Page 58 - Bulletin 23- 2020

54

The Marina Nursery used a large hydraulic “tree-spade” to transfer many wind-tolerant trees
(mainly Manatoka) from remote areas of the site to the Sandvlei public park and road areas.

These are still thriving in the strong wind conditions as occur in the open-grassed and other

public picnic areas around the vlei.

Stabilised Water Level and Water Depth

The management of the vlei water level was considered to be one of the most important

design elements of the project.

After a detailed consideration of the sea tides, the historical vlei water levels, the balance
between inflow, seepage and evaporation, the flood flows, the distribution of aquatic weed

and much else, it was decided to fix the “Operating Water Level” at 0.7m above the LWOST
datum. Once this level had been set other design criteria could be established and bank levels,

housing floor levels, and the dredged depth in the canals were determined. (Fig. 2.10).

The “depth of the water at the edge” was an important issue which was considered in detail
because “shallow” creates problems for water circulation and leads to prolific aquatic weed

growth. The safety of children was the determinant and all banks were designed to be
shallow, with this in mind.

Aquatic weed

Three types of water weed predominated at the vlei at the time of construction. It was

observed that where the water was over 1.8m deep there was little weed growth and this
became the main determinant for the dredged depth in the sailing areas and at the centre of

the canals.

Weir at the sea

At times of low tide, when the mouth of the vlei is open to False Bay, the water level in the

vlei can fall very quickly due to the draining of the vlei to the sea. The water level can

correspondingly drop below the Operating Water Level. To control this, the stone rubble weir
at the Royal Road bridge was set to a level that met the new water management criteria.

54