REHABILITATION OF THE HANSEN DAM LOWER LAKES

I. EXECUTIVE SUMMARY:

The Corps seeks to remove the fill it deposited in the Lower Lakes in 2002, and to restore their original contours to the extent that can be done without significantly disturbing the habitat that has grown back since the fill activities occurred. Its proposed action for rehabilitating the Lower Lakes is the only action consistent with its responsibilities under the Clean Water Act.

II. BACKGROUND INFORMATION:

Figure 1: Lower lake at Hansen Dam

Tujunga wash is one of the most active washes in Southern California. Storm events deposit sediment throughout Tujunga wash as it enters the basin; the sediment deposits gradually and naturally fill the basin. To sustain the dam’s flood control capability, sand and gravel removal operations were undertaken in the late 1980s through the mid-1990s to remove these sediment deposits and maintain the basin’s design capacity. Two lakes were inadvertently created when two sand and gravel borrow pits filled with ground-water and other inflow associated with Tujunga Wash. They have come to be known as the “Large Lower Lake” and the “Small Lower Lake.” (Figure 1). The borrow pits were to be sloped to drain toward the dam before the sand and gravel removal operation ended. For reasons unrelated to the current situation, the sand and gravel removal contract expired before the pits were reconfigured to drain.

The two lakes are located upstream from the dam and lie at the confluence of the Big and Little Tujunga washes. The Lower Lakes are separate and distinct from the Recreation Lake Complex constructed by the Corps in 1999.

The first discharge of fill into the Lower Lakes occurred in May 2002 when approximately 1,650 cubic yards of reinforced concrete rubble were deposited in the northwestern corner (the deepest area) of the Large Lower Lake. The fill material had been gathered from the bottom of the Hansen Dam Swim Lake while it was under repair. This material was capped by approximately 2,200 cubic yards of clean soil from the same swim lake repair project. The fill area measures 150 feet long by 145 feet wide, and projects five feet into the water.

Figure 4: Arundo Piles North of Small Lower Lake

The Corps ceased filling at the Lower Lakes in June 2002. To determine the potential impact of the fill on the water quality, we performed water quality analyses in December 2002, March 2003 and again in October 2003. (Figure 5) These studies show the fill activity did not pollute the lakes or degrade the habitat. The only contaminants that the studies detected were also found in the upstream background surface water samples, and can be attributed to non-point source pollution associated with over a century of agricultural and urban land use in the watershed above the Basin, as well as from horse manure associated with riding near the lakes.

III. CORPS’ CORRECTIVE MEASURES

Figure 5: Water Quality Sampling at the Lower Lakes

Beginning in July 2003, and continuing to the present, we have been removing all rebar visible along the shoreline of the Large Lower Lake. These efforts are consistent with the Corps’ commitment to ensuring the safest possible conditions for people frequenting the area.

The Corps is eager to proceed with the full rehabilitation and mitigation measures we proposed in our Environmental Assessment, dated November 14, 2003, but are prevented from proceeding until the Regional Board revises its November 21, 2003 order to permit us to do so.

IV. RECENT SITE INVESTIGATIONS

Figure 6: Removal of stockpile material next to Small Lower Lake

Additional site investigations conducted after the Corps’ Environmental Assessment have confirmed the accuracy of the area and volume estimates that it sets forth, and support the reasonableness of its remediation and mitigation proposal.

The Environmental Assessment estimated that 1,300 cubic yards of fill were placed in and alongside the Small Lower Lake. The subsequent surveys, including measurements of materials that the Corps subsequently removed from the shoreline of the Small Lower Lake, confirm that this estimate was accurate.

The concrete materials remaining at the Large Lower Lake are more easily identified than the mulch mixed with soil placed in the Small Lower Lake, and the November 21, 2003 order of the Regional Board did not question the Corps’ defined area of impact at the Large Lower Lake.

In December 2003, subsequent to the completion and distribution of its Environmental Assessment, the Corps completed surveys of the Lower Lakes. Results of those surveys were delivered to the Regional Board on January 20, 2004. In addition, the Corps delivered to the Regional Board copies of construction records indicating cubic yard quantities of Swim Lake demolition debris placed in the Large Lower Lake by the contractor. The construction records verify the figures relating to the volumes of reinforced concrete placed in the Large Lower Lake that are contained in the Environmental Assessment.

Figure 7 (left): Area where removal of material occurred – no remaining stockpiled material adjacent to Small Lower Lake

The Regional Board’s initial order questioned the results of water quality testing that the Corps submitted to the Board in its October 15, 2003 submission. In response, the Corps conducted additional water quality testing on October 16, 2003, with the assistance of SOTA. These test results were consistent with the prior test results, and indicated that levels of contaminants were no higher in the areas of the Lower Lakes where the Corps had placed fill than in upstream background water samples.

Fate and transport modeling study, two types of data analysis used by hydrologists and environmental scientists, and site-specific risk assessment are under way. The draft studies further support the conclusions of the earlier water quality studies. We anticipate that the Final Site Assessment Report, with the Fate and Transport Modeling Study and Site Specific Risk Assessment included as appendices, will be completed by the end of April 2004.

V. PROPOSED ACTIONS

Figure 8: Photograph from visual survey of the Lower Lakes

The Corps’ proposed action includes the removal of approximately 2,000 cubic yards of fill material from the top 3 feet of the fill area at the Large Lower Lake and the removal of all identifiable fill material at the Small Lower Lake. All removal actions would be done using a long- reach excavator, like the one pictured in Figure 9. At each of the lakes, the excavation equipment would be positioned to minimize the impact to the surrounding habitat. At the Large Lower Lake, equipment would be placed 5 feet back from the shoreline to begin extraction at the toe of the fill, and would be moved back incrementally to continue extraction. While at the Small Lower Lake, an access pad would be created by partially flattening the unsafely steep slope there. All removal activity will avoid disturbing the densely vegetated fresh water marsh areas that exist at both lakes, and silt curtains will be installed during excavation to trap any suspended particulates and thus prevent adverse turbidity impacts.

Excavated material will be temporarily stockpiled adjacent to each of the extraction sites, to allow excess water to drain from the material prior to removal for proper disposal. After draining, the soil and other materials from the Small Lower Lake and the concrete from the Large Lower Lake will be disposed of in an approved landfill. The clean soil from the Large Lower Lake will be recycled through beneficial re-use.

Figure 9: Example of long-reach excavator proposed for use at Lower Lakes. Photo from Texas Crane Services website

After the removal activities are completed, mitigation would begin. Mitigation activities would include restoration of the shoreline at the Large Lower Lake, and completion of the laying back of the dangerously steep shoreline at the Small Lower Lake to a safer 3:1 slope ratio. Removal of non-native plants and replanting of native species would occur at both lakes. Replanting would be followed by adaptive management, such as irrigation, and even a second round of replanting, if necessary. The mitigation sites would be maintained and monitored by the Corps for 5 years, the standard monitoring period for Clean Water Act mitigation activities. One-and-a-half acres would be restored at the Large Lower Lake, and 0.9 acre would be restored at the Small Lower Lake, for a total restoration of 2.4 acres, an area three times larger than that originally impacted by the Corps’ fill activities.