Project Case Study: Three layers of protection

Flood mitigation system at University of Texas Health Science Center at Houston

Project team
Prime consultant: Walter P Moore, Infrastructure Division
Structural engineer: Walter P Moore, Structural Diagnostics Division
Architects: P&W Architects and Gensler
Landscape architect: Clark Condon
Surveyor: C.L. Davis
MEP engineer: E&C Engineers and Consultants, Inc.

Project summary
Redundant systems, including a perimeter berm, floodwalls, drains, and building improvements, provide 500-year flood protection to a medical center campus.

Flood-proofing efforts result in architectural and aesthetic improvements to
the University of Texas Health Science Center Medical School Building.

Tropical Storm Allison dropped a record amount of rainfall across the greater Houston area in June 2001, flooding much of the city. Central Houston alone received about 18 inches of rain, with 11 inches falling in one 24-hour period. As the flood waters receded, officials of the University of Texas Health Science Center at Houston (UTHSCH) found that the campus in the Texas Medical Center (TMC) was devastated. The basement and ground floor of the Medical School Building (MSB) were flooded and all contents destroyed. Losses of more than $205 million included scientific equipment and supplies, research animals, medical supplies, and priceless scientific data, as well as electrical and other building systems.

Prior to Tropical Storm Allison, many basements, subterranean garages, and tunnels in the TMC and nearby Rice University flooded in a 1976 storm. In response, a berm was constructed along Fannin Street to the west of the UTHSCH to divert sheet flow from the overtopping of nearby Harris Gully away from TMC institutions. This measure, coupled with new deed restrictions and development guidelines, was expected to provide flood protection for new and existing TMC buildings.

Nevertheless, Tropical Storm Allison clearly demonstrated that the existing flood mitigation measures in 2001 were inadequate. After the cleanup, UTHSCH sought to develop a flood hazard mitigation program to reduce the potential for future flooding. To provide adequate funding, the University of Texas initiated a capital improvement effort, which included obtaining a grant from the Federal Emergency Management Agency (FEMA).

UTHSCH retained Walter P Moore to lead a project team to develop the flood hazard mitigation program. This required an in-depth look at existing conditions and development of a mitigation protection system that met the protection expectations of UTHSCH and the scope of work covered in the FEMA grant.

UTHSCH is located in the TMC along the northern edge of Brays Bayou, which drains an urban watershed of approximately 127 square miles. The campus is sited downstream of about 75 percent of the watershed area. Harris Gully, a major tributary to Brays Bayou, drains a heavily urbanized, 4.5-square-mile watershed north of the TMC campus and is channelized in two large box culverts (each 15 feet by 15 feet) running directly under the south end of the MSB.

The city of Houston storm sewer system is generally designed for a two-year storm, and the streets are designed to carry rainfall in excess of this capacity. The combination of street flooding, over-bank flooding of Brays Bayou, channelization of the Harris Gully, and ongoing subsidence of the area created a major susceptibility to flooding.

The first step of the flood hazard mitigation program was to establish the flood protection elevation. A protection elevation of 47.0 feet, corresponding to a 500-year return period flood, was selected based on FEMA requirements.

Next, the as-built condition of the complex and surrounding terrain was documented and reviewed, including existing perimeter conditions, utility locations, and the surrounding topography. The UTHSCH complex consisted of the MSB (a 10-story reinforced concrete facility with a basement), the John Freeman Building (a two-story steel frame) and the Cyclotron Building (a two-story reinforced concrete building with a basement). During development of the mitigation program, UTHSCH decided to replace the John Freeman Building, so protection was not developed for this structure, but it was designed to integrate into the future replacement building.

Layered defense
UTHSCHs specific needs required a new approach to flood protection. Because of the complexity of the site and difficulties coordinating design, construction, and operation with adjacent institutions, the team determined that the overall flood-protection plan for the UTHSCH complex would generally be designed to operate independently without the need for adjoining institutions to instigate their own flood-protection systems. Exceptions were made at certain key locations.

Based on field survey data, schematic flood-mitigation concepts were developed in coordination with UTHSCH staff. During the schematic design phase, the project team decided to design a redundant system with three layers of independent protection.

Perimeter berm—The primary protection system consists of an earthen berm and a structural concrete floodwall around the perimeter of the campus. This system enhances and extends the previous berm system constructed after the 1976 floods. The perimeter berm project included repairing and rerouting existing stormwater utilities outside the protection berm. This system is designed for a 100-year flood event.

The perimeter berm meanders through the existing landscape and roadways along the east and west sides of campus. On the north and south sides of campus, the perimeter berm ties into the secondary protection system because of a lack of setback for the existing facility buildings.

The perimeter berm consists of earthen berms, structural concrete walls, flood walls, flood gates, road humps, and various facility structures. The berm incorporates existing elements of the campus where possible, creating unobtrusive protection that enhances the aesthetics and landscaped character of the campus. The landscaping package was designed carefully to obscure critical elements of the flood-control system, and obvious protective construction has been integrated into other landscaping design features, providing a calm atmosphere in a high-traffic area.

The campus public area, Webber Plaza, is protected from flooding by the construction of the earthen berm and a structural concrete flood wall around the east side of the complex. The landscape architect directed placement of new trees, plantings, and landscape elements to integrate with the existing landscaping. Other improvements included benches and tables placed for faculty, students, and visitors to enjoy.

UTHSCH has a strong green building tradition and the protection of existing oak trees along Fannin Street was designed into the system. Wall design was coordinated with an urban forester to minimize the potential for tree damage, and a structural concrete floodwall supported on drilled piers was designed in this area to minimize tree root damage. Additional green building components included the use of 50 percent fly ash in all reinforced concrete elements on the project, with the exception of floor slabs.

Campus utilities were reconfigured to protect against hydrostatic pressure failures during a flooding event. A video survey found that existing storm drain lines located around the buildings perimeter were badly damaged and inadequately sized. These were replaced with larger lines, and all tie-ins were repaired. Roof drainage flows through a new 24-inch storm drain line directly to Harris Gully. Precipitation falling inside the perimeter berm is collected via sheet flow and landscape drains. Then it is piped to six existing sump pump stations, which were upgraded with dual submersible pumps and pits. The collected rainwater is pumped directly into Harris Gully, and the pumps are sized to function properly even if the gully is pressurized.

Integral floodwall—The secondary protection system is a structural floodwall incorporated into the perimeter columns of the MSB building. This system is designed for a 500-year flood event and is only functional in the event of a failure or overtopping of the primary protection system.

This system extends over the full perimeter of the MSB with the exception of the north side, where the MSB integral flood wall ties into a floodwall previously constructed by Memorial Herman Hospital. The floodwall separates the MSB building and the Memorial Herman Hospital building and, when combined with the MSB integral floodwall, provides full flood protection on 100 percent of the MSB perimeter. The owner required that this system be integrated architecturally into the building in such a way that the wall appeared to be part of the original structure.

The perimeter wall provides protection to an elevation of approximately 7 feet above the ground floor elevation and contains nine flood gates to allow access through the wall. In some locations, submarine glazing was provided below the flood elevation to allow light into the ground floor and to minimize the aesthetic impact of the flood wall. To allow egress from the building during flooding or when the flood doors are closed, eight new elevated exterior stair platforms were added on the stair towers.

The basement walls were checked for strength under negative hydrostatic and earth pressure assuming saturated earth conditions and associated surcharge loads from standing water at the building perimeter. The backfilled walls were found to have adequate strength if the formation of plastic hinges at the supports was allowed. However, while the common wall basement between the MSB and Memorial Herman Hospital was found to have adequate strength under positive loads (protecting Memorial Herman Hospital if the MSB floods), the wall has insufficient strength under negative loads (the design conditions of flooding Memorial Herman Hospital). Therefore, the wall was strengthened by adding additional externally bonded negative reinforcing.

The basement walls contained large numbers of mechanical, electrical, and plumbing (MEP) penetrations. The MEP consultants and structural engineers designed systems to make all penetrations water resistant. Expansion joints and wall cracks with evidence of past leakage were reworked using the injection of expansive urethane grout to prevent water seepage into the basement.

Interior floodwalls—The tertiary protection system consists of interior protection of critical areas and lower basement levels within the MSB. This system is designed for a 500-year flood event and is only functional in the event of a failure of the primary and secondary protection systems.

Interior floodwalls in the basement and at ground level divide the building into two sections, preventing complete flooding of the building in the event of a failure of the secondary system. An additional tertiary protection wall was constructed around the ground level MEP room to prevent power loss in the event of a failure of the secondary system, regardless of the location. Protection of the MEP system is critical because flooding of the MEP room prevents use of the entire building. Because a basement is located under the MEP room, and the flood protection elevation is approximately 6 feet above the first floor elevation, the floor slab of the MEP room was strengthened against hydrostatic uplift.

The UTHSCHs MSB facility presented a unique challenge for the flood mitigation project team because of the clients stringent requirements:

  • Systems had to provide a high level of reliability.
  • Systems had to be architecturally and aesthetically acceptable. The client did not want a flood-protection system that had the utilitarian look of a flood-protection system. The 500-year protection had to be integrated into the face of the building such that it is architecturally invisible and appears to be part of the building structure.
  • The flood protection plan had to be green, with minimal impact on existing vegetation.
  • The site was tight, with extensive mapped and unmapped utilities.
  • The building had to stay in operation during construction without disrupting the occupants.

The three-layer flood-protection system developed by the project team creatively worked the perimeter berm and building improvements into the overall campus landscape and met all of the requirements of the owner. Especially significant, the construction cost was approximately 20 percent less than the cost budgeted during the schematic design phase.

Eric Green, P.E., is a principal in the Structural Diagnostics Services Group of Walter P Moore; and Diego Monroy is a graduate engineer in the firms Civil Engineering Services Group. They can be reached at 1-713-630-7300.

Posted in Uncategorized | January 29th, 2014 by

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