Shoring up Shoal Creek

Jan. 1, 2010

Shoal Creek, in Austin, TX, runs through Pease Park, a popular recreational area near the campus of the University of Texas. Like many urban streams, Shoal Creek exhibits problems related to flooding, erosion, and degraded water quality. Seeking to address these ailments where they occur within the park, the city of Austin is pursuing a comprehensive set of integrated solutions intended to stabilize the stream channel, better manage stormwater, and restore heavily degraded riparian zones. Overall, the project is intended to restore the health of the creek and improve the park experience for all visitors.

Heavy public use has taken its toll on significant sections of the park, including riparian areas. Home to a popular disc golf course, the park sometimes attracts hundreds of players on a single day. The resulting foot traffic has caused extensive erosion, soil compaction, and loss of vegetation, leading to increased sediment loads in Shoal Creek. Other problems to be addressed within the park include channel instability, bank erosion, degraded stream habitat, hillside erosion, dog wastes, and stormwater runoff from neighboring properties.

In September 2008, the city of Austin hired the consulting engineering firm PBS&J to design and prioritize a series of measures to address the problems within Pease Park. The following firms assisted PBS&J with this task: Glenrose Engineering Inc., HVJ Associates Inc., Frank Lam & Associates Inc., MWM Design Group, Landmark Surveying Inc., and Winterowd Associates. After conducting field investigations to identify trees, define soils, and delineate and characterize the creek’s riparian corridor, PBS&J conducted hydrologic and hydraulic analyses to develop preliminary design solutions to stabilize the streambank and channel along approximately one mile of Shoal Creek. Meanwhile, the project team designed 11 “green infrastructure” projects–including biofiltration ponds, flow spreaders, a rain garden, and a pervious parking lot–to treat runoff that enters Shoal Creek untreated and unmanaged from approximately 100 acres of nearby developed land. Measures to stabilize and restore roughly 32 acres of riparian corridor and parkland also were developed.

Photo: PBS&J
Displaying some of the worst bank erosion within Pease Park, this section of Shoal Creek includes high, unstable, unvegetated banks.
Photo: PBS&J
The park’s hike-and-bike trail generally follows Shoal Creek, providing a commuter route as well as access to the various playing fields and other recreation areas.

Setting Multiple Restoration Goals
A greenbelt recreational area, the 43-acre Pease Park mostly extends in a linear fashion along the banks of Shoal Creek, a flashy, flood-prone urban waterway that was once a spring-fed, perennial creek. The hike-and-bike trail that generally follows the creek provides a commuter route, as well as access to the various playing fields and other recreation areas. Given its proximity to Austin’s urban center and its various amenities, the park attracts numerous visitors engaged in activities that are not available at other nearby parks. For example, an 18-hole disc golf course extends among the trees and across Shoal Creek. Part of the park is designated a “leash-free” zone where dogs are allowed to roam. Additional features include three sand volleyball courts, a wading pool, and an area with concrete picnic tables long enough to seat at least 200 people.

The park encompasses a wide range of terrain and vegetation, including open fields, grassed meadows, and shaded woodlands along Shoal Creek. However, the increasing popularity of the park has led to severe ecological damage in many locations. Essentially, Pease Park is being “loved to death” by its many users.

Although some areas of the park receive little human foot traffic, other sections have been trampled to the point that the grass, forb, and understory vegetation is gone. Without these root systems, the soils have eroded, and in many areas only the sturdiest trees remain, surrounded by stony ground. Large and small trees located along the disc golf course display significant bark damage as a result of the sport. Throughout the park, exotic and invasive grasses, shrubs, cane, and trees encroach upon the habitat of native plants.

Meanwhile, Shoal Creek itself has experienced significant degradation, the result of upstream development that has sent increasing amounts of stormwater runoff to the waterway. All told, Shoal Creek extends approximately 9.5 miles from its headwaters to its junction with Lady Bird Lake on the Colorado River in downtown Austin. The stretch of Shoal Creek that passes through Pease Park is about 1.3 stream miles from the mouth of the stream. As a result, most of the nearly 13-square-mile Shoal Creek watershed contributes to the reach that extends through the park.

This relatively long and narrow watershed is highly urbanized and fully developed, exemplified by its 50% to 60% impervious cover. The resulting increase in runoff has caused significant widening and degradation of the creek within Pease Park in recent decades. Meanwhile, older neighborhoods located immediately adjacent to the park were built before the city of Austin implemented its requirements for managing stormwater. As a result, the neighborhoods feature curb-and-gutter systems that whisk runoff into storm drains that discharge directly into Shoal Creek.

Faced with these problems, the city seeks to restore as many of the park’s beneficial uses as possible. By stabilizing the stream channel and installing innovative measures for managing stormwater onsite, the city aims to improve its water quality. By restoring native vegetation throughout the park, the city intends to reduce erosion, increase the habitat value of the park, and improve the park’s aesthetic appearance.

Examining the Stream Channel
Shoal Creek extends for roughly 6,400 feet within the project limits. As part of its investigation, PBS&J documented problems related to channel instability, local scouring, and bank erosion and recommended corresponding solutions. Preliminary design solutions were developed to address each known problem area.

Landmark Surveying Inc. provided surveying services, which included conducting measurements at several specific cross sections of the creek channel and noting details of each storm drain outfall and corresponding inlet within the main channel. Landmark Surveying also located all trees 8 inches or larger in diameter within the defined riparian corridor of Shoal Creek. PBS&J ecologists then identified the trees and developed a tree inventory. HVJ Associates conducted a geotechnical investigation to inform the design of such project elements as mechanically stabilized retaining walls, sloped and terraced embankments, rock riprap revetments, and water-quality structures.

Shoal Creek was divided into four separate reaches within the project limits for the channel stability analysis. The bank and channel stabilization solutions were developed based on a detailed channel assessment and stability analysis of each reach. These solutions were guided by previous studies and recently completed improvements implemented to stabilize the channel immediately upstream of the current project. The previous project served as a reference reach and provided useful data for the current restoration effort.

Photo: PBS&J
This storm drain outfall has been severely scoured and undermined by Shoal Creek.
This proposed biofiltration pond is one of 11 green infrastructure projects recommended for Pease Park.

Stabilizing the Banks and Channel
For each reach, PBS&J proposed solutions for stabilizing the banks and channel of Shoal Creek. A combination of eight grade-control structures and approximately 5,300 feet of bank stabilization was recommended to prevent further channel degradation and widening that threaten infrastructure and park property.

Where bank stabilization is necessary, the design goals are to develop measures that are most cost-effective (considering construction cost and long-term maintenance costs), protect the heritage trees and trail, and create the most “natural” appearance, without increasing the flood potential of Shoal Creek. For Shoal Creek, the proposed bank stabilization measures consist of three basic design approaches: graded slopes with rock toe protection, retaining walls made of stacked limestone boulders, and mechanically stabilized earth embankments.

In another effort to control channel degradation, the design includes eight low-profile grade-control structures to be constructed on the channel bottom using limestone boulders. The boulders will be configured such that the structures function as either an immovable riffle in the channel bed or drop structure. This approach was used successfully on the reference reach immediately upstream of the project site.

Assessing a Wastewater Pipeline
The presence of wastewater pipelines within the channel of Shoal Creek also must be considered as part of efforts to stabilize the stream. Approximately 1,700 feet of 18-inch-diameter wastewater pipeline located within the stream channel is under consideration for abandonment, removal, and relocation. Roughly 820 feet of the pipeline is exposed along the channel bed, while the rest is below grade. Encased in concrete, the exposed sections cause localized channel instability by directing flows into the banks and contributing to scour and bank slope failure. Scour also threatens to undermine the wastewater pipelines.

The MWM Design Group evaluated the feasibility of relocating the wastewater pipeline. The purpose of the study was to identify feasible alternative pipeline alignments outside of the creek channel and determine the associated project costs.

Based on this study, the design team recommends moving the pipeline from the channel and moving the relocated sections to the right of way along the park’s boundaries. The recommended alignment involves installing approximately 1,500 feet of 18-inch-diameter pipe within unpaved areas along the right of way of a nearby arterial that runs beside the eastern edge of the park. The report recommendations include removing the abandoned pipeline structures from the channel and installing grade-control structures to redirect flows toward the center of the channel, so as to limit the potential for head-cutting in channel sections upstream of the current pipeline locations.

Implementing Green Infrastructure
A key component of the Pease Park restoration project involves retrofitting the park’s storm drains and improving the quality of runoff entering Shoal Creek from areas adjacent to the park. Improved runoff quality, combined with the proposed streambank erosion control projects and riparian landscape restoration, will help ensure the success of the overall project.

The project team was asked to review 13 potential green infrastructure projects identified by the city of Austin for use in improving water quality Shoal Creek. Of these, seven projects were small-scale stormwater treatment systems involving flow spreaders, rain gardens, a pervious parking lot, and efforts to restore soil and vegetation in a particularly degraded area of the park. The remaining six projects were larger in scale and sought to provide biofiltration with partial sedimentation. After reviewing the potential projects, staff from PBS&J and Glenrose Engineering eliminated two–a rain garden and a biofiltration system–because of space constraints. Preliminary layouts of the remaining 11 water quality projects were selected to minimize disruption to existing trees (Table 1).

The three flow spreaders will consist of gravel trenches designed to disperse concentrated runoff moving down vegetated hillsides within the park. Ranging in length from 480 to 680 feet, the flow spreaders will control runoff from areas as small as 3,700 square feet to 8,100 square feet. The primary function of the flow spreaders is to help decrease erosion.

Like the flow spreaders, the rain garden also will treat runoff from a hillside. With a total footprint area of 3,200 square feet and a ponding depth of 0.5 foot, the rain garden is designed to handle flows from a one-acre area. In addition to improving water quality, the feature will provide an aesthetically pleasing amenity within the park.

Photo: PBS&J
This section of Pease Park, near hole 16 of the disc golf course, is the proposed site for the biofiltration pond.
Photo: PBS&J
Encased in concrete, exposed sections of a wastewater pipeline in the stream cause localized channel instability by directing flows into the banks and contributing to scour and bank slope failure.

In another effort to facilitate infiltration, an existing parking lot with an impervious surface will be replaced with one having a porous surface. Stormwater runoff from a developed upland area nearby and an adjacent parkway will flow onto the 14,600-square-foot lot, which will have a 4-inch-thick pervious surface underlain by gravel ranging in depth from 8 inches to 2 feet. The design report recommends pervious concrete rather than pervious asphalt pavement because of its longer design life expectancy.

Custer’s Grove Embayment, an approximately 1.3-acre drainage swale parallel to Shoal Creek, has been particularly worn down by foot traffic associated with disc golf. This area, which is so named because it once endured the rigors imposed by Captain George Custer’s encamped troops, will be restored by means of mechanical soil aeration and a host of other efforts to return vegetation to the area. Following the soil aeration, mycorrihizal treatment and mulch will be applied. Subsequently, the area will be revegetated with native seed mixes, shrubs, and woody plants. To protect the restoration, report recommendations include relocation of disc golf activities for hole 18 and engineered soils designed to withstand heavy traffic for any new fairway locations.

The five biofiltration ponds will vary widely in size, ranging in total area from approximately 18,400 to 49,000 square feet. Pond sizes will depend on the size of the drainage basin each pond will serve. Varying in size from 3.5 to 37.2 acres, the drainage areas all feature mainly residential land uses, with some office and commercial use. The estimated percentage of future impervious cover for the different drainage areas runs from 42% to 65%. Depending on the pond, the ponding depth ranges from 2 to 3.5 feet. The desired minimum drainage area capture depth for the ponds is 0.15 inches.

Restoring Riparian Areas
Compared to upland areas within Pease Park, the banks and channel of Shoal Creek generally display impacts from park visitors in more specific locations. Steep topography and creek flows have tempered human encroachment into areas immediately along the creek. However, there are some exceptions.

In places along the riparian corridor, intensive use by park-goers has resulted in severe ecological degradation. In large areas, the natural litter layer, topsoil, and understory vegetation have disappeared under the trampling feet of visitors. Soil compaction and erosion contribute sediment to Shoal Creek from storm runoff, along with seeds, sprouts, and roots of invasive plants. Returning these areas to a healthy, natural condition will require intensive restoration efforts.

The goals of the riparian restoration project along Shoal Creek in Pease Park are to establish a healthy native riparian soil and vegetation system that will

  • Filter storm runoff
  • Enhance rainfall infiltration to support vigorous plant growth and historical seeps and springs
  • Provide a natural and aesthetic landscape that reflects the geographical character of the park’s unique location straddling the Central Texas Balcones Fault Zone

To achieve these goals, the project restoration team–Glenrose Engineering and Winterowd Associates–developed multiple solutions for restoring the riparian areas. The preliminary design report presents restoration plans for 32 acres or approximately 75% of Pease Park.

Removing Invasive Plants and Delineating Trails
Removing invasive plants is a key step toward restoring the riparian areas. Glenrose Engineering conducted an invasive plant survey of designated riparian restoration areas in October 2008. Priority invasive species were inventoried and mapped, and methods for removing the species were recommended.

Delineating pedestrian trails is expected to help reduce damage to soil and vegetation. Some trail sections have been paved or bordered with rock to indicate the desirable path. However, in locations with no such border, the trail has widened unnecessarily. Clearly marking the trail boundary in a manner similar to what has been done elsewhere in the park should address the problem.

Riparian areas with less than 30% canopy cover are suitable for open meadows of native grasses, while in other locations existing oak groves merit protection because of their status as some of the park’s most treasured natural resources. Both of these endeavors will require limiting pedestrian traffic by properly marking trails and alleviating soil compaction. For both types of areas, the project team recommended methods for how best to restore soil fertility and create the ideal conditions for the desired vegetation.

Controlling Foot Traffic and Errant Discs
To help ensure that efforts to restore vegetation succeed, temporary fencing will be used in certain locations. In other locations along the riparian corridor, however, permanent fencing is needed. A low cedar post rail fence is recommended for these situations, and it is important that this fencing be repaired and maintained over time to ensure it functions properly.

Vegetation can serve as a barrier to deter pedestrian traffic, protect trees from golf discs, and guide golf disc throws in certain directions. To minimize the creation of new trails through these vegetation barriers, plant materials will be relatively large when installed so they function as intended from the beginning.

In areas where clusters of trees are in need of protection from errant disc golf throws, netting may be installed in order to protect against defoliation and bark damage. Golf course netting used for driving ranges can be adapted to this disc golf situation. Where it is not feasible to use nets in this manner, individual trees can be netted using hardware wire cloth wrapped around the trunks to cushion the impacts of disks.

The disc golf basket areas receive a large amount of pedestrian traffic, not just in a linear path to the basket, but in a wear pattern from a radius around the basket as a result of “putt” throws from all angles. For any disc golf baskets within the riparian zone, a 30-foot-radius area surrounding the basket will be surfaced with a permeable material that stabilizes the area without vegetation.

Pursuing Comprehensive Solutions
The project team recommended ways to integrate the most feasible solutions into four distinct project phases to minimize disruptions to park users and achieve economies of scale during construction. Significant public outreach is being conducted as part of an effort to survey park users, document their various recreational needs, and achieve consensus on how to proceed. Following the completion of this process, the project team will finalize the design, and the city will begin advertising bids for construction. The current schedule calls for construction to begin within the next two years.

When complete, the restoration of Shoal Creek where it passes through Pease Park will provide a notable example of integrated stormwater management. Shoal Creek faces multiple threats, some of which originate from outside the park, and others that occur within. Rather than simply addressing the varied problems in a piecemeal fashion, the planned restoration attempts to solve them in a comprehensive, holistic manner. As a result, the project stands a better chance of success, and it will confer significantly more benefits than would more limited actions.

Once restored, a rejuvenated Shoal Creek will benefit Pease Park both ecologically and aesthetically, contributing to water-quality gains within the park itself and downstream. At the same time, the revamped natural areas along the creek and throughout the park will provide critical habitat for wildlife, while also improving the park experience for its many users.

About the Author

Jeffrey Kessel

Jeffrey S. Kessel, P.E., is a senior project manager with engineering firm PBS&J in Austin, TX.