Did you know that a good drainage design does not always send rainwater away as quickly as possible? That sounds backwards in a town where sudden tropical downpours can make the pavements gleam and the gutters run full. Yet one of the important principles of modern stormwater management is to slow some of the flow at its source, instead of allowing every hard surface to send its water downstream at the same moment.

Punggol offers a useful setting for understanding the idea. It has dense housing, paved roads, planted landscapes, a major man-made Waterway and reservoir connections. In a built-up town, rainfall does not land on one uniform surface. Some drops reach roofs, some hit roads, some fall on gardens, and others run towards drains. What happens during the first few minutes can influence the peak amount of water arriving downstream.

This guide explains why delaying a portion of stormwater can matter in Punggol. It focuses on the timing of runoff, not a new list of rain gardens or a claim that any particular housing block is flood-proof. It also shows families how to think about rainfall safely and how students can use a simple mathematical model to understand the difference between total water and peak flow.

The answer in 50 seconds

PUB, Singapore’s National Water Agency, explains that modern stormwater management combines approaches at the source, along pathways and at receptors. For new developments and redevelopments of at least 0.2 hectares, on-site measures are required to slow surface runoff and reduce peak discharge to public drains. Such source measures can include detention tanks, ponds and green features such as bioretention basins.

The logic is straightforward: if rain falls across many hard surfaces and rushes into the same drain at once, the downstream system must handle a large short-term demand. Temporarily holding and gradually releasing some runoff helps reduce that peak. Rain gardens and similar features can also filter certain pollutants when designed and maintained for that purpose.

A rain garden is not a guarantee against flooding. No neighbourhood is free from every possible extreme-weather risk. The key idea is managing the timing and quality of water, not pretending rain can be switched off.

The question hidden in an ordinary downpour

Picture a Punggol street during heavy rain. Water hits a roof and moves towards its gutter. Water on a footpath seeks the lower edge. Water on an open planted area may be absorbed, temporarily held or eventually flow elsewhere, depending on conditions. Each small route has its own speed and capacity.

Now imagine many roofs and paths draining into a shared downstream network. Even if the total rainfall is manageable over a long period, it can be challenging if too much runoff arrives at the same location during a short interval.

This reveals a distinction that is easy to miss: how much rain falls and how fast runoff arrives are different questions. A bucket can hold ten litres in total, but pouring all ten litres into a narrow funnel at once can cause overflow. Pouring gradually may let the same funnel carry the water without overflowing.

The bucket-and-funnel picture is only an analogy. Real drainage design involves changing rainfall, soil conditions, slopes, storage, outflow controls, maintenance and engineering safety factors. Still, the model captures why timing matters.

What ‘runoff’ means in plain English

Runoff is rainwater that moves over land or developed surfaces rather than staying where it lands. In a town, it may travel across roofs, pavements, roads and landscaped areas before entering drains, detention features, canals or other parts of the water system.

Different surfaces behave differently. A roof is intended to shed rain. A paved road offers limited direct infiltration compared with some suitable soils. A planted area may let water enter the soil, but not infinitely; the infiltration rate depends on its composition, compaction and how wet it already is.

Children sometimes assume that every green patch behaves like a sponge that never fills up. That is incorrect. A saturated or poorly draining soil can still produce surface runoff. Likewise, a rain garden can temporarily pond water by design, but it has limits and an overflow arrangement.

The useful habit is to look at the path of the water from the point it first lands to the next place it reaches. That provides a more accurate understanding than saying simply, “There is a drain nearby.”

Why paving changes the movement of rainfall

A neighbourhood with many roofs, roads and footpaths often sends water into drainage systems differently from land covered by vegetation and soil. This does not mean that a natural landscape absorbs every storm or that paving is inherently wrong. Towns need safe roads, housing and accessible walking routes.

The change is in how much water reaches the surface network, how quickly it gets there and where it concentrates. Hard surfaces commonly route water towards constructed edges, pipes and drains. A well-designed urban system has to consider those flows together rather than designing each building as if it were alone.

Punggol’s residential landscape therefore presents a familiar modern engineering task: accommodate development while keeping drainage and water management reliable. This is not unique to Punggol. It is a principle relevant throughout built-up Singapore.

The reason it is worth explaining locally is that residents see the interfaces every day: roof edges, landscaped depressions, roadside drains, the Waterway and ponds or planted water-management features in appropriate developments.

The important distinction: volume and peak flow

Suppose two hypothetical developments each produce 1,000 litres of runoff in a simplified storm. Development A releases most of its water into the downstream drain during the same short period. Development B temporarily holds some runoff and releases it over a longer period.

Both places may eventually discharge similar total volumes, depending on losses and design. But their peak flow rates can be quite different. The peak is the highest rate at which water reaches a point over a particular interval.

This matters because drains and downstream systems have capacities. If several upstream sources discharge their largest flows at exactly the same time, the combined peak can be larger than if some flows are delayed. A design may therefore seek to spread discharge over time, within appropriate engineering limits.

There is a subtle but important qualification. Detention is not simply “keep all water as long as possible.” The timing of release must be designed carefully in relation to downstream conditions and the wider catchment. An unsuitable release schedule can create other problems. Professional hydraulic design, not a household guess, determines what is appropriate.

Source, pathway and receptor: a useful three-part model

PUB describes a holistic framework for managing flood risk. Source measures act where runoff begins. Pathway measures concern the systems that convey water, such as drainage channels and improvements. Receptor measures concern protecting people, buildings and other assets that could be affected.

A child can understand these as three questions. First, can we reduce or delay the water entering the public drain? Second, can the drainage route handle the water that reaches it? Third, how do we reduce harm where people or important spaces might otherwise be exposed?

These functions complement one another. A rain garden is not a replacement for every drain. A larger drain does not remove the need for reasonable site management. Measures that protect a building may still be necessary even when upstream controls are present.

The most useful lesson is that a serious problem rarely has a single magic solution. Flood resilience involves multiple layers, and each layer has limitations. Punggol’s planners and residents benefit from understanding that distinction rather than expecting any one attractive landscape feature to solve every storm.

What detention really means

Detention means holding water temporarily and releasing it later at a controlled rate. It differs from using a permanent reservoir simply as a scenic waterbody, and it differs from assuming rainwater vanishes because it has entered a green area.

A detention feature may have storage space that fills during a storm. The water can then leave through a designed outlet, infiltrate where conditions permit or pass through an overflow route under greater flows. The actual mechanism depends on the feature.

For a simple classroom model, imagine a tray that receives water from a jug while releasing it slowly through a small controlled opening. If water arrives faster than it leaves, the amount stored rises. When the rainfall stops, the remaining water continues to discharge until the storage level falls.

That relationship is the heart of detention. But do not turn the model into an outdoor experiment at a real drain. Public drainage infrastructure is not a children’s laboratory; rain events and water conditions can change quickly.

Why rain gardens belong in the explanation

A rain garden, also called a bioretention basin in appropriate technical contexts, is designed to receive runoff into a planted, engineered environment. PUB’s ABC Waters guidance explains how bioretention systems can help with filtration, detention and some biological uptake of certain contaminants.

A well-designed rain garden is more than a decorative flower bed. Its soil and drainage layers are selected for the intended treatment and hydraulic role. The plants are part of the system, but the engineering also includes how water enters, how long it is stored, how it filters and where it exits.

It would be misleading to label every hollow planted area in Punggol as a rain garden. A landscaped depression might serve another purpose entirely. The existence of greenery does not prove that a specific feature was designed or approved for stormwater treatment.

For a verified introduction to the local forms, the earlier Punggol guide to rain gardens and swales is the right companion. This article concentrates on why slowing discharge matters to the broader network.

Why a swale is not identical to a rain garden

The terms are related but should not be collapsed. A vegetated swale can be a planted channel that helps convey water, with suitable designs also supporting infiltration or treatment. A bioretention swale combines conveyance with engineered filtration and detention properties. A bioretention basin or rain garden is more focused on treatment and temporary storage rather than serving as a conveyance channel.

PUB publishes engineering procedures for different ABC Waters design features, showing why the distinctions matter to professionals. The correct feature depends on the site, incoming flows, outfall and required performance.

To a casual observer, all three may look green. But appearance does not reveal the full hydraulic function. Just as a school playground and a football field may both have grass but different uses, planted water-management features can have different roles.

The family-level lesson is to notice form without inventing a technical label. If a sign or official site description identifies a feature, use that verified name. Otherwise, describe what is visible and ask what further information would confirm the design.

Why clean water and flood control are separate goals

An engineered landscape may reduce some contaminants through filtration and related processes. That is one reason PUB’s ABC Waters programme includes bioretention systems. But improving runoff quality and reducing peak runoff rate are not identical measurements.

Imagine two hypothetical systems. One filters water well but releases it too rapidly to make much difference to a downstream peak. Another delays water effectively but provides limited treatment of pollutants. The systems solve different portions of the problem.

Good design can sometimes pursue both objectives, along with greenery and public amenity, but performance must be assessed rather than assumed. A clear-looking outflow is not proof that every contaminant has been removed. The colour of water alone cannot establish its chemical or microbiological safety.

This is an important science lesson for children: one observation cannot answer every question. A system may be attractive, may filter some runoff and may reduce peak flow, but each claim needs its own evidence.

Why Punggol’s Waterway is not the same as a rain garden

Punggol Waterway is a major planned element of the town’s waterfront system. Its landscape and connected reservoirs are part of a much wider water story. It is not interchangeable with a small precinct-level bioretention basin.

Scale matters. A neighbourhood rain garden may manage runoff from a relatively small contributing area. A canal, waterway or reservoir operates within a larger catchment and infrastructure network. Different authorities, designs and operations govern the systems.

A useful way to understand the relationship is to follow the idea of scale. Rain may start on a balcony roof or playground. It may pass through small site-level features or drains. It then becomes part of a broader network that ultimately conveys water through the catchment. The precise route at a particular address must be verified rather than guessed.

When children see the Waterway after rain, invite them to ask where nearby runoff could originate. Do not tell them that every puddle in the town flows directly to the Waterway or that every planted space is hydraulically connected to the same point.

Why extreme rain still requires caution

Even a well-designed system has limits. Rainfall intensity can exceed the capacity assumed in a particular design, maintenance can affect performance, and several contributing conditions may occur together. Urban flood resilience reduces risk; it does not eliminate every possible extreme event.

PUB provides flood-risk and stormwater-management information, including the need to improve drainage systems and strengthen source controls. For residents, the sensible response to heavy rain is to pay attention to official advisories and local conditions rather than assuming a familiar route is safe simply because it has good infrastructure.

Do not enter flooded roads, drains, canals or waterlogged underpasses. Avoid attempting to wade through uncertain water depths. Children should stay away from drainage openings and fast-moving water, especially during storms.

The lesson here is not fear. It is respect for a system that is designed to manage risk within real limits. Engineering and personal safety behaviour work together.

What happens when rain falls faster than a drain can carry it?

Think of water entering a narrow route. If more arrives during a particular moment than can be carried forward, water can temporarily accumulate upstream. Real drainage and flooding involve many factors, including topography, rainfall distribution, obstructions and interactions with downstream conditions.

It is tempting to explain every puddle by saying “the drain is blocked.” Sometimes a blockage is relevant. But visible standing water may also have other causes, and a photograph does not establish which one applies. The correct response is to report a concerning location through suitable official channels and let the responsible authority assess it.

A child can use a simplified model: a wide funnel receives a continuous stream while a narrow outlet can pass only so much per minute. If incoming flow exceeds outgoing flow, the reservoir of water rises. This is a general conservation principle, not a diagnosis of a specific Punggol street.

The concept transfers to traffic, queues and even computer networks. Whenever inflow temporarily exceeds capacity, a backlog builds. Managing demand and capacity is essential.

Why timing can matter more than total rainfall alone

Consider a hypothetical storm that delivers the same total amount of water in two different patterns. Pattern A spreads the rain over several hours. Pattern B releases much of it in a short intense burst. The total may be similar, but the peak load on drains can differ.

Now add two neighbourhoods draining into the same downstream system. If their peaks coincide, the combined peak may be challenging. If one development can safely slow a portion of its discharge, the downstream peak may be reduced. Designers must analyse the exact hydrograph—how flow varies over time—rather than make a rough guess.

This is why the words rate, duration and storage are just as important as the word volume. The public may focus on “how much rain fell,” but a civil engineer must consider the time pattern and location of the flow.

A Secondary 1 Mathematics student can explore this with a simple table of hypothetical litres per minute. A Secondary Geography learner can relate it to impervious surfaces and drainage management. The core reasoning stays the same across subjects.

An easy numerical example: holding water back

Assume a simplified fictional site produces 600 litres of runoff over six minutes. If the discharge were exactly 100 litres per minute throughout, it would be straightforward to model. But suppose the runoff instead arrives unevenly: 50, 150, 200, 100, 70 and 30 litres in consecutive minutes.

The total is 600 litres. The highest one-minute incoming amount is 200 litres. Now imagine a hypothetical storage feature able to release 100 litres per minute whenever sufficient water is available. In a simplified ledger, the feature may build a stored balance during the larger incoming minutes and release it later.

The point is not to design a real drainage system using this six-row table. Real engineering requires rainfall intensity, catchment analysis, outlet characteristics, overflow and safety considerations. The child should instead see why a temporary storage balance can lower the immediate outgoing peak.

Another useful question is what happens if the storm lasts much longer than expected or if the storage volume is insufficient. The answer is that storage has limits, and an appropriate overflow and risk management plan is essential.

Why the outflow cannot simply be blocked

A frequent misunderstanding is that the safest response to heavy rain would be to seal every outlet and hold all water on the site. That would create a new problem. Storage is finite. Once full, water must have a safe route under the conditions for which the system is designed.

PUB’s guidelines distinguish treatment, storage, conveyance and overflow features. That distinction matters because a design must cope with ordinary rain and larger events within the planned performance envelope.

For children, use an empty cup as the analogy. A cup stores water successfully until it is full. Holding the cup does not change how much more it can contain. Once the capacity is reached, additional incoming water overflows unless there is another path.

This is why engineered stormwater features need appropriate dimensions and exits. Do not attempt to “improve” a real public drainage feature by blocking an opening or moving material into it.

Why maintenance belongs to the science

A feature may perform well when first built but less well if it is not appropriately maintained. Sediment, litter, damaged vegetation or clogged filter material can affect intended operation. PUB’s design guidance addresses the need to protect and maintain bioretention systems.

This reveals an easily overlooked point: an infrastructure system is not finished when construction ends. Operation, inspection and maintenance remain part of its real-life function.

For a resident, practical responsibility means not dumping rubbish into drains or planted stormwater features and reporting suspected problems through the proper channels. It does not mean entering maintenance areas, opening grates or performing unqualified repairs.

For students, maintenance is an engineering concept worth learning early. A design can be excellent on paper and still fail if the real-world conditions are ignored. Reliability is a continuing process, not a one-time achievement.

Why the planted edge can also support biodiversity

When water-management features are landscaped, they may contribute to local habitat or visual greenery in addition to their primary hydraulic role. PUB’s ABC Waters programme reflects an intention to integrate water and public environments, with different forms of treatment and landscape.

But one benefit does not guarantee another. A plant chosen for a particular engineered wet-dry setting may differ from a plant chosen for wildlife habitat. Some systems must prioritise drainage performance, maintenance access and safety. The best outcome depends on the full design.

Avoid assuming that a rain garden should always be densely planted with whichever species looks prettiest. Its soil profile, inlet, outlet and vegetation must operate as a system. Changes should be made by the responsible professional team.

Parents can use this to explain that cities often solve more than one problem in the same space. A planned landscape might manage water while providing greenery, but those objectives need to be balanced intelligently.

What residents should actually notice on rainy days

Do not treat a storm as an invitation to explore drainage infrastructure. From a safe sheltered position, a family may notice ordinary things: water running along the edge of a pavement, a depression holding rain temporarily, or a roadside drain carrying runoff.

The educational task is to ask where that water came from and what might happen next. It is not to touch, enter or test the infrastructure. Children can sketch a hypothetical version at home after the weather has passed.

If water is accumulating in a location where it could cause harm, residents should use current official reporting channels. Explain the location, time and observable condition without guessing that a particular design feature failed. Photos taken from safe positions may help, but there is no reason to take risks for a picture.

The most mature observation is both curious and modest. You can see a water level rising; you generally cannot infer the full state of a catchment from that sight alone.

A short explanation of the ‘sponge city’ idea

People sometimes use the phrase “sponge city” for urban approaches that retain, slow or absorb more rainfall through suitable green and blue infrastructure. It is a helpful metaphor, but it can be too broad if used carelessly.

A literal sponge absorbs water until its capacity is reached. Real urban systems combine multiple mechanisms. Some water infiltrates, some is detained, some is treated and some is discharged through engineered pathways. No single planting design can replace all pipes and channels.

For Punggol residents, the more useful vocabulary is detain, filter, convey, release and protect. Each word represents a function that can be examined in a real design. “Sponge” is memorable, but precision explains what actually happens.

This helps children connect a popular environmental slogan to the physical processes that must make it work.

Why this is also a lesson in public policy

PUB’s stormwater-management approach sets requirements for larger new developments and redevelopments. That is an example of shifting part of the responsibility to the place where runoff begins, rather than relying exclusively on public drainage enlargement downstream.

The broader policy principle is that many small upstream contributions can affect a shared system. The drainage network serves more than one individual property. If one site changes how it discharges runoff, the consequences may extend beyond its boundaries.

Rules therefore need technical definitions, qualified design and appropriate enforcement. A general educational article cannot determine whether a particular property complies with the latest requirements. That is a professional and regulatory matter.

A child can still understand the civic idea: living together in a town means our buildings and actions interact. Managing a shared resource is more complicated than each person focusing only on the space inside their own gate.

Three scenarios that bring the explanation to life

Scenario one: a building roof during a storm

A school project imagines a large roof collecting rain. The students are asked whether sending every drop into the public drain immediately is always the only possible design. They investigate the concept of controlled on-site detention without proposing any do-it-yourself modification.

The class identifies three questions: how much runoff arrives, how quickly it arrives and what safe overflow route is needed. The teacher explains that actual drainage design requires professional work and regulatory compliance.

The lesson is that reducing a peak is not the same as stopping rainfall.

Scenario two: the planted depression beside a path

A child notices a planted low area and asks whether it is a rain garden. The adult responds, “It might be, but let’s check the official information rather than guess from appearance.”

If the area is documented as a bioretention feature, the family can discuss water entering engineered soil layers and later leaving through a designed outlet. If it is not documented, they can still discuss the general science of surface runoff without assigning the wrong label.

The lesson is to distinguish a visible observation from a verified technical function.

Scenario three: a community report after heavy rain

Residents notice persistent standing water near a regular walking route. They record the exact safe location and the time, avoid entering the water and use an appropriate authority reporting channel. They do not claim that a specific pipe has failed without evidence.

An engineer or responsible agency may need to inspect the situation and evaluate the actual cause. The residents’ contribution is a precise report of the symptom, not a speculative technical diagnosis.

The lesson is that good observations can help public systems, while unsafe amateur interventions can make matters worse.

What this means for learning Science and Mathematics

For Primary Science, the entry point is properties of materials and observation of water movement. Students can ask why a sloping surface directs runoff, why different soils behave differently and why water collects in low places.

For Mathematics, the central ideas include volume, rate, graphs and the difference between a total and a maximum. A graph of discharge against time can show a sharp peak and a more spread-out pattern with the same or similar total area.

For Secondary Geography, the topic connects to land use, urbanisation, runoff and human management of physical environments. For design and technology, it illustrates trade-offs between performance, safety, maintenance and cost.

None of these school connections requires children to visit flooded areas. The concepts can be explored with teacher-approved classroom materials, diagrams and safe observations in ordinary conditions.

Mistakes to avoid when explaining stormwater

Do not say that all rainwater should leave a site instantly. Do not say that a rain garden removes the need for public drains. Do not assume a green space can absorb unlimited water.

Do not claim that reducing a peak at one point guarantees zero flooding everywhere. Do not assume every ornamental pond or planted depression is part of an approved stormwater system. Do not confuse visibly clear water with verified drinking quality.

Another mistake is treating flood resilience as only a problem for engineers. Residents still have responsibilities in keeping public spaces free of litter, following warnings and reporting concerns. Equally, it is a mistake to tell residents to fix drains themselves; unauthorised interventions can be unsafe.

Finally, avoid invented performance numbers. If a design has an approved runoff target, it should come from a credible source for that exact development, not from a general article about Punggol.

Frequently asked questions

Why would a drain need water to arrive more slowly?

Reducing short-term peak inflow can help manage the demand placed on downstream drainage infrastructure. PUB promotes on-site runoff detention as part of flood resilience.

Does slowing rainwater mean leaving puddles everywhere?

No. Engineered detention has planned storage, treatment and outflow arrangements. Random standing water is not automatically a designed feature.

What is the difference between runoff volume and peak flow?

Volume is a total quantity of water. Peak flow is the maximum rate at which water passes a point during a time interval. Two situations can have similar totals but different peaks.

What is a rain garden?

A landscaped bioretention basin designed to receive and treat runoff through an engineered soil and planting system, subject to the specific design and performance requirements.

Is a swale the same thing?

Not exactly. Different swales and bioretention basins have different conveyance, detention and treatment roles.

Are all planted spaces in Punggol rain gardens?

No. A technical label should be verified from official information for the actual site.

Does a rain garden prevent every flood?

No. It can contribute to stormwater management within its design limits. Extreme events and system conditions still matter.

What is PUB’s 0.2-hectare rule?

PUB states that new developments and redevelopments of 0.2 hectares or more require measures to slow runoff and reduce peak discharge. Project-specific compliance should be checked with current professional guidance.

What should I do if I see water accumulating dangerously?

Avoid the water and use current official reporting or emergency channels as appropriate. Do not enter drains or attempt repairs.

Can children learn about this without going outdoors in heavy rain?

Yes. Safe classroom models, invented data tables and diagrams are better for learning than approaching active drainage infrastructure.

A useful checklist for Punggol residents

• Think about the whole rainwater path, from where it lands to where it is safely conveyed.
• Distinguish total rainfall, runoff volume and peak discharge.
• Use official PUB guidance rather than assuming one landscaping feature solves every storm.
• Recognise that source controls and public drainage improvements work together.
• Never enter, open, block or modify a drainage feature without authorisation.
• Keep litter out of drains and planted stormwater areas.
• Avoid flooded paths and follow weather and flood advisories.
• Report concerning conditions with a precise location and observable facts.
• Teach children through diagrams and safe observations, not storm chasing.
• Verify technical claims about a named site from its actual design information.

Teaching Guide: the bottleneck experiment on paper

Learning aim. A student should explain why the rate of discharge matters separately from total rainfall and why temporary storage can reduce a peak.

First task. Draw a wide rectangle labelled “site” and a narrow arrow labelled “outlet”. Imagine that 20 units of water arrive in the first minute and the outlet can carry only 10 units. Ask what happens to the difference. In the simplified model, it has to be stored or overflow.

Second task. Give four minutes of fictional inflow: 5, 15, 20 and 0 units. Ask students to calculate the total, then identify the highest one-minute inflow. Total is 40 units and peak inflow is 20 units per minute.

Third task. Add a toy outlet that can release 10 units per minute when water is available. Have students keep a storage ledger: opening stored balance plus inflow, minus outflow, equals closing stored balance. Explain that this is a classroom abstraction, not a method for designing actual drains.

Fourth task. Ask what happens if storage capacity is exceeded. Students should recognise that an overflow plan and safety engineering are necessary. That is the point at which the model reveals its limitation.

Fifth task. Ask why planted treatment might be useful in addition to temporary storage. The answer may refer to suitable engineered filtration and other processes, with the caution that treatment performance must be verified.

Transfer test. A cafeteria has a queue because customers arrive faster than service can be completed. Ask how this resembles water entering a drainage system faster than it can be conveyed. Students should identify rates, accumulation and capacity without confusing people with water.

Parent checkpoint. Ask the learner to complete this sentence: “Holding some water temporarily can help because the drain does not have to receive all of it at the same instant.” If they can explain that in their own words, the key mechanism is clear.

Related Punggol guides: different questions, different answers

The earlier Rain Gardens and Swales in Punggol article explains the local landscape features and where stormwater goes. Understanding Punggol Waterway provides the broader reservoir and drainage context. Heavy Rain, Flood Alerts and Coastal Research helps readers separate safety and future developments.

This article answers the causal question behind those guides: why is it sometimes better to slow water at the source instead of simply pushing it into a drain immediately? That is a different learning objective from a site directory or safety notice, and it deserves its own explanation.

Official sources and further reading

• PUB: Stormwater Management
• PUB: ABC Waters Design Guidelines
• PUB: Flood resilience
• PUB: Get Flood-Wise
• eduKatePunggol: Rain gardens, swales and runoff

The one idea to remember

Punggol’s urban water story is not just about getting rain from a roof to a drain. It is about managing the timing of that movement so that many small sites do not place an unnecessarily large demand on a shared system at the same moment. Holding, treating and releasing runoff responsibly is one part of flood resilience—not a promise that storms can never cause trouble.