What is a Distributed Storage

Distributed storage is a stormwater management approach in which temporary storage capacity is spread across multiple locations within a catchment rather than concentrated entirely in one large downstream facility. Each storage element retains part of the runoff generated during rainfall and releases, infiltrates or reuses that water more slowly, depending on its design.

The individual storage volumes can be relatively small, but their combined effect can alter how runoff reaches the drainage network. Examples can include detention basins, ponds, tanks, oversized pipes, below-ground storage structures and other distributed attenuation features. Some sustainable drainage systems can also contribute storage where temporary retention forms part of their hydraulic function.

The defining feature is therefore not a particular type of structure. It is the spatial distribution of storage throughout the drainage catchment and the coordinated effect this has on runoff timing, peak discharge and downstream hydraulic loading.

Distributed Storage Changes When Runoff Reaches the Drainage Network

Urban development changes the way rainfall becomes runoff. Roofs, roads and other impermeable surfaces reduce infiltration and can deliver water to drains much faster than undeveloped ground.

If a large proportion of a catchment responds quickly to rainfall, flows from different areas can reach downstream pipes at similar times. The combined discharge can produce a pronounced peak.

Distributed storage changes this timing. Instead of allowing runoff from each contributing area to enter the downstream network immediately, part of the volume is temporarily retained closer to where it originates.

A simple storage balance can be expressed as:

Change in storage = inflow – outflow

During the part of a storm when inflow exceeds the permitted outflow, stored volume increases. Later, as rainfall and inflow decline, water continues leaving the storage and the stored volume falls.

This means that storage does not eliminate runoff simply by holding it temporarily. A detention facility generally delays discharge. An infiltration system can additionally reduce the volume entering conventional drainage where ground conditions permit infiltration, while reuse or evaporation can alter the water balance in other systems.

Several variables determine the hydraulic effect of distributed storage:

• total storage volume available across the catchment;
• location of individual storage elements;
• contributing impermeable area to each element;
• inflow hydrograph;
• allowable outlet rate;
• infiltration where this forms part of the design;
• initial water level before rainfall begins;
• interaction between individual storage facilities.

The last point is especially important. Ten storage units cannot automatically be treated as one equivalent tank simply because their capacities add up to the same total volume.

Their positions within the network affect when they receive water and when their controlled discharges reach downstream junctions. Two facilities with identical volume and outlet characteristics can have different effects on a critical sewer depending on where they are located.

The objective is often to reduce or delay the peak flow at hydraulically constrained parts of the network. Distributed storage can achieve this by modifying several contributing runoff hydrographs before they combine.

Location Can Be as Important as Total Storage Volume

A large total storage volume does not guarantee effective peak-flow reduction if it is positioned where it has little influence on the critical part of the drainage system.

Consider a simplified catchment containing three branches that combine into one downstream sewer. If only one branch is attenuated, the other two may still generate their peak flows at approximately the same time. Storage distributed across all three branches can alter the combined hydrograph differently even where the total installed volume is similar.

This spatial effect distinguishes distributed storage from a purely volume-based approach.

Timing can sometimes produce less intuitive results. Delaying one flow peak can cause it to coincide with a later peak arriving from another part of the catchment.

For this reason, “slower runoff” does not automatically mean that every downstream location experiences a lower peak. The complete network response has to be considered.

The position of flow controls is also significant. A storage facility with a large volume but an unrestricted outlet may fill and empty too rapidly to provide the required attenuation. Conversely, an outlet that is excessively restrictive can retain water for too long and leave insufficient capacity for subsequent rainfall.

Distributed storage design is therefore a problem of volume, discharge rate and timing rather than storage capacity alone.

This becomes increasingly relevant in larger networks containing multiple subcatchments. Rainfall does not necessarily produce the same runoff response everywhere because impermeable area, slope, drainage density and travel time differ across the catchment.

Different Storage Elements Can Contribute to the Same Network Strategy

Distributed storage does not require every facility to use the same construction. The network can contain storage elements with different geometries and operating principles as long as their hydraulic contribution is understood.

An above-ground detention basin can temporarily store runoff over a relatively large area. A below-ground tank can provide attenuation where surface land is unavailable. Oversized pipes can combine conveyance with storage by holding water above the normal dry-weather or low-flow condition.

Other drainage features may provide temporary storage within depressions, permeable construction layers or vegetated systems. Whether their full physical volume can be counted as effective stormwater storage depends on the design and on how much capacity is actually available when the design event occurs.

This distinction between physical volume and available storage is fundamental.

A tank with a geometric capacity of 100 m³ does not necessarily have 100 m³ available at the start of every storm. If 20 m³ remains occupied by water, sediment or another permanent volume, the immediately available capacity is lower.

The same issue applies across a distributed network. Effective storage depends on the state of each component when rainfall begins.

For hydraulic assessment, it is useful to distinguish:

1. geometric storage, the physical volume within the facility;
2. active attenuation storage, the portion intended to fill and drain during runoff events;
3. permanently occupied volume, where relevant;
4. unavailable volume caused by sediment or other loss of capacity;
5. storage recovered between successive rainfall events.

Recovery between storms can be important. A facility that drains slowly may perform effectively during an isolated event but still contain water when another storm arrives.

Distributed systems make this issue spatially variable. Some facilities may have fully recovered while others remain partly full. The network can therefore begin a later storm with different available capacities across the catchment.

Outlet behaviour can also change as water depth changes. In many hydraulic controls, discharge is related to upstream head, so the release rate is not necessarily constant throughout the event.

For this reason, distributed storage is normally represented dynamically when detailed network behaviour matters. Modelling can account for rainfall, subcatchment runoff, storage level, outlet behaviour and flow routing through pipes and channels over time.

Network Performance Depends on How the Storage Elements Interact

The combined effect of distributed storage becomes most important at points where flows from different areas converge. Each storage facility modifies its local runoff, but the drainage network determines how those modified flows combine downstream.

A system can therefore perform well locally without producing the expected result at every network location.

For example, attenuation upstream may reduce the maximum flow entering one sewer but extend the duration for which that sewer carries an elevated discharge. This can be beneficial where peak capacity is the main constraint, but duration can matter where the receiving system has limited storage of its own or where pumping forms part of the downstream network.

Distributed storage also changes the consequences of individual component performance. Failure or loss of capacity at one small facility does not necessarily remove all attenuation from the catchment, as could occur if the strategy depended entirely on one central structure. However, numerous small facilities create more individual assets whose hydraulic condition can vary.

Sediment accumulation, blocked flow controls and unauthorised alterations can all change the intended storage-discharge relationship. If a flow control becomes blocked, a facility may retain excessive water. If it is removed or enlarged, stored water may discharge faster than the network design assumed.

The effect of such changes depends on where the facility sits within the catchment. Loss of attenuation immediately upstream of a known hydraulic constraint can be more significant than the same volume change elsewhere.

Distributed storage is therefore best understood at network scale. The individual basin, tank or oversized pipe provides the physical volume, but the drainage benefit comes from where that volume is placed and how its inflow and release are timed relative to flows from the rest of the catchment. Two drainage strategies with the same total cubic metres of storage can consequently produce different peak flows because the storage is distributed and controlled differently across the system.