APPENDIX G VOLUME DETERMINATION DRAFT FEASIBILITY STUDY

Transcription

1 PORTLAND HARBOR RI/FS APPENDIX G VOLUME DETERMINATION DRAFT FEASIBILITY STUDY DRAFT Prepared for The Prepared by Anchor QEA, LLC

2 TABLE OF CONTENTS TABLE OF CONTENTS... I LIST OF FIGURES... II LIST OF TABLES... III LIST OF ACRONYMS... IV EXECUTIVE SUMMARY DREDGE VOLUME DETERMINATION General Methods Detailed Methods Neatline Volume Allowable Overdredge Volume Additional Volume for Engineering Factors Residuals Neatline Volume Ratio Reductions to Dredge Volume Results REFERENCES... 8 i

3 LIST OF FIGURES Figure 1a-e Figure 2a-e Figure 3 Figure 4 Figure 5 Alternative B-F Depth of Impact Sub-Thiessen IDs and Slope Buffer Zones Alternatives B-F Dredge Volume Determination Method Slope Buffer Zone Reductions Structure Dredging Access Issues ii

4 LIST OF TABLES Table 1 Table 2 Table 3 Table 4 Table 5 Summary of Technology Implementability Screening Results by Sub-SMA Based on Site Uses and Physical Conditions Sub-Thiessen Dredge Volume Calculations EPA Method Sub-Thiessen Dredge Volume Calculations LWG Method Removal Volume Summarized by SMA EPA Method Removal Volume Summarized by SMA LWG Method iii

5 LIST OF ACRONYMS CAD CDF cy DOI DUI EF EMNR EPA ft 2 FS LWG SL SMA SU Confined Aquatic Disposal Confined Disposal Facility cubic yards Depth of Impact Depth of Unknown Impact Engineering Factor Enhanced Monitored Natural Recovery U.S. Environmental Protection Agency square feet Feasibility Study SubSMA type for confined dredging Sediment Management Area SubSMA type for upland dredging iv

6 EXECUTIVE SUMMARY This appendix: Provides greater detail concerning the methods for estimating removal volume discussed in Section 5.10 of the draft FS text Describes how the removal volumes are estimated based on the two methods considered the U.S. Environmental Protection Agency (EPA) method and the (LWG) method and the differences between the two methods. Provides detailed estimations and summations of removal volume on a subsma, SMA, and total alternative basis for each alternative for both the LWG and EPA methods. A summary of removal volumes by SMA for both the LWG and the EPA methods are shown in Tables 4 and 5 respectively. In general, removal volume estimates based on the LWG method are significantly greater than the removal volumes estimates based on the EPA method. 1

7 1.0 DREDGE VOLUME DETERMINATION Section 5.10 of the draft FS describes development of the sub-thiessen surface area and Depth of Impact (DOI), and then describes how simplified removal volumes are calculated. The purpose of this appendix is to describe in greater detail how the dredge volumes were calculated and modified to account for structural considerations and slope offsets (slope buffer zone). The accuracy of the methodology used for volume estimation in this draft FS is highly dependent upon the concept of environmental dredging presented in Section 6 and includes one production pass followed by the placement of a thin layer of suitable cover material. If further dredging to remove residuals is required (i.e., continued dredging passes until a sediment chemical criteria is met), the volume estimates presented in this evaluation may be significantly lower than the actual quantities dredged during construction. 1.1 GENERAL METHODS Where removal was considered to be an implementable technology by Table 1, sub- Thiessens were flagged for removal for in situ focused or removal focused alternatives. Dredge volumes were not calculated for previously remediated areas, areas where removal was not considered an implementable technology, or for enhanced monitored natural recovery (EMNR) areas in sediment management area (SMA) 17S. Where sub- Thiessens intersected with confined aquatic disposal (CAD)/confined disposal facility (CDF) areas, the sub-thiessen volume was flagged for exclusion from the final dredge volume tabulation if the CAD or CDF was active. A range of dredge volumes was calculated for each sub-thiessen flagged for removal under each alternative. Low and high dredge volume estimates were determined on a sub-thiessen basis using reasonable high and low assumptions, which are discussed in Section 1.2. The basic dredge volume calculation is equal to the sub-thiessen area multiplied by the DOI. In general, this calculation uses the sub-thiessen area, the sub-thiessen DOI, and the subsma type, which informs the type of dredging rate that would be used in the cost estimates. For example SU indicates upland dredging while SL indicates a limited in-water dredging rate to account for structural limitations. Each designation has a different dredging rate. 1.2 DETAILED METHODS The core Thiessen polygon surface was intersected with subsmas and divided into sub- Thiessens. In this way, a subsurface core could contribute DOI information to multiple sub-thiessens and multiple subsmas. Figure 1 shows the Thiessen polygons and DOI. Figure 2 shows the sub-thiessen identifications, and slope buffer zones (discussed in Section ) for each alternative. 2

8 Volumes were calculated on a sub-thiessen basis and then summed to provide overall volumes by SMA. Volumes were calculated using two separate methods, which are referred to as the LWG method and the EPA method. The LWG proposed the LWG method, but EPA (EPA 2011; see Appendix O) a different method. The EPA method is the basis of cost estimates presented in Appendix K. The total volume for each sub-thiessen is the sum of the neatline volume (as defined by the DOI), plus the additional volume for residuals management, constructability considerations like overdredge allowances, and increases for uncertainties associated with design. For the LWG method, this additional volume includes specific adjustments for overdredge allowances, residuals volume, and adjustments for engineering factors as described in Section The EPA method does not necessarily differentiate each of the factors used in the LWG method. Instead, EPA's method uses the neatline volume ratio (discussed in Section 1.2.5) to account for the additional volume. The total volume for a particular sub-thiessen is described subsequently for both the EPA method and the LWG method and is shown in Figure 3. Total dredge volumes for the LWG method (V LWG ) and the EPA method (V EPA ) were calculated as follows: where: V LWG = Volume in cubic yards (cy) as calculated using the LWG method (1) (2) V EPA NV = Volume in cy as calculated using the EPA method = Neatline volume in cy NR = Neatline ratio (1.5 to 2.0) RV AO = Residuals volume in cy = Allowable overdredge volume in cy EF = Engineering factors (30% to 50%) Neatline Volume The portion of the removal volume directly associated with the DOI is defined above as the neatline volume. Because the DOI and the dredge areas are equivalent for the LWG and the EPA methods, the neatline volumes for both methods are equivalent. The neatline volume is typically calculated using the DOI multiplied by the area of the sub- Thiessen. In some cases the neatline volume is modified for the slope buffer zone as discussed in Section The neatline volume is calculated as follows: 3

9 (3) where: NV = Neatline Volume in cubic yards (cy) A s-t = Sub-Thiessen Area of unrestricted Dredging in square feet (ft 2 ) DOI = Depth of Impact in ft A SBZ = Sub-Thiessen slope buffer zone area in ft 2 A s-t = Sub-Thiessen area of unrestricted dredging in ft Depth of Impact The DOI is based on the raw depth of impact for a particular sample location. An additional 1 to 3 feet is added to the DOI if the bottom-most sample of a core has elevated concentration levels. This is known as the Depth of Unknown Impact (DUI) and it is added to the core to determine the final DOI. If the bottom sample of a core does not have elevated concentration levels, the DUI is zero. The DOI is calculated as follows: where: (4) DOI = Depth of Impact in ft DOI raw = Depth of Impact in ft prior to adjustments for cores with elevated bottom samples DUI = Depth of Unknown Impact in ft, which ranges from 1 to 3 feet, or is zero Slope Buffer Zone Area Reductions to neatline volumes associated with the slope buffer zone area apply to both the LWG method and the EPA method. The Slope Buffer Zone area is defined as the area where the depth of the dredge cut transitions from zero feet to the full DOI. Because the slope buffer zone area is dependent on the slope of the mudline and the DOI, slope buffer zone areas are equivalent for the EPA and the LWG methods. This transition in the dredge cut is needed to allow for potential slope stability issues that would be created by very steep cuts, and allows for a more realistic and constructible dredge prism. As a conservative draft feasibility study (FS)-level assumption, a 2H:1V dredge prism cut is considered in the volumes only for sub-thiessens adjacent to the shoreline where removal occurs. Figure 4 illustrates the slope buffer zone area. The slope buffer zones in each SMA are shown in Figure 2. Slope buffer zone areas for each sub-thiessen are shown in Table 2. 4

10 The area of unrestricted dredging,, is defined as the area where all removal operations (i.e., residuals dredging, residuals capping, and removal of the full DOI) are expected to occur. In the slope buffer zone area, A SBZ, only partial removal of the DOI is expected. To account for the impacted sediments left in situ, an engineered cap will be constructed in the slope buffer zone area. The area of unrestricted dredging is calculated as: where: (5) A s-t A raw A SBZ = Sub-Thiessen area of unrestricted dredging = Raw sub-thiessen area, without reductions for the slope Buffer Zone = Slope buffer zone area in sub-thiessen Allowable Overdredge Volume The allowable overdredge estimate only applies to the LWG method. For the EPA method, this additional volume is assumed to be accounted for in the neatline ratio discussed in Section The allowable overdredge, D, is an estimate of the range of additional dredge depth, which will be expected as a result of the contractor s equipment type, varying operator experience, and Site conditions. Allowing overdredge is necessary to account for variance between volumes developed in the estimating process and likely actual volumes. For calculation of the LWG method the allowable overdredge depth was assumed to vary from 0.5 feet to 2.0 feet for low and high volume estimates. The allowable overdredge volume for the LWG method is calculated as: where: (6) D = Additional depth for overdredge allowances of 0.5 to 2 ft A s-t = Sub-Thiessen area of unrestricted dredging in ft 2 AO = Additional volume in cy to account for the overdredge Additional Volume for Engineering Factors Similarly, the estimate of additional volume for engineering factors only applies to the LWG method. For the EPA method, this additional volume is assumed to be accounted for in the neatline ratio discussed in Section For the LWG method, an Engineering Factor (EF) was multiplied by the neatline and overdredge volumes to account for adjustments to the estimated dredge volume expected following final design and 5

11 construction. As part of remedial design, additional cores will be advanced in the active remediation areas. These additional cores will change the actual extent of required dredging. In addition, as part of remedial design, the engineer will develop dredge prisms that are efficient for the construction equipment anticipated to be used. This will entail removing extra materials so that the contractor has longer, flatter dredge cuts to improve efficiency. The optimization of the dredge prism from the GIS-based planned dredge cuts will increase the dredge volumes. Finally, additional dredge material associated with the transition slopes between deep and shallow cuts and around the dredge perimeter will need to be considered. The EF is established to capture the increased dredge volume for all of the factors mentioned. Based on past experience, the range of values of EF was set at 0.3 to Residuals Volume estimates for residuals are explicitly calculated for the LWG method. For the EPA method, the residuals volume is presumed accounted for with the neatline ratio, as discussed in Section For the LWG method, the residuals volume is calculated separately from the removal volume associated with the DOI. For the EPA method, the residuals volume is included in the neatline ratio. In order to complete cost estimates, volume estimates for residuals volume need to be calculated and broken out from the total volume. For both methods, the residuals volume is calculated assuming a 6-inch dredge cut plus a 6-inch overdredge allowance to account for factors similar to those discussed in Section of the draft FS. For the LWG method, the residuals volume is calculated as: where: (7) RV RD = Residuals Volume in cy = Residuals Dredge Depth in ft, equal to 1 ft A s-t = Sub-Thiessen Area of Unrestricted Dredging in ft Neatline Volume Ratio EPA directed LWG to account for the combined factors described in Section to of this appendix by using a neatline volume ratio; as such, the neatline volume ratio only applies to the EPA method. The neatline volume ratio is a factor applied to the neatline volume as shown in Equation 2. As directed by the EPA, the neatline volume ratio ranges from 1.5 to 2.0 for the low volume and high volume estimates, respectively Reductions to Dredge Volume Removal volumes were reduced for structural considerations as discussed in the draft FS Section 5.4, and for the slope buffer zone areas discussed in this appendix. Dredging 6

12 immediately adjacent to structures can potentially impact the structure in the following ways: Removing sediment adjacent to a structure s piling can cause loss of lateral and vertical support, which in turn can weaken the structure. Many waterfront structures have battered piling to resist lateral loads against a structure. Dredging adjacent to structures can potentially damage battered piling. Dredge buckets can strike structures and piling if they are too close to the structure. Offsets prevent unwanted contact. As part of final design many of these elements will be investigated in detail, potentially eliminating the need for offsets. Section 6 of the draft FS describes the evaluation of dredging in, around, and under structures in more detail. Figure 5 summarizes the extent of dredging around structures and shows situations where dredging is limited or restricted. In summary, extensive dredging under robust fixed structures was generally screened out as a feasible technology for the draft FS. 1.3 RESULTS Sub-Thiessen level volume calculations for the EPA method are provided in Table 2. Sub-Thiessen volumes for the LWG method volumes are provided in Table 3. Table 4 summarizes removal volume by SMA for each alternative using the EPA method. Table 5 summarizes removal volume for each alternative by SMA for the LWG method. In general, removal volume estimates based on the LWG method are greater than the removal volume estimates based on the EPA method. The cost estimates presented in Appendix K were generated based on the EPA method. 7

13 2.0 REFERENCES EPA Letter dated July, 15, 2011, from EPA to LWG regarding EPA Comments on Portland Harbor FS Key Elements Check-in (June 21 & 22, 2011). Region 10, Oregon Operations Office. Portland, OR. 8