Heritage Cases

The project intends to improve the Montana Spruit between an area 600m upstream of the Tsamma Road stream crossing and to about 600m downstream of the Tsamma Road crossing in order to reduce the impact of the 1:100-year flood on adjacent properties and houses. The shape of the spruit will be improved to accommodate more flow and to ensure that all the buildings and houses adjacent to the spruit are located outside the 1:100-year flood line. Tsamma street will also be upgraded and realigned to improve on the flood line position, culvert capacity and road layout. The culvert crossing at Tsamma street will be upgraded to accommodate the 1:2-year flow by means of a portal culvert crossing, flows from recurrence intervals greater than 1:2 years will overtop Tsamma street and flow on surface. The project area for the Montana Spruit Channel Improvement – Phase 1 is located within the Doornpoort and Montana Park residential area, south of Pretoria, and is defined to include: the Montana Spruit Flood Management area; an area 600m upstream of the Tsamma road stream crossing and to about 600m downstream of the same crossing; and Tsamma Road between Breed Street and Cassia Street. Proposed new road upgrades The existing Tsamma Street will be reconstructed on the same horizontal alignment as before. The vertical alignment will be changed to lower the road and accommodate the culvert crossing. The lowering of the road is also required to ensure that the 1:100-year flood can overtop the road without the flood lines encroaching against the buildings on adjacent land. The 9 x Ø450mm existing concrete pipe has insufficient capacity to accommodate the minor flood and will be replaced by 20 1500x450mm CL100S portal culverts to accommodate the 1:2-year flow. Flow from storms greater than the 1:2-year recurrence interval will overtop Tsamma Street and run on surface. The roadway will be 6.0m wide consisting of 2 x 3,0m lanes in each direction. The higher side of the road will have a 400mm sloping kerb with a 2m wide paved walkway and the lower side will have a 400mm sloping kerb. The road will have a 3% crossfall with the natural slope to improve on the channel flow and to drain surface water away. The road crossfall at the culvert crossing will be 1%, this is to match the slope of the portal culverts. A 250mm concrete slab will be cast over the portal culverts as the final road surface. 250Ø Concrete pillars of approximately 1000mm high will be constructed on both sides of the concrete slab and road as to indicate to motorists the extend of the road width during floods. The embankments at the Tsamma crossing will be lined with reno mattresses to protect against erosion. Proposed new channel improvement Following the investigation of the hydrology analysis, the following solutions will be implemented: • Channelize the Montana Spruit by changing the existing channel through excavating & shaping and widening of the spruit. • The overall objective of any natural stream channelisation improvement design should be to provide enough space to meet flood conveyance targets, increase vegetation in the channel, improve habitat conditions, and improve water quality in the stream. • The horizontal alignments decided for the proposed Montana Spruit Improvements will follow the existing stream bed line. • The natural low flow area of the spruit will be kept and all improvements will be above the water level of the low flow. This option is preferred because it does not disturb flow and natural ecological activities in the main stream influence. • A berm will be constructed at the old age home to prevent any flood water from entering the estate. The floodline will not encroach onto the berm, the berm is just a precautionary measure implemented to prevent any flood damage. • A well-designed stream channelization will typically have three zones on either side of the watercourse: a streamside zone, a middle zone, and an outer zone. To enhance the ecological conditions, natural bedding and excavated stream flood banks with re-vegetation will be provided for the Montana Spruit Channelisation. To make the proposed Spruit more environmentally friendly, the stream banks will be lined with natural substrates to produce a suitable environment similar to the existing main stream condition as per the figure below. The cross-sectional area of the stream channel will be widening by excavating one or both banks outwards to provide a larger cross-section for flow conveyance. From the hydraulic analysis for a 1:50 year and 1:100 year return period it is evident that some embankment erosion might occur especially along the eastern embankment of the Montana Spruit. The general velocity of the spruit will be around 1,4-1,9m/s with isolated areas where the velocity will be around 2,4m/s. The embankment of the spruit will be 1:5 and hydro seeded for erosion protection. In some sections of the spruit where buildings and walls are close to the waterfront a gabion basket wall will be used for erosion protection. The 1:50 and 1:100 year return period flood may cause extensive surface erosion and it is important that the slope and surfaces be protected by grass. The topsoil will be replaced on the entire disturbed area and will be hydroseeded with a recommended seed mixture. Two types of attenuation ponds aimed at attenuating flow will be excavated. The first is a very shallow pond located within the newly widened channel and the second attenuation pond type is located outside of the widened channel.