Transactions on Ecology and the Environment vol 19, 1998 WIT Press, ISSN

Transcription

1 Water surface profiles and scour analysis for a river reach with multiple bridge proposals C.C. Nuthmann & C.C. Chang Sverdrup Civil, Inc., Two Center Plaza, Boston, Massachusetts, USA, nuthmacc@sverdrup. com R.G. Murphy Massachusetts Highway Dept, Ten Park Plaza, Room 7133, Boston, Massachusetts, USA, rgmurphy@state.ma.us Abstract The Route 146 Project in the City of Worcester and the Town of Millbury, Massachusetts, USA. will replace 6.8 miles of two lane arterial roadway with a grade separated six lane freeway. The new highway parallels the course of the Blackstone River system. Crossing roadways and numerous interchange ramps impact the river and the floodplain. A total of fourteen new river bridges will be constructed, and eleven old river bridges will be demolished. Construction will be staged in phases over a period of eight years. Detailed hydrologic and hydraulic studies were performed to meet environmental regulations, and to provide data for use in bridge scour analysis. The project will require updating the Federal Emergency Management Agency (FEMA) Flood Insurance Studies (FIS) for the municipalities involved. The hydrology of the watershed was studied by several methods, and a generalized watershed model was developed using the U.S. Natural Resource Conservation Service (NRCS) TR-20 program. Water surface profiles were analyzed for the full length of the project using the U.S. Army Corps of Engineers (ACOE) HEC-2 computer model. Bridge scour analysis was performed with spreadsheets using equations given in the Federal Highway Administration (FHWA) Hydraulic Engineering Circular No. 18 (HEC-18), Evaluating Scour at Bridges [3]. The studies of the water surface profiles and bridge scour are summarized. Discussions focus on unique modeling applications, problems encountered, and lessons learned.

2 196 Hydraulic Engineering Software 1 Introduction The Route 146 Project is significant both due to the size and scope of the environmental impact and due to a historic change of approach in structural design. The environmental considerations involve existing and proposed flood elevations, floodplain storage volume, and peak flow rates. The structural design for bridge piers and abutments has been updated to include hydraulic scour depths predicted by new analytical techniques. Environmental concerns were addressed in the preliminary design phase, and were continually revisited as the design progressed. Local and regional impacts were considered due to individual project areas and due to the project as a whole. First, a hydrologic study (Howard, Needles, Tammen, and Bergendorf [1]) was performed in order to develop peak discharge rates for a range of return frequency flood events for use in a steady state, standard step hydraulic model (HEC-2), which covered the entire project area (Sverdrup/Parsons Brinckerhoff [2]). The hydraulic model was then used to develop proposed flood elevations and storage volumes within the project area, which were then input into a generalized watershed hydrologic model [2]. This hydrologic model was used to evaluate the project impact on flow rates downstream of the project area. Scour analysis was based on new equations and methods developed by the Federal Highway Administration [3], and on engineering experience and judgment. Input data was obtained from geotechnical records and existing bridge scour inspection reports along with information from individual hydraulic reports which were prepared specifically for final designs at each proposed bridge. Where bridge superstructures were proposed at elevations lower than design flood levels, a debris accumulation risk analysis was performed and design scour depths were adjusted accordingly. 2. Hydrology for HEC-2 Models Four different sources were evaluated in developing the design flows used in the HEC-2 models. The first source consisted of performing Log Pearson Type III statistical analysis on annual flows measured by stream gauges both upstream and downstream of the project site. The flows from the gauges were then extrapolated to the project site using regional equations developed by the United States Geologic Survey (USGS). The second source consisted strictly of USGS regional equations. The third

3 Hydraulic Engineering Software 197 source consisted of flows developed for the Federal Emergency Management Agency (FEMA) Flood Insurance Studies (FIS) for the two municipalities encompassing the project area. The FIS flows were synthesized by combining discharge peaks determined for four headwater tributaries to the Blackstone River (using four different methodologies) without regard to flood synchrony. The last source came from a study performed for the U.S. Army Corps of Engineers (ACOE) who used unit hydrograph methods along with statistical analysis of local rain gauges. Analysis of the four sources revealed that the most conservative (highest) flow rates were predicted by the FIS. Because the FIS flow rates were conservative, and because federal regulations require that the FIS be updated to incorporate the project improvements, the flow rates from the FIS were used for all of the HEC-2 models developed both project wide and at the individual bridge sites. 3. HEC-2 Models HEC-2 was chosen for the project hydraulic modeling in order to be compatible with the FIS and to eliminate the need for transposition of data into or out of the FIS. 3.1 Project Wide Analysis - Permitting The project wide hydraulic analysis was developed strictly to provide data to meet the environmental design criteria and permitting requirements. The basic design criteria for impact to flood elevations is to show that the proposed design will cause little or no increase in flood elevations on private property, and to show no net volume loss of floodplain storage. In order to create hydraulic models which could be used to compare existing and post development conditions, numerous cross sections were surveyed at proposed bridge locations and at locations where cuts and fills were proposed within the floodplain. These cross sections were added to the cross sections which had already been provided in the FIS, to provide a modified existing conditions model. Hydraulic modeling of the bridges in the project wide case was performed by the special bridge method where appropriate (generally where bridge superstructures are submerged or where a bridge opening could be reasonably modeled as a trapezoid). In cases where superstructures were not submerged or where a trapezoidal

4 198 Hydraulic Engineering Software approximation of the opening was not reasonable, the piers and abutments were modeled as obstructions in the waterway and use of the normal bridge method as described in the HEC-2 manual (ACOE [4]) was not observed. Use of the normal bridge method was incorporated for the final individual bridge hydraulic reports, to provide best accuracy. Comparison of the proposed condition model to the modified existing condition model indicated that the proposed design would not increase flood elevations on private property and that increases were limited to the state right of way. This objective was reached by the removal of existing restrictive bridges and by the provision of extensive compensatory storage areas within the floodplain. 3.2 Individual Bridge Analyses - Permitting and Scour The final individual hydraulic analyses for the proposed bridges was more refined than the project wide case due to the use of the normal bridge method. These analyses were complicated by the fact that some of the proposed final designs (particularly at the downstream end of the project) were radically different from the preliminary designs. Final design changes made the project wide model irrelevant for determining accurate starting water surface elevations for some of the final individual analyses. The issue of where and how to best use the normal bridge method produced considerable discussion among project engineers particularly where skewed bridges and piers were involved. Bridges on the project are skewed at angles ranging from zero to sixty degrees. The HEC-2 manual [4] indicates that bridges with skew angles up to thirty degrees can be effectively modeled by the normal bridge method, either by use of the cosine option given within the program or by projecting the bridge onto sections normal to the flow lines. Based on experience, the projection method was recommended as best. Where multiple column piers are skewed to the river it was assumed that the spaces between the columns would be clogged with debris so the piers were generally modeled as solid structures with the width projected to reflect the skew. In one case projecting the skew from piers would have blocked out most of the river channel, so the projected pier widths were modified using engineering judgment to provide a representative width. In case of the one bridge with a sixty degree skew, use of the normal bridge method was deemed inappropriate because projecting the structure would have meant condensing a stretch of several hundred feet of river. With the abutments at either end of this stretch it was clear that a

5 Hydraulic Engineering Software 199 projected section would not be representative and that the abutments and piers should be modeled as separate entities at their real world locations. Another case involved a configuration in which a bikeway bridge begins at the river bank elevation well below the design water surface and then rises up across the river to an elevation well above the design water surface. The normal bridge method was employed for this case without any difficulty. The problem of developing starting water surface elevations for some of the individual models was handled in different ways. The elevations at the downstream limit were taken directly from the FIS. In two cases where multiple bridges were close together, the project schedule allowed for the bridges to be modeled together in one run. In cases where bridges were sufficiently isolated, the project wide study was looked at and the original FIS elevations were used. In one case the project wide model was modified to more closely reflect the proposed conditions. Given the constraints resulting from design changes and project schedule, reasonable estimates of starting water surface were used, realizing that a project wide run would be developed at the end of construction to satisfy FEMA regulations. Proposed changes in floodplain topography arose during the final design stages resulting from the discovery of contaminated materials in the floodplain area. These changes required revisiting the environmental permitting process with recalculated flood elevations and floodplain storage volumes. Initially it was thought that constricting the channel would increase the flood elevations, however after recomputing the hydraulic model it was found that the reverse was true. 4. TR-20 Models - Project Wide The project wide hydrologic model was developed with the NRCS runoff equations and the unit hydrograph method given in the NRCS TR-20 computer program. The purpose of this model was to determine the relative project impact on the existing flood hydrology. This effort did not constitute an effort to establish a unified and contemporary watershed model of the Blackstone river and its headwaters. The basin is a large and hydraulically complex one, with an extensive network of flood control impoundments and diversions. A large portion of it is intensely urbanized with excess runoff conveyed to the river system through closed drainage systems. Data collection, assembly and calibration of a detailed watershed model was deemed time and cost prohibitive by project planners. It was decided to construct a more generalized model using

6 200 Hydraulic Engineering Software input data for some subwatershed areas developed in previous studies. Input data for the remaining components were developed for the model using standard NRCS procedures. The project wide existing conditions model was calibrated so that the model would reflect the flow rates used in the FIS at the appropriate points along the river. The proposed condition model was developed by examining the changes in land use (runoff curve number) and water surface elevations and storage within the river reaches. Runoff curve numbers and stage end area (storage) values were increased for two of the subwatershed areas. The net result of the model comparison was that the project improvements did not result in any relative increase in flood discharge peaks downstream of the project area. This result is attributed to the extensive provision of compensatory storage areas within the project limits. 5. Bridge Scour Analysis Analysis of hydraulic scour using the methods given in the HEC-18 was mandated by the FHWA. An evaluation study (Sverdrup/Parsons Brinckerhoff [5]) was performed for the project to assess relevant guidelines and the state-of-the-practice with respect to local conditions, and to provide project specific guidelines for scour analysis and design of countermeasures. In addition to HEC-18, the evaluation study reviewed current provisions and guidelines from the American Association of State Highway Transportation Officials (AASHTO) and from the Massachusetts Highway Department (MHD). Engineering experience from the Mississipi and Missouri River Flood of 1993 was also discussed. The study then detailed practical application of the HEC-18 guidelines and provided reasons for using engineering judgment in the application of HEC-18 results. HEC-18 classifies scour into three main categories: aggradation and degradation, contraction scour, and local scour. Aggradation and degradation or long term changes in the river bed were not included in the project scour analysis because examination of flood records and topographic surveys over a period of forty years indicated that the river bed had not significantly aggraded or degraded. Contraction scour due to increased velocity through a restricted flow area was computed at appropriate locations using the Laursen equations given in HEC-18. The computed contraction scour depth was added to

7 Hydraulic Engineering Software 201 the maximum local pier scour depth to obtain the design scour depth for the piers. Local scour caused by vortices at piers and abutments was also computed in accordance with HEC-18. Local scour depths were computed at all proposed pier locations that were considered to be exposed to flood flows using the Colorado State University (CSU) equation. Local scour depths were computed for selected abutments using Froehlich's live bed scour equation. The maximum local scour depth computed for all piers at a bridge location was applied to all of the piers that were considered to be contacted by flood water, in order to account for possible lateral migration of the main channel during a flood. The contacted piers were then designed as unbraced structures to the design scour depth. The local scour computed for the abutments was taken into consideration, but was not directly used in the design of scour countermeasures. It is documented within HEC-18 that for a number of reasons local abutment scour depths using HEC-18 methodology should be considered conservative, and that engineering judgment based on experience should be used in determining the extent of scour protection at abutments. Structural solutions such as designing unbraced foundation structures to the scour depths predicted by the local scour equations for abutments was determined to be prohibitively expensive. Scour countermeasures consisting of surface protection around the abutments were employed in final designs. In one case where a bridge superstructure was proposed below the design flood elevations, a debris accumulation risk analysis was applied to the design. The proposed structure consisted of a simple span, and analysis of the upstream area indicated that debris (uprooted trees) would be a threat. An estimate was made of the size of debris stack that would accumulate at the structure and a corresponding adjustment factor was applied to the contraction scour depth computed for the bridge. 6. Conclusions The hydrologic, hydraulic, and bridge scour modeling problems encountered with the size and scope of the Route 146 project show that there are numerous ways to model different situations and that engineering experience and judgment need to be applied in all aspects of the work. Textbook methods cannot always be strictly applied to the real world situations that arise, and assumptions need to be made with a solid theoretical and practical basis.

8 202 Hydraulic Engineering Software The hydrologic modeling involves examination of the locally available methods and results. Conservative solutions should be chosen that will provide compatibility with past, present, and future regulatory requirements. The comparisons of predevelopment and postdevelopment flow rates need to withstand scrutiny by employing the hydrologic methods accepted by the local regulators. In this case documentation of the project wide condition by NRCS unit hydrograph and routing procedures was required. The hydraulic modeling with HEC-2 may be done by at least three different methods depending upon who is asked, and judgment needs to be exercised in cases not covered "by the book". Cases involving skewed bridges with multiple column piers and with superstructure geometry both in and out of the flood waters need careful attention and finally, the development of a consensus between the involved parties as to how to prepare the best model. Development of bridge scour analysis has undergone significant changes recently and there is currently a significant portion of the work that is up for discussion, particularly in the areas of local scour at bridge abutments, and in debris accumulation risk analysis. The practical application of these methods needs to rely heavily on past experience where reasonable assumptions need to be made. Acknowledgement The authors would like to dedicate this article to the memory of our colleague Anthony W. Sykes, who contributed greatly to the work on the 146 Project and unfortunately passed on before his time. Anthony will be greatly missed and we are thankful for his fine contributions to our profession and for the mentoring he provided.

9 Hydraulic Engineering Software 203 References [1] Howard, Needles, Tammen and Bergendorf, Hydro logic Report for Route 146/Massachusetts Turnpike Project, Blacks tone River and Middle River in Worcester and Millbury, Massachusetts, Massachusetts Highway Department and Massachusetts Turnpike Authority, Boston, September, [2] Sverdrup/Parsons Brinckerhof, Massachusetts Highway Department Massachusetts Turnpike Authority Route 146/ Massachusetts Turnpike Interchange Project U.S. Army Corps of Engineers Application for Department of the Army Permit, Boston, April, [3] U.S. Department of Transportation Federal Highway Administration Hydraulic Engineering Circular No. 18 Evaluating Scour at Bridges Third Edition, Washington, November, [4] U.S. Army Corps of Engineers Hydrologic Engineering Center Generalized Computer Program HEC-2 Water Surface Profiles User's Manual, Davis, September, [5] Sverdrup/Parsons Brinckerhof, Route 146/Massachusetts Turnpike Interchange Project Evaluation and Mitigation of Bridge Scour, Boston, October, 1996.