Sediment

Raul M. Rojas, Director, Public Works
sediment entering storm drain

Erosion is caused by wind, rain, and wear detaching soil particles where they can then be transported as sediment. While erosion and sediment transport is a natural process that is a vital part of the natural stream process, excess erosion and sediment transport from human activities can not only be very damaging to our creeks and wildlife but also clog storm drains, cause localized flooding, and restrict creek channels.

Many pollutants such as heavy metals, pesticides, PAH's, and detergents bind to soil particles so sediment coming from the urban environment also carries many additional pollutants into our creeks, bays, and ocean. These pollutants are toxic to aquatic life and can remain in our waterways for decades.

 

Sources of Erosion and Sediment

While there are many sources of erosion and sediment in the urban environment, listed below are main sources in Marin and ways to reduce and eliminate them.

  • Construction and landscaping
  • Increased development and impervious surfaces
  • Vegetation removal
  • Bank erosion and alterations
  • Fire roads, trails and other private dirt roads
  • Social trails

Construction and Landscaping

Grading, vegetation removal, stockpiles, trenching for utilities and irrigation systems, and vehicles and equipment tracking sediment out into the street are just some of the activities that these types of projects need to address. You can find preventative measures, referred to as Best Management Practices (BMPs), for these and many other construction and landscaping activities on our Resources for Projects During Construction page including helpful brochures such as Dirt Can Hurt, Keep it in Place!

Increased Development and Impervious Surfaces

As areas are developed the native soil and vegetation that used to infiltrate and take up most of the water during rain events is covered up by impervious surfaces such as roofs, driveways, sidewalks, and streets. Where natural ground cover may only have 10% runoff during the rainy season, areas with 10%-20% and 35%-50% impervious surface will have double and triple the amount of runoff respectively!

Increased runoff means increased peak flows in our creeks. This not only leads to more frequent and severe flooding events, but also causes excess scouring and downcutting of the creek bed and increased erosion and sloughing along the creek banks. Less water infiltrating down to the water table also causes decreased spring and summer flows in our creeks.

To mitigate these effects larger new and redevelopment projects are required to install permanent features such as bioswales, cisterns, bioretention facilities, and rain gardens that are designed to infiltrate and treat stormwater runoff after (post) construction and mimic preconstruction conditions. These practices are referred to as Low Impact Development (LID). To learn more about these requirements and how to implement Low Impact Development practices visit our Post Construction Requirements and Low Impact Development Resources page and check out helpful brochures such as Slow the Flow, Keep Rain Onsite! and Slow It. Spread It. Sink It.

Vegetation Removal

Vegetation plays many key roles in preventing rain and wind erosion.

  • Trees, shrubs, and other plants provide a canopy that protects the soil from rain drops striking the soil directly and dislodging particles.
  • Their root structure binds soils and holds them in place which is particularly important for stability on hillsides and along creek banks, and preventing dust from wind. Note - Native plants work the best because they have a much deeper root structure than most non-native plants, require less watering, and are self-sufficient after they become established (typically 2-3 years).
  • They slow the flow of runoff allowing sediment particles to settle out, increasing infiltration, and reducing concentrated flows that cause rilling and gullying.
  • They reduce the amount of runoff by uptaking water and releasing it through transpiration.

Bank Erosion and Alterations

A stable unaltered creek with native vegetation and a riparian corridor will often see minimal bank erosion in an average rain year. Bank erosion is often an indicator of upstream or downstream impacts and/or creekbank alterations. Factors can include:

  • Higher peak flows caused by increased development and impervious surfaces
  • Increased stream velocity caused by upstream armored banks, channelizing, and constrictions (walls, rip-rap, fill)
  • Increased depth and eddying caused by a downstream restriction
  • Removal of trees, shrubs, and other vegetation from the creekbank
  • Failing walls and other armoring

If you are a creekside homeowner and you're experiencing creekbank erosion, it's important to identify why the bank is eroding before developing a solution. Remember that any work done in the creek or on its banks will need permits! There are resources to help you get started on our Everything Creeks! page including helpful brochures such as Repairing Creekbank Erosion or check out the flyer for the Marin Project Coordination meetings.

Fire Roads, Trails, and Dirt Roads

Without proper maintenance fire roads, public trails and other public or private dirt roads can erode or even washout sending large volumes of sediment into our creeks.

Social Trails

Social trails are created by pedestrians, bicycles, and vehicles not staying on designated walkways, paths, trails, and roads. The continual wear on these cut-throughs tramples existing vegetation and compacts soils which greatly inhibits any new plant growth. These bare dirt trails also often go vertically up and down slope making them highly susceptible to erosion or are along roadways and streams so the sediment directly discharges to storm drains and creeks.

Resist the temptation and stay on designated walkways, paths, trails, and roads!

Lagunitas Creek Fine Sediment TMDL

Lagunitas Creek Fine Sediment Total Maximum Daily Load (TMDL)

Lagunitas Creek, the largest watershed in Marin County, flows from its headwaters on Mount Tamalpais to its mouth in Tomales Bay. The watershed supports federally listed populations of coho salmon, steelhead, and California freshwater shrimp. In 1990, based on evidence of widespread erosion and concern regarding adverse impacts to fish habitat, the State Water Board listed Lagunitas Creek as impaired by sedimentation under section 303(d) of the Clean Water Act.

The SF Regional Water Quality Control Board then developed a Total Daily Maximum Load (TMDL) allocation for sediment within the Lagunitas watershed. The TMDL contains specific actions for attaining water quality objectives and supporting identified beneficial uses for that waterbody. In June 2014, the San Francisco Regional Water Quality Control Board adopted a Basin Plan Amendment which officially established a TMDL for fine sediment in the Lagunitas Creek watershed. The Basin Plan Amendment also included an implementation plan to achieve the TMDL and related habitat enhancement goals.

Paved Roads Assessment

As part of the implementation plan a Paved Roads Assessment was completed to identify potential sediment discharge issues along county maintained paved public roads. The report was completed in 2017 and the County is using it as road map to identify and implement projects to repair and replace roadways, roadside ditches, and culvert crossings.

Available Data Sources

The following documents, reports and studies provide further detail on the regulatory background, sediment assessment and road infrastructure in the TMDL project area.

Regulatory Background

Road Maintenance Guidelines