As we mentioned in an earlier post, the reason we are converting the Paseo into green infrastructure is because it receives water from about 20 upstream acres of land and the soils and geology are ideally suited to infiltration to groundwater. During larger storms, water picks up speed as it flows into the alleyway. Thus, we needed to design something to slow the water down before it hits the bio-swale.
The Forebay reduces the erosive force of water entering the bio-swale. It also allows sediment and litter carried with the water to settle out. The forebay elements include a river rock cobble bed, a weir, and a river rock cobble apron for the weir.
The river rock cobble bed at the forebay entry creates an obstacle course of sorts for the water entering the bio-swale; water slows down as it moves over and around the river rock cobble. Slow moving water is less likely to erode soil and has more time to soak into the soil – exactly what we want to happen in the bio-swale.
A weir is a low dam built across a water course to regulate the flow of water. The weir is constructed of concrete and separates the river rock cobble bed from the bio-swale; it regulates the amount of water entering the bio-swale through the channel at the bottom of the weir and its low walls. When there is a small volume of water, it will flow through the 12 inch wide channel at the bottom of the weir. When larger volumes of water enter the river rock cobble bed, water will continue to flow through the channel but the low wall of the weir will hold back water until it is at a height that it is allowed to spill over the walls. Eventually all the water moves through or over the weir.
The cobbleapron on the down-side of the weir protects the bio-swale from the erosive force of water moving through the channel or spilling over the weir. When water flowing through or over the weir hits the cobble apron, its velocity or energy is dissipated or spread out over the cobble so the water entering the bio-swale is moving at a slower, gentler rate that is less likely to cause erosion.
Before the river rock cobble bed and cobble apron could be constructed, we needed to complete construction of the concrete entry and weir. The construction crew refined the grade elevation points for the entry, forebay, and weir. The concrete pour for the entry and weir went smoothly. We have a few more steps to get through before you can see the finished forebay, however, so hold on for more.
Now that the infiltration trench and bio-swale have been constructed, it is time to lay the sidewalk, which goes on top of the infiltration trench. How’s that, you say? Putting a sidewalk on top of the place where water is supposed to soak in? For the Paseo, we installed pervious concrete but even if we had used regular cement, it still would have worked as the system is designed to receive water from the bio-swale underground.
The Pervious Concrete Pathway winds its way from one end of the Paseo to the other, providing a pleasant and safe path. Pervious concrete has ‘voids’ or gaps that are created when the fines are removed from mix thereby allowing water to flow through the concrete. Water falling on or over the pervious concrete will flows through these spaces to the infiltration trench below it. To begin construction of the Pervious Concrete Pathway, a course of gravel was spread out over the infiltration trench and areas of the exposed impervious liner.
Unlike Portland cement concrete, pervious concrete sets up quickly. Many hands were needed to pour, spread, smooth, and score the concrete. The choreography of material and work was impressive that day and kudos to Rudy and his crew at J&M Contractors for a pathway many will enjoy.
To aide in the curing process and protect from errant soil or debris, the completed pathway was covered with plastic sheeting. Below you see the completed pervious pathway along with one of the landscape boulders.
Next up is construction of the Forebay, an important element of the design to protect the bio-swale from erosion and clogged soil.
We don’t have enough parks in Los Angeles. Given the expense of land and the reality of built-out cities, we need to think about how we can create more parks in urban areas. We need to be creative in converting even small scraps of vacant land into places where people can enjoy nature, walk their dogs, and play. Almost five years ago a small group of people in Altadena got the idea to turn a small barren bit of dirt into a park for the community. Just 8,000 square feet and owned by the County’s Roads Division, the parcel was too small to be a true park. Also working against it, the land was on the border of Pasadena and Altadena – in the unincorporated County but with water from Pasadena. But this neighborhood had no park and thus the idea was borne – why not create a “pocket park?”
The design included native, drought-tolerant plants designed to bring the mountains into the city, and dry wells to collect and percolate all of the rainfall, eliminating sheet flow runoff from the site.
Several more grants and lots of hard work later, the hard, barren ground is blooming. We wanted to demonstrate low-water use gardening that people could replicate in their own yards. The park was completely a volunteer, community-led project from start to finish. It would not have happened, however, without the support of Supervisor Michael Antonovich and Metropolitan Water District. Mountain View Cemetery, Foothill MWD, Pasadena Water & Power, plus the members of Neighborhood Church were also major supporters. The list of supporters is long.
Here are a few more photos of the park, which is at the intersection of Marengo and Woodbury. We named it “Old Marengo Park” to acknowledge its origin as left over land when Pasadena and the County aligned Marengo Avenue. What neglected scraps of land can you turn into a park in your neighborhood?
Gardens on Elmer Avenue in Sun Valley hold rainwater and recharge the aquifer, replacing what they use…and they are pretty, too. I’ll be talking more about Elmer Avenue in the next few weeks, telling the story of the street from research and conception to implementation.