A RACE AGAINST TIME: REPLACING A DAMAGED WTP INTAKE CRIB AT THE BOTTOM OF LAKE ONTARIO

Allan Choi, P.Eng. 1, Kimberly Sayers, M.A.Sc., PMP, P.Eng.1, Zoran Filinov, P.Eng. 1, Jim McGilton, P.Eng. 2

1R.V. Anderson Associates Limited, 2Regional Municipality of Durham

Intake Crib Design Alternatives

Evaluation

CriteriaWooden Crib Stainless Steel Crib Concrete Crib

Ease of

Constructability

Long fabrication time

Lightweight and easy to

transport, however must be

weighted or dragged to lake

bottom

Moderate fabrication time.

Steel structure is lightweight,

sinks by gravity.

Irregular shape requires

detailed precasting. Long

fabrication time.

Concrete structure is heavy.

Difficult to transport.

Susceptibility to

Damage

Medium

Boat anchors may catch on

timbers

Low

Smooth / circular surfaces

Low

Smooth surfaces. Heavy weight

prevent displacement

Thermal

Conductivity

Best resistance to frazil ice

formation

Least resistance to frazil ice

formation

Moderate resistance to frazil ice

formation

Construction

Cost

Medium

($140,000 est.)

High

($200,000 est.)

High

($200,000 est.)

Expected

Lifespan30 years 75 years 75 years

Environmental

Impacts

Medium

Pressure-treated wood may

be required

Low Low

Background Intake Design Marine Construction

Newcastle

< Oshawa

Lake Ontario

Newcastle Water Supply Plant

Intake Pipe

Intake Crib

HWY 401

Durham Region is the Owner and Operator of the Newcastle Water Supply Plant (WSP)

which services the communities of Newcastle and Newtonville in the Municipality of

Clarington. The plant uses low lift pumps to draw raw water from Lake Ontario through a

1.2 km long 600 mm diameter intake pipe. The current plant capacity is 8 MLD, and there

are plans to double this capacity in the future. The plant utilizes conventional filtration and

high lift pumps for pumping to the distribution system.

In May of 2014, Operations staff discovered that the wooden intake crib structure located

on the lake bottom was significantly damaged and moved approximately 6 m from its

original location. The raw water sample line and chlorination line for zebra mussel control

were also damaged at the intake. The cause of the accident is unknown.

Damaged wooden intake crib (above left). Unprotected raw water intake pipe (above right).

Durham Region retained R.V. Anderson Associates Limited to complete the design of a replacement intake crib

structure complete with chlorine feed, diffuser and raw water sampling lines, and to ultimately oversee the

construction of the crib replacement before the arrival of the winter season.

The design of the intake crib structure was based on recommendations provided by the Ministry of the

Environment and Climate Change Design Guidelines, and the Hydraulic Division of the American Society of

Civil Engineers (ASCE) guideline for Design of Water Intake Structures for Fish Protection (1981). The design

included considerations for crib maintenance in marine environments, risk of frazil ice formation, low entrance

velocities, and protection from damage due to boat anchors.

Three possible construction materials were evaluated for the structure, including wooden timbers, welded

stainless steel, and pre-cast reinforced concrete.

Based on the evaluation of the available materials, the preferred intake crib

design for the replacement of the Newcastle WSP intake crib was a

stainless steel structure. This option provided the Region of Durham with a

long-lasting solution, and due to its easier constructability, it offered the

fastest fabrication time with the least impact on the construction schedule.

The image on the left shows the final design for the new stainless steel crib

structure, which is bolted on top of the existing concrete pad. Access doors

are provided on the side of the structure for maintenance access to the

chlorine diffuser and raw water sampling piping.

An expedited approval was obtained from Transport Canada, as this work

was considered emergency works under the Navigation Protection Act. Dean

Construction Co. Ltd. was selected to construct the works. A jack-up barge

was positioned on the lake (left), over the intake pipe opening, which provided

a stationary working platform, and allowed the use of a crane to lower the new

structure onto the lake bottom.

The fabrication of the stainless steel intake crib was completed in a shop and

the unit was stored near a dock. However, rough waters on Lake Ontario

throughout the month of November prevented construction crews from

reaching the intake location. In early December, weather conditions finally

allowed the new stainless steel intake crib and the sample line and chlorine

diffuser to be installed, providing Durham Region with a long-lasting solution

at the Newcastle Water Supply Plant, and restoring the security of the

municipality’s water supply

The new crib is pictured in the image below, shown immediately before it was

lowered onto the lake bottom. The water treatment plant was shutdown during

the installation of the intake crib, which occurred at night to minimize

disruptions to the municipality’s water supply.

Jackup barge with crane

(top), Hinged FRP bar

screens (right), Chlorine

diffuser piping accessible

through maintenance

doors (below).

Divers were employed to

perform the installation of

the crib structure at the

lake bottom, and to

remove the damaged crib

(left and above).