Study into the movement of Water in relation to the land

Lag Time

Lag time is the the amount of time it will take for a river to flood after a period of heavy precipitation. Each aspect has an impact on the River Irwell’s lag time as shown by the Catchment Clock below.

Topography of the upper catchment is steep, geology impermeable, precipitation high and land use relatively isolated to the narrow river valleys. As a result this creates high rates of run-off, and short lag times.

There is also a large source of erodible material and debris (superficial geological deposits) that is transported downstream to the lower and flatter parts of the catchment where this material is deposited, further increasing flood risk and localised flooding downstream.

These characteristics of the upper catchment create a flashy river regime, giving the local population insufficient time to react to flood events.

In the lower catchment area, urbanisation of flood plains has decreased opportunities for natural infiltration. This produces a heavy reliance on old sewers for urban drainage that are prone to flood and back up when river levels are high.


Historically Flashy River Regime



Throughout each century, the River Irwell has flooded due to heavy precipitation, siltation (deposits impeding river flow), and exceeded channel capacity. This indicates that the Irwell’s 1 in 100 year possibility of flooding is far too optimistic for a river that frequently does not cope with increased water levels.

Controlling Hydrological Flows

The river has been heavily modified and constrained by culverts, retaining walls and weirs, many of which are hidden underground. They can become blocked with debris transported from the upper catchment and are the cause of much localised flooding during peak events.

In the lower catchment area, urbanisation of flood plains has decreased opportunities for natural infiltration. This produces a heavy reliance on old sewers for urban drainage that are prone to flood and back up when river levels are high.

The image below represents the changing hydrology witnessed in historical maps between 1840 to present.

All maps were sourced from Digimap.


Hydrological Flows as we know them 

The two diagrams below, visualise the variations in the water level flows in upper and lower catchment sections.


Irwell Vale

Mean Flow m³/s


Q10 Exceedance m³/s


Highest flow 2015 m³/s


22/06/2012 m³/s



Irwell at the Race Course

Medlock at London Road

Irk at Scotland Weir


Mean Flow m³/s





Q10 Exceedance m³/s





Highest flow 2015 m³/s





09/12/1983 m³/s





All information was collected from http://nrfa.ceh.ac.uk/data/search

The central section of each diagram represents the mean flow. For the Irwell within the Manchester City Centre, in the lower catchment, this is 20.1m³/s and at Irwell Vale, in the upper catchment, it is 3.1m³/s.

The next section is the Q10 which represents the top 10% of a rivers flow within a year, and in both diagrams this is slightly over double the mean flow rate.

 What this diagram shows is the vast variations in flow levels that the river has to deal with from day to day to extreme circumstances.


The largest flow levels on record from the 9/12/1983 is 20 times larger than the mean flow and can give us an insight into what a 1-100 year flood looks like in terms of volume of water.

 With this being a highly urbanised area, the Environmental Agency has estimated at 3,000 properties are at risk to a 1-100 year flood.

Future estimated runoff characteristics are anticipated to double/triple by 2080 and yet the river systems are not adapted or prepared to carry this increased amount of flow.

Even in todays climate the risk of flooding is not fully understood. The Irwell flows from the city centre through the Manchester Ship Canal, which is privately owned, the only section of the catchment not managed by the Environmental Agency.

The Ship Canal does have a number of defences which can be deployed during times of peak flows, including a network sluices. They are however untested in extreme scenarios which has caused a number of tensions between the two bodies.

If fully functional the sluices nullify any threat of a flooding, however the residual risk endangers 1500 properties in Salford. The contrasting risks can be seen below in the PDFs.




The range of flow levels within the Upper Catchment is much greater than in Manchester City Centre.

The largest flow on record for the Irwell Vale is 44 times larger that the mean flow, which is over double the range of Manchester City Centre. This is largely due to the shorter lag times as previously discussed. This can cause devastating water and flood levels but as space is less of a premium within a rural setting the settlements and infrastructure are kept further away from the river.



The effect of urbanisation on water levels is perfectly captured within our model. The two river stations either side of the Bury show that the mean flow that enters and leaves Bury are similar, but the q10 of the water levels downstream of Bury is much higher. The impermeable nature of the urban landscape and the drainage network that flows into the Irwell causes significant differences to the Irwell’s water levels in times of peak flow.



The other major flooding issue for Manchester City Centre is due to the consequences of the many river modifications that have taken places over the last 150 years.

As we have seen, Manchester has culverted and buried large sections of the the Irk and Medlock but there are many other waterways beneath the surface of the city that can not be tracked through cartography.

For example, the River Tib, which used to define the boundary of the original Roman settlement in Manchester was converted by the Victorians in one of the first sewer systems for the city. During the 1800s several rivers and brooks were converted and have not been seen for over 200 years. The Victorians had many teething issues perfecting the sewage systems and these watercourses are largely leaking into the ground water.

The Victorians only designed their sewers to mange 1 in 5 year floods so they regularly reach their capacity.

The large number of culverts within the city has also contributed to the flood risk largely due to deposition of sediment and debris.

The culverts cause the waterways to flow faster and also reduces the amount of deposition due to the higher energy of the water flow.

This leads to mass deposition once the culvert ends and the river slows. This deposition can cause large flood risks, particularly if it takes places underground where it can go unnoticed.

These are issues that are growing within Manchester City Centre, the last notable floods within Manchester in 1995, 2000, 2002, 2004 and 2006 have all be due to blocked culverts and/or sewer overflow and blocked drains.



Remnants as ruins: the Irk Culvert, Manchester


Manchester Sewer History


SFRA – Manchester, Salford and Trafford Council





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