As Paso Robles built out its infrastructure in the late 1800s, Mountain Springs Creek was buried in a pipe. During large storm events, the stream makes its presence known by flooding the entire street and depositing rocks and sediment in the roadway: an unfortunate byproduct of upstream land use and erosion.

But all of that will soon change. The City of Paso Robles is working with SvR to reverse the course of history. Along five blocks of the street they are creating a green boulevard that will bring the stream’s flow back to the surface, while accommodating all modes of transportation, including one of only four railroad crossings in west Paso Robles. For the community, 21st Street is a critically important connection between the local middle school, city parks and the Mid-State Fairgrounds, so providing a balanced, Complete Green Street solution was critical for the project’s success.

21st Street’s redesign is not only an opportunity for the City of Paso Robles to reinvigorate this corridor, but is also seen as a precedent-setting project for the entire Central Coast of California, which is why it was awarded a nearly $1 million grant by California’s Strategic Growth Council in 2010.

A section of 21st St illustrative improvements plan; illustrative above, infrastructural below

Design Performance

Informed by the Uptown/Town Centre Specific Plan and the Bike Master Plan, SvR set about solving the mobility and drainage problems of the street. Along the length of the 21st Street corridor, the street changes from an 80’ right-of-way to a 50’ right-of-way west of the BNSF railroad line that bisects the site. This variation in width presented SvR’s civil engineers, landscape architects and mobility planners with a number of challenges.

Left: 80' ROW perspective and section without stream; Right: 50' ROW perspective and section

Within the 80’ cross section, we were able to propose porous pavement sidewalks, planting strips, porous pavement parking, bike lanes, travel lanes and a planted boulevard median which can accommodate the un-earthed stream’s flows. Once the street shrank to a 50’ right-of-way, the design became more constrained and modes had to carefully mix. By encouraging traffic-calming through narrow travel lanes and street trees, we proposed changing the bike lanes to sharrows and removing parking to concentrate on efficiently moving pedestrians, vehicles and stormwater through the space.

At the intersections, curb bulbs were used to shorten pedestrian crossing distances and planted traffic circles facilitate circulation for new right in/right out alleys. These interventions also add character and traffic calming for the non-arterial intersecting streets.

Another design challenge was finding a strategy to preserve as much of the existing street tree canopy as possible. The stately live oaks helped inform our placement of curb bulbs, sidewalk meanders and driveway configurations. Complementing these existing trees, was a palette of plant groupings that were tailored to various water regimes. For example, planting strips required drier, more robust plants whereas plantings near the water from the stream needed to be able to adapt to seasonally changing flows and periodic inundation.

Section and perspective with stream, 80' ROW

Technical Performance

It was clear that the street geometry needed to change once the flow of water from the stream was introduced, but how? First, we needed to make sure that we had enough volume in the cross section to accommodate the stream’s more than 150 cfs of flow. Rather than continuing to dig down—making the stream feel less like a creek and more like a ditch—we added 1’ of width to each side of the median by reducing our planting areas, allowing us to accommodate the necessary volumes of water.

A crowned street allowed us to maintain separation between water that fell on the road surface and the cleaner stream flows passing through the site. Not wanting to discharge dirty water to the Salinas River, this street runoff will be slowed and pollutants removed through a number of bio-retention cells at alleys and intersections. To help prevent flooding, we installed box culverts and drains to ensure water flows continue relatively unimpeded to the downstream discharge at the Salinas River corridor.

Collaborators on this project include Skeo and the City of Paso Robles. SvR is currently working with Cannon, a local engineering firm, to develop construction drawings for this project which is expected to break ground in Spring 2012.

Information on and resources from the Complete Green Streets Workshop held in conjunction with this project is available here.