DEBRIS AND MAINTENANCE OF LABYRINTH SPILLWAYS and Maintenance... · Debris and Maintenance of...

Debris and Maintenance of Labyrinth Spillways 501

DEBRIS AND MAINTENANCE OF LABYRINTH SPILLWAYS

Brian M. Crookston, Ph.D., P.E.1 Dustin Mortensen, P.E.2

Tina Stanard, P.E.2 Blake P. Tullis, Ph.D.3

Victor M. Vasquez, P.E.2

ABSTRACT Labyrinth spillways are passive control structures that are commonly implemented in dam projects (i.e., new or rehabilitation) due to their hydraulic performance, flexible design, and cost effectiveness. The accumulation of sediment or floating debris can be problematic at spillways; sediment deposition or log jams can require cleaning and removal at regular intervals to eliminate flow impediments and maintain spillway performance. If neglected, debris or sediment could potentially reduce flow capacity and increase upstream flooding. Floating debris may be more likely to collect upstream of labyrinth weirs than linear weirs due to the reduction in flow depth and flow momentum per unit weir length. This paper presents performance data and general information on the effects of debris and sediment on labyrinth spillways, including survey results of 65 labyrinth spillways in the USA and Portugal. In general, labyrinth weirs were found to be self-cleaning. In some instances, labyrinth weirs that replaced gated structures greatly improved existing maintenance issues with woody debris. Information is also presented regarding a labyrinth spillway with unique sediment and woody debris challenges. The paper also presents the results of a hydraulic model study performed at the Utah Water Research Laboratory that investigated the impact of upstream sediment deposits on labyrinth hydraulic performance. In addition, a dam owners experience with sedimentation at a labyrinth weir is discussed. Finally, the effects of driftwood on a unique labyrinth weir geometry, called a piano key weir (PKW), are also presented. Physical modeling was performed at the Laboratory of Hydraulic Constructions (LCH) located in Lausanne, Switzerland. The results include observations regarding probabilities of debris passage as a function of trunk size and flow depths. The impacts of debris accumulation on the discharge coefficient are also presented, as well as observations related to driftwood collection.

1 Schnabel Engineering, Inc., 1380 Wilmington Pike Suite 100, West Chester, Pennsylvania, 19382, (610) 696-6066, [email protected] 2 Freese and Nichols, Inc., 10431 Morado Circle Suite 300, Austin, Texas ,78759, (512) 617-3100, [email protected], [email protected], [email protected] 3 Utah Water Research Laboratory, Utah State University, 8200 Old Main Hill, Logan, Utah, 84322-8200, (435) 797-3194, [email protected]

502 USSD 2015 Annual Conference

INTRODUCTION Spillway Deficiencies and Issues A spillway is a safety device that may serve a variety of functions, such as: flood control, regulating the upstream water surface elevation, emergency drawdown, and an ecological bridge (e.g., fish passage). Spillways have been classified based upon type of control structure (active, passive, etc.), function (service, auxiliary, emergency), and according to geometry-specific details (e.g., ogee, side channel, shaft, siphon, stepped, labyrinth, etc.) (Khatsuria 2005). In many instances these hydraulic structures provide necessary support for communities throughout the globe. However, no spillway is ageless. Construction of a new spillway or the rehabilitation of an existing spillway can be motivated by dam safety and owner concerns related to hazard creep, structural deterioration or damage, hydrologic revisions, or inadequate performance. Inadequate discharge capacity is a common spillway deficiency. Also, a cumbersome and expensive maintenance schedule may be required to ensure that a mechanical spillway successfully operates when needed. In some instances, spillway maintenance issues have justified the modification or even replacement of a spillway. Labyrinth and Piano Key Weirs A labyrinth weir (Crookston and Tullis 2013a) is a passive-control spillway with a unique geometric shape (Fig. 1). The non-linear nature of labyrinth weirs (folded back and forth in plan view) allows for significantly longer weir lengths than could be achieved using a traditional linear weir for a given spillway width. These spillways are used in a variety of applications (e.g., reservoirs, run-of-river). For example, a labyrinth spillway may be the only spillway operating during a storm, with the crest set at the normal pool elevation. Or, it may serve as an emergency spillway adjacent to spillway gates and engage near the peak of the inflow hydrograph. Labyrinth spillways are particularly well suited for instances where increased spillway capacity is needed or spillway footprint is restricted (Crookston et al. 2013). For a given spillway width, the water depth at the crest of a labyrinth will be less at a given discharge than for other linear free overfall structures due to the additional crest length (relative to a linear weir).


(A) (B) Figure 1. Labyrinth weir geometric parameters (A) and a photo of a prototype example.

The U.S. Bureau of Reclamation constructed the first labyrinth weir in the USA at East Park Dam in California in 1910 (a 9-cycle, curved labyrinth weir). However, research and construction of additional labyrinth spillways did not begin in earnest until the 1970s. Following the publication of the Tullis et al. 1995 systematic labyrinth weir design method, an escalation in labyrinth weir construction occurred. It is estimated that about 100 labyrinth weirs exist today in the United States. Furthermore, there is a current annual trend of 10 or more labyrinth spillways in planning or design in the USA. More detailed information regarding the history of labyrinth weirs, published literature, and hydraulic design methods are presented in Crookston (2010), Crookston and Tullis (2013a,b), Crookston et al. (2013), and Falvey (2003). A relatively new type of labyrinth weir is the piano key weir (PKW) (Fig. 2); the traditional labyrinth design was modified to improve hydraulic performance and decrease construction cost. The PKW is a rectangular labyrinth weir (in plan form) with cantilevered apexes and ramped floors; this geometry increases the allowable weir length for restricted footprint applications (e.g., top-of dam) (EDF-LNHE and University of Biskra, Algeria). A great deal of research has been performed regarding PKW hydraulic performance [Erpicum et al. (2011, 2013); Anderson and Tullis (2012a,b), (2013), Dabling and Tullis (2012)], and a number of structures have been constructed in reservoir or channel applications in Europe, Asia, and Australia.


(5) (B)

Figure 2. PKW geometric detail and a photo of a PKW prototype structure. The accumulation of sediment or floating debris can be problematic at spillways, particularly if there is a flow impediment. Weirs generally have a greater ability to pass floating debris than active-control spillways such as gates (Khatsuria 2005). However, due to the geometry of labyrinth spillways, branches and trees from forested upland areas can be transported to a spillway during flood events and form a blockage, which could potentially reduce spillway discharge capacity and increase upstream flooding. Furthermore, floating woody debris may be more likely to collect upstream of and atop labyrinth weirs than linear weirs due to the reduction in flow depth and flow momentum per unit weir length. The potential for debris blockages and required maintenance to remove sediment or debris would be of interest to designers, operators, regulators, and owners. A few case studies have been conducted specific to labyrinth type weirs (PKW) and driftwood [Ouamane and Lemprir (2006), Laugier (2007), Sogreah (2011)]. In addition, effects of sediment on labyrinth discharge capacity were investigated for the Boardman Labyrinth Spillway (Babb, 1976), and the Garland Canal, yet little field data and design information is available regarding the performance of these spillways with regards to floating debris and sediment. This paper presents information on floating debris at labyrinth spillways, including a case study where sedimentation and debris were of concern. This information comes from the authors experiences with specific projects, and information gathered from USA dam safety officials and colleagues in Europe. This paper also includes a summary of the results of recent laboratory studies conducted in the USA on labyrinth weirs and in Switzerland regarding the influence of woody debris on the discharge capacity of PKWs.


STUDY OVERVIEW Labyrinth Spillways Numerous owners and dam safety officials were contacted to compile their experiences regarding labyrinth weirs, woody debris, and maintenance. The locations of the labyrinth spillways in this dataset are presented in Fig. 3. The surveyed labyrinth spillways are subdivided into structures designed by Schnabel Engineering, Freese and Nichols, and by others. Fig. 3 does not present all labyrinth spillways in the USA nor surveyed labyrinths located in Portugal.

Figure 3. Labyrinth weir locations in the Unites States Surveyed for this current study.

The current study research focused on the following points:

Occurrence of debris and type at labyrinth spillways How labyrinth spillways handle debris (self-cleaning, maintenance required, etc.) Any specific issues with blockages, log jams, etc. Typical maintenance performed For rehabilitation projects, changes in debris handling and related issues before

and after the labyrinth spillway construction Considerations regarding potential debris or sediment issues during design Any other issues or observations


Laboratory Investigations Extensive physical modeling was performed during the design of the Lake Brazos labyrinth spillway. Sectional and full-width models of the dam were tested at the Utah Water Research Laboratory (Utah State University) in Logan, Utah. The sectional models, which featured a variety of weir geometries (wall angle, wall width, cycle width, cycle depth, apex shape, and crest shape), were tested at a ~1:10 scale (Froude similitude). The effects of tailwater submergence on discharge capacity were also explored. The effects of sediment build up on labyrinth weir discharge characteristics were also evaluated; the results are discussed later in this paper. The full-width model (1:15 scale) was used to confirm hydraulic results from the sectional model and assist in evaluating the hydraulic requirements immediately upstream and downstream of the labyrinth weir. One use of the full-width model was to evaluate the ability of the labyrinth weir to pass anticipated sediment and debris. Debris was modeled with sagebrush and fabricated trees with dimensions and size distributions based on site-specific field observations of naturally occurring floating debris. In general, the branching and volume were conservatively selected to evaluate extreme conditions and impacts on discharge capacity. Because of downstream controls, the labyrinth weir operated under submerged conditions at the larger design flows. Physical modeling of three PKW geometries was conducted at the Laboratory of Hydraulic Constructions of the cole Polytechique Fdrale de Lausanne (EPFL) in Lausanne, Switzerland. The principal investigators included Dr. Michael Pfister, Prof. Dr. Anton Schleiss (Director), and Prof. Dr. Blake Tullis. The 1:30 scale PKW tested featured weir heights ranging from 3 to 6 inches. A photograph of one of the models is presented in Fig. 4. The range of tested headwater ratios (upstream total head H to weir height P) were 0.1 H /P 0.5.

Figure 4. Photo of one PKW geometry evaluated by Pfister et al. (2013).


Driftwood element dimensions and size distributions were selected based upon field data presented by Bezzola and Hegg (2007) to represent naturally occurring floating debris. 10 driftwood test groups, comprised of up to 22 elements, were prepared and released sequentially upstream of the spillway. The volume of wood and the impact of each driftwood group on H were documented, resulting in a data set of 120 points.

RESULTS Labyrinth Weir Debris Handling Performance The size and nature of the woody debris and the non-linear weir geometry (e.g., labyrinth or PKWs) influence the spillways potential as a debris collector. The debris handling performance of 75 labyrinth spillways is presented in Table 1 with key observations summarized as follows:

Labyrinth spillways are generally self-cleaning. Large tree logs or stumps may require removal. Labyrinth weirs that replaced existing spillways were observed to significantly

improve debris handling (5 structures). Field observations regarding debris loads or frequency of spillway operation are

incomplete. It can be difficult to observe the debris handling performance of these spillways due to location and accessibility, as well as the infrequency of events generating large debris loads.

In general, surveyed owners expressed satisfaction with the debris handling characteristics of their labyrinth weirs and very little maintenance has been required. Photographs of several examples of labyrinth spillways with typical debris conditions are presented in Fig. 5. According to the survey responses, infrequent removal of large woody debris elements was only required for less than 7% of the labyrinth spillways. Management of large woody debris elements at Dog River dam was reported to simply involve pushing the occasional stump or log off the crest into the downstream channel when it became an aesthetic nuisance. Tongue River and Middle Creek have debris booms upstream to avoid debris collection/removal at the structure. Only one labyrinth, Lake Brazos Dam (unique debris conditions), reported significant debris loading and accumulation under normal conditions. The dam owner reported, however, that the labyrinth weir debris collection and management along with other maintenance issues were less problematic that with the gated spillway structure it replaced. The debris at Lake Brazos Dam is discussed in the results section. Lake Townsend Dam and Linville Dam labyrinth spillways also replaced gated structures that required frequent maintenance to remove debris (Linville Dam shown in Fig. 6). Significant improvements have been reported, with required maintenance greatly reduced. When asked, the Water Supply Division Manager said the following about Lake Townsend, Its been a dream. We have noticed the design does not allow much debris to build up very little maintenance.


For some applications, where the labyrinth crest (auxiliary or emergency spillway) is located at an elevation above typical operating pool levels, a few owners expressed an eagerness for an occasional rainstorm to flush the debris that accumulates at the labyrinth spillway during base flow conditions. One owner, who maintains normal pool at the crest of the labyrinth with little to no discharge passing over, reported an occasional jumping fish and/or climbing turtle that climb/jump over the crest and land on the dry, downstream side of the labyrinth. The resulting fish smell did impact park aesthetics near the structure.

Table 1. Labyrinth Weir Debris Handling Performance Dam Location Cycles Spilling Frequency Debris Handling

Accord Pond Dam MA 2 No issues Agua Branca Portugal 2 No issues Alfaiates Portugal 1 No issues Alij Portugal 1 No issues Alloway Lake Dam NJ 5 No issues Antelope Creek Channel NE 3 No issues Arcoss Portugal 1 No issues Calde Portugal 1 No issues Capital City Country Club Dam GA 1 No issues Concourse Lake Dam VA 2 No issues Cloe Dam PA 7 No issues Crystal Bridges (2 Labyrinths) AR 2 Frequently No issues Dog River raised GA 8 Minor maintenance DRA Detention Structure GA 4 No issues Elmendorf Lake Dam TX 6 Frequently No issues Fort Miller Dam NY 45 No issues Glen Park Dam NY 25 Frequently No issues Greystone Development Dam GA 1 No issues Hard Labor Creek Dam GA 4 No issues Hollis G Lathem Reservoir Dam GA 3 No issues Huntington Hills Dam SC 3 No issues Indian Run Dam PA 2.5 No issues Kauffman Dam PA 5 No issues Kings Falls Dam NY 6 No issues

Lake Brazos Dam TX 24 Constantly Significant maintenance Lake Natalie Dam GA 4 No issues Lake Paupacken Dam PA 2 No issues Lake Sovereign Dam GA 2 No issues Lake Townsend Dam NC 7 No issues Lake Upchurch Dam NC 6 No issues Leaser Lake Dam PA 3 No issues Linville Land Harbor Dam NC 4 No issues Little Blue Run Dam PA 10 No issues Lyman Run Dam PA 8 No issues Meacham Grove Stormwater Facility IL 4 Frequently Minor maintenance Mercer Dam OR 3 Frequently No issues Middle Creek Dam MT 1 Yearly No issues New London Dam MN 4 Minor maintenance North Fork of Smith River Dam MT 2 Yearly No issues


Dam Location Cycles Spilling Frequency Debris Handling

Opossum Dam PA 4 No issues Pawnee Wetlands NE 3 No issues Pelican lake Dam MN Minor maintenance Pine Lake Dam GA 2 No issues Pine Lake Dam NY 3 No issues Pine Run Dam PA 4 No issues Piso Portugal 1 No issues Potato Lake Dam MN Minor maintenance Pye Lake Dam GA 3 No issues Quincy Dam CO 4 Seldom no debris Rapp Run Flood Control PA 6 No issues Roy F. Varner Reservoir Dam GA 8 No issues Ruby River MT 3 Yearly No issues Sam Rayburn Dam TX 16 Never n/a Santa Justa Portugal 2 No issues So Domingos Portugal 2 No issues Shawnee Dam PA No issues South River No. 29 GA 3 No issues South River No. 4 GA 2 No issues Sun Mountain Dam PA No issues Teja Portugal 1 No issues Tired Creek Dam GA 3 No issues Tongue River Dam MT 4 Yearly No issues Treesdale Impoundment No. 1 PA No issues Unicoi State Park Dam GA 3 No issues Upper Dam Rangeley ME 4 No issues Upper Norton Dam VA 3 No issues Upper Owl Creek Dam PA 3 No issues Valentine Mill Pond NE 2 Frequently No issues Wanahoo Dam NE 3 No issues Waterford Lake Dam NC 3 No issues White Heron Dam PA No issues Whitewater Creek Dam GA 12 No issues Yahoola Creek Dam GA 5 No issues Yellow River No. 3 GA 2 No issues Zorinsky Basin No 1 NE No issues


Figure 5. Examples of floating debris collected at labyrinth spillways (photos courtesy Schnabel Engineering).


(A)

(B)

Figure 6. Examples of typical debris collections conditions at one reservoir where the gated structure (A) was replaced with a labyrinth spillway (B)

(photos courtesy Schnabel Engineering). Sediment Impacts on Labyrinth Weirs Labyrinth weir discharge coefficients generally peak at a relatively low head and then decrease as H/P increases. Sediment buildup within the upstream cycles of a labyrinth weir would have the effect of reducing the effective wall height (P) and therefore increasing the H/P ratio, which could potentially decrease spillway discharge. Sedimentation was examined as part of the Lake Brazos Dam design and found to


decrease discharge as predicted by the discharge coefficient curves and the H/P changes. None of the surveyed labyrinth weirs reported sediment deposits in the upstream cycles, despite the fact that several of the reservoirs, including Elmendorf and Valentine Mill, reported sedimentation problems in other parts of the reservoir. As part of the Lake Brazos dam model study, Willmore (2004) conducted 4 model tests to examine the impacts uniform sediment deposition upstream of a labyrinth weir would have on H for a given discharge. The labyrinth weir geometric properties are summarized in Table 2. Sediment deposits were simulated with a horizontal false floor placed on the upstream of the weir (Fig. 7). The false floor elevation was adjusted to reduce the weir height by 20%, 40% and 50%.

Table 2. Sediment effects - labyrinth weir sectional model geometric information Parameter Value

8 P 1.0 ft

Lc-cycle 11.5 ft N 2.0 tw 1.5 in

Crest Shape Partial Ogee

Figure 7. Sediment simulation test with wall height reduced by 50%


Head-discharge rating curves were developed for each model for 0.1 H/P 0.7. Total upstream head values for model tests that included the simulated sediment deposits are identified as Hsed. In Fig. 8, Ps is the reduced weir height due to sediment deposition. The Hsed to H ratio is presented in Fig. 8; sediment deposits that reduce the weir height by 20% or less were found to have a minor impact on the discharge rating. However, for sediment deposits that reduced the weir height by 40% and 50%, the increase in H, and hence upstream reservoir pool elevation, was more than 10% and 25% for about 0.1 H/P 0.6. It should be noted that the applicability of these results are limited as the simulated sediment deposits were not actual, erodible sediments. For higher H/P ratios, the scour and removal of actual sediments at a labyrinth spillway might be expected if the sediment carrying capacity exceeds sediment inflow. Tests conducted in the laboratory using fine sand suggest the ability of the labyrinth to be self-cleaning; velocity profiling in the laboratory also found that flow velocities in the upstream cycles are relatively uniform, which suggests that the self-cleaning nature of the upstream cycles should also be relatively uniform. Bathymetric surveys of Lake Brazos Dam confirmed the structures sediment self-cleaning ability.

0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

0 2 4 6 8 10 12 14 16 18 20 22 24

Hse

d/ H

Q (cfs)

Ps/P=1.0

Ps/P=0.8

Ps/P=0.6

Ps/P=0.5

Figure 8. Influence of sediment deposits on 8 labyrinth weir discharge rating curve. Lake Brazos Case Study The Lake Brazos Dam was originally constructed as a gated spillway by the City of Waco in 1970. It is a run of the river dam and creates a recreational lake. The original drum gates could not maintain a constant reservoir level due to corrosion and debris issues. The drum gate system was replaced with leaf gates and hydraulic cylinders in 1985, but operational problems associated with debris and the gate operating mechanism persisted and often led to lowered lake levels and unappealing reservoir conditions. The continual problems with the gates led the City to replace the gates with a labyrinth weir.


One of the principle project challenges was to complete the spillway replacement without significantly affecting the upstream lake levels. The challenge led to a significant physical modeling effort that resulted in a highly customized design (Vasquez, 2006). Sediment and debris management represented two significant operation and maintenance design concerns. Since the dam is run-of-river, sediment can reach the dam rather than being dropped out when the river enters the lake. The design team did not know if the labyrinth weir would trap more sediment than the previous gated structure. In addition, the 46 river miles of the Brazos River, upstream of the dam site, carries a large debris load consisting of trees with root balls. Much of the debris was passed through the gated spillway although some of it would get stuck on the adjacent concrete-overlaid auxiliary embankment spillway (Fig. 9). The debris that passed over the gates often damaged the hydraulic cylinders located beneath the gates, which would require a lowering of the reservoir for repairs. As with the gated spillway, sediment and debris also had the potential to cause significant operations and maintenance challenges for the labyrinth weir. Therefore, sediment and debris impacts to the labyrinth were investigated as part of the physical model study.

Figure 9. Typical Debris on original Lake Brazos Dam

Photos Courtesy City of Waco


The results of the sedimentation modeling was presented previously. The impact of debris was modeled using two types of debris. The first was with model trees fabricated to replicate the type of tree debris seen at the site. The second method to model debris was with sagebrush (Fig. 10). The sagebrush increased the lake level by 3-inches for the 1-year storm event and by 1-foot for the 10-year event (42,000 cfs). By 58,000 cfs, the debris washed over the labyrinth. The fabricated trees had approximately the same impact on the lake level as the sagebrush and were washed over at approximately 35,000 cfs, a little less than the 5-year event (see Fig. 10).

Figure 10. Debris tests on the full-width model After completion of the Lake Brazos dam labyrinth weir project in 2007, the City has monitored the dam for debris and sedimentation. The spillway passes an average daily flow of 1,000 cfs and debris collection at the dam is common (Fig. 11). The largest flood to date since the completion of the labyrinth weir is 30,000 cfs (< 5-year event), which was not large enough to cause the debris to pass over the weir. The City has removed debris seven times (~ once per year on average) since the end of construction, resulting in a total expenditure of ~$300,000 (current cost is about $25k per cleanup). Since 2012, the City has retained a contractor for on-call services to assist with the debris removal on an as-needed basis. Prior to 2012, the removal was performed by the City and contracting or schedule difficulties resulted in the debris accumulating upstream of the dam for several months before it was removed.

Photos Courtesy Freese and Nichols, Inc.


The debris may have increased the river level slightly during storm events. There is a USGS flow gage downstream of the dam but not a reliable upstream lake level gage. The City has a lake level gage at the dam but operation is inconsistent and the accuracy is unknown. Data from this lake level gage is shown in Fig. 12 along with the design rating curves.

Figure 11. Debris on Lake Brazos Labyrinth Weir


Figure 12. Lake Brazos Dam Rating Curve with Historical Performance A bathymetric survey of Lake Brazos was performed in 2003 during the labyrinth weir design phase. Surveys were also performed in 2007 after the labyrinth was completed and five years later in 2013. The City removed floating debris prior to the 2013 survey to facilitate surveyor access. The most recent survey found significant debris accumulation on the river bottom upstream of the dam up to nine feet thick. Fig. 13 depicts the difference in elevation (or the thickness of the sediment/debris) from 2007 to 2013. As shown, the cycles were relatively free of sediment and the majority of accumulation is immediately upstream of the cycles along the original river channel. There does not appear to be accumulation inside the cycles; therefore, it is assumed the accumulation consists of woody debris, which floated to the dam and subsequently became waterlogged. The debris accumulation extends approximately 150 feet upstream of the dam.


Figure 13. Accumulated sediment/debris channel bottom layer thickness in Lake Brazos

Debris Blockage Sensitivity of Piano Key Weirs As previously discussed, a PKW is a modified labyrinth weir with narrow rectangular cycles or keys. The narrow design of the PKWs inlet and outlet keys produce an increased potential for debris collection (e.g., the weir functions similarly to a trash rack for longer woody debris). If the catchment is forested, the potential effects of driftwood accumulation on the PKW head-discharge relation should be considered as part of the hydraulic design (Leite Ribeiro et al. 2012a, b). Pfister et al. (2013) conducted systematic model testing of the effect of debris (i.e., wooden trunks and rootstock) on the PKW discharge efficiency for three different laboratory-scaled PKW geometries (see Fig. 14 and 15). The first test series determined the blocking probability of individual debris elements from seven different size classes of driftwood trunks; at a 1:30 model scale, the trunk diameters ranged from ~2.5 to 26 inches prototype scale. These tests found that the trunk diameter and upstream head represent the primary dimensional parameters influencing floating debris blockage probability. The second test series more closely approximated real-world conditions by investigating the nature of individual driftwood element accumulation on the PKWs and the effect on discharge efficiency. For each individual debris batch, the number and sizes of the passing and trapped elements were noted along with the corresponding H. Without removing the debris trapped on the PKW from previous batches, subsequent debris batches were introduced upstream of the flume; this was repeated for all 10 debris batches. The effects of debris accumulation on the upstream head varied with the value of the no-debris upstream head reference conditions. At lower upstream head values, the cumulative debris tests produced increases in the debris-associated upstream head of up to 67%; higher upstream reference head values produced upstream head increases limited to approximately 16%.


Fig. 14 shows photographic examples of the debris accumulation tests for one of the PKW geometries tested following the addition of the 10 driftwood batches.

(a) (b)

Fig. 14 Model debris including (a) trunks, and (b) rootstocks

Figure. 15. Example of a blockage configuration following the addition of 10 debris batches.

It was found that despite the blockages, flow continues to pass under the floating debris and through the inlet cycles, with a maximum reduction in Cd of about 25% as a suggested rule of thumb or rough estimate for typical situations. Pfister et al. (2013)


proposed Eq. 1 to estimate the new discharge coefficient impacted by woody debris, with a selection of results presented in Table 3.

23

=

HH

CC debris

d

debrisd (1)

Table 3. Driftwood impact ratios results sample

Hdebris / H Cd-debris / Cd 1.1 0.87 1.2 (typical cond.) 0.76 1.7 (meas. maximum) 0.45

In equation 1, Cd and H are the discharge coefficient and total upstream head, respectively, of a PKW without woody debris. Cd-debris and Hdebris are the discharge coefficient and total upstream head of a PKW with debris. For more detailed results, please see Pfister et al. (2013).

CONCLUSIONS The increased weir length provided by a labyrinth weir facilitates an increase in discharge efficiency and lower upstream head for a given discharge relative to linear weirs of equal spillway width. Consequently, concern existed that this type of weir may have a higher potential as a debris collector. However, 74 of 75 surveyed dam owners/operators of labyrinth weirs in the USA and Portugal reported no significant debris accumulation or cleaning effort. Catchments that contribute significant amounts of large debris may not allow passage over the spillway; therefore, some maintenance is required. The results of a laboratory study on three PKW geometries (a type of labyrinth weir) in a reservoir scenario indicate a 16% increase in upstream head for higher flows, and a 67% upstream head increase for lower flows. Similar trends were also reported in a physical model study for a large labyrinth weir. Consequently, the potential impacts of debris on the discharge capacity of a labyrinth spillway during a large storm event may be less than expected. However, the design and maintenance of labyrinth spillways must certainly consider the effects of debris for situations where a heavy debris load is anticipated.

ACKNOWLEDGEMENTS This study was supported by Freese and Nichols, Utah State University, and Schnabel Engineering. The authors are grateful to the many individuals who provided information regarding labyrinth spillways in the USA and abroad, with a special thanks to Prof. Jorge Matos (Universidade de Lisboa, Portugal) and Antnio Pereira da Siva (Consultores de Enganharia e Ambiente).


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