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A Morphological and Economic Examination of Plunge Pools As Energy Dissipaters in Urban Stream Channels1

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A Morphological and Economic Examination of Plunge Pools As Energy Dissipaters in Urban Stream Channels1 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION FEBRUARY AMERICAN WATER RESOURCES ASSOCIATION 2005 A MORPHOLOGICAL AND ECONOMIC EXAMINATION OF PLUNGE POOLS AS ENERGY DISSIPATERS IN URBAN STREAM CHANNELS1 Craig J. Allan and Christopher J. Estes2 ABSTRACT: Naturally formed plunge pools (scour holes) are a com- reinitiating a larger cycle of instability in an other- mon morphologic feature in many urban stream systems where the wise stable “built-out” watershed. Increasing urban- transition between a pipe and a natural channel occurs. Plunge ization upstream of many culvert crossings has pools serve as significant stream energy dissipaters, increasing flow resistance and enhancing stream channel stability. Such features resulted in the erosion of the original armoring riprap may also improve habitat diversity and serve as refugia for stream apron, as erosive flows have become more frequent biota during low flow periods. The morphologic characteristics of and intense. The long term maintenance of degrading several naturally formed plunge pools associated with road crossing riprap aprons at culvert crossings is expensive, and culvert outlets in the metropolitan Charlotte, North Carolina, area the resultant crossing instability often impacts water are presented. Plunge pool dimensions surveyed include maximum depth, length, and width, and longitudinal and side slopes as well quality and erodes streambanks on public and private as bed material. Culvert outlet dimensions and hydraulic charac- property. Culvert outlets that have not been modified teristics of the scouring jet for each study site are also reported. in more than 10 years and have become adapted to Design equations developed from flume studies generally failed to the urban watershed display geomorphic characteris- predict the naturally formed plunge pool dimensions. Pool volume tics of stability. Historic surveys at culvert outlets was significantly correlated with drainage area, with pool depth being the least sensitive dimension to changes in the magnitude of (Keeley, 1963; A.C. Scheer, 1968, unpublished report, the scouring flow. The excavation costs for designed plunge pools University of Montana, Missoula, Montana) identified compare favorably to initial construction costs of traditional culvert both the presence of gully scour that migrated outlet riprap aprons. upstream or localized scour holes below road culvert (KEY TERMS: surface water; hydrodynamics; plunge pool; scour; crossings. Gully scour was usually associated with culvert; urban stream morphology; geomorphology.) unstable downstream channel slopes, while localized Allan, Craig J. and Christopher J. Estes, 2005. A Morphological and Economic scour was associated with more stable downstream Examination of Plunge Pools as Energy Dissipaters in Urban Stream Channels. channel slopes. The development of an apparently Journal of the American Water Resources Association (JAWRA) 41(1):123-133. stable plunge pool. is a recurring theme at culvert outlets underlying urban road crossings where the periodicity of the maintenance of downstream riprap or other channel armoring is infrequent. Often when INTRODUCTION such pools are simply filled in with more riprap, a common maintenance practice, significant localized In 1997 the City of Charlotte began investigating bank erosion and channel down cutting has resulted. the implications of its current maintenance practices The prediction of scour geometry at culvert and for urban channels at road culvert crossings. The cantilever pipe outlets is a regular component of cul- investigations were initiated after it became apparent vert or dam design methodology in determining the that certain aspects of these practices could destabi- need for erosion protection. Federal Highway Admin- lize downstream channels and accelerate erosion, istration procedures (FHWA, 1983) present outlet and 1Paper No. 03065 of the Journal of the American Water Resources Association (JAWRA) (Copyright © 2005). Discussions are open until August 1, 2005. 2Respectively, Associate Professor, Department of Geography and Earth Sciences, University of North Carolina-Charlotte, 9201 University City Blvd., Charlotte, North Carolina 28223-0002; and President, Estes Design Inc., 515 Pembroke Drive, Wakhaw, North Carolina 28173 (for- merly City of Charlotte Stormwater Services, Charlotte, North Carolina) (E-Mail/Allan: [email protected]). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 123 JAWRA ALLAN AND ESTES scour geometry relationships for scour depth, width, a developing urban environment, thereby reducing length, and volume as a function of discharge intensi- long term maintenance costs. Also plunge pools are ty (Q/D2.5) where Q is the discharge in cfs and D is preferable to riprap channel linings in natural chan- the pipe diameter in feet. Similar relationships were nel design stream restoration projects, providing proposed earlier by researchers from the U.S. Army stream habitat diversity and in some instances Corp of Engineers (USACE) (Bohan, 1970; Fletcher stream aeration. and Grace, 1972). Modified relationships to account Here the geometry of several plunge pools that for variations in culvert shape are presented by Abt have formed naturally below road culverts as a result et al. (1987). U.S. Department of Agriculture (USDA) of riprap channel lining failure in the Charlotte researchers have developed design criteria for pipe metropolitan area is characterized. Natural plunge spillway outlets suitable for farm pond outlets or flood pool dimensions are compared to the culvert scour control reservoirs. geometry predicted by FHWA, USACE, and the Initially, plunge pool dimensions were determined USDA design equations. The costs associated with for free flowing cantilevered pipe outlets (USDA, plunge pool excavation are compared to those for ini- 1986; Blaisdell and Anderson, 1991). These design cri- tial construction of traditional riprap outlet aprons teria were later modified for fully submerged (Rice currently required under present design standards. and Kadavy, 1994) and partially submerged pipe out- lets (Rice and Kadavy, 1995). Several limitations exist in the research carried out to date. In all instances, model flume studies were used to predict scour geom- METHODOLOGY etry with little or no field measurements. Such stud- ies cannot account for the influence of vegetation and A field survey of several naturally formed plunge streambed and bank material heterogeneity on scour pools was conducted in the Charlotte metropolitan hole geometry. The influence of multiple culvert open- area. The pools were surveyed with a stadia rod and ings on scour geometry, a relatively common feature theodolite or by chaining the dimensions of each pool. in urban settings, has not been investigated. Most of Plunge pool survey dimensions were collected by wad- the above investigations primarily examined channel ing the plunge pool or from a small boat in larger scour in noncohesive sediments; only the FHWA plunge pools. Culvert dimensions and outlet configu- investigators analyzed scour in cohesive sediments rations were also recorded at this time. Streamside (Ruff et al., 1982); and only Blaisdell and Anderson gauge boards were installed at the distal end of most (1991) considered the effect of impeding layers on plunge pools and read manually during high flow con- scour geometry. Perhaps most importantly, the scour ditions to establish the relationship between tailwater geometries generated by these different methodolo- and culvert elevations. Sediment samples were col- gies have not been compared with each other or with lected from the streambed and banks and where pre- examples of localized channel scour in the field. sent from the outlet bar at the downstream end of Armored culvert outlet aprons designed to protect each plunge pool. Samples were analyzed at the Uni- the road crossing and prevent downstream erosion versity of North Carolina (UNC)-Charlotte sedimen- may consist of costly, elaborate concrete structures or tology laboratory for grain size distribution. riprap channel lining sized to the culvert opening and Sediment samples were first dried at 105˚C for 24 design discharges. Failure of armored culvert outlets hours prior to analysis. The bulk sediment sample can be attributed to any or all of these conditions: was then weighed and sieved through progressively incorrect installation, lack of maintenance, and dis- smaller screens at 0.5φ intervals until only the < 4φ charges that exceed initial design specifications. Ulti- fraction remained. The < 4φ fraction was then ana- mate scour dimensions will be determined by the lyzed with a Spectrex Laser Particle Counter, and erosive power of the stream, channel materials, those results were combined with sieve results to streambank vegetation, and pipe outlet or tailwater determine the complete grain size distribution for geometry. The use of preformed plunge pools below each sample. culvert openings as an energy dissipation structure An extensive literature review was conducted to may represent a viable alternative to riprap channel explore the use of plunge pools as energy dissipaters linings or other structures in settings where the in fluvial systems and to investigate plunge pool downstream channel slope is stable. Plunge pools are design criteria for different hydraulic settings. In a commonly used energy dissipation structure for addition, storm water and engineering utilities of dams of all sizes (Hager, 1998), and riprap lined out- major
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