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.

Download a PDF version of our 2017/2018 pricing structure by clicking on this link:  AED Quotation RSA Floodlines 2017-2018

View AED’s flood line experience CV by clicking on this link:  WG Krige Floodline and Bridge Backwater Computation Experience

A Computer- Generated Image of a river, the 50- and 100-year flood lines (blue and yellow lines), a double line railway bridge and the existing surface infrastructure in the vicinity of the river

How flood lines are determined

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.