Stilling Basins

CIVE 401 Hydraulic Engineering Term Paper 11/18/2014

Sam Plaza Alex Potvin

Ahmed Sahab

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Introduction

Stilling Basins are used to dissipate energy from high velocity water to minimize the scouring effects which can occur downstream of the flow. This can be due to water falling from the upper portion of a dam or in the vicinity of a hydraulic jump. Local Scouring occurs immediately downstream of stilling basins and is unavoidable because local water velocity exceeds the incipient motion velocity of the sediment. This leads to a difference between the level of the concrete floor of the basin and the riverbed. In such cases, different basin designs can be used to protect against the development of a scouring hole that could destabilize the structure and cause widespread damage to the region.

Stilling basins are designed with special appurtenances that help to direct flow and maximize the efficiency of the structure. Depending on the properties of the incoming flow, chute blocks, sills, and baffle piers can be designed trigger a hydraulic jump for the required tailwater condition. Chute Blocks are used to form a serrated device at the entrance of the stilling basin. They function to furrow the incoming jet, thus lifting a portion from the floor of the spillway to create a shorter length of hydraulic jump. The sill is located at the end of the basin and acts to reduce the length of the hydraulic jump and minimize scour. Baffle Piers are blocks placed in the midway across the floor of the basin and function to dissipate energy through the impact action from the flow. They are useful in small structures with low incoming velocities but are unsuitable where high velocity flows could induce cavitation. For a properly designed stilling basin, the velocity leaving the basin is equal to the velocity of the inlet channel. A schematic for a stilling basin has been shown in Figure 1.

Figure 1: Stilling Basin design for USBR type 3 (USDOT FHA)

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The higher the Froude number at the entrance to a basin, the more efficient the hydraulic jump thus requiring a shorter necessary basin. To increase the Froude number as the water travels from the culvert to the basin, an expansion and depression section can be implemented. This geometric modification converts the depth or potential energy into kinetic energy by allowing the flow to expand or drop. The result of these changes is that the depth decreases and the velocity and Froude number increases as shown in Figure 2. Depending on the specific design, stilling basins can operate over a wide range of approach flow Froude

numbers typically from 1.7 to 17. The selection of a stilling basin is dependent on several considerations including hydraulic limitations, feasibility of construction, basin size, and cost. Stilling Basin Classifications

Stilling Basins are divided into two types: basins with Froude Number greater than 4.5 and basins with Froude number less than 4.5. The stilling basins with the lower Froude numbers are usually encountered with weirs and barrages, while those with higher Froude number are used in medium to high dams. The higher the Froude number is, the shorter the basin is, and the more economical design it is. Several typical designs are set for each category.

For Froude number < 4.5, there are four type of design can be simply listed as:

i. R.S. Varshney ii. Indian Standard Stilling Basin iii. U.S.B.R. Stilling Basin IV iv. S.A.F. (Saint Anthony Falls) Stilling

Basin.

Figure 1: Cross section schematic of a USBR stilling basin (USDOT FHA)

What is the Froude Number? The Froude Number describes the nature of the flow and is equal to the ratio of the flow velocity to the square root of the acceleration of gravity times the flow

depth.

Open channel flow can be described as either a subcritical flow, , or supercritical flow, . For subcritical flow, the fluid has a greater depth and lower velocity whereas a supercritical flow is characterized by the fact that it “hugs” the channel bed with a high velocity. The boundary between these two flow types is defined by, , and is where a hydraulic jump will occur.

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The U.S.B.R. stilling basin IV is recommended for Froude number ranges from 2-4.5, which usually occurs in canal structures and diversion dams. For this low range of Froude numbers, an oscillating jump will be produced generating a wave that is hard to dampen. Basin IV solves this

problem by narrowing D1 (preferably 0.75*D1) and by setting the tailwater (Tw) equal to (1.05-1.1)*D2 (see Figure 3). This design seems to be the perfect solution; however, it is only applicable for rectangular cross section. For Froude number > 4.5, there are three main types of design, which are:

i. S.A.F. Stilling Basin. ii. Indian Standard Stilling

Basin II. iii. U.S.B.R Stilling Basin II.

These designs are recommended for high Froude numbers. They are often used in large and medium spillways and large canal structures. These types of stilling basins are very economical, as the

length of the basin is reduced by 33% with the use of appurtenances (including chute blocks and/or a dentated sill.) However, baffle piers are not used because high velocities might cause cavitation that damages the piers. Based on United State Department of Interior Bureau of Reclamation, Basin VI has certain standards to be followed. The basin’s dimensions have been standardized based on the basin width. The limits are shown in Figure 4. The basin size is determined by the width

Figure 2: Proportions of Froude numbers 2.5 to 4.5 (Basin IV) (Beichley)

Figure 3: General design of the type VI stilling basin (Beichley)

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of the basin and the quantity and velocity of flow. Figure 5 below will explain the limit of the basin size. “C” is the coefficient that varies for maximum, minimum, and intermediate flow.

Figure 4: Flow Capacity and Velocity for Various C Values

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Case Study I – Deadman’s Basin Failure, October 2010, Shawmut, Montana

Deadman’s Basin, located near Shawmut, Montana, diverts water from the nearby Mud Creek for the purposes of irrigation. The basin supplies 110 irrigators and also provides a popular

fishing site (Montana Fish, Wildlife & Parks). Deadman’s Basin has a total capacity of 72,220 acre-feet, and operates at an average capacity of 40,500 acre-feet (Deadman’s Basin Water Association). The basin was originally connected to an 8’ x 8’ box culvert conduit (with a capacity of 970 cfs) and to an outlet canal system (with a capacity of 560 cfs); the normal operating flow was between 200 and 300 cfs (Grabinski et al.). Due to damage in the outlet structure, a replacement box culvert extension was

designed and constructed between 2009 and 2010, with a design flow rate of 560 cfs (Grabinski et al.). A distinctive feature in this structure was a hydraulic jump stilling basin which culminated in an end sill that flowed onto Articulated Concrete Blocks (ACBs) before entering the outlet canal system (see Figures 6 and 7.)

The ACBs were used at the outflow of the stilling basin to prevent scouring of the canal bed as an alternative to riprap. However, while the use of ACBs proved effective at the similarly designed Nilan reservoir (Grabinski et al.), they were unable to sufficiently dissipate the energy in the flow, leading to scouring underneath the sheet

Figure 5: Articulated Concrete Blocks (Grabinski et al.)

Figure 7: Implementation of ACBs in Deadman's Basin outflow structure (Grabinski et al.)

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of ACBs (see Figure 8).

Figure 8: Progression of Failure in outflow ACBs (Grabinski et al.)

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Several alternatives were considered to replace the ACBs, the foremost of these being riprap (grouted or ungrouted) or 48” A-Jacks. The riprap variations proved to be unacceptable alternatives, however, as each would require a large enough diameter so as to make procurement difficult; furthermore, for the riprap to effectively prevent scour, “significant sandstone bedrock excavation” would be required as well, increasing the costs of the repair even more (Grabinski et al.). Since the A-Jacks did not require nearly as much bedrock excavation, they were selected to replace the ACBs (see Figure 9). After installation of the A-Jacks, tests proved that they were sufficient to quell the scouring at the outflow structure’s base.

While the A-Jacks eventually proved effective at dissipating the energy in the outflow, another possibility that could have been implemented would be redesigning the stilling basin to dissipate the energy before it reached the canal. The design of the stilling basin appears to have been insufficient to reduce the energy entering the canal, as alternative measures were required to prevent scouring, which even then were inadequate, in the case of the ACBs. To increase the amount of energy dissipated in the stilling basin, the hydraulic jump effected by the basin would need to be augmented: this could be accomplished by deepening the drop in elevation at the stilling basin, or adding baffle blocks to the design. While each of the these selections would increase the Froude number sufficiently to supplement the energy dissipation of the stilling basin, the baffle blocks would seem to be a better approach given the difficulties in excavating the sandstone bedrock expressed above.

Case Study II – El Guapo Dam Spillway, December 1999, El Guapo, Venezuela

The El Guapo Dam, constructed between 1975 and 1980 near El Guapo, Venezuela, was designed to provide safe drinking water, flood mitigation, and irrigation water to the surrounding area (United States Department of Reclamation). The dam was fashioned to hold a volume of 33,000 acre-ft., and originally included an uncontrolled ogee crest spillway ending in a concrete hydraulic jump stilling basin. The spillway had a width of 40 ft., a length of 925 ft., and a design discharge of 3600 ft.3/s; an additional tunnel spillway 820 ft. from the original was added after the spillway overtopped during construction. The design of the dam and primary spillway were based on a hydrologic study of a basin similar to the Rio Guapo, but not on the Rio Guapo basin itself. The overtopping of the initial spillway led to a new flood study, which resulted in the tunnel spillway’s construction.

Figure 9: A-Jack implementation (Grabinski et al.)

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Figure 11: Breach of the El Guapo Dam (United States Department of Reclamation)

During December, 1999, the water elevation of the reservoir rose considerably, peaking on December 15th with a water elevation .66 ft. below the dam crest. The spillway chute walls began to overtop near spillway crest, and the cities in the perceived flood path were evacuated at this time (United States Department of Reclamation). Following inspection via helicopter the next morning, the danger due to flooding was determined to have ceased, with the dam’s water elevation cresting at 2.5 ft. below the dam crest. However, by that afternoon, the water’s had rose considerably and quickly. The spillway began to overtop, which led to a sweepout of the stilling basin (see text below). This sweepout, concurrent with overtopping in the spillway, eroded the soil foundation of the stilling

basin, spillway chute, and crest structure, and within an hour headcutting had progressed to the reservoir itself, causing a breach and simultaneous failure of the El Guapo Dam (see Figures 10, 11, and 12).

Though overtopping of the spillway chute was the main cause of the erosion and subsequent failure, this overtopping was primarily the result of the stilling basin being improperly designed. The stilling basin was not designed to withstand the capacity of flow it experienced during the rainfall event, which not only caused the sweepout of the stilling basin, but the backlogging of water in the spillway chute,

Figure 10: Sweepout of the El Guapo stilling basin (United States Department of Reclamation)

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which then overtopped and accelerated the failure of the dam. This could have been prevented by properly examining the El Guapo basin, and determining the maximum discharges the stilling basin may experience. The hydrologic study that accompanied the initial design was not of the El Guapo basin, but of a different basin, and while the El Guapo basin itself

was studied during construction due to the spillway overtopping, the study seems to have focused primarily on the spillway discharge capacity, rather than the stilling basin discharge capacity. This is evidenced in that the spillway was the structure to fail amidst construction, and in that the eventual result of the study was a supplemental tunnel spillway, which increased the discharge of the spillway with no apparent complementary addition to the stilling basin discharge. This resulted in a stilling basin with more water flowing into it than it was designed to discharge. Had a full and comprehensive hydrologic study of the El Guapo basin been conducted, the incommensurate discharge of the stilling basin may have been detected, and suitable alterations may have been made to prevent the catastrophic failure at the El Guapo Dam.

Figure 12: Overtopping of the El Guapo spillway (United States Department of Reclamation)

What is a Stilling Basin Sweepout? A stilling basin sweepout is a phenomena in which “the tailwater is insufficient to allow a hydraulic jump to develop or be maintained” (United States Department of Reclamation, italics theirs). Since a hydraulic jump stilling basin’s primary mode of energy dissipation is the creation and prolonging of a hydraulic jump, the failure to preserve a hydraulic jump can cause one of two failure modes. The first of these begins with downstream erosion that results in headcutting, which then can lead to failure in the stilling basin, spillway, or reservoir itself due to degradation of the foundation materials. The second failure mode occurs when the tailwater has overtopped the spillway and surrounded the stilling basin, in addition to preventing the formation of a hydraulic jump. In addition to the headcutting and erosion failures of the first failure mode, failure can also transpire due to “flotation of the stilling basin due to uplift pressures” (United States Department of Reclamation).

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Works Cited

1) Beichley, G. L. "Hydraulic Design of Stilling Basins for Pipes or Channel Outlets." January 1, 1978. Accessed November 12, 2014.

2) Deadman's Basin Water Users Association. Accessed November 12, 2014. http://www.lmcd.mt.nacdnet.org/DBWUA/.

3) Grabinski, Kevin, and Clayton Fawcett. "Lessons Learned - Successes & Failures with Articulated Concrete Blocks in Stilling Basins." Montana Association of Dam and Canal Systems. Accessed November 12, 2014. http://madcs.org/files/Lessons_Learned_Successes_Failures_with_ACBs.pdf.

4) Hydraulic Design of Energy Dissipators for Culverts and Channels,Hydraulic Engineering Circular Number 14, Third Edition http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/hec14ch08.cfm

5) Montana Fish, Wildlife & Parks. "Deadman's Basin." Accessed November 12, 2014. http://fwp.mt.gov/fishing/siteDetail.html?id=283279.

6) United States Department of Reclamation. "Overtopping of Walls and Stilling Basin Failures." Accessed November 10, 2014. http://www.usbr.gov/ssle/damsafety/Risk/BestPractices/24-OvertoppingOfWallsAndStillingBasinFailure20121028