Case study 8.1 – Channel regrading following mining subsidence
Effect of mining subsidence
The gradual collapse of old mine workings causes
subsidence of the land above (including rivers
and streams). Often the water levels are
unchanged, because they are controlled by levels
downstream of the area of subsidence. As a
result, flow depths increase and the lowered
riverbanks give less freeboard, increasing the
flood risk to adjacent to land and property.
The diagram below shows how a series of mining
‘panels’ can result in a lowered riverbed with
‘spikes’ between the areas of subsidence
impounding a series of pools of deeper water.
Regrading
Original bed profile
Subsided bed
Reprofiled bed
Elevation
Regrading involves re-establishing the gradient of
the riverbed over a long enough length to restore
the channel’s flow capacity and reduce the flood
risk to the adjacent land and properties.
Regrading is not feasible in all cases, so
alternative means of restoring the channel
capacity, such as constructing flood
embankments, may be necessary.
Regrading works require particular attention to
structures along the length of works such as
bridges, weirs and pipe crossings. These
structures may require underpinning or replacing
to suit the new bed profile.
Distance
Silverdale Brook (Staffordshire)
Subsidence caused by underground workings at
the Silverdale colliery affected the bed gradient of
the brook. The lack of freeboard meant that
flooding occurred on a regular basis inundating a
major road and residential area.
3
This project involved the excavation of 6000m of
silt over a 1km length of the brook. A footbridge
was replaced with a precast concrete box culvert
unit as illustrated. It was also necessary to reroute electric cables running along the bed.
New box culvert to replace footbridge
Typical water level at culvert before regrading
Entrance to same culvert after regrading
Case study 8.2 – Vertical retaining wall to arrest erosion
Stevenage Brook
The Stevenage Brook is very flashy, and can
produce high velocity flows with little warning.
This erosion took place just downstream of a
bridge after a road outfall headwall fell out of the
bank and caused an obstruction in the channel.
Debris built up rapidly on the obstruction, forcing
the floodwater around each side. The left bank
had previously been protected by concrete
bagwork, but the right bank had no such
protection and the resulting erosion removed part
of the garden.
The solution chosen was a vertical retaining wall
formed by dropping timber sleepers into steel Ibeams tied back to anchors, as shown below.
Case study 8.3 – Bank protection to arrest erosion
River Kennet at Thatcham
A sharp bend was causing severe bank erosion, undermining previously installed toe boarding and causing the
bank behind to slip. The solution was to install steel
sheetpiling to water level, and lay rock rolls in front of this to
fill any erosion holes that might subsequently develop.
The bank was re-profiled to a stable angle and protected by a seeded coir/polypropylene geotextile,
with a planted coir roll at the toe, just behind the piling. The photographs below show the site
immediately after completion and exactly one year later.
Case study 8.4 – De-culverting and river restoration
River Pinn in Hatch End
The river ran through an old brick culvert, approximately
1.0m in diameter, which was inadequate for the flood
flows being experienced. Immediately downstream the
river ran in several lengths of vertical concrete-sided
channel.
The opportunity was taken to divert about 600m of this
constrained river by creating a new course in playing
fields and open space.
The soil strata mainly comprised heavy clay, although
some bands of gravel did exist along parts of the new
course. It was realised that the new channel would take a
long time to create a natural regime, so it was designed
with meanders, pools, riffles, varying bank slopes, and a
lowered wetland area where space was available.
The two photographs below were taken almost exactly a
year apart. All vegetation has been self-established.
Case study 8.5 – Weir removal
Before the weir removal
In the early 1990s, a weir at Tanhouse Farm,
Colnbrook, Slough, was rebuilt as part of a flood
defence scheme. It was a fixed crest weir with
stepped downstream face, incorporating a pooland-traverse fishpass. The brickwork is cladding
to the reinforced concrete structure.
This weir ponded water upstream, leading to
significant silting and excessive summer weed
growth.
It was therefore decided to lower the weir and
remove the fishpass, in conjunction with other
works on adjacent channels, in order to lower the
retained water level in the upstream reach.
During the removal works
Most of the fishpass has now been removed and
already a narrower stream can be seen upstream.
Note the safety precautions being taken despite
the shallow depth of water (<0.25m). Note also
the good practice of providing oil pollution booms,
designed to catch any accidental spills before
they pass downstream, although at the time of the
photograph only air tools were in use.
The completed scheme
With the water level now lowered fully, a
meandering watercourse appears upstream of the
weir, without undertaking any intervention works
on the channel whatsoever.