Managing flood risk and reservoir safety at Havant Thicket

The challenge

The proposed Havant Thicket Reservoir, located north of Havant, is a major infrastructure project designed to enhance regional water supply resilience. In order to deliver the proposed reservoir safely, it was necessary to understand any flood risk implications. Accordingly, the Environment Agency required a robust hydraulic modelling study to assess baseline flood conditions and the impact of the proposed reservoir embankment under various scenarios.

Key challenges included:

• Complex catchment: The relatively small catchment is underlain by clay and includes multiple watercourses, which results in responsive runoff characteristics.
• Scenario diversity: Modelling needed to cover baseline, temporary construction phases, reservoir design conditions (with and without the spillway), as well as sensitivity testing the unlikely event of a reservoir breach.
• Climate change: Upper-end climate change allowances were applied to ensure conservative risk assessments.

The solution

Haskoning developed a high-resolution 1D-2D linked ESTRY-TUFLOW hydraulic model, incorporating:

• LiDAR-derived Digital Terrain Models (1m resolution) for accurate topographical representation.
• Direct rainfall modelling using FEH13 and ReFH2.3 methodologies to simulate multiple storm durations.
• Comparable scenarios covering baseline, temporary construction phases, reservoir design conditions (based on Section 73 documents, including spillway details), and breach conditions.
• Sensitivity tests included variations in surface roughness, inflow, and downstream boundary conditions.

Three primary embankment scenarios were modelled:

1. Without spillway – assessing the total reservoir capacity.
2. With spillway – evaluating downstream flood risk from controlled outflow via the spillway culvert.
3. With breach – simulating the failure of the embankment during a storm (0.1% AEP Wet Day Embankment Breach).