“Rehabilitation of an Existing Channel Dam Engineering and Environmental Issues Rio Vista Dam – San Marcos, Texas” by John N. Furlong, P.E., Project Manager Halff Associates, Inc. 1201 North Bowser Road Richardson, TX 75081 Laurie Anderson, P.E., City Engineer City of San Marcos San Marcos, TX 78666 Wes Birdwell, P.E., Project Manager Halff Associates, Inc. 4030 West Braker Lane, Suite 450 Austin, Texas 78759-5356 Melani Howard Environmental Coordinator City of San Marcos San Marcos, TX 78666 Lewis Yates, P.E., Special Projects Manager Fugro Consultants, Inc. 8613 Cross Park Drive Austin, TX 78754 ASCE – Fall 2006 1 Rehabilitation of an Existing Channel Dam Engineering and Environmental Issues Rio Vista Dam – San Marcos, Texas I. Introduction A. Project Setting The Rio Vista Dam located on the San Marcos River is an existing channel dam which was built in 1904. Figure 1 shows the dam prior to repair. Figure 1 – Existing Rio Vista Dam and Spillway Figure 2 shows the location of the structure on the San Marcos River just downstream from Aquarena Springs, in Rio Vista Park. It is one of several low dams on the river, but a favorite for “river running” with tubes, kayaks, and canoes due to a slot near the middle of the structure. Late in 2005, kayakers noticed that existing cracks had expanded and reported the problem to city Parks and Recreation staff. A local engineer inspected the dam on November 15, 2005 and found the dam to be “in a state of relatively slow failure due to undermining of the foundation material under the downstream portion of the dam…in particular, a 50 foot long section of the concrete apron located on the right side of the dam had cracked in several places and had dropped approximately 12 inches.” The City then placed several signs to warn the public to keep away from the dam. Proactively, the City informed the Texas Commission on Environmental Quality (TCEQ) of the problem. They decided to inspect the dam and determine its adequacy. The concrete apron was found to be separating from the body of the dam in fissures as large as a foot deep. The ASCE – Fall 2006 2 TCEQ subsequently issued a letter stating safety issues and public welfare concerns. The City then decided to go forward with an emergency repair of the structure. B. Project Understanding The existing Rio Vista Dam is about 10 feet high and 102 years old. It is a channel dam built with cedar and cypress posts in a log crib fashion which was later cover plated with concrete. The dam is located about 400 feet upstream from the Cheatham Drive bridge. The San Marcos River flows continuously at about 120-150 cubic feet per second over the dam. Four endangered and threatened species exist in this reach of the San Marcos River including native rice which grows above and below the dam structure. An old mill race exists on the left bank looking downstream. The dam site is protected by the San Marcos River Corridor Ordinance. On December 14, 2005 the San Marcos City Council heard a presentation by Mr. Scott Shipley of Recreational Engineering and Planning, Inc. (REP). At the meeting, the City Council determined both to address the dam safety concerns and add recreational enhancements as proposed by REP. The council further determined to finish the project this spring and re-open the area by May 31, 2006. The city selected Halff Associates, Inc. to work with its Engineering and Parks and Recreation departments to address dam safety and right bank concerns. The City retained REP to address in-stream improvements. Fugro Consultants was retained to assist with geotechnical concerns. Rio Vista dam Figure 2 – Location of Rio Vista Dam ASCE – Fall 2006 3 Due to the location of the dam and its park setting, the general public placed great value on this structure. The City of San Marcos sees this area as a valuable economic asset to the community. Any construction activity to repair the structure would have to occur before the height of the tourist season beginning on Memorial Day and lasting through the summer months. This timing issue placed a great emphasis on scheduling the repair to occur prior to May 26th 2006. This date was a “drop dead” date for project completion. Previous inspections of the dam were conducted by: 1) the State of Texas in 1982 and 2005, 2) the City by Freese & Nichols, Inc. in 1996 and 1999, and 3) TCEQ by R.J. Brandes, Inc. in November, 2005. The latest inspection letter to the City of San Marcos from TCEQ prompted the City staff to convene an initial project meeting on January 10, 2006 at which City staff, REP, Halff Associates, Fugro and other regulatory agencies were present. This meeting helped determine what course of action should be taken to repair the dam, what the repair would look like, and to some degree how much time and money would be involved for the repair to occur. C. Project Approach Several dam safety issues addressed in the design included: • Stabilizing the existing dam by grouting the interior mass and constructing a new buttress section adjacent to and downstream of the existing structure • Removal of part of the right abutment which had failed • Addition of two downstream drop structures to reduce turbulence and velocities • Laying back the right abutment to provide better public access. • Diversion during construction of river flow • Protection of threatened or endangered species Several state and federal agencies had permitting issues to be addressed. The agency and their area of concern is summarized below: • Texas Commission on Environmental Quality – Dam Safety Issues and NPDES permit. The TCEQ’s dam safety unit has responsibility for oversight of dam safety issues. Design and construction issues with regard to dam safety will be reviewed by their staff. The state of Texas now reviews construction plans for Storm Water Pollution Prevention Plans through the National Pollution Discharge Elimination System (NPDES) permit process. This process requires the contractor to notify the TCEQ 24 hours in advance of the start of construction with a Notice of Intent and further requires that a SWPPP be submitted for review as construction proceeds. • U.S. Army Corps of Engineers (USACE) 404 Permit – The USACE has responsibility for reviewing any instream activity which may impact waters of the United States. The San Marcos River qualifies as a “water of the United States.” This work activity falls under Section 3 of the USACE 404 permit process. Out of stream work activity, i.e., along the streambank, is covered by a Nationwide Section 27 permit under the 404 permit process. The USACE would review any work activity which impacts the streambank and abutments along the San Marcos River. • U.S. Fish & Wildlife (USF&W) Department – Threatened and Endangered Species Act - The USF&W has responsibility for preservation of any threatened and endangered species in the construction area. Two known species are the native rice and fountain darter which reside in the San Marcos River. City of San Marcos staff are working ASCE – Fall 2006 4 closely with the USF&W to assure adequate protection of the these two species and in the event of any disruption to the native rice area, it is our understanding that environmental groups are ready to transplant the rice to another water course for protection. • Texas Parks & Wildlife Department – Sand & Gravel Permit – TP&W requires the contractor to obtain a sand & gravel permit since there will be some disruption the beds and banks of the San Marcos River. The City of San Marcos will work closely with the TP&W to assure that there is minimal disruption in the work area. • Archeological Investigation – The staff at Texas State University has been requested to be present when the right bank area is cleared to check for any items of archeological significance. All permits were obtained on an emergency basis due to the threat to public safety as a result of potential dam failure. As much information as possible was submitted to all permitting agencies including maps, drawings, project description, best management practices designs, monitoring protocol, and site assessment. Emergency permits were issued with the majority of the paperwork to be accomplished after project completion. Weekly project status reports were submitted by the City to all permitting agencies. Interim issues such as intervals of turbid water, and concerns about the wild-rice were immediately addressed by the City. To avoid or minimize take of the two endangered species at this site (Texas wild-rice and fountain darter), protective barriers were placed immediately upstream of the wild-rice stands to keep construction activity away and to maintain a pool of water for the plants during site dewatering. As the area was dewatered, City of San Marcos, Texas Parks and Wildlife, and US Fish and Wildlife staff removed biota and released them upstream of the dewatered area. US Fish and Wildlife also ran transects to establish a count of fountain darters in the area of impact. Take numbers will be based on these population samples. City staff worked from sunrise to sunset capturing and releasing the wide diversity of biota in the dewatered area. The site was continually pumped to keep it relatively free of water. A detention pond was built to contain the pump discharge long enough for sediment to settle out before releasing this water back into the river. Figure 3 – City and US Fish & Wildlife staff seining the San Marcos River for fountain darters. ASCE – Fall 2006 5 City of San Marcos staff assumed responsibility for permit compliance for most of the oversight agencies mentioned above.