What is a Time of entry

Time of entry is a key concept in hydrology and drainage design. It refers to the time taken for surface runoff generated by rainfall to travel from its point of origin on the ground surface to the point where it enters a drainage system, typically through an inlet such as a gully, channel drain, or kerb opening. This time is an essential parameter in determining the time of concentration for a catchment and directly affects how quickly runoff reaches pipe networks, swales, or stormwater attenuation systems.

In urban drainage systems, understanding time of entry is fundamental to the accurate modelling of stormwater behaviour, the sizing of pipes and inlets, and the prevention of flooding. Misjudging this seemingly small interval can result in under-designed systems, surcharging, or overflows.

This article explores the definition, significance, factors influencing time of entry, and how it is applied in drainage engineering.

The Role of Time of Entry in Surface Water Hydrology

Surface runoff does not reach a drainage system instantaneously once rain begins to fall. Initially, water must travel overland, moving across different surface types such as roads, pavements, lawns, or roofs, depending on the nature of the site. The time of entry encompasses this initial overland travel time, from the point of rainfall impact to the point where water physically enters a formal drainage infrastructure element.

This concept is most relevant in urban environments where surface runoff must be collected and conveyed efficiently to prevent standing water, property damage, or traffic hazards.

Time of entry is typically measured in minutes and is considered as part of the time of concentration, which includes both the time of entry and the time required for water to flow through the pipe network to a point of interest, such as a manhole, junction, or outlet.

In practical terms, the shorter the time of entry, the faster the runoff reaches the drainage system, and the greater the potential peak flow. Therefore, time of entry directly influences the peak discharge rate and the design criteria of pipes, channels, and detention structures.

Key Factors Affecting Time of Entry

Several variables influence how long it takes for runoff to enter the drainage system. These include natural, engineered, and site-specific conditions.

1. Surface Type and Roughness

The nature of the ground surface plays a major role in the speed of runoff:

  • Impervious surfaces (e.g. asphalt, concrete, roofs) offer minimal resistance to flow, leading to shorter times of entry.

  • Pervious surfaces (e.g. grass, gravel, bare soil) absorb some water and slow down flow, increasing the time of entry.

  • Surface roughness and texture also impact flow speed. Rougher surfaces cause friction that delays movement.

2. Slope and Gradient

Water moves faster downhill. Steeper slopes reduce the time of entry by allowing gravity to accelerate runoff. Flat or slightly graded areas slow down overland flow and may even allow for some infiltration or ponding.

3. Flow Path Length

The horizontal distance from the furthest point of runoff origin to the nearest inlet significantly affects the time of entry. A longer overland path means more time is required for water to reach the drainage point.

4. Obstructions and Surface Features

Barriers such as kerbs, landscaping elements, speed bumps, and vegetation can interrupt or divert flow, increasing travel time to the drainage point.

5. Inlet Spacing and Location

Well-positioned and closely spaced inlets reduce time of entry, as water does not have to travel far before entering the system. Conversely, widely spaced inlets may allow water to accumulate before it finds a path into the network.

6. Rainfall Intensity

High-intensity storms can produce more runoff in less time, reducing the impact of surface characteristics and accelerating entry into the system. Low-intensity rainfall may infiltrate or evaporate before reaching a drain, especially on pervious surfaces.

Importance in System Design

Understanding time of entry is essential when performing hydrological and hydraulic calculations, especially for:

  • Drainage design in urban areas

  • Sizing of pipework and inlets

  • Attenuation and storage system modelling

  • Surface water management plans (SWMPs)

  • Flood risk assessments (FRAs)

Incorrect estimation of time of entry can lead to:

  • Underestimated peak flows that result in undersized pipes

  • Overestimated storage requirements, leading to unnecessary construction costs

  • Inadequate inlet design, causing local ponding or surface flooding

By accurately accounting for time of entry, engineers can design more efficient and resilient systems.

Methods of Estimating Time of Entry

There is no universal formula for calculating time of entry, as it depends heavily on local conditions. However, the following approaches are commonly used:

Empirical Estimation

For small catchments or where detailed data is unavailable, simple empirical values are often applied based on surface type:

  • Roofs and roads: 1 to 2 minutes

  • Pavements and hardstanding: 2 to 5 minutes

  • Grass or open ground: 5 to 10 minutes

These values are typically added to the pipe flow time to determine the total time of concentration.

Kinematic Wave and Overland Flow Models

For more complex or critical designs, overland flow equations such as the Manning’s equation for sheet flow or the NRCS (formerly SCS) method can be used. These models consider surface slope, roughness, and rainfall intensity to estimate flow velocities and travel time.

Computer Modelling

Modern hydrological modelling software such as MicroDrainage, InfoDrainage, and Civil 3D includes tools to simulate overland flow and calculate time of entry with high precision. These models allow for variable inputs and are often used in large-scale developments or in flood-sensitive areas.

Practical Applications

Time of entry is used across a range of drainage design and planning scenarios, including:

  • Road and highway drainage: Ensuring runoff enters the carriageway drainage before reaching junctions or low points

  • Housing developments: Calculating how quickly water from roofs and drives enters the main system

  • Commercial sites: Modelling large paved areas such as car parks

  • Greenfield to brownfield transitions: Comparing natural and developed flow paths

Engineers and drainage designers must account for the variability of time of entry based on the layout, materials, and usage of a particular site. Site surveys, digital terrain modelling, and rainfall data all contribute to refining the estimates.

Design Optimisation Using Time of Entry

By factoring time of entry into early-stage design, professionals can improve the efficiency and resilience of surface water management systems. Key opportunities include:

  • Optimising inlet locations to intercept flow efficiently

  • Designing swales and kerb channels to provide controlled flow paths

  • Sizing pipes and culverts based on actual peak flow timing

  • Improving urban flood prevention by limiting the accumulation of runoff

A well-calculated time of entry also supports accurate hydraulic simulation, which is vital for ensuring that drainage systems operate within their design thresholds during both common and extreme weather events.

Time of Entry vs. Time of Concentration

It is important to differentiate between time of entry and time of concentration, although they are often used together in design calculations.

  • Time of entry: Refers specifically to the time it takes for surface runoff to reach the drainage network or channel inlet from the point of generation on the ground.

  • Time of concentration: Includes the time of entry and the time taken for water to flow through the drainage system to the point of interest (e.g. outfall or storage tank).

Understanding both helps engineers model flow peaks more accurately and avoid systemic underperformance during high rainfall events.

Maintenance and Operational Implications

While time of entry is primarily a design parameter, it has implications during the operational phase of a drainage system. For example:

  • Blocked inlets increase the effective time of entry by delaying access to the system

  • Surface wear or compaction may reduce infiltration and change flow paths

  • Vegetation overgrowth can alter flow speeds across grassed areas or swales

As such, regular inspection and maintenance of surface drainage features help preserve the assumptions made in design, including those related to time of entry.

Conclusion

Time of entry is a fundamental but often underappreciated element in the design and assessment of surface water drainage systems. It defines the initial period of runoff travel before entering a pipe or formal drainage network and directly influences the calculation of peak flow rates, system sizing, and flood mitigation strategies.

Accurate estimation of time of entry, supported by site data and hydrological modelling, leads to better-performing, cost-effective, and sustainable drainage systems. Whether in urban road design, residential development, or industrial sites, understanding this key metric ensures that stormwater is managed safely and efficiently, even under challenging weather conditions.

As urban development continues and climate variability increases, the role of precise surface water modelling — starting with parameters like time of entry — will remain central to building resilient infrastructure that protects people, property, and the environment.