What is a Hydraulic analysis

Hydraulic analysis is the process of assessing the behaviour and performance of water (or other fluid) flow within a system, using principles of fluid mechanics to evaluate how the system functions under various conditions. It is a critical aspect of the design, verification, optimisation, and troubleshooting of infrastructure such as drainage networks, sewers, stormwater systems, pipelines, channels, and treatment facilities.

The goal of hydraulic analysis is to understand how water moves through a system — considering factors such as flow rate, velocity, pressure, depth, and capacity — to ensure the system performs as intended, especially under peak or extreme flow conditions.

Applications of Hydraulic Analysis

Hydraulic analysis is widely used across the water engineering and infrastructure sectors. Common applications include:

  • Design of drainage and sewer systems for urban developments
  • Flood risk assessments and modelling of overland flow paths
  • Sizing of pipes, culverts, and channels to accommodate design storm events
  • Assessment of combined sewer overflows (CSOs) and overflow frequency
  • Verification of Sustainable Drainage Systems (SuDS) performance
  • Pump station design and force main (rising main) sizing
  • Hydraulic optimisation of treatment plant process flows

It also plays a crucial role in retrofitting, rehabilitating, and expanding existing networks by identifying bottlenecks, under-capacity segments, or areas at risk of surcharge and flooding.

Key Elements of Hydraulic Analysis

A complete hydraulic analysis typically involves the evaluation of:

1. Flow Rates (Discharge)

  • The volume of water passing a point in the system over time, usually expressed in litres per second (L/s) or cubic metres per second (m³/s).

2. Hydraulic Grade Line (HGL)

  • The level to which water would rise in vertical pipes connected to the system — essential for determining whether pipes or manholes will surcharge.

3. Pipe and Channel Capacity

  • Determines whether infrastructure can accommodate flows during design storm events (e.g. 1-in-30 or 1-in-100 year rainfall).

4. Velocity and Shear Stress

  • Assesses whether flow is fast enough to prevent sediment build-up (self-cleansing), but not so fast as to cause erosion or pipe wear.

5. Water Surface Profiles

  • Especially important in open channel systems, to model flow depths along the channel under varying flow conditions.

6. Backwater Effects

  • Analysis of how downstream constraints (e.g. outfalls, tide gates) can influence upstream system performance.

7. Inflow/Infiltration (I/I) Contributions

  • Assessment of excess water entering foul sewers due to groundwater infiltration or illegal surface water connections.

Tools and Methods Used

Hydraulic analysis can range from manual calculations using established formulas to advanced computer-based simulations. Common methods include:

  • Manning’s Equation: Used for open channel and gravity pipe flow analysis
  • Bernoulli’s Equation: For energy conservation in pressurised systems
  • Continuity Equation (Q = A × V): Fundamental flow relationship
  • Hydraulic Modelling Software:
    • InfoWorks ICM (urban drainage and flood modelling)
    • MicroDrainage (UK standard for drainage design)
    • SWMM (Storm Water Management Model) – widely used in the US
    • HEC-RAS (river and channel hydraulics)
    • MIKE URBAN/MIKE FLOOD – for complex integrated modelling

These tools allow simulation of real-world scenarios, including rainfall events, system blockages, pump failures, and surcharge conditions.

Inputs and Data Requirements

Accurate hydraulic analysis depends on high-quality input data, including:

  • Topographic and survey data
  • Network layout and pipe attributes (diameter, material, slope)
  • Design rainfall data (intensity-duration-frequency curves)
  • Catchment characteristics (impervious areas, runoff coefficients)
  • Existing flow records or monitoring data
  • Design standards and regulatory requirements

Inaccurate or incomplete data can significantly affect analysis outcomes and lead to incorrect conclusions.

Outcomes and Deliverables

A hydraulic analysis typically produces the following outputs:

  • Flow tables and profile diagrams
  • Flood maps showing surcharge and overflow locations
  • Capacity assessments of individual pipe segments
  • Design verification reports for new or proposed infrastructure
  • Recommendations for upgrades, attenuation, or flow control

These deliverables support decision-making for developers, engineers, water authorities, and planning bodies.

Regulatory and Design Context

In the UK, hydraulic analysis is required to demonstrate compliance with:

  • Building Regulations Part H – for drainage design in new buildings
  • Sewerage Sector Guidance (SSG) – for adoptable sewers
  • Lead Local Flood Authority (LLFA) planning requirements
  • Environment Agency flood risk management frameworks
  • SuDS Manual (CIRIA C753) – for sustainable drainage systems

A robust hydraulic analysis helps ensure systems meet design criteria not only under typical conditions but also under climate change-adjusted scenarios.

Conclusion

Hydraulic analysis is a foundational element of modern water infrastructure planning, enabling engineers to design systems that function safely, efficiently, and sustainably under a wide range of operating conditions. By understanding and predicting how water behaves in both gravity and pressurised systems, it supports flood prevention, system resilience, and long-term infrastructure performance. Whether for new development, rehabilitation, or regulatory compliance, hydraulic analysis is essential for informed, evidence-based decision-making in the built environment.