Atherton consultants recommend pumped dewatering system and interim infiltration fixes for El Camino Real flooding

Consultants hired by the Town of Atherton on Wednesday recommended a pump-based dewatering system to clear localized flooding along El Camino Real and proposed short-term infiltration trenches at priority intersections while the town pursues longer-term projects.

Douglas Kim, project manager for the El Camino Real Complete Streets project, introduced the presentation and said the study aimed to identify “longer term solutions rather than shorter term solutions that might go away with the future improvements along El Camino Real.” Eric DeLand, a consultant with Friar and Loretta, presented the technical work and options.

Why it matters: repeated ponding along El Camino Real has created safety and accessibility problems for pedestrians, bicyclists and drivers in parts of the corridor. Council members and residents said they want measures that produce a noticeable improvement before the next rainy season.

What the team found: the consultants mapped existing drainage and found that the principal large outlet in the study area is the Atherton Channel, which crosses El Camino Real near Carton Field and discharges to the Bayfront Canal. South of that channel, a county-owned drain captures some flows; north of it the corridor lacks a continuous downstream outlet. The study noted a catch basin and an apparent pipe at Selby that “is walled off and not connected to anything,” and the team scheduled a closed-circuit TV inspection at Selby to determine the pipe’s extent.

Options considered: the consultants compared (1) a conventional gravity storm-drain extension, (2) expanded use of infiltration (dry) wells and trenches, and (3) a mechanical dewatering (pumped) system that would collect standing water from drain inlets and pump it into the Atherton Channel when channel levels are low. The presentation said a new gravity storm system would only be effective if a downstream outlet could be built or upgraded all the way to the bay; otherwise it would transfer flooding downstream.

Consultant rationale for pumps: DeLand said the Atherton Channel can be at capacity during peak storms and “can’t take any more water” during those short, intense peaks, but it has available capacity before and after those peaks. A dewatering system with a small lift station and controls tied to channel-level sensors would pump local ponding into the channel only when channel levels allow. DeLand described the system as a dewatering approach rather than a full storm-drain network: "It's a dewater system. This thing will take down puddles and every see along the El Camino."

Performance and sizing notes: the team presented order-of-magnitude figures. Storage in a continuous 12‑inch conveyance could be roughly 48,100 cubic feet along the proposed line; an 18‑inch line about 93,100 cubic feet. The consultants said a 12‑inch pipeline paired with pumps could draw down visible puddles in about an hour under the study assumptions. Typical pumping rates cited in the presentation were on the order of 200–300 gallons per minute (gpm); the consultants stressed velocity/maintenance tradeoffs (larger pipes reduce maintenance but raise cost and pump size).

Short-term measures: several council members and residents asked about quicker fixes. DeLand and staff said the town can pursue targeted infiltration trenches or deeper dry-well installations at key intersections as interim measures, but they cautioned that clay soils and a water table about 25 feet deep in the corridor limit infiltration effectiveness. The presentation noted regulatory constraints for deep infiltration (a standard guideline cited was keeping the bottom of an infiltration feature at least 10 feet above the water table).

Costs and schedule: the consultants provided order-of-magnitude costs and construction approaches. They showed an estimate of roughly $2.7 million for the horizontal-drilling construction method to install the main conveyance and manholes, plus soft costs and contingencies for permitting and environmental work. The team also said about $275,000 of design/permitting effort would be needed to reach a shovel‑ready stage (final design and permit pursuit). Staff emphasized that permitting, regulatory review and identifying funding partners will influence timing and whether the town pays construction costs up front.

Regulatory and risk considerations: speakers cautioned the town must avoid adding flow to the Atherton Channel while that channel is at or near peak levels; the consultants proposed float-controlled interlocks that prevent pumping when the channel is at its peak. DeLand noted that regulators could require FEMA or flood‑plain analyses if the town proposes to add to channel flows, potentially lengthening review. Staff also listed likely coordinating agencies: Caltrans (for the state right‑of‑way), the Regional Water Quality Control Board, the Bay Area Water Quality Control Board, Fish and Wildlife and possibly the U.S. Army Corps of Engineers for channel or habitat impacts.

Direction from the council: staff said consultants will continue the investigation, complete the TV inspection at Selby, finalize recommendations for the northern portion of the corridor and return to council before moving into final design. Council members asked staff to include short‑term infiltration or trenching options where feasible and to pursue funding partners once a shovel‑ready package is available.

What’s next: staff will share updated investigation results through the town’s project staff (George) and will return with recommendations for phasing and funding. The consultants recommended limiting construction to the dry season and coordinating permits to avoid wildlife and nesting constraints.