Interpretation of "1 in 100 Year Flood -Line"

Land used in development of project should fall in in the region where the possibility of flood is once in 100 Years, It is an interpretation not my know how.it may be wrong,

I agree, "once".

There probably does exist a legal definition of this somewhere, buried deeply in local legal folklore. Maybe the highest flood that occurred in the last hundred years that records, if any, were kept? Or some hypothetical construct that pertains to some allegedly representative block of 100 years? Elephino.

Yes, our laws in the U.S. have similar terms, about as well/ill defined as above. Despite the almost imponderable variables, it does make sense to try to plan ahead for the worst circumstance that might be expected to happen in a fair length of time. Perfect precision is impossible, and even good statistics might be hard to come by.

You need to contact your local weather bureau. At least in the US, they keep statistics about things like that, the worst flooding that occurred in the last 100 years (and, iiuc, they convert it into a prediction (that could be wrong)) about the worst flood to be expected in a 100 year period. (My language probably does not exactly reflect what their statistic really means.)

If all you have to do is show the 100 year flood line, that seems simple (and, it doesn't seem to restrict you from having your building on the flood side of that line). It may bother your customer(s), and at some level of risk, they may be required (or may choose to buy) flood insurance. The cost of flood insurance will, I'm sure, vary based on things like where the development is with respect to that 100 year flood line.

(In the US, there is some movement (or at least talk) about being more restrictive about allowing people to build in flood prone areas. This is because flood insurance is typically subsidized by the government. Some people seem to take advantage of that. Or, as opposed to restricting building in flood prone areas, the requirements may be raised so that they become ineligible for the government subsidized flood insurance.)

The weather statistic of any specific disaster occurring 1 in 100 years are loosely defined. Everything depends on your specific local conditions of elevation above the nearest large body of water, anticipated weather conditions and already scheduled land usage up and down stream of your location. My and your interpretation of what this means is not worth a pile of beans. You need an authorized report. Here in the USA this comes from the US Geological Survey group. I would expect that in South Africa there is likely a similar group doing these surveys.

Here's a reference for you.

http://www.bolandenviro.co.za/Romanze%20Estate%20PDF%20report/Floodline%20study/1-Report.pdf

Know how it is geustimated know how to use it, know why it was written into the Act.

My question was only about the interpretation of the words.

My interpretation is that is the high level mark where the water would most probably flow on the biggest flow in period of 100 years or the Pessimistic view - you have a 100% chance of flooding once in 100 years.

I disputed a flood line on one recent occasion because the flood line on the map ran perpendicular to the contours for a short distance (a drop of 3 m) and then miraculously missed the buildings.

In another A friend of mine contacted the authority because the stand was below the flood line (actually stretching between 1:20 year and 1:100 year} and was told that it ment there is only a 1% chance of flooding.

And in addition to the flood lines the state also have to warn these persons in writing and in some cases introduce an early warning system.

In that case I'd go along with your interpretation.

There is a high statistical probability of this level of flooding occuring during the period stated.

the entire united states was topoed by the us government years ago. The flood plains are clearly delineated and copies of the maps are usually available through your county government. The last time I had to face this was 3 months ago and the map sections ran me 50 cents an acre. Any changes were donr by the qarmy core of engineers. The flood zones are based on statistical probability not history. The current reasoning is that fire trucks etc could not reach you in an emergency. There are jurisdictions that allow encroachment for individuals but I wouldn't reccoment it. Private flood insurance is many times the price of the FEMA plan and no structures built after 1994 are covered by FEMA if they encroach. That means you or any purchaser will have to pay a large yearly premium or you can't get financing. have the Fema on to properties and the premiums are less tha 250 a year each.

Thanks Kevinm it does help.

The house has been standing for 40 years with 2 floods that inundated the bottom 10% of the property. Nothing major. The stream is naturally protected from floods due to the high retention factor of the vacant catchment area.

The new flood lines that is fought against is due to the planned development in that area of about 6000 housing units , commercial and light industries.

I will post the plan some or other time.

We started a friends of the wetland group some years ago (and won two prizes for our conservation effort.) At least we have managed to protect the stream from being channeled.

I bought a lot, in Ventura County, California, 25 years ago--It consisted of a flat lot of 1.25 acres--I spent much time trying to get building permits for it, but was told I was on a One Hundred year Flood Plain--After months of going back and forth, I found out from locals, that this designation was a way for the County to regulate building and development. Notice, that the property was at the end of a County Road, that had NEVER flooded, and was NEXT to the Water District's main wells and Pumps--Yes --This was the same County's Water pumping facility, and yet they seemed to think that there was no problem with providing water to 10,000 residents, but MY property was a potential hazard. Do your homework--I am not saying that all designations are Politically driven, but, then again, I would talk to anyone who will listen (Oh--BTW--When the County became mired in Obligations debt--Guess what? Building permits abounded , and now the area looks like a upscale sub-development- Not an unbuilt lot to be found---Be wary) Best, Mac

In Western Australia each council has maps showing the 50 yr, 80yr and 100yr flood level. They refer to these when you apply for development. I have never had the need to ask for a copy but i presume they are readily available from either the council or the water authority.

The 100 year storm has a 1% chance of occuring in any year. For computation methods, google TR-55 , HEC-RAS and Rational Method. The FEMA web site has the ablity to produce firmettes which show the FLOOD HAZARD line which may be accepted depending on use and jurisdiction.

You need to review state and local ordinances to see what's required for your area.

So many time 'laws' exist on the books that are somewhat outdated and are enforced for the wrong reasons..Flood stage in Okla rivers and stream have become outdated due to the dilligent building of dams small and large to reduce the flow of such streams. The flood stage and the highwater marks have been reduced greatly by careful planing..Nations without flood control must warn of the possible danger..In Okla. also on the 'laws of the state' that the land between the highwater marks of a running stream belong to the state, try laying that on a game warden...I ask and went to the top and they had not heard of such a thing...When you look for an answer from the powers that be you must show how the conditions have chanfged in the last 100 years

Check the national flood insurance maps. The FEMA maintains these for 1 in 100 year flood stage for flood insurance mapping. However, some counties do not keep this mapping current, and modelling for areas that lie near the 1 in 100 year flood stage is frequently needed if there is some possibility that grading could change the elevation relative to the flood stage.