Peter Wilcock Geography and Environmental Engineering National Center for Earth-surface Dynamics Johns Hopkins University SEDIMENT TRANSPORT IN STREAM RESTORATION 19 September 2012

Sediment transport is complicated, predictions are highly uncertain. But with a few basic concepts, and some tools for incorporating uncertainty … We will propose coherent strategies for incorporating sediment transport and its uncertainty in stream restoration Classic concepts from fluvial geomorphology dominate steam channel design We will evaluate these concepts and their utility and … suggest their appropriate role in stream design Two broad channel types were defined by drainage engineers a century ago: threshold and alluvial channels. We will update these definitions and … add another!

Lane/Borland Balance (USBR ) Sediment Supply Transport Capacity

Sediment supply > Transport capacitySediment supply < Transport capacity

Does the sediment balance matter in this stream? It has super capacity with respect to supply, but it is also unable to entrain sediment from the bed.

Flow competence Will a flow move the grains on the bed? Transport Capacity. At what rate can the flow transport sediment? (hint: think of the sediment supplied, not what is in the bed!) There are two basic transport problems These are different problems!!! Define Q c the water discharge at which grains on the bed begin to move Q  Q c does not mean that the sediment supplied can be transported! Q < Q c does not mean there will be no transport!

Competence v. Capacity Flow Competence Can a flow entrain the grains on the bed? Applied to the channel bed Leads to a threshold channel Transport Capacity At what rate can a flow transport sediment? Compare to sediment supply Leads to a mobile channel Will a channel accumulate or evacuate sediment ? How much sediment do we need to add to restore streams below dams? Can we mine sediment from a stream w/o causing downstream problems? How will a sediment slug move through a channel? How far downstream will changes occur? How long will it take? Will channel bed and banks remain stable (static) at a design flow? Will a channel will need ‘repair’ in the next 25 yrs? What flow will mobilize the bed surface, in order to flush fines from subsurface? Will the frequency of bed disturbance change with alterations to the flood regime?  climate, land use, reservoir operation, fire)

Transport model for a threshold channel is based on a definition of incipient sediment motion Uncertainty Exercise For a simple, wide, prismatic channel, find critical discharge Q c for incipient motion hydraulic geometry momentum Manning’s eqn. continuity 5/16/20158

What if you are not too sure about some of the values needed to determine Q c ? Like n, D, and  * c – what do you do? 5/16/20159

Suppose your best estimate of Manning’s n is and that you are pretty sure that the real value falls between 0.03 and We could approximate your assessment of the value of n with a normal distribution with mean = & standard deviation = % of this distribution falls between 0.03 and 0.04, as can be seen in the cumulative frequency plot, so we are saying that the real value of n is 95% likely to fall between 0.03 and 0.04 and that it is more likely to be around the center of the distribution (0.035) than in the tails. We use this distribution to pick values of n in our Monte Carlo simulation. How does that work? We use a random number generator to pick a number between 0 and 1 and then use this number to find a value of n for the cumulative frequency distribution. For example, for 0.88, n = for 0.23, n = /16/201510

The Monte Carlo simulation 1. Pick values of n,, and D from specified frequency distributions. 2. Calculate critical discharge and transport rate. 3. Repeat 1000 times. 4. Distribution of calculated values gives estimate of the effect of input uncertainty on calculated critical discharge and transport rate  Monte Carlo 5/16/201511

Threshold Channel Find critical discharge Q c at which grain motion begins Mobile Channel Find transport capacity for different water discharge Q Estimating uncertainty in sediment transport It’s the input, not the formula !!! These terms have lots of uncertainty !!

2x 2x – 10x Now, what do we do with this uncertainty?

5/16/201515

SvSv 5/16/201516

SvSv Calculated slope 3x bigger!  RISK 5/16/201517

QDQD Failure in a threshold channel = grain entrainment

The core questions: What is the supply of water and sediment? What do you want to do with it? 1.What is the water discharge Q(t) and sediment supply rate Qs(t) and grain size D(t) delivered to the upstream end of the design reach? 2.How will the available flow move the supplied sediment through the design reach? More precisely, Sediment Transport in Channel Design How do we incorporate transport in channel design? When do we need to? 5/16/201519

The imaginary: What is the dominant discharge? Why only one flow? The wishful : Q bf (field)  Q bf (DA)  Q eff ≈ Q 1.5 ? Basis for connecting to core questions? The core questions may be difficult to answer But we cannot wish them away & ignoring them is the basis for project failure The core questions are often replaced by other questions 5/16/201520

“Stream stability is morphologically defined as the ability of the stream to maintain, over time, its dimension, pattern, and profile in such a manner that it is neither aggrading or degrading and is able to effectively transport the flows and sediment delivered to it by its watershed. Sediment Transport Capacity Sediment Supply Why do we hope for this convergence? Why do we expect this similarity to produce a “stable” channel Nicely stated, but why is it that one would think that a channel sized to the 1.5 yr flow, or some field indicator of such a flow, would neither aggrade or degrade and be able to transport the flows and sediment delivered to it by its watershed? 5/16/201521

When does a disturbance here show up here? Is that before, during, or after the impact from a disturbance here? Where is steady state found in a real watershed? In many cases, there is no steady state, & there is no template 5/16/201522

Morphology: Choose bankfull geometry from a template: a reference reach, regional hydraulic geometry Process: specify flood frequency AND sediment supply resistance eqn.  bankfull flow flood frequency curve  flood frequency incipient motion, transport criteria  flow competence, capacity flood frequency curve  bankfull flow + hydraulic & transport relations  channel slope & width + channel shape relations  bankfull geometry Design channel from a template, then check for transport? OR use drivers to develop the design? In either case: is hard to get an accurate estimate of sediment supply. Template vs. Prediction 5/16/201523

But there are more fundamental problems! At the core of the template approach is a correlation between channel geometry, flow, and sediment supply The correlation requires that the channels have adjusted to their water and sediment supply. But what if channel is currently adjusting, or perpetually adjusting? How would you know? A template approach provides no basis for linking cause and effect in a logically complete and testable framework. I II If a template-designed project “fails”, how is the method to be improved? ! This correlation is remarkable: The flow that moves the most sediment, over time, tends to just fill the channel and occurs ever year or few. The width of channels increases very consistently with the square root of discharge. 5/16/201524

Connecting sediment supply to the design problem 1.Reconnaissance phase: What is the trajectory of the stream? How has it responded to changes in water and sediment supply over the years? {Henderson relation  mixed-size seds} 2.Develop flood series, specify flood frequency  Q bf. {Select Q bf for flood frequency specified to maintain riparian ecosystem & prevent vegetation encroachment} 3.Estimate sediment supply 4.Planning phase: What slope S is needed to carry the sediment supply with the available flow? {How does S vary with Q s and width b?} 5.Develop flow duration curve 6.Design phase: Evaluate trial designs. Will the sediment supply be routed through the reach over the flow duration curve? {Build 1-d hydraulic model for trial design. Calculate cumulative transport over flow duration curve at each section; evaluate sediment continuity.} 5/16/201525

Borland’s stable channel stability relationship illustrated by James Vitaliano, BOR, in From Pemberton, E.L. and R.I. Strand, 2005, “Whitney M. Borland and the Bureau of Reclamation, 1930–1972”, J. Hydraulic Engineering, May 2005, pp The Lane/Borland Stable Channel Balance ReconnaissanceReconnaissance 5/16/201526

The Lane Balance, quantified 45 yrs ago by Henderson (1966, Open Channel Flow) What if q b increases and D decreases? Lane’s balance is indeterminate. ReconnaissanceReconnaissance 5/16/201527

Steady state: sediment supply balanced by transport capacity. Slope is stable. Increase sediment supply Sediment supply > transport capacity S 2 > S 1 sediment accumulates Increase water supply Sediment supply < transport capacity S 2 < S 1 sediment evacuates Interpretation, for evaluating stream history ReconnaissanceReconnaissance We will add a version for mixed-size sediment shortly 5/16/201528

Given Water discharge and sediment supply Find channel slope, depth & width (& velocity & shear) We have enough general relations to solve for all but one of these unknown variables If we specify channel width, we can solve for the rest of the variables What slope is needed to transport the supplied sediment with the available water? How big the channel? PlanningPlanning 5/16/201529

Hydraulic Design of Stream Restoration Projects September 2001 RR Copeland, DN McComas, CR Thorne, PJ Soar, MM Jonas, JB Fripp For a specified supply of water and sediment, what slope is needed to transport the supplied sediment with the available flow? We find the Slope varies little with sediment supply except at larger rates of supply Mobile channel design = match transport capacity to sediment supply

Sometimes, yesDoes sediment supply matter? Sometimes, no 5/16/201531

So, there must be a boundary between cases where sediment supply matters or not ThresholdAlluvial Bed & banks immobileActive transport Easier to model & design Bed & banks must only be strong enough Harder to design Requires a balance between transport capacity & sediment supply Extend Threshold definition to include small sediment supply rates requiring a slope negligibly larger than the zero supply case Focus on cases in which slope is sensitive to supply Nothing new under the sun … see SCS in the ’30s 5/16/201532

Why we can ‘neglect’ small sediment supply rates 1.Small sediment supply rates  many storms (and many decades) req’d to produce significant aggradation and degradation. 2.Small sediment supply rates  channel morphology and slope required to transport the supplied sediment can be negligibly larger than that of a threshold channel. 5/16/ Stress (Pa) Transport Rate (kg/hr) Sediment Supply (kg/hr) Slope

So, what is a SMALL sediment supply rate? That sounds dangerously like a real question, so first, lets deal with real sediments, which contain a mixture of sizes But for mixed-size sediment, there are complications … Grain size of bed  grain size of transport Bed is sorted spatially and vertically Transport is a function of the changing population of grains on the bed surface 5/16/201534

48 flume runs w/ 5 sediments Incorporates sand And effect of sand on transport of other sizes Tested against field data Transport Function Hiding Function Sand Interaction Function 5/16/201535

Surface-based transport model can be used in both forward & inverse forms Forward: predict transport rate & grain size as function of  and bed surface grain size Inverse: predict  and bed surface grain size as function of transport rate & grain size Don’t try this with a subsurface –based model! We can use an inverse transport model to forecast, or design, a steady state channel that will transport a specified sediment supply rate and grain size with the available flow (!) 5/16/201536

1. State Diagram I – transport v. discharge, lines of constant slope 2. State Diagram II – transport v. slope, lines of constant discharge 3. Channel Stability Diagram Presenting …. iSURF Inverse Model: predict  and bed surface grain size as fn(transport rate & grain size) Specify discharge and basic channel geometry and solve for slope (& depth) 5/16/201537

iSURF Channel Stability Diagram what slope is needed to transport a specified transport rate of specified size distribution with a specified discharge through channels of different widths? PlanningPlanning 5/16/201538

Channel Stability Diagram As a bonus, you find out how armored the bed becomes ! PlanningPlanning And get a measure of where you are relative to the threshold/alluvial channel boundary ! 5/16/201539

If your sediment supply is safely below the boundary between “low” slope and “high” slope, channel slope is relatively insensitive to sediment supply – you are less likely to accumulate sediment given an error in estimating sediment supply threshold alluvial Is an accurate sediment supply estimate needed? 5/16/201540

Strategy for a mobile channel (i)Determine if the sediment supply is a big number or a little number (a) if big, invest in more accurate estimate of sediment supply be prepared for a dynamic channel reserve riparian corridor and let the stream go or plan to trap and remove sediment (b) if little,design a threshold channel (ii)Estimate uncertainty and account for the consequences esp. potential for aggradation, degradation Little Number Big Number

OR, Design a flume Make your channel (i) steep enough: transport capacity exceeds supply and (ii) strong enough: bed material immobile … a washload threshold channel Design Basis:Flow Competence Competence & Capacity Transport Capacity Channel TypeThreshold Channel Threshold Channel w/ washload Alluvial Channel Topography & Bed Material Static Dynamic Flumes: an increasingly common & safe design option, may provide acceptable aesthetics. Note: does not provide anything like natural structure & function

“small”“large” Threshold channel design Use risk assessment and P(failure) to guide design Alluvial channel design Allow for dynamic stream Invest in improved sediment estimate Build a flume Is the sediment supply small or large?

1.Reconnaissance phase: What is the trajectory of the stream? How has it responded to changes in water and sediment supply over the years? 2.Develop flood series, specify flood frequency  Design Q. {Select Q bf for flood frequency specified to maintain riparian ecosystem & prevent vegetation encroachment} 3.Estimate sediment supply 4.Planning phase: What slope S will transport the sediment supply with the available Q bf ? Calculate (b, S) combination {S and valley slope determine sinuosity} Check if alluvial v. threshold channel 5.Develop flow duration curve 6.Design phase: Evaluate trial designs. Will the sediment supply be routed through the reach over the flow duration curve? {Build 1-d hydraulic model for trial design. Calculate cumulative transport over flow duration curve at each section; evaluate sediment continuity.} 7.Bottlenecks or blowouts? Adjust for sediment continuity Design steps incorporating sediment supply iSURF State Diagrams 5/16/201544

Objective sediment & nutrients property & infrastructure biological recovery aesthetic penance What needs fixing? Stormwater control Nothing Channel change Introduced species Disturbance Internal or external? InternalExternal Fence out the cows! Remove the concrete! Template approach can work Small Channel Design Sediment supply large or small? Large Estimate flood frequency Design threshold channel Estimate sediment supply & flow duration Design mobile channel Mobile or Threshold Channel? … Sediment Transport in Stream Restoration Environmental Drivers

You don’t always have to consider sediment transport in stream restoration e.g. if the problem does not involve channel change There are two types of transport problem – competence and capacity (threshold and mobile bed) (and flume) Most error in transport calculations is in the input, not the formula. If you need an accurate of sediment transport, you must make field observations. They need not be fancy, but it takes care Often, you can avoid this effort … Uncertainty can be estimated AND you can incorporate that uncertainty into your design strategy If the sediment supply is not “large”, you can go to the simpler threshold design problem If the sediment supply is “large”, you can let the channel go design a flume channel (if you have enough slope) do the work to properly design a mobile-bed channel

(1) Chapters 1 and 2 in Wilcock, Peter; Pitlick, John; Cui, Yantao Sediment Transport Primer: Estimating Bed-Material Transport in Gravel-bed Rivers, Gen. Tech. Rep. RMRS-GTR-xxx. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. (2) Chapters 2, 7, 8, 9 in NRCS, Stream Restoration Design Handbook (NEH 654), USDA. You can either download individual chapters of NEH 654 or request a free cd. There are no paper copies. (I) Download the book chapter by chapter from navigate to Handbooks, Title Engineering, National Engineering Handbook, Part Stream Restoration Design (II) To request a CD, go to and search for NEH-654. The CD version is free and includes navigation bookmarks, is fully searchable with keywords, and has high quality files for selective printing. The CD also contains a copy of Federal Interagency Stream Restoration Working Group (FISRWG) Stream Corridor Restoration: Principles, Processes and Practiceshttp://policy.nrcs.usda.gov/index.aspx Readings (3) RiverRat Skidmore, P. B., C. R. Thorne, B. Cluer, G. R. Pess, J. Castro, T. J. Beechie, and C.C. Shea. In review Science base and tools for evaluating stream engineering, management, and restoration proposals. U.S. Dept. Commerce, NOAA Tech. Memo. NMFS-NWFSC. 5/16/201547

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