What is a Intermittent Flow

Many drainage systems are designed on the assumption that water is constantly moving through the network. In reality, this is often not the case. Domestic wastewater arrives in short bursts when toilets are flushed or appliances discharge. Stormwater systems may remain completely dry for weeks before carrying large volumes of runoff during a single rainfall event. Industrial processes frequently release wastewater only during production cycles, while pumping stations intentionally create alternating periods of flow and no flow as pumps switch on and off. This operating pattern is known as intermittent flow, and it has a significant influence on hydraulic performance, sediment transport, maintenance requirements and infrastructure design.

Intermittent flow describes any hydraulic condition in which water moves through a pipe, channel or drainage structure at irregular intervals rather than as a continuous stream. The interruptions may follow predictable operational cycles or occur randomly depending on rainfall, equipment operation or human activity. Although the average daily flow volume may be accurately known, the timing of that flow often has a greater impact on drainage performance than the total quantity of water transported.

Understanding intermittent flow is important because many hydraulic problems arise not during the periods when water is moving, but during the intervals when the system remains inactive. Sediment settles, biofilms develop, gases accumulate and flow conditions change before the next discharge arrives. Designing for intermittent operation therefore requires different considerations from designing systems that carry continuous flow.

Where intermittent flow occurs

Intermittent flow is encountered throughout both public and private drainage infrastructure. Some systems experience it by design, while others operate intermittently simply because of the way wastewater is generated.

A domestic foul sewer provides a familiar example. Water enters the pipe for only a few seconds during each toilet flush, shower discharge or washing machine cycle. Between these events, sections of the drainage system may carry little or no flow. Stormwater drainage behaves even more dramatically. A pipe that remains completely dry for several weeks can become fully surcharged within minutes during an intense storm.

Industrial facilities introduce another form of intermittent operation. Food processing plants, vehicle wash facilities and manufacturing sites often generate wastewater only while production is taking place. Outside operating hours, drainage systems may remain inactive despite being capable of handling very large flow rates when required.

Pumping stations also create intermittent flow intentionally. Rather than operating continuously, pumps start only after water reaches a predetermined level within the wet well. They then discharge at a relatively high flow rate before stopping again, producing repeated cycles of movement and stagnation throughout the rising main.

These examples illustrate that intermittent flow is not an abnormal condition. In many drainage systems, it represents the normal operating regime.

Why intermittent flow behaves differently from continuous flow

The hydraulic characteristics of intermittent flow differ fundamentally from those of continuous flow because the drainage system repeatedly alternates between active and inactive conditions. Each period without flow allows physical, chemical and biological processes to develop inside the pipework before they are disrupted by the next discharge.

Sediment transport provides one of the clearest examples. During continuous flow, particles may remain suspended because the water velocity consistently exceeds the self-cleansing threshold. Under intermittent conditions, however, solids begin settling almost immediately after flow stops. If the next discharge is too small or too short in duration, these deposits remain in place and gradually increase over time.

Water quality also changes during stagnant periods. Dissolved oxygen concentrations decline, temperatures may increase and biological activity continues even though the water itself is no longer moving. In foul drainage systems, these conditions encourage anaerobic decomposition, increasing the likelihood of odour generation and hydrogen sulphide production.

Hydraulically, the first discharge following a prolonged inactive period often behaves differently from later flows. Air trapped inside dry pipework, accumulated debris and partially settled deposits all influence how water enters and travels through the system.

Engineers therefore analyse intermittent systems not only during peak flow but also during the periods between discharge events.

Typical sources of intermittent flow

Although the hydraulic pattern varies considerably, intermittent flow generally originates from a relatively small number of operational scenarios.

Common sources include:

• domestic sanitary appliances
• pumping stations
• stormwater drainage systems
• industrial batch processes
• irrigation networks
• vehicle washing facilities
• pressure sewer systems
• temporary construction drainage

Each source produces a distinctive flow pattern. Domestic wastewater usually consists of numerous short discharges spread throughout the day, whereas industrial batch processes may generate large volumes over a relatively short period followed by several hours of inactivity.

Stormwater systems differ again because both the timing and magnitude of flow depend entirely on rainfall. A drainage network designed for intermittent stormwater flow must therefore remain functional after extended dry periods without assuming that continuous flushing will prevent sediment accumulation.

Recognising the origin of intermittent flow helps engineers predict how the system will behave over its operational life rather than relying solely on average daily flow figures.

Engineering challenges created by intermittent flow

Designing for intermittent flow requires attention to issues that are less significant in continuously flowing systems. Many operational problems develop because periods without movement allow changes to occur inside the drainage network.

One issue frequently encountered in older drainage systems is the gradual reduction of hydraulic capacity. During routine inspections, operators sometimes discover significant sediment accumulation in pipes that appear adequately sized according to design calculations. In many cases, the problem is not insufficient diameter but insufficient flow frequency to keep solids moving.

Intermittent operation can also affect instrumentation. Level sensors, flow meters and automatic valves may remain inactive for extended periods before being required to operate immediately. Equipment that functions reliably under continuous use may behave differently after long intervals without movement, making routine testing particularly important.

Designing systems for intermittent operation

Successful design begins with recognising that average flow alone provides only part of the hydraulic picture. Engineers evaluate not only how much water enters the system but also how frequently it arrives and how long inactive periods are likely to last.

Several design parameters deserve particular attention:

• minimum flow velocity during discharge
• duration of inactive periods
• expected solids content
• pipe gradient
• storage capacity
• ventilation requirements
• flushing frequency
• accessibility for maintenance

Pipe gradients often become especially important. Where intermittent flow transports wastewater containing suspended solids, maintaining sufficient slope helps maximise self-cleansing during each discharge event. In some industrial systems, automatic flushing arrangements are installed to supplement natural flow and remove deposits that would otherwise accumulate during prolonged inactivity.

Storage structures may also require different sizing strategies. A balancing tank receiving intermittent discharges must accommodate rapid inflow without assuming that downstream flow will occur continuously.

Inspection and operational management

Intermittent flow systems benefit from maintenance strategies that reflect their operating pattern rather than treating them as conventional continuously flowing networks. Experience from pumping stations and industrial drainage shows that inspections are often most valuable immediately after operating cycles, when evidence of poor hydraulic performance remains visible.

Sediment surveys should focus on locations where flow repeatedly starts and stops, such as upstream of pumping stations, within low-gradient pipe sections and inside balancing tanks. These areas experience the greatest opportunity for deposition during inactive periods.

Where odour has become a recurring issue, engineers frequently investigate whether prolonged stagnation rather than excessive loading is responsible. Improving ventilation, adjusting pump operating levels or introducing periodic flushing may solve the problem without requiring structural modifications to the drainage network.

Monitoring also plays an increasingly important role. Modern control systems can record pump operating frequency, flow duration and inactive periods, allowing operators to identify gradual changes in hydraulic behaviour before maintenance problems become severe.

Intermittent flow in modern drainage design

As sustainable drainage systems, water reuse schemes and smart pumping stations become more common, intermittent flow is receiving greater attention during hydraulic modelling. Traditional design methods often assumed relatively steady operating conditions, but many contemporary systems intentionally store, delay or batch water before releasing it downstream. As a result, intermittent hydraulic behaviour has become a normal design consideration rather than an exception.

Engineers now increasingly assess how periods without flow influence sediment transport, water quality and equipment performance over many years of operation. Computational modelling can simulate not only peak discharge but also repeated start-stop cycles, helping designers identify locations where maintenance requirements are likely to increase.

This broader approach recognises that drainage systems spend much of their service life outside peak operating conditions. Understanding what happens during those quieter periods often provides the key to improving long-term reliability.

Intermittent flow is more than simply water that starts and stops. It represents a hydraulic regime with its own design principles, operational challenges and maintenance priorities. Systems carrying intermittent flow must cope not only with the movement of water but also with the consequences of repeated inactivity, including sediment deposition, changing water quality and fluctuating hydraulic conditions. Designing with these factors in mind allows drainage infrastructure to operate more reliably over decades of service, even when water is present for only a small proportion of the time.