About the author:
William Chandler is project engineer for Lockwood, Andrews & Newnam Inc. Chandler can be reached at [email protected].
Like much of the central and southern coast of the U.S., Austin, Texas, is regularly exposed to powerful storms moving inland from the Gulf or overland from the Pacific. Combined with the flat geography and underlying strata of silt and limestone, these meteorological characteristics create an environment that responds to precipitation in a way that often is dramatic, with the scarred hill country terrain serving as a reminder of the forces of erosion.
Austin’s growth has not stopped accelerating since the turn of the millennium. Thankfully, Austin leadership has long known that such relentless growth must be coupled with careful forethought as the city limits creep across previously undeveloped terrain. Not only can storm events devastate communities with inadequate infrastructure, but unchecked development can magnify the threats posed by these storms by disrupting the equilibrium of local riparian ecosystems.
A Fragile Ecosystem
Cottonmouth Creek is an ephemeral headwater stream that is fundamental to relieving storm water from the growing number of communities in southeast Austin. Bound by subtly rising ranchland and the slopes of the extinct Pilot Knob volcano, flows collected in the Cottonmouth Creek basin are discharged first to Onion Creek and then the Colorado River.
Brookfield Residential Properties Inc., the developer, has worked to make Easton Park a reality since the early 2000s. Easton Park is a 10,000-unit residential development in southeast Austin that borders the Cottonmouth Creek floodplain immediately upstream of its confluence with Onion Creek. Much of the land in the Cottonmouth Creek basin is representative of the Texas Blackland Prairie Ecoregion, which the state considers a highly threatened ecoregion. Unfortunately, historical agricultural uses have resulted in deteriorated ecological conditions throughout the Easton Park site. Environmental assessments conducted while planning the Easton Park site determined that active restoration and ongoing management practices were necessary for the Blackland Prairie to survive.
Both the city of Austin and Brookfield Residential are committed to minimizing impacts of development on the natural environment. During initial planning discussions, the city and the developer worked together to establish special development conditions that would ensure that the proposed construction activities would not jeopardize the valuable ecoregion or storm water conveyance functions of Cottonmouth Creek. Because of this coordination, the entities agreed to the following two conditions for approving Easton Park’s construction: the developer would be responsible for restoring the Cottonmouth Creek ecoregion to that of a healthy headwater Blackland Prairie, and the work in the Cottonmouth Creek riparian corridor would need to provide significant, demonstrable environmental benefit to the creek, riparian area and floodplain.
Starting in 2013, Brookfield Residential retained the services of Lockwood, Andrews & Newnam Inc. (LAN), a national planning, engineering and program management firm, to masterplan, design, permit and construct the major water and wastewater infrastructure. Initially headed by a different team of consultants, the Cottonmouth Creek restoration project had been in permitting for a year and a half when progress came to a stand-still because of design disagreements between the design team and the city. To break the deadlock, in mid-2017 Brookfield Residential gave the reins to LAN’s Austin team of floodplain experts to bring the permits to fruition. LAN overhauled the design packages and diligently worked with the city to rebuild their confidence in the project. The city issued construction permits for the restoration work within six months of LAN taking on the project.
Much like the inherent symbioses that support an ecosystem's equilibrium, the city’s conditions are related and mutually beneficial. Renewing the native headwater Blackland Prairie ecoregion to the Cottonmouth Creek basin provides significant, demonstrable environmental benefit to the creek’s riparian area because it is a naturally sustainable ecosystem that begets creek resilience and longevity. As such, the essence of LAN's proposed restoration involves planting species that comprise the Blackland Prairie ecoregion. The restoration design improves the Cottonmouth Creek riparian corridor in three ways that are critical to establishing a hospitable environment for the burgeoning ecosystem:
- It promotes an ideal mosaic of distinct vegetative habitats;
- It increases the complexity of hydrologic connectivity in the floodplain; and
- It provides short- and long-term structural streambank protection.
Restoring a Prairie
Many of the Blackland Prairie species require more consistent saturation than could be maintained with the existing vegetation. Through years of agricultural inactivity and proliferation of invasive species, the existing Cottonmouth Creek was overrun by non-native grassland, such as Johnsongrass, sumpweed and Japanese brome in the upland, while the riparian corridor was dense with hackberry and giant ragweed. These invasive plants outcompete the native species for resources, creating an environment that is hostile to many of the essential species of the Blackland Prairie.
Therefore, the first step in ensuring success of the restored headwater Blackland Prairie ecoregion was to implement a rigorous sequence of invasives management and vegetation treatment to cull the woody plants and herbaceous annuals that would otherwise stifle the new growth. This sequence comprised a six-month regimen of mechanical disking and mowing, iterative application of non-selective herbicide, and manual removal of giant ragweed and grubbed vegetation.
Following the vegetation treatment, seeding and planting of three primary vegetation zones was proposed to establish a diverse habitat of structural and native species characteristic of the Blackland Prairie. The vegetation zones included riparian forest (e.g., cedar elm, wax myrtle, etc.); wet meadow (e.g., cottonwood, indigobrush, etc.); and, upland savanna and grassland (e.g., bur oak, redbud, etc.). This arrangement provided a maximum canopy cover of approximately 85% to facilitate herbaceous undergrowth in the riparian corridor and evenly transition to an anticipated 15% canopy density in the upland savanna and grassland.
In addition to reducing resource competition imposed by invasive species, the normal availability of water in the riparian corridor needed to be increased to sustain the newly planted vegetation. A network of seepage berms was designed to provide consistent and abundant water supply to sustain the Blackland Prairie habitats.
The seepage berms will promote infiltration and reduce storm water erosion. A total of five seepage berms will receive flows from storm drain outfalls as they discharge from the residential areas to the floodplain. Each seepage berm is designed as an earthen wall with a reinforced section where overflow is expected to occur. The seepage berms will retain storm water received from the subdivisions during a 100-year storm event.
By collecting and retaining precipitation, the seepage berms promote recharge of the groundwater by infiltration. This increases the complexity and hydrologic connectivity of the floodplain by creating a riparian area with a consistently higher degree of saturation that benefits the hydrophilic plant species of the Blackland Prairie ecoregion. Additionally, by locating the seepage berms in the flow path of the residential storm drain outfalls, the risk of streambank and channel erosion due to scour by overland flow is substantially reduced, thereby further enhancing the structural protection of the streambank.
Channel improvements, comprising cobble riffles and natural structures, will be installed to provide structural protections and increase water retention in the corridor. The primary function of the cobble riffle arrangement is to obstruct flow in the channel to facilitate formation of riffle-pool sequences. Riffle-pool sequences impose control over channel velocities to abate widespread scour in the floodplain and increase streambank resilience.
Large woody debris collected during restoration activities will be installed between cobble riffles to further inhibit channel velocities. Root wads, logs and tree trunks will be partially buried in the creek channel so that localized scouring can deepen the channel and produce pools. In combination with the velocity restrictions imposed by the cobble riffles, these pools increase hydraulic retention time through the creek. Longer hydraulic retention time in the creek directly improves groundwater recharge by extending the opportunity for standing water to saturate the ground. Additionally, these more frequent and resilient pools of water further develop the complexity and hydraulic connectivity in the floodplain by creating protected locations for infiltration.
Meeting Goals
Overall, the restoration improvements will substantially benefit the riparian corridor by increasing frictional resistance on floodwaters. Although the existing vegetation is particularly dense in the channel, the plantings scheme and location of habitat structures are designed in an arrangement that more consistently opposes flow velocity across the floodplain. Similarly, the seepage berms and cobble riffles will reduce peak rates of flow discharged into and through the channel. These design elements obstruct creek flows to reduce streambank erosion and further support a sustainable and structurally stable creek for years to come.
The restoration work was split into three contracts to minimize construction impacts to the creek. Each of the three phases of the work is contracted to be completed within 14 months. Such long construction windows are necessary to allow adequate time for effective implementation of pre-vegetation and invasive species management before the Blackland Prairie species are sown. Having begun at the end of 2018, construction is currently underway and is estimated to be complete by the end of 2020.
