Surface Drainage and Road Infrastructure: Design and Maintenance
Surface Drainage and Road Infrastructure
Surface drainage is crucial for preventing flooding, platform deterioration, pavement deterioration, and ensuring traffic safety and protection. Surface drainage involves three key stages:
- Collection: Gathering rain or snowmelt from the platform and its margins using gutters, scuppers, and catch basins.
- Evacuation: Transporting collected water through a network of pipes, culverts, and ditches to natural waterways or sewage systems.
- Restitution: Restoring the natural flow of watercourses intercepted by the road through refurbishment and construction of cross-drainage structures.
Functional Criteria for Drainage Systems
- Current Speed: Maintaining an appropriate flow rate to prevent erosion or silting.
- Water Level: Ensuring minimum clearance between the platform and water level.
- Raised Water Level Check: Assessing potential damage to adjacent properties or cross-drainage works, considering the embankment as a dam.
Return Period and Risk Assessment
The return period (T) is the average time interval within which a specific flow rate (Q) is exceeded. The risk of obstruction, caused by debris carried by water, is particularly significant in catch basins and buried collectors. Factors influencing obstruction risk include the characteristics of rivers and floodplains. Risk categories are:
- High: Potential for large debris like trees to block drainage.
- Medium: Risk of blockage from reeds and shrubs.
- Low: Minimal risk of significant obstruction.
In high-risk areas, cross-drainage works should not operate at full capacity, maintaining a minimum clearance of 1.5 meters and a minimum floor width of 12 meters. In medium-risk areas, these dimensions can be halved. If these conditions are not met, the potential water level increase due to obstruction must be considered in the design.
Consequences of Drainage Failure
Drainage failure can lead to:
- Damage to the drainage element itself.
- Interruption of traffic flow.
- Damage to adjacent properties (non-catastrophic or catastrophic).
Proper drainage prevents water accumulation on the road, reducing the risk of hydroplaning, ice formation, and water spray from vehicles.
Types of Drainage Devices
- Longitudinal: Parallel to the road’s axis.
- Lateral: Perpendicular to the road, allowing water to cross the road’s path.
Homogeneous sections should be defined to ensure consistent surface drainage. Special attention should be given to areas with low platform slopes and the impact of drainage elements. Solutions should be efficient, simple, robust, and easy to maintain.
Caz-Strip Shaped Drainage
A narrow, lined channel with minimal depth, typically located at the edge of the platform. Its drainage capacity is limited by its depth. To prevent excessive water width, a collector is needed, either through a continuous drain or a series of isolated sinks. These must discharge before any changes in camber to prevent water from crossing the road.
Longitudinal Gutter-Ditch
An open channel adjacent to the platform, typically following the road’s longitudinal slope. Gutter type selection depends on:
- Conditions for safe crossing of the ditch.
- Small ditches are only suitable for rough terrain and must be covered or protected by safety barriers.
- If there’s a risk of water infiltration affecting the road’s stability, the ditch should be deep, and its surface should be waterproofed using concrete, prefabricated parts, stone, or bituminous materials.
Waterproofing
Waterproofing is necessary in cases of high water velocity, very low water velocity to prevent leaks, and in areas where maintenance is difficult and expensive. The maximum water level in the ditch should not reach the road’s surface. A minimum depth of 30 to 40 centimeters is recommended. The maximum velocity should be considered, and steep slopes (greater than 4%) should be lined. A minimum longitudinal slope of 0.5% is advisable, estimated using Manning’s equation.
For slopes greater than 0.5%, the most unfavorable element is downstream. For lower slopes, the water height increases upstream. In these conditions, the greatest depths are found in the initial section.
Drainage Flow Estimation
Estimate the flow, select a gutter type, determine the slopes, calculate the water height and velocity, and validate the results.
Drains and Scuppers
Drains collect water from gutters, while scuppers are direct discharge devices (e.g., on bridges). Catch basins are buried water pipeline devices. Types of catch basins include lateral curb, horizontal, mixed, and continuous. Each isolated catch basin must have a chest beneath it to connect to the collector. Inlets are more efficient but pose a greater traffic hazard and are more prone to clogging.
The design of drains and scuppers should prioritize traffic safety and minimize obstruction risks. The spacing of catch basins is approximately one every 200m2 of platform and one every 25 to 50 meters in urban areas with curbs.
Catch basins on the platform must not disrupt traffic flow. In high-speed roads, they should be located at the edge of the platform. The surface should be stable, and the finishing of the adjacent area should ensure water flows into the basin. The grid should be difficult to move, have a good seat, and be strong enough to withstand vehicle traffic. Where there is no risk of vehicles passing over the basin, depressions and inclined planes can be used to facilitate water channeling.
Isolated catch basins located on inclines should generally be horizontal, intercepting the bottom of the ditch or caz, with their bars preferably aligned with the flow. The free length (L) of the bars, when parallel to the flow, should be no less than a specific formula.
In a lateral drain, its capacity can be increased by increasing its length. Intermediate supports can reduce the opening but significantly reduce drainage capacity. Where there is no risk of vehicles passing over the basin, depressions and inclined planes can be used to facilitate water channeling.
Collectors-Manifolds
Tubes can be made of precast concrete, asbestos cement, or galvanized corrugated steel, either alone or in combination. They must be supported on a bed of hydraulic concrete. Corrugated steel hoses should not be placed on rigid beds and require careful compaction to prevent crushing during construction. The coating should be sufficient to prevent damage from movement. Collectors should have a minimum diameter of 30 cm, with 40 cm recommended. Avoid low or low-sloped sections that can accumulate sediment, unless unavoidable and equipped with appropriate sandboxes.
Caskets
Caskets should be installed to facilitate inspection and maintenance of buried drainage devices. They should allow for easy cleaning, especially those crossing the road. The distance between caskets should not exceed 50 meters. Collection boxes should be placed at locations such as sinks, pipe confluences, reloading devices, oil separators, sandboxes, and sewage infiltration points. Avoid blind or obstructed caskets.
Cross-Drainage
The presence of a road interrupts the natural drainage of the terrain. The main purpose of cross-drainage is to restore the continuity of the natural drainage network, allowing water to pass beneath the road under suitable conditions. Cross-drainage works can also be used to drain the platform and its margins. If natural drainage points are too far apart, it may be necessary to install dedicated cross-drainage works.
Types of Cross-Drainage Works
- Sewers: Up to 1 meter.
- Tajeas: Up to 1-meter sections (not O).
- Culverts: 1 to 3 meters.
- Small Bridges: 3 to 10 meters.
- Bridges: Spans over 10 meters.
Properly designed and maintained drainage systems are essential for the longevity and safety of road infrastructure.