City of Hermosa Beach
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CIP 438 - Storm Water Dry Wells
Summary
Hermosa Beach, in coordination with the Beach Cities Watershed Group, must implement storm water pollution‑reduction projects to meet Municipal Separate Storm Sewer System (MS4) Permit requirements by 2031 and avoid substantial fines. After the cancellation of the 2016 Greenbelt Infiltration Project, a 2022 feasibility study identified much smaller, decentralized dry wells—capturing only Hermosa Beach runoff—as the most cost‑effective and least disruptive alternative. Updated analysis in 2026 refined the concept to 13 dry wells located at South Park and either the Greenbelt or Bicentennial Park, significantly reducing project scale, cost, and construction impacts. The City, jointly with Redondo Beach, will apply for Safe Clean Water grant funding in July 2026 to advance design, with local funds available for initial phases but additional funding needed if grants are not awarded.
Storm Water Pollution Reduction Regulation
The cities of Hermosa Beach, Torrance, Redondo Beach, Manhattan Beach, and the Los Angeles County Flood Control District formed the Beach Cities Watershed Group (Watershed Group) to develop a Watershed Management Plan (WMP) to comply with the Regional Water Quality Control Board’s (Water Board) 2012 MS4 Permit. The MS4 permit is the guiding document that directs agencies to develop the projects and programs to help reduce pollution from storm water runoff. The Watershed Group entered into a Memorandum of Understanding (MOU) to cost share the development of the WMP and a Coordinated Integrated Monitoring Plan (CIMP) in 2013 which were updated in 2021 and 2023, respectively. The WMP was approved by the Water Board and the Watershed Group was directed to begin implementation of the WMP immediately, including construction of identified capital projects to capture run off and reduce pollutant discharge ahead of the 2031 compliance deadline. Failure to complete the projects would likely result in significant fines and increased regulatory oversight.
Former Greenbelt Infiltration Project (2016) – Cancelled
The WMP had identified the “Greenbelt Infiltration Project”, that was proposed to install a large concrete infiltration chamber underneath the Greenbelt, between Herondo and 2nd Street. In 2016, the City of Hermosa Beach, in partnership with the other Watershed Group parties, submitted a successful application to the Water Board for funding for the design and construction of the Greenbelt Infiltration Project, which was awarded a Proposition 1 Storm Water Implementation Grant in the amount of $3,099,400. In September 2017, City Council awarded a professional services agreement to begin the design of Project. Shortly thereafter, in response to significant community concerns, the project was cancelled by City Council in March 2019. Some of the major community concerns were:
- Overall scale and size of the project;
- The fact that it included runoff from other cities; and
- Fears about the structural and hydrological effects of the project.
As a result of the cancellation, the City forfeited the grant funds, returning $267,963.33 in project costs to the partner cities, and was required to make a payment of $160,000 to the Watershed Group to fund a new feasibility study that explored alternate, replacement sites and projects.
Former Greenbelt Infiltration Project Concept (cancelled)
New Feasibility Assessment for Replacement Project
As required by the Water Board, and in response to prior community concerns, the Watershed Group developed a new pre-feasibility study in 2022 to identify a suite of smaller projects distributed throughout Torrance, Redondo Beach and Hermosa Beach that together would capture storm water runoff volumes equivalent to the cancelled Greenbelt Infiltration Project. For Hermosa Beach, installation of dry wells was proposed as the most efficient, cost-effective, and least impactful strategy for achieving this goal. The revised plan as conceived would be accomplished in strategic locations that would capture runoff only from within Hermosa Beach – and not from other neighboring cities.
A dry well is a below-grade structure that allows run-off to be quickly and efficiently captured and infiltrated deep into the soil. These are typically concrete tubes, approximately 4 to 7 feet in diameter, that extend 20-30 feet into the ground and are filled with gravel and other media to help filter the storm water run-off and improve general hydrology and structural integrity. The run-off reaches the dry well through a typical catch basin inlet connected by a pipe to the dry well. Media filters are installed to eliminate the majority of larger pollutants, such as trash, from entering the system. Construction involves excavating sufficiently to place the pre-cast concrete structure in the ground. After construction, the structure is buried and is only visible as a manhole at the surface to provide access for maintenance and inspection. Dry wells are a popular option for storm water capture that have been installed in many locations throughout the region. Redondo Beach has installed 16 dry wells since 2021 and has experienced no issues with operation nor detrimental effects on the surrounding areas. The recently completed Beach Cities Green Streets project installed 27 new dry wells throughout Manhattan Beach, Torrance and Redondo Beach.
Dry wells are more cost effective and logistically manageable than the previously proposed concrete chamber and allow much greater flexibility for siting. Dry wells generally only require about 15 feet of separation from adjacent building footings based on LA County Public Works Low Impact Development Stormwater Infiltration Guidelines GS200.1 and are commonly installed on residential properties throughout Hermosa Beach as part of low impact development (LID) storm water requirements.
The 2022 pre-feasibility study identified potential locations east of Pacific Coast Highway (PCH) for the installation of 23 dry wells across residential neighborhoods east of PCH between 1st Street and 10th Street. However, staff had been concerned that the proposed 23 dry wells were too widely dispersed across the city requiring significant construction period coordination and impacts.
2022 Pre-feasibility Study Concept
In March 2026, staff advanced a more detailed feasibility assessment of the proposed project with the goal of applying for Safe Clean Water grant funding to produce a final design in partnership with the City of Redondo Beach; the application is due by July 31, 2026. Staff has reassessed the initial proposal for 23 dry wells east of PCH and are instead proposing a smaller project with only 13 dry wells which will further reduce the overall cost and construction period impacts. There are two potential options under consideration:
- Option A (recommended): 8 dry wells in South Park and 5 dry wells along the Greenbelt north of 2nd Street; or
- Option B: 8 dry wells in South Park and 5 dry wells in or near Bicentennial Park.
Two Los Angeles County Flood Control District storm drains run through this area and all three sites are efficiently located to be able to intercept runoff from each of these lines. Unlike the prior concept, these dry wells would be located further downstream and would not require extensive installation of catch basins and would only require construction of a diversion structure and connection line at each location. This smaller project would also greatly reduce construction period impacts versus the prior alternative. The proposed drywells could also all be placed at least 40 feet from any adjacent buildings, well beyond drywells recommended 15 feet, and will be installed at depth of 20 feet to 30 feet. The bottom of dry wells will remain outside the 1:1 structural plane line. Therefore, the Project would not have any impact on adjacent foundations.
South Park Site – Under either option being considered, the Project proposes to construct 8 dry wells, including 1 pre-treatment device, at South Park along the existing concrete walkway near the eastern edge of the park. The result would be a new concrete walkway and would not impact the use of the park. The design of the project would ensure that the manhole lids would be installed in a way to provide a flush and smooth walking surface on the pathway.
Greenbelt Site – the proposed site is located in the Greenbelt, just north of 2nd Street along the western side between the pathway and Valley Drive and would be combined with the South Park site under Option A. The proposed 5 dry wells, including 2 pre-treatment devices, would be installed in the planted area and the project would also include landscaping and irrigation improvements to minimize any visual impacts of the manholes.
Bicentennial Park Site – The Bicentennial Park site option would feature 5 dry wells, including 1 pre-treatment device, proposed to be installed on 4th Street and the western edge of the park and would be combined with the South Park site under Option B. The proposed installation at the Bicentennial Park site would also bring other benefits to the area by paving of the dirt section of 4th Street and Cochise Street; resurfacing 4th street west of Ardmore Street and Ardmore Street between 3rd Street and 4th Street; and tying in the existing catch basin at the end of 4th Street (that currently results in runoff into the Greenbelt) into the storm drain system. Additionally, the City could request possible park amenities (e.g., swing set, benches, etc.) as part of a grant application with Measure W, but would be subject to successful award and approval. While the project would bring other non-storm water benefits to the area, the construction period impacts would be much more challenging than the Greenbelt location (Option A) due to its distance from the existing storm drain line and the resulting trenching and paving that would be required to install an intercepting line. Underground utilities and overhead power lines also pose costly challenges.
South Park Concept (Option A & B)
Greenbelt Concept (Option A)
Bicentennial Park Concept (Option B)
Public Meeting Information
Staff will be presenting the proposed project concepts at the Public Works Commission meeting on July 15 at 6:00 p.m. in the Council Chambers at City Hall and seeking community input. Meeting details, including participation and viewing options, will be posted on the City website at least 72 hours beforehand.