Galveston County, Algoa/Alta Loma Culvert Replacements

SPI was selected by Galveston County to provide design and construction phase services for culvert replacements in the Algoa/Alta Loma communities. The County Road and Bridge maintenance division identified ten locations where metal culvert crossings over channels that showed high levels of erosion, corrosion and settlement. SPI coordinated the surveying and geotechnical analysis at these locations and reached out to private utility companies to identify any potential conflicts. SPI conducted a number of site visits in dry and wet conditions to confirm the data received from these efforts and to establish flow and ponding characteristics around the culverts. Further observations at the sites helped identify damaged features to existing structures, channel and roadway that could be repaired and mitigated in the project.

Being so near the bay along the northern Texas Gulf Coast, the area has a high water table, receives tidal backflow and storm surge during tropical events inundates the channels with salt water. Given these conditions, SPI designed these culverts to be replaced with reinforced concrete box culvert crossings, as these would have greater stability and longevity than metal culverts. Furthermore, the county did not want the roads raised above the existing top of pavement at the crossings, so boxes with longer widths and lower heights could be used to accommodate the anticipated flows without the roadway being overtopped during rain events up to the 50-year.

SPI designed culvert crossing configurations varying from single 4’ wide x 3’ tall, double 9’ wide x 5’ tall and triple 8’wide x 8’ tall box sizes. Each required analyzing the flow rate and velocities resulting from constriction of the channel upstream of culvert inlet and expansion of the channel downstream of the culvert outlet. The appropriate headwall or wingwall structure and erosion protection was then designed to mitigate effects from scour at the culverts’ inlet and outlet. Proper catchment of adjacent drainage ditches flowing into the channel were designed using inlets or culverts with safety end treatments, such that their discharges would not erode the channel slope.

Additionally, SPI designed the realignment of roadway that curved across two culvert crossings. This improved the conveyance of the channel, removed roadway obstacles and improved the safety and visibility for traffic. Other obstacles, such as steep roadside slopes and concrete headwalls, that could not be removed, required designing traffic guard fencing with appropriate end treatment, signage, pavement markings and reflective posts. SPI designed a traffic detour plan, to plan on full road closure during construction. This included a temporary truck route and coordinating a temporary road with the County Road and Bridge maintenance division. SPI’s storm water pollution prevention plan specified inlet protection and rock filter dams, type 1 and type 3.