Understanding Restoration within river restoration design

Dr Jenny Mant Dr Philip J. Soar Overview

• accounting for in river restoration • -forming • what to think about for restoration design • class exercise

• FORM, PROCESSES AND DYNAMICS (change over time)

Context

 An emerging science  Interest rapid during the 1980s  NRA/EA R&D reports in 1990s geomorphological approach to river management  River processes, sediment transfer and physical habitats vital for ‘sustainable’ river management and restoration  UK few consultant geomorphologists (changing?)  No standard approaches to river restoration Accounting for Sediment in river restoration Starting point

Guidebook of Applied Fluvial Geomorphology Technical Report FD1914 (Thorne, Sear and Newson 2003)

Starting point

Applied Fluvial Geomorphology for and Management (Thorne, Hey and Newson 1997)

Starting point

Fluvial forms and process A new perspective

( Knighton, D 1998) Sustainable restoration design

 Why should we ‘account for sediment’?  What do we mean by ‘continuity’ (or connectivity)?

Sediment Continuity…  as a design ‘principle’  as a design ‘process’  in the design ‘procedure’

Define Fluvial Geomorphology...

 Fluvial geomorphology is:

Fluvial is defined as: found in, or produced by a river or Morphology is defined as: the scientific study of form and structure Geo relates to the surface of the earth

Fluvial geomorphology is the study of sediment sources, fluxes and storages within the river channel over short, medium and longer timescales and of the resultant morphology (Sear and Newson, 1993)

Why ‘account’ for sediment in restoration

The amount and type of sediment supplied to a .

Why? Sediment is part of the balance between energy (stream power, shear stress or amount of energy in the flow (discharge = m3/s) and load: determines channel stability.

Lack of sediment relative transport capacity= of channel boundary of an alluvial channel.

Oversupply relative to transport capacity=reach

Why ‘account’ for sediment in design

For sustainable channel restoration design...

Modification of the size of a cross section and sinuosity needs to be analysed to ensure that upstream sediment loads can be transported through the reconstructed reach with minimal deposition or erosion ( if that is what is required – not always the case).

River restoration objectives

 What is the ‘primary’ objective of river restoration?  Support a diverse biodiversity!  Improve fisheries  Improve conservation value of the river landscape  Restore !

 Further objectives might include protection and recreation

 Geomorphology and are often given a lower priority or not considered at all!

Form without function

 Without accounting for sediment transport and river processes = risk of designing ‘form without function’

 By imposing an unsustainable condition, the designed channel might not be able to support the targeted habitats over the long term (dynamic equilibrium)

 The river can reject the imposed changes very quickly, especially if there is a high sediment load and sufficient energy, resulting in complex responses ( understand you catchment) River management NOT working with natural processes

‘Detroit riprap’

Geomorphology offers a better solution! And near to home! Discharge

Velocity distribution in channel velocity profile bed roughness

Channel Bed material Bed shear geometry characteristics stress

availability selective transport of sediment competence & capacity erosion & Bedload deposition transport

Sediment supply after Ashworth and Ferguson (1986) The ‘sediment system’

Schumm, 1977 Characterising the sediment system (relate to previous slide)

Hawkcombe Stream, Somerset

Channel-slope coupling

a) Gorge Fully coupled: delivery of slope material to channel at all flows

b) Confined channel / floodplain Partially coupled: delivery of slope material to channel. Spatially discrete and sporadic activity of sources. Some limited storage.

c) Unconfined channel / floodplain Uncoupled /weakly coupled: delivery of slope material to channel at high flows or where join. Storage on floodplain. Sediment transfer (Soar and Thorne, 2001)

Reference Transfer Downstream Reach Reach Reach (Supply) (Demand)

Stream type Local variability conditions

Width Channel stability

Channel-forming discharge

Sediment load Depth Sediment and gradation Slope load Sedimentation

Braided channel?

Time

0 Channel Change Channel Unstable Channel Design Stable Channel Design Erosion Impact of channelisation on sediment transporting capacity

discharge sediment discharge

QwS+ ~ Qs+D50 bed slope median bed material size

A . Planform .B

.A .B Long profile Impact of channelisation on and degradation

Brookes, 1988  The channel is trying to ‘recover’ a new equilibrium through a combination of degradation and aggradation: negative feedback

Know your river!

 Where in the fluvial system?  ‘Type’ of river (meandering? mobile bed? ? etc)  Channel typing and characterisation:  Reference reach(es)  Natural morphologies to use as analogues?  Supported sediment forms and features  Supply reach  Sediment transfer and connectivity  Flow regime ( i.e. stream power)  Bed and bank materials/vegetation/typology  Project reach  Bed and bank materials/vegetation  gradient and site constraints Where in the fluvial system? Stream Power

Stream Power: f(QS)

Q = discharge S = slope

FISRWG, 1998

Q = width x mean depth x mean v Independent and dependent variables controlling channel form Sediment transport classification

 Sediment load: which component? Predicting Sediment Transport Rate

 Measured sediment transport data not available other than for research sites in UK  Numerous equations to calculate the sediment (bed material) load in a stream.  Accuracy ranges +/- 50% of actual transport rates unless calibrated against measured loads – LOTS OF DATA!  Key to selecting an appropriate equation and improve accuracy = find an equation developed for conditions that match those under which it is to be applied. Sediment Transport Rate

Bed material size Basis Sample Applications Comments Formula

Bagnold (1980) sand, gravel Stream Mimmshall Brook, Performed well in power R. Sence, R. Idle, tests against field Shelf Brook data using reach- average values. Both under and over predicts.

Bathurst, Graf gravel, cobble Discharge Shelf Brook, (Newson Performed well for and Cao and Bathurst 1991) steep, headwater (1987) (S > 0.1). Over-predicts and can produce negative loads. Ackers-White silt, sand Shear stress R. Sence, Usk, Performed well in tests (1973) updated gravel Colne, Stour, based on and by HR Ecclesborne (HR rivers. Much better when Wallingford Wallingford 1992) calibrated against data (1990) from site in question. Over-predicts.

Newson (1986) silt, sand Catchment Shelf Brook, Sence, Provides estimate of gravel area Tawe, Idle Dunsop, annual sediment yield to Whitendale river. (Newson and Bathurst 1991) ‘Typing’ the bed and banks

 Bed type: implications for slope, bed material composition, roughness and stable channel dimensions

Bathurst, 1997  Bank material and vegetation: implications for roughness and stable channel dimensions River profile

-pool sequence characteristic of both straight and meandering channels with heterogeneous bed material in the size range 2- 256mm (mostly gravel-cobble bed streams). Slope range generally 0.001 to 0.02. Important sediment storage and sorting for channel stability and high ecological value.

 Step-pool sequence slopes are steep > 0.03 – 0.1, typically formed from accumulation of boulders and cobbles in confined valleys. Riffle-pool spacing measured in rivers

 Diagram from Leopold and Wolman (1957).  Riffle spacing in low sinuous channels has a spacing (on average) or 5-7 times the channel width. Or does it? Pool-riffle characteristics

 Keller and Melhorn (1978) spacing can range between 1.5 and 23.3 times the channel width with a mean of 5.9. – related to stream types and equlibrium

generally absent from sand bed rivers and cobble- boulder bed streams, where they are replaced by step- pool units.

 Riffles have a coarser bed material than pools suggesting that a sorting mechanism is present. How is this maintained? Riffles

River Cole, nr Birmingham

River Gaunless, Co Durham WANDERING

POOL- RIFFLE PLANE BED

STEP-POOL

CASCADE

Transport Capacity 40 Know your river

Stream Reconnaissance (Thorne, 1998)

Fluvial Audit (Defra FD1914)

And … RIVERS MOVE!

Mississippi River, Mississippi And…. move!

? and… MOVE!

Gilwiskaw Brook, Leicestershire Add

Stability and Equilibrium

‘Geomorphological’ stability:

 A river that maintains the same average cross section dimensions and planform characteristics while adjusting its position on the floodplain is said to exhibit ‘dynamic equilibrium’.  The width, depth and slope are adjusted so that there is neither net aggradation or degradation over time.  Bank erosion and changes to forms are natural features of this dynamic equilibrium (and vital ecologically!).  SPACE FOR NATURAL ADJUSTMEN T

Types of morphological ‘adjustment’

Downs, 1992 Types of morphological ‘adjustment’

Downs, 1992 Conclusions

Accounting for sediment continuity will provide more sustainable (and economic) solutions while meeting flood defence and biodiversity objectives

How?  Problem is approached ‘holistically’ within the catchment context  Designing ‘form with function’  Thus, ‘minimum maintenance’ requirement  Geomorphological processes create dynamic and diverse habitats

Channel Forming Discharge representations and variability Overview

 Concept and use in river restoration  Representations of channel forming discharge  Bankfull discharge  Discharge of a fixed return period  Effective discharge

Definition

 ‘At this discharge, equilibrium is most closely approached and the tendency to change is least. This condition may be regarded as the integrated effect of all varying conditions over a long period of time.’ (Inglis, 1947).

Important because…

 Incomplete understanding of the ‘formative’ properties of all flows.

 Flows (Q) at and near the bankfull stage have morphological significance. They are ‘effective’

 In channel restoration design, a ‘design discharge’ is required that corresponds to the conveyance capacity of the restored channel

Purpose

In river restoration, the channel-forming discharge concept provides a relationship between the:

 hydrologic characteristics of the catchment;

 the hydraulic characteristics of the channel, and;

 the geomorphic characteristics of the project reach.

Influence of Hydrology

HYDROLOGY (The natural distribution and sequence of flows) The ‘driving’ parameters

HYDRAULICS The mechanistic processes of open channel flow

SEDIMENT TRANSPORT Sediment erosion, transportation and deposition

MORPHOLOGY Channel configuration, forms and features

CHANNEL STABILITY Stable channel restoration designs Representations

In a natural river the most appropriate definition is the discharge conveyed at the elevation of the active floodplain (Wolman and Leopold, 1957).

Three definable flows Represent the 'dominant' flow based on the application of repeatable geomorphological and hydrological techniques

 Bankfull discharge  Discharge of a fixed return period  Effective discharge

1. Bankfull discharge

 Maximum flow that the channel can convey without overflowing onto the floodplain.

 Requires estimation of the bankfull stage and determination of the discharge at that stage

 A frequently applied method is to estimate the discharge corresponding to the channel width at the minimum width-to-depth ratio (Wolman, 1955) Bankfull discharge Bankfull morphology Bankfull discharge

Bankfull Indicators:  Elevation of the ‘active’ floodplain  Highest elevation of active channel bars  Lowest elevation of perennial vegetation  Marked break in bank slope or materials

 Requires stream reconnaissance and knowledge of geomorphic stability criteria

 BUT accurate location of bankfull indicators is not a routine procedure and is problematic and subjective Bankfull in an incised channel?

Long Creek, Mississippi Where is bankfull here? Bankfull discharge (main) methods

 Direct gauging  Use a stage-discharge rating curve from a nearby gauge  Use an empirical relationship (eg. predict from catchment area)  Use appropriate flow resistance (roughness) equations which can synthesize a stage-discharge curve

Bankfull discharge from catchment area

Emmett,1975 (in FISRWG, 1998)

Flow resistance equations There are many equations in the literature  Manning equation: AR0.67S0.5 WD1.67S0.5 Q  Q  n n where: d1/ 6 n  50 21.1

n = Strickler for static beds with no bedforms; look-up charts or Barnes book (1967) for different Roughness estimator UK

Q is bankfull discharge, A is bankfull section area, R is hydraulic radius (Area / ), S is slope, n is Manning roughness coefficient, W is width, D is depth, d50 is median particle size

 Other equations in Applied Fluvial Geomorphology 2. Discharge of a fixed period

Rivers remain dormant for long periods(low flow conditions)

Most active in higher than normal flow.

As the process of channel adjustment is purely mechanical the determining factors must persist for sufficient time for this adjustment to take place and stability to be reached.

Discharge of a fixed period (return)

In general, bankfull discharge has a return period of between 1 and 2 years (Leopold et al., 1964)

However, there is a range of possible frequencies...  1.0 to 5.0 years (Wolman and Leopold, 1957)  4 to 10 years (Pickup and Warner, 1976)  50% of sites had return periods less than 1.25 years or greater than 1.75 years (Andrews, 1980)  0.56 to 3.44 years on Partial Duration Series (Hey and Heritage, 1988)  1 to 10 years (USACE, 1994) and others... 3. Effective discharge (Andrews 1980) (sediment stability)

 The dimensions of a dynamically stable river adjust in accordance with sediment balance

 Perennial rivers adjust their bankfull capacity to the flow that transports the greatest quantity of sediment load over a period of years (usually based on the period of flow record)

Frequency of effective discharge

Effective Discharge corresponds to...

 2 year flow event (Biedenharn et al., 1987)  2 year flow event (Watson et al., 1997)  1.46 year flow event (Orndorff and Whiting, 1999) and others…

All in the range of bankfull discharge return periods!  Effective Discharge Calculation: A Practical Guide. ERDC/CHL TR-00-15 (August, 2000)

 Requires a combination of empirical, statistical and mathematical methods

 The method is designed to have general applicability, have the capability to be applied consistently and to integrate the effects of physical processes responsible for determining the channel dimensions Channel forming flow ?

Bankfull Discharge of Effective = = Discharge Fixed Return Discharge Period Channel Restoration Design Geomorphological approaches What level of detail can we design to?

Frissell et al. (1986) Representative cross sections Restoration Approaches

 Natural recovery: Do nothing

 Prompted recovery: Rehabilitation of natural processes and forms

 Managed recovery: Channel restoration design

Natural Recovery

Hawkcombe Stream, Somerset, UK

‘Prompted’ Recovery

Deflector 6A in the River Idle, Nottinghamshire, UK ‘Full restoration’ design methods

 Historical reconstruction: the carbon copy solution

 Reference reach geometry: the natural analogue solution

 Regime analysis: the empirical design solution

 Analytical channel design: the rational design solution

Channel Restoration Design i) Historical reconstruction: the carbon copy solution Historical reconstruction

 Reinstatement of a previous channel configuration that possessed the type of channel configuration and range of forms and features required in the target restored channel  ‘Carbon copy’ approach may replace meanders as found prior to disturbance  Widely practiced in northern European countries.  Two techniques are available:  replicate planform from historical sources (e.g. air photographs, maps);  excavate old river courses on the floodplain. Historical reconstruction

BUT  Assumes rainfall-runoff/sediment discharge patterns in the catchment have not significantly varied over time (stationary controls)

 If land-use patterns and hydrology have changed - restored channel configuration likely to be unstable and could result in progressive aggradation or degradation. Channel Restoration Design ii) Reference reach geometry: the natural analogue solution Reference reach(es)

 ‘Pristine’ reaches are very rare to find in catchments targeted for river restoration; stable segments of river can usually be found.  Undisturbed reaches close to the target restored channel may be used as channel restoration design blueprints  If stable reaches can be identified in catchments with similar hydrological and physical characteristics/valley type, then channel geometry data could be scaled to the restored reach (eg. Adopting the same width-depth ratio)

Reference reach(es)

BUT  Difficulty and subjectivity in locating ‘stable’ reaches and sites with similar boundary conditions as the target restored channel  Difficulty identifying bankfull dimensions from field indicators.

 Despite this, reference reaches should always be sought to examine the morphological characteristics that can be supported  To find out more about reference sites…

Harrelson, C., Rawlins, C. L. and Potyondy, J. P. 1994 Reference Sites: An illustrated guide to field technique

Channel Restoration Design iii) Regime analysis: the empirical design solution Regime analysis (see handout)

‘A self-formed alluvial channel is ‘in regime’ if there are no net changes in discharge capacity or morphology over a period of years’

 Cross-sectional form is inherited from the imposed natural sequence of flows and boundary . Discharge usually explains most of the variance in geometry

 Downstream Hydraulic Geometry equations (after Leopold and Maddock (1953): 0.5 0.4 W  aQb D  cQb

Qb is bankfull discharge, W is bankfull width and D is bankfull depth. ‘a’ and ‘c’ are constants for different ‘types’ of channel Meander wavelength 100000

Lm = (11.26 to 12.47) W within 95% confidence limits

10000

(m)

m L

1000

100

90% single response limit 10 Meander Meander Wavelength, 95% mean response limit Regression 1 1 10 100 1000 Bankfull Width, W (m) Regime equations

BUT  Only limited equations available for different ‘types’ of channel (especially sand-bed rivers)  Extreme caution to be exercised when applying regime equations to rivers with different characteristics to those used to derive the equations  The equations are empirically derived and not based on flow/sediment mechanics (although some justification from analytical methods)  Limited predictive capabilities (especially for depth and slope)

Channel Restoration Design iv) Threshold channel design: the engineered design solution Static Boundary Design

 Some river beds are static and coarse sediment transport is negligible  Considerations of shear stress and ‘initiation of motion’ can be used in the design, based on critical depths and maximum permissible velocities  Can be used to size sediment for riffles in a restoration project. As a guide: d  11DS d is smallest particle size that can be moved, D is critical depth (assume bankfull) and S is the slope

 Mechanics are beyond the scope of this workshop Channel Restoration Design v) Analytical channel design: the rational design solution Stable channel design procedure Supply Reach Phase 1 Assessment

Project Reach Phase 2 Reach-Average Assessment Channel Dimensions

Local Morphological Phase 3 Channel Design Variability

Sediment Impact Phase 4 Design Brief Assessment  To find out more about the design approach and procedure…

Soar, P. J. and Thorne, C. R. 2001 Channel restoration design for meandering rivers

ERDC/CHL Report CR-01-1 United States Army Corps of Engineers, Engineer Research and Development Center, Vicksburg, Mississippi, September Meander features

Straight Channel Meandering Channel

Pointbar

Riffle or Crossing

Pool Local morphological variability

Lm

q Rc

A’ A A m Z C’ B’

B

Apex C Local morphological variability

A A’

Wi W

D Inflexion Point  m D

B B’

Wp

Maximum Pool Scour Dmax

C C’

Wa

Bend Apex Design conclusions

 No a ‘cook book’ for all types of river  Supply Reach Assessment and understanding the type of channel must be supported by boundary conditions info  Approaches that ‘account’ for sediment transfer in the design and with a sediment impact assessment ensure that restored river morphology and assemblage of physical habitats are sustainable  Does ‘full’ river restoration occur in the UK?