Volume 2: Modelling Flood Inundation in the Mlazi River under Uncertainty
Dec 2003

Report No 1217/2/04

Two studies were undertaken on Flood Nowcasting for the eThekwini Metro. The first which focussed on the Umgeni River was started in early 2001 (contract K5/1217), the second was started in early 2002 (contract K8/456) and focussed on the Mlazi River. Both were completed in mid 2003. The study comprises the two projects. They appear in two volumes describing the two parts.


The Disaster Management Group of the Durban Metro attends to a range of possible disasters including fires, large road accidents, chemical spillages, building collapses in addition to floods which are the topic of this report.

The Disaster Management personnel are not trained to understand the hydraulics and hydrology of rivers. The consequence is that to inform them that a flow of say, 1200m3/s is going to occur at a particular part of a river in an hour’s time has no meaning for them, by their own admission. What they need to know is: which part of the river water course or flood plain is going to be affected by the water so that they know who to advise, where to go, and what steps need to be taken to evacuate and prepare for disaster mitigation. The way to do this is to show these lines on a map or, even better, dynamically on a GIS representation of the area in a readily updatable form. This project forms the second part of a larger initiative reported in K5/1217: Umgeni Nowcasting using Radar - an Integrated Pilot Study. The purpose of this project was to take the rainfall measured, modelled and forecast in the parent study, convert these data into river flows computed throughout the Mlazi basin and interpret these river flows as levels of inundation in sensitive areas.

The project originated from a contract between the Durban City Engineer and the Civil Engineering consulting firm, Arcus Gibb Inc, who were contracted to provide estimates of the 20 year, 50 year and 100 year flood lines in the Mlazi River Basin. The Commission extended that study under this contract to enable the information coming from the Nowcasting study to be interpreted meaningfully by Disaster Managers. So what was originally envisaged as a planning study for the City Engineer was enriched and turned into a tool of direct and immediate use in saving life, property and mitigating flood damage.

The report describes the modelling techniques employed for the Mlazi River in the context of flood analysis and flood forecasting. These techniques are applicable to an environment where there is uncertainty due to lack of historical data input for calibration and validation purposes. The process involved the integration of GIS technology, a physically based hydrological model for flood analysis, the conceptual forecasting model for real time forecasting and the hydraulic model for computation of inundation levels. The integration of modelling techniques is better explained by summarising the process into three phases:

Phase 1 Desktop catchment modelling: A continuous physically based model (US Army Corps of Engineers’ HEC-HMS Model) was set up using GIS technology. The model applied the SCS-Unit Hydrograph method for the estimation of peak discharges. Synthetic hyetographs for various recurrence intervals were used as input to the model. A sensitivity analysis was implemented and subsequently the HEC-HMS model was calibrated and peak discharges simulated. The synthetic hyetographs together with results from the HEC-HMS model were used for validation of the Mlazi Meta Model (MMM) used for real time flood forecasting.

Phase 2 Implementation of the Inundation Model: The hydraulic model (US Army Corps of Engineers’ HEC-RAS) was created using a Digital Elevation Model (DEM). Field survey was conducted for the purpose of capturing the roughness coefficients and hydraulic structures, which were incorporated into the model and also for the confirmation of the terrain cross sections from the DEM. Flow data for the computation of levels of inundation were obtained from the HEC-HMS model. The levels of inundation for the natural channel of Mlazi River were simulated under the one dimensional steady state analysis whereas for the canal overbank areas simulation was conducted under unsteady state.

Phase 3 Creation of the Mlazi Meta Model (MMM): The MMM used for real time flood forecasting is a linear catchment model which consists of a semi-distributed three reservoir cell model (Pegram and Sinclair, 2002). The MMM parameters were initially adjusted using the HEC-HMS model so that it became representative of the Mlazi catchment.

This work was completed within the project timetable and now provides an additional information source to the Disaster Management Centre of the eThekwini Metro.


There have been two facets of Capacity Building that have sprung from this project – indirect (people being exposed to the ideas and concepts but not working on the project) and direct (those people personally involved with aspects of the project). In addition, there has been a strong component of Competency Building as a direct result of the project.

Indirect capacity building.

In the Hydrology Section of Umgeni Water, where Scott Sinclair worked during 2001 and 2002, there are two PDIs who came into contact with him and his ideas on a regular basis, because they shared an open plan office with Scott. The first was Scott’s immediate superior, Percy Sithole, who was one of the original Umgeni team that got involved with this project and gave it the go-ahead. The other person was Sihle Shange, a recent graduate who worked at the desk next to Scott. They were kept abreast of the developments of the project by Scott’s dissemination of information in both informal and formal ways, via reports and presentations.

The 2002 final year class of 28 Civil Engineering Students in Hydrology at the University of Natal, Durban, contained 16 PDIs (of whom 5 are women) and 2 white women. The Project leader made frequent reference to the Flood Nowcasting project in class and repeated the oral presentation given at the European Geophysical Society in Nice in April 2002 entitled “Umgeni Flood Nowcasting – an Integrated Pilot Study”. (Incidentally, a second presentation arising from this study was also given in Nice which was a joint paper by Pegram, Seed and Sinclair, mostly put together by Sinclair using Seed’s and Pegram’s ideas – “Comparison of Methods of Short-Term Rainfield Nowcasting”). These presentations tempted students to undertake dissertations under the project leader’s supervision in the second semester of 2002 and 2003.

Direct Capacity Building.

In the second semester of 2001, a female final year student, Deanne Everitt, undertook a dissertation study under the supervision of the project leader entitled “Flood Impacts: Planning and Management”. This was an overview study with special focus on the Mlazi catchment in Durban, and was a direct spin-off from the present study. Deanne has now joined the Centre for Research in Environmental, Coastal and Hydrological Engineering (CRECHE) in the Civil Engineering Programme at the University of Natal as an MScEng student. In 2003, a Black student, Bahla Nkoko, undertook a dissertation on the effect of sampling density of raingauges on the estimation of the Area Reduction Factor. He made use of radar images which were collected for this project and therefore gained an introduction to the techniques of spatial rainfall measurement and estimation. He submitted his dissertation in October 2003.

Nokuphumula (Phums) Mkwananzi, who works as an Engineer with Arcus Gibb, and registered for an MScEng at Natal University under the supervision of the project leader. Phums came to the project via a WRC contract K8/456 called “Extension of Research on River Flow Nowcasting to include Levels of Inundation”, with particular focus on the Mlazi river which runs between the Durban International Airport and the SAPREF refinery complex. This work is reported under Part 2 of the project and was conducted by him almost single-handedly. Phums started work on the project in January 2002 (officially on the contract in April) and completed it in June 2003. He submitted his MScEng dissertation based on the project in September 2003, 20 months after starting, which is a remarkable achievement for any post-graduate student. He graduated MScEng in December 2003.

Competency Building

Because of the nature of the Research, a number of people in Umgeni Water, Durban Metro/eThekwini Municipality, METSYS/SAWS and the University of Natal have been exposed to new ideas and potentials for ameliorating flood damages; new technology has been developed and existing technology has been improved and refined. Every individual involved has grown in competence and benefitted from the project; in the long run the wider community in the region will be beneficiaries. It was expected that the successful completion of this project would encourage other Cities and Catchment Management Authorities to adopt the methodologies for the greater good of the nation. This has already happened in the follow-on WRC project: K5/1429: A National Flood Nowcasting Initiative: Towards an Integrated Mitigation Strategy.

Technology Transfer

The presentations made as a result of these projects include:

Pegram, G.G.S .and Seed, A.W., (2002). 3-Dimensional Kriging using FFT to Infill Radar Data. Oral presentation at 27th EGS Assembly, Nice, France, April.

Pegram, G.G.S., Seed, A.W. and Sinclair, D.S. (2002). Comparison of Methods of Short-Term Rainfield Nowcasting. Poster presentation at 27th EGS Assembly, Nice, France. April.

Sinclair, D.S., Ehret, U., Bardossy, A and Pegram, G.G.S., (2003). Comparison of Conditional and Bayesian Methods of Merging Radar & Raingauge Estimates of Rainfields, Presentation at EGS - AGU - EUG Joint Assembly, Nice, France, April.

Pegram, G.G.S., (2003). The Design and Implementation of a Real-Time Flood Forecasting System in Durban, South Africa. Poster presentation, EGS - AGU - EUG Joint Assembly, Nice, France, April.

Mkwananzi, Phums, Geoff Pegram & Scott Sinclair, (2003). Modelling Flood Inundation in the Mlazi River under uncertainty, 11th South African National Hydrology Symposium, Technikon Port Elizabeth, September.

In yet other ways, these flood studies benefited substantially from international exchanges of knowledge. Initiatives to present data and results led to fruitful discussions and the pursuit of new ideas. In particular, Professor Geoff Pegram was active in fostering Australian and European links, as marked by the following personal invitations:

The knowledge gained by these interactions has benefited not only the participants in Umgeni and Mlazi Flood Nowcasting, but has already realized its potential to benefit the post-graduate students working on on-going projects which are out-growths of the Water Research Commission’s investment in these projects.