Hydromorphological Appraisal of the Use of

Large Woody Debris in the Restoration of the

River Lathkill, Derbyshire

John M. E. Cowx & Ian B. Drew

Adapted from Gordon et al., 2004

Focus on Hydromorphology – but recognising

the link with ecology

Raven et al., (2002) - the importance of the physical

(hydromorphological) dimension as a supporting element in the

ecological restoration of rivers is profound.

Chessman et al., (2006) indicate that rehabilitation of geomorphic

condition can be critical for the improving the biodiversity.

Clarke et al., (2002) argues that both morphological and

ecological components of a river system are inherently linked and

that ecological goals can only be achieved through geomorphic

improvements.

Successful river restoration can only be achieved by a

multidisciplinary approach, fully understanding ecological,

hydrological and geomorphological process and form

Lathkill Dale 1 of 5 valleys comprising the Derbyshire Dales National Nature

Reserve managed by Natural England

Historical channel modification by human

intervention

18th and 19th century lead mining was associated with channelisation

and the excavation of drainage soughs.

Diagrammatic profile through the edge

of the Derbyshire plateau, showing a

sough cut to drain the limestone for

deeper mining access (Ford and

Rieuwerts, 2007). Note: contrary to this

diagram the sough at Lathkill Dale was

driven in below the level of the river.

The drainage provided by the sough combined

with the permeable limestone geology has

causes surface flow to dry up in summer

months.

Environment Agency daily discharge data for

the River Lathkill (1997-2009) (data provided

courtesy of Professor John Gunn).

The River Lathkill indicating the perennial and intermittent springs. The reach of the river in

which this study was conducted is highlighted in red (adapted from Wood et al., 2005).

In the Victorian era the river channel was straightened, clay lined

and controlled by weirs in order to establish good conditions for

trout fishing.

Natural England & Wildlife Trust

Restoration began in 2003 by

removing the grasses and reeds

which occupied the river and by

digging a narrower channel

through the otherwise flat

topography of the river bed.

In 2006 the topography of the river bed

was still relatively even and bed material

uniformly distributed.

The Lathkill site characteristics offered a unique opportunity to

study & sample the river bed around LWD in detail

5 LWD structures of various design, stabilised and anchored with

timber stakes and wire, were installed in 2008 There are five

anchored wooden structures in this reach all of which are

comprised of locally sourced sycamore and elm.

The Project The aim of the research was to undertake a post project analysis in

order to evaluate the success of LWD in creating a

hydromorphologically diverse river channel with a view to

identifying its potential impact on the ecology of the stream.

To identify the hydromorpholoical characteristics of the river, a

programme of detailed field mapping was undertaken.

Spring 2010: Flow direction & velocity was measured at up to 14

points across 41 cross-sections in the 30m reach. Readings were all

taken at a height above the bed equivalent to 0.6 of the water

depth.

Spring 2010: River bed elevation was determined along the 14 cross

sections to produce a topographic map of the river bed

Summer 2010: bed material around the woody debris was mapped

and sampled for analysis.

Initially visual characterisation of the bed material size was used to

identify bed units. These were mapped and sampled. From each

bed unit a sample was taken up to a trowels depth (approximately

15cm). There was no division into surface and subsurface samples.

Trowel sampling was chosen rather than an infield grid pebble

count grid technique because it was thought a sub-surface sampling

of the river bed would be more representative rather than just a

surface sample (Rice and Haschenburger, 2004).

The samples were subsequently analysed in the lab using standard

techniques: dry sieving of the whole sample, followed by axis

measurements and sediment shape and angularity analysis for the

coarsest fraction.

Flow Patterns - 17th of June 2010

Mapping confirms that the LWD structures are having an influence on flow patterns and shows precisely how

direction and magnitude are modified.

At the entrance to the reach the channel plan form and flow is relatively uniform in direction but with the line

of fastest flow towards the left hand bank. Subsequently the first LWD structure seems to have a minor

influence in deflecting flow, while further downstream the effects are more marked demonstrating how the

course of the thalweg can be manipulated. This will consequently influence the plan form of the channel

which has been made less uniform in the LWD reach with bank erosion has occurring where water flow

velocity is increased and deflected towards the channel side. Flow has also forced its way round stream side

structures resulting in additional areas of bank erosion – only the first side structure built well into the bank

has avoided this. Flow is generally slowed down behind the LWD where in some case areas of still water or

reversed flow have been created.

Bed Topography Summer 2010

The contours shown depth BELOW bank height. They identify areas of deeper channel which

indicates scouring is taking place, particularly beside channel side LWD structures and in front

of the mid channel structures and generally reflecting the new line of fastest flow (as indicated

previously). In future the scour could result in undercutting of the structures which would result

in greater uncertainty in regards to the nature of longer term channel changes. Bed elevation is

generally higher downstream of the structures as a consequence of deposition associated with

the lower flow velocities. The patterns of bed topography seem to increase in asymmetry when

associated with LWD set at an angle to the flow rather than perpendicular to it – the middle

structure showing the most symmetrical pattern.

Bed Material Size Distribution

The map suggests that the bed material size distribution reflects the increased complexity of flow

once the LWD reach is entered. Before the first structure the pattern of bed material distribution is

more uniform. Once the reach is entered finer sediments are generally located downstream of

LWD structures which also correspond with shallower areas of channel and areas of low/no flow.

Bed units over 4 mm in composition share general patterns being generally located in the areas of

fastest velocity seen in figure 4 associated with deeper sections of the river. Roundness index

numbers were relatively low probably due to the discontinuous flow.

Sample 25 contained high levels of clay in a scour zone, this could represent the Victorian channel

lining efforts.

The tufa deposits are calcareous precipitates which can cement materials river bed materials; their

position could indicate incision of the channel into a older bed.

Unfortunately plans to undertake further study of flow around the

LWS at different river stages during winter 2011 were not possible

due to a delay in flow becoming re-established.

Creating pool habitats - Enhanced

scour has created deeper areas directly

in front of and beside the LWD structures.

Promote riffle and bar formation through induced sediment

deposition - Sediment deposition is evident in areas of the riverbed

with lower flow and located directly downstream of LWD structures.

These shallow areas were identified as having potential for riffle and

bar formation.

Diversion of flow – has resulted in some evidence of plan form

variation from the straight cut section.

In summary it is clear that geomorphic thresholds for sediment

transportation, erosion and deposition vary throughout the channel as

a result of the diverse hydraulic conditions promoted by the LWD.

Success in meeting objectives of

hydromorphological diversity

Implications for Habitats

Bed type Range of

particle size

(mm)

Relative

frequency of

bed

movement

Density of

benthic

macro-

invertebrates

Diversity of

benthic

macro-

invertebrates

Fish use of

bed

sediments

Boulder –

Cobble

≥64 Rare High High Cover,

spawning,

feeding

Gravel –

pebble –

cobble

2-64

64-256

Rare to

periodic

Moderate Moderate Spawning,

Feeding

Sand 0.063-2 Continual High Low Off-channel

fine deposit

used for

feeding

Fine

material

<0.063 Continual or

rare

High Low Feeding.

The material of the riverbed has been highlighted as being fundamental to

the aquatic habitat of invertebrates and fish (Robert 2003).

Gurnell (2006) determined that LWD were sites that serve as food for

grazing organisms with high organic matter retention, nursery habitat for

fish and perches for birds and other animals.

References Chessman, B. C., Fryirs, K. A., and Brierley, G. J. (2006) Linking geomorphic character,

behaviour and condition to fluvial biodiversity: Implications for river management. Aquatic

Conservation: Marine and Freshwater Ecosystems 16, 267–288.

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hydromorphic approach to sustainable river restoration. Aquatic Conservation. Marine and

Freshwater Ecosystems 13, 439–450.

Ford, T.D., and Rieuwerts J.H., (2007). Lead mining in the Peak District. The Peak District

Mines Historical Society

Gordon, N. D. McMahon, T. A., Finlayson, B.L., Gippel, C. J. and Nathan, R. J. 2004, Stream

hydrology: an introduction for ecologists, 2nd edn, John Wiley and Sons Ltd, West Sussex.

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biocomplexity of river corridors. Frontiers in Ecology and the Environment 7, 377–382.

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qualitative comparison of three survey methods. Aquatic Conservation: Marine and Freshwater

Ecosystems 12, 405–424.

Rice, S. P., and Haschenburger, J.K. (2004). A hybrid method for characterization of course

subsurface fluvial sediments. Surface Processes and Landforms 29, 373–389

Robert A. (2003) River processes. Arnold. London