San Pablo Dam Road Culvert Replacement
Storm Drain Repair & Hydraulic Analysis
Orinda, California
PEC Water served as the hydraulics and hydrology lead on the San Pablo Dam Road Culvert Replacement project, providing storm damage response support to Contra Costa County following the major 2023 winter storms that severely corroded and deformed two 72-inch CMP culverts beneath the roadway. Working as a subconsultant to Mark Thomas, PEC Water developed a HEC-HMS hydrologic model for the 400-acre tributary watershed, established controlling design flows of 449.5 cfs (Q10) and 704.2 cfs (Q100), and evaluated nine culvert size and material alternatives using HY-8 hydraulic software — providing the sizing criteria and headwater analysis needed to support a trenchless replacement approach.
Client
Prime
Subconsultant
Site Conditions and Field Investigation
PEC Water conducted a site visit in August 2024 to document existing conditions at the culvert crossing before the hydraulic analysis began. The visit allowed the team to observe the inlet and outlet structures firsthand, assess the extent of the damage to the pipe inverts, and evaluate channel geometry and drainage patterns that informed key modeling assumptions. Field observations confirmed the roadway dip failure location and captured the inlet loss coefficients, Manning's roughness conditions, and outlet geometry used in the HY-8 hydraulic model.
The crossing includes both a main culvert barrel and an overflow structure — the overflow pipe contains multiple bends that increase its susceptibility to clogging, a finding documented in the field and incorporated into the H&H Analysis Report as a maintenance consideration for the final design. The August 2024 site visit established the physical baseline that grounded the hydraulic analysis in real observed conditions, rather than assumptions alone.
The Project
San Pablo Dam Road is a two-lane roadway winding through the rolling hills west of Wildcat Canyon Road in Orinda, carrying local and regional traffic through the East Bay hills at elevations ranging from approximately 330 to 390 feet above sea level. The two culverts beneath it manage stormwater runoff from a 400-acre watershed that drains from Tilden Regional Park — a significant contributing area with high-runoff Hydrologic Soil Group C soils throughout. When the 2023 storms drove severe invert corrosion and deformation in both pipes, the County faced not just a structural failure but a drainage capacity question: were two 72-inch culverts still the right solution for this watershed, and if so, what material and configuration would perform best?
Contra Costa County's decision to use trenchless replacement — rather than an open trench approach — added further technical constraints. Trenchless methods limit the degree to which pipe geometry can be changed, so the hydraulic analysis had to be precise: confirming that the replacement sizing would convey design flows within acceptable headwater limits while working within the dimensional envelope that the trenchless method required. PEC Water's H&H analysis provided the hydraulic foundation for those decisions, covering everything from watershed delineation and hydrologic modeling through culvert sizing alternatives and headwater evaluation.
Contributing watershed — 0.623 sq. mi. draining from Tilden Regional Park to the San Pablo Dam Road culvert crossing
What we Delivered
PEC Water served as the hydraulics and hydrology lead for the San Pablo Dam Road Culvert Replacement project, delivering a complete H&H Analysis Report that provides the technical foundation for Contra Costa County's storm damage repair. Our site visit and watershed delineation established the physical and hydrologic baseline; our HEC-HMS model produced controlling design flows across three storm durations for both the 10-year and 100-year events; and our HY-8 alternative analysis evaluated nine size and material configurations, giving the design team the headwater and capacity data needed to confirm replacement sizing and support the trenchless installation decision.
The H&H Analysis Report produced by PEC Water provides not just the sizing results but the complete technical record — watershed parameters, precipitation inputs, modeling assumptions, and the full alternative analysis table — supporting Contra Costa County's design decisions and satisfying the documentation requirements for county and federal project review. When the new culverts are installed, San Pablo Dam Road will have a drainage system sized for the storms the watershed actually produces, restoring the reliable access that residents and emergency responders depend on
Storm damage response with lasting results — hydraulic analysis that right-sizes the replacement and protects the road for the next century.
Watershed with all hydrologic layers — soil types, drainage paths, and contributing area delineation used in HEC-HMS modeling
Culvert Sizing Alternatives
With controlling design flows established, PEC Water performed hydraulic analysis using HY-8 culvert software, evaluating nine combinations of pipe size and material for the main culvert paired with various overflow culvert configurations. Alternatives ranged from dual 66-inch steel pipes to dual 84-inch steel pipes for the main culvert, with overflow culverts from 36 to 66 inches in steel and corrugated steel. Each combination was evaluated for headwater elevation and pressurization above the pipe crown under both Q10 and Q100 conditions.
The analysis showed that the dual 72-inch configuration — matching the original pipe sizing — performs adequately under the 10-year storm in free-surface (non-pressurized) flow when paired with an appropriate overflow culvert, while larger alternatives provide additional freeboard margin under the 100-year event. PEC Water flagged an important maintenance consideration for the overflow culvert: the pipe contains multiple bends that increase clogging susceptibility, and the final design should ensure adequate maintenance access and debris management at the overflow inlet to protect against capacity loss over time.
Watershed Delineation and Hydrologic Analysis
PEC Water delineated the contributing watershed for the San Pablo Dam Road culverts using GIS-based hydrologic tools, establishing a drainage area of 0.623 square miles (approximately 400 acres) draining from Tilden Regional Park through the project site. All soils within the watershed are classified as Hydrologic Soil Group C — moderately high runoff potential with low infiltration rates — meaning a significant fraction of each storm event reaches the culverts as surface runoff, placing high demands on conveyance capacity.
Hydrologic analysis was performed using HEC-HMS in accordance with the Contra Costa County Flood Control and Water Conservation District (CCCFCWCD) methodology. Peak discharge was determined for the 10-year and 100-year return periods, evaluating three storm durations — 3-hour, 6-hour, and 12-hour — for each event. The 12-hour storm duration produced the controlling peak discharge for both return periods, yielding a Q10 of 449.5 cfs and a Q100 of 704.2 cfs. These values established the hydraulic performance targets against which all culvert replacement alternatives were evaluated.