What are the challenges and procedures in the construction of river and watercourse crossings?

A river crossing consists of installing a pipe, cable or gallery under or across a watercourse without compromising its hydraulic regime or the environment. The main challenges are grouped into four blocks:

• Hydraulics: floods with high return periods, slope and slope scour, cross currents and pipeline buoyancy.

• Geotechnical: saturated soils, poorly competent alluvial soils, presence of blocks or irregular bedrock.

• Environmental and regulatory: turbidity, protection of ichthyofauna, spawning windows, Public Hydraulic Domain and authorizations from Hydrographic Confederations or Maritime Authorities.

• Logistics: limited access, daily hydraulic level variations, river or road traffic that must be kept operational.

Standard procedure for a river crossing

1. Previous studies
   Topobathymetric survey, geotechnical soundings and hydraulic models (e.g. HEC-RAS) to calculate runoff, scour and maximum water level.

2. Selection of construction method

   • Microtunneling or Direct Pipe for deep, straight crossings where millimeter slope and minimum turbidity are required.

   • Horizontal Directional Drilling (HDD) for long curved paths and cohesive soils, avoiding excavation in the bed.

   • Open sky with channel detour (cofferdams, sheet piles or inverted siphon) when the sheet of water is small and the impact is assumable.

3. Environmental management and permits
   Presentation of the Impact Study, turbidity control plans, sediment curtains, and definition of "biological windows" to be executed during minimum flows.

4. Detailed engineering
   Design of jackets, joints and ballasts against buoyancy; calculation of anti-erosion protections (gabions, breakwater, articulated blocks).

5. Execution
   Laser stakeoutThe installation of topographic monitoring points and hydraulic control stations.
   - In microtunnel/HDD: closed bentonite mixture and pressure monitoring to avoid "frac-out".
   - In the open air: temporary channeling, excavation, placement of the pipeline and compacted select backfill.

6. Protection and restitution
   Placement of geotextile blankets, wildlife ramps, revegetation of riverbanks and removal of temporary infrastructure.

7. Commissioning and follow-up
   Sealing and geophonic tests; annual bathymetry and piezometers to monitor scour or subsidence.