Outeniqua Coast Water Situation Study
By Dudenski, Christo Grange, Aldu Le; Thompson, Isa
IN MAY 2005 THE DEPARTMENT of Water Affairs and Forestry (DWAF), Directorate: National Water Resource Planning commissioned the Outeniqua Coast Water Situation Study (OCWSS), which was completed in December 2007. The need for the study was driven by the fact that a number of towns in the study area were experiencing serious periodic water shortages, mainly because of substantial growth in urban water usage; high ecological reserve requirements associated with the ecologically important coastal rivers; insufficient yield of the existing water sources; and inadequate capacity of the bulk water supply infrastructure. These shortages have resulted in prolonged water supply restrictions, which impact on the potential for future economic growth. Certain municipalities have imposed moratoria on the approval of applications for new developments, pending the implementation of water source augmentation projects.
The purpose of the OCWSS was to investigate the nature and the extent of the water balance problems in the area, to identify the problematic areas and to evaluate possible reconciliation strategies needed to alleviate the yield deficit and to make adequate provision for the ecological Reserve, taking into consideration the current and future situation in the more seriously affected areas, such as the Mossel Bay, George and Knysna municipalities.
OVERVIEW OF THE STUDY AREA
The study area covers about 7 000 km^sup 2^ of land in the coastal belt between Stilbaai and Knysna, which is bounded by the Goukou River catchment to the west, the Noetzie River catchment to the east and the Outeniqua and Langeberg mountain ranges to the north. The area falls under the jurisdiction of the Eden District Municipality and includes significant portions of the Hessequa, Mossel Bay, George and Knysna municipalities.
The total population of about 285 000 (2005) is expected to grow to about 700 000 in 2025 and resides predominantly in urban settlements. The additional seasonal population is estimated at 360 000 (2005) people, which results in exceptionally high seasonal peak water requirements.
The economy is characterised by significant agricultural developments, associated with crop production and afforestation. The only large industrial development of note is PetroSA near Mossel Bay. A notable increase in golf course developments has been observed during recent years, with 17 new developments in the process of implementation.
The study area is drained by 14 perennial rivers, which flow into the Indian Ocean. From a hydrological perspective, the study area covers portions of primary drainage regions H, J and K and is subdivided into 29 quaternary catchments. The area forms part of the Gouritz Water Management Area (WMA) and covers portions of its Gouritz and Coastal subcatchments.
The runoff in the study area is currently regulated by 42 large dams with a total storage capacity of about 65 million m^sup 3^. In addition, the area includes an estimated 11 300 small privately owned farm dams with a combined storage capacity of about 56 million m^sup 3^.
WATER REQUIREMENTS AND LAND USE
A summary of the estimated present and projected future water requirements per user sector for the entire study area is provided in table 1. The distribution of the water requirements between key catchment areas is illustrated in figures 1 and 2.
The urban and rural water requirements were estimated on the basis of the methodology developed for the National Demographic Study (Schlemmer et al, DWAF, 2001). Provision was made for the water requirements associated with the additional seasonal population and all known planned developments, including golf courses and other housing projects. Various scenarios with regard to population and economic growth were developed and evaluated.
Detailed desktop studies were undertaken to determine the present and projected future land-use for irrigation, farm dams, afforestation and invasive alien plants (IAPs) and various scenarios with regard to their future growth were evaluated. The information was used to model the water requirements and to update the hydrological model for the study area.
The area under irrigation is projected to increase from 397 km^sup 2^ in 2005 to 451 km^sup 2^ in 2025. The irrigation is mostly opportunistic, supplied from small farm dams and run-of-river abstraction works. As a result of that, the assurance of supply is relatively low, about 60 96.
The area of the existing forestry plantations in 2005 has been estimated at 472 km^sup 2^. For economic reasons, in 2000, the large forestry growers in the area considered withdrawing significant portions of their plantations. However, in recent years these plans have changed owing to the substantial growth in demand for timber. Forestry managers have recommended that the best practice would be to assume that the present plantation size would remain unchanged until 2025.
The condensed area covered by IAPs is estimated to increase from 221 km^sup 2^ in 2005 to 478 km^sup 2^ in 2025. IAPs are widespread throughout the study area, but the most severe infestation is evident in the catchments of the Klein Brak, Great Brak, Maalgate, Gwaing, Kaaimans and Wolwe rivers, which cover most of the Mossel Bay, George and Knysna municipal areas.
SURFACE WATER RESOURCES
The stream flow analysis for the study area was undertaken using the Water Resources Simulation Model 2000 and the simulated runoff sequences were calibrated against flow records from 12 flowgauging stations. Land use, infrastructure and water use data sets were updated. The naturalised and 2005 MARs for the study area were estimated at 777 million m^sup 3^/a and 558 million m^sup 3^/a respectively.
The Water Resources Yield Model (WRYM) was used to determine the individual yields for the major dams in the study area. Various scenarios with regard to the ecological water requirements (EWR) were also modelled.
The water sources for the Mossel Bay Regional Water Supply Scheme (RWSS), which include a number of large dams (the Wolwedans, Klipheuwel, Ernst Robertson and Hartbeeskuil dams) and run-of-river schemes (Kleinbos Weir and Searles Furrow), are largely interlinked and interdependent. For that reason, these schemes were modelled as a combined system and the system operating rules were optimised.
The system WRYM for the Mossel Bay Regional Water Supply System was configured to include all existing sources, as well as three additional future water source development options, which include the raising of the existing Klipheuwel Dam, the construction of the proposed Bottelierskop Dam on the Moordkuil River, and the construction of a large farm dam on the Varings River. In order to test the sensitivity of the yields, the system was analysed for a number of scenarios, which were composed on the basis of different combinations of the water sources and provision for the EWR.
The yield from all existing sources with no provision for EWR is estimated at 24,3 million m^sup 3^/a. For EWRs of 1 million m^sup 3^/ a and 6 million m^sup 3^/a for the Great Brak Estuary, the system yield is reduced by 1,49 million m^sup 3^/a and 3,58 million m^sup 3^/a, respectively. If the ecological reserve for the Moordkuil River is implemented in future, the yield will be further reduced by 3,18 million m^sup 3^/a. The Klipheuwel Dam, an off-channel dam close to the Moordkuil River, is an existing scheme and the full EWR for the Moordkuil River is at present not supplied.
From a regional perspective, the groundwater recharge is estimated at 351 million m^sup 3^/a. Of this, an estimated 141 million m^sup 3^/a discharges into rivers as the groundwater contribution to base flow and 210 million m^sup 3^/a is potentially available for abstraction. The groundwater resources in the study area are currently not being utilised to their full potential, as the present groundwater use is estimated to be only in the order of 9,21 million m^sup 3^/a.
A local-scale investigation was undertaken to assess the potential for development of groundwater sources to supply either alone, or in conjunction with surface water sources, the water requirements in specific areas. The investigation concluded that groundwater would be a feasible source of supply of the long-term peak requirements for Albertinia, Buffelsbaai, Sedgefield, Stilbaai, Gouritzmond, Vleesbaai and Boggomsbaai.
Limited exploration drilling was undertaken in the Mossel Bay area (quaternary catchment K10A). The results confirmed and exceeded the projections made during the desktop study.
The yield balance for the study area was modelled using the WRYM. The term ‘yield balance’ is defined as the difference between the assured yield of the water resources and the water requirements in a specific area.
Separate ecological reserve determination studies are currently being undertaken by DWAF’s Directorate: Resource Directed Measures (RDM) in order to obtain a better understanding of the ecological water requirements of certain sensitive rivers and estuaries.
The yield balance estimates were undertaken for a number of scenarios based on variations of two parameters, namely the land and water use (present or future scenario) and water sources, for which the yields were determined. In all instances the EWRs were considered and determined on the basis of either the desktop model or as provided by DWAF: RDM. The presentday yield balance for the study area is estimated to be in deficit of about 1,3 million m^sup 3^/a. If no additional water source developments are implemented in future, this deficit will increase to 28,3 million m^sup 3^/ a by 2025. The distribution of the yield balance between key catchment areas for the years 2005 and 2025 is illustrated by colour coding in figures 1 and 2. Even if all presently known proposed surface water development options are implemented, the yield balance in 2025 will still be in deficit of about 8,4 million m^sup 3^/a. The development of alternative sources such as groundwater, desalination of seawater and the use of treated effluent is therefore required.
The water supply situation in the Mossel Bay and Hessequa municipal areas was assessed during the course of this study and possible options for the reconciliation of the water requirements with availability were developed and evaluated. The George and Knysna municipalities recently completed feasibility studies, which evaluated various options for the augmentation of the existing sources of supply for their main water supply centres. The recommendations for all areas are briefly summarised below.
Mossel Bay Municipality
The water requirements for all users supplied by the Mossel Bay RWSS are estimated to grow from 16,8 million m^sup 3^/a in 2005 to 23,8 million m^sup 3^/a in 2025, while the total licensed allocation for the system from all existing water sources is 15,6 million m^sup 3^/a. The system is already in deficit of about 1,2 million m^sup 3^/ a, which is expected to grow to about 8,2 million m^sup 3^/ a in 2025. However, it was estimated during the OCWSS that there is a surplus yield of about 5 million m^sup 3^/a available from the Wolwedans and Klipheuwel dams which has not been allocated yet. Various water source development options were assessed and the recommendations for reconciliation interventions are:
* The DWAF should increase the allocation for Mossel Bay Municipality from the Wolwedans Dam from 2,34 million m^sup 3^/a to 5,8 million m^sup 3^/a
* The municipality should undertake a project to develop and implement a comprehensive Water Conservation and Water Demand Management (WC/WDM) strategy and programme
* The municipality should undertake a feasibility study to identify, evaluate and implement possible supply-side interventions to reconcile the water requirements with water availability. The study should consider the development of groundwater sources, the raising of the Klipheuwel Dam, the construction of the Bottelierskop Dam on the Moordkuil River, the use of treated effluent for irrigation, industrial and potable use, and the desalination of seawater.
Measures to augment the existing source of supply to Riversdale are required. Hessequa Municipality should undertake a study to reconcile the yield of the Korentepoort Dam with the allocations for all users. If surplus yield is available, the municipality should apply for licensing of additional abstractions. The study should also investigate options to increase the yield of the dam by raising of the dam wall and/or clearing of IAPs in its catchment to increase run-off, as well as an investigation into the availability of groundwater sources.
Desktop investigations undertaken during the course of this study have revealed that there is high potential for further development of the groundwater sources to meet the future water requirements for Albertinia, Stilbaai and Gouritzmond. The municipality should undertake further studies to develop additional groundwater sources in these areas.
George Municipality should undertake the following actions:
* A comprehensive WC/WDM study and implementation of all feasible intervention
* Further studies and implementation of the following most feasible surface water source development options: revitalisation of the Kaaimans Weir scheme (already completed), combined with the raising of the Garden Route Dam and the implementation of the Malgas Dam scheme
* Further studies to select the most feasible development options, which will be required to meet the requirements beyond 2022
Knysna Municipality should undertake the following actions regarding the augmentation of the water sources for the Knysna RWSS:
* Expand the existing WC/WDM interventions, identify and implement further interventions
* Raise the Akkerkloof Dam and increase the pumping capacity from the river
* Investigate the refurbishing/upgrading of the Charlesford/ Eastford pumping scheme to utilise the full permitted capacity
* Investigate and implement the best scheme, which will provide storage on or near the Knysna River
* Investigate the feasibility of further use of treated effluent for irrigation
* Undertake further investigations and develop groundwater sources
Water source augmentation options for other towns in Knysna Municipality are:
* Sedgefield Possible augmentation options include the run-of- river abstraction from the Hoekraal River, the construction of an off-channel storage dam at the farm Swartrivier and the development of groundwater
* Rheenendal Augmentation options include the clearing of IAPs in the upper reaches of the Homtini River catchment, the construction of an off-channel storage dam at the farm Suurvlakte and/or the development of groundwater sources
* Buffelsbaai The development of groundwater has been identified as the obvious solution for the augmentation of the existing water source. Potential drilling targets to the north of the town have been identified
* Karatara The development of a new run-of-river abstraction works on the Karatara River, close to the town, has been identified as a possible water source augmentation option. The existing bulk water supply infrastructure needs to be upgraded
CONCLUSIONS AND RECOMMENDATIONS
* The ecological reserve requirements are the key for the determination of the available system yield and yield balance in the area. When the results from the current reserve determination study undertaken by DWAF become available, they should be used to update and refine the conclusions of the OCWSS regarding yields and yield balances
* DWAF should undertake further studies to verify the allocation and lawful water use for irrigation and to update and refine the estimated areas covered by irrigation, afforestation and IAPs
* IAP clearing projects in the area appear to have huge potential for preservation of the water resources
* In order to enhance future hydrological modelling, DWAF should refurbish certain flow-gauging stations in the area, which yielded inadequate flow records
* The groundwater resources can play a meaningful role in addressing many of the water supply challenges in the study area
* All municipalities in the study area have implemented some WC/ WDM interventions. However, the municipalities should undertake the necessary formal and comprehensive WC/WDM studies, identify specific goals, and develop and implement a defined and structured WC/WDM strategy and programme to reduce their water requirements
The study was undertaken by UWP Consulting (Pty) Ltd, in association with BKS (Pty) Ltd, as the lead professional service providers, supported by various sub-consultants and DWAF directorates providing specialist services
Text Christo Dudenski
UWP Consulting (Pty) Ltd
Aldu le Grange
BKS (Pty) Ltd
Department of Water Affairs and Forestry
Copyright The South African Institution of Civil Engineers Jun 2008
(c) 2008 Civil Engineering : Magazine of the South African Institution of Civil Engineering. Provided by ProQuest Information and Learning. All rights Reserved.