What is a Hydraulic performance

Hydraulic performance refers to the effectiveness of a drainage or sewer system to convey water — typically stormwater or wastewater — under varying flow conditions. It is a measure of how well the system performs hydraulically, particularly in terms of its capacity, flow conveyance, and resistance to surcharge, flooding, or blockage.

While hydraulic capacity refers to the theoretical maximum flow a pipe or structure can carry, hydraulic performance is a broader and more practical measure. It assesses how the entire system — including pipes, manholes, gullies, flow controls, and outfalls — behaves during real-world operating conditions, such as heavy rainfall, partial blockages, or long-term degradation.

Key Elements of Hydraulic Performance

Hydraulic performance considers multiple interacting factors that influence the movement of water through a system. These include:

  • Flow rate and velocity: Whether the system maintains appropriate velocities to transport water and self-clean solids without causing erosion or sedimentation.
  • Surcharge behaviour: How the system reacts when flows exceed pipe-full capacity, and whether water remains contained or leads to surface flooding.
  • Freeboard and headroom: Whether manholes and chambers provide sufficient space above the hydraulic grade line to prevent spills and backups.
  • Backwater effects: The impact of downstream conditions, such as high river levels or tide gates, on upstream system performance.
  • Structural integrity: The ability of the system to maintain performance under physical stress, root intrusion, or deterioration.

Good hydraulic performance means that a system reliably handles designed flows without unacceptable levels of flooding, surcharging, or pollution.

Evaluating Hydraulic Performance

Assessment of hydraulic performance is often done through modelling, site inspection, and flow monitoring. Engineers may use hydraulic models such as InfoWorks ICM, MicroDrainage, or SWMM to simulate how a system performs during specific rainfall events — such as a 1-in-30 year or 1-in-100 year storm — and to identify any shortfalls or risk areas.

In addition, CCTV surveys, manhole inspections, and level loggers are used to compare observed performance against predicted outcomes. These assessments help determine whether the system performs as intended or requires maintenance, upgrading, or additional attenuation.

Design and Regulatory Context

Hydraulic performance plays a central role in the design and approval of new drainage systems. In the UK, for example, the Sewerage Sector Guidance (SSG) and Building Regulations Part H require that systems:

  • Convey foul water without surcharge in a 1-in-30 year rainfall event.
  • Prevent internal property flooding in more extreme events (e.g. 1-in-100 year).
  • Manage surface water sustainably using SuDS principles, where feasible.

New developments must demonstrate — often through hydraulic modelling — that the proposed drainage systems will not overload existing infrastructure and that they meet performance criteria under both current and climate-adjusted rainfall conditions.

Factors That Degrade Hydraulic Performance

Even well-designed systems can experience a decline in hydraulic performance over time. Common causes include:

  • Silt and debris build-up reducing pipe capacity
  • Fats, oils, and grease (FOG) causing blockages in foul sewers
  • Root ingress into joints and manholes
  • Structural failure such as pipe collapse or misalignment
  • Illegal connections increasing flow volume beyond design assumptions

For this reason, long-term performance is supported by planned preventative maintenance and asset management strategies.

Importance in Asset Management and Investment Planning

Hydraulic performance is a critical parameter used by water utilities, developers, and local authorities to:

  • Prioritise capital investments
  • Justify funding for sewer upgrades or new infrastructure
  • Evaluate the feasibility of new property connections
  • Plan surface water management strategies
  • Comply with environmental regulations and flood risk targets

Monitoring and improving hydraulic performance ensures that drainage infrastructure remains fit for purpose in the face of urban growth and increasing rainfall intensity.

Conclusion

Hydraulic performance is a key indicator of how well a drainage or sewer system functions in practice. It goes beyond static capacity figures to account for the dynamic, real-world behaviour of water within the system — including how it responds under stress, during storm events, or in deteriorating conditions. Ensuring strong hydraulic performance is essential for protecting public health, preventing flooding, and delivering sustainable water infrastructure in both new and existing developments. For engineers and asset managers alike, it is a foundational metric in the effective planning, operation, and maintenance of modern drainage networks.