Challenge

The original dam upgrade, completed in 2016, was designed as a 13 m high earth embankment with a clay core and a water storage volume of 1.1 million m³.  The facility includes an intake tower and outlet tunnel which feeds water to the adjacent water treatment plant. Subsequent high rainfall identified leakages at the dam foot and through the concrete of the intake tower.

In 2022, Eastern Water & Sanitation Company appointed SMEC to implement Katete dam rehabilitation works. The client’s objective was to secure the facilities long-term stability whilst enhancing operational management – in accordance with international standards. The project was staged as follows:

• Stage 1: Detailed Design and Tender Documents
• Stage 2: Assistance During Tendering & Procurement of the Contractor
• Stage 3: Construction Supervision & Commissioning of remedial works (Supervising of Testing and Commissioning of the Works)
• Stage 4: Preparation of Final Project Report
• Stage 5: Preparation of Operation and Maintenance Manual, Emergency Preparedness Plan and associated training.

Solution

SMEC provided engineering design, tender documentation, construction supervision and training of staff in operations and emergency procedures. The construction phase was tendered in two lots to reduce risk, simplifying procurement and contractor appointment.

The team applied a downstream focus to minimize construction risks and interference with the dam’s primary water supply function. SMEC also designed for dry season construction. The strategic timing of the work would ensure a safer and simplified construction processes.

Engineering interventions

Lot 1

• Construction of a rock drainage berm to control the seepage in a safe manner.
• Improvements to the stilling basin to prevent excessive turbulence and to protect the opposite bank of the Katete River downstream of the dam.
• Implementation of safety and monitoring infrastructure.

Lot 2

• Modifications to the spillway and road culverts to make them more resilient to high water flow. Detailed hydrological analysis and simulations were undertaken to determine the velocity range and material selection for lining the spillway. Finite Element Analysis of the structural components was undertaken to understand different loading conditions.
• The spillway arrangement comprises a pointed masonry lined channel and is 410 m long. The approach channel and spillway are lined with Chute Blocks to dissipate water velocity and reduce erosion in high velocity occurrences.
• The rock berm design and construction sequencing were another key consideration for the team. Building a carefully designed and structurally sound rock berm provides additional reinforcement to the downstream side of the dam.
• Construction of a supplementary wave wall to limit wave damage to the dam crest during flood events.

Environmental and social due diligence

• Screening of environmental, social, health and safety risks and opportunities.
• Preparation of an environmental and social management plan according to KfW Development Bank Sustainability Guidelines.

Management procedures

• Production of an Operations and Maintenance Plan including dam safety plans
• Development of operation and maintenance procedures and training staff in the new procedures
• Preparation of the Emergency Preparedness Plan, with the participation of all stakeholders

Impact

Reservoir Intake Tower, Spillway and Rock Berm

Leakage monitoring instruments installed around the intake tower create an early warning system for structural issues. Detecting leaks early allows for timely repairs, preventing more extensive damage to the tower and the dam structure. Formal training to staff on how to manage the reservoir will reduce stress on the intake tower. This includes controlling the water level and the rate at which water is drawn, minimizing the potential for leaks.

The spillway design increases stability and offers further protection against erosion. A drainage trench on the rock berm also improves water management, preventing water buildup that could weaken the berm or the dam itself. These measures proactively protect the integrity of the dam, ensuring reliable water supply whilst reducing the risk of catastrophic dam failure. The entire design has been simulated against a more extreme 1000-year flood scenario.

Community benefits and Knowledge Transfer

Katete Dam is a vital water resource for the people of Katete. The design measures protect the downstream area which also reduces any risk of destabilizing the dam foundation. Approximately 100,000 people will benefit from the enhanced safety measured implemented.

SMEC has curated a bespoke program to upskill twenty-five people from the dam operator’s disaster management committee and selected staff from the Eastern Water Sanitation Company.  The program enhances knowledge and skills required for the effective implementation of the Katete Dam Operation and Maintenance (O&M) and Emergency Preparedness Plan.