| Main Page | General Information | Catchment Characteristics | Land and Estuary Uses | Ecology |
| Threats | References | Water Quality | Hydrodynamics and mouth dynamics | Climate |
|
KNYSNA ESTUARY: Physical Features |
||||
|
|
Topography |
 |
|
|
|
Bathymetry |
|
The maximum depth of the Knysna Lagoon is 40 feet. Below Leisure Island the lagoon narrows and the channel reaches a maximum depth of 51 feet. The bar between The Heads is charted as 13 feet deep. Day's map indicated depths from 4 to 48 fee and these depths were from the Admiralty Chart corrected in 1938. Day indicated depths up to 6 m deep but this should be 16m. An aerial survey of the lagoon was made in July 1970 and a contour map with 0,5m vertical interval contours from low water (0,5GMSL) to +5,0 GMSL was made. Depth surroundings in the lagoon were carried out in February 1971 and combined with the aerial survey. Twelve metres is the greatest depth shown. The average depth at the mouth is 4,59m. (Ref 1) |
|
|
Obstructions |
|
Old drift: Below Charlesford there was once a stony ford or drift. This has long since been abandoned but rubble remains; now known as the Old Drift. (Ref 1) |
|
|
Red Bridge: This old steel girder road bridge which is now abandoned is referred to as Westford Road Bridge by Day et al., (1952) A causeway 400m long crosses the marshes on the west bank. (Ref 1) |
|
National Road Bridge: This concrete bridge carrying the N2 across the estuary has a 750m long causeway on the east bank and a 100m long causeway on the west bank. The bridge has five spans with vertical clearances of between 5,6 and 7,1m above MSL (that is minimum of 3,6m above maximum flood level). (Ref 1) |
|
Railway Bridge: This
steel bridge has a 1 700m long causeway across the marshes on the west
bank and a 700m long causeway along the west bank.
All three bridges have solid embankments which restrict tidal flow and encourage the deposition of sediments. (Ref 1) |
|
Thesen's Causeway: The road from Thesen's wharf crosses a narrow side channel. A rubble causeway approximately 420m long carrying a narrow tarred road links Long Street to Thesen's Island. There are no culverts but on the flood-tide water flows throught the rubble to the west while on an ebb-tide the flow is reversed. It has been recommended that part of this causeway be replaced by a bridge. Grindley and Snow recommended that large box culverts be introduced and this is being done. A proposal for a small boat harbour adjoining this causeway on the west side has be prepared. (Ref 1) |
|
|
Leisure Island: This is a long narrow causeway across saltmarshes. This 500m long causeway had nine culvert pipes under it to allow a limited tidal flow beneath the road. Concern was expressed about the adequacy of the flow because of excessive sand accumulation in the 'Green Hole' area. To improve the flow large culverts were introduced in the early 1980s but such measures are inadequate for the removal of the accumulated sand. The film record of the NR10 physical model indicates a net inflow into the Green Hole area over a tidal cycle. (Ref 1) |
|
|
Woodbourne Causeway: The conrete main road to the Eastern Head traverses a causeway which cuts off a permanently flooded area to the east of the road. This area which receives drainage from the Hornlee stream and from the golf course has variable intermediate salinities. This shallow sheltered area with varying range of salinity is perhaps the most important bird habitat adjoining Knysna Lagoon and requires conservation. The range of salinities varies with the season but in March 1984 salinities of 0, 2, 22 and 25 parts per thousand were recorded. A new culvert has increased tidal exchange to this area. (Ref 1) |
|
|
Moored boats can be an
obstruction in the open waters of the lagoon so the following
guide-lines were proposed in the Guide Plan:
|
