ESTUARIES IN SOUTH AFRICA

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HARTENBOS ESTUARY: Physical Features

	Topography

	Bathymetry
	A deep water channel of 1 to 2m depth, depending on the conditions of the tide, ran along the south-western bank from the Riviera Hotel down to the parking area on the south-western sandpit. From there the channel turned sharply across the estuary towards its north-eastern bank, and then broke out to sea in a S-shaped bend, where the water depths decreased to approximately 1 m. All the other areas of the lower estuary are dominated by relatively shallow areas and sand shoals, of less than 0,5 m water depth. From the railway bridge upstream to the southern drift depths varied from 1,3 to 1,2m on a high spring tide (ECRU survey). (Ref 1)
	Obstructions
	In the upper reaches of the estuary, approximately 2,5 km from the mouth road causeways cross the two main watercourses of the Hartenbos. The southern causeway is a low-lying concrete structure with several pipe culverts for the passage of water. At the time of the ECRU survey, this causeway had recently been damaged (probably during the 1981 floods) and repaired. The northern causeway had been completely washed away on its southern side and could no longer be traversed by vehicles at the time of the ECRU survey. The banks of the channel on the southern side of the causeway had also been badly eroded by floodwaters. Attempts to curb this have been made by filling in the eroded part with derelict vehicles. (Ref 1)
	The old national road single span bridge built before 1940 crosses the Hartenbos about 2km upstream of the mouth. It has substantial supporting embankments built across the floodplain on either side of the estuary. The main pipeline for the fresh water supply (dams on the Klein Brak and Groot Brak to the north-east of the Hartenbos) to Mossel Bay, is attached to the old national road bridge. The new national road bridges were constructed in 1973 and are situated about 1.6km from the mouth. They are also supported by embankments across the floodplain on either side of the estuary. At the time of the ECRU survey, construction rubble was present on the bottom of the estuary, under the bridges, making boating hazardous. (Ref 1)
	About 1,2km from the mouth a causeway supporting the old pipeline for freshwater supply (from the catchment of the Groot Brak to the north-east) to Mossel Bay and Hartenbos, crosses the estuary. The pipe is buried in the floodplain on either side of the causeway. At the time of the ECRU survey this pipeline, which was constructed in 1940 (GL Lambrecht, GP Kellerman and GL de Lange, pers.comm.) was still in use. The northern side of its causeway had recently been damaged (probably during the 1981 floods) and repaired. According to Messrs Lamprecht, Kellerman and De Lange, this old causeway and pipeline are to be removed soon. (Ref 1)
	The present railway bridge, constructed in 1956 and carrying the main Cape Town/Port Elizabeth line, is situated about 800m from the mouth of the Hartenbos. It has a substantial supporting embankment across the floodplain on the northern side. Immediately downstream of it are the remains of the old railway bridge. Theses consist of disused embankments on either side of the constriction in the estuary at this point and the remains of the old pylons situated in the main channel. The old railway bridge was made of steel and due to rust, became unsafe by the early 1950's, necessitating its replacement. The old bridge pylons, instead of being removed were dynamited and capped with concrete cylinders, creating both a hazard to boats and an obstruction to the flow of water. (Ref 1)
	There are also developments on the floodplain of the Hartenbos, both on the northern and southern banks, which obstruct floodwaters. Of particular noe is the Riviera Hotel on the southern bank, situated about 200m downstream of the railway bridge. Floodwaters are channelled directly towards the hotel and in the past have flooded it and the buildings behind it. (Ref 1)