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e e . Phase I of the Consent Decree improvements were initiated in 1993 with a site characterization reconnaissance. The original Phase I objective included Sand Creek channel improvements, wetlands restoration, and a program to control invasive, non'native vegetation. Phase I Sand Creek channel improvements followed Sand Creek Master Pian recommendations with the following modifications: . Embankments were to be regraded to a maximum sideslope of 3 feet horizontal to 1 foot vertical with toe protection consisting of boulders over a subsurface foundation of concrete rubble and biotechnical treatments including biologs, willow staking, and coir ropes and mattresses. . Distribution of the recommended 7 -foot drop over a series of four lower sloping chute drops to allow for fish migration through the site. Design of Phase I improvements commenced in late 1993 with construction scheduled for 1994. However, just before mobilization, a vinyl chloride seep was discovered on the site entering Sand Creek from the north bank. This seep precluded any work on the north bank until it was remediated. The Department of Aviation immediately prohibited any access to the north bank and began a voluntary cleanup program (VCUP) to remediate the contamination. However, in order to placate the Sierra Club and continue to meet the conditions of the Consent Decree, the Department of Aviation also requested that improvements be implemented in areas not affected by the contamination. These areas included the drop structure at the east property boundary and embankment work along the south bank. This work was undertaken in late 1994 and completed in 1995. The VCUP was not completed until early 1998. During the interval of 1995 to 1998, CDM conducted Phase 11 of the site improvements that included construction of park facilities consisting of an entranceway, parking lot, restrooms, a handicapped- accessible path from the entrance to the lake, a boardwalk into the wetlands adjacent to the lake, an open-sided sheller, and xeriscaping around the facilities. These facilities were completed in early 1998 and the park began hosting Denver Public School students. In August 1997, Sand Creek experienced a series of high fiows resulting from afternoon thunderstorms that were estimated to range from 3,500 cfs to greater than 5,000 cfs. These fiows ranged in the 2- to 10-year return interval based on hydrology developed for the Flood Hazard Area Delineation. It appears that fiows in this range are being experienced on a yearly basis, indicating that hydrology is changing, probably due to urbanization of the watershed. As a result of these fiows, as well as the fact that improvements to the Sand Creek channel were only partially completed, the channel experienced severe damage. In an attempt to increase its sinuosity, the channel circumvented the upper drop structure to the north, developed a 15-foot high cutbank downstream of the drop along the south bank, and migrated up to 120-feet north towards the VCUP slurry wall further downstream. Additional localized bank erosion also occurred downstream of the VCUP. The objective of Phase 111 of the Bluff Lake improvements was to mitigate the stream degradation caused by the 1997 high fiows and complete the improvements begun in 1993. In addition, the Department of Aviation was approached by the Colorado Department of Corrections (DOC) with a request to realign a reach of Sand Creek in the western half of the Bluff Lake site that was encroaching upon DOC property to the north. The DOC is building detention facilities on the property and was concerned that the stream would further migrate onto the property and compromise the integrity of the facility. The Department of Aviation agreed to realign approximately 1,000 feet of Sand Creek further south so the entire channel will be contained on Bluff Lake property. Other Phase III improvements included rebuilding the upper drop to also move the channel south and away from the Aurora Wastewater Reuse Plant, remediate bank erosion through the site, and build a second drop structure at the beginning of the channel realignment. This design was to start In the spring of 1998 with construction scheduled to begin that fall.