Benefits and Outcomes of the Miller Avenue Streetscape Project: Water-Related Infrastructure Gets a Makeover

October 20, 2017

The Miller Avenue Streetscape Project, an overhaul of approximately two miles of one of Mill Valley’s two main arteries and far and away the biggest road improvement project the City of Mill Valley has undertaken in decades, is now in its fifth month. While we have seen considerable progress above-ground with new sidewalks, curbs and gutters, a large portion of the work done to date has been happening underground, out of sight for most passers-by.

One of the main components of the Miller Avenue Streetscape Project involves improving our infrastructure, particularly as it relates to the flow of water through town. With direction from the City Council, the City has placed a high priority on upgrading our sewer pipes and our storm drains and implementing innovative techniques to reduce flooding and pollution of our network of waterways.

“This is a road improvement project, make no mistake about it,” Mayor John McCauley says. “But we’ve been very focused on upgrading our infrastructure below and taking a forward-thinking approach to managing how the new Miller Avenue will interact with our creeks and waterways.”

The project is built around the “Complete Streets” principle of design, addressing safety for pedestrians and bicyclists, providing better transit access and improving motorist safety, all while retaining and enhancing Miller Avenue’s distinct character. The approved Streetscape Plan will provide continuous bike lanes, improved sidewalks, enhanced crosswalks, and comprehensive replacement and repairs to pavement, storm drains, and sanitary sewers along Miller Avenue.

The following is a look at some of the sewer- and water-related improvements stretching from Almonte Boulevard near Tamalpais High all the way to Sunnyside Avenue near downtown:

Overhauling Our Sewer System
Reducing “infiltration and inflow” or “I&I” – that is, groundwater and stormwater seeping into the sewer system through cracks in the pipes, particularly during the rainy months – is a critical part of reducing damage to the long-term integrity and safety of the City’s sewer system. Infiltration and inflow increase the load placed on public sewer pipes and make it more likely that the larger system will fail, and a failure of the system can lead to serious environmental damage.

To address I&I, the Miller Avenue project is replacing the sewer system infrastructure under Miller Avenue and many surrounding streets, part of the City’s estimated $13 million, five-year overhaul of its sewer system.

Additionally, the project is replacing sewer laterals connecting homes and businesses to the sewer system, as these service sewer laterals account for as much as 36 percent of the groundwater and stormwater seeping into the overall sewer system, according to the City’s comprehensive study of the system. The normal quantity of sewage that is generated by homes and businesses in the sewer treatment plant service area is approximately 2.3 million gallons per day. This volume increases more than 10 fold to almost 30 million gallons per day during peak storm events – a clear indication of a significant I&I problem.

The sewer lateral work on Miller is part of a larger effort to upgrade laterals City-wide. The City Council approved an ordinance in 2014 designed to improve the efficiency and operational integrity of the sewer system and the wastewater treatment plant, and to enhance the quality of groundwater, creeks and the Bay.

Protecting Our Creeks
In the early 90’s, the City of Mill Valley began addressing stormwater pollution, along with 10 other Marin cities and towns, the County of Marin, and the Marin County Flood Control and Water Conservation District. In 1993, they created the Marin County Stormwater Pollution Prevention Program (MCSTOPPP), with goals to prevent stormwater pollution, protect and enhance water quality in creeks and wetlands, preserve beneficial uses of local waterways, and comply with State and Federal regulations.

With this in mind, the City is dedicated to minimizing and filtering stormwater runoff into our creeks, both to reduce pollution quality during the Miller Avenue Streetscape project and after its completion. During construction, contractors are adhering to the City’s stringent state-of-the-art Storm Water Pollution and Prevention Plan (SWPPP).

The City’s SWPPP is designed to, among other things, control pollutants and their sources, including sources of sediment associated with construction, construction site erosion and all other activities associated with construction activity.

The finished Miller Avenue Streetscape Project will include a number of low impact development tools to reduce the rate of runoff and improve the quality of our creeks, such as rain gardens, bio filters, drop inlets, energy dissipators, and pervious pavements. For instance, rain gardens will be created at Miller Avenue’s intersections with Willow Street, Locust Avenue, La Goma Street and Evergreen Avenue, among other locations. Examples of rain gardens and pervious pavements are seen below:

“I’m very pleased to see that there are rain gardens and detention basins in the project – those are good [water quality features],” says Betsy Bikle, a board member of Mill Valley Streamkeepers. “Improving the quality of the water in the creeks during and after the project is incredibly important in order to protect the threatened steelhead fish. These are the type of forward thinking elements a project like this needs to have.”

Moving to Address Flooding, Sea Level Rise
While the City begins the development of a City-wide master plan on drainage needs and flood control, there are some intermediate gains in the Miller Avenue Streetscape Project.

Those gains are the result of the City garnering $1.06 million in funding earlier this year from the County of Marin’s Flood Control and Water Conservation District, allowing the City both to make immediate improvements in one of Mill Valley's most flood-prone areas and undertake a comprehensive drainage Master Plan for Mill Valley.

The District’s eight flood control zones cover distinct geographical areas and receive funding via taxes and fees applied to annual property tax bills. The City’s funding request for 2016 covered $650,000 for immediate Miller Avenue Streetscape Project drainage measures, the largest component of which focuses on the flood-prone area of Miller and Montford avenues near the 2am Club and Joe’s Taco Lounge. At an estimated cost of $250,000, contractors are creating a bypass storm drain to divert stormwater away from Miller and Montford avenues, where the Arroyo Corte Madera del Presidio creek is quite narrow and thus serves as a frequent drainage “pinch point,” causing flooding around the businesses at that intersection. The bypass will divert stormwater down the middle of Miller Avenue to the Reed Street and Valley Circle intersection, discharging it into Reed Creek, which runs into a section of Arroyo Corte Madera del Presidio that is much wider and can accept the extra flow.

The second component of the Miller Avenue-area improvements is the installation of new tidal flap gates along Miller between The Redwoods and the adjacent Pickleweed Apartments, replacing the existing 30-inch diameter culvert with two 48-inch diameter culverts with headwalls. That improvement is to prevent high tide flooding at Miller and Camino Alto. Its cost is estimated to be $197,000.

The project also calls for a pair of improvements on Miller Avenue just north of Almonte Boulevard near Tam High, a section of the road that often floods during high tides, forcing road closures: 1) installing two check valves on the storm drain pipes in the area and 2) raising the pavement of Miller Avenue just north of Almonte Boulevard by about 10 inches, as well as a 200-foot portion of the median in that area by about 12 inches, to prevent flooding during high tides. The cost of those two improvements is estimated at $203,000.

In addition to the Miller Avenue projects, the District’s budget allocation also includes $265,000 for comprehensive evaluation of the City’s vast inventory of creeks, storm drains, pipes and culverts, providing additional analysis that will inform the City’s creation of a master plan for drainage improvements. The study and resulting master plan would include extensive community outreach efforts to gain agreement about priorities and funding options for the much larger, long-term measures to be taken to addressing flooding and sea level rise.

Thank You
The Miller Avenue Streetscape project is deeply aligned with Mill Valley's unique charm, character, and small town values, reflecting the work of the many committee members and residents who participated in the planning and design process. We recognize that construction is challenging and presents multiple issues for daily activates. We thank you for your patience and understanding.

Learn more on our project page.