LOCATION:

Latitute: - 25⁰ 08’ 13"

Longitude - 31⁰ 56’ 42"

Catchment Area size: 5713 Km²

INTRODUCTION

Gauging station X3H015 is situated in the Sabie River and approximately 5.6 Kilometers downstream of the Lower Sabie Camp in the Kruger National Park. Being the last gauging station in the Sabie River before entering Mozambique, it plays an important role in the management of the river especially during flood occurrences when early warning to the Mozambique authorities is crucial. The station is therefore equipped with a satellite communication system in order to obtain real time data and to be pro-active in the adhering of international operating rules.

The flood event of February 2000 was the result of rainfall figures that exceeded 400 mm over a period of 48-hours in the upper catchment area. A Tropical Low Pressure System caused rainfall patterns to be extremely intense and because of an already saturated catchment, run-off has been described as the worst in living memory. A flood peak of 7100 m3 /sec was measured which has been determined as an event with a return period greater than 100 years. Since the flood peak was measured as 9.361 meters, all measuring equipment in the pump house was submerged and therefore no complete hydrograph is available. Although no major damages to the structure were caused, bank protection on the left bank was severely damaged but no outflanking occurred. The pump house together with all coupled pipe network, electric installations and other infrastructure were severely damaged.

CONSTRUCTION WORKS:

1. Bank protection on Left Bank Although the bank was covered with Armour-flex protection bricks during the original construction of the weir, proper foundation was not provided on the edges of the protected area. This caused the surface protection to be undermined by the floodwaters and resulted in the bare area to be exposed. Besides the erosion problems that could be caused by this, the structure stood a chance of being outflanked completely.

All existing protection material was removed and thorough foundation was provided on the edges of the protected area to ensure the proper affixing of the Armour-flex bricks. The protection area was extended into the downstream river canal in order to maintain slopes that are not vulnerable for erosion and further to prevent any undermining of the flank wall.

Since the recorder hut on the Left Bank was abandoned due to the instrumentation being installed in the pump house on the Right Bank, it was decided to demolish the structure and to burry it in an existing erosion trench. The artificial berm that was solely build to camouflage the recorder hut was also flattened and the surrounding area was rehabilitated into its original state. All the works were covered with topsoil and debris that was left from the flood were used to protect and contribute to the proper rehabilitation of the embankment.

 2. Fish pass, pump house and recorder hut:

2.1 Observations revealed that certain fish species had difficulties in overcoming the weir as obstacle during their emigration process and a fish pass was therefore recommended. Solid rock foundation and the position of the low notch favored the positioning of the structure on the right bank and were eventually constructed next to the scour chamber approximately 5 meters away from the low notch. According to Dr P. Wessels the design of the intake to the fish pass is as such that the influence will not exceed 3% of the discharge at any given flow rate at the weir and corrections will therefore be minimum. The structure consists of various chambers at a maximum height difference of 300mm, which eventually opens into a pool close to the spill of the low notch. (See drawings attached)

2.2 Due to the extensive flood damage to the pump house and the fact that it has and will never again been utilized for the purpose of transferring water to Lower Sabie Camp, a decision was made by the Park Board Authorities to demolish the structure. The SADF (Defense Force) was approached in this regard and on 09/05/2001 the structure was exploded as part of a manoeuvre exercise. The Department had no participation in this regard and the Parks Board executed the clean-up process.

2.3 Because the pump house accommodated the measuring equipment, the demolishing thereof compelled the Department to erect a new recorder hut. In collaboration with the Park Board, a new site for the recorder hut was identified and eventually the concrete structure was camouflaged with artificial rock.

INLET SYSTEM AND INSTRUMENTATION:

Since the beginning of the station in December 1986, the inlet system consists of a traditional inlet pipe that stretched from the low notch to the recorder tower on the Left Bank. Besides the usual problems of silt and blockages, the length of the inlet system caused the data to be suspicious. Access to the recorder was very poor and even dangerous during rainy season and resulted in the station to be visited very infrequently. Due to logger technology, instrumentation was installed in the pump house on 06/10/1999 but the mechanical recorder was still kept in place for a back-up system. The pressure probe was installed in a vertical pipe attached to a manhole that was mend for abstraction to the pump house. Damages caused by the floods of February 2000 resulted in instrumentation being absent since 07/02/2000 to 09/08/2000. Since then the probe was installed direct into the manhole and connected to an Ott-logger in the pump house. Construction works caused the data not to be continuous and sporadic periods of data is available up to 09/05/2001 when the pump house was demolished.

After the construction of the Fish Pass and the new recorder hut, the probe was installed in a vertical pipe in the manhole and an Ott-Logger was again installed on 07/06/2001. A Nera Satellite communication system was also installed and real time data is ever since updated on the DWAF-Website on a daily basis.

FLOW RECORD:

Flow record exists as follow: 1. 2000/01/19 - 2000/08/09: Permanent no record due to flood damage 2. 2000/08/09 - 2000/10/11: Data available 3. 2000/10/11 - 2001/06/07: Sporadic periods of data available due to construction works. 4 2001/06/07 - Date Data available but problems experienced with logger. Wave action in record is incorrect.

Data missing due to flood damage: 2000/01/19 to 2000/08/09

SURVEYS AND CALIBRATION:

Although Survey Services executed a complete terrain survey on the 11 November 2001, changes to the calibration can be expected to be minimum. Besides the influence of the Fish Pass and possible changes to the upstream pool, no drastic modifications to the calibration are expected. As mentioned, the effect of the fish pass will not exceed 3% at any given flow rate. Attached is a detail sketch of the fish pass but the survey and drawings from Survey Services are still outstanding.

The flood survey was done at a terrain approximately 6 Kilometers upstream of Lower Sabie Camp but will be representative of the flow rate at the weir since there are no tributaries in between.

Conclusions from the flow duration curve are that an average flow rate of 10 m3 /sec are measured over a period of 11-years and that the mean annual run-off is calculated as 344 Million m3. The weir capacity of 88.476 m3 /sec will cater for 99% of the flow rate which will represent more than 99% of the volume. This statistics do not include the February 2000 floods and flow run-off there after. See next figure for more detail.

COSTS AND BUDGET

An amount of R 1 500 000.00 was originally budget for the construction works and eventually an amount of R 1 424 000.00 was spent. Directorate Construction executed the works and all expenditure was directly booked against a specific Flood Fund over which Hydrology had no jurisdiction. Mr H. Muller was the acting site agent and various Hydrology personnel made inputs during numerous site visits.

Control Industrial Technician: Hydrology-Mpumalanga 

21 February 2002