Scour Analysis and Countermeasure Design

Transcription

1 Scour Analysis and Countermeasure Design for UNION STREET RAILROAD BRIDGE (Trail Connection Project) Key No ODOT, Region 2 Local Agency On-Call ATA WOC 1 City of Salem, Urban Development HDR Project No : October, SW 5 th Ave., Suite 1800 Portland, OR Phone Fax

2 Table of Contents Introduction... 1 Site Description... 2 Field Observations... 2 Previous Studies... 2 Design Criteria... 3 Hydrology... 3 Hydraulics... 3 Model Review... 3 Model Refinement... 3 Hydraulic Analysis... 4 Scour... 5 Scour Protection... 6 Riprap Sizing... 7 Riprap Blanket Dimensions... 7 Permitting... 8 Conclusion... 8 References... 9 Appendix A HEC-RAS Output Appendix B FEMA Flood Insurance Study Map... 19

3 List of Figures Figure 1. General project area Figure 2. Existing Union Street Railroad drawings, bridge profile Figure 3. Existing Union Street Street Railroad drawings, pier detail Figure 4. the "Christmas Flood" of Figure 5. Aerial view of the 1964 Flood on the Willamette River, looking downstream Figure 6. in foreground, looking upstream towards the Marion and Center Street Bridges Figure 7. Cross-sectional comparison of USACE HEC-RAS model and Underwater Inspection Report measured pier depths Figure 8. HEC-RAS Screen Capture Figure 9. Approximate location of the model cross-sections List of Tables Table 1. Design Flows for the Union Street Bridge Hydraulic Analysis..3 Table 2. Union Street Bridge Existing Conditions...5

4 Introduction This report describes the approach and presents the results of the hydraulic and scour evaluation of the in Salem Oregon. The bridge, which crosses the Willamette River in downtown Salem, is to be converted into a multi-use path as a part of a joint City of Salem/ODOT Railto-Trails project. The ultimate goal of these analyses is to determine if scour countermeasure are required and provide appropriate, site specific recommendations as needed. The existing bridge, built in 1912, is composed of a five (5) span truss section crossing the river and a timber trestle section which crosses through Wallace Marine Park on the west side. The topics addressed in this report include: General site description Regulatory standards and design criteria Site hydrology and historic flooding Bridge hydraulic analysis Scour evaluation Countermeasure design UnionStreetBridgeScourReport_Final Page 1 of 19

5 Site Description The is located along NE Union Street in Salem, Oregon. The bridge crosses the Willamette River downstream of Minto-Brown Island and the Marion and Center Street Bridges that carry Highway 22 east and west, respectively (Figure 1). The bridge itself consists of five major piers within the bankfull channel and a trestle-style abutment on the lesser sloped western bank. The westernmost piers, labeled in the original drawings as Piers 1 and 2, stand in a gravel bar. Piers 3 through 5 lie in the main river channel with Pier 5 approximately centered in the deepest part of the river channel (Figures 2 and 3). The eastern abutment is concrete and sits below the intersection of SE Water Street and Front Street near Marion Square Park. The bridge and piers are perpendicular to the river channel. Union Street Bridge is situated in a 4,000 foot wide floodplain between historic downtown Salem, Oregon and West Salem. Wallace Park, which is located on the west bank, makes up a large part of the floodplain. During extreme flood events, 1964 being the most recent (Figure 4), the floodplain may have been submerged to an elevation of 145 feet (NGVD) or greater according to both the updated US Army Corps of Engineers (USACE) HEC-RAS model used for this scour analysis and the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) of Salem, Oregon (Appendix B). Figure 5 shows the floodplain during the 1964 flood, a 200-year event, inundated well into West Salem. Field Observations The bridge was observed on September 2, 2005 and the river flow rate was approximately 7,120 cfs according to the local USGS gage (# ). The tops of footings of all five piers were exposed at this water level. On the date of the field reconnaissance, the footing of Pier 1 was exposed with the original framing timbers still in place. The pier was sitting in a slack water pool in the gravel bar with several driftwood trees snagged on the pier and adjacent timber trestle-style bents. Pier 2 was also on the gravel bar, except that a semi-circular depression approximately the size of the footing has scoured out on the west side of the pier to a depth of roughly three feet. Debris has collected in front of Pier 2 as well, although it was much less than on Pier 1. Piers 3 and 4 were in the middle of the river channel at this water level and had no sign of collision or debris collection. Pier 5 showed signs of a previous collision approximately four feet above the current water level. Figure 6 shows the bridge on the date of the field reconnaissance looking upstream. In Wallace Park near the boat ramp on the west bank of the river, a concrete/riprap scour-control measure was constructed there most likely to protect the picnic area from scour. A High water line was also noted during the site visit. The eastern bank is much steeper than the western side and there are signs of previous riprap and concrete dumping on this portion of the bank. Previous Studies An underwater inspection report, dated April 2004, was reviewed for pertinent information at Piers 3, 4 and 5 which are inundated essentially year-round. According to the report, riprap was installed as bridge construction finished and consists of 2 to 6 foot diameter rock along with dumped railroad steel ; this scour protection measure appears to collect woody and other debris. Riprap surrounds the entire footing on Pier 5, with a higher concentration of larger rocks on the downstream side. Pier 4 has loose riprap with a steep slope (possibly 1:1) on the upstream face, but no riprap on the downstream face. Pier 3 has a pile of riprap on the southwest corner that tapers out at the southeast and northwest corners. Piers 2 and 1 have no riprap since their footings are embedded in the gravel bar that stretches to the west bank. The report also describes the surface of the streambed as containing both sand and gravelly sand. UnionStreetBridgeScourReport_Final Page 2 of 19

6 Design Criteria According to ODOT guidelines (Lulay, Tom. Memo: 1999), scour analyses at bridges are to utilize 500- year flows as design criteria as it is required that bridge structures remain stable through such events. Additional design criteria used in this analysis include average channel velocity to calculate pier scour, the evaluation of the dominant scour types: contraction and pier, and evaluation of scour countermeasures using HEC-11 Design of Riprap Revetments. Hydrology The US Army Corps of Engineers (USACE), in corporation with the Federal Emergency Management Agency (FEMA), prepared a Flood Insurance Study (FIS) for the Willamette River in Salem, Oregon in The FIS presents streamflow estimates at various return periods for the Willamette River at Salem. It is important to note that these estimates represent regulated flow rates in the river. That is, the USACE owns and operates a number of large flood-control reservoirs in the Willamette River basin above Salem that regulate flows in the lower reaches of the river. It is this regulation, and the operating rules that govern how flows are released during periods of flooding, that has been incorporated into the statistical streamflow estimates summarized below in Table 1. Table 1. Design Flows for the Union Street Bridge Hydraulic Analysis Average Return Interval 10-yr 50-yr 100-yr 500-yr Willamette River, at Salem, Design Flows (cfs) 190, , , ,000 Source: Federal Emergency Management Agency. Flood Insurance Study, Marion County, Oregon, and Incorporated Areas CV001 Hydraulics The purpose of this analysis is to investigate the scour potential of the as it currently exists. This scour analysis was analyzed using the US Army Corps of Engineers (USACE) HEC-RAS model (Version 3.1.3). Model input was developed using the original Marion County Flood Insurance Study as a baseline. The model itself was supplied by the USACE Portland District. This section details the status of the model as supplied and alterations made to perform the scour analysis. Model Review The baseline model used in this analysis was in the original HEC-2 format and was provided by Bruce Duffe from the United States Army Corps of Engineers on August 24, This model was used for previous Marion County, Oregon, Flood Insurance Studies (FIS) and was originally created in Discharges were revised from 1969 values to 1982 values and Manning s n values were updated in As provided, the model did not contain any bridges because it was assumed that they would not obstruct flow during high water events (Marion County, Oregon and Incorporated Areas FIS 2003, 41047CV001, page 149). The model contains the 2-, 10-, 50-, 100- and 500-year frequency flood profiles plus the profile of the December 1964 flood. The model extended from river mile 77.0 to 89.6, totaling 12.6 miles in length. Model Refinement To perform the scour analyses, it was necessary to first convert the model to HEC-RAS and update the model geometry to include the Union Street Bridge and the surrounding Highway 22 bridges. The Union Street Bridge information was added using a combination of the original as-built drawings (provided by ODOT) and information regarding water levels at each pier from the recent underwater inspection. The UnionStreetBridgeScourReport_Final Page 3 of 19

7 depths recorded in the underwater study corresponded well with the pier depths from the original drawings. The United States Coast Guard (USCG) also completed a sounding of cross-sections of the Willamette River up- and downstream of the Union Street Bridge in This data, along with the underwater survey measurements and the as-built drawing section, was plotted verses the USACE crosssections from the HEC-RAS model. The net result of this process indicated that the USACE model crosssections still provide a good estimate of the channel shape in the immediate project area (Figure 7). In addition to the cross-section geometry, the bridge itself was added to the model. Each of the five major piers consists of a rectangular footing, 15 by 35 ft, and a rounded nose pillar that tapers from the footing up to the steel structure. The pier systems were modeled as a round nose pier 15 ft wide from the riverbed to the top of each footing (between 111 and 115 ft (NGVD) and 8.5 ft wide up to the low chord elevation, 153 ft (NGVD). The wood bents on the 1,000 foot timber trestle section of the bridge (west bank) were modeled as groups of cylinders since four bents exist at each pier. The bents are spaced 15 ft on center starting at a station of 2695 and heading west. The trestle superstructure is 19 ft wide (out-to-out) with 15 ft for the train or future multi-use path and 36.5 ft tall (out-to-out) with an interior height of 24 ft. For the Marion Street Bridge revised 1980 drawings were used to add bridge geometry and for the Center Street Bridge site photos were used to approximate the pier configuration. A screen capture of the HEC-RAS model at the Union Street Bridge is shown in Figure 8. In order to more accurately model the channel hydraulics in and around the bridges, it was necessary to add additional cross-sections into the model. A total of 9 sections were added in the project area so that at least four sections (two on either side) were present for each bridge with additional cross-sections added in regions specified by the model to increase efficiency. All new cross-sections were produced using the built in cross-section interpolation function in HEC-RAS. The approximate locations of the crosssections are shown in Figure 9. Channel roughness values (Manning s n ) are consistent with the previous FEMA studies and verified by field inspection and aerial photography. The channel and overbank roughness factors varied from 0.03 to 0.1. The contraction and expansion coefficients were also revised from the original model to include the newly added bridges. Lastly, ineffective flow areas were added to the model in the vicinity of the three bridges as per the HEC-RAS documentation. Hydraulic Analysis A subcritical, steady-state flow regime was selected to model the Willamette River at the Union Street Bridge. The model contains the steady-state flow rates for the 10-, 50-, 100-, and 500-yr events as given in the FEMA FIS plus the 2-yr storm, the 1964 Christmas Flood (200-yr event) and the flow on the day of the site visit for a reference while building the bridges in the model. The results of the hydraulic analysis are summarized in Table 2. These results represent the 500-yr flood event, which is recommended for calculating scour by ODOT (Lulay, Tom. Memo: 1999). UnionStreetBridgeScourReport_Final Page 4 of 19

8 Table 2. Union Street Bridge Existing Conditions 500-yr Event Total Discharge (cfs) 363,000 Recurrence Interval (yrs) 500 High Water Elevation at Upstream Face of Bridge (ft) High Water Elevation at Downstream Face of Bridge (ft) High Water Elevation at 250 ft Downstream of Bridge (ft) Average Velocity at Upstream Face of Bridge (ft/s) Average Velocity at Downstream Face of Bridge (ft/s) Average Velocity at 250 ft Downstream of Bridge (ft/s) Channel Depth at Upstream Face of Bridge (ft) Channel Depth at Downstream Face of Bridge (ft) Channel Depth at 250 ft Downstream of Bridge (ft) Scour Based on field observations and survey data, no evidence of significant channel or bridge-induced scour was apparent at the existing bridge structure. The extent of long-term aggradation or degradation at the project site is unknown. According to the underwater inspection report, the riprap appeared to have been placed soon after the bridge was constructed has remained fairly static and has been an adequate amount of protection although the diver concluded that the riprap was minimal in amount, crudely placed, and ranged in diameter from 2 to 6 feet. Scour for the Union Street Bridge was calculated using the HEC-RAS model based on FHWA 2001 (HEC-18) criteria/methods. The equations utilized by the model are expounded upon in this section. As per ODOT requirements, scour depth were calculated using the 500-yr event, which correspond to a flow of 363,000 cfs, and normal depth as the downstream boundary condition. The model output is tabularized in Appendix A. Bed Material Bed material in the vicinity of the project area was assumed to be very coarse gravel with a D 50 and D 90 of 45.3 and 60.0 mm respectively. This bed material was specified on both the original Union Street Bridge drawings and revised Marion Street Bridge drawings with test hole details; the corresponding grain size distributions were estimated from the HEC-RAS users manual (Table 12-6, chapter 12). The bank material was approximated as a coarse silt with a D 50 and D 90 of and 0.06 mm respectively using the same table. Pier Scour Pier scour was calculated assuming 5 reinforced concrete piers with a width of 15 feet plus the 1,000-foot timber trestle section which was modeled as grouped cylinders 1 foot wide and spaced 15 feet on center (Figure 8). A maximum pier scour depth of 12.9 feet at the main piers and 5.3 feet for the timber bents was estimated from the bridge pier geometry using the Colorado State University (CSU) equation (FHWA, 2001): UnionStreetBridgeScourReport_Final Page 5 of 19

9 y s = K K K K Fr y1 y1 a where K 1 accounts for pier shape (1.0 for round nose), K 2 accounts for flow angle (1.0 for no skew), K 3 accounts for the bed condition (1.1 for clear-water), K 4 accounts for bed armoring (0.42 when the D 95 is 60.0 mm), a is the pier thickness (1.0 ft for the timber bents, 15 ft for the main piers), y 1 is the maximum flow depth at the upstream face of the bridge (48 ft), and Fr is the Froude number at the upstream face of the bridge (0.33). Contraction Scour Contraction scour was calculated as clear water scour. The clear water approach was determined by using Laursen s equation (FHWA, 2001): ( y ) ( )3 V c = D 1 50 where y 1 is the depth of the approach channel section (37 ft), D 50 is the median diameter of bed material (45.3 mm, ft) and V c is the critical velocity for incipient motion of the bed material in the approach section (10.8 ft/s). Since the pier approach velocity (10.5 ft/s) is less than the critical velocity (10.8 ft/s), the clear water scour equation was used to compute contraction scour. A contraction scour depth of 1.1 feet was estimated using the clear water contraction scour equation (FHWA, 2001): y 2 = CD 2 Q2 2 3 m W where y 2 is the average depth in the contracted section (35.6 ft), Q 2 is the flow within the contracted channel (363,000 cfs), C is a constant (130 for Standard units), W 2 is the bottom width of the contracted channel excluding the pier width (864 ft) and D m is the diameter of the smallest non-transportable particle (1.25 * D 50, ft). The contraction scour (y s ) is then y 2 y 0 where y 0 is the existing depth at the downstream cross-section (34.5 ft) and y s is 1.1 feet. Total Scour Total scour depth was calculated at the proposed bridge crossing by summing the contraction scour depth with the pier scour. Output from the HEC-RAS model indicates a total scour depth of approximately 14 feet at the main piers and 6.4 feet at the timber trestle. Scour Protection A HEC-11 (FHWA 1989) analysis was conducted to examine the size of riprap that could be used to protect the Union Street Bridge piers if revetment protection were to be selected. Given the location of the five primary piers in the Willamette River and expected average channel velocities of up to 11 ft/s, UnionStreetBridgeScourReport_Final Page 6 of 19

10 other less-traditional pier protection measures would not likely be feasible or economical. ODOT guidelines require that bridge structures be stable during 500-yr events. Furthermore, the Highway 22 bridge piers located just upstream of Union Street Bridge each use riprap as scour protection measures. As-built drawings indicate that the middle piers in the main channel have riprap blankets of 100 feet by 130 feet and 3 feet deep (1450 yd 3 per pier) of Class 700 Standard. HEC-RAS model results for these bridges estimate an average channel velocity of 9.6 ft/s. Riprap Sizing An evaluation of the appropriate riprap class for the Union Street Bridge was completed assuming ODOT and HEC-11 criteria using the 500-yr flood and the following equation for sizing riprap on bridge piers in fresh water: D V = 2 ( S 1)2 g where D 50 is the median riprap diameter (1.8 feet), V is the expected channel velocity plus safety factor (11 ft/s x 1.5), S is the specific gravity of the riprap material (2.65), and g is the force of gravity (32 ft/s 2 ). The above calculations estimate that the median diameter of riprap for the Union Street Bridge be 1.8 feet (22 inches) or, according to the ODOT riprap classification, Class 2000 Standard. This calculation is sensitive to changes in velocity. Consequently the 1.5 ft/s difference between the average channel velocities upstream of the Highway 22 bridges and Union Street Bridge is enough to changes the riprap class for the Union Street Bridge to Class Riprap Blanket Dimensions The HEC-18 manual suggests that the top width of the scour hole be calculated using the following equation for which W, the calculated top width (30 ft), is equal to 2 times y S, the scour depth (15 ft). W = 2y s The HEC-11 manual suggests that riprap be placed no less than 1.5 x D 95 in thickness. For the Union Street Bridge this results in a 30 foot radius, 4 feet deep, riprap blanket surrounding each pier, totaling 980 yd 3 for each of the five primary piers. For the timber bents, the estimated scour is 6.5 feet which results in Class 50 Standard riprap in a continuous blanket 1 foot thick. There may be other more ecologically appropriate scour countermeasures such as the placement of woody debris or geotextiles. Specific design of these countermeasures would need to occur in parallel with the permitting process. Discussion The riprap sizing presented to this point has been designed as required by ODOT to maintain bridge stability through the 500-year event. To date there is little evidence of scour at the bridge piers. Additionally, the flood of 1996, which had an approximate return period of 50 years, does not appear to have created any long-term scour or undermining of the existing structure, although scour holes may have developed during the crest of the flood and filled in later as the river levels receded. That said, extreme flood events that may cause scour or failure of the existing bridge cannot be predicted. Consequently, the long term scour protection solution should reflect an acceptable level of risk; the 500- year event according to ODOT. And to meet this criterion, placement of appropriately sized riprap is the UnionStreetBridgeScourReport_Final Page 7 of 19

11 recommended approach. In the interim, if the city chooses not to place riprap, an effective way to evaluate the immediate need for additional scour protection could be to monitor the condition after high flow events. If significant scour is observed, either via scour holes or missing riprap, immediate placement of new riprap is recommended. Permitting Permitting for scour countermeasures has been addressed in a separate memorandum (Regulatory Compliance Technical Memorandum) and is summarized as follows: A [Federal Rivers and Harbors Act of 1899,] Section 10 permit from the Corps is required for any construction in or over navigable waters of the United States which could affect the course, location, condition or capacity of these waters. This includes any excavation or dredging in the Willamette River, placement of rip rap, or construction and repair of piers and pilings. The permit is issued by the Corps, based on information provided in the CWA Section 404/Removal Fill joint application. Conclusion Due to the velocities expected in the Willamette River during a 500-yr event (11 ft/s), addition of riprap should be considered as a scour countermeasure on the main bridge piers (Piers 1 through 5). Current insitu countermeasures will likely provide adequate protection during lesser flood events, however during the design conditions (500-yr) they may not be adequate. Consequently, it is recommended that new riprap with a median diameter of 22 inches (Class 2000) be placed in a 30-foot radius surrounding the five major piers, to a depth of 4 feet. UnionStreetBridgeScourReport_Final Page 8 of 19

12 References U.S. Army Corps of Engineers (USACE). HEC-RAS Hydraulic Reference Manual, Version 3.1, January Federal Emergency Management Agency (FEMA). Flood Insurance Study, Marion County and Incorporated Areas, Oregon. Community Number Revised January 2, Federal Highway Administration (FHWA). Design of Riprap Revetment. Hydraulic Engineering Circular No. 11, Fourth Edition. March Evaluating Scour at Bridges. Hydraulic Engineering Circular No. 18, Fourth Edition. May Oregon Department of Transportation (ODOT). Hydraulics Manual, Hydraulics Unit. Hydraulic Design and Report Writing for Projects Designed by Consultants. June Scour Guidelines. Natural Resource Conservation Service (NRCS). Soil Survey for Marion County, Oregon, accessed September 10, 2005, OBEC Consulting Engineers. Willamette River Underwater Inspection Report for The City of Salem April Lulay, Tom. Memo: Use of Riprap as Embankment Armoring Material, December 22, Historic Photographs. Salem Public Libraby Collection, accessed September 21, 2005, Figure 4, photo ID: HSTC122; Figure 5, photo ID: SJ340; UnionStreetBridgeScourReport_Final Page 9 of 19

13 Figure 1. General project area UnionStreetBridgeScourReport_Final Page 10 of 19

14 Figure 2. Existing Union Street Railroad drawings, bridge profile Source: Waddell and Harrington Consulting Engineers, Kansas City, MO. File No , Sheet No. 12; 1912 Figure 3. Existing Union Street Street Railroad drawings, pier detail Source: Waddell and Harrington Consulting Engineers, Kansas City, MO. File No , Sheet No. 2; 1912 UnionStreetBridgeScourReport_Final Page 11 of 19

15 Figure 4. the "Christmas Flood" of 1964 (~308,000 cfs, ~200yr Flood) UnionStreetBridgeScourReport_Final Page 12 of 19

16 Figure 5. Aerial view of the 1964 Flood on the Willamette River, looking downstream UnionStreetBridgeScourReport_Final Page 13 of 19

17 Figure 6. in foreground, looking upstream towards the Marion and Center Street Bridges UnionStreetBridgeScourReport_Final Page 14 of 19

18 Profile Comparison Station USACE HEC-RAS Profile Underwater Inspection Report Profile Elevation (ft) Figure 7. Cross-sectional comparison of USACE HEC-RAS model and Underwater Inspection Report UnionStreetBridgeScourReport_Final Page 15 of 19

19 USRRB Existing conditions Plan: WithBridge 9/20/2005 Union Street Bridge Station (f t) Legend WS 500-y r Ground Ineff Bank Sta Elevation (ft) Figure 8. HEC-RAS Screen Capture UnionStreetBridgeScourReport_Final Page 16 of 19

20 Figure 9. Approximate location of the model cross-sections UnionStreetBridgeScourReport_Final Page 17 of 19

21 Appendix A HEC-RAS Output HEC-RAS Output - Bridge: Union Street RR Profile: 500-yr River Discharge Total Min Ch El W.S. Elev E.G. Elev E.G. Slope Avg. Channel Velocity Flow Area Top Width Channel Mile (cfs) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft) Froude # , ,373 8, , ,386 10, , ,443 12, , ,702 9, , ,454 9, , ,398 8, , ,849 8, , ,725 8, , ,477 8, , ,830 7, , ,526 7, , ,586 6, , ,135 6, , ,155 6, , ,856 6, , ,629 4, , ,928 5, , ,107 4, , ,370 3, , ,950 3, Center Street Bridge , ,312 3, , ,933 3, , ,777 3, Marion Street Bridge , ,916 3, , ,553 3, , ,965 3, Union Street Railroad 83.9 Bridge , ,540 3, , ,721 3, , ,127 3, , ,016 3, , ,144 7, , ,890 11, , ,309 11, , ,368 14, , ,712 11, , ,514 12, , ,092 7, , ,943 13, , ,254 13, , ,751 12, UnionStreetBridgeScourReport_Final Page 18 of 19 10/28/2005

22 Appendix B FEMA Flood Insurance Study Map UnionStreetBridgeScourReport_Final Page 19 of 19 10/28/2005