Storm Water

Carmel-by-the-Sea Department of Community Planning and Building

ASBS Dry Weather Diversion


Most of the stormwater runoff from the City of Carmel-by-the-Sea flows through outfall pipelines onto Carmel Beach and then into Carmel Bay. Carmel Bay is one of 34 coastal water bodies that have been designated by the California State Water Resources Control Board (SWRCB) as an “Area of Special Biological Significance” (ASBS). On March 20, 2012 the SWRCB adopted special requirements pertaining to stormwater discharges into ASBSs. These are contained in the SWRCB’s “Special Protections for Selected Storm Water and Nonpoint Source Discharges into Areas of Special Biological Significance.”  

One of the requirements is that the only allowable discharges to the ASBS are those that occur during wet weather and which are composed only of storm water runoff. To meet this requirement, the City has been awarded an SWRCB Grant to build and operate a diversion system which will eliminate the discharge of flows to the ASBS during dry weather, when flows are composed largely of non-storm water. Dry weather flows may contain oil and other fluids from cars, detergent and grime from car washing, litter, and fertilizers and pesticides from overwatering landscaping.

To accomplish dry weather diversions some other cities have constructed, or plan to construct, systems consisting of permanently installed pump stations, elaborate pipeline and manhole connections, electrical power, telemetry equipment, and flow meters. These systems are expensive to construct and operate, and pump all of the dry weather flows from the storm drainage system into the sanitary sewer system which combines these flows with wastewater (sewage) that then must be treated at the local wastewater treatment plant.

Project Description

Recently completed flow monitoring has shown that the dry weather flows into the City’s storm drainage system are minimal.  Therefore, the City is installing an innovative lower-cost and easier to maintain system to comply with the dry weather diversion requirements. With the facilities being installed, runoff that would normally be discharged from the outfalls during dry weather (non-rainfall) periods will instead be diverted in one of two ways:

1.  By temporarily plugging the outfall pipes during dry weather (summer months) and retaining the runoff water within the storm drain inlet structures (catch basins) immediately upstream of the outfalls. The water that is captured and retained in the catch basins will be periodically removed using portable pumping equipment, and trucked to the City's Corporation Yard where it will be discharged into the sanitary sewer connection at that location. 


2.  In a few locations where there are summer-long flows of groundwater into the storm drainage system, by diverting the runoff water through a diversion manhole and piping it into an underground percolation bed, similar to the percolation beds used with septic tank systems. In some locations along Scenic Road sump pumps or sump drains from residential basements and underground garages that presently flow into the City’s storm drainage system will be repiped to discharge into City-owned landscaped areas where the flows will percolate into the ground.

Public Cooperation is Essential

In order for the City’s dry weather diversion system to operate efficiently and reliably, residents and businesses within the City are asked to lend their support by minimizing the amount of dry weather flows that get into the storm drainage system. They can help in the following ways:

  • Keep car washing water from flowing into the streets where it will go into the storm drainage system and add to the water that has to be diverted. Instead wash cars on an unpaved area or in such a manner that the runoff water flows into landscaping or open areas where the water can percolate and evaporate without reaching the street. 
  • Perform landscape irrigation such that excess water does not flow into the streets. This is easily done when landscape is manually irrigated with a hose by simply only applying the amount of water that can soak into the ground without running off.  With automatic irrigation systems, the duration of the irrigation cycle should be adjusted to be short enough such that no runoff will occur.
  • Cleaning sidewalks, driveways, patios and other outside surfaces by sweeping or blowing them with a leaf blower, rather than washing them down with water. If some washing needs to be done, minimize the amount of water that is used and divert it into landscaping or open areas where the water can percolate and evaporate without reaching the street.
  • Not performing any other activities that would result in water being discharged into the street, or directly into a gutter or inlet, where the water will flow into the storm drainage system.
  • Never dumping paint, anti-freeze, motor oil or any other substance down a storm drain. Remember, in Carmel they all "flow to the sea."

For More Information

You may contact Sean, Planning and Building Services Manager, at (831) 620-2010 or by email at if you have questions or would like more information about the City’s dry weather diversion project.

Low Impact Development Guidance Manual Developed by the City of Carmel-by-the-Sea

Discharges from the City’s storm drainage system are regulated under a National Pollutant Discharge Elimination System (NPDES) permit issued by the Central Coast Regional Water Quality Control Board (RWQCB) of the State of California. The City became subject to this permit on May 1, 2008. Recently, the RWQCB expanded its requirements and has directed all cities to develop and implement a set of Best Management Practices (BMPs) to reduce storm water runoff and storm water pollution emanating from new development and redevelopment projects. This is to be accomplished by using “Low Impact Development” (LID) concepts during the design and construction of those projects.

LID is a development approach that (1) reduces the amount of storm water runoff by retaining a portion of the storm water on the project site where it can infiltrate into the ground, and (2) in some instances reduces the amount of pollutants in storm water through natural or manmade treatment processes.

In order to comply with the RWQCB’s requirements, the City has developed a Low Impact Development Guidance Manual.  Because the City has been progressive over the years in implementing ordinance and policy requirements to help safeguard the environment, nearly all of the RWQCB’s requirements had already been adopted by the City and were already being applied to development projects. These requirements are primarily found in Chapter 17 of the City’s Municipal Code and in the document titled “BMP Guidance Series” which the City Council adopted in November 2009. The newly developed Guidance Manual presents all of these existing requirements in a single compact document, which design professionals such as architects, building designers, and engineers can use to refer to when they are designing their projects. City staff involved in reviewing plans and issuing permits for a project will also use this Guidance Manual to determine whether or not the project has been designed to meet these requirements.

Low Impact Development Guidance Manual (Click to open)