African Environmental Development is one of a few South African companies with the capacity of modelling flood lines under South African rainfall conditions. This also includes the calculations needed to determine Bridge Backwater and to model the flow under a bridge during floods.

The flood lines AED produces are accepted by all authorities throughout South Africa. We have done many flood line studies throughout South Africa and in many other Sub-Saharan African countries.

What is the cost of having flood lines modelled?

Download a PDF version of our 2018/2019 pricing structure by clicking on this link: AED Quotation RSA Flood Lines 2023-2024

View AED’s flood line experience CV by clicking on this link: WG Krige Flood Line and Bridge Backwater Computation Experience 2012 – 2019

Legal Considerations

In South Africa, in terms of Section 144 of the National Water Act of 1998 (Act 36 of 1998), a flood line, representing the highest elevation that would probably be reached during a storm with a return interval of 100 years, must be indicated on all plans for the establishment of townships. The term, “establishment of townships” includes the subdivision of stands or farm portions in existing townships, if the 100-year flood lines are not already indicated on these plans, or when the land-use category of a particular portion of land is changed.

The purpose of this section of the act is to inform developers/landowners or residents/occupants/tenants/land-users of the dangers of flooding.

An example of a flood line set on an elevation map

What is a flood line?

Note: Although this section mostly refers to a 100-year flood, it is true for floods with any return interval (RI).

A 100-year flood is a flood event that has a 1% probability of occurring in any given year. The 100-year flood is also referred to as the 1% flood, since its annual exceedance probability is 1%, or as having a return interval of 100-years. The 100‑year flood is generally expressed as a flow rate (m³/s). Based on the expected 100-year flood flow rate in a given stream or river, the flood’s water level can be mapped as an area of inundation. The resulting floodplain map is referred to as the 100-year floodplain, which may be very important in how close to the stream buildings or other activities are allowed.

A common misconception exists that a 100-year flood is likely to occur only once every 100 years. In fact, statistically, there is an approximately 63.4 % chance of one or more 100‑year floods occurring in any given 100-year period. The Probability (Pe) of one or more of a specifically sized flood occurring during any return interval, exceeding the specifically sized flood severity, can be expressed as:

…where Pe is the probability, T is the return interval of a given storm (e.g. 100-year, 50-year, 20-year, etc.), and n is the number of years. The exceedance probability Pe is also described as the natural, inherent, or hydraulic risk of failure when, e.g. when referring to dams, bridges, etc. However, the expected value of the number of 100-year floods occurring in any 100-year period is 1. In other words, 100-year floods have a 1% chance of occurring in any given year (Pe = 0.01), 10-year floods have a 10% chance of occurring in any given year (Pe = 0.1), 50-year floods have a 2% chance of occurring in any given year (Pe = 0.02), etc. The percent chance of an x-year flood occurring in a single year can be calculated by dividing 100 by x.

How are flood lines determined?

The determination of flood lines is done in two steps, 1) modelling a succession of “design storms”, each of them with a specific duration (1 hour, 2 hours, 24-hours, etc.) and producing a particular discharge in m³/s and, 2) routing the highest discharge produced in Step 1 through cross sections across representative reaches of the river/streams at the study area, which then assigns an elevation on each side of the centreline of the stream to which the floodwaters would rise at that particular cross section. The flood lines are then drawn using these elevations at the cross sections as guides. The flood lines indicate the area that will be inundated during a 100-year flood event at the study area.

Initially we model a series of “Design Storms” with durations ranging from 1 up to 100 hours (or more, depending on the size and location of the catchment). This is done using methods described in the University of the Witwatersrand Dept. of Civil Engineering Hydrological Research Unit Report No. 1/72 “Design Flood Determination in South Africa”. The modelling techniques referred to above were streamlined by software developed in-house by AED. Depending on various factors built into the simulation, the model provides us with a water discharge rate in m³/s and a time-frame over which the peak of the flood would occur.

The next step is to have the banks of the river or stream surveyed in order to produce a number of cross sections through the stream or river valley. These will then be used to plot the elevation points of the flood line at that particular cross section.

Gamogara River 100-year flood lines
Several cross sections through the river valley are used to plot the flood line on either side of the river. This is the Gamogara River in the arid northwestern region of South Africa

Once a profile of the stream or river valley has been developed, the flood volume determined above is routed through the cross sections, considering factors such as slope, Mannings roughness coefficients, etc. The actual flood elevation is derived using methodology described in Report No 1/74 “A Simple Procedure for Synthesizing Direct Runoff Hydrographs” of the University of the Witwatersrand Dept. of Civil Engineering Hydrological Research Unit. This is also done using software developed in-house by AED.

The flood elevations are finally plotted on either side of the stream/river and the points are linked intelligently to produce two continuous lines, one on either side of the stream.

100-year flood lines of the Moshaweng River in the Northwest Province

The flood lines AED produce are done in accordance with official modelling techniques developed specifically for South African conditions and are accepted throughout South Africa by all Government Departments. We can produce flood lines with return intervals from as short as 2 years up to 100 (or even more) years. The flood lines are made available in Shape File, DXF/DWG format (or any other CAD format) to the client, using whichever co-ordinate system the client prefers.

The 100-year floodplain of a river in the DRC near a proposed new tailings dam
Certified Drawing
An example of a drawing, certified in terms of Section 144 of the National Water Act (Act 36 of 1998), showing the 10-, 50- and 100-year flood lines of the Klip River in Gauteng Province

Contact us for a quotation for flood lines

For a quotation for the determination of flood lines, the calculations for bridge backwater, or for more information relating to flood lines, please contact us at Alternatively, call Garfield Krige directly on +27 83 657 0560.

Even though this intersection is totally flooded with a river bursting its banks, the traffic lights still work!
Inerestingly, even though this intersection is totally flooded by a river bursting its banks, the traffic lights still work!