THE DESIGN OF A SIDE DISCHARGE ST. ANTHONY’S FALLS

STILLING BASIN

LCE Engineers, Inc., Lovick C. Evans, PE, RLS

Background Information

The McEachernLake and dam is a small lake of 3.5 acres in size with an embankment

height of 19 feet and a drainage basin of 265 acres. The pond was originally constructed as a

rural farm pond many decades ago.Since that time, the drainage basin has been heavily

developed and the pond would now be considered an urban pond. Because of this, the storm

flows into the lake of 1,500 cfs, are reasonably large in relation to the pond size. The

existingprimary and emergency spillways for the lake are a concrete flume type spillway. The

embankment and outlet channel are also constructed adjacent to a downstream property line

and abuts a downstream residential subdivision,(Figure#1),which severely limited any repair

work which needed to be done. The lake is also being used as a storm water regional

detention pond to handle the storm water flows from adjacent schools and housing

developments. Upgrades within the last 14 years whichoccurred to the spillway and

embankment, were performed when the school and housing developments were constructed.

These upgrades included a metal weir being placed in the concrete flume spillway, a gabion

outlet channel,(Figure #2a,b,c), and the embankment being raised. From recent floods of

2009 within the drainage basin, the spillway and outlet channel were unable to withstand the

flood waters and started failing. This failure was mainly the result of an inadequate design

utilizing the modern design storm criteria.

Figure 1.

As shown on (Figure 1), the flow from the existing concrete flume spillway had two 90-

degree turns which had to be considered. Because the downstream property line is only 54

feet from toe of the dam embankment, construction of a plunge pool area, which would handle

the design flows, would be a challenge. Also, a permitting rule required that all the flows from

the site be returned to a natural condition before leaving the site unless expensive downstream

drainage easements were acquired, if even possible,(Figure#3). With these constraints on the

flow regimen, selection of a replacement spillway system was very limited.

Figure #2a, Existing Concrete Flume and metal weir plate

Figure#2b, Collapsed Gabion outlet channel downstream of concrete

flume

Figure#2c, Collapsed Gabion outlet channel along toe of dam

Figure#3, Existing receiving creek at downstream property line

Spillway Selection and Design Process

The final schematic of the spillway was to design and construct, the spillway system to

correctly convey the storm water through and around the small dam and pond utilizing the

most current design flows for the basin. Since the drainage basin is mostly developed, this

design was first started by tabulating the existing development characteristics of the basin.

Because of the amount of flows and the location of the dam to the downstream property line,

the spillway selection was very limited.Controlling energy dissipation coupled with the visual

effects of a new spillway structure, created a high level of concern of having the storm flows

being returned to the receiving creek at a natural condition and the physical appearance to be

witnessed by the downstream homeowners. When the design criteria was weighted against

the site constraints, a concrete chute with a concrete St. Anthony’s Falls stilling basin was

selected for the energy controlalong with the stilling basin being lowered into the adjacent

ground to reduce the visible appearance of the concrete walls of the still basin. The difficulty

was with the use of the concrete St. Anthony’s Falls Stilling basin.

The standard design of the St. Anthony’s Falls stilling basin has the concrete chute

spillway entering the basin, the energy dissipation occurring in the basin, and then the water

flows out the end. Thrust blocks and deep tail water depths are used for the means to causes

the energy dissipation,(Figure#4a,b). However, because of the site constraints, the flow from

the stilling basin could not exit the end but had to exit the side of the basin and the deep tail

water depths could not be achieved. Because all of the standard equations for the stilling basin

design dealt with the depth of the tail water, location of the hydraulic jump and the thrust block

size and locations, a search was done to determine which equations would be applicable for

the side discharge condition. The search revealed very little information on this side discharge

condition.

Figure#4a, Typical St. Anthony’s Falls Stilling Basin

Figure#4b, Typical St. Anthony’s Falls Stilling Basin

The first item was to determine what could be used from the normal stilling basin

design. The flow dynamics for the start of the basin design were as followed: Flow rate of

984cfs, Velocity of flow of 23 ft/sec, and the Froude Number of 2.3. Because the basin design

also would require a tailwater depth, the depth that was selected was slightly lower that the

anticipated wall height of 11 feet or a tailwaterdepth of 10 feet. With this information, the initial

sizing of the basin dimensions were then determined.Realizing that in a normal St. Anthony’s

Falls stilling basin, the forward direction for the water’s velocity remained the same, although

changed in magnitude. With the site constraints, this could not happen and the forward velocity

of the water had to be stopped and restarted in a 90 degree direction. Of course having an end

wall would accomplish this but a concern was that a run over of the end wall would occur if the

assumed tailwater depth was not correct. Without any found research on this condition, a

method was needed to try to introduce more energy dissipation into the stilling basin to reduce

the water’s forward velocity.

Figure#5, Proposed Design of Spillway

Because of the side discharge situation of the stilling basin, one of the side walls would

be openand a side slope of the stilling basin floor was set to direct the flows toward the

opening.With this side wall opening an opportunity would exist to allow the ability of introducing

air entrainment into the flow regime. Therefore, a step drop from the chute into the stilling

basin was set to provide a vented nape for the flow,(Figure#5).The next step was to computer

model the stilling basin design. A review of the available computer programs that could be

used to model the basin was then looked at. Based on the flow complexity, the HECRAS

computer program was selected and the spillway geometry was incorporated into the program.

The model was initially setup to model the spillway as a single unit and bent the cross sections

through the stilling basin to try and calculate any tail water effects. The model was set to run in

a mixed flow regime. This uses both subcritical and supercritical flows profiles in the model.

When these results were reviewed, it did not appear to provide results that were anticipated.

A change in the modeling approach was then initiated. This approach was to look at and

analyze the basin as closely spaced cross sections with a lateral weir flow. This was done to

try and imitate the flow leaving the stilling basin at different rates based on the distance in the

basin and the speed of the water. By using this approach, multiple computer runs were done

and from these studies, a hydraulic jump was shown to occur. In order to test the robustness of

the computer modeling, small adjustments to the lateral weir flow amount was done. When

these small adjustments were made, the hydraulic jump was not stable and was shown to

move back and forth through the stilling basin. Because the convergence of the results from

the different computer runs would not focus in to a single result, the confidence level of a

successful design still was not high enough. The path for the computer modeling appeared to

be correct but the computer modeling was still not satisfactory and an understanding of the

flow dynamics in the stilling basin was in doubt.

A scale model test had to be done,(Figure#6).From this testing, a better knowledge of

the flow profile would be revealed. The scale model was constructed utilizing the initial design

geometry for the spillway on a scale of 1”=4’. This model incorporated the thrust blocks, chute

spillway, step drop,end sill, side slope of the basin floor, and wall heights. Flow rates were

simulated by increasing the flow rates until the depth of flow was achieved as reported by the

HECRAS modeling within the chute spillway. From this modeling, distances were measured at

critical points of the flow regime within the stilling basin. From the flow test, it revealed a

moving “falling back wave” developed,(Figure#7),which moved and created a wash over at the

end of the basin. This was similar to the computer model that showed the hydraulic jump

moving through the stilling basin. At this point , it was surmised that air entrainment would be

required to break the flow regime to reduce the velocity necessary to produce a stable flow.

Figure#6, Scale model 1” = 4’

Figure#7, Moving “Falling Back Wave”

Revisions were then made to the model to improve the air entrainment into the flow and

increase the energy dissipation within the basin. A major change was to place chute blocks at

the top of the drop,(Figure#8). This changed disrupted the flow by splitting the flow at the drop

allowing half of the flow to continue down the chute and have the other half elevate over the

chute flow.This allowed for a doubling of the air entrainment surface to occur in the

chutespillway flow. By allowing more air entrainment to enter the flow, the thrust blocks

became more effective at dissipating the energy throughout the stilling basin and not just at the

rear of the basin. On the initial flow test, the hydraulic jump was occurring near the end of the

stilling basin because of the limited tailwater depths and bypassing most of the thrust blocks

which resulted in the unsteady wave, causing the end wall spillover. When the chute blocks

were installed, the hydraulic jump occurred within the center of thestilling basin and on top of

the thrust blocks because with the increase of the air entrainment, a higher water depth was

achieved much faster in the flow profile. When this situation occurred, the hydraulic jump

became steady and did not move.

From the scale model flow test, it was also shown that about ¼ of the open portion of

the spillway was not being used for discharging flow out of the stilling basin, (Figure#9).

Because this is a critical item needed to be known for integration into the HECRAS computer

modeling for lateral weir flows,the distance was measured from the model and scaled up to be

included in the HECRAS model. With this information, adjustments were made to the

HECRAS model reflecting the location at where the lateral weir flows would be utilized. The

HECRAS modeling was re-run and good correlation was derived between the computer

modeling and the scale model testing. With multiple computer runs, a convergence of the

different computer modeling runs was achieved and a robust, stable model was developed,

(Figure#10).

Figure #8,Chute block location

Figure#9, Model Flow Test

Figure#10, HECRAS Profile of stilling basin

Inclusion of Air Entrainment on the flow regime shown from Scale

Model Testing

Some additional design items which need to be mentioned were shown from the scale

model testing. With the inclusion of the chute blocks, which allowed the flow to be split as it

was entering the stilling basin, a higher volume of air entrainment occurred. With this higher

volume of air entrainment, much more resistance was introduced to the flow regime, which

allowed the thrust blocks to become much more efficient at reducing the energy levels. When

the chute blocks were not included, the high velocity flow extended through the basin creating

a higher velocity flow near the bottom of the flow profile and a lower velocity at the top. This

resulted in a wave forming would fall back onto itself allowing for an unstable water surface,

and caused the run over of the end wall. By the inclusion of the higher volume of air

entrainment, the flow regime appeared to slow at a more uniform pace through the flow profile

and did not create the wave which resulted in a very stable water elevation at the end of the

basin.

The higher air entrainment also allowed the hydraulic jump to occur in a more

predictable and stable location. Without the air entrainment, the resulting “back falling” wave

would cause the hydraulic jump to move within the stilling basin based on the unsteady

resistance of the wave. The flow test also revealed that a very stable flow condition appeared

along the back wall of the stilling basin. This stable flow area showed very little turbulence and

air entrainment. This appears to be a result of a lower energy state with the flow profile. With

this lower energy state, the water becomes denser in relation to the highly air entrained areas

and therefore would not allow the air entrainment to enter this area, creating a cushion along

the stilling basin’s end wall. From the results of the multiple flow tests, this condition built up

through the increasing flow rates and was repeatable with each test. Based on the scale model

testing, it revealed that the side discharge, St. Anthony’s stilling basin, having the ability to

introduce high levels of air entrainment, allowed quicker and more efficient energy dissipation,

without the high levels of tailwater elevation necessary by the normal stilling basin

design,(Figure #11).This exiting flow condition would also have to extend downstream into the

design of the plunge pool area beyond the stilling basin. Because the flow would enter the

plunge pool area at different rates, the plunge pool would then have to be designed to address

this condition where the majority of the flow would be at the back half of the stilling basin.

Figure #11

Design Nomagrapghs

A design nomagraph was developed utilizing the information derived from the HECRAS

computer modeling on this project. This graph gives a percentage of flow versus the

percentage of side opening lengthto the total opening length, starting from the chute drop or

the opening starting point. This relationship would be used to prepare a computer model of this

type of spillway system, after the stilling basin was initially sized by the normal St. Anthony’s

Falls basin design. We note that this nomagraph,is based on the design guidelines which

were used in this design. Any changes to the design items would most likely cause

adjustments to the exiting flows and would have to be tailored to the specific design items.

(Figure #12).

Figure #12

Summary

Flood events of 2009 determined that the changes in land use within the drainage basin of an existing dam resulted in partial failure of the structure. Because the dam was located near the property line, design alternatives for the spillway and stilling basin were extremely limited. The resulting design included a concrete chute and a St. Anthony’s Falls stilling basin with side discharge. Working within the limited site footprint, this innovative design was developed to accommodate site constraints in the absence of established design methodologies. Evaluation of flow characteristics was conducted using the HECRAS computer model and subsequently adjusted and validated through us of physical model testing. Based on this data, a unique design nomograph was developed for use on this and other projects with similar chute and stilling basin characteristics.