U.S. Department of the Interior

U.S. Geological Survey

Is there enough san

Evaluating the fate of Grand Canyo

Scott A. Wright, John C. Schmidt, Theodor

David J. Topping, David M. Rubin

Manuscript status: Approved by USGS, in review 

8 April 2008

GCDAMP TWG meeting, PHX
Motivation

 Science Symposium Panel: Several were de

lost cause and that sediment augmentation 

 Knowledge Assessment Workshop: Primary 

identified for sediment was “Is there a ‘flow-sediment augmentation) operation that will rmaintain sandbar habitats over decadal time 

Goal here was to take a “big pictu

approach to try to answer the ques

Is there enough sand?
Approach

 Define and evaluate the “best case sc

terms of hydrology and dam operatiorebuilding and maintaining sandbars scenario “optimal operations”)

 Why this approach?

 It’s the logical first step - If the "optimal o

“work”, then non-flow measures may be e"optimal operations" do “work”, it sets thexpectations and leads into more complex 

 The "optimal operations" are the easiest t

available information, leading to the least 
Definition of "optimal opera

Sand transport is non-linearly related to water dischafollows that lower flows transport less sand, and for a volume, steady flows transport less sand than fluctuaThus, for maximizing sand retention:

- “optimal hydrology” is minimum releases (8- “optimal flows” are steady year-round- result: 8.23 MAF steady = 11,400 cfs

Any accumulation would be below 11,400 cfs stage. Redistribution to higher elevations is required by perflows (we assume an annual high flow with a hydrogdependent on the supply condition for a given year)

* This analysis was done before shortage 
Δ io = − MMM ioin

Approach

=Δ in − MMM out

Redistribution of sand to bars can only happ

accumulation occurs during “intervening opera

Framework for analysis is the annual sand budget fo

First step is to evaluate potential accumu

during intervening operations, ΔMio

Min is tributary inputs, Mout is 

If ΔMio > 0, then some fraction can be redistribut
Annual tributary inputs Annual tributary inputs

Δ io

=

− MMM ioin

Min – annual tributary inputs

Paria ~ 1,500,000 metric tons per y

(Topping et al., 2000)

Others ~ 290,000* metric tons per y

(Webb et al., 2000)

Total ~ 1,800,000 metric tons per ye

* recent data indicate this may be as much 
Δ io = − MMM ioin

Mio – annual export (non high flow)

Annual export (non high flo

1

10

100

1,000

10,000

0 200 400 600 800 1,000

Water discharge (m3/s)

Suspended-sand concentration (mg/L)

0

20406080

100120140160

To compute Mio, we need to estimate the sand concentr

Lower Marble Canyon gage Using availabl

professional ju

estimated ~ 

Thus, Mio ~ 500

tons per 
ΔMio = 1,800,000 – 500,000 = 1,300,000 metri

Annual accumulation

Δ io = − MMM ioin

The “optimal operation” has the potential to accumu

1,300,000 metric tons per year (on average

This sand would be below the 11,400 cfs stage; r

redistribution to higher elevations by high flow

How much can be retained in sandbar
High flow redistribution

(

)

+

+

ΔMFFF ioecb

As pointed out by Rubin et al. (2002), high flows aredged sword” because, in order to redistribute sand elevation, a substantial quantity must be exported:

Fraction transferred to bars

Fraction left behind in channel

Fraction exported
High flow redistribution

Estimates of 

Fb

 Topping et al. (2006b) estimated that ~10-20% of the av

sand retained in the upper 50 km of Marble Canyon wareach following the 2004 high flow event, presumably in 

 Though the 1996 high flow event resulted in an overall n

sediment from sandbars (Schmidt, 1999, Hazel et al., 2a gain in high-elevation volume that was ~20% of the lolow-elevation portions of sandbars and the channel

 Hazel et al. (2006) estimated the potential active storag

Marble Canyon to be ~13,000,000 metric tons, which is 

total pre-dam fine sediment load (sand and finer)

we chose Fb ~ 0.15
High flow redistribution

How does this compare to post-dam erosion 

Thus, we estimate that ~200,000 of the ~1,300,000 of accumulated sand could go towards bar-buildingbased on “optimal” operations, it can be considered bound for expectationsIf gains are maintained until the next high flow (an imassumption in our approach), response would be cuthrough time

Sandbar accumulati

=

ΔMFon iob
Post-dam erosion rates

 In April-May 1965, ~16,000,000 metric tons wa

from Marble Canyon (Rubin and Topping, 200came from sandbars, this equates 1,600,000 mwhich is about 8 times our estimated potential accumulation

 Schmidt et al. (2004) and Hazel et al. (2006) e

loss of fine sediment from sandbars from the pthrough the 1990s to be ~6,000,000 metric tonapproximately 40 years since dam constructionequates to an average annual erosion rate of ~metric tons per year

For Marble Canyon, we have relatively good 

potential accumula

~200,000
Conclusions

 The “best case scenario” for hydrology and dam o

some viability for rebuilding and maintaining sand

 The maximum rate at which sandbars could poten

rebuilt is comparable to their erosion rate over the 

since dam construction

 Deviations from “optimal conditions” (e.g. wetter h

fluctuating flows) will decrease the rate of accumuresult in net erosion

 The largest uncertainty is in estimating Fb. Can hig

hydrographs be “optimized” to promote sandbar bminimizing export?
Future work

 Variable hydrology – we assumed minimum releases 

MAF) – if releases are higher, export increases and leavailable for building sandbars. The difficulty in analyz

scenarios is predicting future hydrology.

 Fluctuating flows – To evaluate a year-round steady 

discharge and one concentration are required. To evafluctuating flows, hourly hydrographs, relations betweedischarge and sand concentration, and time series of inputs are required. We are working on models to do t(LTEP), but they are not quite ready. 

 Optimized high flow hydrographs – Need models c

evaluating various hydrograph scenarios and supply c
Final comments

Is there a ‘flow-only’ (non sediment aug

operation that will rebuild and maintain 

habitats over decadal time scale

We think so, yes...

But, it is likely dependent on low volume 

And, it will take time even under "optimal o