USING A LABYRINTH WEIR TO INCREASE HYDRAULIC CAPACITY Dustin Mortensen, P.E. 1 Jake Eckersley, P.E. 1 Plum Creek Floodwater Retarding Structure No. 6 is located in an area of Kyle, Texas, that is currently undergoing a transition from rural to suburban development. As a result of the development the structure has been reclassified as a high hazard dam and is in need of upgrades to comply with current dam safety and performance criteria. Plum Creek Conservation District (District), in collaboration with the National Resource Conservation Service (NRCS), evaluated several alternatives to rehabilitate the structure. Their efforts considered the presence of residences near the dam s downstream toe and others located within the upstream flood pool. The District and NRCS ultimately selected to rehabilitate the structure by replacing the existing vegetated auxiliary spillway with a concrete labyrinth weir spillway and installing a larger principal spillway conduit. This alternative limits the required construction footprint and maintains the maximum upstream water surface elevation. The proposed 210 feet wide labyrinth weir spillway consists of 7 labyrinth cycles and a Saint Anthony Falls stilling basin at the downstream end. The structure will be built into the existing earthen embankment and will require the realignment of an existing Hays County road as part of the project. County coordination has been required to realign the road and satisfy floodplain requirements due to the increased discharge through the larger principal spillway. The project is being performed under the NRCS s Small Watershed Rehabilitation Program, with construction funding being provided by the NRCS, Plum Creek, and the Texas State Soil and Water Conservation Board (TSSWCB). This paper presents the overall design and coordination with Hays County and Texas Parks and Wildlife Department. Introduction Throughout the 1960s and 1970s, the Natural Resource Conservation Service (formerly Soil Conservation Service) constructed over 2000 small watershed dams in Texas and over 10,000 nationally under the authority of the Watershed Protection and Flood Prevention Act of 1954 (Public Law 83 566), Flood Control Act (Public Law 78 534), Pilot Watershed Program 1953/1954 and the Resource Conservation and Development RC&D) Program. These structures were designed to protect downstream agricultural areas and to prevent the adverse economic and physical effects of flooding through the entire watershed community. The dams were designed by NRCS and are operated and maintained by local sponsors. Plum Creek Conservation District is the local sponsor for twenty eight dams, including Plum Creek Floodwater Retarding Structure No. 6 (Dam 6). At the time of construction, the economy of the Plum Creek Watershed was almost entirely agricultural. However, in the last 10 to 15 years the area surrounding Dam 6 has developed into a suburban area. Existing Conditions When Dam 6 was designed and constructed in 1967 it was considered a low hazard dam due to the minimal impact it would have on lives lost and property lost if it were to fail. However, due to the development around Dam 6, it is now considered a high hazard structure. Several features at Dam 6 led to the high hazard classification. There are six residential properties, including a four plex, located at the downstream toe that would be inundated in the event of a dam 1 Freese and Nichols, Inc., 10431 Morado Circle, Suite 300, Austin, Texas,78759, (512) 617-3100, dgm@freese.com, jwe@freese.com
failure. Additionally, Goforth Road, a county owned and operated thoroughfare, runs parallel to the dam embankment 25 feet downstream. The as built drawings from the original dam construction indicate this road was to be abandoned when the structure was built. Instead, the usage of Goforth Road by the local community has increased and it now handles approximately 8,000 vehicle trips per day and serves as a primary route for local school buses. Goforth Road crosses the auxiliary spillway and creates breaches in the spillway dikes. There are also many residences located around the reservoir below the top of dam elevation. Figure 1 shows the dam and the location of Goforth Road and structures. Structures located in lake area below top of dam Principal Spillway Auxiliary Spillway Homes and businesses along downstream toe Figure 1: Aerial view of Dam 6 and surrounding area. For high hazard dams, TR 60 criteria require that the principal spillway pass the runoff associated with the 100 year rainfall without engaging the auxiliary spillway. The existing Dam 6 principal spillway is a 30 inch diameter concrete conduit which passes beneath Goforth Road and discharges into a plunge pool on the downstream side. The existing principal spillway does not meet the current TR 60 criteria for high hazard dams as the auxiliary spillway engages during storms larger than the 75 year, 24 hour storm event. The resulting flows through the auxiliary spillway inundate Goforth Road during flood events smaller than the 100 yr event, thus making it a public hazard. TR 60 also requires that the auxiliary spillway pass the runoff associated with the probable maximum precipitation (PMP) without overtopping the dam. Hydrological analyses indicate that Dam 6 would be
overtopped by the PMP storm event. This could lead to dam failure and would put approximately 42 homes, 8 businesses and several roads in harm s way. Rehabilitation Once it was determined that Dam 6 does not currently meet the appropriate dam safety guidelines, NRCS and the District began evaluating alternatives to rehabilitate the dam. In order to increase the reservoir s ability to accommodate a 100 year rainfall event before engaging the auxiliary spillway, the principal spillway capacity will be increased by replacing the existing 30 inch diameter conduit with a 42 inch diameter conduit. This will increase the maximum principal spillway discharge from approximately 100 cfs to over 220 cfs, allowing the principal spillway to pass more water earlier in the storm event. The increased discharge through the principal spillway will prevent discharge through the auxiliary spillway prior to the 100 year storm event. This larger principal spillway also provided enough capacity to allow the auxiliary spillway crest to be lowered by approximately 8 inches, which reduced the required width of the auxiliary spillway, while still maintaining the current top of dam elevation. The next step in designing the modifications to the dam was to determine how to allow the structure to handle the full PMP storm event to prevent overtopping of the dam. The proposed alternatives were the following: 1. raise the top of dam, increasing the storage capacity of the reservoir, or 2. increase the discharge capacity of the auxiliary spillway enough that the PMP water level would not exceed the top of the current dam. In most situations in Texas, the height of the dam would be increased in order to store the excess water. Several factors at Dam 6, however, made raising the top of the dam unrealistic. Houses located in the upstream floodplain would be further inundated if the maximum water surface elevation were raised. Additionally, raising the dam with earthen fill would require increasing the dam s footprint in the downstream direction. Houses currently located along the downstream toe prevent this footprint increase. Since raising Dam 6 was not considered feasible, increasing the spillway discharge capacity was the next dam modification considered. The increased spillway capacity could be added through an additional auxiliary spillway, widening the existing open channel auxiliary spillway, or through lowering the spillway crest. In order to pass the design discharge of 26,000 cfs, a vegetative spillway of approximately 650 ft width would be required at Dam 6. However, there was not sufficient land available adjacent to the existing 350 ft wide spillway. Furthermore, an additional spillway on the right end of the dam is not feasible due to the presence of homes and businesses. With no room on either end of the dam, encroaching into the embankment was the last remaining option available to add the additional spillway capacity. In order to minimize the impact to the embankment, NRCS evaluated and ultimately selected a concrete labyrinth weir placed into the existing embankment to replace the existing auxiliary spillway. A labyrinth weir is a type of spillway which increases the discharge weir length without increasing the channel width by folding the weir in a zig zag pattern (accordion like) in plan view within the set confines of training walls. A series of trapezoids are formed which typically increase discharge capacity for a given channel width by approximately three times the discharge capacity of a straight weir. Plum Creek 6 will be the first labyrinth weir used by NRCS in Texas. While the labyrinth weir is innovative and a significant change from a typical vegetative lined,
earthen spillway, the concept has been utilized since the 1970s and there are now over 80 labyrinth weirs in the United States. Due to the increased efficiency of the labyrinth weir vs an earthen/vegetated spillway, the labyrinth weir will be 210 ft wide, rather than the 650 ft feet needed for a linear weir. The labyrinth weir provides the entire spillway capacity and allows the existing earthen spillway to be eliminated. This new auxiliary spillway also provides an increased level of protection to Goforth Road. Figure 2: Proposed Labyrinth Weir at Plum Creek Dam 6 The flow velocity through the auxiliary spillway will be over 40 feet/second as it drops 23 feet from the labyrinth crest to Goforth Road. In order to protect the road and downstream channel from the flow, a Saint Anthony Falls (SAF) stilling basin will dissipate the water s energy. The SAF basin incorporates chute blocks, baffle blocks and an end sill to dissipate the energy. The SAF basin floor elevation will be 8 feet below Goforth Road to provide adequate tailwater and to allow two 5 ft x 5 ft culverts to pass flow beneath the road and increase the level of protection of Goforth Road. The labyrinth spillway will engage at the 100 year storm event and Goforth Road will be not be overtopped until the principal and auxiliary spillway discharge exceeds the culvert capacity of approximately 500 cfs, which is more than twice the peak discharge of the 100 year storm. Hays County and Texas Parks and Wildlife Department Coordination In the planning phase and throughout design, NRCS and Plum Creek Conservation District coordinated with various government entities to ensure the proposed design and dam rehabilitation would comply with County and State regulations. These entities included Hays County Transportation Department, Hays County Floodplain Administrator and the Texas Parks and Wildlife Department (TPWD). Hays County Transportation Department performed a geotechnical investigation along the proposed roadway alignment, provided the pavement design, and requested that the project include gates to restrict access to the portion of Goforth Road that will be inundated when the spillway discharge exceeds the culvert capacity. As the design of the modifications progressed, the Transportation Department
reviewed the proposed horizontal and vertical alignments for compliance with local regulations. The County will also have a construction inspector observing construction of the roadway. The project modified the discharge characteristics of the dam and had the potential to impact the FEMA regulated 100 year floodplain. An analysis of the project s impact on the 100 year floodplain was evaluated and submitted to the Hays County Floodplain Administrator. While the new principal spillway doubles the peak discharge during the 100 year storm event, the water levels remain within the existing FEMA Zone A floodplain. Coordination with TPWD was required due to the necessity of draining the lake in order to perform the rehabilitation construction. The drained lake will necessarily displace the fish and aquatic wildlife. TPWD requested that an aquatic resource relocation plan (ARRP) be prepared and implemented to reduce the impact that the project will have on the state s fish and other aquatic life. The ARRP will be developed in coordination with TPWD and will identify the purpose of the activities, method of collecting and moving the aquatic life, documentation methods to be used, and the disposal methods required to remove dead and non native fishes. As part of the plan preparation, TPWD requested that a mussel survey be performed to determine if any threatened or endangered mussels live in the lake. It is anticipated that the ARRP will be prepared for Dam 6 in the summer of 2015. The fish in the lake at Dam 6 will be collected and transported to a nearby lake. Conclusion The rehabilitation of Plum Creek Dam 6 uses an innovative design to rehabilitate Plum Creek Dam 6 so that it will meet current dam safety criteria. The project will increase public safety and provide economic benefits due to flood control. The labyrinth weir auxiliary spillway is a first for NRCS in Texas. The project also successfully addressed the concerns of multiple public agencies through relocation of the Goforth Road, evaluation of impacts on the floodplain and protection of aquatic resources.