Gerrit Basson

• PhD Hydraulic Engineering
• Professor emeritus and expert consultant at Stellenbosch University and ASP Technology (Pty) Ltd

A scour hole in the pre-excavated plunge pool bed downstream of a dam can develop if the energy dissipation of the plunging jet from a spillway is underestimated. The objective of the research was to predict the equilibrium geometry of the scour hole downstream of a high-head dam to safeguard the stability of the dam foundation. A study incorporati...

This study quantified transfer losses over the 2021/2022 water year for irrigation releases from Darlington Dam into the Sundays River, which are diverted at the Korhaans Drift Weir. A one-dimensional (1D) hydrodynamic model was set up and calibrated to simulate the transfer losses which were assumed to consist primarily of evaporation and evapotra...

Rock scour caused by plunging jets could endanger the foundation of the dam. Accurate prediction of scour hole geometry is crucial for the hydraulic design, yet no universally accepted method exists despite extensive research since the 1950s. This research aims to contribute to the understanding and prediction of equilibrium scour hole geometries c...

Rock scour formation near the foundation of a dam due to a plunging jet could compromise the safety of the structure. It is therefore essential to predict the geometry of the equilibrium scour hole during the hydraulic design of the dam. Rock scour is normally predicted by analytical-empirical formulae and methods. Despite extensive research si...

The safety of dams is often endangered by rock scour formation near the dam's foundation due to a free-falling jet from the spillway plunging into the plunge pool. The prediction of the scour hole geometry is essential in the hydraulic design of high head dams. The purpose of the research is to contribute to the knowledge on predicting the equilibr...

Due to the increasing demand in potable water for the City of Cape Town Metropolitan area additional water sources was investigated. One of these sources, the Berg River Voëlvlei Augmentation Scheme (BRVAS) was investigated to abstract water during the rainy season, at a rate of up to 6 m³/s, increasing the yield of Voëlvlei dam by 23 million m³. A...

This paper highlights the progress made towards the development of guidelines for the hydraulic design of a combined fishway-canoe chute at weirs, with the aim to complete this study by the end of 2023.This study investigates a combined fishway-canoe chute that was designed as part of the Berg River Voëlvlei Augmentation Scheme (BRVAS) for the pass...

This study provides an independent review on a large automatic TOPS gate, developed by AmanziFlow Projects. The hydraulic evaluation was done with a physical model of the gate on a scale of 1:20 based on the Froude model laws. The prototype dimensions of the gate were 10 m high by 17 m wide installed on a 3 m high ogee weir. The prototype was desig...

High sedimentation rates at Thwake Dam poses a threat to the dam's design life span. The SHETRAN hydrological model was utilised to estimate the long-term sediment loads. Three sets of scenarios were considered; observed rainfall + the European Space Agency (ESA) land-use changes for (a) the year 2000, for (b) the year 2016 and for (c) the predicte...

Nqweba Dam is one of the oldest dams in South Africa and was commissioned in 1925 for irrigation. The original reservoir depth was 31 m, but over the years sedimentation has reduced the water depth to 9 m. The original storage capacity of 79 million m 3 has decreased to 44 million m 3 by 2020. The dam supplies the town of Graaff-Reinet and due to t...

The paper presents the extreme sedimentation case study of Welbedacht Reservoir, South Africa, and the respective RESCON 2 analysis for screening of sediment management alternatives. The reservoir is located in a semi-arid region characterized by increased hydrologic variability and therefore the provided flow regulation services are much needed fo...

Reservoir sedimentation can represent a major threat to water supply and should be considered when establishing a reservoir's design life. This study optimized reservoir management using a Dynamic Programming Neural Network by incorporating an optimal operation rule devised to minimize the gap between water releases and water irrigation demands. Th...

The Nqweba Dam is one of the oldest dams in South Africa and was commissioned in 1925 for irrigation. The original reservoir depth at the dam wall was 31 m, but over the years sedimentation has reduced the water depth to 9.8 m. The original storage capacity of 79 million m ³ has decreased to 46 million m ³ by 2020. The dam supplies the town of Graa...

A 1:60 scale physical hydraulic model study was used to investigate and optimize the proposed design of the left bank additional spillway of the Corumana Dam in Mozambique. A portion of the reservoir, the Piano Key Weir (PKW) control structure, which was 3D printed, the chute, the stilling basin and the downstream river were constructed from concre...

Water is at the core of sustainable development and is critical for socio-economic development. Inconsistent rainfall in the recent past has affected water security for basic consumption as well as economic use within the Limpopo province. The Musina-Makhado Special Development Zone (MMSEZ) a State Owned Company (SOC) has been collaborating with va...

Sediment accumulation in reservoirs leads to a loss in water storage capacity, potential contamination or dam safety issues, and low assurance of water or power supply. Typical sediment management strategies such as watershed erosion control, sediment traps (check dams) or sediment bypass systems reduce incoming sediment loads but do not directly a...

This paper discusses the detail design of the Goodenough Abstraction, Desilting and Pumping System on the Umkhomazi River, KwaZulu-Natal (KZN). This system will form part of Umgeni Water's proposed Lower Umkhomazi Bulk Water Supply Scheme feeding up to 130 Mℓ/d into the existing KZN South Coast System. A hydraulic model at a scale of 1:40 (undistor...

The proposed ungated spillway of the Polihali Dam was optimised by a 1:40 physical model study for the Matla a Metsi Joint Venture (MaMJV) as part of Phase II of the Lesotho Highlands Water Project. The floods predominantly approach from the left side where a curved wingwall guides the flow over a 98 m long ogee weir with a 2075 masl Full Supply Le...

Pongolapoort Dam is a double curvature arch dam on the Phongolo River in KwaZulu-Natal. The dam was completed in 1973 and has a wall height of 89 m, making it the third-highest dam in South Africa. Flood control is provided by a centrally located uncontrolled ogee spillway and three radial gate-controlled chute spillways with a flip-bucket energy d...

Armorflex is an articulating concrete block erosion protection measure that has been used as an alternative to riprap for many years. Even though extensive research and hydraulic testing have been conducted on Armorflex, the principal constraint on the use of concrete blocks has been the lack of information on prototype performance. Furthermore, th...

The effect of varying vortex settling basin (VSB) outlet configurations on the removal of sediment particles >75 μm was computationally investigated using ANSYS Fluent's Euler-Lagrange approach, which incorporated supervised optimization and was validated by physical modeling. A rectangular centroidal outlet was established with a low outlet weir a...

This paper evaluates the possible future impacts of climate change on sediment yield by incorporating predicted future climate data and a physically-based hydrological and sediment yield model, SHETRAN. The Nqweba Dam catchment (3681 km2) on the Sundays River, located in the semi-arid region of the Eastern Cape of South Africa, was identified for t...

This presentation evaluates the possible future impacts of climate change on sediment yield by incorporating predicted future climate data and a physically-based hydrological and sediment yield model, SHETRAN. The Nqweba Dam catchment (3681 km2) on the Sundays River, located in the semi-arid region of the Eastern Cape of South Africa, was identifie...

Boksburg Lake in the City of Johannesburg, South Africa, used to be a very popular and important public amenity and has now decayed into an unsightly, unsafe and contaminated water body. Of greatest concern to the local residents is the poor personal safety, the unpleasant odour of the lake water and aesthetically unattractive litter that lines the...

Boksburg Lake in the City of Johannesburg, South Africa, used to be a very popular and important public amenity and has now decayed into an unsightly, unsafe and contaminated water body. Of greatest concern to the local residents is the poor personal safety, the unpleasant odour of the lake water and aesthetically unattractive litter that lines the...

Bridge pier scour was simulated by a coupled fully 3D hydro-morphodynamic model and validated against 24 experimental datasets for a combination of different pier shapes, approach velocities and sediment bed material. The proposed numerical model adopts an Eulerian multi-fluid model to account for rolling and saltating particles and computes bed sh...

Diverted sediment causes a wide range of problems at small diversion works. The difference in water levels between low flows and floods, low investment capital and presence of cohesive sediment and fine non-cohesive sediment coupled with a requirement for a high level of supply assurance makes the design of such hydraulic structures complex. Vortex...

The erosive power of a free-falling high-velocity water jet, flowing from a dam spillway, could create a scour hole downstream of the dam, endangering the foundation of the dam. Despite extensive research since the 1950s, there is presently no universally agreed method to predict accurately the equilibrium scour depth caused by plunging jets at dam...

The erosive power of a free-falling high-velocity water jet, flowing from a dam spillway, could create a scour hole downstream of the dam, endangering the foundation of the dam. Despite extensive research since the 1950s, there is presently no universally agreed method to predict accurately the equilibrium scour depth caused by plunging jets at dam...

Executive Summary More than a decade ago, a SA Water Research Commission (WRC) study was completed by Stellenbosch University and guidelines were generated for the hydraulic design of river abstraction works: "Considerations for the Design of River Abstraction Works in South Africa" (Basson, 2006). The guidelines have been used in the designs of ma...

Piano key weirs (PKWs) are nonlinear hydraulic structures that exhibit unsteady hydraulic phenomena. A study incorporating both physical and numerical modeling was undertaken to identify instabilities that are generated by flow. Several excitations were identified and a portion of their nature is characterized in this paper. These include a system...

The existing Sir Lowry's Pass River downstream of the N2 Bridge is relatively small with a very wide floodplain that is inundated during major floods and makes new development unfeasible. It is proposed that the main river channel is rehabilitated to accommodate a flatter 1:230 slope, a 84 m width and 4 drop structures that consist of 38.5 m long o...

This paper aims to provide design guidelines for optimising a postulated configuration for a sediment flushing system of a low-level outlet at a dam for control of reservoir sedimentation to protect the hydropower intakes. The postulated design, which comprises of a hybrid low-level outlet conduit and low weir, is optimized by means of numerical an...

Future climate change could increase flood peaks by 50% to 100%. Dam spillways will therefore have to deal with large unit discharges. Flaring Gate Piers (FGP), which can deal with high unit discharges were developed in China as energy dissipation/aeration structures, with the flares located upstream of the RCC steps on a smooth ogee crest. This re...

This paper investigates local sediment flushing under pressure at dams through a low-level outlet for the control of reservoir sedimentation to protect the hydropower turbines. A state-of-the-art three dimensional fully hydrodynamic computational fluid dynamic (CFD) model was developed with user defined functions which link the sediment mass balanc...

The Holsloot Diversion Works is required to replace a number of temporary rockfill diversion weirs as part of a more sustainable river management plan. It is located on the Holsloot River, 9.7 km southeast of Rawsonville and 11 km downstream of Stettynskloof Dam. The paper covers the weir's hydraulic design and hydraulic model testing, as well as g...

The erosive power of a free falling high-velocity water jet, flowing from a dam spillway, could create a scour hole downstream of the dam, endangering the foundation of the dam. The main objective of the study discussed in this paper was to compare the different equilibrium scour hole geometries for different bed rock materials, for a range of tail...

A Flaring Gate Pier (FGP) is a type of energy dissipator that was developed and applied on dam spillways in China. These FGP's are constructed by building triangles on the sides of piers that will contract the flow of the water passing by the sides of the piers. This sudden contraction causes an increase in the velocity of the water which in turn c...

A vortex settling basin (VSB) offers a promising alternative to conventional sediment settling structures, such as sand traps, for the removal of fine non-cohesive sediment for potable water use at small river abstraction works of less than 100 l/s pump capacity (7.2 Ml/d at 20 h/d). The hydraulic design of a suitable VSB was carried out by numeric...

Sediment traps are structures designed to allow the suspended sediment load, which enters river abstraction works or diversion canals, to deposit whilst relatively clean water passes through. The deposited sediment is then removed by making use of gravitational flushing. The sediment traps discussed in this paper are settlers and sand traps. The fo...

River abstraction works designed for abstracting water for potable use from alluvial rivers in Africa requires a high assurance of supply. Abstraction works that are located in alluvial rivers are susceptible to dealing with coarse sediment and therefore need to consider sediment management features in order to ensure structural and operational sus...

Reservoir sedimentation is an ongoing critical concern worldwide with a recorded global average of 33% of reservoir storage capacity already been lost therefrom, which is expected to exceed 50% by 2050 (i.e. not considering new dam projects) (ICOLD, 2009). Drawdown or pressure flushing through low-level dam outlets below hydropower intakes are comm...

River abstraction works designed for abstracting water for potable use from alluvial rivers in Africa requires a high assurance of supply. Abstraction works that are located in alluvial rivers are susceptible to dealing with coarse sediment and therefore need to consider sediment management features in order to ensure structural and operational sus...

Sediment deposition in reservoirs can affect flood levels, drainage from agriculture land, the firm yield of a dam, hydropower operation and navigation. Turbulent suspended sediment transport is the dominant mechanism of sediment transport through most reservoirs. This paper focuses on the simulation of flow and sediment transport patterns utilisin...

Pier scour has been cited as the main mechanism responsible for the failure of bridges spanning rivers. Despite extensive research since the 1950s, there is no universally agreed upon procedure to accurately predict the equilibrium scour depth. Experimental data was generated by 48 tests with four flows and three pier shapes to evaluate the capabil...

Stepped spillways are one of the oldest spillway designs dating back to 500 B.C. With technical advances in Roller Compacted Concrete (RCC) construction, the stepped spillway has become increasingly popular over recent decades. However, the use of this spillway is limited to a maximum safe unit discharge of 25 m²/s due to the risk of cavitation. In...

The determination of extreme flood levels in an estuary should consider river floods and waves penetrating the estuary. The Breede River estuary in South Africa, 50 km in length, is discussed as case study in this paper. A two-dimensional hydrodynamic model Mike21C of the DHI Group of the 50km estuary (including the estuary mouth) was set up and ca...

In order to develop an integrated water resources development plan, accurate prediction of sediment yields in a basin is important. The study area is one of the four major sub-basins in the Eastern Nile Portion of the Nile basin. It is located in the southernmost portion of the Eastern Nile Basin contributing about 26 billion m 3 of water every yea...

Regional potable water supply schemes require a high assurance of supply and the river abstraction works (pump station) has often been found to be the most critical component in the scheme. River abstraction works are hydraulic systems which consist of different components, such as a weir, boulder trap, gravel traps and pump canals, and is used to...

For 60-odd years, dredge spoil was artificially dumped in the natural course of the uMfolozi River (near the mouth of the St Lucia estuary), by then conservation managers as it was thought that silt exacerbated from the canalisation of the uMfolozi River was the biggest man-induced risk to the 350km2 estuarine Lake St. Lucia system in South Africa....

The Orange River Re-Planning Study (ORRS, 1997) recommended that the proposed Noordoewer/Vioolsdrift Dam (NVD) site be further investigated as possible dam development downstream of Vanderkloof Dam, in the lower reach of the 2200 km long Orange-Senqu River. The Lower Orange River Management Study (LORMS, 2005) recommended that a re-regulating dam b...

This paper discusses the possible impacts on the Orange River Estuary, of the proposed Noordoewer / Vioolsdrift Dam (NVD) project on the lower Orange River located 300 km upstream of the estuary. The flow patterns and sediment dynamics at the estuary were evaluated as supporting information to the feasibility study of the proposed NVD site. A stati...

Sediment deposition in reservoirs is an important research topic in engineering practice. Reservoir sedimentation has the potential to affect flood levels, drainage for agricultural land, pump station and hydropower operation as well as navigation. This paper describes the development of a coupled fully three-dimensional (3D) numerical model for th...

This paper investigates the hydraulics of raising the existing diversion weir and canal on the Holsloot River. The key findings of this study are that the raised Holsloot diversion weir (0.3 m) has a significant benefit to increase the diverted flows when the Holsloot River flows are between 30 and 120 m 3 /s. The raised weir will have limited impa...

This paper discusses the optimization by physical modelling of the hydraulic design of the stilling basin for the proposed Xhora Dam in the Eastern Cape (currently under construction). The key hydraulic components of the dam are: a side channel spillway 25 m in length, a discharge carrier consisting of a trapezoidal concrete lined chute (5 m wide a...

This paper discusses the hydraulic design of a proposed new 13 km long flood levee in Malawi. The design addresses an avulsion event which included the wash away of the railway line along the lower Ruo River by the extreme flood of January 2015 and led to the river being diverted permanently in a westerly direction towards the Shire River, away fro...

Sediment accumulation behind dams reduces storage capacity and threatens the intake works of hydropower reservoirs, which can lead to turbine damage and generation losses. In order to protect the hydropower intake, a low-level flushing outlet placed below the hydropower intake can be used to scour out deposited sediment under pressure. Pressure flu...

The proposed Smithfield Dam is located 187 km upstream of the river mouth and the estimated total sediment load trapped in the reservoir is 1.27 million t/a, of which the sand fraction is about 10%. Reservoir sedimentation prediction of the proposed Smithfield Reservoir was carried out by using a two dimensional hydrodynamic model for a 100 year pe...

Hydrological modelling (daily rainfall-runoff-sediment transport) was carried out to predict the sediment yield of the proposed 164 m high Polihali Dam in Lesotho as part of the Phase II detail design of the Lesotho Highlands Water Project (LHWP). Land use scenarios were considered for the years 2009 and 2070. Following model calibration, the long...

The objective of this paper was to evaluate the expected future land use change impacts on the long term sediment yield of a catchment in Southern Africa. The Usuthu River sub catchment (Ngwempisi catchment) was selected as case study to produce a sediment yield map for the current and possible future land use scenarios. A physical process based ra...

Piano Key Weirs generate complex flow patterns in their vicinity. These patterns are not only variant in space but also fluctuate over time. Time variant dynamics in the inlet key originate from the presence of separation bubbles which form at the entrance edge to the inlet key. This turbulent bubble periodically sheds vortices which cause regular...

Roberts splitters is an effective means of energy dissipation for dam spillways. Roberts' (1943) standard unaerated splitter design procedure is, however, limited to a spillway head (H) of 3.0 m (q ~ 12 m²/s). In order to avoid cavitation at higher design spillway heads, this study investigated the artificial aeration of the flow by local air vents...

Stepped spillways are one of the oldest spillway designs dating back to 500 B.C. With technical advances in Roller Compacted Concrete (RCC) construction, the stepped spillway has become increasingly popular over recent decades. However, the use of this spillway is limited to a maximum safe unit discharge of 25 m²/s due to the risk of cavitation. In...

The key aim of the study was to determine whether a hydro-morphodynamic model, recently developed by Sawadogo (2015), could be applied to simulate local scour around a cylindrical bridge pier. ANSYS Fluent solved flow equations for the Eulerian multiphase and k-e turbulent models which were coupled with sediment transport and bed deformation sub-mo...

Boegoeberg Dam near Groblershoop on the Lower Orange River in South Africa is one of the oldest dams in the country, commissioned in 1931. The dam is only 10 m high and was constructed for irrigation water supply. The original storage capacity of 35 million m 3 was soon lost due to sedimentation and reservoir surveys of 1959, 1983 and 1999 indicate...

The Berg River Dam is equipped with the first multi-level draw-off environmental flood release outlet in South Africa and can release flows up to about 200 m3/s. The outlet is controlled by a radial gate at the outlet end, and is protected by a vertical emergency gate near the inlet end. Commissioning tests of the emergency gate in 2008 found that...

Liu's theory of 1957 to estimate the point of incipient motion, derived assuming a flat bed slope and uniform flow, states that for large particles the movability number (defined as the ratio of the shear velocity to the settling velocity), is constant. Data collected from 12 undistorted and unscaled laboratory tests was used to investigate the app...

Extreme flood hydrology in Namibia for the past 30 years has largely been based on the South African Department of Water Affairs Technical Report 137 (TR 137) of 1988. This report proposes an empirically established upper limit of flood peaks for regions called the regional maximum flood (RMF), which could be associated with an annual recurrence in...

SYNOPSIS The Berg River Dam is equipped with the first multi level draw off environmental flood release outlet in South African and can release flows of up to 200 m 3 /s. The outlet is controlled by a radial gate and is protected by a vertical emergency gate. Commissioning tests of the emergency gate in 2008 found that large volumes of air were exp...

Particle image velocimetry scalar measurements were carried out on the body of a stably stratified density current with an inlet Reynolds number of 2,300 and bulk Richardson number of 0.1. These measurements allowed the mass and momentum transport between the current and the less dense ambient fluid to be investigated. Reynolds stress, Reynolds flu...

SA Water Research Commission Study on Freeboard for dams in collaboration with the South African Committee on Large Dams

SA Water Research Commission Project Vol1 report Appendices on Freeboard for Dams

SANCOLD Guidelines for Freeboard for Dams since 2011

WRC Sediment yield: GIS map files

Sediment yield 2010 research: pdf maps generated from GIS

WRC Sediment yield prediction: Excel file to calculate the sediment yield

Regional Sediment Yield Prediction for Southern Africa

Worldwide experts will convene in South Africa in September 2010 to deliberate on the growing problem of river sedimentation. High rates of sedimentation in many reservoirs and better care of long term sustainability have emphasized the importance of reservoir sedimentation. Based on the ICOLD World Register on Dams, hydropower dams make up 81.5% o...

This paper describes two-dimensional mathematical modelling of reservoir hydrodynamics and cohesive sediment transport processes, using advection-dispersion theory. Creating such a numerical model involves setting up a suitable curvilinear grid and requires data of the bathymetry, recorded inflows and water levels. It also requires sediment charact...

2D numerical modelling of the breaching process of the Klein River Estuary in South Africa was carried out. The model was calibrated on field data and performs reasonably well, and is able to simulate the ebb and flood channels that form upstream of the mouth. The focus of the simulations was to determine the effectiveness of flushing of sediments...