north diversion channel physical modeling

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North Diversion Channel Physical Modeling:

Alameda Outlet Structure

Between Alameda and Rio Grande

May 20th, 2012

Prepared for

Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA)

By: UNM Hydraulics Laboratory

Julie Coonrod, Ph.D., Professor

Emile Kareem Saint-Lot & Tyler Gillihan, Graduate Research Assistants

Department of Civil Engineering

University of New Mexico

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Introduction:

The North Diversion Channel (NDC), built by the Corps of Engineers and maintained by

Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA), intercepts storm water

from multiple open channels and carries the water north for discharge into the Rio Grande. A

500-yr storm event in the Albuquerque area results in a runoff estimate of 44,000 ft3/s through

the NDC at the Alameda Outlet Structure (Figures 1 &2). A 1:40 scale physical model of the

Alameda Outlet structure was built in 1987 at the Waterways Experiment Station (WES). The

WES model exhibited danger zones near the railroad bridge affected by standing waves. As a

result of this model two 1-foot high diagonal sills were placed just upstream of the bridge which

allowed the hydraulic jump to pass under the bridge while minimizing danger. The 1987 WES

model focused on improving hydraulic conditions near the Camino Arroyo Inlet and Railroad

Bridge (USACE, 1987).

Figure 1: Alameda Outlet Structure Vicinity Map

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Figure 2: Zoom on Project Location

Newly, observed flow conditions near the railroad bridge have initiated safety concerns

regarding a hydraulic jump which occurs just before the railroad bridge at lower flows and

moves downstream just past the bridge at higher flows. Velocities near the railroad bridge were

observed to be higher towards the right side (north side) of the channel as a result of the super-

elevated curve in the North Diversion Channel. A 1:84 scale model was constructed for

AMAFCA and used to determine flow conditions through the Alameda Outlet structure with

both the current, and a new proposed design for the bath tub. AMAFCA requested a prototype

flow of 34,000 ft3/s for the model and wanted to re-create the observed flow conditions in the

channel from the 1987 WES model. They also requested a comparison of the danger zones for

the current and proposed designs near the railroad bridge. Danger zones were defined by flow

conditions which created standing waves that encroached on the railroad bridge support beam.

The Corps of Engineers requires that channel modifications not jeopardize necessary storm water

conveyance at the bridges. The University of New Mexico Hydraulics Lab was used to model

both the old and new proposed design for the Alameda Outlet Structure. The subjects of this

study are the “bath tub” near the railroad bridge and the baffle blocks just downstream of the

bridge (Figure 3). The baffle blocks, which were designed to dissipate energy, cause sediment

accumulation between and around the blocks. In an effort to reduce maintenance costs

AMAFCA has proposed a design which removes the baffle blocks from the channel cross

section. The “bath tub” is a section in the channel which begins at an elevation of 5005 ft.

upstream of the railroad bridge, falls to an elevation of 5000 ft. just after the railroad bridge

abutment, and continues at that elevation for several hundred feet before rising back up to an

elevation of 5005 ft. (Figure 4).

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Figure 3: NDC Bathtub and Baffle Blocks

Figure 4: Bath Tub Details

The hydraulics near the bridge are complicated by the Camino Arroyo confluence, the super-

elevated curve, and the “bath tub”. Tributary flows from the Camino Arroyo confluence result in

larger standing waves through the curve (USACE, 1987). According to the 1987 WES report, the

super-elevated curve results in standing waves through the curve and an oblique standing wave

near the railroad bridge. The oblique waves were of concern because the bridge has a clearance

of approximately 14 ft. from the bottom of channel. Therefore the diagonal sills were constructed

to lower the maximum wave heights in the channel. The bath tub was designed to allow the

hydraulic jump to pass beneath the bridge at higher flows while minimizing danger to the bridge.

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Model Details

A 1:84 scale model was completed in May of 2012 which captured the super-elevated curve with

enough upstream channel length to create steady flow. The model was constructed primarily of

¼ inch corrugated plastic and aluminum tape. ½ inch chicken wire screen painted brown was

used to simulate sand roughness in the bottom of the channel just after the bath tub. Regular

household door screen, also painted brown, was used to simulate rip rap roughness on the sides

of the channel just downstream of the baffle blocks. Both screens were tested in trapezoidal

channels to determine Manning’s n values. Baffle blocks were constructed of corrugated plastic

and tape then glued onto a plastic binder divider (to allow for easy insertion and removal) cut to

channel dimensions. AMAFCA crews assisted in building wooden frames to support the channel.

Model walls were constructed higher than prototype walls to prevent spillage during modeling.

The black lines on the channel sides represent the top of channel for the prototype.

The 1:84 model includes both the Edith and the railroad bridges. Using Froude similitude, 224

gal/min is required to simulate 34,000 ft3/s in the North Diversion Channel. This model captures

the transition from a trapezoidal to rectangular channel as the North Diversion Channel enters the

super-elevated curve. It also shows the complete construction of the proposed design in the

North Diversion Channel (Figure 5).

Figure 5: Alameda Outlet Structure Model

Flow

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Figure 6: Current Bath Tub Conditions

Figure 7: Proposed Bath Tub Design

The current “bath tub” stretches 7.5 ft. (approximately 730 ft. in the NDC) (Figure 6). The

proposed design stretches the bathtub for 10.5 ft. (approximately 880 ft. in the NDC) and

eliminates the baffle blocks (Figure 7). The current bath tub conditions were constructed as an

insert which was placed in the model to represent as-built conditions. The white duct tape on the

bottom sides of the channel was used to keep the insert in place while the model was running.

Aluminum tape was also used around the baffle blocks to keep them in place during modeling.

Manning’s n

Desired Roughness Calculated Roughness

Rip-Rap (Household Screen) 0.040 0.042

Sand (1/2 inch Chicken Wire) 0.030 0.037

Modeling was broken up into 3 different scenarios to allow for a thorough analysis of the

proposed design. Scenario 1 modeled the Alameda Outfall in its current condition. The second

scenario used the proposed design, but also included the baffle blocks. The third scenario

modeled the complete proposed design for the outfall structure, which removed the baffle blocks

and extended the bath tub.

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Scenario 1 Model Results: Current Design

This scenario required the bath tub insert to be placed into the channel and taped down. The

screen on the sides of the channel exposed the tape to water causing it to peel off quickly, which

only allowed the model to be run for a short period of time. For these reasons the model was only

run at the requested flow rate of 34,000 ft3/s. Results in Figure 8 shows the hydraulic jump

downstream of the railroad bridge.

Figure 8: Current Design @ 34,000 ft3/s

Scenario 2 Model Results: Proposed Design with Baffle Blocks

The proposed design with the baffle blocks in place was also effective in moving the hydraulic

jump downstream past the bridge. Figure 9 shows the model running at a flow rate of about

12,000 ft3/s where it is clear the hydraulic jump is upstream of the bridge. Figure 10 is at a flow

rate of 15,000 ft3/s and shows the hydraulic jump occurring just after the diagonal sills. The

oblique standing wave in Figure 10 is a result of uneven velocity and flow distributions between the north and south side of the model due to the super-elevated curve upstream. Running 23,000 ft

3/s through the model moves the hydraulic jump to the location of the railroad

bridge shown in Figures 11 & 12.

Figure 9: Proposed Design with Baffle Blocks @ 12,000 ft3/s

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Figure 12: Proposed Design with Baffle Blocks @ 23,000 ft3/s looking upstream

As the hydraulic jump passes beneath the bridge the water gets close to touching the bridge support beam. Figure 13 is looking upstream and demonstrates how close the water encroaches on the railroad bridge support beam. Under these flow conditions it is evident that a danger zone exists.

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Once the flow rate is increased to 34,000 ft3/s the hydraulic jump moves further downstream past

the bridge, and remains in place just before the channel flares out (Figure 14).


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Scenario 3 Model Results: Proposed Design without Baffle Blocks

The proposed design without the baffle blocks in place proved to be effective in moving the

hydraulic jump downstream past the bridge. Figure 15 shows the model running at a flow rate of

12,000 ft3/s where it is evident that removing the baffle blocks has no visible effects on the

hydraulic jump. However, at a flow rate of 15,000 ft3/s it is clear that removing the baffle

blocks from the proposed design causes the jump to propagate further downstream just past the diagonal sills (Figure 16). At this flow rate, the hydraulic jump exhibits the effects of the diagonal sills with a fairly uniform jump. The flow rate was then increased to 23,000 ft

3/s, which caused the hydraulic jump to propagate downstream and reach the railroad

bridge.

Figure 15: Proposed Design without Baffle Blocks @ 12,000 ft3/s



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From Figure 17 it is clear that removal of the baffle blocks has no visible effect on the location of the hydraulic jump at the 23,000 ft3

/s flow rate. No change was noticed when

comparing danger zones between the two scenarios of the proposed design with and without the

baffle blocks at a 23,000 ft3/s flow rate. Figure 18 is looking upstream at the hydraulic jump

occurring at the railroad bridge. Figure 19 is a close-up of the bridge and hydraulic jump at the

23,000 ft3/s flow rate. In this photo it is evident that a danger zone still exists near the railroad

bridge.

Figure 18: Proposed Design without Baffle Blocks @ 23,000 ft3/s looking upstream

Figure 19: Proposed Design without Baffle Blocks @ 23,000 ft3/s looking downstream

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Increasing the flow rate to 34,000 ft3/s pushed the hydraulic jump past the railroad bridge to the

section where the channel begins to flare out. Figure 20 clearly shows the hydraulic jump

occurring after the bridge during the design flow rate. Notice that removing the baffle blocks

causes the jump to move further downstream providing more distance between the hydraulic

jump and the railroad bridge.


Tributary Flow:

AMAFCA requested the Camino Arroyo Confluence be added to the model to better represent

current channel hydraulics. The confluence had no negative effect on channel hydraulics near the

super-elevated curve under the current modeled prototype flow rate of 34,000 ft3/s. AMAFCA

presented a new box culvert design for the Camino Arroyo Confluence which was also modeled

to determine potential hydraulic effects. The box culvert was placed downstream of the current

confluence design and inserts were built to allow for comparison between the two designs.

Modeling the new proposed box culvert design uncovered a flaw shown in Figure 21 that

allowed water flowing through the super-elevated curve to cause back flow through the

confluence outlet. After comparing the two designs, it was decided the new box culvert was

inadequate due to its adverse effects on channel hydraulics through the super-elevated curve.

Figure 22 shows the current design modeled at the prototype flow rate of 34,000 ft3/s.

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Figure 21: New Box Culvert Camino Arroyo Confluence Design @ 34,000 ft3/s

Figure 22: Current Camino Arroyo Inlet Design @ 34,000 ft3/s

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Conclusion:

A previous physical model, built and reported on in 1987 at the Waterways Experiment Station

(WES), showed the current design to be effective with some danger zones near the bridge

(USACE, 1987). Danger zones were defined by flow conditions which created standing waves

that encroached on the railroad bridge support beam. Although the 1987 WES model did identify

danger zones, the design was deemed to be acceptable with minor alterations which minimized

the danger zones.

The North Diversion Channel Outfall was modeled for AMAFCA at a 1:84 scale to determine

the hydraulic impacts of a proposed design for the bath tub near the railroad bridge. AMAFCA

requested the model to simulate 34,000 ft3/s in the NDC. Ideal design flow conditions for the

model showed the hydraulic jump to remain in the same relative location in both scenarios one

and two, which included the baffle teeth. Removal of the baffle teeth in scenario three caused the

hydraulic jump to progress further downstream past the bridge. Although on some occasions, as

the flow rate was increased in the three scenarios, the water encroached on, and sometimes

touched the railroad bridge support beam. This is likely due to the unsteady flow through the

pump, as the water did not always touch the bridge support beam during modeling.

The danger zone was improved with the final proposed design, resulting in a hydraulic jump

occurring further away from the railroad bridge. The UNM Hydraulics Laboratory model

showed the proposed design of the Alameda Outlet Structure to most effectively minimize

danger zones. Table 1 below shows the measured flow rates during modeling.

Table 1: 1:84 Model Flow Data

1:84 Model @ 34 Hz 1:84 Model @ 35 Hz

Velocity (ft/sec.) 1.14 Velocity (ft/sec.) 1.40

Prototype Velocity (ft/sec.) 10.45 Prototype Velocity (ft/sec.) 12.83

Water Depth (in.) 2.19 Water Depth (in.) 2.38

Flow (gpm) 77 Flow (gpm) 104

Prototype Flow (cfs) 11,155 Prototype Flow (cfs) 14,970

1:84 Model @ 36 Hz

1:84 Model @ 37 Hz

Velocity (ft/sec.) 2.10 Velocity (ft/sec.) 2.68

Prototype Velocity (ft/sec.) 19.25 Prototype Velocity (ft/sec.) 24.56

Water Depth (in.) 2.44 Water Depth (in.) 2.63

Flow (gpm) 160 Flow (gpm) 219

Prototype Flow (cfs) 23,025 Prototype Flow (cfs) 31,606

north diversion channel physical modeling

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