The City of Silverton, OR, has turned a potential environmental and economic nightmare into an attractive and popular wetlands project. Silverton was ordered to stop discharging effluent from its wastewater treatment plant into Silver Creek during the summer because the amount of effluent exceeded the creek’s ability to assimilate it during low-flow summer conditions. The city considered several alternatives, including diverting the effluent to another river as well as storing effluent during the summer and discharging it to the creek or river during the winter. The city concluded, however, that in the long term, river or creek discharge was not the best way to dispose of treated effluents.
At the same time, Silverton also faced other environmental concerns. Construction of an industrial park required that more than 7 ac. of mitigation wetlands be created, and the city also wanted to create a “wetlands bank” for future mitigation. The Oregon Gardens Project, the brainchild of the Oregon Association of Nurserymen, addressed all these issues. Oregon Gardens was conceived as a world-class botanical display, garden, and nature area that would showcase plants of the Northwest, providing a hands-on educational tool, a tourist destination, and a source of inspiration and ideas for gardeners.
Working with HDR Engineering, the City of Silverton devised means to pipe summertime effluent from the wastewater treatment plant to Oregon Gardens, where the effluent is further treated as it passes through a cascading ponds wetlands system. The treated water is used to irrigate the 240-ac. site, sustaining the plants during the dry summer months without drawing on drinking-water resources.
The wetland system, designed by Interfluve Inc. of Hood River, OR, occupies 16 ac. of the garden. Three interconnected complexes contain a chain of 25 cascade ponds or cells linked together by numerous discharge swales. Water elevation in each cell is maintained by a vegetated berm until it flows over a spillway from one cell to the next. The specially selected vegetation in the spillways acts as a natural filter to help improve water quality. The hydraulic roughness characteristics of vegetation reduce flow velocities, hindering the ability of sediments to stay suspended in the flow from one pond to the next. Vegetation also plays a key role in the phytoremediation of various pollutants attached to sediment as it settles out of the flow.
The use of vegetation in the overflow swales posed several concerns, however. First, the imposing flow-induced shear forces in the steep bed slope swales might exceed the permissible shear of the vegetation and underlying soil. Second, potentially large discharges would occur as the amount of effluent entering the pond system increased over time, possibly increasing shear stress beyond the limits of natural vegetation. Finally, as the discharge and frequency of flows increased, the swales would be exposed to longer periods when the soils were saturated, potentially reducing the effective vegetal cover. When these factors were compounded it appeared that vegetation alone would not be the best solution or meet the design requirements for the swale liners.
With assistance from Melissa Chapla, an erosion control specialist with ACF West in Portland, OR, Bill Norris, the hydraulic design engineer for Interfluve, determined that vegetation reinforced with composite turf reinforcement mat (C-TRM) would negate these concerns. North American Green C350 C-TRM was chosen. Although they are permanent rolled erosion control products, C-TRMs differ from 100% synthetic TRMs by incorporating an organic matrix material-in this instance coconut fibers-into a permanent three-dimensional net structure. The matting also provides a more cost-effective option to-and is perceived as environmentally friendlier than-100% synthetic mattings and hard-armor alternatives.
The coconut fibers and surface installation of the C-TRM allowed the Oregon Gardens project to skip the soil infilling process typically specified for 100% synthetic TRM installations. It was important that the soil infilling step be removed to limit the risk of downstream sedimentation should a flow event occur before vegetation became established. If left exposed on the channel-liner surface, infill soil migrating downstream would have reduced the effective life of the pond system and the efficiency of the wastewater treatment.
The specified C350 C-TRM uses coconut fiber matrix stitch-bonded to an ultraviolet-stabilized, permanent, three-dimensional net structure. The fibers in the matting afforded scour protection for the channel’s soil surface against flow-induced shear forces immediately after installation and continued furnishing protection until the vegetation became fully established. The net structure increases the permissible shear stress of the vegetation up to 384 pascal (8 lb./ft.2), a level once thought only achievable with rock riprap or hard-armor alternatives. The fibers also functioned as an excellent mulching material to regulate environmental extremes at the seedbed. The wide variety of grass and wetland plants specified for the swales had different germination rates, requiring the matting to provide extended cover until all the seeds germinated and the plants became fully established.
The wetland complex was constructed in 1998. After all land excavation activities were completed on the wetlands and the swales were shaped to the appropriate grades and sizes, matting installation began. The entire garden construction proceeded in phases, with C-TRM installation and continued plantings throughout the summer of 1999. A combination of grass, sedge, and rush species were hydroseeded on the soil surface, and the matting was installed over this surface-anchored in place with 9-gauge, 12-in. U-shaped metal staples-to provide erosion protection while allowing the vegetation to establish up through the matting.The crimped middle net of the C-TRM increased the initial unvegetated hydraulic roughness coefficient of the overflow swales to reduce water velocities should a flow occur before vegetation was established. All of the swales achieved a thick cover of vegetation only two months after matting installation, with no discernable signs of erosion.