A Better Way to Accommodate Floodwater in Kentfield

by Sandy Guldman

On January 4, 1982, the 10-year-old Corte Madera Creek flood-control channel in Marin County, California, faced its first significant test. As rains fell, record flood flows occurred upstream from the channel. Most of the overbank floodwater was diverted from the project, leaving only about 5,000 cfs—equivalent to the 20year flood—to enter the channel. But the waters overtopped the channel’s banks, even though the flow was well below design capacity. The Corte Madera incident illustrated the problems of traditional flood-control channelization—converting a natural stream to a uniform channel cross section.
Source: Philip B. Williams. 1990. Rethinking flood-control design. Civil Engineering, American Society of Civil Engineers, New York.

The US Army Corps of Engineers (USACE) Corte Madera Creek Flood Control Project began in the late 1960s. The lower part of the project includes an earthen channel in Larkspur and Kentfield with College of Marin (COM) athletic facilities on one side and residential and recreational areas on the other. Upstream, a concrete channel begins by the COM softball field, passes beside Kent Middle School (KMS), crosses under College Avenue, divides the core of the COM campus, and reaches into the Town of Ross. In 1971, after the concrete channel had been built as far upstream as Ross, the project was stopped by lawsuits and was not completed. Over the last several years there has been an on-again-off-again effort to officially complete the project, complicated by the cumbersome USACE processes and the need for Congressional authorization to fund the work. Now the USACE and Marin County Flood Control District are attempting to complete the project and improve its performance. The alternatives they have presented for reducing flooding in Kentfield include combinations of setback walls, berms, and lowering the existing multiuse path, with the concrete channel left intact or modified.
It has long been the position of Friends of Corte Madera Creek Watershed that it would be more beneficial to the local community and environment to remove the channel than to retain it and add walls and berms to provide more capacity. If these barriers are added, the concrete will probably remain in place for decades more. To provide an alternative, Friends decided to fund conceptual designs for total or partial removal of the concrete channel in Kentfield to improve flood management and achieve major environmental benefits. Matt Smeltzer and Scott Walls produced the preliminary concept designs shown below, to be implemented in three phases. The alternative requires widening the channel on one side, replacing the concrete wall with a vegetated slope, and removing the concrete bottom of the channel. A multiuse path will be retained, probably on the side of the creek where the concrete wall will be retained. If approvals from landowners needed for widening cannot be obtained, then walls and/or berms must be used to contain the water entering the channel upstream. The COM Board of Trustees has authorized continued design and evaluation for Phase 1 and Phase 3 on COM property. The Kentfield School District has requested less channel widening in Phase 2; designs have not yet been revised to reflect those changes.

Concrete Removal Alternative
Phase 1: Removes the channel floor and bank of the channel beside COM athletic fields from the Stadium Way Footbridge to the downstream end of the channel (approx. 400 feet); removes both channel walls in the downstream 150 feet
• Produces about 20% of the total flood protection benefit of the three phases
• Provides major visual screening for the COM Maintenance & Operations, Police Department, and Repro facilities scheduled for construction beginning May 2018.
Phase 2: Removes the channel floor and concrete wall adjacent to KMS—if a design acceptable to the Kentfield School District can be developed—from the College Avenue Bridge downstream to the Stadium Way Footbridge (approx. 1,200 feet); however, the KMS gym sits close to the creek and a wall is required on both sides of the channel in this area
• Produces about 45% of the total flood protection benefit of the three phases
• Requires increased capacity at College Avenue (alternatives are being developed by the USACE)
• Requires replacing Stadium Way Footbridge (in collaboration with Marin County Parks)
• Probably requires modification of a Marin Municipal Water District 20” water main adjacent to the existing Stadium Way Footbridge.
Phase 3: Removes the channel floor and concrete channel wall on the COM parking lot side of the creek, extending from the College Avenue Bridge to the upstream end of the campus (approx. 1,100 feet).
• Produces about 35% of the total flood protection benefit of the three phases
• Provides visual enhancement of the core of the COM campus
• Requires replacing the old footbridge leading to core of campus, already in COM’s plans, but retains the new footbridge leading to the COM science building.
Project Components
At the present time, the conceptual designs for all three phases assume that the concrete wall on the generally north or east side of the creek across from KMS and several COM parking lots (the left side of the channel, looking downstream) would remain in place because a large RVSD sewer was installed next to the wall when the current channel was constructed. The only exception is the 150 feet of Phase 1 furthest downstream, where the sewer leaves the wall and is routed to RVSD’s Kentfield Pump Station. As budgets and designs are developed, the feasibility of moving the sewer in some locations will be studied. If there is adequate space and no conflicts with other infrastructure, it may be feasible to move and replace the 50-year-old sewer, and eliminate that part of the wall. This would most likely happen upstream of College Avenue.

Local Benefits
Implementing all three phases would provide significant lowering of water surface elevations compared to the setback walls and berms. The capacity of the larger channel would reduce frequency of flooding in Granton Park, on the KMS campus, and at the COM athletic facilities, parking, and geothermal field. The additional capacity would also allow for lower walls in residential areas upstream of the campus. These are benefits not only for KMS and COM, but also for their neighbors.

Watershed Benefits
Several bridges in Ross, San Anselmo, and Fairfax are scheduled for replacement beginning in 2020. Most of these bridges constrict stream flow and cause flooding behind them; the new bridges will keep more flow in the channel and reduce uncontrolled out-of-bank flow (i.e., flooding). The new channel would accommodate 5,400 cubic feet per second, the proposed design flow for the USACE’s Corte Madera Creek Project, with lower water surface elevations and without any setback walls or berms.

Educational, Environmental, and Aesthetic Benefits
Students could have hands-on experience with restoration. The KMS and COM campuses, instead of hosting a textbook example of a failed flood control project from the 1960s, would demonstrate effective, properly engineered, environmentally responsible flood management. Tidal wetland and channel habitat would be restored, providing more carbon sequestration. These habitats also benefit plants, fish, and wildlife, including steelhead trout and Ridgway’s rail, endangered species that use the creek and its wetland habitats. Finally, there would be aesthetic benefits of trading a concrete box for a more natural environment.

Next Steps
There are a number of challenges ahead. To meet a 2020 deadline for completion of construction, quick action is needed in approximately this order: property owners agree to the design, funding is obtained, the USACE approves modifications to the channel, permit applications are submitted, environmental review is completed, the designs are completed, permits are issued, and, finally, the project is constructed.
Let the Board of Supervisors, which also sits as the Flood Control District Board of Directors, know if you support removal of part of the concrete channel!

A Sea Change: Consequences of Bay Level Rises

by Ann Thomas

Changes to Marin’s bay shore-line that rising sea levels could bring about by century’s end are detailed in a recently completed report, previewed in May at several community workshops. BayWAVE (Bay Water-front Adaptation & Vulnerability Evaluation) anticipates extensive physical and fiscal impacts of rising water around the bay, along with high water associated with storms. It was prepared over two years with the participation of staff and elected officials from all of Marin’s jurisdictions fronting the bay.
Although there is a wide range of forecasts regarding when, and how sea-level rise will occur, the report bases findings on estimates of impacts possible under six specific scenarios: sea level increases of 10, 20, and 60 inches, and in addition, these same three elevation changes in combination with a 100-year storm event. The report studied buildings, utilities, roadways, environmental resources, recreational resources, and other services along the bay shoreline. The report does not anticipate specific timing for any one scenario.
Residential Parcels are Most Affected Under a 10-inch rise, tidal flooding could reach 5,000 acres, 1,300 parcels, and 700 buildings in Marin County. The areas in this watershed most immediately impacted would be bayfront Corte Madera including Mariner Cove and Marina Village homes and commercial areas on Paradise Drive, the Greenbrae Boardwalk, and low-lying roads and Highway 101 in Corte Madera and Larkspur. Water supply, sewage treatment, transportation, and electricity networks could be damaged, causing impacts to extend far upstream.
With regard to utilities, for example, sanitary, stormwater, water supply, and natural gas pipelines under roadways could be squeezed between rising groundwater and overhead pavement, which could cause pipes to buckle or break. Laterals on private property could also suffer.

In the decades to come, low-lying neighborhoods such as Mariner Cove in Corte Madera, will frequently be awash with rising bay waters. Photo by Ann Thomas

Longer Term Changes Extend Inland As longer-term projections materialize, flooding and storm effects would reach further inland and affect thousands more acres. There could be regular flooding of areas also including: Madera Gardens in Corte Madera; Kentfield Gardens, Bacich and Kent schools, and the College of Marin in Kentfield; Larkspur Marina, Hillview, Larkspur Plaza, and Heather Gardens neighborhoods; and schools on Doherty Drive in Larkspur; and Larkspur and Corte Madera retail centers.
Under the most extreme scenario studied, about 30 percent of Corte Madera could experience tidal flooding. Greenbrae Boardwalk tops the list of communities that would be most compromised by sea-level rise.
Marsh and Creek Impacts All likely scenarios would result in extensive changes to marshes along the bay shoreline where these marshlands change from high marsh to low marsh to mud flat, and eelgrass beds shrink under deepening waters. The report lists Madera Gardens lagoons, Corte Madera Ecological Reserve, and Triangle Marsh as locations where tidal wetland and marsh habitats could be vulnerable to increased storms and higher bay waters.
Storms analyzed in this report include only bay storm surge, and the report does not try to predict flooding that could be triggered upstream along the freshwater creeks that flow through Fairfax, San Anselmo, Ross, and Kentfield. However, under the most extreme scenario, a 60-inch rise in sea level along with a 100-year storm, salt water could reach eight miles inland up the creek channel, well into San Anselmo.
Marsh and freshwater creek changes due to rising bay waters would affect plant, fish, and animal species. Species that could be threatened by habitat changes include the harbor seal, otter, salt marsh harvest mouse, Ridgway’s rail, Chinook salmon, steelhead, and many more.
Conclusions Despite uncertainty about when changes could occur, tidal gauges at the Golden Gate documented a rise in sea level of about eight inches in the 20th century, and this appears to be accelerating. Even if carbon emissions stabilized or declined, the report acknowledges, sea levels will likely continue to rise for decades due to past activities, and it could be hundreds of years before levels begin to drop again.
As an informational document BayWAVE does not require any action to prepare for changes. However, it cites sources which indicate that the cost of doing nothing is four to ten times the cost of adapting to sea level rise, and suggests that information in BayWAVE could be useful as jurisdictions update relevant land use and hazard mitigation planning documents. BayWAVE is available online at www.marinco unty.org/main/baywave/vulnerability-assessment.

Dealing with Creek Bank Erosion

by Gerhard Epke

Heavy rains filled our creeks to capacity several times this winter and the sustained high flows caused noticeable creek bank erosion through-out the watershed. This was a nor-mal and healthy purge for the creek habitat. But after years of drought it can be alarming to see all that freshly exposed soil and roots for those of us that live or own property along the bank, particularly because engineering a fix can be very expensive and time consuming. There are a number of landscaping practices that can help protect your streambank, but sometimes it may need a structural fix as well.
Rivers and creeks in undeveloped watersheds meander around their floodplains by eroding along their outside bends, jumping their banks at debris jams, or scouring multiple channels. If, decades ago, we hadn’t begun confining the creek in its current location, the channels would probably be much shallower and wider. Instead, look off a bridge today and you see the creek channel inset in a narrow area that barely qualifies as a floodplain. Since natural floodplains aren’t confined in this way, the concentrated flows have immense power and erosive capacity.
Water erodes banks in several ways. Due to the three months of high flows this winter, most of what we saw this year was scouring action. Water and all the rocks and dirt it carries scour, or ‘rub’ the soil away. This effect is exaggerated where velocities are high, particularly on the outsides of turns or downstream of walls and smooth surfaces. Another way the bank erodes is by failing, or collapsing under its own weight. This is often, but not always, associated with scouring or erosion that under-mines the base of the slope. Some-times a tree falls into the creek and pulls a big piece of bank in with it.

An erosion-control project on Fairfax Creek included a woven willow fence, whose posts will grow into bushes to protect the slope. Photo by Vicki Burns

What to do about erosion? That depends on its scale and significance.
Small scale or non-threatening erosion might not need to be ad-dressed at all. Scouring is the channel coming into equilibrium with its dis-charge, which reached channel capacity several times this year. So the slope may not erode any further. Mi-nor issues can often be solved by establishing a thicket of strong willow trees or shrubs whose roots support the bank and whose branches slow and buffer the impact of the water. There are myriad techniques to do this: willow stakes, willow walls, willow fascines, and brush layering. The plant material should be installed in the late fall when the soil is wet and the plants do not have to supply water and nutrients to the leaves; that allows roots to develop over the winter and establish the plants. Once they do get established, willows can grow vigorously and constrict the channel. So, depending on the type of willow you plant, pruning may be necessary later. “Groundwork: A Handbook for Small-Scale Erosion Control in Coastal California” is a good resource for learning more on this, available online through the Marin County Stormwater Pollution Prevention Program (MCSTOPPP).
Large projects require the hiring of design professionals and contractors and then obtaining regulatory permits. Most large-scale bank stabilization projects that get approved are designed to incorporate vegetation along with the traditional hard engineering structures such as piers, rocks, and metal cable. Even large pieces of wood can be included to mimic a natural creek bank. If you feel that you might have to under-take a project of this size, you should present a concept for feedback at a monthly Marin Project Coordination meeting, hosted by MCSTOPPP. See the brochure under Creeks and Watersheds, Creek Permits.
These meetings are attended by regulators from the San Francisco Regional Water Quality Control Board, California Department of Fish and Wildlife, US Army Corps of Engineers, and local public works departments. Their comments can help you avoid costly delays and re-design when you formally apply for a permit. However, it is helpful to enter the meeting with some preliminary engineering and restoration planning done on your concept.

Talking Trash

by Ann Thomas
Spurred by a new and stringent directive from the State Water Resources Control Board, Marin County communities are joining together to combat litter: the food containers, cigarette butts, plastic pellets, and other discarded waste which can wash into waterways, even be ingested by fish and other aquatic species. A countywide Clean Marin effort is being organized to comply with new state requirements that communities must reduce the amount of trash entering creeks and storm drains, and eventually the bay and ocean.
Clean Marin hopes to engage citizen volunteers to help with this effort. Volunteer “Clean” programs are already underway in Mill Valley, San Rafael, and Novato, and other towns. Outreach will begin later this year to residents around the county interested in helping to keep their neighborhoods litter-free.
The countywide cleanup pro-gram is supported by the Marin County Stormwater Pollution Prevention Program (MCSTOPPP), a network of county, city, and town representatives who enforce the water board directives. Clean Marin and MCSTOPPP hope to bring together anti-litter advocates, and share tools for Clean Marin volunteers. Practices being considered include “Adopt a Spot” and Green Business projects.

Some litter is thrown into our creeks, but most finds its way there driven by the wind or via storm drains, and sooner or later reaches the bay. Photo by Charles Kennard

The State Water Resources Control Board’s 2015 staff report describing stringent new trash dis-charge reduction requirements, states that: “The presence of trash in surface waters, especially coastal and marine waters, is a serious issue in California.”
The report goes on: “Trash is a threat to aquatic habitat and life as soon as it enters state waters. Mammals, turtles, birds, fish, and crustaceans are threatened following the ingestion of, or entanglement by, trash. (These) can be fatal for fresh-water, estuarine, and marine life….”
The final water board order, which arrived in June, requires the county, its cities and towns, to undertake one of two trash programs: 1) install devices in all storm drains located in priority land use areas that are capable of capturing particles that are five mm or larger; or 2) install the devices only in higher traffic areas, while also implementing programs that achieve the same benefits as the full-scale use of trash capture devices. Most Marin communities will opt to adopt the second alternative. Municipalities must submit their plans by the end of 2018.
If you’re interested in being involved in Clean Marin efforts, or are already doing cleanups in your neighborhood, contact MCSTOPPP staff at mcstoppp@marincounty.org. Residents of Fairfax or Larkspur also can contact their public works departments at the addresses below.
Fairfax Staff at the Good Earth grocery store do an exceptional job keeping their general vicinity clean, reports town public works manager Mark Lockaby, and he hopes to engage the broader business community in that effort. Anyone interested in volunteering to help is invited to contact Mark Lockaby at MLocka-by@townoffairfax.org.
San Anselmo The town recently installed four new fountains with water bottle filling stations to reduce litter from plastic water bottles, and plans to install more waste and recycling cans. In the future, they hope to get the community more involve through education and awareness.
Larkspur Scott Metcho will coordinate the Clean Larkspur effort. He would appreciate hearing from anyone who might help. Email him at SMetcho@cityoflarkspur.org and put the words “trash” or “clean Larkspur” in the subject line.