If so, then ask your EAP (Environmental Assessment Practitioner who will manage the EIA process) if a floodline is required. Also, if you’re planning any activity in or near a river (e.g. build a bridge, pump water) then you might require a water use license which might include, among other things, a floodline. If that’s the case, talk to a water use license specialist.
What to expect from a floodline study
You can expect a floodline study to take a few weeks to complete. You can expect the cost for a simple floodline to come in below R 100 000 (2021 prices). The price can be much higher if multiple rivers are included, or various development scenarios need to be modelled (e.g. a large bridge versus a smaller bridge and an attenuation dam). Costs can also increase if complex rainfall modelling is required either because no rainfall data is available for the location or to account for climate change or if extremely large events need to be modelled, like the 1 in 10 000-year rainfall event which is required by some mining standards.
A signed floodline is one of the must-have deliverables of a floodline study and without it, the authorities (e.g. the City of Cape Town) will not accept the floodline. So, who is allowed to sign a floodline? Either a professional engineer who is registered with the Engineering Council of South Africa (ECSA) or a hydrologist who is a professional scientist with the South African Council for Natural Scientific Professions (SACNASP). Another deliverable of a good study is a report explaining the basis of the floodline, which authorities frequently request.
Still unclear?
If you’re still not sure whether you need a floodline then contact a floodline specialist such as myself or my colleagues at SRK.
