The short path seepage area is between the dam embankment and the bend in the existing drainage tunnel. This area is highlighted in red on the

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3 The short path seepage area is between the dam embankment and the bend in the existing drainage tunnel. This area is highlighted in red on the picture in the top left hand corner. Water in this pathway seeps through fluvial sediments and rockslide debris and into downstream dam embankment. Recent drilling indicates that silt, sand and gravel materials are continuous from the upstream side to downstream side across the bedrock septum saddle. This is important because these materials are highly erodible.

4 Construction drawings show an area along the right abutment where drain material was not placed indicated by the white circle above. Design documents explain that seepage was not anticipated by engineers to happen in this location. The red and blue dots indicate where the Corps has conducted various types of exploration using auger and roto sonic drilling methods. During our exploration, we took samples and conducted analysis to determine if the engineered materials are suitable to filter water passing through the abutment. 4

5 Hydraulic modeling calculates the top of the underground water table. The area of concern is where a steep water line crosses the downstream abutment area. Since this area is not properly filtered, a steep water gradient could potentially erode and pipe soil. 5

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7 During the January 2009 flood pool of record ( feet), several symptoms of erosion developed along the right abutment. To test for the existence of preferential water seepage paths, the Corps used food grade dyes along the right abutment during the summer conservation pool. The dye locations are shown above in the picture by the red, green, blue and orange dots. The arrows point to where the dyes traveled. The Corps placed activated charcoal packs at key locations in the existing drainage tunnel and at other drain locations in the right abutment. Activated charcoal is a very sensitive way to collect samples and can detect very low concentrations in parts per billion. Sensitive collection methods are necessary since the dyes dissolve into the water table, and they travel through the abutment in very low concentrations. The Corps determined that dyes traveled very quickly through the abutment. In some cases, dye traveled over 400 feet in as little as 20 minutes. 7

8 Preliminary results of testing in 2010 indicate that many preferential seepage paths detected in the 2009 dye test have been significantly changed. A single test conducted with the reservoir at elevation 1,170 was conducted by introducing dyes at various locations along the right abutment. Activated charcoal packs were placed at key drain locations to absorb even trace amounts of dye. Samples were sent to a laboratory and results were compiled by the Corps. Our findings indicate that preferential seepage paths have been reduced as a result of the placement of the 2009 grout curtain. 8

9 A piezometer is a device that measures water level in the ground. The Corps has installed many of these devices in the right abutment and logs the data year round. We conducted a comparison between these water levels before we placed the new filtered drains (the blue lines in the picture above) and the grout curtain (also shown above). The size of the blue dots indicate specific water level differences between 2009 and The larger the dot, the larger the difference between water levels. As shown in the picture, many of the instruments in the critical area show reduced water levels. This is a positive sign that the installed horizontal drains are safely removing water out of the area of concern. 9

10 Willowstick is a company which uses electricity to find the pathways that water follows underground. By placing electrodes upstream and downstream, scientists can measure the intensity of electrical current at survey points along the right abutment. Since both water and electricity follow the path of least resistance, more electrical current at a given survey location would mean that water is flowing under there. The illustration above shows that most water is flowing around the end of the grout curtain. This is a positive sign, since water now has to flow nearly three times as far as it did in the past in order to reach the short path seepage area. The line which apparentlygoes through the grout curtainis is actually flowing underneath the grout curtain, using an isolated lower aquifer that does not pose a significant seepage risk. 10

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13 Water follows the path of least resistance. The Corps will add filtered drains to the short path seepage area to provide water an easy path to safely flow out of the right abutment. Like a sponge, the right abutment soaks up a great deal of water from the reservoir especially during high pools. By adding a row of filtered drains across water s path, the drains will intercept the water and provide it a way to safely exit from the abutment. If these filtered drains did not exist, water would exit naturally through unfiltered springs. This presents a danger, because the unfiltered exit would potentially allow water to move dirt out of the abutment. If enough dirt underneath the abutment is removed, the abutment could potentially collapse. The Corps is racing ahead to install these filtered drains with pumps in order to remove excess water from the short path seepage area. After the drainage tunnel spur is complete, the pumps would be removed and the drains tied in with the tunnel to allow gravity to do the work of the pumps. 13

14 Adding a tunnel spur and tying in the filtered drains will allow gravity to pull water through the filters and into the tunnel. Water can then flow out the tunnel works and into the Green River. The drains will be installed before the tunnel spur. The Corps believes filtered drains and a tunnel will be effective, as the additional horizontal drains installed in the existing tunnel (completed in 2009) have been proven to work effectively. Design of the tunnel spur is currently at the conceptual level. Design and construction schedules are estimates. Proposed location of the spur is indicated in red on the picture and will generally spanan underground bedrock saddle andtie into two bedrock knobs on either side of the saddle. 14

15 This is a cross section of the short path seepage area. The brown area at the bottom is bedrock. The grainy material above that is the landslide material that water moves through. The Zone 3 Material and Rock Shell are engineered materials that are designed to filter the water before it leaves the downstream face of the abutment. The Corps is concerned that these engineered materials are not adequate to filter water exiting from the abutment during high pools in the reservoir. The blue area represents water that saturates the landslide material and the engineered materials. 15

16 The Corps plans to install filtered drains and pumps into the short path seepage area of the abutment. Like a kid sucking juice through a straw leaving the ice behind, the pumps and filtered drains will draw the water out of the ground, leaving the soil behind. By safely pumping the water out, the water level in the landslide material is reduced. Lower water levels will prevent pressures from building up near the base of the abutment. 16

17 The Corps plans to add a tunnel spur to the existing drainage tunnel in the right abutment. By adding this tunnel, the Corps will tie in the filtered drains and use gravity to pull the water out of the landslide material. Compare this to pumping that we did at first. We can use gravity just like a pump to remove the water, with the benefit that gravity is free and does not need to be maintained like a pump. The tunnel will provide the water safe passage out to the Green River. 17

18 There are other things that need to be fixed at the dam. A new and improved debris boom is necessary to prevent logs and other debris, that washes off of the hills into the reservoir, from reaching the dam spillway. If the spillway got clogged, it could cause the dam to be overtopped in an extreme flood event. The spillway has never been used, because it is reserved for that extreme flood event that would generate a high reservoir pool. In the event of an extreme flood, our engineers believe that large quantities of water flowing over the spillway would cause it to lift up and become unstable. By putting in rock anchor bolts, the spillway concrete would be bolted to the bedrock it lays on and stay in place. Also, the rock currently on the face of the dam near the spillway is weathered and needs to be replaced (see area in red above). Should we need to use the spillway in an extreme flood event, there would be large volumes of water that could erode the face of the dam near the entrance to the spillway. Although the chances of an extreme flood event are very small, these issues are things the Corps needs to fix. 18

19 As stated earlier, the Corps is racing ahead to install filtered drains in the short path seepage area as soon as possible. The tunnel spur is currently being designed and we anticipate starting construction on that project towards the middle of next year. Other projects would begin this winter. Certainly, as is always the case, beginning these projects depends on the availability of funds to support design and construction commitments. 19

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21 This "La Niña" forecast was provided Aug. 5, 2010, by NOAA Climate Prediction Center. Last year was El Niño, and that pattern tends to suppress flooding weather patterns in the Northwest, and sure enough it did. This year is La Niña (strength unclear at this point), and that pattern tends to see more storms and flooding than El Niño; therefore the odds for flooding are higher this upcoming winter, but it is not a guarantee. The picture above is a satellite photo of an atmospheric river or pineapple express where the air flow draws moisture from Hawaii and dumps it over the Pacific Northwest. They are called atmospheric rivers because these weather events are narrow (like a river) and are estimated to carry more water than the Mississippi River. They are like a fire hose as they focus their abundant precipitation over our relatively small watersheds, engorging rivers in as little as 48 hours. This atmospheric river produced the historic Northwest flooding of early February

22 The Corps has people who work at the dam every day. They monitor the dam, make frequent inspections and take instrument readings. They compile reports and send them to scientists and engineers at the Seattle District for regular analysis. If a storm event begins to fill the reservoir, we intend to increase the frequency and intensity of monitoring. If we think we are going to be managing a flood pool, we will place engineers and geologists directly on the dam. That way they can make first hand observations and measurements of how the dam is performing. In terms of how much water we would ultimately store above 1170 feet, COL Anthony Wright, the District EngineerandCommander Commander, will make real time decisions based on the weather forecast, river flows in the lower Green River Valley and information about how the dam is performing while holding back the flood pool. 22

23 We have come a long way in terms of understanding what is wrong with Howard Hanson Dam, and how we are going to fix it. All the work performed last summer and our testing this summer have improved our confidence in the dam. However, there is still around a 1:60 chance of flooding in the lower Green River Valley. La Niña, with her frequent and possible back to back storms, may lead to a decision to rapidly draw down a flood pool. PLEASE keep in mind that even with the dam restored to full capacity, the residents of the Green River Valley live on a flood plain and are always at risk for flooding. We hope to restore Howard Hanson Dam to its full 1:140 capabilityby by mid 2012, but that does not prevent flooding for ALL storms. 23

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25 Completing the Dam Safety Modification Study is very important and is considered a top priority to the Seattle District. We are currently on schedule to deliver a completed Dam Safety Modification Study. The study will identify several measures we think will provide a lasting fix to Howard Hanson Dam. This study is required before any repair can be considered permanent. The study can recommend that temporary measures become permanent, but in order for them to be considered so, the study has to completed and approved. 25

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