IRA MARK ARTZ ANALYZES PIGGYBACK YARD HYDROLOGY
Providing Channel Capacity for River Revitalization
The Piggyback Yard is one of the most valuable land areas in the entire City for river restoration because of its immense water storage potential compared to other single-owner parcels of land.
This is where the Piggyback Yard Project provides its most significant contribution to the hydrology of the River: By storing peak flow, the volume of water in the channel is reduced. This in turn allows additional space in the channel for habitat plantings and related revitalization features, both adjacent to the Piggyback Yard and downstream.
In designing the site, it is important to save most of the storage capacity for when it’s really needed: for detention of the flood peak. If the storage or detention basin fills up prior to the peak occurring, the effective storage capacity would already be used when the greatest and potentially most damaging flows occur in the River. The beauty part is that flood peaks occur so infrequently that most of the site can be used for compatible uses most of the time…uses such as recreation, habitat plantings, lakes, trails, aesthetic treatment, and development around the fringe.
Taking the Peaks off the Reaches Downstream
“Taking the peaks off the reaches downstream” refers to reducing the volume of water that would otherwise flow in downstream segments, or “reaches.” The importance of capturing the “peak flow” is related to that portion of the storm that fills the topmost area of the channel.
The storage of this volume effectively reduces the downstream peak, and in turn, both reduces the potential for flooding downstream and increases the ability to add additional habitat restoration. From a purely hydrological perspective, it would be more advantageous to use the entire site for storage. But the need for a balanced strategy is recognized due to the overall benefits – not just hydrologic – that need to occur with project implementation.
Affecting the Speed of Water Flow
The speed of flow of water in the channel is affected by two main factors: (1) the amount of water (volume), and (2) the cross-sectional area of flow. For any given amount of water flowing in a channel, a smaller cross-sectional area will result in a greater speed of flow, similar to putting your finger halfway across a hose. No additional water comes out; it’s just faster because the cross-sectional area is smaller. Similarly, a wider channel will slow down the water.
Slowing down the flow in the River is critical to allowing habitat planting to take place. Currently, with flow approaching 25-30 feet per second (about 20 miles per hour), a soft-bottom channel would erode, and vegetation would get ripped out. This can be avoided by making the channel larger and/or storing water to reduce the peak flows, both of which reduce the flow velocity.
The roughness of the channel (concrete vs. vegetation) also has an effect in slowing down the water due to friction, as well as taking up flow space in the channel. So, the Broadened River would result in slower flow in the river section compared to the River Strand since the channel area is wider. I have previously noted in verbal discussions that whatever proposal is adopted would require enough conveyance capacity to maintain the existing rate of water moving past the site. Therefore, having more wetland habitat in the main conveyance channel would require the balancing effect of a larger channel- or less water.
Comparing Piggyback Wetlands
The hydrologic value of the site for storage depends on how much storage is available when it’s needed, that is, when the peak flow occurs in the River. The “River Strand” proposal ensures more effective storage since it has a higher berm compared to the “Broadened River.” It therefore offers more control of when flow enters the site. The “River Strand” also offers more available storage since the storage footprint is larger. The “Broadened River” is very attractive in its ability to demonstrate habitat value that could be achieved within a wider Los Angeles River, given enough space.
Of course any strategy would have to undergo detailed modeling studies related to channel capacity, storage volume, and inflow/outflow characteristics from the site.
Ira Mark Artz, Divisional Vice President, Tetra Tech, and Project Manager of the consulting team on the Los Angeles River Revitalization Master Plan
JESSICA HALL EXPLORES THE ECOLOGICAL DIFFERENCE BETWEEN THE TWO PIGGYBACK YARD WETLANDS PROPOSALS
How Rivers Work
In general, ecological assessments look at how the natural processes of the landscape function and evaluate habitat based on that, without human management or intervention; i.e., what’s happening with water, sun, soil, nutrients? How does it rebound from natural disturbances? The preference there is to re-establish the system so that it is self-maintaining for the species that are suited to it. However, some governmental agencies evaluate habitat in other ways, focusing on suitable habitat for specific (threatened or endangered) species. In these cases, an evaluation may prioritize the management of habitats for the continuation of these sensitive species in highly constrained environments. This could play into the differences between the two concepts.
River restoration involves restoring the River’s processes: the relationship between the movement of water and sediment, and the dissipation of energy as water and sediment are moved by gravity down the river to the sea. Channel form, meanders, braids, floodplains etc. are responses to this, and habitat takes root in relation to the natural shifting and adjusting that occurs. An artificial change to sediment load, stream slope, etc. kicks off changes in the river as it attempts to reestablish equilibrium – often leading to things we don’t like (erosion, scouring out new meanders, sudden changes in channel shape, etc).
Additional functions served by rivers include flood storage (this is the role of the flood plain), groundwater recharge, conveyance, sediment transport and biochemical processes. The extent to which a river does these things varies depending on its location in the watershed, its slope, geology of the watershed etc., but they are the foundation of how I evaluate ecological functioning of a site or proposal.
The Broadened River Concept
The Broadened River concept is based on the restoration of river processes and dynamics, of which a flood regime is a central element. Allowing flooding in the river means the full cycle of life, destruction and regeneration is allowed to occur within the bounds of the flood walls. Large floods spill over into an off-channel storage area.
Sediment transport is fairly straightforward here, meaning the channel will be mostly self-regulating. The active channel will migrate, build banks, wipe them out and rebuild them. It is likely that finer sediments will be washed downstream or be deposited on higher floodplain (terrace) areas, maintaining a cobbly or gravelly channel bed and sandy flood plain. This facilitates groundwater recharge within the channel and geomorphic floodplain.
The roughness of vegetation in the floodplain will slow flows, which also facilitates some percolation, although the benefit shouldn’t be over-stated – water is also still being conveyed downstream. Depending on the slope and ability of the river to meander (naturally blow off energy, some control structures may be needed, or other devices to protect the river’s floodwalls.
Over time, willows, mulefat, cottonwood and possibly sycamore will begin to stratify along a cross-section of the river, in terms of species and age, based on proximity to the active channel and tolerance to water. This will establish more diverse habitat that prefer different levels of cover and density to nest and forage. For example, a natural flood cycle maintains a class of lower, young willow trees and low herbaceous scrub in the floodplain that the endangered Least Bell’s vireo often uses for nesting, and the Los Angeles River was once a likely population center of this bird. Raptors on the other hand prefer tall snags and canopy trees, more likely to be found in the older trees.
Our native fish have specific requirements in terms of slopes, water depths, stream widths, riparian cover, etc. If non-native species can be controlled, frogs such as the Arroyo Toad and Pacific Chorus Frog have a good chance of showing up. Birds will likely increase in the populations of the specific species that currently use the River, and we may be able to see the re-establishment of other species that are more willow-dependent. Small mammals will definitely benefit.
The River Strand Concept
With the River Strand, diversion and detention of water into a wetland side channel will create wetland habitat, and the goal as I understand it is to also allow a natural Los Angeles River main channel that is narrower than in the Broadened River plan, so the River Strand will also provide some flood-related habitat benefits, although on a more limited scale, more comparable to the Los Angeles River in the Glendale Narrows – which is still an exciting level of naturalization to achieve.
I expect there would also be a willow bosque around the edges of the diversion wetland/channel, which will increase habitat for many river and wetland birds already present on the river. If flood storage is more managed, it is likely that finer sediments will accumulate at either the inlet, or within the wetland area (depending on design) rather than washing downstream, requiring regular removal to maintain infiltration value over time. This will also affect the main river’s channel-forming ability, which may require more management within the main channel. With more controlled capture of floodwaters aquifer re-charge will be more intensive (water will have more time to seep into the groundwater). Nutrients and bacteria could accumulate within if the wetland area includes ponded water.
What is to be Gained?
In both plans it is likely that amphibian and some fish population may turn up. Native river fish habitat is optimal, however, where cobbles and gravels are clean of fine sediments, suggesting that native fish will do better in the natural channel reaches of both schemes, given the appropriate conditions for them (slope, water temperature, pools and riffles, etc.) and seasonal fluctuations in water levels are present.
Both plans propose upland habitat areas, which would be re-vegetation projects. Over a period of years, it is likely for these uplands to support birds and small mammals. In general, from a habitat standpoint it is preferable for these new “wildlands” to be contiguous, without interruptions of landscape elements that are managed primarily for people.
In simplest terms, the Broadened River plan provides relatively self-maintaining habitat through restoration of natural processes; the River Strand more directly manages natural processes and habitats through its flood management systems.
Jessica Hall, Senior Associate, Restoration Design Group
DAN ROSENFELD ON THE KATRINA WEST PHENOMENA AND DOWNSTREAM LEVEES
The Katrina West Phenomenon and Downstream Levees
One of Los Angeles County’s biggest concerns is the adequacy of the levees downstream in minority communities – the Katrina West phenomenon. Just as they’ve done in other communities, FEMA [the Federal Emergency Management Agency] is about to issue a notice informing people that they’ll need flood insurance, which will scare them to death. According to FEMA, Compton Creek is likely to flood in a once-in-66 year storm event, and that’s not a very comforting thought. The ability to retain and absorb storm water anywhere upstream, not just for green space but for detention, is critical. The Piggyback Yard is one of the few places we can get even half the flood detention capacity of a Sepulveda Basin [in the San Fernando Valley].
Dan Rosenfeld, Senior Deputy, LA County Supervisor Mark Ridley-Thomas