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INTERNATIONALWATER POWER & DAM CONSTRUCTION June 2013

Overtopping spillwaysmake a comeback

Once shunned by the dam safety community, spillway overtopping dams are becoming morecommonplace as a means to address existing infrastructure rehabilitation. Greg W Huddock fromGolder Associates demonstrates how this has successfully provided additional hydraulic capacity inthe US state of Georgia.

D am owners, engineers, and regulatorshave struggled with the use of spillwaysovertopping dam embankments duringrecent decades. Typical dam safety practice is toavoid spillways overtopping earthenembankments because the structures cover thesurface of the embankment and can mask thevisual indications of dam safety issues such asseepage, slope instability, internal erosion andanimal burrows.

The consequences of poor design for anovertopping spillway chute can be disastrous,

since chute malfunction can cause failure of thedam. Nevertheless, overtopping spillways can bea very viable solution for existing dams whereadditional hydraulic spillway capacity is needed,and further area on the abutments is not available.

Spillway history in GeorgiaThe State of Georgia, located in the southeasternUS, has been actively utilising overtoppingspillways as a means to bring low hazardstructures in compliance with Georgias rules fordam safety related to hazard creep.

Hazard creep occurs when developmentdownstream in the dam breach inundation zoneof an existing low hazard dam structure results inthe dam becoming high hazard, because there isnow a high probability of loss of life. In Georgia,low hazard dams are not regulated and do notneed to comply with a particular design storm,but high hazard dams are regulated and the damsafety rules require a specific fraction of theprobable maximum precipitation (PMP) as thedesign storm. The specific fraction of the PMPrequired is based on the size of the structure and

Aerial view of the Yellow River Dam No. 14 located in GwinnettCounty which is a perfect example of downstream hazard creep(as evidence by the subdivisions in the downstream valley).

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varies from one-quarter PMP for small dams to thefull PMP for very large dams.

For dams in an urban environment,overtopping spillways may be the only solution toremedy deficient spillway hydraulic capacity dueto hazard creep. The Metropolitan Atlanta areacurrently ranks 9th in population among US cities,with an estimated population of 5.4M. In 1960,Atlanta wasnt even ranked in the top 20 mostpopulous cities in the US. In the last 50 years,Atlantas population has doubled two and a halftimes, leading to a total growth of nearly 350%.

Rural areas that were once agricultural arenow sprawling with homes, roads, schools,businesses, parks and other heavily useddeveloped areas. Dams that were built foragricultural purposes or flood control are nowintegral focal points of communities, providingrecreation, water supply, flood control, waterquality enhancement and biological habitatbenefits. There are literally thousands of damsacross Metropolitan Atlanta that are subject tohazard creep and will require rehabilitation tocomply with the state rules, as the result ofinsufficient spillway capacity.

In Georgia, the following tenets are consideredby dam engineers and owners which governspillway design projects:

Riparian water rights.Floodplain ordinances disallowing additionaldownstream discharge during the 100-yearstorm.Legal case law requiring dam owners tomaintain existing lake levels.Limits on dam raises because of impingingdevelopment around the lake.

Overtopping spillways are ideal because thespillway solution is easily customised to addressdownstream flow requirements.

Overtopping spillways are constructed of non-erodible materials and are predominatelyreinforced concrete, roller-compacted concrete(RCC), or articulated concrete blocks and mats. Inonly a few instances would non-concrete materialslike riprap, gabions, or vegetated channels be used

for an overtopping spillway. A spillway located onthe dam is often more cost-efficient than abutmentspillways since the dam location is a more directlayout for discharge to the downstream andutilises less construction materials. Depending onthe design layout, stakeholder concerns likeaesthetics and safety are even readily addressed.

For most dam types, overtopping spillways areonly recommended at existing dam sites where amajority of the consolidation settlement of the soilmaterials has occurred. Furthermore, the risk ofovertopping spillway use is minimised if thestructures are not service spillways. Rather, theywould be emergency spillways activated duringstorm events with a low probability of occurrence,such that the spillway and dam can be inspectedregularly for visual indications of dam safetyconcerns.

A majority of the overtopping spillway projectsaccomplished in Georgia were completed in thelast decade a Natural Resources ConservationService (NRCS) watershed structures. These areflood control dams constructed mostly in north

Georgia from the 1950s to the 1990s, including inand around Metropolitan Atlanta.

These NRCS dams were originally exemptfrom the Georgia Rules for Dam Safety, but in2000, the exemption expired. There are 357 NRCSdams in Georgia and, in 1999, with the exemptionexpiration looming, the NRCS Georgia, togetherwith the State Soil andWater ConservationCommission, piloted a rehabilitation project fortwo dams, Potato Creek Dam No. 6 and PinelogTributary Dam No. 86.

RCC chute spillways were selected for bothstructures as the most cost-effective solution toprovide the spillway capacity needed to route thefull probable maximum flood (PMF) through thestructure and still maintain the project floodcontrol benefits. Golder Associates designed theRCC spillways for both of these structures thatconsisted of the following:

Potato Creek Dam No. 6 100m wide steppedRCC straight chute spillway with a reinforcedconcrete ogee weir control section.Pinelog Tributary Dam No. 86 152m widestepped RCC straight chute spillway with abroadcrested weir control section.Potato Creek Dam No. 6 was constructed in2003 for approximately US$1.1M by DPSIndustries.Pinelog Tributary Dam No. 86 was neverconstructed due to a lack of funding.

Beginning in 2000, Gwinnett County, Georgiafunded a capital improvement project to upgrade14 NRCS watershed dams to comply with theGeorgia Rules for Dam Safety. Similar to thePotato Creek and Pinelog Tributary projects, theGwinnett structures were thoroughly studied todetermine the most cost-effective rehabilitationsolution to route runoff from the required designstorms.

Six of the Gwinnett County NRCS damswarranted an overtopping spillway to provide theadditional spillway capacity needed:

Completed RCC spillway overtopping thedam embankment at the Potato CreekDam No. 6 project located nearThomaston, Georgia.

Big Haynes Dam No. 3 hydraulic model study of apreliminary design concept for a converging RCC

spillway. Note the spillway flows leaving the chutetraining walls downstream of the ogee weir.

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Yellow River Dam No. 14 (Y-14) GolderAssociates designed a 68.5m wide steppedRCC spillway with an ogee weir that routes thefull PMF through the structure. ThalleConstruction Company completed constructionof Y-14 in 2004 for a cost of US$1.33M.Yellow River Dam No. 17 (Y-17) The US ArmyCorps of Engineers designed a 158.5m widestepped RCC spillway with an ogee weir thatroutes the full PMF through the structure. ASIRCC completed construction of Y-17 in 2005 fora cost of US$1.5M.Yellow River Dam No. 15 (Y-15) GolderAssociates designed a 146.3m wide steppedRCC spillway with a short-crested weir thatroutes the full PMF through the structure. ASIRCC completed construction of Y-15 in 2008 fora cost of US$4.3M.Yellow River Dam No. 16 (Y-16) SchnabelEngineering designed a 33.5m wide stepped

RCC spillway with a sharp-crested weir whichroutes 50% of the PMF through the structure.ASI RCC completed construction of Y-16 in2008 for a cost of US$1.8M.Big Haynes Dam No. 3 (H-3) GolderAssociates designed a 38m wide stepped RCCspillway with a short-crested weir that routes50% of the PMF through the structure. ASIConstructors completed construction of H-3 in2011 for a cost of US$1.7M.

How aesthetics and design come into playSecondary to dam safety concerns, aesthetics hasbeen a major design constraint. Since thesestructures are in a populous suburban areasurrounded by homes and green space, GwinnettCounty opted to cover the Y-17, Y-15, Y-16, and H-3 structures with soil and grass. The combinationof the RCC design, which does not have verticaltraining walls, and the covered spillway chuteswas strategic to successful stakeholdernegotiations for the projects.

Model studies completed by the USDepartment of Agriculture (USDA) AgriculturalResearch Service for the H-3 project, and the USBureau of Reclamation (USBR) for the Y-15 project,

helped to establish design criteria for steppedspillway projects that converge flows from a widespillway crest to a narrow stilling basin because oftopographic constraints.

In general, the H-3 model identified concernswith typical RCC spillway design configurations,but the Y-15 model proved that the spillwaygeometry could be designed to mitigate theseissues. Design constraints that were identified tobe addressed as part of the design include theseflow patterns::

Flows converging in an RCC overtoppingspillway have a tendency to run out of thechute at the training walls and erode theinterface of the RCC and soil embankment.Flow bulking occurs along the convergingsidewalls but the formation of supercriticalwaves was not observed.If converging flows intersect downstream ofthe stilling basin, significant scour may occurin the stream bed.Converging flows that intersect within thespillway have a tendency to form high-velocityjets that may cause scour in the stream bedwhere the flow concentrations exit the stillingbasin.

Beginning with the Potato Creek Dam No. 6through to the Big Haynes Dam No. 3 projects,additional design features were added to eachRCC spillway. These were based on lessonslearned from the previous projects, ongoingresearch on RCC spillway performance, and a

Placement of RCC at the Big Haynes Dam No. 3 RCC spillway located near Snellville, Georgia. The RCC steps are 0.3m high.

Yellow River Dam No. 15 RCC spillwaylocated near Lawrenceville, Georgiaengaged during the September 21, 2009flood event. The spillway overtopped by0.84m and flowed for 36 hours.

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desire to improve the visual appearance of theRCC finished product. As a result, the cost of RCCincreased from US$126/m3 at the Potato CreekProject to US$188/m3 at the Y-15 project. SinceRCC is placed with earthfill moving equipment,the construction methods used to build thestepped spillway typically require approximatelythree times as much RCC as reinforced concrete.As the price increases, it becomes lessadvantageous to use RCC in lieu of traditionalreinforced concrete. Beginning in 2010, the effectsof the US recession significantly reduced the priceof concrete construction. Spillway projects thatwere planned to be RCC were re-evaluated anddesigned as reinforced concrete chutes.

Building on the success of the NRCS and theGwinnett County dam rehabilitation projects,Georgia began upgrading several NRCS watersheddams in 2010. The state projects were facilitatedthrough funding by the American Recovery andReinvestment Act of 2009 (ARRA, also known asthe stimulus bill). State revenue bonds were soldto secure the 35% cost share that is required forNRCS dam rehabilitation projects, and four NRCSdams in Georgia were rehabilitated.

Two of these projects involved construction ofspillways overtopping the embankment. Theseprojects were planned as RCC chutes, butbecause of the favorable reinforced concretecosts, traditional reinforced concrete chutes wereselected for prices analogous to the RCC options.

Sandy Creek Dam No. 15 (SC-15) GolderAssociates designed a 55m wide reinforcedconcrete spillway with a labyrinth weir thatroutes the full PMF through the structure.Brasfield and Gorrie General Contractorscompleted construction of SC-15 in 2011 for acost of US$2.7M.South River Dam No. 4 (SR-4) SchnabelEngineering designed a 32m wide reinforcedconcrete spillway with a labyrinth weir thatroutes the full PMF through the structure.Sunbelt Structures completed construction ofSR-4 in 2011 for a cost of US$1.9M.

When flooding hitsMetropolitan Atlanta experienced a significantflood event on 21 September 2009, which wascaused by a stalled low pressure system pullingmoisture from the Gulf of Mexico. Many areasacross the region experienced 24-hourprecipitation amounts in excess of the 100-yeardesign storm. Areas to the west of Atlantaexperienced extreme rainfall amounts that wereapproximately 50% of the Probable MaximumPrecipitation.

The Yellow RiverWatershed in Gwinnett Countyreceived approximately 203mm of rainfall in a 24-hour period and nearly 381mm of rainfall in a week.The Y-14, -15, -16 and -17 RCC spillways engagedfor approximately 36 hours, with a maximumovertopping of 0.84m.With the exception ofdamage to the soil covers over the RCC, the YellowRiver spillways performed exceptionally well andattenuated downstream flooding for close to a weekafter the flood event. No subsequent repairs wererequired for these structures.

Hydrologic studies completed by GolderAssociates after this flood event indicate that the

Y-15 and Y-17 structures would have failed if thestructures had not been recently rehabilitated toprovide additional hydraulic spillway capacity.

More commonplaceWhile once shunned by the dam safetycommunity, spillways overtopping dams arebecoming more commonplace as a means toaddress existing infrastructure rehabilitation. Theuse of these in Georgia to provide additionalhydraulic capacity has been successfullydemonstrated. The state, Gwinnett County, andother municipalities in Georgia continue to pursueovertopping spillway projects for damrehabilitation as a means to increase hydraulicspillway capacity in developed areas across thestate.

Currently, North Georgia Concrete isconstructing a 23.8m wide reinforced concretechute with a labyrinth weir design by SchnabelEngineering at the South River Dam No. 29 for acost of US$1.9M. Additional projects are alsocurrently under design throughout the state thatwill be overtopping spillways.

First and foremost, understanding the historicperformance and future use of a structure isessential. Exhausting all other design options withstakeholders is key before pursuing theovertopping spillway option. Owners shouldengage experienced dam hydraulic engineers andgeotechnical engineers to complete the designand retain experienced contractors to constructthe project. As a team, the owner, engineer, andcontractor should work together to enhance thesafety of the dam structure through the use ofovertopping spillways that can extend the life andbenefits of the structure for many years.

Author Information

Gregg W. Hudock is a Senior Consultant& Associate at Golder Associates Inc.Email: ghudock@golder.com

Labyrinth weir with a concrete chuteovertopping the Sandy Creek Dam No. 15 damnear Commerce, Georgia. The chute spillway

is 55m wide and was completed in 2011.

Yellow River Dam No. 15 overtopping spillway after the 21 September2009 flood event. The soil cover eroded in concentrated flow areas butno damage to the RCC or embankment was noted.