Hydraulic Analysis of Little Monteocha Creek Box Culvert Replacement Under NE 21st Street (SR31A)
The Northeast 21st Street bridge-culvert is located within the Little Monteocha Creek Watershed in the jurisdiction of the St. Johns River Water Management District (SJRWMD). The purpose of this project was to design a new box culvert to replace the existing box culvert that will allow for adequate conveyance of flood waters to protect the roadway and not adversely impact surrounding properties.
DRMP conducted an analysis of the project needs in order to design a replacement box culvert that would meet Alachua County criteria and would not adversely impact the surrounding properties.
Geographic Information Systems (GIS) shapefiles of SJRWMD basins were obtained and the drainage area was compared against contours derived from LiDAR, as well as existing drainage patterns found in the construction plans of NE 21st Street. Changes to the contributing drainage basin were made where appropriate and an updated drainage basin of 4,961-acres (7.75-square-miles) was determined.
In order to efficiently evaluate the hydrologic and hydraulic systems for the NE 21st Street crossing the flood study prepared and used for the floodplain analysis was obtained from Federal Emergency Management Agency (FEMA). This study contained the existing culvert as well as flows for various storms up to including the 100-year event.
DRMP coordinated with the County to review any design plans and calculations that might be available for the originally constructed culvert. It is apparent from a site review that the original plan design of three box culverts was not constructed and a bridge-culvert with approximate cross-section dimensions of 24-feet by 6-feet (span by height) was constructed in place.
The replacement culvert proposed for this site is triple 12-foot wide by 8-foot high concrete box culvert. This culvert size was chosen to convey the 50-Year event with adequate freeboard and contain the 100-year event without overtopping the roadway. Construction of this culvert will require regrading and side slope construction in order to allow the existing channel to effectively transition to the proposed structure at the upstream and downstream locations.
