ENVIRONMENTAL IMPACT REPORT DONE BY NEMAI CONSULTING ON THE AREAS SURROUNDING KELLAND BIRD SANCTUARY
1.1. Introduction
Urban development within the catchments of the Fairlands Spruit and Darrenwood Spruit has significantly affected local flow patterns and has caused extensive damage at the confluence of these watercourses with the Klein Jukskei River, at the Kelland Bird Sanctuary.
The Johannesburg Roads Agency (JRA) is currently developing a master plan for their roads infrastructure, and they have taken a strategic decision to include environmental issues in all their future planning. JRA appointed Nemai Consulting to undertake an environmental and engineering study of the abovementioned watercourses and their confluence to prevent further degradation and to propose measures to ameliorate extant adverse conditions at the Kelland Bird Sanctuary.
The study area comprises of the following:
Willow Estate;
Kelland Bird Sanctuary;
Section of Klein Jukskei River within Randpark Ridge Golf Course and Kelland Bird Sanctuary;
Section of Darrenwood Spruit flowing through the Willows Estate, Kelland Bird Sanctuary and the Randpark Ridge Golf Course;
Fairlands Spruit from the open park land (Willowview drive) to the confluence in Kelland Bird Sanctuary; and
Randpark Ridge Golf Course.
An initial situation analysis report was compiled to provide information on the status quo of the confluence and a section of the Fairlands Spruit, Darrenwood Spruit and Klein Jukskei River. The situation analysis report served as baseline knowledge from which. the proposed mitigation measures and management plan, was compiled.
1.2. Aims and Objectives
The primary aims of this study are listed below:
Propose rehabilitation measures for the Fairlands Spruit to prevent further damage to the river course;
Ensure that the Kelland Bird Sanctuary is not further degraded;
Address the impact of stormwater in the Fairlands Spruit; and
Obtain inputs from the various stakeholders.
The intention of this report is to grant an overview of the existing state of the confluence and sections of the three watercourses, with the purpose of informing subsequent management proposals.
2. FAUNA
Riparian areas are used as green corridors for the movement of animals, and provide scarce habitat opportunity.
African Black Ducks (Anas sparsa), Grey Herron (Ardea cinerea), Sacred Ibis (Threskiornis aethiopicus), Black Smith Plover (Vanellus armatus), Egrets (Egretta ardesiaca) and various wetland associated bird species like the Red Bishop (Euplectes orix) were spotted during the foot survey. The wetlands provide valuable habitat for birds, and allow for breeding and feeding activities.
Small mammals like mongoose and rodents are expected to occur in the area. The wetland at the bird sanctuary again serves as significant habitat for mammal types.
3. FLORA
Exotic vegetation is common in the study area, and includes Weeping willow (Salix babylonica), Black wattle (Acacia mearnsii), Silver Wattle (Acacia dealbata), kikuyu grass (Pennisetum Clandestinum) and a host of garden varieties. In the bird sanctuary, indigenous trees such as Shepherd’s tree (Boscia albitrunca), White stinkwood (Celtis africana), Fig Trees (Ficus species), Milkwood trees (Mimusops species), Olive trees (Olea species), Karree (Rhus lancea), Boer-bean trees (Schotia species), Pear trees (Sclopia species) and Tamboti (Spirostachys africana) were planted to promote biodiversity. The indigenous common reed (Phragmites autralis) and the Spanish reed (Arunda donax) occur in the wetland.
4. HYDROLOGY
Three streams flow in the study area namely: the Fairlands Spruit, Darrenwood Spruit and the Klein Jukskei. The confluence of the three streams is situated at the Kelland Bird Sanctuary.
5. MANAGEMENT REACH
5.1 Management Goal:
Safeguard biodiversity; maintain sanctuary as educational and recreational asset.
Flood control
Ensuring that water flows through the wetlands
Prevention of pollution
5.2 Area description:
The Kelland Bird Sanctuary is situated at the confluence of the Darrenwood Spruit, Fairlands Spruit and the Klein Jukskei River, between the Randpark Golf Course and the Willows Estate. Refer to layout in Appendix A.
The Kelland Bird Sanctuary consists of a wetland system, which is fed by the Fairlands Spruit and the Darrenwood Spruit. The wetland is predominantly underlain by clay. The indigenous common reed Phragmites autralis is most dominant. The exotic Spanish reeds (Arunda donax), are currently being removed by the management of Kelland Bird Sanctuary. Twenty-four wetland associated bird species have been recorded in the sanctuary. Small mammals occur in the area. The Kelland Bird Sanctuary has made various improvements to the sanctuary at their cost (refer to Appendix B).
The wetland is drying up due to the lack of supply of water to the wetland. A duck pond used to exist at the bird sanctuary but has dried up because of the deposition of silt and lack of water. The abovementioned activities at the Willows estate, in particular the removal of the silt trap, contributed to silt deposition at the Kelland Bird Sanctuary. The modifications to the channel reduced the flow of water into the wetlands. The removal of the ground level altered the natural flood planes of the wetlands.
5.3 Habitat Assessment
According to the report: Habitat Assessment and Reserve Determination for the Kelland Wetland by Wetland Consulting Services, the wetland is currently highly degraded as a result of erosion and silt deposition. The Present Ecological Status Category (PESC) is E, which implies that the wetland has been seriously modified and the losses of natural habitat and ecosystem functions are extensive. The report indicated that intervention to stop the erosion was required to prevent the wetlands from being lost.
A letter from Pulles Howard and de Lange Inc. (PHD) (Appendix C) emphasised that urgent interventions were required. It was suggested that gabions be placed upstream to arrest the erosion and a wall be built to restore the duck pond.
6. HYDROLOGY
The most pressing problems in the study area pertain to hydrology, the recommendations revolve around the management of water flow. Below follows an extract from the hydrological report prepared by Professor Stephenson (2004), as contained in Appendix H.
The aim of this project is to provide attenuation of floods by on-site or in-channel storage or other means. This will have the two-fold effect of reducing the erosion and increasing dry weather flows. However, the very nature of the urbanization is such that everything has increased the runoff and therefore the runoff velocity, and this has resulted in shorter concentration times and more intense design storms. If planning is caught at an earlier enough stage, then on-site detention is the most effective way to ameliorate the various effects. That is, building, garden and road planning should be such as to retain water by overland flow and infiltration. Various solutions are tabulated below and the more promising ones discussed later.
On-site retention, i.e. roof drainage and road drainage diverted to porous areas. It is probably too late to change town planning now.
Retardation of road drainage. This means more diversion of minor floods to roads, which have a retarding effect because the flow is shallower. This is again difficult to revert to without realigning roads to reduce the flow velocities down hills.
Changing road inlets. This means partly blocking curb inlets to create overland flow. This will aggravate traffic and annoy residents.
Off-channel storage. Park areas could have flow diverted to them, but there has been a development of previously open areas in the Fairlands suburb and we have not identified any possible off-channel storage.
Instream detention. This means small dams at regular intervals along the stream. These would have to be relatively shallow and narrow and the volumes would not be adequate unless many such dams were built.
Downstream attenuation storage. This means a large dam or dams in the wetlands area. For the volume however to have any effect on the three tributaries, they would be large and many of the undesirable effects would already have taken place upstream
Dredging of channels both along the spruit and in the wetland. The cost of providing sufficient storage thus ie 100000 cubic metres is estimated to be R2 million. It is very likely that the channels along the spruit and in the wetland may be re-silted if the upstream activities are not controlled. A possible practical and logical solution would be for a few flexible dams to be constructed across the streams. These dams would not have sufficient surface area or volume to create all the desired attenuation; therefore another series of dams would need to be constructed in the wetland area itself. These could be more flexible and in fact, owing to the foundation conditions, would have to be flexible. They could be low cost dams with wide spillways as the danger of subsidence or flooding is not as severe as along the contributing streams.
What is envisaged is a series of gabion weirs, which will do the following:
Create a surface area, which is adequate to route the incoming floods, i.e. for attenuation purposes. Although it is a bit late for changing up-stream conditions, it will improve the conditions in the bird sanctuary area and further downstream all the way to Hartbeespoort Dam.
The dams will need heavy cut off walls to avoid erosion around the gabions and careful design of filters. They will need erosion protection downstream and minor energy dissipation works.
The dams will no doubt arrest sediment brought down by the streams and fill up to an extent but they will eventually reach an equilibrium depth. The nature of the wetland would therefore change, i.e. there would be more open water expanses and this should be considered favourable from an ecological perspective.
The critical flood flows for the three tributaries were estimated. As an example, the 100-year flood for the Fairlands spruit on present development conditions is estimated to be 150 m 3 /s due to a ¾ hour storm. This flow is about double the flood, which would have occurred from the natural, undeveloped catchment and that is the figure we aim to reduce the floods to with the attenuation facilities.
The volume of the flood hydrograph is approximately 100 000 m 3 as will be seen the computations and there is inadequate storage capacity along the tributaries to contain this. A large surface area is also needed, i.e. approximately 40 000 m 2 , to cause the attenuation and it is the combined volume and surface area which does the flood routing.
A series of small dams should be built upstream in the tributaries bringing the flood water level to the maximum height possible without endangering buildings. This will need more careful surveys to ascertain but it is envisaged that 4-5 m high gabion dams could be constructed along the streams.
These dams would have a stepped, notched spillway, with the lowest notch being only some 1 or 2 metres above bed level in the case of the tributary dams. In this way, the dams would have a large capacity for catching floods as well as a bigger surface area when they are full for routing the floods. A sketch of the envisaged dam is attached.
The three dams in the bird sanctuary area possibly would have a higher bottom notch, i.e. approximately 3-4 metres dead water depth as they would also be designed to catch sediment in that dead water depth. The final design of the dams would also require an accurate tache survey although the whole project could be phased, i.e. one dam at a time built as the area is cleared and remodeled.
An interesting concept is used extensively in Europe, China and Japan and is proposed for this project. That is, the use of inflatable rubber barrages for stormwater detention. These rubber sausages can be built up to 4 metres diameter and filled with water or air. They automatically deflate when the upstream water level increases to pass a greater flood and then reflate as the flood recedes. This type of dam would be more suitable for the in-channel detention along the spruit.
The dam area capacity in relation for maximum flood attenuation was optimised. It is estimated that approximately five dams with a combined surface area of 40 000 m 2 would reduce the 150 m 3 /s flood to about 75 m 3 /s. There is no single site big enough for this area but in fact a succession of dams has the same effect and may be cheaper because of the lower dam wall height. The problem with a number of dams is always that more spillways are required and the spillway is often the most costly component in a dam. However, with gabion weirs the flow would be permitted to overtop the gabion and, provided adequate downstream erosion protection and energy dissipation is constructed, there is no separate special structure needed for the spillway. The same would apply to inflatable rubber barrages.
The width of the spillway also affects the routing capacity and we have optimized this as a 10m wide spillway per dam. This is at the top height of the spillway and the spillway would narrow down to a bed level minimum width of approximately 1m. In this way, the dams can increase in depth by up to 5m as the flood comes in and then drop in level as the flood passes. The alternative spillway widths and surface areas were investigated and have minimized the dam surface area in the computations.
Once accurate contours are available each dam will be optimized subject to flood plain width available. The urban streams were probably planned for 50 year floods whereas the new Act requires planning with the 100 year flood, making planning more difficult.
7. WETLAND
Wetlands play an important role in an urban environment. Wetlands usually have the capacity to store floodwater, thus serving to offset flood events, diminish erosion of flood flows, and filter sediments. Slow release of water stored in wetlands recharges groundwater, which feeds streams during dry periods—an essential function in regulating the timing and quantity of runoff and in sustaining aquatic life (Jukskei EMF Report, March 2002)
Wetlands also provide a platform for recreation and breeding areas for various species of birds. The roots of wetlands help to stabilise stream banks, and this vegetation also
8. NEMAI CONSULTING
Nemai Consulting appointed Exigent Engineering Consultants cc to undertake an investigation of the wetland for the following purposes:
Review applicability of the recommendations of the previous wetland study conducted by Wetland Consulting Services (Habitat Assessment and Reserve Determinations for the Kelland Wetland, August 2001);
Review applicability of Pullard Howard & De Lange;
Identify the distribution of reed species on site;
Identify the impacts of the existing reeds
9. CHANGING THE STRUCTURE OF THE WETLAND
Reduction in Stream Energy, Water Level Increase and Silt Deposition The root of the problem is the increased flood flows into the wetland. These are caused by increasing urbanisation of the catchments, particularly that of the Fairland Spruit. Urbanisation creates impervious surfaces, which allow rainwater to run freely through drainage lines into rivers. This effect occurs in the entire catchment such that water flows are all concentrated within a short space of time. This concentration leads to increased stream energy, which is the cause of the deep incised channels in the wetland.
The only practical solution, given the level of urbanisation in the catchments and the impossibility of re-engineering the town planning of the area, is to catch the large stream flows and slow them down. This will reduce the energy in the stream and thus the stream’s erosive power.
Catching the stream flows is achieved through weirs. Weirs collect water behind the wall and thus slow it down. In the process, any silt loads in the stream are dropped to the reservoir floor as the energy required to suspend it dissipates. By slowing the water down, the time taken form the flood volume of water to flow through the system is increased, thus reducing the stream flow rates and energy in the stream.
A result of the reduction in stream energy through the installation of gabion weirs in the wetland will be increased rates of silt deposition. Transported silt will be deposited into the incised channels behind the gabion weirs over time. This silt deposition is essential to the re-establishment of wetland function. It is expected that silt will deposit in the channels, gradually filling them up. This will have a negative impact upon the weir’s capacity for flood routing, however once the wetland’s functionality is restored, its flood routing capacity will be much enhanced when compared to its current status.
Smaller weirs should be constructed in the channels on an ad hoc basis to ensure that silt deposition takes place where it is most needed and in areas that are not affected by the proposed gabions weirs.This option is preferred over the option of mechanically reworking the topography of the wetland to fill the incised channels with the resultant cut material. The mechanical filling option has the advantage of speed – the wetland topography would be changed within months. It has the disadvantage of being vulnerable to the same processes that created the incised channels in the first place. The root of the problem – that of high stream energy is not addressed by the mechanical filling option. If the mechanical filling option were to be pursued without there being any stream energy reduction measures put in place, the incised channels would be recreated as the flood seasons pass.
In addition, the mechanical filling option has the disadvantage of completely destroying the existing wetland. The fauna and flora in the wetland will be severely disturbed may take may years to re-establish. This destruction of the wetland is the main purpose of the project, and as such cannot be supported.
10. CONCLUSION
The options will be presented to all the stakeholders. Immediate action is required to rehabilitate the Kelland Bird Sanctuary and the Fairlands Spruit
