Excerpts from the Hydrotechnical Report for Second Peace Bridge

Buffalo and Fort Erie Public Bridge Authority
Peace Bridge Plaza
Buffalo, New York, U.S.A.
Fort Erie, Ontario, Canada

Excerpts from the
Hydrotechnical Report
for
Second Peace Bridge

November 1998
BT-3412

DELCAN

1.3 Proposed Bridge

The proposed bridge is to be located parallel to and immediately upstream of the existing bridge as shown on Exhibit 1.2. The new bridge will be a steel arch structure similar to the existing bridge with the span over Black Rock Canal also being a steel arch. The centreline of the new piers will be in line with the centreline of existing piers. The pier width (generally 9 m (30 ft) wide) is narrower than the existing piers as the ends of the new arches will be located closer together. The proposed pier length of approximately 37 m (120 ft) is similar to the existing piers. The upstream ends of the new piers will have a triangular shape, including a triangular shaped ice breaking wedge similar to the existing piers.

The separation between the bridge decks will be approximately 24 m. At the water level the separation between the upstream end of the existing ice breaking wedges and the downstream end of the new piers will be approximately 6 m.

5.1.1 Mitigative Measures

The simulation results in Section 4.7 indicate that the proposed bridge will increase the water level in the Niagara River by 26 mm at the Fort Erie/Buffalo gauges. As increased water levels could have an adverse impact on Lake Erie, measures to reduce the potential increase in water levels were investigated. The model was used to assess the increase in water level, relative to the existing condition, for the proposed bridge in conjunction with a variety of mitigative measures.

Each simulation incorporates one or more mitigative measures. The combination of measures used in each simulation and the resulting water level at the upstream end of the model are summarized in Table 5.1. The terms used to identify each mitigative measure are described in the following. Pier numbers are as per Exhibit 1.2.

Join Piers - connection of the proposed piers with the corresponding existing piers using a smooth transition.

The "river" piers refers to proposed piers 6, 7, and 8. The "bank" piers refers to proposed piers 5 and 9 located adjacent to the Canadian and American shorelines respectively. Joined river piers are shown in Exhibit 5.1.

Tail on Existing Piers - changing the downstream end of the existing piers from a semi circular shape to a triangular shape. The "angle" refers to the angle at the downstream end of the proposed tail. Three angles - 90, 60, and 45 degrees - have been used in the simulations. Exhibit 5.1 shows a combination of joined river piers with a triangular shaped tail.

Channel Excavation - removing bed material to improve the conveyance of the river channel. Three channel excavations were simulated:

remove mound located between proposed piers 6 & 7. Noted in table as "Mound 6/7". See Exhibit 5.2.
remove bed material adjacent to proposed pier 5. Noted in table as "Bed P5". See Exhibit 5.3.
deepen channel between proposed piers 6 & 7. Channel 0.5 m deep by 30 m wide by 100 m long. Noted in table as "Channel 6/7". See Exhibit 5.4.

By-Pass Culvert - construct culvert around bank pier on the Canadian side to increase "river" capacity. Culvert 10 m wide by 3 m high by 395 m long. See Exhibit 5.5.

Streamline Bank Piers - completely "streamlining" the existing and proposed bank piers by providing a smooth transition between the shoreline and the upstream and downstream ends of the piers. This includes joining the existing and proposed bank piers.

Variations on the streamlining of the bank piers were simulated using the individual streamlining components as follows:

join the upstream end of the proposed bank pier to the shoreline. This variation is noted in the table as "Upstream";
join the existing and proposed bank piers with a smooth transition. This appears under the column - Join Piers, Bank.

The above two components - partial streamlining of bank piers - are combined in Exhibit 5.6.

Reduce Existing Pier Width - the existing piers appear to be over designed and could be made narrower. The total reduction in the width of each pier - half of the total is taken from each side - that was used in the simulation is shown in the table. The reduction applies only to the three existing river piers. The bank piers were not changed.

5.1.2 Simulation Results

The simulation results indicate that the water level increases are due primarily to the convergence and divergence of flow around the piers rather than the area/volume displaced by the proposed piers. The gap between the proposed existing piers results in significant eddy losses which translates into an increase in the upstream water level.

The losses due to convergence and divergence are a function of velocity and therefore the greatest losses occur around the river piers (proposed 6, 7, and 8) where the velocity is the highest - 2.5 to 4.0 m/s. The velocity at the bank pier is generally less than 1.5 m/s and losses are therefore considerably lower at the bank piers.

The greatest mitigation is provided by streamlining the three river piers (Scenarios 7 and 8). Joining the three existing and proposed river piers and streamlining the downstream end of the existing river piers reduces the increase in water level by 23 mm; from an increase of 26 mm with no mitigation to an increase of 3 mm.

Due to the lower velocities along the shoreline, streamlining the bank piers only reduces the water level by an additional 3 mm (Scenario 15). While the additional decrease is quite small for the work involved, streamlining of all the existing and proposed piers - both river and bank piers - will effectively eliminate any impact due to the Second Peace Bridge.

Changing the angle of the tail on the downstream end of the existing piers can further reduce the impact on the upstream water level. Changing the angle from 60º (Scenario 15) to 45º (Scenario 17) results in a 1 mm change in the water level and a water level that is 1 mm below existing conditions.

The key advantage of changing the angle of tail on the downstream end of the existing piers is that with the additional 1 mm reduction in water level, only partial streamlining of the bank piers (join bank piers and connect upstream end to the shore) is required to maintain a zero impact on the upstream water level (Scenario 16). The partial streamlining leaves the area downstream of the existing piers in an open/existing condition and reduces the impact on fish passage at the bank piers as discussed in Section 5.2.

The model results with the joining and streamlining of the piers are consistent with experimental data on drag coefficients for various cylindrical shapes (Lindsey, 1938). Drag coefficients are used to estimate the force due to the water moving around the piers, the separation of the flow and the resulting wake that occurs downstream. The drag coefficient (C_D) for various pier shapes are as follows:

elongated piers with semi-circular ends 1.33
elliptical piers with 2:1 length to width 0.60
elliptical piers with 8:1 length to width 0.29
triangular nose with 30º angle 1.00
triangular nose with 90º angle 1.60

The upstream portion of the existing pier can be considered to be an elliptical pier with a 2.5:1 length to width ratio and a semi-circular downstream end. By joining the new and existing piers together the length to width ratio will improve to approximately 10:1. The existing drag coefficient will be reduced by streamlining the downstream end of the existing piers. The net result is an over drag coefficient (energy loss) for the new bridge that approximates the existing condition.

Modifying the channel bathymetry (Scenarios 1, 3, and 4) is an attempt to increase the channel cross section and conveyance in order to compensate for the loss in cross sectional area due to the piers and the energy losses around the piers. These channel modifications provide only a minimal reduction in the water level increases that would result from the proposed bridge. The simulation results confirm that energy losses due to flow around the proposed piers rather than a reduction in channel capacity are the primary cause of the increased water levels.

Similarly, providing additional capacity by constructing a bypass channel/culvert (Scenario 2) does not mitigate the potential impact on water levels.

Scenario 6 indicates that a combination of excavation and joining the piers could work together to lower the increase in water level. Although not specifically modelled, it is surmised that extensive channel excavation (in excess of Scenario 4) in conjunction with streamlining the river piers (Scenario 8) could be used to eliminate any increase in upstream water level. As the river bottom at the Peace Bridge is comprised of rock, blasting would be required to excavate/deepen the river channel. Due to the possibility of damage to the existing piers from the blasting and the associated environmental impacts, this alternative has been eliminated from further consideration.

Reducing the width of each of the existing river piers by 600 mm was included in Scenarios 12 and 13. The change in upstream water level from Scenario 8 to 12 is only a decrease of 1 mm. Reducing the width of the existing piers is therefore not a significant component in minimizing the water level impacts associated with the new bridge.

elongated piers with semi-circular ends 1.33
elliptical piers with 2:1 length to width 0.60
elliptical piers with 8:1 length to width 0.29
triangular nose with 30º angle 1.00
triangular nose with 90º angle 1.60

Similarly, providing additional capacity by constructing a bypass channel/culvert (Scenario 2) does not mitigate the potential impact on water levels.

5.4 Bridge Construction

The hydraulic model was used to assess the water level impacts during construction of the recommended layout for the piers and mitigative works as described in Section 5.3 and as shown on Exhibit 5.13.

There are various combinations of river and/or bank piers that could be constructed at any one time. Three basic construction phases were investigated as outlined below. These phases are based on 2 piers being constructed at the same time in Phase 1, an additional 2 piers in Phase 2, and the final pier constructed in Phase 3. The results from the various simulations are summarized in Table 5.3.

TABLE 5.3
CONSTRUCTION IMPACT SUMMARY

					Water Surface Elevation m	Increase mm
Existing conditions					174.342	--
Proposed Bridge - No Mitigation					174.368	26
Proposed Bridge - Full Mitigation					174.342	0
Construction Phase	Piers Under Construction		Piers Completed
Construction Phase	River	Bank	River	Bank
1	one	one	--	--	174.354	12
2	one	one	one	one	174.353	11
3	one	--	two	two	174.349	7

Cofferdams will be used to construct the new piers and to modify the downstream end of the existing river piers. The cofferdam will completely enclose the existing and proposed piers. At the bank piers, the walls connecting the existing and proposed bank piers and the shoreline will be constructed after the cofferdams have been removed.

The cofferdams will be located approximately 2 m beyond the edge of the existing pier. The total width will therefore be 4 m wider than the existing piers - an increase in pier width of approximately 35%. In order to minimize the hydraulic impacts the ends of the cofferdams will have a triangular shape to streamline the cofferdams as much as possible. A 90º angle has been used at the upstream end with a 45º angle on the downstream end of the river piers.

The recommended final - as constructed - configuration for the existing and proposed Peace Bridge eliminates any water level impacts due to the proposed bridge. However, during construction the water level will increase due to the additional width of the cofferdams. During Phases 1 and 2, there will be 2 piers under construction and the increase in water level will be approximately 12 mm. There will be only one pier under construction during Phase 3 and the increase will be 7 mm. After completion of the final river pier and removal of the cofferdam the water level will return to the existing condition.

It is anticipated that four of the piers - Phases 1 and 2 - would be constructed in the first construction season from June to December 1999. Due to the potential for ice flows to damage the cofferdams during the winter season and spring runoff the piers would need to be completed and the cofferdams removed from the river during that time.

The final pier - Phase 3 - would be constructed in 2000 after the ice has left Lake Erie and the Niagara River.

An increase in water level in the Niagara River upstream of the Peace Bridge will result in an increase in the water level of Lake Erie, however, due to the large surface area of Lake Erie the lake level would increase quite slowly. Using stage - discharge and surface area information for Lake Erie, the change in water level on Lake Erie during the construction period can be estimated. The monthly changes in water level associated with the above construction schedule are shown in Appendix H.

Two cofferdams in the river during Phases 1 and 2 would increase the Niagara River water level by 12 mm, however, at the end of the 7 month (June to December) construction period the increase in the water level on Lake Erie would be less than 10 mm. During the winter and spring periods the Niagara River would be back to existing conditions (no cofferdams) and the water level increase on Lake Erie would drop to approximately 3 mm.

Installation of the final cofferdam would increase the Niagara River water level by 7 mm and the Lake Erie water level would increase to approximately 5 mm. About 7 months after removal of the final cofferdam, the water level on Lake Erie would have returned to existing conditions (less than 1 mm increase in water level).

While the construction schedule may vary somewhat from the above, the calculations indicate that the temporary increase in water level on Lake Erie during the construction of the Second Peace Bridge will be less than the increase in the Niagara River water level. After construction is completed both the Niagara River and Lake Erie will return to existing conditions.

6.1 Conclusions

A SMS/FESWMS hydrodynamic model has been developed to simulate Niagara River water levels in the vicinity of the Peace Bridge. The model was calibrated using two sets of water level data from events in 1997. The validation using two sets of water level data from 1996 indicates that the model is suitable for the analysis of existing and proposed conditions in the Niagara River.

The hydraulic analysis indicates that the Second Peace Bridge will increase the water level in the Niagara River at the Fort Erie/Buffalo gauges by 26 mm. The increase in water level is primarily a result of the energy losses associated with the divergence and convergence of flow around the proposed and existing piers.

In order to maintain the existing water levels the following mitigative measures as shown in Exhibit 5.13 will be implemented:

join the proposed piers - both river and bank piers - with the corresponding existing pier. The joining is to provide a smooth transition between the existing and proposed piers and is to extend from the river bottom to above the water level;
modify the downstream end of the existing river piers (6, 7, and 8) from a semi-circular shape to a triangular shape. The angle at the downstream end of the tail to be 45º and
provide a smooth transition between the shoreline and the upstream end of the bank piers.

To maintain upstream fish passage along the shoreline the following features as shown in Exhibits 5.11 and 5.12 will be incorporated into the design of the new bridge piers and associated works:

provide a gap for fish passage between the upstream face of the existing bank piers and the wall connecting the bank piers together;
provide a gap for fish passage between the shoreline and the wall connecting the new pier to the upstream shoreline.

With the above mitigative measures, the Second Peace Bridge will not change the existing water levels in the Niagara River and upstream fish passage can continue similar to existing conditions.