Simulating Sediment Transport to Evaluate Dam Removal Restoration Strategies

Klamath River looking upstream, showing Iron Gate Dam in the background.

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Downstream of Copco 1

Downstream of Iron Gate

Downstream of Seiad Valley

Downstream of Orleans

Copco 1 Pool Level

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Many large-scale restoration actions are characterized by signifi-cant uncertainty in their eventual outcome, leading to consid-erable concern on the part of agencies and stakeholders about whether and how to pursue the proposed restoration strategy. Dam removal is an excellent case in point: removing large dams typically releases large volumes of sediment to downstream reaches that can potentially cause significant environmental and economic consequences, or be quite benign, depending on the circumstance.

Accurate scientific information regarding the dynamics of released sediment is critical to guide the

engineering approach to dam removal.

In this regard, engineers and scientists have long relied on one-dimensional sediment transport models for predicting aggrada-tion and degradation in rivers. These models have usually require detailed channel cross section survey as model input despite the fact that one-dimensional models inherently cannot simulate the output of detailed features such as bars and pools.

However, over the last 15 years a new generation of one-di-mensional models has been developed based on far simpler data input. These models have the advantage of being a) highly cost-effective because they require less field data input; b) com-putationally more versatile and capable of handling sub-, super- and transient flow conditions; and c) able to run with minimal calibration; and d) proven to provide accurate predictions. Below, we illustrate several case studies that assisted in selecting an ap-propriate dam removal strategy.

Introduction

Yantao Cui, John K. Wooster, Peter W. Downs, Scott R. DusterhoffStillwater Sciences, Berkeley, CA

DREAM-1 and -2 were devel-oped based on the sediment pulse model of Cui and Park-er (2005) and the data col-lected to model dam removal for Soda Springs and Marmot dams. DREAM-1 applies to the case where reservoir depos-its are composed primarily of non-cohesive fine sediment, and DREAM-2 applies to the case where the top layer of the reservoir deposit is com-

posed primarily of gravel and coarser materials. The two models were developed with the intention of applying the minimal amount of field data possible so that they can be set up for specific field studies relatively quickly. The models were run using the Sandy River as the prototype, combined with a few different reservoir sediment deposit scenarios for sensitivity tests. Results of the

sensitivity tests not only pro-vide guidance in planning field data collection for dam remov-al projects but also provide valuable information on poten-tial impacts of dam removal under different reservoir sedi-ment deposit conditions.

Dam Removal ExpressAssessment Models

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(1). Light colored lines are initial profile;(2). Vertical grid size = 0.05 m.

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Simulated flux at flume exitSediment feed

FIGURE 2. Comparison of measured bed elevation and sediment flux at the exit of the flume with DREAM-2 simula-tion: (a) measured bed elevation; (b) simulated bed elevation; and (c) sediment flux (measured and simulated) at the exit of the flume.

DREAM-2 simulation

A very simple calibration was conducted so that the simulated equilibrium slope is identical to observed equilibrium slope for a previous run, resulting in an adjustment of the reference Shields stress in Parker (1990) from 0.0386 to 0.0444. This adjusted reference Shields stress was used for all the runs with good agreement between model simulation and flume observation.

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FIGURE 1. Comparison of measured bed elevation and sediment flux at the exit of the flume with DREAM-1 simulation: (a) measured bed elevation; (b) simulated bed elevation; and (c) sediment flux (measured and simulated) at the exit of the flume.

Details for model development and examination can be found in Cui et al. (2006a); model sensitivity tests that provide some important information for dam removal managers can be found in Cui et al. (2006b).

Ok Tedi Mining Ltd. in Papua New Guinea operates a gold and copper mine that has discharged over 1.3 billion metric tons of rockwaste and tailings into the adjacent val-leys of the Ok Tedi-Fly River fluvial system.

In conjunction with Drs. Gary Parker, William Dietrich, and Geoff Pickup, numerical model simulations have been developed since 1996 as part of an environmental impact study of over 600 km of river, ranging from steep headwaters with minimal flow to lowland reaches with extremely high runoff. Some tributaries have experienced sediment de-position of 80–100 m over the past 25 years, due to a sediment pulse far larger than any available case study of sediment release re-lated to dam removal.

Simulating sediment transport in both the gravel-bedded upper and mid Ok Tedi, and the sand-bedded lower Ok Tedi and Fly Riv-er has provided an excellent case study for simulating sediment transport dynamics and channel aggradation in a situation analogous to dam removal and other large sediment pulse applications.

Modeling results indicate accurate predic-tions of the observed aggradation in both gravel- and sand-bedded reaches of the Ok Tedi, the Fly River and their tributaries.

O k T E D I / F ly R I v E R , PA U PA N E w G U I N E ASediment Transport Study

The mid Fly River, courtesy of OTML.

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Comparison of simulated and measured channel aggradation at two villages along Ok Tedi and Fly River.

M A R M O T D A M R E M O vA l S T U Dy Sandy River, OR

Marmot Dam was a 47-ft tall dam located the Sandy River, Oregon about 30 miles upstream of the Columbia River confluence. It was re-moved in the summer of 2007, potentially re-leasing the approximate 1 million cubic yards of coarse and fine sediment stored upstream of the former dam into the Sandy River, home to several listed salmonid species.

A one-dimensional model (a precursor to the DREAM models) was developed to predict sedi-ment transport dynamics under various dam removal alternatives. The study took about 6 months including field data collection, model development (coring of reservoir sediments was previously conducted) and peer review of the technical report. The rapid analysis was possible because the river’s longitudinal profile was acquired via photogrammetry, and bank-full channel widths were estimated from aerial photographs, eliminating the need for field cross-section survey.

The modeling and subsequent interpretation of biological impacts played a central role in lead-ing a diverse stakeholder group to adopt the highly cost-effective strategy of single season dam removal with minimal sediment excavation, colloquially, the “blow-and-go” alternative. This was the first time in US history that such a large volume of gravel and sand was consciously released into a major salmonid bearing river. Post-removal data collection has indicated that the extent of channel aggradation and degradation was within the predicted range, and all other major predictions proved to be reason-able (see figure below).

The downstream-most four dams on the Klamath Riv-er, ranging in height from 38 to 177 ft and storing an estimated 20 million cubic yards of sediment, are un-der consideration for removal.

To help stakeholders understand the potential for sediment deposition downstream of the dams follow-ing removal, an initial sediment transport model was developed under a tight schedule to simulate poten-tial sediment release under worst-case-scenario as-sumptions. It was concluded, and accepted by a peer review of national experts, that removal would result in very limited channel aggradation within a short distance downstream of Iron Gate Dam (the down-stream-most dam) despite the huge amount of sedi-ment deposit trapped in the reservoirs, relieving most of the concerns for potential flooding issues associ-ated with dam removal.

A more detailed model has since been completed, using refined field data, to examine different dam re-moval options under the concurrent drawdown alter-native (i.e., drawdown Copco 1 and Iron Gate reser-voirs concurrently).

In addition to confirming the conclusions of the earlier study, that there would be minimal channel aggrada-tion downstream of the dams following removal, time series of predicted suspended sediment concentra-tions for the lower 200 miles of river were provided to fisheries biologists and water quality specialists to facilitate their respective evaluations of potential im-pacts.

D A M R E M O vA l S T U D yKlamath River, CA

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Estimates by Andrew Marshall and Geoff Pickup based on field data

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Field measurement by Portland General Electric

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The Waipaoa River Basin drains 2,200 km2 of the east coast of New Zealand’s North Island, ultimately emptying into Poverty Bay. Euro-pean colonization during the late 1820s re-sulted in forest clear cutting over almost the entire watershed and a conversion to grazing land. Subsequently, the Waipaoa watershed has been subject to intense erosion, and there is now elevated concern for the poten-tial impact of the sediment slug stored in the fluvial network under possible alterations to the hydrologic regime due to climate change.

In collaboration with Dr. Basil Gomez, a nu-merical model was developed to simulate sediment dynamics for the lowermost 50 km of the Waipaoa River. First, the model was applied retrospectively, and produced a very reasonable simulation of the observed sedi-ment dynamics over the period of 1948–2002 (see illustrations), including general patterns of aggradation and degradation and bed ma-terial median grains size and sand fraction. Then, using a hydrologic and climate model developed by collaborators, the model was used to predict sediment transport dynamics for the next 100 years under various climate change scenarios.

The results indicated that significant sedi-ment deposition in the lower most reach of the river would occur regardless of the climate change assumptions, as the large sediment slug gradually works its way down-stream.

wA I PA O A R I v E R , N E w Z E A l A N DSediment Transport Study

Top diagram indicates that the model closely reproduced the aggradation and degradation patterns and their general magnitudes observed for the period of 1948 – 2002.

Middle diagram shows that the model closely reproduced the downstream change in bed material median size on a reach-averaged basis.

Bottom diagram shows that the model closely reproduced the downstream increase in sand fractions in the deposit on a reach-averaged basis.

American Rivers • American Trout • Bureau of Reclamation • California Bay-Delta Authority • Environmental Protection Agency • Friends of the River • National Center for Earth-surface Dynamics • National Marine Fisheries Service • National Science Foundation • The Nature Conservancy • Ok Tedi Mining Ltd. • PacifiCorp • Portland General Electric • St. Anthony Falls Laboratory • Trout Unlimited • University of California at Berkeley • University of California at Davis • University of Minnesota

This poster would not be possible without the cooperation and contributions from various organizations and individuals.We gratefully acknowledge:

A c k N O w l E D G E M E N T SCopper mine operated by Ok Tedi Mining Ltd. in Papua New Guinea.

Large ship navigating the Fly River.

Dam removal is a relatively recent addition to the suite of strategies for river manage-ment, and there is considerable uncertainty surrounding its potential short-term impacts on water resources, hazard avoidance, and river conservation. Concern over the release of a sediment pulse after dam removal is fre-quently the single largest environmental con-straint, and its avoidance is potentially the greatest single cost element as well.

Numerical modeling can provide cost-effec-tive methods to assess possible morphologic response and bed texture change due to a dam-released sediment pulse. One-dimen-sional numerical modeling of sediment-pulse behavior provides an effective, simple means of determining the reach-averaged down-stream morphological impact of a migrating sediment pulse. Numerical modeling results can provide invaluable information when evaluating and selecting dam removal alter-natives, as demonstrated by case studies pre-sented herein.

In addition, we also find that physical model-ing (i.e., flume experiments) is a cost-effective means of calibrating and verifying numerical models to improve confidence in their output as well as address two- and three-dimension-al aspects of the sediment pulse to answer questions relevant to engineering, biology, and ecology.

Additional information on Stillwater Sciences’ flume experiments pertaining to dam removal

and sediment transport can be found at:

http://flume.stillwatersci.com

Discussion