Colorado neighborhood re-establishes vegetation while resizing its storm water culvert
When a new housing development is built, the area’s infrastructure often must be upgraded to handle the change in land use. In the Leyden Ranch neighborhood of Arvada, Colo., a storm water discharge system required a redesign to handle increased runoff caused by the influx of new housing. The storm water culvert needed resizing to handle the increased flows and prevent scour of the outfall and pollution of area waterways.
Resizing storm water culverts, like those at Leyden Ranch, typically leaves soils disturbed and vulnerable to erosion. While scour protection has traditionally been addressed using rock riprap, new vegetative technologies have emerged to lessen cost and improve aesthetics, performance, and safety. The proper solution would allow for vegetation establishment while safeguarding against soil erosion.
The Leyden Ranch neighborhood is a new housing development built on former grasslands in the rolling hills of Arvada. The increased runoff caused by the new housing has pushed the current storm water system to capacity.
To address the site requirements, Todd Sanville of Nilex USA in Denver considered the culvert size, peak discharge, and the culvert’s down-slope gradient. Sanville used North American Green’s ECMDS design software to properly size the area the scour-protection mat would need to cover. Peak discharge from the resized 36-in. culvert pipe was determined to be 168 cu ft per second at a 3% gradient. These values suggested the scour protection mat needed to be at least 15 ft by 15 ft.
ShoreMax scour protection mats were chosen because they are very flexible, permit vegetation establishment and protect the culvert outfall from scour. Since the scour protection mats have a higher specific gravity than normal erosion control blankets or turf reinforcement mats, they act as a ballast, preventing uplifting in turbulent storm water and improving the performance of the underlying mat. In addition, the flexible scour mat allows vegetation to establish, which creates a soft-armored system, as opposed to traditional hard-armored designs.
The ShoreMax was applied to a 225 sq ft area using 18-in. rebar staples and installed over the C350 turf reinforcement mat (TRM). The TRM was extended 20 to 30 ft beyond the culvert outfall to further armor the outfall area. The fiber matrix of the TRM helped prevent soil erosion from occurring prior to vegetation establishment. After the vegetation is completely established, the combination of the scour protection mat and the TRM will deliver permanent protection from soil erosion by reinforcing both the stems and roots of the vegetation. To help protect the upper slopes surrounding the culvert outfall, a temporary Bionet erosion control blanket (ECB) was installed and to help re-establish vegetation in an environmentally and wildlife-friendly manner.
Six months later, vegetation on the site is almost completely established, and the system has remained stable through record rainfall in April and May 2015.