Highway Drainage

Sub-surface water (high water table, seepage from higher ground, and capillary and vapour movement) weakens the subgrade and must be kept at a safe depth below the formation level. IRC and NRS 2070 recommend that the subgrade top be at least about 0.6 to 1.0 m above the highest water table (more in silty soils), or that a measure below be provided.

1. Lowering the water table

• Longitudinal (trench) sub-surface drains: a trench below the side drain, filled with graded filter material and a perforated pipe, lowers the water table beneath the pavement.
• Transverse drains: trenches at an angle to the road (herring-bone pattern) in wet cuttings or low-lying lengths.
• Deep side ditches below the formation level in flat country.
• Well points / sand drains: used in soft clay or temporary works.

   Pavement
 ___________________
 ___|_______________|___  Formation level
   \ gravel filter  /
    \  [o o o o]  /     o = perforated
     \_geotextile_/        pipe
      water table lowered ~~~~~~

2. Controlling seepage flow

• Interceptor (cut-off) drains on the uphill side, a trench filled with filter material down to an impervious layer, intercept lateral seepage in hill cuttings.
• Horizontal (bore-hole) drains at the toe of slopes release water from within the slope.
• Drainage blanket (a free-draining sand-gravel layer, or the granular sub-base extended to the slope) drains water under the pavement; a geotextile separates it from the subgrade.
• Counterfort / chimney drains behind retaining walls, with weep holes.

    uphill ground
       \   seepage ->
        \      __ road
   ======\____|__|______
     [filter trench + pipe]
     ======= impervious layer

3. Controlling capillary and vapour rise

• Capillary cut-off: a layer of coarse sand or gravel, 150 to 300 mm thick, between subgrade and sub-base breaks capillary rise (the pores are too large to lift water).
• Impermeable membrane: a bituminous layer, plastic sheet or geomembrane stops both capillary and vapour movement.
• Raising the embankment above the capillary fringe (the height of capillary rise is about $h_c\approx C/(e D_{10})$, large for silts and clays).
• Lime or cement stabilisation of the top subgrade reduces permeability.

 Surfacing
 Base / Sub-base
 ###### coarse sand cut-off ######   <- breaks capillary rise
 Subgrade  (fine soil)
 ~~~~~ water table ~~~~~

The measure depends on the cause: seepage needs an interceptor drain, a high water table needs trench drains or a higher embankment, and capillary rise needs a cut-off layer or membrane.

The moisture content of subgrade soil changes with time, and a wet subgrade has a lower CBR and bearing strength. The main causes of moisture variation are:

1. Rain-water infiltration through the pavement surface, cracks, joints, permeable shoulders and unlined side drains.
2. Seepage of ground water laterally from higher ground, springs and leaking canals or ponds, especially in cuttings and hill roads.
3. Fluctuation of the ground-water table, seasonal rise during the monsoon, and floods.
4. Capillary rise of water from the water table into fine soils such as silt and clay, which can reach 1 m or more above the water table.
5. Vapour movement and condensation. Temperature differences between day and night, and between the pavement and soil, drive water vapour upward or downward. It condenses beneath the impermeable pavement and builds up moisture.
6. Ponding and flooding of the road side because of poor surface drainage, blocked culverts or low embankments.
7. Evaporation and drying at the shoulders, which makes the edge zone drier than the centre. This differential moisture causes cracking and edge failure.
8. Moisture trapped during construction (compaction on the wet side of optimum) and the effect of tree roots and vegetation.

The moisture content under the centre of a covered pavement slowly reaches an equilibrium value, while the edges vary seasonally. The design therefore uses the soaked CBR (worst moisture state) of the subgrade (IRC 37, NRS 2070).

A good highway drainage system should satisfy these requirements:

1. Quick removal of surface water. Rainwater should drain off the carriageway immediately through camber and cross slope, with no ponding on the surface.
2. Safe disposal. Water collected from the road and nearby catchment must be taken through side drains and cross-drainage works to a natural outfall (stream or gully) without damaging the road, adjacent property or farmland.
3. Proper flow velocity. Velocity in drains should be high enough to prevent silting (non-silting velocity, about 0.6 to 0.75 m/s) and low enough to prevent scouring (non-scouring limit depends on the soil and lining).
4. Interception of outside water. Catch drains should stop water from the upslope area from reaching the road.
5. Control of sub-surface water. The water table should be kept well below the subgrade (about 0.6 to 1.0 m) and seepage and capillary rise should be controlled.
6. Protection of embankments and slopes from erosion and from saturation, using lining, turfing and energy dissipating structures.
7. Adequate capacity for the design flood (return period of 5 to 25 years for the structures, as per NRS 2070/IRC) and a free board.
8. Economy and ease of maintenance. Drains should be simple, easy to clean and made of locally available material, without causing hazard to traffic (safe slopes, no deep open drains near the carriageway).

Highway drainage is the process of removing and diverting surface water and sub-surface water from the road and its surroundings, by a system of structures and channels, so that the pavement and the subgrade stay stable, dry and safe for traffic.

The drainage system collects rainwater falling on the carriageway, shoulders, slopes and the catchment above the road, and transports it to a safe outlet. It has three parts:

• Surface drainage (camber, side drains, catch drains)
• Cross drainage (culverts, causeways, minor bridges)
• Sub-surface drainage (control of water table, seepage and capillary water)

Water is the greatest enemy of a road. Proper drainage is necessary for the following reasons:

1. Pavement strength. Water entering the pavement and subgrade reduces the soil strength (CBR falls sharply when soaked), causing rutting, potholes and failure. Drainage keeps the subgrade in the design moisture state.
2. Pavement life. Water in bituminous layers causes stripping of binder, and in cement concrete causes pumping and erosion of the sub-base. Good drainage lengthens the life and lowers the maintenance cost.
3. Safety. Water on the carriageway leads to skidding and hydroplaning, and reduces visibility.
4. Stability of embankments and slopes. Saturated slopes slide, and runoff causes gully erosion, scour and landslides. This is especially serious on hill roads in Nepal during the monsoon.
5. Prevents flooding and washouts of the road and avoids damage to adjoining land and property.
6. Frost heave and swelling damage are reduced, since less water is available in the soil.
7. Continuous service. The road remains open in the rainy season, which is vital for the economy of the area.

Studies show that a poorly drained pavement can fail in a fraction of its design life, so drainage is a part of road design from the beginning.

Surface drainage design of a longitudinal drain (side drain) proceeds as follows (IRC SP:42 / NRS 2070).

1. Collect data: road alignment and longitudinal gradient, cross-section, catchment area and its slope, soil and land use, rainfall records, outfall locations.
2. Fix the design return period (for example 5 to 10 years for side drains) and find the rainfall intensity $I$ for the time of concentration $t_c$.
3. Time of concentration: $t_c=t_{inlet}+t_{flow}$, where $t_{flow}=L/V$ (the length of drain divided by the flow velocity).
4. Calculate discharge by the rational method:

$$Q=\frac{C\,I\,A}{360}\ \ (\text{m}^3/\text{s})$$

where $C$ is the runoff coefficient, $I$ in mm/hour and $A$ in hectares. 5. Select the shape (trapezoidal, rectangular, V-shaped or U-shaped) and the material or lining, and the longitudinal gradient (normally the road gradient, but not less than about 0.5%). 6. Design the section by Manning's equation:

$$Q=A\,V,\qquad V=\frac{1}{n}R^{2/3}S^{1/2}$$

Assume a section, find $A$, wetted perimeter $P$ and $R=A/P$, and iterate until the capacity equals or exceeds $Q$. Add a free board of about 0.15 m. 7. Check the velocity: it must be greater than the non-silting velocity (about 0.6 to 0.75 m/s) and less than the permissible non-scouring velocity for the material (for example about 0.6 to 0.9 m/s for sandy soil, 1.2 to 1.5 m/s for hard clay, 3 m/s or more for stone masonry or concrete lining). 8. Provide lining or checks if the velocity is too high, and increase the gradient or the flush if it is too low. 9. Fix the spacing of cross drains and outfalls (turnouts or mitre drains) so that the drain length does not exceed the capacity. 10. Prepare drawings and quantities.

Cross-drainage structure

A cross-drainage structure is a structure built across or under the road to carry water from one side of the road to the other. It lets streams, gullies and the drainage collected from the road pass the alignment without damaging the road.

Types and application

The main types, in increasing order of discharge and span, are listed below (IRC/NRS 2070).

1. Pipe culvert

Concrete (Hume) or steel pipes laid under the embankment in one or more barrels. Minimum diameter 600 mm (900 mm preferred for ease of cleaning), minimum cover about 0.6 to 1.0 m. It is cheap and quick to build, but its capacity is limited and it may choke with debris.

 ====== road level ======
 ====== fill (cover) ====
    ___________
   (  pipe     )  -> flow
    -----------
 inlet headwall   outlet apron

2. Box culvert

RCC rectangular section, single or multiple cells. The slab acts together with the walls and the base, so it spreads load over the soil. It suits soft ground and large flows and needs little cover.

 ====== road ======
 |  _____________  |
 | |      |      | |  <- 2 cells
 | |______|______| |
 |_________________|  base slab

3. Slab culvert

RCC slab deck on masonry or concrete abutments with wing walls. The waterway is open, so it is easy to clean, and it is used where the bed is firm and the stream has a clear span up to 6 m.

   ____deck slab____
  |                 |
  |abutment  abutment|
  |_water way_______|

4. Arch culvert

Masonry or concrete arch, which acts mainly in compression and so uses stone effectively, but needs strong abutments and a good foundation.

5. Causeway (drift) and Irish bridge

The road is carried across the stream at bed level, with or without small pipes (vented). The road is submerged during floods. It is used for broad streams that rise only for short periods and where a bridge is too costly. Lines of protection (aprons and cut-off walls) are needed.

 road ----\__________/---- road
    (dip)   vents  ==  low-water flow
     flood level ~~~~~~~~~~~~~

6. Minor bridge

Used for spans above 6 m (generally up to 30 to 60 m), when a culvert cannot pass the discharge. Designed to the HFL with adequate free board (soffit clearance) as per IRC/NRS bridge standards.

7. Aqueduct and siphon

Aqueduct carries a canal across a drainage channel or road; an inverted siphon carries drainage water under a canal or a road in a pipe running full.

Selection depends on discharge, stream slope, nature of the bed and foundation, height of embankment, availability of materials, and cost.

When water leaves a drain, culvert or steep channel, it has high velocity and energy. This energy scours the bed, undermines structures and erodes slopes. Erosion control and energy dissipating structures reduce the velocity, spread the flow, and protect the soil.

A. Erosion control measures (reduce velocity and protect the surface)

1. Lining of drains: stone pitching, brick/concrete lining, or grouted masonry in steep or erodible drains.
2. Check dams: small stone, gabion or concrete walls across a steep drain. They reduce the effective gradient, trap silt and slow the flow.
3. Turfing / bio-engineering: grass, vetiver, shrubs, brush layering and fascines on slopes bind the soil. Jute nets and geotextiles protect the slope while plants grow.
4. Rip-rap and gabion aprons: loose stones or wire-crate mattresses laid at inlets, outlets and stream banks.
5. Mitre / turnout drains and cascades which lead water away from the road at short intervals and divide the flow.

B. Energy dissipating structures

1. Drop structure / stepped drain (cascade): the drain is built as a series of steps. The water falls over each step and loses energy in the pool below.
2. Stilling basin (plunge pool): a lined depression at the foot of a drop or culvert outlet where the jump forms; baffle blocks and an end sill help the hydraulic jump.
3. Splash apron and rip-rap apron downstream of culvert outlets. Loose stones spread the flow and resist scour.
4. Baffle blocks or dentated sills at the end of an apron break up the jet.
5. Rapids (rough channel) on a steep slope: a channel with rough bed (boulders, cross ribs) which keeps the velocity within safe limits.
6. Flip bucket / trajectory bucket for high drops.

  Drain ----\
             \  drop
              |_____  stilling pool
                    \___baffle__/ -> safe outflow
                     rip-rap apron

Functions

• Reduce the velocity to a non-scouring limit.
• Prevent undermining of culvert outlets, bridge abutments and embankment toes.
• Prevent gully formation and landslides on hill roads.
• Safely release the water into a natural stream.

The type is chosen by discharge, drop height, soil, and available material (stone is often used in Nepal for gabions and masonry check dams).

Surface drainage removes water that falls on or flows over the ground surface and the carriageway. Sub-surface drainage controls water in the soil beneath the surface: the water table, seepage and capillary water.

Both are needed together: surface drainage stops water entering the road structure, and sub-surface drainage removes the water that has already entered or comes from below.

Surface water must be collected from the pavement and the area around it and taken to a safe outlet.

Rural roads

1. Camber and shoulders: the cross slope (about 2.5% for bituminous, 3 to 4% for gravel/earth) sends water from the carriageway to the shoulders, and the shoulder slope (3 to 4%) sends it to the side drain.
2. Side (longitudinal) drains: open earth, stone-pitched or lined drains along both sides in cutting, or on the hill side. In embankment, water flows down the side slopes or into drains at the toe.
3. Catch (intercepting) drains: placed above the cut slope to stop hill water entering the road.
4. Mitre / turnout drains lead water away from the side drain into the natural ground at intervals (every 100 to 250 m or so, depending on gradient).
5. Cross-drainage works: culverts, causeways and minor bridges carry the collected water across the road at natural streams and at fixed spacing.
6. Disposal: at stable natural outlets, with energy dissipation if the fall is steep.

 catch drain   ____ cut slope
 ----\_/-----\      \ side drain
              \______\___ road ___/ shoulder
                 --> culvert --> natural stream

Urban roads

1. Kerb and channel (gutter): the road is bounded by kerbs; water flows along the gutter at the edge of the road.
2. Gully inlets / catch basins: grated inlets set in the gutter at regular spacing take water into underground pipes. A silt trap catches sand and debris.
3. Underground storm sewers / box drains carry the water, with manholes every 30 to 90 m for inspection.
4. Outfall into a river, a larger storm drain or a soak pit. Combined with the footpath drains, and the drains are often covered by slabs.

 footpath |kerb| road ---> gutter -> [gully inlet]
                                        |
                              storm sewer (pipe) ===> outfall

In both cases the design uses the rational method for runoff and Manning's equation for the capacity of the drains.

Classification

Cross-drainage structures carry streams and road-side drains across the road. As per NRS 2070 (Nepal Road Standard) and IRC practice, they are classified by span and discharge as:

1. Culverts (clear span up to about 6 m):
   • pipe culvert (single or multiple barrels),
   • box culvert (RCC, single or multi-cell),
   • slab culvert (RCC slab on masonry or concrete abutments),
   • arch culvert (masonry or concrete arch).
2. Causeways / drifts (submersible crossings): vented (with pipes) or unvented (fords, Irish bridges).
3. Minor bridges (span above about 6 m).
4. Major bridges and special structures: aqueducts and inverted siphons are used where canals meet the road.

Suitability

General rules: use pipe where flow is small, slab where a clear opening is needed, box where the soil is weak, and arch where stone is available and the foundation is rock or hard soil. Culvert location should follow the natural stream line, with minimum skew, and a gradient that prevents silting.

A culvert is a short cross-drainage structure with a clear span up to about 6 m (IRC/NRS 2070) that carries a small stream or the water from side drains below the road embankment. Larger structures are called bridges.

Components

• Waterway (barrel / opening), abutments and piers, deck slab or arch, wing walls and return walls, foundation and bed protection (apron), headwalls, parapet and wearing surface.

Types

1. Pipe culvert: Hume (RCC) or steel pipes, single or in rows. Cheap and easy; for small discharge.
2. Box culvert: RCC rectangular cells; for larger discharge and weak soil.
3. Slab culvert: RCC slab on abutments; open waterway.
4. Arch culvert: stone or concrete arch; for hill roads.

 road surface =====================
 ====deck slab=====================
 |abutment|    waterway   |abutment|
 |________|_______________|________|
          apron / bed protection

Location and design points

• Place along the natural stream line, at right angles to the road if possible.
• Provide a design gradient 0.5 to 2% to avoid silting and scour.
• Design discharge from rational/Dicken or Ryves empirical formulas, with the return period of 25 years for the culvert (and a check for higher flood).
• Provide minimum cover as required, inlet and outlet protection, and a free board.
• Spacing of culverts is more frequent on hill roads (every 100 to 250 m) to avoid long drain runs.

Culverts are generally cheaper and simpler than bridges, but they must be cleaned regularly to avoid blockage by debris.

Aqueduct

An aqueduct is a structure that carries a canal or open channel over a natural drainage, river, or road. The canal bed is above the High Flood Level of the drainage. The canal flows in a trough supported on piers or abutments, and the drainage water passes below, like a bridge for water. When a road crosses an irrigation canal, an aqueduct (canal over road) allows the road to pass freely beneath.

Types: (i) the canal trough of RCC/steel on piers (like a bridge); (ii) a siphon aqueduct where the HFL of the drain is above the canal bed so that the drain flows under pressure through the barrels beneath the canal.

   canal trough ===============   (canal over drain)
   ============================
   |pier|     drain flows     |pier|
   |____|~~~~~~~~~~~~~~~~~~~~~|____|

Inverted siphon

An inverted siphon is a closed conduit (pipe or barrel) that runs below the obstacle, so the water goes down, runs full under pressure, and rises again on the other side. It is used where a drain, stream or canal needs to cross a road, canal or valley and the head is not enough to pass above. In a road, a drain crossing beneath a canal/road at a low level that is below the hydraulic gradient forms an inverted siphon.

  inlet |\               /| outlet
        | \_____________/ |    <- pipe full, under
        |  (below road/canal)    pressure

• Velocity should be 2 to 3 m/s to prevent silting.
• Needs a silt trap and an air vent.
• Head loss must be checked.

An aqueduct is chosen when the canal level is higher than the HFL of the drain, and an inverted siphon when the canal or road is lower than the HFL (or the drain level is much higher than the canal level).

Highway drainage is classified as follows.

1. Surface drainage: removes the rainwater from the carriageway and the surroundings.
   • Longitudinal drainage: camber and shoulders, side drains (open or covered), catch drains, kerb and channel, gully inlets and storm sewers.
   • Cross-drainage: culverts (pipe, box, slab, arch), causeways, minor bridges, aqueducts and siphons, which carry water from one side of the road to the other.
2. Sub-surface drainage: controls the ground water, seepage and capillary water, using trench drains, interceptor drains, drainage blankets, cut-off layers and horizontal drains.
3. Special drainage (for slopes, hills and rivers): erosion control and energy dissipating structures, drainage of slopes and retaining walls (weep holes), and river training works.

Based on location, the drainage may also be urban (kerb, gully, storm sewer) or rural (open side drains, culverts).

The components of a highway drainage system are:

1. Camber and cross-slope of the carriageway and shoulders: first stage of removing the surface water.
2. Side drains (longitudinal drains): open or lined drains along the road edge; kerb and channel in urban areas.
3. Catch / intercepting drains: above cut slopes to stop hill-side runoff reaching the road.
4. Mitre (turnout) drains and outfalls: take water from side drains into natural ground.
5. Cross-drainage structures: culverts, causeways, minor bridges (and inverted siphons or aqueducts where needed).
6. Sub-surface drainage: trench drains with perforated pipe, drainage blankets, cut-off layers, horizontal drains and weep holes in retaining walls.
7. Erosion control and energy dissipating structures: check dams, drop structures, aprons, rip-rap, stilling basins, bio-engineering.
8. Urban components: gully inlets, catch basins, storm sewers and manholes.
9. Disposal: natural streams, soak pits or storage ponds.

Sub-surface drainage is the system of structures that intercepts, lowers and removes water below the ground surface, such as ground water, seepage flow, and capillary water, so that the road subgrade and pavement layers stay dry. It includes longitudinal and transverse trench drains with perforated pipes, interceptor drains, drainage blankets, horizontal drains and capillary cut-off layers.

Importance

1. Keeps the subgrade strong. A subgrade at high moisture content loses its CBR and bearing value, so the pavement deflects and fails.
2. Prevents pavement damage: pumping of fines, stripping of bitumen, frost heave and volume change of expansive soils.
3. Stabilises slopes in cuttings and hills by lowering pore-water pressure and so preventing slides.
4. Controls seepage and springs, which cause wet patches, weak formation and erosion.
5. Increases pavement life and reduces maintenance cost, since a drained pavement carries many times more traffic than a poorly drained one.
6. Allows lower thickness design because a higher soaked CBR value can be used.

Surface drainage is the system that removes rainwater from the road surface and its surroundings and leads it to a safe outlet before it can enter the pavement, ponding or erode the road. It is the most important part of highway drainage.

Elements

1. Camber (cross slope) on the carriageway (about 2.5% on bituminous roads, steeper on earth roads) drains water quickly to the edges.
2. Shoulders with a cross slope of about 3 to 4% carry the water from the pavement to the side drain, away from the pavement edge.
3. Side drains: longitudinal open drains (triangular, trapezoidal or rectangular, lined or unlined) along the road on one or both sides. In urban areas, these are replaced by kerb, gutter, gully inlets and storm sewers.
4. Catch drains above cut slopes intercept water from the hill side.
5. Cross-drainage structures (culverts, causeways and bridges) pass water under the road at streams and at regular intervals.
6. Outfall and disposal: mitre drains, natural streams, and energy dissipation where the fall is steep.

 catch drain
   \_/        hill
     \   cut slope
      \_ side drain
       \___ shoulder ___ carriageway ___ shoulder
                  camber (crown)
 slope of road -> side drain -> culvert -> stream

Design points

• Discharge by rational method: $Q=\dfrac{CIA}{360}$.
• Section by Manning's equation, with the velocity kept between the non-silting and non-scouring limits.
• Longitudinal gradient of side drains not less than about 0.5%.
• Lining where the velocity is high or the soil is erodible.

A side drain carries the runoff from the carriageway, shoulder and nearby catchment along the road. The design steps are:

1. Collect data: catchment area ($A$), slope, soil/ground cover, rainfall intensity for the chosen return period (5 to 10 years for side drains), and the longitudinal gradient of the road.
2. Find the time of concentration $t_c$ and read the rainfall intensity $I$ for that duration.
3. Compute the discharge with the rational formula:

$$Q=\frac{C\,I\,A}{360}\ \ (\text{m}^3/\text{s})$$

with $I$ in mm/h and $A$ in hectares, and $C$ the runoff coefficient. 4. Select the shape and material: trapezoidal (side slope 1:1 to 1.5:1 in earth), rectangular (masonry or concrete, in hills) or V-shaped (small flows), and fix the bed slope, equal to road gradient but not less than 0.5%. 5. Size the drain by Manning's equation:

$$V=\frac{1}{n}R^{2/3}S^{1/2},\qquad Q=A\,V$$

Assume dimensions, find $A$, $P$, $R=A/P$, and revise until $Q_{capacity}\ge Q_{design}$. 6. Check the velocity is more than the non-silting velocity (about 0.6 to 0.75 m/s) and less than the non-scouring limit of the soil/lining. 7. Add a free board (about 0.15 to 0.3 m) and provide lining or check dams where velocity is too high. 8. Fix the discharge points (mitre drains, cross drains) so that the length of drain does not exceed the capacity, and design the outfall protection.

Lining of drains means covering the bed and sides of an earth drain with a durable, smooth material such as stone pitching, brick or stone masonry, plain or reinforced concrete, or precast blocks, so that it can carry water without erosion or seepage.

Necessity of lining

Lining is required when:

• the flow velocity is more than the non-scouring velocity of the soil, for example, on steep gradients (more than about 3 to 4%) in erodible soil;
• the drain runs through erodible, sandy or loose soil, or dispersive soil;
• water seeping from the drain could weaken the subgrade or slope (drain close to the pavement or in an unstable hill slope);
• the drain is narrow because of lack of space (rectangular lined section carries more flow);
• the drain is in an urban or built-up area, or needs to be cleaned easily.

Lining also improves the flow capacity, because of the lower roughness, which reduces the section needed and the cost of maintenance.

A slab culvert is a cross-drainage structure in which a reinforced concrete slab (deck) rests on two abutments, with an open rectangular waterway below. It is used for spans of up to about 6 m when the foundation is firm and the stream bed is stable, and it needs little height between the bed and the road level. In Nepal it is widely used on hill and Terai roads.

Elements

1. Deck slab: RCC slab designed for IRC wheel loads (Class A / 70R, as adopted in DoR practice). Bearing on abutments through a bearing strip (a simple bitumen/felt layer).
2. Abutments: stone masonry or concrete walls that support the slab and retain the road fill.
3. Wing walls and return walls: guide the flow, retain the embankment, and protect from scour.
4. Foundation: strip or raft footing on firm soil or rock, taken below the scour depth.
5. Bed protection (floor/apron, curtain walls and cut-off walls): upstream and downstream to prevent undermining.
6. Parapet / kerbs and wearing coat: provide safety to traffic and a riding surface.
7. Drainage: weep holes in the abutments.

 parapet |-- wearing course --| parapet
         |=== RCC deck slab ===|
  wing   | abutment   abutment | wing
  wall   |_____waterway_______| wall
          apron + cut-off walls

Advantages: the open waterway is easy to inspect and clean, and the structure is quick to build. A disadvantage is that it needs abutments on good foundation, so it is not suited to soft soil (where a box culvert is better).

A minor bridge has a span of about 6 to 30 or 60 m (IRC/NRS 2070). Its hydraulic behaviour depends on the flood level relative to the bridge deck.

Un-submerged (free-surface) condition

The High Flood Level (HFL) stays below the underside (soffit) of the deck, with a free board (vertical clearance, normally 0.6 m or more for small discharges, increased for larger discharges and floating debris). The flow under the bridge is open-channel flow with a free surface. The road is not overtopped. This is the normal design condition.

Design: afflux (rise in water level upstream due to the contraction), velocity, and scour depth are computed (by Lacey's regime theory), and the foundation is taken below the scour level.

Submerged condition

The water level rises above the deck soffit or even over the road. The flow then becomes pressure (orifice) flow under the deck, or flow over the deck when overtopped. The structure is then subject to hydrostatic and drag forces, buoyancy and uplift, debris impact, and heavy scour. The road may be damaged and traffic is interrupted.

A submerged bridge (a "submersible bridge" or a vented causeway) is accepted only on low-traffic roads where floods are rare and of short duration, as it is much cheaper than a high-level bridge. It is designed with:

• strongly anchored decks (to resist uplift),
• smooth soffit and rounded edges,
• no parapet (or collapsible railings),
• heavy bed protection and downstream aprons.

 Un-submerged                Submerged
 ____deck____ soffit          ~~~~~~~~~~~ HFL
 ~~~~~ HFL ~~~~ clearance     ____deck____

1. Rapids

A rapid is a steep, long, rough-bedded channel (a chute) used to carry a drain down a steep slope over a long distance. Roughness elements (boulders, cross ribs) keep the velocity within the non-scouring limit. It ends in a stilling basin.

 drain \___rough bed___ \
                         \ chute (slope steep)
                          \____stilling pool__

2. Drop structures

A drop structure (stepped drain or cascade) lets the water fall vertically in one or more steps where the ground is too steep for a normal gradient. The jump forms in a lined pool at the foot, dissipating energy. It protects the drain from scour and erosion.

 ______
       |  step 1
       |____
            |  step 2
            |____ pool

3. Causeways

A causeway (drift or Irish bridge) is a road crossing built at or just above the stream bed, allowing floods to flow over the road. It may be unvented (a ford, with paving only) or vented (with pipes or small openings for low flows). It is cheap and used on low-traffic roads across wide, shallow streams whose floods are short and rare, but traffic is blocked during floods. It needs aprons and cut-off walls on both sides.

 road ---\_____ low-level slab _____/--- road
          =vents=  <-- low flow

4. Inverted siphon

An inverted siphon is a closed pipe or conduit that dips below a road, canal or valley and rises again, flowing full under pressure. It carries drainage water across depressions or under another structure where it cannot pass above. It needs a velocity of about 2 to 3 m/s to prevent silting and a silt trap/air vent.

 inlet \                  / outlet
        \________________/
          (pipe full, pressure flow)

i) Catch drain

A catch (intercepting) drain is a channel dug above the cut slope on the hill side. Its necessity:

• It intercepts runoff from the upslope catchment and carries it to a stream or a culvert before it reaches the road.
• It prevents erosion and gullying of the cut slope.
• It keeps the side drain from overflowing, and the slope and subgrade from becoming saturated, which reduces the chance of landslides.

ii) Causeway

A causeway is a low-level crossing built on the stream bed. Necessity:

• Where wide, shallow streams carry flow only during short monsoon floods, a bridge would be uneconomical.
• It is cheap, quick to build, and low in maintenance on low-traffic roads.
• It lets the flood flow over the road without damaging the embankment, which is protected by aprons and cut-off walls.

iii) Energy dissipating structure (EDS)

Needed where water flows with high velocity down steep slopes or leaves a culvert or drain on a steep outfall.

• It reduces the velocity and energy so that the flow does not scour the bed or undermine foundations.
• It prevents gully erosion and slides on the downstream slope.
• Examples: drop structure, stilling basin, rip-rap apron, check dams, baffle blocks.

iv) Sub-surface drainage

Needed where the water table is high, seepage emerges in cuttings or springs are present, or the subgrade soil is fine-grained.

• It lowers the water table and intercepts seepage, so the subgrade stays in the design moisture state (soaked CBR is not lost).
• It prevents pavement failure, pumping, frost heave and slope failures.
• It extends the life of the road and reduces maintenance cost.

Soil erosion is the detachment and transport of soil particles by running water, wind or gravity. Along roads it appears as rills, gullies, scour at culvert outlets and washing of embankment and cut slopes.

Causes

1. High velocity of runoff due to steep slopes and long drain lengths, exceeding the non-scouring velocity of the soil.
2. Concentrated flow from culvert outlets, drains and mitre drains discharging onto loose soil.
3. Removal of vegetation and exposure of bare soil during construction, with steep cut and fill slopes.
4. Heavy rainfall (monsoon in Nepal) with raindrop impact and surface wash.
5. Erodible soil such as silt, fine sand and dispersive soil, with poor compaction.
6. Poor or missing drainage, saturated slopes and seepage.
7. Stream scour near bridges and bank-side roads.

Preventive measures

• Proper surface drainage: catch drains, side drains with controlled velocity, mitre drains at frequent intervals, and well-spaced cross drains.
• Lining of drains (stone pitching, masonry or concrete) where velocity is high.
• Slope protection: turfing, vetiver and other bio-engineering (brush layering, fascines, live check dams), jute net and geotextile, and stone pitching or shotcrete on steep slopes. Flatter slopes and benches.
• Retaining and breast walls, gabions at toes.
• River training: spurs, guide banks and bank protection near bridges.
• Minimise the exposed area during construction and restore vegetation quickly.

Energy dissipation

• Check dams, drop structures (stepped drains) and rapids for steep drains.
• Stilling basins, splash aprons and rip-rap at culvert outlets.
• Baffle blocks and dentated sills which break up the jet.
• Release the water at stable natural outlets.

 catch drain
   ___\                 turfing / bio-engineering
      \__ slope ________\____ side drain
          stone pitching     check dam
            outlet -> apron + rip-rap