Morning Glory Spillway: The Giant Overflow Hole in a Reservoir

How a morning glory spillway works, step by step

A morning glory spillway, also called a bell-mouth or shaft spillway (a bellmouth overflow in Britain, as at Ladybower Reservoir) and nicknamed the glory hole, is named for the funnel-shaped flower it resembles. In its usual form it is an uncontrolled overflow: with no gates, the lake level alone decides whether it runs. The best-known example serves Monticello Dam, the Bureau of Reclamation’s 304 ft (93 m) concrete arch completed in 1957, and sits in Lake Berryessa just upstream of the dam.

• The crest sets the full level. The circular concrete lip, 72 ft (22 m) across, sits at elevation 440 ft. The spillway cannot lower the lake beneath its own crest; supply releases and drawdown use separate outlet works.
• Water enters from every side. Once Lake Berryessa passes 440 ft elevation, water flows radially inward over the whole circumference as a smooth sheet, the nappe, that clings to the curved crest.
• The sheet accelerates and contracts. Falling water speeds up, and by continuity (Q = A·v) a faster stream needs less area, so the funnel tapers with it: the shaft narrows to ~28 ft (8.5 m) at the bottom.
• A ~200 ft (~61 m) vertical drop. Ignoring friction, v = √(2gh) = √(2 × 9.81 × 61) ≈ 35 m/s, about 113 ft/s or 77 mph, at the foot of the shaft.
• A 90° elbow and a tunnel. The shaft turns into a near-horizontal tunnel that runs partly full, with air above the water, and discharges into Putah Creek below the dam.

The layout suits narrow canyons like Monticello’s, where there is little room for a long chute spillway beside an arch dam.

Three flow regimes: crest, orifice and pipe control

As the head H above the crest rises, control of the discharge Q passes through three regimes, so the rating curve (lake level versus flow) climbs steeply, then bends almost flat.

• Crest control (low heads). The lip behaves as a circular weir: Q = C₀·(2πR_s)·H^1.5, where 2πR_s is the crest circumference and C₀ is a discharge coefficient. The 1.5 power is depth times velocity: the sheet thickens in proportion to H while its speed grows with √H. Doubling H multiplies flow by 2^1.5 ≈ 2.8 at fixed C₀, though in practice C₀ falls as H/R_s rises, because the converging sheets crowd one another.
• Orifice control (intermediate heads). Eventually the sheets deliver more water than the throat can pass. Water backs up the shaft, the crest drowns, and the narrowest section governs as a submerged orifice: Q = C_d·A·√(2gH_a), with H_a measured from the lake surface down to the throat. Weir flow Q ∝ H^1.5 has become orifice flow Q ∝ √H_a, and because H_a already includes the throat’s depth, each extra foot of lake adds very little flow.
• Pipe control (high heads). If the lake keeps rising, the tunnel runs full and the whole conduit sets the flow: Q ≈ A·√(2gH_t/K), where H_t is the total drop to the outlet and K sums entrance, bend, friction and exit losses. The curve flattens further, and full flow brings air slugs, blowback and pressure swings, so designers avoid it.

A well-designed glory hole passes its design flood under crest control or right at the transition to orifice control, never under pipe control.

Worked example: running Monticello’s numbers

Monticello’s rated maximum, 48,400 ft³/s (1,370 m³/s), arrives with the lake just 15.5 ft (4.7 m) above the lip. A few quick calculations show what that means.

• Implied coefficient. Taking R_s as the 36 ft lip radius, the circumference is 2π × 36 ≈ 226 ft and 15.5^1.5 ≈ 61.0, so C₀ = 48,400 ÷ (226 × 61.0) ≈ 3.5 in foot-second units (about 1.9 in metric). Overflow crests typically run about 3 to 4 in the same units, so that is a sensible value. The head is also a sizeable fraction of the radius, H/R_s ≈ 0.43. At a fixed coefficient, half that head would pass only 48,400 ÷ 2.83 ≈ 17,100 ft³/s.
• Room in the shaft. Near the foot of the ~200 ft (~61 m) drop, free fall gives about 113.5 ft/s (34.6 m/s). Carrying 48,400 ft³/s at that speed takes 48,400 ÷ 113.5 ≈ 426 ft² of water, while a 28 ft circle offers π × 14² ≈ 616 ft², so the water fills roughly 70% of the section even before friction slows it and entrained air bulks it up: a tapered shaft with modest spare area.
• What an extra 5 ft buys. Suppose a flood lifts a glory hole’s lake from 15 to 20 ft above its crest. With the crest still in control, flow rises by (20/15)^1.5 ≈ 1.54, about 54%. If the crest has drowned and a throat 50 ft below the crest governs (an illustrative depth), H_a goes from 65 to 70 ft and flow rises by √(70/65) ≈ 1.04, only about 4%.

That flat upper curve is the type’s defining weakness: a straight overflow crest keeps adding capacity as a flood grows, but a glory hole pushed past its design point adds almost none.

Crest shapes, swirl control and air: the design standards

The shape of the lip is not arbitrary. In 1956 W. E. Wagner of the Bureau of Reclamation published Morning-Glory Shaft Spillways: Determination of Pressure-Controlled Profiles in Transactions of the ASCE, vol. 121, based on laboratory measurements of water falling over sharp-edged circular weirs. The concrete crest copies the underside of that aerated nappe at the design head, so pressure on the concrete stays near atmospheric. Well above design head, crest pressures drop below atmospheric, inviting separation, vibration and cavitation. Wagner’s profile tables were carried into Reclamation’s Design of Small Dams, the standard reference for the type.

• Swirl control. Wind, shoreline shape or an off-centre approach give the water slight rotation. Angular momentum is conserved as it moves inward, so tangential speed scales as 1/r: a drift of 1 ft/s at 100 ft from the centre becomes about 7 ft/s at a 14 ft radius. The resulting vortex puts an air core where water should be, cuts capacity and shakes the structure, so designers add radial piers or guide vanes, or shape the approach, to keep inflow radial.
• Keeping the tunnel free-flowing. Design of Small Dams advises sizing the conduit downstream of the throat to run no more than about 75% full at maximum discharge, leaving an air passage above the water so the tunnel cannot seal, siphon or surge. Air vents supply the air the fast water drags along with it.
• Gated versions. Some glory holes carry a cylindrical steel ring gate that rises from a recess around the crest, letting operators store water above the fixed lip. Owyhee Dam in Oregon and Hungry Horse Dam in Montana use this arrangement; Monticello’s is purely uncontrolled.

How designs are tested and rating curves proven

Glory holes are too sensitive to geometry to trust equations alone, so large ones are proven in physical scale models, historically at Reclamation’s hydraulics laboratory in Denver. Because gravity drives the flow, the models follow Froude similarity: velocities scale with the square root of the length scale, and discharge with its 2.5 power.

• Scaling example. In a 1:30 model the 72 ft lip becomes 2.4 ft (0.73 m) across. Discharge scales by 30^2.5 ≈ 4,930, so the 48,400 ft³/s maximum needs only about 9.8 ft³/s (≈280 L/s) of laboratory water, and a 113 ft/s prototype velocity becomes about 21 ft/s.
• What gets measured. Engineers step the model through a range of heads, recording discharge, the level at which the crest drowns, piezometer pressures along crest and elbow, swirl, and where the tunnel stops flowing freely. The product is the rating curve of reservoir elevation against discharge.
• Scale effects. Surface tension and viscosity do not follow Froude scaling, so small models entrain too little air and understate air demand and cavitation risk, which are extrapolated conservatively.
• In service. An uncontrolled spillway needs no flowmeter: the reservoir gauge reading, looked up on the rating curve, gives the discharge. Dam-safety reviews periodically recheck that curve and the freeboard against updated flood estimates.

History, incidents and failure modes

Monticello Dam was built from 1953 to 1957 for Reclamation’s Solano Project; Lake Berryessa drowned the town of Monticello and holds about 1.6 million acre-feet at full pool. The glory hole runs only in wet winters; its February 2017 spill, the first since 2006, and another in 2019 produced viral footage widely misread as a drain swallowing the lake.

• A flood beyond the design. Once the crest drowns, extra lake height brings little extra release, so the reservoir climbs toward the dam crest. A glory hole relies on sound flood estimates, freeboard or a second spillway rather than reserve capacity of its own.
• Debris. Logs that float easily over a 72 ft lip can wedge across the much narrower shaft or elbow, shrinking flow area exactly when flow is highest, so floating debris is best managed upstream.
• Full-flow surging. If the tunnel seals with water, trapped air escapes in slugs, flow can blow back up the shaft, and pressure swings shake the structure: the regime the 75% guideline prevents.
• Cavitation at bends. High-velocity tunnel spillways have been badly damaged where the flow turns. At Hoover Dam in 1941 and Glen Canyon Dam in 1983, cavitation near the bends of inclined tunnel spillways tore through the concrete lining into the rock. Those were not glory holes, but a glory hole’s elbow faces the same physics, and Reclamation’s fix, aeration slots that mix air into the flow, is now standard for high-velocity conduits.
• People. Inflow speeds up sharply as it converges on the lip. In 1997 a swimmer, a UC Davis graduate student, was pulled into Monticello’s glory hole and drowned, and boaters and swimmers are warned to keep well clear of the intake.

Not a whirlpool, not a siphon: how it differs from look-alikes

Viral ‘plughole’ clips suggest the lake is being sucked down a whirlpool. A healthy glory hole looks nothing like a draining bathtub: water slides in radially as a smooth, glassy sheet, and the open core in the middle is the hollow centre of a falling ring of water, not the eye of a vortex. Visible swirl is a defect that costs capacity.

• Siphon spillway. A hooded conduit that must prime: once its air is expelled it runs full, driven by the drop from lake to outlet, and can cycle on and off. A glory hole stays open to the air and is designed never to flow full.
• Vortex drop shaft. Deep storm and sewer tunnels, such as London’s Thames Tideway Tunnel, use spiral inlets that deliberately spin water down a shaft around a stable air core, shedding energy on the walls. There the vortex is the design; in a glory hole it is a defect.
• Diversion intakes. Portugal’s much-photographed bell-mouth at Covão dos Conchos, built in the 1950s, sends overflow through a tunnel to another reservoir, Lagoa Comprida, rather than past a dam.
• Drop-inlet risers. The pipe risers in small flood-control and farm-pond dams belong to the same family on a tiny scale, but many are designed to run full, with anti-vortex plates on top.

The morning glory spillway compared with other ways a dam passes floodwater

Spillway type	What controls the flow	Flow as the lake rises	Main trade-off
Morning glory (bell-mouth shaft)	Circular ungated crest at full level; the throat and tunnel take over at high heads	Q ∝ H^1.5 while the crest runs free, then close to Q ∝ √H once the crest drowns	Compact and gate-free for narrow canyons, but little reserve beyond the design flood, and debris can choke the throat
Ogee overflow or chute spillway	Straight crest discharging down the dam face or a concrete chute	Q = C·L·H^1.5 keeps climbing as the head grows	Generous reserve capacity, but needs a long crest and room for a chute
Labyrinth or piano key weir	Zig-zag crest folds several times more crest length into the same width	Weir-type flow; the advantage over a straight crest shrinks as the head grows	Passes large floods with a small rise, at the cost of more concrete and structural complexity
Siphon spillway	Hooded conduit that primes, expels its air and runs full	Q ∝ √(lake level − outlet level) once primed	Very large capacity for a tiny rise, but it can cycle on and off and must stay airtight
Gated radial (Tainter) spillway	Steel gates hold water above the crest and are opened by operators	Set by the gate opening; orifice-type flow under partly raised gates	Stores extra water, but depends on power, staff and maintenance: Folsom Dam’s gate 3 failed in 1995
Vortex drop shaft	Spiral inlet deliberately spins the flow down a shaft around an air core	Designed around a stable swirl	Standard in storm and sewer tunnels; in a glory hole the same vortex is a defect