Feature image: One example of the Garden’s commitment to conservation, the design accommodated existing trees on-site and strict criteria was developed for working around them. During constriction, contractors had to work equipment strategically around a tree protection fence to ensure they did not damage the existing trees.

Thirty years ago, a small contingent of ambitious Houstonians witnessed firsthand the positive impact botanic gardens were having in different communities around the globe. Their determination and dedication eventually resulted in the Houston Botanic Garden, a crown jewel in the city’s revitalization of its green spaces.

In January 2015, the goal of bringing a botanic garden to Space City became a reality when the Houston City Council unanimously approved a long-term lease agreement and partnership with the Houston Botanic Garden to transform the former Glenbrook Golf Course.

“They had a desire that Houston join in the growing interest in botanic gardens and add to the city’s ecosystem of cultural institutions by creating a world-class garden in Houston,” says Claudia Gee Vassar, president and general counsel at the Houston Botanic Garden.

Located near Hobby Airport, the 132-acre Houston Botanic Garden offers easy access for the greater Houston region and the opportunity to develop the garden around the Sims Bayou. Before the construction of the Garden, the area saw very little investment in green spaces or cultural amenities.

“The influence and integration of the bayou in the Garden allows visitors to understand these beautiful waterways that are integral to the resiliency of the Bayou City,” Vassar says.

The Garden was designed so visitors would be inspired to explore the spacious grounds. Upon entering the Garden’s gate, the live oak-lined Botanic Lane, which offers an immediate calming effect on visitors, provides a glimpse of what lies beyond. Once inside, the Garden includes eight different facets, each of which serves an educational component: Global Collection Garden, Susan Garver Family Discovery Garden, Culinary Garden, Pine Grove, Community Garden, Woodland Glade, Stormwater Wetlands, and Coastal Prairie.

The Garden also features 2.5 miles of walking and hiking trails, a lagoon, and a nature play area for children.

Addressing Design Challenges

The Sims Bayou channel runs through the middle of the Houston Botanic Garden dividing it into two separate areas: the Island and the South Gardens. Because the site was a former golf course on an island off Sims Bayou, many of the project’s difficulties involved balancing the desire to maintain the existing tree canopy while constructing the landscape and structures of a botanic garden.

“Instead of resisting the vestigial golf course layout, we strategically designed the layout and systems to avoid as many existing obstacles as possible,” says Donna Bridgeman, senior landscape architect at West 8.

Global engineering firm Walter P Moore was involved in the initial site review and served a critical role on the building team that created the more than $100 million master plan for the Houston Botanic Garden. Construction on Phase I of that master plan—which incorporated existing natural features and elements to mitigate potential flooding—commenced in spring 2019 and was substantially completed in fall 2020.

“Walter P Moore was instrumental in spearheading the challenges of permitting a multiphase project while leading the design of challenging structural, grading, and drainage systems,” Bridgeman says. “Their team was a key collaborator among a larger group of consultants working with the client and contractor to navigate technical complexities and field conditions with creative engineering, allowing the project to realize unconventional spaces and systems.”

Detention and Drainage

To develop the property, which encompasses an area of Sims Bayou and includes the floodway as well as 100- and 500-year floodplains, permitting required coordination with the U.S. Army Corps of Engineers, the Harris County Flood Control District, and the City of Houston.

The U.S. Army Corps of Engineers’ responsibility included the normal high water of the bayou for Section 404 permitting of wetlands as well as for Sims Bayou as a navigable waterway. The Harris County Flood Control District has jurisdiction over the bayou and maintains the floodplain maps for the county. Finally, the City of Houston’s Floodplain Management Office provides floodplain jurisdiction, as well as permit jurisdiction, for development within the city.

During the design phase, and in response to Hurricane Harvey in 2017, the city changed the detention and floodplain mitigation criteria for any development within city limits, which required additional coordination with the Houston Public Works Department on the project.

Because the Houston region is susceptible to flooding, the city and Harris County carefully reviewed the development criteria within the floodplain to address the flooding issues in the region.

“The permitting process of this project was at the time when Houston was about to update its detention and floodplain requirement,” says Chong Wei Ooi, senior associate at Walter P Moore.

As a result, the new detention and floodplain volume mitigation were incorporated into the project’s master plan. The floodplain mitigation on the site was designed as the Coastal Prairie, which is a major ecosystem of the Houston region, to handle the inundation of stormwater during major rain events.

“After the water is recessed, naturally formed depressed areas in the Coastal Prairie temporarily hold water, and, over time, a unique ecosystem is developed in this area for flora and fauna,” says Ooi.

Additionally, a detention pond—designed jointly with West 8 and Texas A&M AgriLife Extension’s Texas Community Watershed Partners (TCWP)—serves as a Stormwater Wetlands exhibit using the permanent body of water to allow for planting of aquatic plants.

The Houston Botanic Garden invited TCWP to help improve the wetland. Working in concert with the building team, TCWP helped tackle the problem of creating wetlands within the detention area. A volunteer group from TCWP did the planting and consulted on the wetland design.

One of the university professors involved, Fouad Jaber, an associate professor and extension specialist in the biological and agricultural engineering department at Texas A&M AgriLife Extension Service, specializes in wetland drainage, which helped further the chance of success. Working in concert with Walter P Moore, they helped determine how long the water needs to stay within the wetland, how to control the water level, how much elevation drop was needed, and where to put the planting shelf, so it works as a wetland and for detention.

Typically, offsetting the fill volume within the floodplain by removing the same amount through excavation addresses floodplain mitigation requirements. In most other developments, the areas excavated for floodplain mitigation are seeded and left unattended after completion.

Furthermore, the Garden’s innovative drainage structure in the Stormwater Wetlands was designed so it could be partially hidden within the bank of the pond’s slope. During a rain event, the amount of water that fills the Stormwater Wetlands is dependent on the size of the storm event. Water above the Stormwater Wetlands area becomes detention volume. The structure’s weir wall controls stormwater release to drain within 24 to 48 hours.

Included in the weir wall are small orifices designed to control the water level and regulate release. The orifice is a means of water flow control to imitate the pre-development storm runoff condition. During a rain event, the pond water level will rise beyond the Stormwater Wetlands, and the detained stormwater will be released slowly into a nearby stream. A valve system allows for additional control of the water level for maintenance purposes. One valve drains the entire Stormwater Wetlands, and a second valve controls the water level of the Stormwater Wetlands.

Additionally, the stormwater structures provide special use needs of the Stormwater Wetlands and the children’s lagoon including water quality, education, and erosion control. The structure incorporated functions for pond water level control, outflow restriction, and filtration for debris with a trash screen. The point of discharge from the outfall structure is also protected with riprap for erosion control. The special structures were designed to facilitate maintenance of the water features. The special structures also help to create the sustainable wetland and aquatic features in the Gardens that aid in water quality, education on the benefits of wetlands and ponds, and to control erosion in the bayou downstream.

Topography

The Houston Botanic Garden has a wide variety of topography that made the design of the storm system to collect runoff quite challenging. The building team worked with landscape architect Clark Condon Associates Inc. to design adequate drainage. Clark Condon designed the subsurface drainage within the landscape bedding and provided recommendations for landscaping suitable for local climate. Additionally, Walter P Moore continued to coordinate with TCWP for its expertise related to the design of the wetlands.

To address the topography issues of the island created by the divergence of the Sims Bayou meander and channel, as well as areas of the Garden to the south of the channel, Walter P Moore designed a storm detention ponds to collect runoff for 100-year storm events before draining to the bayou through existing storm pipe to prevent impacting existing bayou bank that may lead to erosion. Additionally, in the Global Collection Garden, an underdrain was included in order to provide additional drainage to help counteract the slow-draining native clay soils, providing for plant materials that require better drainage. Some plants in the Garden can withstand inundated soil for a longer period of time. The underdrain allows moisture in the soil to drain out quickly. Finally, multiple raised walls were made into tiered planter boxes for the Culinary Garden. Using underdrain perforated pipes connecting from planting wells to point of discharge, the design ensured there was enough drainage to collect runoff within the planter boxes.

Valuing Conservation

Knowing the importance of water to the Garden as a valuable natural resource, an additional riser structure was designed for the lagoon near the children’s play area. A small aquatic garden surrounds the lagoon, which is separated into two sections.

The riser structure for this lagoon posed an unusual challenge to the design team. City storm water detention criteria required a weir structure to control flow leaving the detention for different storm events, but the location of this structure required it to be concealed from public view in order to reduce the aesthetic impact to the garden. Therefore, the weir was incorporated inside a storm vault under a proposed decomposed granite path. However, the storm vault is quite deep, because the weir elevation needed to close to the pond elevation, while the bottom of the vault needed to intercept the existing storm pipe connected to Sims Bayou for ultimate discharge.

Reusing Harris County Flood Control District infrastructure allowed the project to reduce the construction impact to Sims Bayou and its banks. Walter P Moore worked closely with the Harris County Flood Control District to protect and not disturb the original outfall pipes by designing an intercept structure upstream of the existing pipe into Sim Bayou. The engineered openings on the box allowed for the control and release of runoff from the proposed site. Working with the Garden’s facility group, Walter P Moore designed a drainpipe and valve system at the bottom of the control structure that allows the Garden to drain down the lagoon for maintenance purposes.

Meanwhile, conservation goals also had an impact on how the floodplain was constructed. The Garden required excess soil material to be reused elsewhere on the project. To accomplish this, Walter P Moore used Civil3D to create multiple earthwork and grading design iterations of the entire project. As a result, excess material was limited to approximately 2,000 cubic yards of earthwork, which was recycled in the Global Collection Garden.

“The team designed with the existing soil employing an extensive rehabilitation process of harvesting layers of existing soil, stockpiling, and restructuring soils with cover crops and amendments,” Bridgeman says. “We avoided significant imports of soil volumes this way and improved an existing condition to support the wide variety of plants and trees.”

In another example of the Garden’s commitment to conservation, designs accommodated, and strict criteria were developed, for working with existing trees.

“We had to work equipment strategically around the tree protection fence to ensure we did not damage the existing trees on the site,” says Carolina Hernandez, project manager at Harvey Builders.

The trees are a significant and stunning part of the entry drive and provide shade in the Picnic Grove. The landscaping designs highlighted the proper spacing from these trees so they could continue to thrive and flourish in a new setting. The designs contained no grading around the trees, creating a design problem because of all the landscaping additions and a preference for adjacent areas with higher elevations. The result is a blend of new and existing flora and fauna.

Bridge Design and Maintenance

Because the site included the original Sims Bayou meander and a newer channelized bayou cutting through it, connectivity across the bayou provided a unique problem for the building team in creating access to and throughout the garden.

As part of the site review, five pedestrian bridges were evaluated for structural integrity. Improvements were recommended to extend the life span of the existing bridges. Some of the issues included deterioration from deferred maintenance, incomplete existing bridge drawings and documentation, and scour.

Ultimately, a new entry bridge, which allows vehicle, bicycle, and pedestrian access from Park Place Boulevard to the Island, was designed and constructed to cross the Sims Bayou meander.

To accommodate the bridge, areas at both ends were filled to raise the approach elevation achieving the required floodplain clearance from the city and Harris County Flood Control District. (Approval was also required from the U.S. Coast Guard because Sims Bayou is considered a navigable waterway).

“The low chord elevation is 25.65 feet above sea level and approximately 2 feet above the 100-year flood event,” says Muna Mitchell, managing director, bridge engineering at Walter P Moore. “The City of Houston provides minimum freeboard requirements so new bridges do not block stream flow during flood stages or flood events.”

The raised approaches to the new bridge resulted in an additional prairie area just beyond the entry garden, featuring native coastal grasses and plants that can tolerate occasional inundation.

Positive Reaction

At the onset of this project, the goal was to give Houstonians another serene setting where they could enjoy nature and experience the physical and emotional health benefits of spending time outdoors.

“The Garden is an enhancement to the city’s infrastructure and will serve as a place for inspiration and education for all Houstonians and its visitors,” says Houston Mayor Sylvester Turner.

Since opening its doors, the Houston Botanic Garden has admitted more than 26,000 ticketed visitors and continues to welcome first-time and returning guests every day.

“For being the first botanic garden in Houston, the reaction from the public has been overwhelmingly positive, especially during a global pandemic,” Turner says.

Related Resources

Houston Botanic Gardens: https://hbg.org/
Mayor Turner: https://www.houstontx.gov/mayor/
West 8: https://www.west8.com/
Walter P Moore: walterpmoore.com
Harvey Builders: https://www.harveycleary.com/
Muna Mitchell: https://www.walterpmoore.com/muna-mitchell
Ted Vuong: https://www.walterpmoore.com/ted-vuong

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Ted Vuong PE, LEED AP, ENV SP is a principal and managing director of Civil Engineering Services at Walter P Moore. He can be reached at [email protected].