July 2010

Hahn Arroyo Island Project:

Hydraulic and Debris Removal Performance

Based on

Physical Modeling Studies at U M Hydraulics Laboratory

Prepared for Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA)

Julie Coonrod, Ph.D., Associate Professor

Nelson Bernardo & Kyle Shour, Research Assistants

Department of Civil Engineering

University of New Mexico

Introduction:

Draining approximated 6.5 square miles before entering the North Diversion Channel, the Hahn Arroyo

plays a vital role in conveying flood water in north e

1960s, the channel has deteriorated significantly (

Flood Control Authority (AMAFCA) has begun

AMAFCA would like to include measures to improve water quality

AMAFCA engineers have designed a structure to be installed as an island in the channel (

However, due to the unpredictable nature of supercritical flow occurring in the Hahn Arroyo,

engineers have requested the assistance of the University of New Me

AMAFCA Engineers and UNM research assistants have tested

structure. The design objectives for th

storm and conveyance of flows up to t

Figure 1: Severely Deteriorated Concrete in Hahn Arroyo

Figure 2: Project Vicinity Map

Draining approximated 6.5 square miles before entering the North Diversion Channel, the Hahn Arroyo

le in conveying flood water in north eastern Albuquerque. Since its construction in the

has deteriorated significantly (Figure 1). The Albuquerque Metropolitan Arroyo

Authority (AMAFCA) has begun rehabilitation of the Hahn. As part of this rehabilitation,

AMAFCA would like to include measures to improve water quality—best management practices (BMPs).

AMAFCA engineers have designed a structure to be installed as an island in the channel (

to the unpredictable nature of supercritical flow occurring in the Hahn Arroyo,

the assistance of the University of New Mexico’s (UNM’s) Hydraulics Lab.

research assistants have tested and improved three designs for the

for the structure included efficient removal of debris up to the 1

conveyance of flows up to the 100-year storm (Table 1).

: Severely Deteriorated Concrete in Hahn Arroyo (from AMAFCA)

: Project Vicinity Map (from http://12.23.244.78/amafcapublic/

Draining approximated 6.5 square miles before entering the North Diversion Channel, the Hahn Arroyo

astern Albuquerque. Since its construction in the

). The Albuquerque Metropolitan Arroyo

the Hahn. As part of this rehabilitation,

management practices (BMPs).

AMAFCA engineers have designed a structure to be installed as an island in the channel (Figures 2-4).

to the unpredictable nature of supercritical flow occurring in the Hahn Arroyo, AMAFCA

xico’s (UNM’s) Hydraulics Lab.

designs for the

up to the 1-year

(from AMAFCA)

http://12.23.244.78/amafcapublic/)

Figure 3: First Structure Installed in Flume

Figure 4: First Structure Installed in the Flume

Flow

Flow

Table 1: Hahn Arroyo Flow Recurrence Values at Project Site

First Model

AMAFCA engineers constructed the first structure and provided dimensions for fabrication of the

channel sections. The model had a 1:16 scale. Construction of the first model was completed on April 8,

2010. The design intended to capture the majority of low flows and first flushes through a ramp

preceding the structure inlet. The inside of the structure was to act as a detention basin, removing both

buoyant and non-buoyant debris. Rapid reductions in velocity would cause debris to fall out in the

structure. Typically, this was accomplished by a transition from supercritical to subcritical flow with a

hydraulic jump occurring at the structure inlet. Treated water would then return to the arroyo through

a series of weir wall and hanging wall pairs.

Early testing of the structure revealed that, even at low flows, much of the flow bypassed structure’s

inlet ramp. Bypassed water rooster-tailed significantly as it passed the structure. Three-quarter inch tall

walls (1 foot in actual design) were installed on either side of the ramp, and the ramp was lowered from

1.5 inches (2 feet) to 0.75 inches (1 foot) to increase flow into the structure and remove rooster-tailing.

The modification was effective at lower flows; however, at high flows, the hydraulic jump forming at the

inlet exceeded the height of the opening. The bottom of the ramp of dropped the remaining 0.75 inches

(1 foot) and the lower hanging wall above the inlet was removed to increase the structure’s inlet

capacity (Figure 5).

Figure 5: First Structure after Ramp and Inlet Improvements

After these modifications, the hydraulic jump occurred on the ramp instead of at the inlet. This caused

the walls along the ramp to be readily over-topped. Additionally, the overflow rate out of the structure

was too high (i.e. the retention time in the structure was too low) and debris was not retained. To

Recurrence Interval: 0.5 1 2 10 100

Peak Flow (cfs): 85 276 458 843 1626

Flow

remedy this, sloped baffle walls were installed along the inside of the hanging walls, and the orifice area

below the lowest, outermost hanging wall was significantly reduced (Figure 6). This did not significantly

improve debris retention but did force the hydraulic jump further up the ramp causing the walls to

overtop at even lower flows.

Figure 6: Sloped Baffles Installed and Lower Orifice Plugged

Second Model

AMAFCA provided a second, 1:16 scale model (Figure 7). The dimensions of the model’s footprint were

the same as the previous model. One of the weir wall and hanging wall pairs was removed. The model

featured adjustable walls around its entire circumference. The wall heights and orifice heights could be

varied for all walls. UNM graduate students constructed multiple ramp configurations that could be

quickly changed in the model. This included a ramp that was flush with the floor of the structure, a

ramp ending in a 0.75 inch (1 foot) drop, and a ramping ending in a 1.5 inch (2 foot) drop. The second

model was installed on May 6, 2010 and tested for the first time on May 7th

.

Slope Baffle

Walls

Plugged Orifices

Figure 7: Second Structure Installed in Flume

Testing on the second model used the ramp with 1.5 inch drop. The second model was tested using the

1-year peak flow rate (Table 1) of 276 cfs which was significantly higher than flow rates analyzed in the

first model (115-180 cfs). The new model performed better hydraulically. The hydraulic jump moved

back into the entrance of the structure, no water overtopped the ramp walls, and no splashing or

rooster-tailing was observed (Figure 8).

Figure 8: Second Structure Operating at 276 cfs

Flow

Optimum wall and orifice heights were determined by qualitatively observing hydraulics and measuring

water depths within the structure. After finding optimum wall heights, the second structure’s debris

removal capabilities were tested. Addition of debris and dye revealed one very beneficial quality of the

structure; water in the spaces between the weir and hanging walls flowed from the downstream to the

upstream end of the model, implying that debris would have an increased retention time in the

structure. The model performed well, removing floatable debris and most neutrally buoyant objects.

However, smaller debris was only detained and not removed. Smaller debris was simulated with coffee

grounds and paper confetti. To improve small debris removal, model alterations were made to decrease

velocity in the structure. The first of these modifications was three rows of baffles installed immediately

downstream of the ramp (Figure 9). However, testing showed no significant improvements in debris

removal.

Figure 9: Baffles Installed in Structure

The baffles were removed. The orifice height under the center wall was reduced to 0.25 inches, the

outer wall height increased to 2.25 inches (3 feet), the center wall height to 3.75 inches (5 feet), and the

inner wall to 5.25 inches (7 feet). These adjustments provided hydraulic characteristics most conducive

to debris removal. Finally, for both debris removal and liability, engineers decided to add bar screens to

the entrance and top of structure (Figure 10).

Figure 10: Second Model with Screens Installed

Third Model

Using the final configuration of the second model, a third, and final, model was constructed with fixed

wall heights (Figure 11). The final model included the wall and orifice heights described above with

greater wall lengths caused by a lengthening of the structure’s footprint. Final model alterations

comprised creating a new screen over the structure and installing screens between inner and middle

walls and in front of middle wall orifice (Figure 12). The orifice area below the middle wall was reduced

by approximately 43 percent to decrease the flow over the outer weir and increase the flow over the

middle weir. This forced the hydraulic jump upstream of the structure entrance at low flows, causing

debris to flow over the ramp walls and bypass the structure. 6 inch (8 foot) long wingwalls were

installed parallel to the flow to prevent water from overtopping the walls at low flows (Figure 13). The

wingwalls also reduced rooster-tailing at the 100-year flow (Figure 14).

AMAFCA engineers raised the ramp to match the slope of the adjacent portions of the cross-section and

altered the ramp into the structure to match the slope of the drop on either side of the structure (Figure

13). This improved the design constructability and allowed for easier retrofit work in the future. This

design improvement produced no adverse hydraulic effects.

Figure 11: Third Model

Figure 12: Third Model with Screens Operating at 276 cfs

Figure 13: Third Model with Wingwalls and New Ramp Configuration

Figure 14: Third Model Conveying 100-yr Event (1626 cfs)