Flood Prone Land Development

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So you’ve been told your property is in flood prone land? Development within flood prone land can often be confusing and complicated. This page aims to provide a simple summary of some of the common requirements of undertaking a development within flood prone land in NSW as well as some of the key concepts and terminology used in floodplain management. It is by no means comprehensive and expert advice should be sought for individual circumstances.

It is important to note that flooding as discussed here does not involve household flooding such as a roof or pipe leaking or overflow from washing machines, tubs etc.

Some Key Flood Concepts

When rainfall occurs, it will tend to soak into the soil or run in to the storm water drainage system, however if the rainfall is heavy enough or lasts for a long time, the water will begin flowing downhill and concentrating into flow paths such as creeks and rivers. These flow paths can then become overwhelmed and the water spreads out, creating a flood.

While this is a fairly simple concept, there are a number of misconceptions about flooding. For example, many people assume that flooding occurs only along the edges of rivers or near the coast. In reality, flooding can occur in almost any location and the way we have developed our cities has exacerbated the problem. Another key misconception is the relative danger of flooding, some people assume that if you can swim then you will be Ok to wade through flood waters, or if you have a four wheel drive then you can drive through it. This is often a fatal mistake and it is never safe to enter flood waters. More than 50% of the flood related fatalities in Australia are as a consequence of people voluntarily entering flood water.

The size of a flood is generally spoken of in terms of the frequency of it occurring and it is important to understand how this works. Flood frequency is all about probability. The 1 in 100 Year flood is often mentioned in both the media and planning documents. Some people take the “1 in 100 year” to mean that a flood will occur every hundred years, however, what it really means is that there is a 1% chance of a flood this size occurring in any given year. In reality the “1 in 100 year” flood can occur multiple times per year, or not occur for a thousand years. A better way to describe a flood is to use the percentage chance that it will occur in a year. So the “1 in 100 year” is the 1% AEP flood (annual exceedance probability). This AEP terminology is now the correct way to describe a flood event.

Planning arrangements for flooding are usually associated with a 1% AEP flood. It is important to know what this means in terms of time scales and what we are planning for. Quite often a residential developer will be surprised that their Council says their property is flood affected and I have often heard something along the lines of “I’ve lived here for 30 years and have never been flooded”.  Lets assume that your property is only flooded by the 1% AEP event, in that case there is only a 26% chance that a flood of size would have occurred during the 30 years. If you live in the same house for 80 years, there is only a 55% chance that the house will be flooded or alternatively, there is a 30% chance that it would be flooded twice in that time. So if you haven’t been flooded, it doesn’t mean that it can’t and won’t happen.

The 1% AEP flood is never the largest flood that can occur. In both the 2011 Queensland floods and 2007 Newcastle Flood (Pasha Bulka storm), many places experienced flooding that has a probability of occurring 0.1% in any given year (or a 1 in 1,000 year flood). Similarly, in 1867, the Hawkesbury-Nepean Rivers flooded to a level that has a 0.5% chance of occurring in a given year. The largest possible flood is generally referred to as the Probable Maximum Flood (PMF) these floods have a much smaller probability of occurring, however they are generally far more severe than a 1% AEP flood.

Flooding can occur in three different ways:

  1. Storms along the coast cause large waves (coastal flooding)
  2. Rainfall causes a river or creek to water level to rise and break its banks (mainstream flooding)
  3. Rainfall causes water to flow overland in undefined flowpaths (overland flooding)

Coastal flooding is often treated differently and the focus of this page is on mainstream flooding and overland flooding. In many ways mainstream and overland flow flooding is similar, however there are a few critical differences that you need to be aware of:

  • Mainstream flooding is generally well defined and understood, whereas overland flow paths are more difficult to locate and in much of Sydney (and other parts of NSW) the overland flowpaths have been developed and the flowpaths now weave their way through and around houses within suburbs
  • Mainstream floods often have some warning that they will occur, this is especially true of rivers in Western NSW where it may be known for several days that a flood will occur prior to it arriving, in Sydney the warning time is generally less than 6 hours. Flooding through overland flowpaths will occur far quicker and usually at the same time as the rainfall is occurring, there will be little to no warning time and emergency services won’t have time to prepare and issue evacuation notices
  • In many places mainstream flooding can completely submerge houses and is generally far deeper than overland flowpath flooding

If you consider how a creek or river is formed, you can understand the difference between overland and mainstream flooding. Rivers are formed by water eroding the soil or rock to form a channel, the more land upstream of a point means more water contributing during rainfall and therefore more energy to erode a channel. So generally, in the upstream parts of a catchment, there is relatively less water and not enough energy for that water to scour out a channel, however the water still needs to flow downhill somehow… this is how an overland flowpath occurs. As you move downstream there is more water, more energy and therefore a channel is developed, when that channel is overwhelmed in extreme rainfall, you have mainstream flooding.

Is my development within a floodplain?

It is probably a fairly safe assumption that those reading this have already been told that their property is considered flood prone land, which is to say that it is within a floodplain. However it is always good to verify this from official information. There are two ways to determine whether your development is officially within a floodplain and it is important to do both. These are:

  • Request a copy of your Section 149 Certificate, often referred to as the S149. This will contain details of flood constraints on your property. Councils usually offer a S149(2) and a more detailed S149(5) which is generally more expensive but has more detail. In some cases the flooding information is in the S149(2) but in others it is only in the S149(5). Councils are constantly updating their information, so if you have a S149 from when you bought the property 10 years ago, there is a chance that Council has since updated the S149 to place flood constraints on it.
  • Look up your property on Councils Local Environment Plan (LEP) maps. An LEP is used to define what type of development is allowed in different locations (land use zones). However they also contain maps of flooding and other constraints. These are free to access online, usually through the Councils website which will redirect you to a page called AUSTLII. Sometimes the maps can be a bit tricky to find inside the AUSTLII page, however they are there if you look through the table of contents. These are usually in a PDF format so you will then have to locate your property by comparing it to a street directory or Google maps. Some Councils now have a Google maps type website where you can search directly for your property.

Often Councils also offer a “flood certificate” which contains more details than the S149 or LEP maps. These flood certificates can provide useful information for the planning of your development.

Councils generally have a good understanding of which locations within their area are subject to flooding. This understanding has been developed from a number of sources over the years, primarily through undertaking “flood studies” and also their experience of historical floods. The flood studies are generally funded  by the Council and NSW State Government and are undertaken by consultants. Most completed flood studies are available for the public on Council’s websites. Note that flood studies are highly technical documents that detail hydrological and hydraulic modelling that has been developed in order to simulate rainfall and flooding. These models determine where flooding occurs and what are the associated depths and velocities for a particular AEP.

What Should I do?

As soon as you start to consider a development, you should try to find out what the requirements are in terms of flooding, as well as any other potential constraints such as ecology or bush fire. Please note that the earlier you know what your constraints are, the less it will cost you. For example, if you design your new house and then find that the flood constraints require a re-design then you will be spending far more money than if you had incorporated those flood constraints in the original design.

The best source of information will always be your local Council and it is always better to speak to them upfront than to submit a DA and then have it rejected. Councils have a number of documents explaining their requirements around flooding, and most will be more than happy to discuss these with you and spell out what you need to do, try to get this in writing so that you can refer back to it or pass it along to a consultant if necessary. This is something that anyone can do and you do not need to be an engineer, if you are especially nice and also lucky, the Council may even do the work that they would otherwise ask you to engage a consultant for.

It is worth noting that in professional consulting “time is money” has a very literal meaning. We have often been contacted by would be developers that have found out that their lot is flood prone and immediately called a consultant to sort it out. That consultant (i.e. us) will then call Council and spend time working out what exactly is required, this time will then be charged to you. Alternatively, a consultant may assume what is required from Council and undertake a costly study that may have been avoided had they spoken to Council. In either case it would have been better to talk to Council directly.

Some general requirements for development within floodplains

If you they have determined that your property is within the floodplain, Council may ask for some variation of the following types of study:

  • A flood study: This will involve some form of hydraulic modelling to determine what the flood levels are on site. In many cases Councils have already undertaken a large scale study (as described above) and this will not be required, in other cases you will need to engage a hydraulic engineer to do this for your site.
  • A flood impacts analysis: This study involves using a hydraulic model to determine whether your proposed development has an impact on neighbouring properties. For example, if you build a house across a flow path, not only will your house get flooded, it will slow or stop the water from flowing upstream and potentially flood, or increase flooding of your neighbours. Most Council have rules that a development must have either no impact, or a minimal impact on surrounding properties. In these studies the existing case is modelled and then the development is put into the model and the difference in water level is calculated.
  • A flood risk study: A flood risk study generally involves examining the impact of the flooding on the people that will be living or working within the proposed development. It will look at whether the occupants have enough warning time to safely evacuate and if not whether there is a safe place to shelter. Other considerations will include how often an evacuation is triggered or if the occupants need to shelter in place, is the length of time reasonable.

In some cases these studies can be undertaken fairly quickly and cheaply and others may be expensive and take a long time. The time and cost is partially dependent on the complexity of the development but is generally much more reliant on the location of the development in the floodplain and the amount of data that is readily available.  At a minimum, expect that each study will take two days worth of work and it will take at least two weeks for all of the required data to be compiled and a report prepared. Feel free to contact us for advice on the associated costs.

It is important to note that different Councils have different requirements for each of these studies and what may be acceptable for one Council may be inadequate for another. Councils set out their requirements in their Development Control Plans (DCP), Councils may also have a number of other documents that are outside of the DCP but are also enforceable (such as a flood prone land policy). You should ensure that when you engage a consultant to undertake a study that they guarantee their work will meet the requirements of the relevant Councils DCP and other floodplain management documents. Again – it is important to determine the exact requirements from Council prior to undertaking any work.

In addition to, or possibly in lieu of, these studies, Council may set out the constraints for your development. A common one will be that all habitable floors should be above the level of the 1% AEP flood plus 500mm freeboard. Freeboard is added to account for the possibility of errors in the model as well as other factors such as waves caused by passing cars or wind. The constraints will also be determined on your development type, for example a nursing home will have far stricter constraints than a warehouse. This development type is often referred to as the “Land Use” in the DCP.

How does the Exempt and Complying Development Codes SEPP apply?

The Exempt and Complying Development Codes SEPP (often referred to as the Codes SEPP or CDC) is a NSW State law that allows certain types of development to occur without the need to go through the local Council, instead the development is assessed by a Private Certifier. The aim of the Codes SEPP is to reduce the workload on Council by eliminating the need for them to look at small or uncontroversial developments and at the same time speed up the process for the applicant.

Flooding is considered within the Codes SEPP under Clause 3.36(C). Generally what this entails is the Private Certifier will determine whether your lot is considered a “flood control lot” using the same sources as described earlier. If the lot is a flood control lot, the requirements for the development are largely similar to what a Council will require and also refer back to the Councils requirements. Additionally, the Certifier will request that you engage a hydraulic engineer to determine whether the proposed development is within; a flood storage area, a floodway area, a flow path, a high hazard area or a high risk area. If the hydraulic engineer finds that it falls into any of these categories than the development will not be considered acceptable under the Codes SEPP and you will need to submit a development application to your Council.

For these reasons trying to get your development through using the Codes SEPP is not always advisable where your property is within a floodplain. The Codes SEPP should not be seen as a way of cutting Council out of the process if you do not like the advice that they are giving you, especially as there is a good chance your development will end up back there.

What are the key documents?

There are a number of key documents that govern the way that floodplain management and development is undertaken in NSW. For your interest we have provided a brief description and link for each below.

The Floodplain Development Manual is a NSW state government document that sets out both the policy position of the government on flooding and also the floodplain management process that Councils need to undertake. If you are interested in further detail on how floodplain management operates in NSW, or are involved in a Floodplain Risk Management Committee, it is an interesting read.

Australian Rainfall and Runoff is essentially the textbook for how flood calculations are undertaken by engineers across Australia.

Managing the Floodplain is a guide to best practice for floodplain management and provides supporting and more up to date theory to the Floodplain Development Manual

ABCB Construction of Buildings in Flood Hazard Areas is the standard required for construction of buildings within the floodplain. It details requirements such as the forces that buildings must withstand.

The NSW SES has a number of publications that will help you prepare for and give guidance on how to respond to flooding