Floodline map Johannesburg: how to do a floodline check before you buy

Johannesburg has no great river, which leads many buyers to assume flooding is not a local issue. The city is in fact threaded with streams and spruits — the Jukskei, the Braamfontein Spruit, the Klein Jukskei, the Klip and their many tributaries — and summer thunderstorms can turn them from trickles into torrents within an hour.

A floodline check shows whether a property lies in the path of that water. It takes minutes, and it affects safety, insurance, building rights and resale.

What a floodline is

A floodline marks the edge of the area expected to be covered by water in a flood of a given size. Floods are described by their statistical frequency:

• the 1:50 year floodline — a flood with a 2% chance of occurring in any year;
• the 1:100 year floodline — a flood with a 1% chance in any year.

"1:100" does not mean once a century. It means a one-in-a-hundred chance every single year, and two such floods can occur in successive seasons.

The 1:100 year floodline is the one planning law is built around. Under national water legislation, township layouts must show the 1:100 year floodline, so that people are made aware of the risk.

How to read a floodline map

A floodline map of Johannesburg shows these lines along watercourses, laid over property boundaries.

When you look at a property:

1. Does a floodline touch the erf? If no line comes near, the property is outside the mapped flood area.
2. How much of the erf is affected? A line clipping a back corner is very different from one that crosses the house.
3. Which side is the building on? The area between the watercourse and the floodline is the flood-prone part.
4. Where is the access? A house above the floodline with a driveway below it can still be cut off.

On checkproperty.co.za, the property report for a Johannesburg erf includes site constraints, with floodlines among them, alongside dolomite and soil information. The layer map shows the layers across a wider area for plan subscribers.

What a floodline means for building

Land within the floodline is restricted for good reason.

• New buildings are generally not permitted within the 1:100 year floodline, and the municipality will not approve plans for habitable structures there without specialist justification.
• The area still counts towards your erf size, but it is not buildable. A 2,000 m² erf with 600 m² inside the floodline is, for development purposes, a 1,400 m² erf at best.
• Building lines may be applied from the floodline or the watercourse.
• Fill and walls that would alter the flow of water are restricted, because they push flooding onto neighbours.
• Environmental rules may apply to work in or near a watercourse or wetland, requiring authorisation before anything is done.

For developers, a floodline is one of the first things to plot, because it directly reduces yield. See what can I build on my property.

Floodlines and existing houses

Many older Johannesburg houses were built before today's rules, and some stand within what is now the mapped flood area. If you are buying one:

• Ask about flooding history. Sellers must disclose known defects; ask the question directly and in writing.
• Look for evidence — tide marks on walls, new skirtings, raised electrical points, sandbags in the garage.
• Check the insurance position. Ask an insurer for a quote before you sign, and ask specifically about flood cover and excesses.
• Check the bank's attitude. Lenders may be cautious.
• Check for approved plans. Additions built inside a floodline are less likely to have them. See unapproved structures.

Why floodlines change

A floodline is a calculation, not a permanent feature of the land.

• Upstream development adds hard surfaces, which sends more water downstream faster. As the catchment urbanises, flood levels rise.
• Blocked culverts and bridges cause water to back up.
• Changes to the channel — erosion, silt, walls built by neighbours — alter where water goes.
• Rainfall patterns shift over time.

A floodline determined decades ago may understate today's risk. For a serious purchase or development beside a watercourse, a current determination by a qualified engineer is worth commissioning.

Beyond the mapped floodline

Not all flooding comes from rivers.

• Stormwater overwhelms drains in heavy rain and floods low-lying streets and properties well away from any stream.
• Overland flow follows natural low points, which may run straight through a garden.
• Wetlands and seepage areas stay waterlogged and carry their own restrictions.

Look at the lie of the land. A property at the bottom of a slope, or in a dip in the road, takes water from everything above it. See floodlines and wetlands: the other risk overlay.

A floodline check in five steps

1. Search the property and open its report.
2. Look at the site constraints for any floodline affecting the erf.
3. On the map, note the nearest watercourse and how the erf sits relative to it.
4. Compare with aerial imagery over several years: has the channel moved, has upstream land been developed? See reading aerial imagery history.
5. If a floodline touches the erf, get professional advice before you commit.

When to bring in an engineer

• the floodline crosses or comes close to buildings;
• you intend to build, extend or subdivide on a property beside a watercourse;
• the existing floodline determination is old;
• there is a history of flooding;
• a bank or insurer asks for a flood certificate.

A civil engineer experienced in hydrology can determine the floodline for the site and certify levels.

Floodlines as part of the wider picture

Flood risk is one environmental check among several. Ground conditions are another. See dolomite risk and the overview in property risk reports.

Frequently asked questions

Can I build inside the floodline if I raise the house? Not as a matter of course. It needs engineering justification and municipal approval, and is often refused.

Does being outside the floodline mean no flood risk? No. Stormwater and overland flow can still affect a property.

Will a floodline affect resale? It can narrow the pool of buyers and affect finance and insurance.

The bottom line

A floodline check tells you whether water is a factor before you fall for the garden running down to the stream. If a line touches the erf, find out how much land it takes, whether the buildings are clear, and what an insurer says — before you sign.

Run a floodline check on a property.

This is general information, not engineering or legal advice. Floodline information is indicative; consult a qualified engineer for a floodline determination.