By Mike Johnson

One of the treasures of Texas tourism, San Antonio's Paseo del Rio - or River Walk - dates back to 1536. Beginning in the 1920s, the citizens of San Antonio authorized a series of projects that have resulted in the River Walk as it is today. The current River Walk encompasses a 2.5-mi. portion of the San Antonio River that runs through the heart of the city.

The city and other stakeholders have begun a 10-year, $140 million improvements project aimed in part at further restoring portions of the San Antonio River.

As part of that continuing evolution, the City of San Antonio, Bexar County, the San Antonio River Authority, the San Antonio River Oversight Committee, the US Army Corps of Engineers, and several other stakeholders have initiated a number of projects to extend elements of the River Walk and reclaim and restore portions of the river.

Those projects comprise the River Improvements Project, a 10-year, $140 million investment in flood control, amenities, ecosystem restoration, and recreational improvements to the river, both north and south of downtown.

Improving the Museum Reach

The project's northern section - known as the Museum Reach due to the presence of two of San Antonio's finest museums along its length - extends approximately 4 mi. from the existing northern terminus of the River Walk to just south of the river's historic headwaters. This reach is divided into the Urban and Park Segments.

The Museum Reach's Urban Segment will mirror and extend many of the elements found in the existing River Walk. The Park Segment is intended to revitalize the river in a more rural context, with softer sloping banks containing more opportunities for ecosystem restoration and parklike activities.

Overall, the primary goal of the Museum Reach improvements is to provide stable, maintainable flood control while adding recreational opportunities.

Diversity Challenges Design

The need to balance flood conveyance and control with a sustainable, linear park concept presented many challenges to the design team of HDR and Ford, Powell & Carson.

Both Urban and Park Segments must incorporate potentially diverse and competing features - such as architectural amenities - that will create a recreational, linear park environment for residents and tourism; ecological features that will enhance benthic, fish, and riparian habitat opportunities; flood conveyance capabilities that will handle 100-year flood flows; and maintenance and operational features that will allow for efficient, cost-effective, long-term operation of the improvements.

The hydrology and hydraulics of the San Antonio River in these areas are critical design factors for all features. Consequently, a thorough hydrologic and hydraulic analysis of the river - and particularly the Urban and Park Segments - was necessary to characterize the impact of base and flood flows on the planned improvements.

Hydrology, Hydraulics Provide Basis for Improvements

HDR updated, modified, and calibrated the existing hydrologic and hydraulic models for the entire San Antonio River watershed to the October 1998 storm - one of the largest storms of record for the area. The updated models were then used to quantify the flood magnitudes and elevations for both existing and proposed conditions during a variety of storm events, including 10-, 25-, 50-, 100-, and 500-year design events.

This information was used to set expected flood levels, establish potential bed shear and tractive force quantities, and establish the expected current and future sediment load characteristics. Flood-flow magnitudes and elevations are important in the establishment of sidewalk elevations, bridge impact analyses, and the evaluation of existing and proposed condition floodplains. As part of the design criteria and in relation to responsible floodplain management, the project cannot raise the existing 100-year floodplain elevations or create flooding conditions worse than those that already exist.

The bed shear and tractive force information was used in the design of a stable channel template and bank stabilization options. A sediment transport analysis was also compiled to establish expected sediment aggradation or degradation areas and a sediment budget. This will be used to evaluate the long-term ecological and maintenance impacts of the improvements affected by excess sediment accumulation or removal.

Ecological Solutions for Urban, Park Segments

Both segments of the Museum Reach portion of the project will feature enhancements to the river itself.

Both segments must incorporate provisions for ecological restoration and sustainable, natural wildlife populations. This is a particular challenge in the Urban Segment, where the river edges must be armored against flood damages, pedestrian influences, and wave action generated by barge traffic.

To solve this problem, the project will incorporate several standard structural features that create locations to harbor wildlife, such as fish “lunker boxes” and shallow water marsh areas for small fish and foraging riparian bird populations. Any vertical concrete surfaces below water will be “roughened” using form liner techniques to enhance the establishment of benthic populations. Finally, the river bottom itself will not include any “hard” surfaces and will be graded to include deep pools at the outside of bends and shallow areas at the inside of some bends to simulate natural geomorphic features. To allow for maintenance access, the river bottom will also incorporate a broken rock pathway to allow for operation of rubber-tire or track-type equipment.

Dam and Lock System Is Key to Urban Segment

The master plan for the Urban Segment calls for the ability to allow existing barge traffic to venture more than a mile north of the current northern River Walk limits, a section of the river not now navigable. The improved Urban Segment must be able to accommodate barge traffic throughout most of its length while at the same time allowing pedestrian access and flood conveyance for the 100-year storm.

The Urban Segment will incorporate a small dam and lock system to lift the barges from a lower pool elevation of 631.2 ft. to an upper pool elevation of 640 ft. The entire structure must fit within a total width of less than 100 ft. To optimize aesthetics and the ability for people to interact with the structure, the dam and lock must maintain as low a profile as possible. Therefore, the design team has set the top of the structure at a little more than 3 ft. above the upper pool elevation and designed the entire structure to survive overtopping during a flood event.

The lock system will consist of a gravity-fed, dual-chamber system. The normal base flow of the San Antonio River at the lock location will average only 20 ft.³/sec. The dual-chamber system can be optimized to limit upper and lower pool surging by emptying one chamber while filling the alternate chamber. The elimination of flow and elevation surges in the upper and lower pools is important for passenger comfort and barge navigation. The gravity-fed lock system also has the fewest moving parts: two pairs of hydraulically actuated miter gates and four hydraulically actuated slide gates for filling and drawdown operations. This all translates into increased reliability and minimized operations and maintenance costs.

Restoring a Channel in the Park Segment

The Park Segment improvement is primarily the restoration of a small, concrete-lined trapezoidal channel - known as the Catalpa-Pershing Channel - to revive its ecology and provide additional flood conveyance and some bank stabilization. The vision for the restored Catalpa-Pershing Channel is to remove as much of the existing concrete as possible and restore a more natural channel meander and planform to this segment. Non-native plant species that have invaded the channel corridor will be identified and removed, if practical, and native riparian plant communities will be reestablished. The flood capacity of the existing channel is degraded by two small, existing bridge structures that impede flood flows and cause localized flooding. These bridges will be reconstructed to adequately pass the 100-year flood flows.

The Future of the River Walk

The preliminary design for the Museum Reach was slated for completion in early 2003. Final design activities will be broken down into three projects, scheduled for construction over the next seven years. The Urban Segment will be designed in two separate packages scheduled to start construction in 2004 and 2007. The Park Segment is scheduled for construction in 2009.

Mike Johnson, P.E., is with HDR's San Antonio, TX, office. This article originally appeared in the spring 2003 issue of Innovations, a technical publication of HDR.

SW - July/August 2003

Home + About + Subscribe + News + Calendar + Glossary
Talk + Images + Advertise + Contact Us + Search + Email + Services

Erosion Control Magazine | MSW Management Magazine
Grading & Excavation Contractor | ForesterPress | StormCon | Forester Media

© 2000 - 2001 FORESTER MEDIA, INC.