Expert Engineering Independent Third-Party Review Briscoe-Desimone Levee Design Green River Basin State of Washington

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1 Expert Engineering Independent Third-Party Review Briscoe-Desimone Levee Design Green River Basin State of Washington Prepared for King County Flood Control District Prepared by Robert B. Gilbert, Ph.D., P.E., D.GE The University of Texas at Austin February 16, 2013

2 Third-Party Review Report 1 February 16, 2013 Executive Summary This report contains the findings of an independent third-party review concerning the engineering aspects for two alternative concepts to improve levees on the Briscoe-Desimone Levee between S. 180 th Street and S. 200 th Street, Green River Basin, State of Washington. The objective in improving the levees is to reduce the risk of flooding to public safety and property in an efficient and environmentally beneficial way. The alternative proposed by the City of Kent addresses the right bank of the river and consists of the existing levees combined with new levees setback on the outside bends of the river channel using landside retaining walls to avoid the need for property acquisition. It achieves a 100-year design level for the right bank, which is the standard of practice in the United States. A short-term alternative proposed by King County follows the same approach as Kent, except that the new levees setback on the outside bends of the river channel extend onto private property because they do not utilize retaining walls and armoring is installed at the river bank to minimize scour and erosion. A long-term alternative proposed by King County addresses both the right and left bank of the river, sets the levees back as much as 300 to 600 feet from the river channel, and raises all of the levees to a 500-year design level. The findings from this review are summarized as follows: There are no critical flaws in the designs for the Kent approach that have been certified by two independent and qualified engineering firms. While the King County approach has not yet been developed to the point of certification, there is no reason to expect difficulty in certifying this approach. There is no discernible difference in the residual life-safety risks for any of the approaches because the life-safety risks are controlled primarily by the Howard Hanson Dam located upstream from these levees. There is no discernible difference in the residual property-damage risks for the Kent approach and the King County short-term approach. A reduction in residual propertydamage risks of several hundred thousands of dollars per year could be realized with the King County long-term approach. All approaches will pave the way for reducing the cost of flood insurance for property owners. The estimated implementation costs are $17 Million for the Kent approach and $63 Million for the King County short-term approach. The majority of the cost difference in the two approaches is due to property acquisition. The Kent approach could be implemented within one year, the King County short-term approach is estimated to require three years to implement. The estimated implementation costs are $400 Million to $900 Million for the King County long-term approach, with cost increasing the further the levees are set back from the river. The majority of these costs are associated with property acquisition and road reconfiguration. The King County approach, with an armored river bank, will lower operation and maintenance costs associated with repairing the river bank from erosion and scour. At

3 Third-Party Review Report 2 February 16, 2013 most, this reduction is estimated to be on the order of tens of thousands of dollars per year for these levees. Both the Kent approach and the King County short-term approach could benefit endangered fish species by improving riparian vegetation. The Kent approach further benefits fish species by leaving a natural, un-armored river bank. The King County long-term approach with large levee setback distances adds green space, benefits fish species by adding rearing habitat, and could substantively reduce flood water elevations by increasing the width of the floodway if it is implemented everywhere along the river. The following recommendations are provided based on this review: The approach proposed by the City of Kent is the most cost-effective approach for managing the risk of property damage from flooding for this section of levee in the immediate future. This approach could be improved further if a consistent design flood level is used throughout the system and if the system is designed to be resilient in the event of overtopping. With this approach, it is recommended that deformation and erosion of the river bank be monitored in real-time to provide warning information if a breach is possible and scientific data for future projects. It is also recommended that a specific plan be developed for making repairs so that there are minimal delays caused by permitting, acquisition of materials and availability of equipment. The alternatives proposed for the long-term by King County with 300-foot and 600-foot levee setbacks provide additional benefits in adding green space and improving fishrearing habitat. These larger levee setbacks also increase the size of the floodway, which if combined with similar levee setbacks throughout the valley could reduce the demands on the levee system and the risks from flooding. Regardless of which approach is implemented, there is a residual risk of flooding from both normal and emergency operation of the Howard Hanson Dam, underscoring the continual need for preparing the public to protect human life and minimize property damage in the event of a flood. A system-wide plan for managing flood risk in the Green River Valley, which includes the dam and land use as well as levees, would be valuable.

4 Third-Party Review Report 3 February 16, 2013 Introduction This report contains the findings of an independent third-party review concerning the engineering aspects for two alternative concepts to improve levees on the Briscoe-Desimone Levee between S. 180 th Street and S. 200 th Street, Green River Basin, State of Washington. This section of levee is located along the right bank of the lower Green River in the cities of Kent and Tukwila. A site location map is shown in Figure 1. GREEN RIVER Cross-Section in Figures 2, 4, 6 and 7 Figure 1 Site Location and Plan for Briscoe-Desimone Levee between S. 180 th Street and S. 200 th Street (adapted from GEI 2012). The flood plain 1 in the lower Green River Valley contains 24,000 permanent residents and 70,000 workers and residents during the day, it houses $12 billion in property, and it supports nearly 100,000 jobs including the second-largest industrial park on the west coast (DSES ). The Green River provides important habitat for Chinook salmon and steelhead trout, both listed as threatened species under the Endangered Species Act, and it hosts one of the largest summer/fall runs of Chinook salmon in the Puget Sound (Washington State Department of Ecology 2011). 1 Flood plain is used here as the area susceptible to flooding according to the regional flood impact assessment exercise performed in 2010 by the federal government (DSES ).

5 Third-Party Review Report 4 February 16, 2013 Flood protection along this stretch of the river is provided primarily by the Howard Hanson Dam, which is located about 50 miles upstream from the project location and has been in operation since Protection is also provided by levees, such as the project levee, which are generally located coincident with the natural levees that existed along the river in 1962 when the dam began regulating flow. At the project location, the existing river bank is marginally stable on the outside bends of the river. It is susceptible to slope failures when the toe of the slope is scoured during high flow events in the river or when the water level in the river is drawn down rapidly after high flow events. Because the flood-protection levees are very close to the river bank, these slope failures in the river bank have the potential to damage the levees. Figure 2 shows a typical cross-section of the right-bank levee with the zone of marginal stability demarcated. As an example, a slope failure occurred on the river bank in 2006 at the outside bend just south of Briscoe Park. This slope failure did not cause a breach of the levee, and it was repaired by setting the levee further back from the river bank along a 600-foot long stretch in Figure 2 Cross-Section of Right Bank Levee Looking Downstream (see Figure 1 for location). Two independent engineering firms have assessed the stability of the project levee on the right bank: GeoEngineers (2011) and GEI (2012). Both firms concluded that the marginal stability of the river bank on the outside bends would preclude their ability to certify the levee for the purposes of accreditation from the Federal Emergency Management Agency (FEMA). Accreditation by FEMA allows for the levee to be included in developing flood plain maps that are used for insurance purposes. Without accreditation of the project levee, approximately 500 structures worth about $650 Million (USACE 2007) would be required to purchase flood insurance under the National Flood Insurance Program at a total cost of approximately $3 Million per year 2. 2 A rough approximation provided to me by an executive with FM Global, a flood insurance company.

6 Third-Party Review Report 5 February 16, 2013 Description of Alternatives The objective in improving the project levee is to reduce the risk of flooding to public safety and property in an efficient and environmentally beneficial way. Two alternatives for improving the project levee were reviewed. For the purposes of this report, they will be designated as follows: Kent Levee Setbacks with Walls; and King County Levee Setback. Kent Levee Setbacks with Walls The Kent approach for improving the project levee on the right bank of the river is described in GeoEngineers (2011) and GEI (2012). The basis for this approach is to design the right-bank levee for a 100-year flow in the river, which corresponds to a water elevation that has a onepercent chance of being exceeded annually and that is exceeded once every 100 years on average. This water elevation is shown in Figure 2. A 100-year design is the standard of practice for levees in the United States and it is the design basis required for FEMA accreditation. The required height of the levees is set to provide at least two feet of freeboard 3 above the design water level. With this freeboard, the chance that the levees will be overtopped is small, on the order of 0.2 to 0.4 percent per year (nhc 2011 and nhc 2013). In addition, the levees are designed to be stable in the design flow event and in other possible extreme events such as scour of the river bank or an earthquake in accordance with standard practice, design guidelines developed by the U. S. Army Corps of Engineers (USACE 2000, 2001, 2003 and 2010). The practical significance of this design basis is that the levees are very likely to survive a design flow event, with a typical reliability of greater than 99 percent (e.g., DWR 2008 and IPET 2009). The Kent approach to improve the existing levee focuses on the three outside bends along the right bank where stability is a concern (Figure 3). They recommend setting the levee back from the river far enough so that the zone of marginal stability at these locations is outside of the levee prism (Figure 4); in other words, they have designed it so that a slope failure along the river bank is unlikely to impact the flood protection function of the levee. The landward side of the levee is retained with a vertical wall in order to avoid impacts to private property. This wall is a steel sheet pile that is driven into the ground surface deep enough to support the weight of the levee soil on the river side under normal conditions and the weight of the ground on the landside in an extreme event of scour in the river bank. A concrete facade is cast around the sheet pile above the ground surface, and soil is backfilled on the river side to make the ground level with the existing levee (Figure 4). In addition to the three outside bends, they recommend raising a short section of the levee near the S. 200 t0h Street Bridge by several feet to provide adequate freeboard for the design flood event (Figure 3). A sheet pile wall on the landward side of the levee is also used here to raise the levee without impacting private property. In anticipation of upgrading the design basis to a 500-year flow in the river at a future date, the tops of the walls have been set to be 3 feet above the 500-year design water level (Figure 4). The total length of levee setbacks proposed by Kent is approximately 4,500 feet, or about 25 percent of the total length of the project levee. Note that the remaining 75 percent of the project levee length that is not set back is designed for the lower 100-year water elevation versus the 500-year water elevation. 3 Freeboard is the distance between the design water elevation and the top of the levee.

7 Third-Party Review Report 6 February 16, 2013 West Valley Highway Raised Levee Figure 3 Kent Levee Setbacks with Wall Site Plan Showing Locations of Levee Setbacks (adapted from GEI 2012). Figure 4 Kent Levee Setbacks with Wall Cross-Section of Right Bank Levee Looking Downstream (see Figure 1 for location; wall is shown at alternate location proposed in GEI 2012 and recommended below in Table 2).

8 Third-Party Review Report 7 February 16, 2013 The Kent approach includes a 1,000-foot long segment on the north side of the project levee where the levee is coincident with the West Valley Highway, SH 181 (Figure 3). On this segment, the embankment for the highway also acts as a levee. The engineers have investigated and analyzed this segment and concluded that it satisfies the design basis (i.e., provides adequate freeboard and stability) and is therefore capable of serving as a levee. Operation and maintenance for this segment will require a cooperative agreement with the Washington State Highway Department. A notable feature of the Kent approach is in how it addresses vegetation on the river bank. In its natural state, the river bank is vegetated with a variety of woody species, including large trees. In the aftermath of Hurricane Katrina in 2005, the U.S. Army Corps of Engineers established a nationwide policy to not allow woody vegetation with diameters greater than 2 inches on levees that qualify for the federal Rehabilitation and Inspection Program (USACE 2009). The engineers have certified the levee with the provision that vegetation is allowed to develop naturally on the river bank in the three setback areas and the short section where they are raising the levee (Figure 3), meaning that woody vegetation with diameters greater than 2 inches will be allowed. This riparian vegetation is important in providing shade to maintain the cooler water temperatures in the river that are most healthy for the fish (Washington State Department of Ecology 2011). The certifying engineers contend that vegetation should not be limited here since it is outside the boundary of the levee (since the wall is designed to act as a flood barrier on its own without depending on the stability of the riverbank). The consequence of allowing natural vegetation, however, is that this levee may not qualify for federal funding to assist with repairs if the levee is damaged in a flood event. An approach of not participating in the federal Rehabilitation and Inspection Program in order to allow for woody vegetation is being adopted on other levee systems, such as in California. Another notable feature of the Kent approach is in how it addresses erosion and sloughing of the river bank. In its natural state, the river bank periodically erodes and sloughs, particularly on the outside bends of the river. One alternative to manage this erosion is to armor the river bank with large rocks (Figure 5). While this alternative minimizes the long-term cost of maintenance, it is detrimental to the health of the fish (e.g., Schmetterling et al. 2001). Another alternative to manage this periodic erosion is to allow it to occur naturally and then repair it before it threatens the integrity of the levee. While this alternative is best for the health of the fish, it increases the cost of long-term maintenance. In the Kent approach, the engineers have certified the levee without armoring the river bank, providing as natural as possible of a habitat for the fish.

9 Third-Party Review Report 8 February 16, 2013 (Ordinary High Water) Figure 5 Schematic Cross-Section Showing a Typical Scheme for Armoring a River Bank to Minimize Erosion (from Wissmar 1996). The estimated construction cost for the Kent approach, based on a 35-percent level design, is $17 Million. This construction could be completed in one construction season. The cost estimate includes about $1 Million to re-vegetate the river banks where the levee is improved (the four segments identified in Figure 3). The cost for operation and maintenance (O&M) will be similar to what it has been historically since the river bank may still periodically erode and subsequently require repairs. King County Levee Setback Short Term The King County approach for improving the project levee in the short term is to address the same areas identified by Kent as needing improvement, the three setback areas on the outside bends of the river and the one segment where the levee needs to be raised (see Figure 3). The county s approach differs from Kent s approach in the following ways: The King County levee setbacks are created using a full levee prism rather than a levee prism that is truncated with a retaining wall, as shown in Figures 6 and 7. The King County setback levee extends beyond the footprint of the existing levee (see Figures 6 and 7) and encroaches on private property in numerous locations. The proposed lengths along the river for the King County levee setbacks are longer than those proposed by Kent by approximately 3,000 feet (based on King County Internal Memo 2012). The King County levee setbacks include armoring the river bank to reduce the potential for erosion. A schematic 4 cross-section with this armoring is shown in Figure 6. As with the Kent approach, this approach in the short term will not improve the levee on the right bank at locations outside of those identified as Setbacks and Raised Levee in Figure 3. 4 I do not have a project-specific cross-section of this approach for the proposed setback areas on the right bank. I have used the typical section for the PL84-99 repair in 2007 on the Briscoe School Levee to develop this schematic cross-section (see Figure 9).

10 Third-Party Review Report 9 February 16, 2013 Subsequently, the remaining 75 percent of the project levee that is not set back is designed for a 100-year water elevation versus a 500-year water elevation with this approach. In addition, this approach will include the 1,000-foot long segment on the north side of the project levee where the levee is the highway embankment for the West Valley Highway (Figure 3). A note on a spreadsheet providing cost information for this approach (King County Internal Memo 2012) indicates that a 150-foot long section with a sheet pile wall similar to Kent s approach (Figure 4) will be required where the levee intersects with the West Valley Highway. Figure 6 Schematic King County Levee Setback Cross-Sections of Right Bank Levee Looking Downstream (see Figure 1 for location). Figure 7 Comparison of King County and Kent Levee Setback Cross-Sections of Right Bank Levee Looking Downstream (see Figure 1 for location).

11 Third-Party Review Report 10 February 16, 2013 I do not have detailed information about the plan for vegetation on the river bank with the setback levee. I understand that King County intends to be in compliance with the Corps of Engineers Rehabilitation and Inspection Program, and that they intend, if necessary, to negotiate a variance with the Corps of Engineers to allow large diameter woody vegetation on the river banks. However, if a variance is not possible, it has not yet been decided whether the county will support forgoing participation in the Rehabilitation and Inspection Program. The county has estimated the implementation cost for this approach at $63 Million 5, with $27 Million in construction, which includes building demolition and utility relocation as well as levee construction, and $36 Million in property acquisition. The county estimates that implementation of this approach could be completed in 2015 (King County Internal Memo 2012), with the two years of delay presumably due to the time required to negotiate property acquisitions. The county intends that the cost for operation and maintenance (O&M) will be lower than what it has been historically since the toe armoring will minimize erosion and the flatter river bank slopes will be less susceptible to sloughing. King County (2012) estimates that this cost could be reduced by one-third or more. Long Term The King County approach for improving the project levee in the long term is described in King County (2012). This long-term approach is larger in scope than the Kent approach or the shortterm King County approach. It addresses both the right bank and the left bank of the river (versus the right bank alone in the Kent and King County short-term approaches), and it uses a 500-year flow in the river as the design basis (versus a 100-year flow in the Kent and King County shortterm approaches). The King County long-term approach is developed at present to a conceptual level. 5 King County staff have also quoted this estimate at $71 Million, which is intended to include a contingency similar to that used to develop the Kent estimate.

12 Third-Party Review Report 11 February 16, 2013 Figure 8 King County Long-Term Levee Setback Site Plan Showing Locations of Levee Setbacks (King County 2012). The King County long-term approach is to set the levees back everywhere along both banks (Figure 6). This approach includes three alternative levee alignments: setback levees adjacent to the existing levee (in-place replacement levee), setback levees that are 300 feet back from the river channel (300-foot setback levee), and setback levees that are 600 feet from the river channel (600-foot setback levee). Figure 9 shows a possible schematic cross-section of the King County in-place setback levee, assuming it is similar to the design developed for the 2007 repair of the Briscoe School levee (USACE 2007).

13 Third-Party Review Report 12 February 16, 2013 Figure 9 Typical King County Levee Setback Cross-Section of Right Bank Levee Looking Downstream (from King County 2012). The King County long-term approach is to reconfigure roads and bridges where they conflict with the levee system, such as the West Valley Highway (Figure 3). Reconfiguring means either elevating roads within the levees or relocating them to be outside of the levees. This approach is in contrast to the Kent approach and the King County short-term approach, which both incorporate the existing roads and bridges into the levee system. Plans are provided in King County (2012) to re-vegetate the riverbank and widen the floodway (for the 300-foot and 600-foot setback levees). However, this effort is not included in the base cost estimates. As with the short-term approach, I understand that King County intends to be in compliance with the Corps of Engineers Rehabilitation and Inspection Program, and that they intend, if necessary, to negotiate a variance with the Corps of Engineers to allow large diameter woody vegetation on the river banks. The estimated cost to implement the King County approach is summarized in Table 1. It is important to emphasize that these costs are based on a conceptual level of design; the actual costs could be significantly smaller or larger depending on details that are sorted out as the design is refined, such as specifics for how the roads and bridges are reconfigured. Table 1 Estimated Implementation Cost for King County Setback Level (King County 2012) In-Place 300-foot 600-foot Replacement Setback Setback Item Levee Levee Levee Property Acquisition $165 Million $261 Million $379 Million Road Reconfiguration $201 Million $304 Million $419 Million Levee Construction $50 Million $72 Million $122 Million Total $416 Million $637 Million $920 Million

14 Third-Party Review Report 13 February 16, 2013 An O&M cost was estimated by assuming that setback of the levees would make the need for periodic repair of the river bank less urgent, and therefore less frequent and less costly. The O&M cost was estimated to be reduced to $58,000 6 per mile per year for the setback levees from $144,000 per mile per year for the existing levees (King County 2012). The basis for the $144,000 per mile per year estimate for the existing levees is the total cost of improving Green River levees over a 13-year period, which constitutes $56 Million for 58 projects, divided by 13 years and the length of levees under the responsibility of the county (King County Personal Communication 2013). 6 The time value of money and the assumed schedule of O&M costs versus time are not reflected in this average annual value.

15 Third-Party Review Report 14 February 16, 2013 Engineering Evaluation of Alternatives This section summarizes my evaluation of the engineering aspects for each alternative, addressing design certification for FEMA accreditation, levee stability, estimated costs and potential improvements. Certification for FEMA Accreditation There are no critical flaws in the designs for the Kent approach that have been certified by two independent and qualified engineering firms, GeoEngineers (2011) and GEI (2012. In my professional opinion, these certifications are sufficient to achieve accreditation by FEMA. While the King County approach has not yet been developed to the point of certification, I do not anticipate any difficulty in certifying this approach. While the design for the Kent approach has been certified for FEMA accreditation, there are additional requirements that would need to be met in order for the land protected by this levee to be considered outside of the flood plain for flood insurance purposes. First, all of the right-bank levees upstream from S. 200 th Street that potentially contribute flood water to this area need to be accredited as well. Second, an agreement needs to be reached with the State Highway Department so that the West Valley Highway embankment south of S. 180 th Street could be operated and maintained as a levee. Third, an operation and maintenance plan that satistifes all requirements for the U.S. Army Corps of Engineers Rehabilitation and Inspection Program, with the exception of allowing large woody vegetation on the river banks, needs to be implemented. Levee Stability The investigations and analyses for levee stability that have been conducted to certify the Kent approach are appropriate and sufficient, and they satisfy the standard of practice for levee design in the U.S. I reviewed in detail the most recent certification report for the Kent approach, GEI (2012), with respect to levee stability. This review included assessing the methods used to develop the design, performing independent stability calculations, and considering the historical performance of the levees. My findings from this review are the following: The methods used to characterize the properties of the soils, the parameters selected to represent these properties in stability analyses, and the calculation procedures used to assess stability are appropriate. The approach to consider slope failures that might compromise the integrity of the levee in performing stability checks is practical and reasonable. The factors of safety 7 used in designing the levees and walls are consistent with the design guidance from the U. S. Corps of Engineers. 7 Results from design analyses for levee stability are typically expressed as a factor of safety. The factor of safety is the shear strength of the soil divided by the shear stress required for equilibrium; a factor of safety of one means the soil is just at equilibrium, a factor of safety less than one means the soil is unstable, a factor of safety greater than one means the soil is stable. The factor of safety is used in combination with a method of analysis, which constitutes assumptions concerning the mechanisms of failure and the properties of the soil. For atypical conditions, such as a rapid drawdown event or an earthquake, the design assumptions are typically conservative (i.e., representative of a worst case) and factors of safety near or equal to one are used in practice. The goal in

16 Third-Party Review Report 15 February 16, 2013 The approach to consider stability for rapid drawdown of the river level after a high flow event by using undrained shear strengths for poorly draining materials (the Corps of Engineers three-stage method) is appropriate. The approach to consider stability in the event of extreme scour in the river bed by removing a prism of soil down to a conservatively established depth of scour (Figure 10), which is 50-percent greater than that estimated in a design flow event, is reasonable and conservative. The approach to consider stability in the event of an earthquake by estimating strength losses due to liquefaction and then checking the static stability of the levee and wall is appropriate. The results of stability analyses for the existing conditions are consistent with the historical performance. These analyses indicate that the river banks are marginally stable on the outside bends of the river, which is consistent with slope failures that have occurred on the river banks in high flow events such as These analyses also indicate that these types of localized slope failures will not necessarily have an immediate impact the flood-protection function of the levees, such as was the case in In my review of the design for the Kent approach, I considered the following specific concerns that have been raised about the stability analyses for rapid drawdown of the river level after a high flow event: (1) the required minimum factor of safety should have been greater than 1.0 and (2) a faster drawdown rate should have been used in the stability analysis. Required Minimum Factor of Safety for Rapid Drawdown Analysis: Guidance from the Corps of Engineers on the minimum factor of safety states that a required minimum value of 1.0 applies to pool levels prior to drawdown for conditions where these water levels are unlikely to persist for long periods preceding drawdown. In my experience, long periods of time applies to cases where high water levels before drawdown are typical operating conditions, such as in a dam reservoir, and not to cases such as this one where the high water levels are temporary conditions in extreme events that may last at most for about 10 days (GEI 2012). I confirmed this interpretation with one of the primary authors of the Corps of Engineers design guidance 8. Furthermore, the method of analysis for rapid drawdown, which assumes that steady-state seepage conditions develop during the high water level and that there is then no drainage for poorly draining materials during drawdown, is conservative and represents a worst-case condition. Therefore, in my professional opinion, the use of a required minimum factor of safety of one in this design is reasonable and is consistent with the standard of practice for levee design in the U. S. Drawdown Rate Used for Rapid Drawdown Analysis: Guidance from the Corps of Engineers on the method of analysis for rapid drawdown is to assume that steady-state conditions develop during the high water, that the soils fully consolidate under these conditions, and that poorly draining soils experience no drainage (undrained conditions) when the water level is rapidly drawn down. This approach is conservative because these analyses is to develop a sense of confidence that the levee is likely to be stable even under these extreme conditions. Guidance for methods of analysis and corresponding factors of safety are provided in guidance documents, such as USACE These documents provide guidance, not rules or standards. It is ultimately the responsibility of the certifying engineer(s) to decide what methods of analysis and corresponding factors of safety are most appropriate for a specific design. 8 Prof. Stephen G. Wright, Emeritus Professor, The University of Texas at Austin.

17 Third-Party Review Report 16 February 16, 2013 steady state conditions may not fully develop under the high water and partial drainage may occur in poorly draining soils during drawdown, both of which will increase the available shear strength and, subsequently, the factor of safety in the event of rapid drawdown. This method of analysis was used by GEI (2012) in developing the certified design for the Kent approach. This method of analysis does not assume a particular drawdown rate; rather, it conservatively assumes that drawdown is rapid enough so that there is no drainage in poorly draining soils. Figure 10 Kent Levee Setbacks with Wall Cross-Section of Right Bank Levee Looking Downstream and Showing Design Case for Extreme Scour. There are two notable differences between the design methodology proposed for the King County approach (King County 2012) versus that used to design the Kent approach (GeoEngineers 2011 and GEI 2012): (1) the factor of safety used to design for rapid drawdown and (2) the treatment of scour potential in the river bed. Factor of Safety for Rapid Drawdown: King County proposes to use a required minimum factor of safety of 1.4 versus 1.0 for rapid drawdown in design in order to satisfy a King County Flood Control Design guidance 9 recommendation. This higher factor of safety is beyond the standard of practice for levee design elsewhere in the U. S. While use of a higher factor of safety may reduce the probability of a rapid drawdown failure, I believe that the probability of a levee breach due to rapid drawdown is tolerably small already with a required minimum factor of safety of 1.0 because the analysis is conservative and because a localized rapid-drawdown failure is unlikely to threaten the levee itself. River Bed Scour: In the conceptual design, King County does not explicitly address the stability of the levees in the event of scour in the river bed. Based on the description of the slope failure that occurred on the right bank in 2006 (USACE 2007), I suspect that this rotational failure was due at least partially to erosion of the river bed at the toe of the 9 I am not familiar with the basis for or source of this recommendation. No reference is provided for it in King County (2012).

18 Third-Party Review Report 17 February 16, 2013 slope. While armoring the toe is intended to reduce the possibility of scour, I believe it is still warranted to consider the effect of scour on levee stability. I believe the approach used by Kent to set the levee and wall back away from the typical zone of scour and to check stability in the case of an extreme loss of soil on the river side is reasonable (Figure 10). Based on the available information about scour and slope stability (e.g., GEI 2012), the assumed extent of scour for this stability analysis is more representative of what might happen if there is a decades-long neglect of the levee system versus what might happen in single or multiple flood events within a short period of time. While I consider that the overall design methodology for levee stability used to certify the Kent approach is appropriate, it is important to emphasize that the river bank slopes may continue to be marginally stable and that they may slough periodically after high flow events due to rapid drawdown. The levees have been designed so that these localized, surficial failures of the river bank should not degrade the flood-protection function of the levees (i.e., they are within the area labeled Zone of Marginal Stability in Figure 4). However, there will be a continuous, longterm need to maintain and periodically repair the river bank when these sloughs occur so that they do not progress back far enough to impact the flood-protection function of the levees. In addition, achieving the standard of practice in design does not guarantee that the levees will survive all flood events. The residual risk associated with the project levee is addressed in the next section of this report. Cost Estimates The construction cost estimate for the Kent approach, based on a 35-percent level design, is reasonable. It is based on a comprehensive list of items and appropriate values for quantities and unit costs. As the design is finalized and if the project is bid, the actual cost is expected to be within +/-20 percent of this estimate. The cost estimate for levee construction in the King County short-term approach is reasonably based on the actual cost for the 2007 repair of the Briscoe School levee (USACE 2007), about $2,600 per foot. However, the cost for property acquisition is difficult to estimate until discussions with property owners are initiated. Kent estimates the cost of property acquisition to be more than double the county s estimate of $37 Million (Kent Internal Memo 2012). The implementation cost estimate for the King County long-term approach, based on a conceptual level design, is reasonably conservative. Given the uncertainty in the major contributions to the total cost, property acquisition and road reconfiguration, I recommend considering these estimates to be accurate to within +/-30 to 50 percent for decision-making purposes at present. I do not have directly relevant information with which to estimate the operation and maintenance (O&M) cost for either approach. The county s approach of estimating this cost by summing up all levee improvement expenditures in the past 13 years seems overly conservative since many of these repairs were capital improvements. For context, the levee system in the California Delta has an average annual repair cost of about $6 Million for about 1,100 miles of levees, or an average cost of $5,000/mile/year (DWR 2011). I would expect the California levee system to

19 Third-Party Review Report 18 February 16, 2013 require a much greater level of intervention for repairs since there have been more than 150 breaches of the system in the past 100 years, since the levees act essentially as dams with the water level higher than the landside ground elevation under normal conditions, and since the levees are subjected to regional subsidence. Therefore, I conservatively estimate that the annual cost of repair for this system going forward will be less than $10,000/mile/year. With the King County approach, it is reasonable to expect that this cost will be reduced due to the armored river bank. Potential Improvements Based on my evaluation of the two approaches, I have identified several potential improvements for each in moving forward. These potential improvements are summarized in Table 2.

20 Third-Party Review Report 19 February 16, 2013 Approach Kent Levee Setbacks with Wall King County Levee Setback Table 2 Potential Improvements for Each Alternative Potential Improvements 1. Make the top of levee and top of wall elevations the same, the 100-year design level. This modification will improve the performance of the system in the event that the levee is overtopped because it will minimize the potential for flow concentrations. This modification will minimize the potential for confusing the public by having dissimilar heights along the levee, particularly since there is no definitive plan in the near future to raise the entire levee to the 500-year design level. It will also eliminate the possibility that delays in finalizing the 500-year design level (only preliminary analyses have been conducted to date) would delay the project. Lastly, reducing the wall heights may reduce the construction cost. 2. Provide for resiliency of the landside levee and wall in the event of overtopping. This provision will be required for the walls if they are made the same heights as the adjacent levee, and it would be beneficial to identify and mitigate any potential erodible areas on the levee slope as well. 3. Move the setback wall back from the toe of the existing levee in the northernmost setback segment near the West Valley Highway (Figure 3), as described as an alternative by GEI (2012). A deeper and larger sheet pile is needed at this location compared to the other setback segments if the wall is not moved approximately 8 feet back from the existing toe. This modification will make all of the setback walls similar in construction and similar in performance, providing a more homogenous system. The construction cost will be reduced with a smaller wall, although the cost of acquiring the necessary property (parking spaces) may more than offset the construction cost reduction. 4. Include periodic inspection for corrosion of sheet pile walls in the operations and maintenance (O&M) activities or protect the piles from corrosion with coating. 5. Actively monitor the riverside slopes on the outside bends of the river for deformation and erosion to provide warning information in case a breach is possible and to provide scientific data for future projects. 6. Develop a specific plan for making repairs so that there are minimal delays due to permitting, acquisition of materials and availability of equipment. 1. For the short-term approach, make the top of the levee elevation the same, the 100-year design level. 2. Consider the effect of scour in the river bed on the stability of the in-place setback levees. Even with armoring, there is still a potential for scour and erosion. 3. Provide for resiliency of the landside levee in the event of overtopping.

21 Third-Party Review Report 20 February 16, 2013 Risk Assessment for Alternatives This section summarizes an assessment of the risks to life and property for the Green River Valley below the Howard Hanson Dam and for the project levee. Green River Valley below Howard Hanson Dam I have approximated the risk for fatalities due to flooding in the Green River Valley below the Howard Hanson Dam (see Appendix A for details). The results are shown on Figures 11 and 12. These figures plot the frequency of a flood versus the expected consequences that might occur in that flood. The lowest risk in Figures 11 and 12 is in the lower left corner of the graph, and the greatest risk is in the upper right corner of the graph. This type of graph is used commonly in managing risks for dams, nuclear power plants, chemical process facilities and other major engineering systems. Figure 11 Assessment of Life-Safety Risk due to Flooding in the Green River Valley in Comparison to Benchmarks. For benchmarking purposes, I ve included two lines on Figure 11 that represent risk thresholds established for major dams by the U. S. Bureau of Reclamation (USBR 2003). Points above the line labeled Risks are Intolerable for Dams represent dams where there is an Urgent Justification to Expend Resources to Reduce the Risk. Points below the line labeled Risks are Tolerable for Dams represent dams where there is Diminishing Justification to Expend

22 Third-Party Review Report 21 February 16, 2013 Resources to Reduce the Risk. The band between these two lines represents dams where there may be Justification to Expend Resources to Reduce Risk, and further study is warranted. In addition, information is shown on Figures 11 and 12 for comprehensive risk analyses that were conducted for the hurricane levee system in New Orleans (IPET 2009) and for the riverine levee system in the California Delta near Sacramento (DWR 2008). Figure 12 Assessment of Property-Damage Risk due to Flooding in the Green River Valley in Comparison to Benchmarks. The existing life-safety and property-damage risks for the Green River Valley are below those for the other levee systems (Figures 11 and 12). This result reflects the significance of the dam in regulating flows in the Green River and also the numbers and locations of people and property vulnerable to flooding; the valley is dominated by commercial and industrial property with workers who are there primarily during the day who are generally capable of evacuating (see Figure A.6 in Appendix A). At present, the life-safety risk is below what is considered intolerable for major dams in the U.S. (Figure 11). Figure 11 illustrates how risk can be effectively managed: the chance of flooding can be reduced by increasing the level of protection and/or the consequences of flooding can be reduced by either keeping people from flood-prone areas or evacuating people when floods occur. The reduction in risk for levee system in New Orleans from 2005 to 2012 is due as much to a smaller population and more effective evacuation as it is to stronger and higher walls and levees.

23 Third-Party Review Report 22 February 16, 2013 Project Levee Information to assess the property-damage risk associated with the project levee, a small subset of the Green Valley below the Howard Hanson Dam, is available from an ongoing study (King County 2012). This study is considering the risks of flooding for both right and left banks of the project levee. I have used this information to approximately and conservatively estimate the expected annual cost of property damage due to flooding for the project levee (see Appendix A for details). These results are summarized in Table 3 for a variety of configurations for the flood control system (dam and levees). Table 3 Property Damage Risk for Project Levee Case Description Average Annual Property Damage ($ Million) Existing Conditions Probability of Breach for Design Case = 0.2 (based on King County 2012) Year Design Probability of Breach for Design Case = Year Design with Perfect Levees No Possibility of Breach Year Design Probability of Breach for Design Case = Year Design with Perfect Levees No Possibility of Breach 0.6 Existing Conditions without Dam Natural River Flow without Dam 260 The results in Table 3 highlight the following points: The dam is a very important component in reducing risk; the risk at present is about 1/100 th of the risk if the dam did not regulate flow. The property-damage risk at present is on the order of $2 to $3 Million per year. This risk is consistent with an estimated flood-insurance cost of about $3 Million if the properties impacted by this levee are mapped in the FEMA flood plain (Flood Insurance Executive Personal Communication 2013). For context, the property-damage risk associated with the project levee is about one-half of the property-damage risk in the valley as a whole. Implementing a 100-year design for the project levee on both the right and left banks reduces the risk by more than 50 percent (from $2.7 Million to $1.1 Million). Implementing a 500-year design for the project levee on both the right and left banks reduces the risk by several hundred thousand dollars per year (from $1.1 Million to $0.9 Million). This risk-reduction is relatively small because most of the risk is due to the possibility of flow rates above the 500-year case, the assumed limit for the dam being able to regulate the flow (see Appendix A). Therefore, the greatest potential for risk reduction beyond the conventional 100-year design would be to either increase the capacity of the dam or make the levees significantly higher than the 500-year flow. Implementing a design with perfect levees (that is, levees that do not breach no matter how much flow they contain), could reduce the property damage risk by $300,000 to $400,000 per year.

24 Third-Party Review Report 23 February 16, 2013 Comparison of Alternatives The Kent approach and the King County short-term approach have the following features in common: 1. Both approaches setback the levee where it is marginally stable at present. 2. Both approaches achieve a 100-year design level for the entire stretch of the project levee, with isolated improvements being designed to a 500-year level. 3. Both approaches rely on the existing levees for more than half of the length of the project levee. 4. Both approaches rely on cooperation of the State Highway Department to use a 1,000- foot long stretch of the West Valley Highway as a levee. 5. Both approaches may not qualify for federal funding in making repairs if woody vegetation with diameters greater than 2 inches is allowed to grow on the river banks. The primary differences between the Kent approach and the King County short-term approach are the following: 1. The Kent approach avoids (or minimizes) property acquisition, while the King County approach requires property acquisition. 2. The Kent approach leaves the river bank in its natural state, while the King County approach armors the river bank. Additional differences between the Kent approach and the King County long-term approach are the following: 1. The Kent approach relies on cooperation of the State Highway Department to use a 1,000-foot long stretch of the West Valley Highway as a Levee, while the King County long-term approach reconfigures roads and bridges to avoid conflicts with the levees. 2. The Kent approach achieves a 100-year design level with the intent to raise it to a 500- year design level at an unspecified time in the future, while the King County long-term approach achieves a 500-year design level. 3. The Kent approach does not substantively increase the floodway or green space, while the King County long-term approach could increase the floodway and green space with the 300-foot and 600-foot setback distances. 4. The Kent approach is for the right bank, while the King County long-term approach is for both banks. A summary comparison of the two approaches is provided in Table 4. In summary, the comparison between the Kent approach and the King County short-term approach is boiled down to the following question: Do the benefits of lower O&M costs with the King County short-term approach justify the greater implementation cost due mostly to property acquisition and the degradation of the fish habitat caused by armoring the toe? From a life-cycle cost perspective with a 50-year design life for the Kent approach and conservatively neglecting the time-value of money, the annual O&M cost would need to be reduced by more than $900,000 per year to justify the added implementation cost.

25 Third-Party Review Report 24 February 16, 2013 Likewise, the comparison between the Kent approach and the King County long-term approach is boiled down to the following question: Do the benefits of the increased green space and floodway with the King County longterm approach (assuming 300 or 600-foot setback distances) justify the greater implementation cost? The answer to this question depends on the value system for the stakeholders in this decision.

26 Third-Party Review Report 25 February 16, 2013 Table 4 Summary Comparison of Two Alternatives for Project Levee Life Safety Kent Levee Setbacks with Wall (Right Bank) Relatively small risk due to Howard Hanson Dam Most effective means to reduce risk further are to reduce residential development in floodsusceptible areas and to provide for effective public preparation and evacuation King County Short-Term Levee Setback (Right Bank) Relatively small risk due to Howard Hanson Dam Most effective means to reduce risk further are to reduce residential development in floodsusceptible areas and to provide for effective public preparation and evacuation King County Long-Term Levee Setback (Right and Left Banks) Relatively small risk due to Howard Hanson Dam Most effective means to reduce risk further are to reduce residential development in floodsusceptible areas and to provide for effective public preparation and evacuation Property Protection Provides for FEMA accreditation for flood insurance purposes Residual risk of about $1,000,000 per year Provides for FEMA accreditation for flood insurance purposes Residual risk of about $1,000,000 per year Provides for FEMA accreditation for flood insurance purposes Residual risk of about $800,000 per year Construction Cost $17 Million (Assuming Cooperation of Highway Dept.) $63 Million (Assuming Cooperation of Highway Dept.) $420 Million (In-Place Setback) $640 Million (300-ft Setback) $920 Million (600-ft Setback) Maintenance Cost Long-Term Concerns $30,000 per year Less than $30,000 per year Less than $60,000 per year Needs significant O&M commitment to inspect and periodically repair erosion and sloughing of the river bank Fish Welfare Benefits fish by providing potential to improve vegetation on river bank Benefits fish by maintaining natural river bank Benefits fish by providing potential to improve vegetation on river bank Degrades fish habitat by armoring river bank Benefits fish by providing potential to improve vegetation on river bank Degrades fish habitat by armoring river bank Benefits fish by increasing off-channel habitat for rearing with larger setback distances Additional Benefits Avoids property acquisition Reduces O&M commitment to periodically repair erosion and sloughing of the river bank Reduces O&M commitment to periodically repair erosion and sloughing of river bank Increases floodway and green space with larger setback distances

27 Third-Party Review Report 26 February 16, 2013 Recommendations The approach proposed by the City of Kent with levees setback on the outside bends of the river channel using landside retaining walls to avoid the need for property acquisition is the most costeffective approach for managing the risk of property damage from flooding for this section of levee in the immediate future. This approach could be improved further if a consistent design flood level is used throughout the system and if the system is designed to be resilient in the event of overtopping. With this approach, it is recommended that deformation and erosion of the river bank be monitored in real-time to provide warning information if a breach is possible and scientific data for future projects. It is also recommended that a specific plan be developed for making repairs so that there are minimal delays caused by permitting, acquisition of materials and availability of equipment. The alternatives proposed for the long-term by King County with 300-foot and 600-foot levee setbacks provide additional benefits in adding green space and improving fish-rearing habitat. These larger levee setbacks also increase the size of the floodway, which if combined with similar levee setbacks throughout the valley could reduce the demands on the levee system and the risks from flooding. Regardless of which approach is implemented, there is a residual risk of flooding from both normal and emergency operation of the Howard Hanson Dam, underscoring the continual need for preparing the public to protect human life and minimize property damage in the event of a flood. A system-wide plan for managing flood risk in the Green River Valley that includes the dam and land use as well as levees would be valuable. Development of this plan could help establish the optimal design water elevation (i.e., 50 year, 100 year, 200 year, 500 year, or something else) for levees below the dam given current and possible future conditions with respect to the dam and land use. If the vision for the future of the valley involves sustaining or increasing human development that is vulnerable to flooding, then the best way to manage flood risk may well involve improving the dam. If the vision for the future involves adding green space and trying to return the valley to a more natural condition, then the best way to manage flood risk may well involve better adapting the human development to accommodate periodic flooding. Neither of these visions will necessarily be best achieved by making higher levees. The exercise of systemwide planning could also engage and promote empowerment for all stakeholders who are at risk from flooding in the valley and/or are affected by measures taken to reduce that risk.

28 Third-Party Review Report 27 February 16, 2013 References DSES-10 (2011a), Regional Baseline Report, Green River Valley, 2010 Dams Sector Exercise Series, Sponsored by U.S. Department of Homeland Security and U.S. Army Corps of Engineers, May DSES-10 (2011b), Regional Consequence Assessment Report, Green River Valley, 2010 Dams Sector Exercise Series, Sponsored by U.S. Department of Homeland Security and U.S. Army Corps of Engineers, May DWR (2008), Delta Risk Management Strategy Phase 1, Risk Analysis Report, California Department of Water Resources. DWR (2011), Delta Risk Management Strategy Phase 2, California Department of Water Resources. GEI (2012), FEMA Accreditation Report, Green River Right Bank Levee, Briscoe-Desimone Levee System, Kent and Tukwila, Washington, Submitted to City of Kent, Prepared by GEI Consultants, Inc., Draft Report, April GeoEngineers (2011), Stability and Certification Report, Briscoe/Desimone Levees, Kent, Washington, Prepared for City of Kent, Prepared by GeoEngineers, Inc., October IPET (2009). Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System - Volume VIII Engineering and Operational Risk and Reliability Analysis, Final Report of the Interagency Performance Evaluation Task Force, U.S. Army Corps of Engineers. Kent Internal Memo (2012), Cost Estimate for King County Improvements, Provided in November King County (2012), 180 th to 200 th Street Levee Setback Study, Prepared for King County Water and Land Resources Division, Prepared by TetraTech, November King County Internal Memo (2012), Briscoe-Desimone Levee Setback Cost Estimate Table, Developed by King County Staff, June King County Personal Communication (2013), King County Staff, January nhc (2011), Risk Based Analysis of Reduced Freeboard: Kent Levees Accreditation River Mile , Prepared for City of Kent, Prepared by northwest hydraulic consultants, September nhc (2013), Personal communications with Vaughn Collins, northwest hydraulic consultants.

29 Third-Party Review Report 28 February 16, 2013 Schmetterling, D. A., Clancy, C. G. and Brandt, T. M. (2001), Effects of Riprap Bank Reinforcement of Stream Salmonids in the Western United States, 26(7):6-13. Washington State Department of Ecology (2011), Green River Temperature, Total Maximum Daily Load, Water Quality Improvement Report, Publication No , Draft Report, May Wissmar, R. C. (1996), Recommended Guidelines for Developing Bank Stabilization Facilities of Rivers in Western Washington, Federal Emergency Management Agency (FEMA), Region X. Fisheries Research Institute, School of Fisheries, University of Washington, Seattle, Washington, FRI-UW USACE (2000), EM , Design and Construction of Levees, U. S. Army Corps of Engineers. USACE (2003), EM , Slope Stability, U. S. Army Corps of Engineers. USACE (2005), ETL , Design Guidance for Levee Underseepage, U. S. Army Corps of Engineers. USACE (2007), Project Information Report, Rehabilitation of Damaged Flood Control Works, Seattle District, Green River, King County, WA, Briscoe School Levee, U. S. Army Corps of Engineers, Seattle District, April USACE (2009), ETL , Guidelines For Landscape Planting and Vegetation Management at Levees, Floodwalls, Embankment Dams, and Appurtenant Structures, U. S. Army Corps of Engineers, April USACE (2010), EC , USACE Process for the National Flood Insurance Program (NFIP) Levee System Evaluation, August USACE (2012), Assembly of Design Flood Hydrographs for the Green River Basin, Summary Report for Flood Plain Management Services Program, Seattle District Army Corps of Engineers, September USBR (2003) Guidelines for Achieving Public Protection in Dam Safety Decision Making,, Dam Safety Office, United States Bureau of Reclamation, Denver, Colorado.

30 Third-Party Review Report 29 February 16, 2013 Appendix A Risk Assessment Approach The goal of this risk assessment is to provide information about the risks to people and property in the Green River Valley and how these risks might be changed with improvements to the project levee. I have assessed these risks using available information and employing simplistic and rough approximations. The results from this risk assessment are to be considered preliminary and approximate and more likely to overstate rather than understate the risk. Risk is the possibility of adverse consequences from a flood. The consequence used to quantify risk to people is fatalities, and the consequence used to quantify risk to property is the direct cost of property damage. The possibility is quantified by the probability or frequency of the consequence happening. The risk is then represented by a range of possible consequences and their associated frequencies, such as shown in Figures A.1 and A A single measure to quantify the risk is the expected consequence, which is the average value of the consequence and is obtained by summing up the product of each possible consequence and its associated frequency. Figure A.1 Assessment of Life-Safety Risk due to Flooding in the Green River Valley in Comparison to Benchmarks. 10 Frequency in these figures is the frequency that an event with at least that number of consequences will occur.

31 Third-Party Review Report 30 February 16, 2013 Figure A.2 Assessment of Property-Damage Risk due to Flooding in the Green River Valley in Comparison to Benchmarks. Green River Valley I used DSES-10 (2011) for information about the consequences of a 25,000-cfs peak flow at Auburn in a flood event 11. This discharge was associated with an annual frequency of once per year when the operation of the dam was impaired. This information is plotted as single markers labeled Green 2009 in Figures A.1 (life-safety risk) and A.2 (property damage risk). I used information from recent hydraulic studies for the Green River (nhc 2011) to extrapolate the 2009 risk to existing conditions with the dam operating at full capacity. This hydraulic information is plotted in Figure A.3 as the peak flow at Auburn versus the annual frequency of it being exceeded. For consistency, I have chosen to use the information for the 500-year peak discharge at the dam that was used in developing the designs developed by Kent (e.g., GEI 2012) 11 In this assessment of the consequence, it was assumed that, in addition to overtopping of the levees, a breach would open up on the left bank of the Tukwila 205 federal levee near the S. 180 th Street Bridge.

32 Third-Party Review Report 31 February 16, 2013 and the studies performed by King County (King County 2012) 12. I made the following simplifications to extrapolate flow conditions to different scenarios: The annual peak flow at Auburn without the dam follows a lognormal distribution. The information on Figure A.3 is re-plotted on Figure A.4 and extrapolated using this assumption. The resulting extrapolation is shown with the dashed line on Figure A.5 for the Natural Flow without Dam case. The dam provides no capacity and no attenuation of the peak flow beyond the 500-year flow. This assumption is conservative. The resulting extrapolation is shown with the dashed line on Figure A.5 for the Flow Regulated by Dam case. The consequences of flooding are a function of the peak flow at Auburn. With these assumptions, the annual exceedance frequency for the consequences of the 25,000-cfs peak flow event assessed in the DSES-10 (2011) study can be extrapolated to the dam operating at full capacity. Figure A.5 shows that this frequency is approximately per year. This extrapolation is then plotted as the markers labeled Green 2013 in Figures A.1 and A.2. For benchmarking purposes, I ve included two lines on Figure A.1 that represent risk thresholds established for major dams by the U. S. Bureau of Reclamation (USBR 2003). Points above the line labeled Risks are Intolerable for Dams represent dams where there is an Urgent Justification to Expend Resources to Reduce the Risk. Points below the line labeled Risks are Tolerable for Dams represent dams where there is Diminishing Justification to Expend Resources to Reduce the Risk. The band between these two lines represents dams where there may be Justification to Expend Resources to Reduce Risk, and further study is warranted. In addition, information is shown for comprehensive risk analyses that were conducted for the hurricane levee system in New Orleans (IPET 2009) and for the riverine levee system in the California Delta near Sacramento (DWR 2008). The existing life-safety and property-damage risks for the Green River Valley are below those for the other levee systems (Figures A.1 and A.2). This result reflects the significance of the dam in regulating flows in the Green River and also the numbers and locations of people and property vulnerable to flooding; the valley is dominated by commercial and industrial property with workers who are there primarily during the day and who are generally capable of evacuating (Figure A.6). At present, the risk is below the threshold for what is considered intolerable for major dams in the U.S. (Figure A.1). I would treat the estimates for the current risk in the Green River Valley (the markers labeled Green 2013 in Figures A.1 and A.2) as upper bounds because the dam will have capacity to attenuate peak flows beyond the 500-year level. A more accurate estimate will require a more comprehensive analysis of both the hydrology, using paleohydrologic information, and the dam under more extreme conditions than have been used to date. In addition, it will be necessary to assess the consequences for other flood conditions with peak flow rates at Auburn that are less than and greater than 25,000 cfs to develop a complete picture of the risk. I recommend a 12 The Corps of Engineers has recently updated the estimated 500-year peak discharge from the dam, increasing it from 15,280 cfs to 18,800 cfs (with confidence bounds between 12,000 and 27,000 cfs). This new information will increase the design elevations and the costs for both approaches. It will also increase the risk for a 100-year design more than for a 500-year design since the 500-year design elevation will be increased to match the updated estimate for the 500-year peak discharge.

33 Third-Party Review Report 32 February 16, 2013 comprehensive analysis be performed because this conservative assessment for the life-safety risk with the dam currently falls in a range for major dams where there may be Justification to Take Action to Reduce Risk according to the U.S. Bureau of Reclamation (USBR 2003). Figure A.3 Peak Flow at Auburn versus Annual Frequency of Exceedance for Natural Flow without Dam and for Flow Regulated by Dam. Figure A.4 Extrapolation for Peak Flow at Auburn versus Annual Frequency of Exceedance for Natural Flow without Dam based on Lognormal Distribution.

34 Third-Party Review Report 33 February 16, 2013 Frequency for 25,000-cfs Peak Flow with Dam at Full Capacity Figure A.5 Extrapolation for Peak Flow at Auburn versus Annual Frequency of Exceedance for Natural Flow without Dam and for Flow Regulated by Dam.

35 Third-Party Review Report 34 February 16, 2013 Figure A.6 Current Land-Use Near and Downstream from Kent (from King County 2012).