Salem
Water Works — Multiple Water Resources & Facilities
Lake Salinda Dam Improvements
The City of Salem owns and operates the
Lake Salinda Dam and the 70-acre lake it impounds for
water supply and recreation purposes. Lake Salinda is
capable of producing one million gallons a day of drinking
water, and is a critical component of the community as
well as the surrounding Washington County area. Construction
of the 40- foot earthen embankment and concrete spillway
were originally completed in 1947.
The dam gradually deteriorated over the
decades and was in desperate need of repair when a 20-foot
portion of the concrete ogee-weir spillway failed on
May 27, 2004 following several days of severe rainfall.
Given the magnitude and urgency of the
project facing the City, The City of Salem quickly appointed Earth
Tech, a distinguihed consulting, engineering and
construction corporation that serves the water / wastewater,
transportation, environmental, and facilities markets,
to oversee and manage this major undertaking.
An overview summarizing the Lake Salinda
Improvements follows.
ROLE OF EARTH TECH IN THE PROJECT
- Dam Inspection
- Hydrologic and hydraulic study
- Design of dam improvements including a labyrinth
spillway, embankment stabilization berm, and self-priming
drawdown siphon
ROLE OF OTHER CONSULTANTS IN
THE PROJECT
- Aerial Photography: Lobo Aerial Surveys, Inc.
- Geotechnical Engineer: ATC Associates
- Construction Engineer: Saegesser Engineering
- Contractor: Temple and Temple Excavating
BRIEF DESCRIPTION OF EARTH TECH'S
CONTRIBUTION TO THE PROJECT
At the City’s request, Earth Tech
visited the site the next day to assess the condition
and provide recommendations for emergency stabilization
of the spillway. Based on the recommendations, the City
took immediate action to stabilize the failed portion
of the spillway with a rock buttress and to lower the
reservoir elevation with a temporary drawdown siphon.
A local contractor began work on these recommendations
within days.
Sustainable Design.
Previous inspections of the dam before
the failure had noted several deficiencies in the embankment,
intake tower, and spillway that were not directly related
to the failure. After the emergency stabilization and
drawdown were completed, Earth Tech and the City discussed
the need for a long term solution as opposed to a repair
that would target only the failure.
Earth Tech performed a hydrologic and
hydraulic study as the first step, and designed dam embankment
and spillway system improvements. The hydrologic and
hydraulic study revealed that the existing spillway system
had a peak discharge capacity of 8,400 cubic feet per
second (cf/s) meaning it could only pass 43 percent of
the Probable Maximum Precipitation (PMP) before overtopping
of the dam embankment would occur.
IDNR requirements for high hazard dams,
such as this dam, require dams to be capable of passing
the full PMP before overtopping of the dam embankment
occurs. To meet the full PMP design, the spillway system
would have to be improved to pass a peak discharge of
more than 30,000 cf/s, more than three times the existing
discharge capacity. Because of this disparity, increasing
the overall spillway system capacity was considered a
key objective for permanent improvements to the dam.
Economic Considerations.
The hydrologic and hydraulic study determined
that making all of the improvements necessary to bring
the dam into full compliance with IDNR requirements would
cost more than $1.7 million. The City could not afford
the full cost in the short term. Postponing the improvements
until the City could raise all of the funds was not an
option due to the risk associated with leaving the spillway
in its failed condition longer than necessary.
Earth Tech worked with the City and the
IDNR to achieve a reasonable compromise between improving
the condition of the dam and staying within the City’s
budget constraints. It was agreed that the spillway capacity
should at least be improved to pass 50 percent of the
PMP in the short term with future improvements bringing
the dam into full compliance for high hazard dams. Earth
Tech split the improvements into two phases and prepared
a bid package for the first phase that focused on replacing
the failed spillway and providing additional spillway
capacity. Earth Tech also included several smaller alternate
items in the bid that the City could add or delete depending
on the contractor’s cost and the City’s available
funding.
Complexity
Site Conditions.
The geology and topography of the area
around the dam as well as the condition of the dam itself
created complexity in providing an economical solution
that would address the failed spillway and the lack of
spillway capacity. Site conditions precluded the construction
of an auxiliary spillway (the most common approach for
increasing spillway capacity) through either abutment.
The presence of rock at each abutment
and steep topography made construction of an auxiliary
spillway or widening of the existing spillway infeasible
due to the cost of excavating such a large volume of
rock. The complexity of the site conditions led Earth
Tech to conclude that improved spillway capacity would
have to be achieved without widening the existing spillway
channel.
Constructability.
The ability to construct a replacement spillway was
challenging because of the need to be able to control
the lake level and divert flow around the structure for
at least several weeks while the existing spillway was
demolished and the replacement was constructed. The temporary
siphon that was installed immediately after the spillway
failure was only capable of lowering the lake one to
two inches per day during dry weather. Due to nonfunctioning
of the existing drawdown gates, the City and the IDNR
expressed the need for a replacement drawdown mechanism
that could be used in the future for both routine maintenance
and emergency purposes. The objective was to address
both constructability requirements and permanent drawdown
needs.
Originality or Innovative Application of New or Existing
Techniques
Innovative Spillway
Earth Tech’s design solution was to replace the
ogee weir spillway with an innovative labyrinth weir
spillway. The labyrinth weir increases the effective
length of the spillway by creating a series of “W” shaped
weirs in plan view. The existing 150-foot wide spillway
channel was replaced with 780 feet of labyrinth weir
length.
The result is a 35 percent increase in spillway peak
discharge capacity compared to the original ogee weir.
The dam is the first and only application of a labyrinth
weir spillway in Indiana.
This spillway has greatly improved the safety of the
dam by reducing the probability that the dam embankment
will be overtopped. The new spillway system is able to
pass 62 percent of the PMP before dam overtopping occurs.
Future Phase 2 improvements will allow the spillway system
to pass the full PMP design storm, but the labyrinth
weir spillway was the first critical step in achieving
full compliance. Drawdown System Utilizes Existing Intake
Tower. A siphon designed and constructed through the
existing water supply intake tower was used to control
the lake level during spillway construction and for maintenance.
The intake tower houses water supply pumps, which supply
water to the onsite treatment plant, but are not capable
of lowering the reservoir at an acceptable rate.
The siphon is capable of discharging flow at a rate
of approximately 25 cf/s which translates to a reservoir
drawdown rate of approximately one foot per day under
dry flow conditions. The drawdown rate provided by the
siphon was sufficient to allow the contractor to quickly
lower the lake well below the elevation of the construction
limits, thus providing sufficient flood storage capacity
and time to reconstruct the spillway.
Future Value to the Engineering Profession and Perception
by Public
Demonstration of New Technology.
This dam is by no means the only structure that could
benefit from construction of a labyrinth weir spillway.
Many other structures throughout the state and country
face similar situations where the spillway capacity is
undersized and yet it is not feasible to widen the spillway.
The labyrinth design has paved the way for other consultants
and dam owners to implement this new approach to provide
an economical solution to increasing spillway capacity
and creating safer dams.
Safety.
The Salinda spillway improvements have greatly reduced
the potential that the dam could be overtopped during
an extreme flood event. The integrity of the spillway
structure has also been dramatically improved, minimizing
the likelihood of a repeat spillway failure.
Additional safety improvement to the dam included the
installation of a stabilization berm and toe drain along
the downstream slope of the dam embankment to address
the embankment’s insufficient factor of safety
against sliding. The improvements to the dam and spillway
provide tangible safety benefits as well as improving
public perception of the dam’s safety.
Exceeding Client/Owner Needs
Earth Tech exceeded the needs of the City by responding
quickly to an emergency situation, listening to the client’s
needs, and providing an innovative solution to a complicated
problem. Earth Tech was on site within hours of hearing
from the City about the spillway failure. Earth Tech
then continued this responsiveness by accelerating the
planning, design, and permitting processes. Several meetings
were held with the City’s key stakeholders to listen
to their needs. Many smaller maintenance and safety items
were incorporated into the design which resulted in a
lower total cost than if the smaller items had been completed
individually. Earth Tech exceeded the expectations of
the City by providing a cutting edge solution for a difficult
site condition. Lake Salinda Dam Improvements.
| P HOTOGR A P H S - Click
Image to View Larger Version |
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Downstream face of
original ogee spillway |
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Downstream face of
ogee spillway after failure on May 27, 2004 |
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Downstream face of
ogee spillway after emergency repairs |
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Spillway area after
demolition of original ogee spillway |
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New labyrinth spillway
during placement of rebar for spillway apron |
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Downstream face of
new labyrinth spillway |
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Upstream face of new
labyrinth spillway |
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Self-priming siphon
pipe through existing lake intake tower |
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New stabilization berm
on downstream slope of dam embankment |
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