Download NCHRP 2016 - ProblemStatement_Gravel

I.
PROBLEM NUMBER
To be assigned by NCHRP staff.
II.
PROBLEM TITLE
Develop a Formula for Determining Scour Depth around Structures in Gravel-bed Rivers
III.
RESEARCH PROBLEM STATEMENT
In the current state of engineering practice, there is a recognized lack of formulas for
predicting the scour depth around hydraulic structures, such as bridge piers, abutments and
barbs, in gravel-bed rivers. The majority of scour depth prediction formulas presently in
use have been developed for sand-bed rivers, and as such do not account for fundamental
processes and characteristics that are unique in gravel-bed rivers (e.g., Raikar and Dey,
2005; Holnbeck, 2011). For instance, the inhomogeneity of gravel particles promotes
particle interlocking, armor development. At the same time, unique turbulent structures
result from the interaction of the non-homogeneous material with the approach flow
(Papanicolaou et al., forthcoming). Even after correcting their scour depth predictions
using empirically derived coefficients to account for these processes, the traditional scour
prediction formulas originating from sand-bed rivers still overpredict scour depths in
gravel-bed rivers. Recent insights into gravel-bed river dynamics along with advances in
sensing technologies, such as RFIDs and Ultrasonic Transducer Arrays, make have
improved understanding of these gravel-bed river processes and for the first time allow the
development of a scour prediction formula that directly accounts for their effects (Tsakiris
et al., 2016).
IV.
LITERATURE SEARCH SUMMARY
The majority of the formulas used in current engineering practice for predicting scour depth
around hydraulic structures have been developed for sand-bed rivers, which are
characterized by near-uniform bed material (e.g., Dey and Raikar, 2005; Ettema et al.,
2011; Guo et al., 2012; Manes and Brocchini, 2015). As a result, parameters such the
inhomogeneity of bed material have been excluded from their formulations, and the
empirical coefficients appearing in these formulas have been derived from laboratory
experiments conducted with near uniform sand-sized sediment. Due to these limitations,
when applied in gravel-bed rivers, where the more inhomogeneous material exhibits
different mobility compared to uniform sand, these formulas significantly overestimate the
scour depth (e.g., Dey and Raikar, 2005; Holnbeck, 2011). Recognizing this shortcoming
of current scour prediction formulas, researchers have attempted to include the effects of
gravel particle inhomogeneity using empirically derived correction coefficients. However,
the failure of existing scour prediction formulas to fundamentally account for the unique
characteristics of gravel-bed rivers in their formulation, translates to large scour depth
prediction errors, even after their correction.
Scour around hydraulic structures is the outcome of the dynamic interaction between the
approach turbulent flow, the hydraulic structure and the bed sediment (e.g., Guo et al.,
2012; Manes and Brocchini, 2015). In gravel-bed rivers, the interaction of the
inhomogeneous, large gravel particles with the approach turbulent flow favors the
generation of coherent turbulent structures near the boundary. In addition to amplifying the
Page 1 of 4
shear stress applied by the flow onto the bed, these structures create a highly variable bed
shear stress field thereby increasing the gravel bed mobility (e.g., Tsakiris et al., 2014;
Sarkar et al., 2016). On the contrary, the larger and less mobile gravel particles offer hiding
effects to the smaller, more mobile gravel particles, therefore reducing the overall gravel
bed mobility. Furthermore, as the smaller gravel particles are entrained during competent
flows, the remaining larger gravel particles create an armor pavement on the gravel bed
river surface, thereby reducing the probability of scour occurrence (e.g., Guo et al., 2012).
Despite the recent progress made in understanding the role that the aforementioned
processes, unique in gravel-bed rivers play in scour around hydraulic structures (e.g., Dey
and Raikar, 2005; Guo et al., 2012; Manes and Brocchini, 2015), a formula that
comprehensively accounts for these processes still remains to be developed.
V.
RESEARCH OBJECTIVE
The objectives of this research project are:
1. To investigate the effects of gravel interlocking and of turbulent structures, such as the
horseshoe and wake vortices, generated in gravel-bed rivers on scour around hydraulic
structures.
2. To develop a physically-based formula for predicting scour depth around hydraulic
structures in gravel-bed rivers that accounts for the sediment and flow conditions in
the gravel bed rivers.
The research tasks for each study phase are synoptically presented in the following:
PHASE 1: Literature Review and Experimental Investigation.
Task 1: Perform critical review of the literature for identifying the processes and related
variables, which govern scour in gravel-bed rivers.
Task 2: Develop flume experiments representing typical sediment and flow conditions
encountered in the field.
Task 3: Perform the designed experiments, where two types of measurements will be
conducted. First, the turbulent flow structures and second, the scour hole geometry.
Task 4: Develop a formula for predicting scour depth around hydraulic structures that will
be specifically applicable in gravel-bed rivers. The new formula will include the role of
particle inhomogeneity, as well as the effects of the turbulent flow structures that develop
from the interaction of the approach flow with the hydraulic structure and the gravel bed.
PHASE 2: 2D Numerical Modeling and Dissemination of the Results
Task 5: Establish a 2D numerical model of the experimental setup. The 2D model will be
calibrated and validated using the measurements performed during Phase 1 of the project.
In addition, the scour formula developed from the experimental measurements of Phase 1
will be implemented in the 2D model.
Task 6: The established 2D numerical model will be used to examine the effects of gravel
interlocking and turbulence structures for various configurations of the selected hydraulic
structures, such as spacing, and angle relative to the flow direction.
Page 2 of 4
Task 7: The findings of this study will be documented in a final report, which will
document the new scour depth prediction formula, detail its development method, range of
applicability, as well as present the data input requirements for its application and make
recommendations for using it in numerical models.
Task 8: The study results will be disseminated and the knowledge gained from this project
will be made available to DOT engineers. Remote training webinars will be organized and
training videos will be made available. The outcomes of this project will also be published
in high-impact, peer-reviewed engineering journals and presented in engineering
conferences at the national and international levels.
VI.
ESTIMATE OF PROBLEM FUNDING AND RESEARCH PERIOD
Recommended Funding: $600,000
Research Period: 30 months
VII.
URGENCY, PAYOFF POTENTIAL, AND IMPLEMENTATION
The development of a scour prediction formula that has been developed specifically for
gravel-bed rivers is of immediate importance and use to DOT and practicing engineers.
Because of the lack of formulas for predicting the scour depth around hydraulic structures
in gravel-bed rivers, DOT and practicing engineers resort to using formulas, which have
been developed for sand-bed rivers and subsequently correcting their predictions with
empirically derived coefficients. These formulas, cannot fundamentally provide accurate
predictions for the scour depth in gravel-bed rivers and even their corrected estimates are
riddled with uncertainty (Papanicolaou et al., forthcoming).
Adoption of a scour prediction formula that is specifically developed to account scour
processes in gravel-bed rivers will be highly valuable for DOT and practicing engineers.
Such a formula will be the product of this research and will for the first time account for
scour processes in gravel-bed rivers and will provide more accurate predictions of the
scour depth. This enhanced accuracy can, in turn, reduce the cost for the design and
construction of hydraulic structures, by reducing the hours required for translating scour
predictions with sand-bed formulas to gravel bed rivers. The new equations will allow for
a reduction in the depth of scour in gravel-bed rivers as opposed to using sand-bed
equations, which will reduce infrastructure foundation cost and environmental impacts
such as unnecessary excavation and materials for constructing hydraulic structures.
VIII.
PERSON(S) DEVELOPING THE PROBLEM
1. Casey Kramer
Principal Engineer
Northwest Hydraulic Consultants, Inc.
711 Capitol Way South Suite 607
Olympia, WA 98501
2. Prof. Thanos Papanicolaou
University of Tennessee
Page 3 of 4
Professor and Henry Goodrich Chair of Excellence in Civil and Environmental
Engineering
Department of Civil and Environmental Engineering
412 John D. Tickle Engineering Building
Knoxville, TN 37996
IX.
PROBLEM MONITOR
To be assigned by AASHTO TCHH.
X.
REFERENCES
Ettema, R., Constantinescu, G., and Melville, B. (2011). NCHRP Web-Only Document
175. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions,
Transportation Research Board, Washington, DC.
Guo, J., Suaznabar, O., Shan, H., and Shen, J. (2012). Pier scour in clear-water conditions
with non-uniform bed materials (No. FHWA-HRT-12-022).
Holnbeck, S. R. (2011). Investigation of pier scour in coarse-bed streams in Montana,
2001 through 2007 (No. Scientific Investigations Report 2011–5107). US Department of
the Interior, US Geological Survey.
Kothyari, U. C., Hager, W. H., and Oliveto, G. (2007). Generalized approach for clearwater scour at bridge foundation elements. J. Hydraul. Eng., 133(11), 1229-1240.
Manes, C., and Brocchini, M. (2015). Local scour around structures and the
phenomenology of turbulence. J. Fluid Mech., 779, 309-324.
Papanicolaou, A.N., Bressan, F., Fox, J.F., Kjos, L., Ettema, R. and Kramer, C. (2016).
Scour around a barb in a gravel-bed stream: Effects of submergence J. Hydraul. Eng., (in
preparation).
Papanicolaou, A.N., and Tsakiris, A.G. (2016) “Boulder effects on turbulence and bedload
transport.” In: Gravel-bed Rivers: Processes and Disasters: Tsutsumi, D. and Laronne, J.B.
(eds.), John Wiley & Sons, Chichester, West Sussex, UK.
Sarkar, S., Papanicolaou, A.N., and Dey, S. (2016). Turbulence in gravel-bed stream with
an array of large gravel obstacles. J. Hydraul. Eng., DOI: 10.1061/(ASCE)HY.19437900.0001191, 04016052.
Raikar, R. V., and Dey, S. (2005). Clear-water scour at bridge piers in fine and medium
gravel beds. Can. J. Civil Eng., 32(4), 775-781.
Tsakiris, A.G., Papanicolaou, A.N., Hajimirzaie, S.M., and Buchholz, J.H.J. (2014).
Influence of collective boulder array on the surrounding time-averaged and turbulent flow
fields.J. Mountain Sci., 11(6):1420-1428. doi:10.1007/s11629-014-3055-8
Page 4 of 4