1. TERMS OF REFERRENCE .............................................................................. 1 2. IRRIGATION DEMAND .................................................................................... 1 3. SOURCES OF IRRIGATION WATER .............................................................. 2 4. STORAGE NODES ........................................................................................... 4 5. BULK SUPPLY ROUTE FROM SOURCE TO STORAGE NODE .................... 5 6. DISTRIBUTION ROUTES TO END USERS ..................................................... 5 7. RECOMMENDATIONS ..................................................................................... 6 7.1 Proposed irrigation scheme: ................................................................................ 6 7.2 Water Resource Study......................................................................................... 6 7.3 Wetland crossings ............................................................................................... 6
ANNEXURE A – IRRIGATION DEMANDS ............................................................ 7 ANNEXURE B – BULK SUPPLY ROUTES ........................................................... 8 ANNEXURE C – DISTRIBUTION ROUTES ........................................................... 9 LIST OF FIGURES LIST OF TABLES Table 2-1: Total Irrigation Demand ................................................................................... 2 Table 3-1: Irrigation Sources ............................................................................................ 2
Concept Design Report for Cornubia Urban Irrigation-February 2015
Tongaat Hulett Developments (THD) has appointed SMEC to investigate irrigation possibilities and shortlist scheme options for the irrigation of urban landscapes in Cornubia Phase 1 & 2. The investigation focussed on identifying the following key elements of the irrigation scheme. 1. 2. 3. 4.
Sources of irrigation water Storage nodes Bulk supply routes from source to storage nodes Distribution routes to end users.
The identification of suitable sources was directed by the estimated abstraction volume required to support the all year irrigation schedule proposed for urban landscaped areas within Cornubia phase 1 & 2. The abstraction volume was derived from a storage/yield model for Marshall Dam where drafts set equal to the estimated irrigation demand for 15 developments within the Cornubia footprint.
2.
IRRIGATION DEMAND
The irrigation demand was determined by the type and extent of urban irrigation required at planning level for Cornubia phase 1 & 2. The irrigation of private parks, road medians, sidewalks & verges as well as sport fields were identified as key elements and was confirmed in meetings held with City Parks, THD and Landscape Architects Uys & White. A list of 15 developments was identified for urban irrigation as well as a selection of minor arterials, residential collectors and commercial collectors. Unit water demands were based on recommended irrigation depths required for vegetation foreseen and advised by Uys & White Landscape architects. The water demand was adapted for seasonal variation where needed. An irrigation application rate of 25mm to 30mm per week was adopted as provisional design value. The irrigation demand calculated for 15 developments, minor arterials, collectors and sport fields amounted to 648 ML/year that includes 2.5% system losses. Table 2-1 illustrate the total irrigation demand broken up according to property ownership and irrigated land types. It is important to notice that the irrigation of arterials and collector roads amount to approximately 45% of the irrigation demand for Cornubia phase 1 & 2.
Concept Design Report for Cornubia Urban Irrigation-February 2015
The following irrigation sources were identified and evaluated, see Table 3-1 below for a summary of the investigations: Table 3-1: Irrigation Sources Identified Irrigation Source
Evaluation
Potable Water
Found to be too expensive and not financially viable Option was eliminated for reasons of low aquifer yield, water quality and capacity concerns and high cost of sparsely spaced boreholes
Groundwater
Surface Water/Marshall Dam
Marshall Dam. Insufficient yield as stand-alone scheme/dam relying on natural run-off only.
Existing N2 River Abstraction – Ohlanga River
Option was eliminated, abstraction works has limited capacity, abstraction point too far, pumping cost high, financially not viable.
Re-use of treated effluent, Phoenix Waste Water Treatment Works
Option was not considered further for reasons: Concern regarding consistent quality of effluent and hygiene concerns.
Ohlanga River
The Ohlanga River was provisionally identified as only feasible source of irrigation water for the Cornubia Irrigation Scheme.
Concept Design Report for Cornubia Urban Irrigation-February 2015
The study concerning the identification of sources concluded that the Ohlanga River is a feasible source of irrigation water for Cornubia 1 & 2 based on the following information available:
•
Quality of water was generally good for irrigation as reported by Scheme Operators of URTC/Izinga scheme abstracting from the Ohlanga River at the N2 River bridge just downstream of the Cornubia development footprint.
•
Adapted simulated records of river flows in the Ohlanga River abstracted from the WR90 Surface Water Resources of SA 1990 Report by the Water Research Commission dated 1994 indicated towards sufficient river flow during low flow winter periods. An initial storage/yield model for Marshall Dam indicated that by abstracting approximately 25% of the simulated low flow (WR 90 report) from the Ohlanga River during winter months would be sufficient to bridge dry winter months when irrigation demand is high compared to available river flow. The lowest river flow was estimated at 320 m3/hour assuming average hourly flows during the lowest month. A flow frequency diagram will need to be developed through a Water Resource Study for the Ohlanga River as part of the Water Use License Application.
•
The licenced abstraction rate of 782 Ml/day at the N2 river bridge abstraction works (see drawing Appendix B) exceeds by far the current annual abstraction volume of ±101 Ml. It is believed that the full licensed volume will not be utilized for the ultimate URTC/Izinga development meaning surplus capacity in the river system.
Two possible abstraction sites from the Ohlanga River was provisionally identified that would supply the proposed irrigation scheme with water, they are:
Option 1: New abstraction works/weir situated ±300m upstream of the future Dube West arterial bridge crossing the Ohlanga River. See Appendix B for locality details. Option 2: New abstraction works/weir on Ohlanga River situated ±250m downstream of the future Dube West arterial river crossing (Ohlanga River). See Appendix B for its locality. Both abstraction works options will entail a formal structure and inlet works complete with pump station and telemetry. The irrigation scheme requires an estimated abstraction volume of 690 Ml/year from the Ohlanga River to support the irrigation demand required for the 15 development within Cornubia phase 1 & 2. Concept Design Report for Cornubia Urban Irrigation-February 2015
The irrigation scheme required to support the adopted irrigation rates of 25mm to 30mm per week for various developments and roads within Cornubia phase 1 & 2, was provisionally modelled by means of a storage/yield model of Marshall Dam as storage node.
The model confirmed that the system requires storage to bridge the dry winter months. Estimated pump rates from Marshall Dam to end users reaching up to 700 m3/hour exceeds the estimated available low flows in winter periods. The low flow in winter periods is estimated at 320 m3/hour. The allowed abstraction rate from the Ohlanga River for the purpose of the storage/yield model was set at 80 m3/hour or 25% of the estimated low flow.
Storage nodes were investigated based on the storage/yield model prepared for Marshall Dam. The model indicated a required storage volume of at least 125,000 m3 for the scheme to supply irrigation at the calculated demand of 648 Ml/year and abstraction from the Ohlanga River at 690 Ml/year.
The construction of holding dams of this size was ruled out given the limited availability of land and restrictions that apply to the construction of new dams within wetlands.
Marshall Dam was identified as the only suitable storage node of sufficient capacity for the urban irrigation scheme within the Cornubia Footprint.
Furthermore, the draft from Marshall Dam has reduced to a minimal with the closing of the supply of water to the Mount Edgecombe Golf Course as well as the domestic supply of the SASA Experimental Farm being supplied by Durban Metro and not from Marshall Dam any more.
Marshall Dam is ideally situated within the Cornubia Footprint and reasonable high to reduce pumping cost of the distribution system.
Concept Design Report for Cornubia Urban Irrigation-February 2015
The urban irrigation scheme requires water to be pumped from the preferred water source (new abstraction works on Ohlanga River) to the storage node (Marshall Dam) from where it is distributed to end users.
Both options 1 & 2 (See Chapter 3) will require a rising main from the Ohlanga River following the wetland area parallel to the river until it reaches the Dube West arterial road reserve.
The rising main for both options 1 & 2 will follow the Dube West arterial in a southerly direction until it reaches Cornubia Boulevard. The rising main will then follow Cornubia Boulevard in a westerly direction and discharge into the natural stream just east of the Cornubia Phase 1A Housing Complex.
An alternative position for an outlet works off Dube West arterial close to Marshall Dam has been identified, see Appendix C for details.
6.
DISTRIBUTION ROUTES TO END USERS
Water pumped from the Ohlanga River and stored at Marshall Dam will be distributed through a booster station sitting downstream of the dam. The booster station will distribute irrigation water to end users in three irrigation clusters within the Cornubia phase 1 & 2 footprint.
The three Irrigation clusters comprise a total of 7 irrigation zones supplying a total of 15 developments within Cornubia. Please see Annexure C for a drawing detailing the proposed bulk distribution network.
Distribution pipes will residential collectors, commercial collectors and minor arterials and will be located within the road reserves of these.
Concept Design Report for Cornubia Urban Irrigation-February 2015
Proposed irrigation scheme: It is recommended that the proposed urban irrigation scheme discussed in this report comprising the following elements are approved.
• Water source: o
New Abstraction Works on Ohlanga River
o
Option 1 or Option 2 position
• Storage node: o
Marshall Dam with volume of 250,000 m3 and top water level of ±84 m
• Bulk supply route o
•
Distribution routes to end users o
7.2
Rising mains between New Abstraction Works and Marshall Dam (Option 1 or 2) to follow wetland sections and road reserves as described in Chapter 5. Water is released from outlet works upstream of Marshall Dam. Two options for the position of outlet works apply. Distribution routes from booster pump station at Marshall Dam delivering through a minimum of three rising mains to 3 irrigation clusters of 7 zones supplying 15 developments.
Water Resource Study It is recommended that a detailed Water Resource Study is conducted for the Ohlanga River and proposed abstraction points to confirm the availability of water and confirming the level of assurance of abstraction rates for the Cornubia Irrigation Scheme.
The Water Resource Study will be required as supporting study for the Water Use License Application for the proposed river abstraction points.
7.3
Wetland crossings It is recommended that way leaves and servitudes in wetlands for abstraction works, bulk supply lines and distribution lines are incorporated in the Water Use License Application for this scheme.
Concept Design Report for Cornubia Urban Irrigation-February 2015
