RE: WETLAND SPECIALIST LETTER: MOTIVATION FOR BIO...

The Biodiversity Company 420 Vale Ave. Ferndale, 2194 Cell: +27 81 319 1225 Fax: +27 86 527 1965 [email protected] www.thebiodiversitycompany.com

Simple Operating Systems (Pty) Ltd). Co. Reg. No. 2012/190711/07 Trading as The Biodiversity Company

15 May 2019

To whom it may concern Directorate Water Abstraction and In-stream Use Section: Instream Water Use Sedibeng Building, 4th Floor 185 Francis Baard Street Pretoria, 0001 RE: WETLAND SPECIALIST LETTER: MOTIVATION FOR BIO-RETENTION PONDS WITHIN WETLAND BUFFER Dear Sir / Madam, Exigo Sustainability was appointed (in 2016) by Atterbury Property Fund Managers (Pty) Ltd to conduct a wetland delineation study that forms part of the Basic Assessment process for the proposed mixed use commercial development to be known as the Erasmus Park Development situated on Portion 22 and the remaining extent of the Farm Waterkloof 378 JR, City of Tshwane, Gauteng Province. The study included a wetland delineation and functionality assessment, with descriptions of the anticipated impacts (risks) associated with the proposed development activities and mitigation to reduce impacts. The assignment was interpreted as follows: Compile a study on the wetlands of the site according to guidelines and criteria set by the Gauteng Department of Agriculture and Rural Development (GDARD) and the Department of Water and Sanitation (DWS)”. The Biodiversity Company was commissioned to provide a motivation for the placement of bio-retention ponds within the wetland buffer for the Erasmus Park Development. This motivation is in response to comments made by the Department of Water and Sanitation (DWS) dated 6 March 2019. The particular comment to be addressed in this motivational letter is the following: It must be motivated why the bio-retention ponds can’t be planned and positioned outside the wetland buffers. The details and findings of the Exigo Sustainability (2016) report form the basis for the motivation, and it has been assumed that all information therein is true and accurate for the purposes of this motivation. The following is summarised from the 2016 wetland study:

• The wetlands of the proposed development site can be classified as an unchannelled valley bottom wetland and man-made depressions (dams) with riparian woodland, while the smaller drainage canals are classified as watercourses (Figure 1).

• The wetlands at the proposed development site are in a ‘Moderately Modified’ condition. The wetlands have been modified through impoundments, alien invasive species, sedimentation and road crossings. These impacts also play a major part in the LOW Ecological Importance & Sensitivity (EIS) of the unchannelled valley bottom wetlands.

• An impact assessment was conducted for the wetlands on site in addition to the buffer zones recommended to ensure the protection of the wetlands.

MOTIVATION FOR BIO-RETENTION PONDS WITHIN WETLAND BUFFER MAY 2019

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• Provided that all recommendations and mitigation measures provided in this report are adhered to the planned project can be supported.

Figure 1: The delineated wetland area and 30m buffer (Exigo Sustainability, 2016) and the respective project area. The bio-retention system designed by EDS (Project no, 2016-048) for the proposed development is an underdrain system or filtration facility. This design is suitable for areas where high filtration and partial recharge of runoff would be beneficial. This design is also suitable for areas and land uses that are expected to generate nutrient and metals loadings, such as mixed-use developments. Note: For the purposes of this motivation, a bio-retention pond is defined as the following: Structural stormwater controls that capture and temporarily store water using soils and vegetation in order to remove pollutants from stormwater runoff. The objectives for these ponds are two-fold. Firstly for the control (or management) of stormwater, and secondly for the enhancement of water quality. According to the Water Sensitive Urban Design (WSUD) for South Africa, swales, bio-retention ponds and detention ponds are used for the management of stormwater (WRC, 2014). A case study presented for the Prince Henry Hospital Development (WRC, 2014) presents the incorporation of bio-retention systems into public open spaces. The Exigo Sustainability (2016) report depicts the extent of development for the proposed project, with the wetland and the proposed buffer comprising portions of the limited remaining open spaces (Figure 2). Based on this proposed layout, there is inadequate space or opportunity for the establishment of bio-retention ponds for the development, other than for the wetland and buffer areas which will not be developed. In addition to this, owing to the fact that these ponds are used for the management of stormwater, and taking into account the dominant surface flow direction is from the north-



east to the south-west of the project area, it would be beneficial and most effective to position the ponds at the receiving end of the catchment, which is associated with the delineated wetland system. According to the Sustainable Drainage System (SuDS) a development typically reduces the natural permeability characteristics of the developed area, by replacing free draining surfaces with impermeable surfaces. This also results in the loss of biodiversity, vegetation and amenity. One key feature of bio-retention is its ability to mimic the pre-development hydroperiod of an undeveloped watershed and thus help to maintain a natural water cycle in urban areas (Liu, 2014). Based on this, this is further motivation for the placement of the bio-retention ponds in the receiving end of the catchment adjacent to the wetland. The design and placement of these ponds will mimic the hydroperiod of an undeveloped watershed before water enters the wetland system.

Figure 2: The detailed layout map (Exigo Sustainability, 2016) for the proposed project. According to Macfarlane and Bredin (2017) the definitions of buffer zones vary depending on their purpose. In the context of the Macfarlane et al. (2014) guideline, buffer zones have been defined as a strip of land with a use, function or zoning specifically designed to protect one area of land against impacts from another. The management of stormwater is a critical for urban planning (Macfarlane & Bredin, 2017). According to Macfarlane & Bredin (2017) buffer zones can contribute effectively in the management of stormwater if these zones are carefully designed and managed. This will require the implementation of unconventional stormwater management measures towards a more holistic approach as advocated in the South African Guidelines for SuDS (Armitage, et al., 2013). Buffer zones (often referred to as filter strips) are recognised as one of a number of effective “local” controls and should ideally be integrated into the stormwater management system. Note (Macfarlane and Bredin, 2017): It should be noted that primary stormwater



treatment structures should generally be located outside of buffer zones, with buffer zones then providing a post-attenuation treatment function. A buffer zone width of 30 m was recommended by Exigo Sustainability (2016), but according to Macfarlane and Bredin (2017) the minimum recommended buffer zone for the “worst case” scenario for mixed-use/commercial/retail/business land activity is 15 m. This minimum width would typically include a commitment to rehabilitate and manage buffer zones to ensure that these areas function optimally. This recommended buffer zone width would allow for the design and placement of the bio-retention ponds within the remaining extent of the buffer zone. This remaining extent would then provide a post-attenuation treatment function. In support of this, according to:

• The KwaZulu-Natal Department of Agriculture and Environmental Affairs (2002) of minimum fixed buffer width of 15 m if required for stormwater outlets.

• The Roads and Stormwater Department of Cape Town (2008) a minimum buffer width of 10 m is required for stormwater control.

Bio-retention ponds (Vice 2011) require a grassed filter strip (or buffer zone) (Figure 3), which would allow for the placement of bio-retention filters within the 30 m buffer area, with the ponds only encompassing a portion of the buffer. Due to the extensive development of the area, the buffer zone is the only area which would allow for a grassed filter strip. The design would allow for the placement of bio-retention ponds within the buffer area, allowing a filter strip between the pond and the proposed development area. The remaining extent of the buffer (downslope area) would encompass a vegetated area (or grassed filter strip) of gently sloping ground designed to drain water evenly and filter out insoluble pollutants. Filter strips (or buffers) are most effective as pre-treatment options in treatment trains, especially to aid the stormwater management processes of bio-retention areas, infiltration trenches and swales (Melbourne Water Corporation, 1999). Filter strips can also be used to intercept and redirect stormwater runoff in order to spread it as sheet flow, thus attenuating flood peaks (Field & Sullivan, 2003; Melbourne Water, 2005). Based on this, and taking into consideration the proposed development and the limited remaining open spaces, the buffer zone is the only area which would allow for a pre-treatment filter strip.

Figure 3: Conceptual design and placement of a bio-retention pond (Vice, 2011) The remaining extent of the buffer area would be a grassed area of land that is used to manage shallow overland stormwater runoff through infiltration. These “grassed areas‟ are typically densely vegetated and generally uniformly graded (Debo & Reese, 2003). The vegetation associated with these ponds should preferably be indigenous vegetation species because these should not only be adapted to the local climate but will also assist in preserving the amenity and



natural biodiversity of the immediate environment (Vice, 2011). According to Vice (2011) bio-retention areas are typically more aesthetically pleasing than most other SuDS options and/or technologies. The correct design and placement of bio-retention ponds within the buffer could in actual fact support greater diversity and species richness (Liu et al., 2014), due to the evidence observed on site of transformation of the floristic characteristics of the valley bottom wetlands (Exigo Sustainability, 2016). Studies have found bio-retention ponds had a significant increase in plant and invertebrates’ taxa (Liu et al., 2014). In closing, it is the opinion of the specialist that although Macfarlane and Bredin (2017) refer to stormwater treatment structures generally being located outside of buffer zones, this does also allow for the placement of structures within buffers in exceptional and acceptable circumstances. The greater extent of the project area will be developed, with limited public and private open spaces remaining, and with these spaces being the only suitable areas (pre-treatment grass filters) for the placement of bio-retention ponds. The dominant surface flow direction is from the north-east to the south-west of the project area, which makes the location of the buffer area the most suited for effective stormwater management. According to Macfarlane et al. (2014) buffer zones are specifically designed to protect areas against impacts, which would be primarily the management of stormwater for the proposed development. In this instance the management of stormwater would require the combination of a carefully managed buffer zone and unconventional stormwater management measures, which would be the placement of bio-retention ponds within the buffer zone. Not only would the bio-retention ponds “bolster” the effect of the buffer zone in management stormwater, but these ponds would contribute to biodiversity and aesthetic value of these open spaces. It is also worth noting that the proposed bio-retention ponds would also contribute to the enhancement of water quality, fulfilling the second objective of these structures. I trust the content of this letter serves to provide sufficient motivation from the wetland specialist for the DWS to allow for the placement of bio-retention ponds within the delineated wetland buffer. Regards, Andrew Husted

Wetland Specialist SACNASP (Pr Sci Nat: 400213/11) Armitage, N., Vice, M., Fisher-Jeffes, L., Winter, K., Spiegel, A. and Dunstan, J. 2013. Alternative technology for storm water management: The South African guidelines for sustainable drainage systems. WRC Report No. TT 558/13. Water Research Commission, Pretoria. Debo T.N. and Reese A.J. 2003. Municipal Stormwater Management. Lewis Publishers. Florida. Exigo Sustainability. 2016. A wetland delineation report for the proposed mixed use commercial development on the remaining extent of the farm Waterkloof 378, Pretoria, Gauteng province. Compiled by: Dr. BJ Henning. Field R. and Sullivan D. (2003). Wet-weather flow in the urban watershed. Lewis Publishers. Florida. Liu, J., Sample, D.J., Bell, C. and Guan, Y. 2014. Review and Research Needs of Bioretention Used for the



Treatment of Urban Stormwater. Water 2014, 6, 1069-1099; doi:10.3390/w6041069. Macfarlane DM, Bredin IP, Adams JB, Zungu MM, Bate GC, Dickens CWS. 2014. Preliminary guideline for the determination of buffer zones for rivers, wetlands and estuaries. Final Consolidated Report. WRC Report No TT 610/14, Water Research Commission, Pretoria. Macfarlane, D.M. and Bredin, I.P. 2016. Buffer zone guidelines for rivers, wetlands and estuaries. Part 1: Technical Manual. WRC Report No (tbc), Water Research Commission, Pretoria. Melbourne Water Corporation, 1999, Urban Stormwater: Best Practice Environmental Management Guidelines, Environment Protection Authority – Melbourne Water Corporation, CSIRO Publishing, Collingwood. Melbourne Water, 2005, WSUD Engineering Procedures: Stormwater, CSIRO Publishing, Collingwood Victoria Roads and Stormwater Department. 2008. Floodplain Management Policy: Balancing flood risk, ecological and social considerations adjacent to watercourses and wetlands (Version 2.0 Draft). Cape Town, Roads and Stormwater Department: 1-17. Vice, M.A.P. 2011. Century City as a case study for Sustainable Drainage Systems (SuDS) in South Africa. CIV5000Z MASTERS DISSERTATION: CIVIL ENGINEERING. Dissertation submitted in partial fulfilment of the requirements for the degree of Master of Science in Engineering in Civil Engineering (MSc Eng). Department of Civil Engineering, University of Cape Town, Private Bag Rondebosch, 7700, South Africa Water Research Commission (WRC). 2014. Water Sensitive Urban Design (WSUD) for South Africa: Framework and guidelines. Neil Armitage, Lloyd Fisher-Jeffes, Kirsty Carden, Kevin Winter, Vinothan Naidoo, Andrew Spiegel, Ben Mauck &Daniel Coulson. WRC Report No. TT 588/14

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