A network of consultants in hydrology, hydrogeology, water quality and water resources

Client: Dar es Salaam Water and Sewerage Authority

Financed by: The World Bank

Period of assignment: Jan 2005 – Jul 2006

Location: Tanzania

Project Value: Unknown

WRA services: £33 900

In co-operation with: Norconsult AS, Norwegian Institute for Water Research (NIVA) and Norconsult Tanzania Ltd

Background

Water Resource Associates assisted the lead consultant Norconsult AS by conducting the surface water hydrology studies in this project, in conjunction with local staff from Norconsult Tanzania Ltd.

The present supply to Dar es Salaam city is from 3 water treatment plants: Upper Ruvu (81 Mld), Lower Ruvu (182 Mld) and Mtoni (9 Mld). This is supplemented with a flow of 36 Mld abstracted from shallow boreholes within the city perimeter. The two largest treatment plants are located on the Ruvu river, about 60 km west of the city, from where the water is transferred through 2 separate pipelines to the city supply network.

The headwaters of the Ruvu river are located in the Uluguru Mountains, far to the west near Morogoro town, and during the dry seasons the perennial flow originating from this area flows across dry plains to reach the treatment plants situated near its downstream end.

This surface water source to Dar es Salaam is inadequate during the low flow seasons and the city has for the last few years been experiencing water shortages during the driest periods of the year.

 Dried-up stream bed of Ngerengere river at end of dry season

Location of all known major water rights in Ruvu basin

Scope of work by Water Resource Associates Ltd

A total of 15 different surface water resource options, of a technical and institutional nature, were evaluated. Their ability to assist in reducing the risk of shortfall at the Ruvu river intakes and to serve new expanding areas of the city were the main objectives.

For the unregulated river case, it was necessary to collect information on upstream licensed abstractions, estimate the future needs for irrigation, public water supply, and downstream environmental flow, conduct low flow frequency analysis and water balance studies. These theoretical studies were supplemented by discharge measurements observed by field teams during the low flow seasons; these surveys were concentrated both in the headwaters in the Uluguru Mountains and in the downstream flood plain.

For the regulated river case, reservoir simulation studies were conducted at the most promising 4 sites. Studies were also made of conjunctive schemes, in which groundwater sources, from both shallow and deep aquifers, were evaluated to see if they could alleviate the dry season shortfall in the Ruvu river.    

Results

Analysis of water levels in the Ruvu river at the Upper and Lower Ruvu Intake Works during 1997-2006 confirm that the present unregulated river does not provide a reliable source of surface water for a major city such as Dar es Salaam. There have been incidents in three separate years during this period when the low flows have been unable to meet the full demand at the two intakes. Comparison of recent and historical discharge measurements indicate a decline in dry season perennial flows, which started in the late 1980s. The reasons for this decline could include climate change, reduction in forest cover, or unknown upstream abstractions, but no definite factor has yet been identified on the ground.

Various options for regulating the river were considered, including impounding dams on the perennial channels of the main stream and larger tributaries, pumped-storage to smaller reservoirs located on ephemeral tributaries, and bunded reservoirs situated on the flood plain in the vicinity of the treatment works. The most promising option was for a reservoir at Kidunda, where the most downstream of four possible dam locations was recommended, in order to alleviate any environmental concerns arising from the adjacent Selous Game Reserve.

If a dam does not prove feasible, the fall-back surface water option would be a transfer of water from the Great Ruaha river along a 15 km route, with pumping to the ridge separating the two basins, from where the water would be gravitated to the Msoro river, which is an upstream tributary of the Ruvu river.