Road Drainage and Culverts

Importance of highway drainage

Highway drainage is the process of removing and controlling surface water and subsurface water in and around the road.

Water is one of the main causes of pavement failure:

• Reduces the strength (bearing capacity) of subgrade soil — the CBR of many soils falls sharply on soaking.
• Causes erosion of shoulders, side slopes and embankments.
• Causes stripping of bitumen from aggregates, ravelling and potholes.
• Leads to mud pumping in rigid pavements.
• Causes frost heave in cold regions and swelling/shrinkage in expansive soils.
• Water on the surface reduces skid resistance and causes hydroplaning.
• Waterlogged ground weakens embankments and causes slope failures (especially in hill roads).

Requirements of a good drainage system

1. Surface water from the carriageway and shoulders should drain quickly without ponding.
2. Side drains should have adequate capacity and gradient to carry water to natural outlets.
3. Water from adjoining land should not flow onto the road; cross drainage structures must pass it under the road.
4. Flowing water should not erode slopes, shoulders or drains.
5. The highest water table should be kept well below the subgrade (commonly at least about 0.6–1.0 m).
6. Seepage and capillary rise into the subgrade should be controlled.
7. In hill roads, water flowing down the hill slopes should be intercepted and safely led across the road.

Surface drainage

Camber and shoulders

• Camber (cross slope) sheds water from the carriageway to the edges (see Geometric Design).
• Shoulders are given a cross slope about 0.5% steeper than the camber so water drains off quickly; earthen shoulders should be well compacted or paved to avoid erosion.

Longitudinal side drains

• Located on both sides in cutting and at the toe of embankments (in plains, often trapezoidal earthen drains; lined drains where velocities are high or soils erodible).
• The bottom of side drains should be kept below the subgrade level so that water does not stand in contact with the pavement layers.
• A minimum longitudinal gradient is needed for self-cleansing (flat gradients cause silting); steep gradients need lining or drop structures to prevent erosion.
• Discharged into natural watercourses, culverts or outfall drains.

Other surface drains

• Median drains on divided highways with depressed medians or superelevated sections.
• Catch water drains (intercepting drains) — on hill slopes above the road to intercept water from the hill side.
• Chutes on high embankment slopes to carry concentrated flow down safely.
• Urban roads — kerb and gutter system; water flows along gutters to inlets (grated or kerb-opening) at intervals and into underground storm water drains.

Estimation of runoff

IRC:SP:42 (Guidelines on Road Drainage) and IRC:SP:50 (urban drainage) give procedures, including methods of estimating runoff and drain design for Indian conditions.

Hydraulic design of side drains

1. Estimate peak runoff $Q$ for the catchment draining into the drain.
2. Choose a section (trapezoidal, rectangular, V-shaped) and longitudinal slope $S$.
3. Find the flow depth using Manning's formula:

4. Check velocity — above the minimum (non-silting) value and below the maximum permissible (non-scouring) value for the lining/soil.
5. Add freeboard.

Components of a bridge

Classification of bridges

Structural forms — key features

• Slab bridges — solid or voided slabs for short spans.
• T-beam and box girder bridges — common RCC/PSC bridges for medium spans; box girders have high torsional stiffness (curved bridges, flyovers).
• Truss bridges — steel members in triangulated frames — railway bridges and medium to long spans.
• Arch bridges — carry loads mainly in compression; need strong abutments to resist thrust.
• Balanced cantilever bridges — built segmentally outward from piers.
• Cable-stayed bridges — deck supported by inclined cables from towers; efficient for medium to long spans.
• Suspension bridges — deck hung from main cables anchored at ends; longest spans.

Site selection and investigations

Ideal site: a straight reach with a narrow, well-defined channel and stable, high banks; firm foundation strata at reasonable depth; square crossing (flow perpendicular to the bridge); absence of whirls and cross currents; minimum training works; good approaches without costly embankments; proximity to the planned alignment; availability of materials and access.

Investigations: topographic survey (catchment and site plan, cross-sections upstream and downstream), hydrological data (rainfall, flood records, HFL, LWL, velocity, discharge), geotechnical investigations (boreholes, soil and rock properties, scour characteristics), navigational requirements, seismic zone, traffic and road/rail data.

Hydraulic design

Design discharge

Estimated by empirical formulas (Dickens, Ryves, Inglis), the rational method (small catchments), area–velocity method from flood marks (slope–area method with Manning's formula), unit hydrograph method, and flood frequency analysis. IRC:5 and IRC:SP:13 guide the design discharge and return periods; the maximum of several estimates is normally checked.

Linear waterway

• For alluvial streams, the regime waterway (Lacey's) $P=4.75\sqrt{Q}$ (m; $Q$ in m³/s) is taken as the effective linear waterway.
• For quasi-alluvial/rocky streams, the waterway is fixed by the natural stream width at HFL; contraction is limited to control afflux and scour.
• Effective linear waterway = total width between abutments minus the effective obstruction of piers.

Afflux

Afflux is the rise in water level upstream of a bridge due to contraction of the waterway.

Excessive afflux can flood upstream areas, cause high velocities through the bridge and deep scour.

Scour depth (IRC:78)

Foundations are taken sufficiently below the maximum scour level (with minimum grip lengths prescribed in IRC:78) and are protected by aprons, flooring or pitching where needed.

Clearance and freeboard

• Vertical clearance — between HFL (with afflux) and the lowest point of the superstructure, to pass floating debris; increases with discharge (IRC:5 values).
• Freeboard — the height of the formation level of approaches/guide bunds above HFL.
• Navigational clearance — for navigable waterways, as required by the waterway authority.

Economic span

For a bridge of a given total length with equal spans, the total cost is minimum when the cost of superstructure of one span ≈ cost of substructure (one pier with its foundation).

If the superstructure cost of a span varies as $k{L}^2$ and the cost of a pier with foundation is $P$ (independent of span), the economic span is:

Deep or costly foundations favour longer spans; shallow foundations and cheap piers favour shorter spans.