I 4.
THE ROAD IN CROSS SECTION Glossary of Terms
4.1
Some main terms are defined below and shown in Figure 4.1. Camber is the normal corrfiguratlon of the cross section far a single carriageway where each half of the carriageway dopes away from the centre of the road (the crown) to allow the water to run to the outside edges of the wrnageway where it can discharge onto the verge or into a positive drainage system, See also crossfall. Carrfageway The width of the tight of w a y dedicated for use by vehicles while travelling.
Catchpit A chamber provided in a surtace water drainage system at an accessible point, for collecting grit (which is pen'odically removed) to prevent it from blocking inaccessible pafls of the drains. The invert of the chamber is set approximately 500mm below the incoming and outgoing pipes to aet as a settlement zone.
CBR (Callkrmia Bearing Ratio) An empirical test used to gauge the strength of aggregate base, subbase and subgrade. The soil is first compacted in a mould and then soaked for four days with a load on its surface. The expansion due t o moistening is then measured, Finally the resistance of the soil to a standard plunger of area 19.35crnz which has penetmted 2.5mm and 5.0rnrn is measured. The ratio uf this resistance to the corresponding resistance in crushed rock is then calculated. This ratio, expressed as a percentage, is the C8R. An acceptable figure for subbase is 30% or more, while a subgrade with a CBR of less than of 8% will require a subgrade layer (see pavement). Crassfall is the slope of the carriageway perpendicular to the direction afthe road which directs rain water to the edge of the road (preferably a minimum of 1 in 50). Where the road is curved the crossfall is varied to make the fall follow the sense of the curve (superalevatlon). This helps vehicles to stay on the road. Where the curve is very gradual, it is permissible to allow the road to fall against the sense of the curve (adverse crossfall or adverse camber).
SULTANATE OF OMAN HlGHWAY DESIGN MANUAL - VOLUME 1
FEBRUARY 1994
Flyover A bridge, usually at an interchange, carrying one road over another. Sldewalk The area of the Fight of way dedicated for use by pedestrians. In UK practice footway is used when it is directly adjacent to the camageway, footpath when it is separated from the carriageway. Confusingly b also generally referred to as "pavement" by many non-technical English speakers. The sidewalk need not necessarily be directly adjacent to the carriageway, and indeed a remate footpath is safer and mare attractive to the pedestrian, and more visually pleasing t o all road users. I
Grade In US terminology can be the formatfon, the gmdfent (also known as profile grade), or ground level (see stgrade).
At-grade Junction is one where traffic flaws cross at the same level at traffic lights, roundabouts, and uncontrolled cross roads. Grade separated is where the twa flows cross each other without interruption at different levels (e.g.flyovers, underpasses, footbridges, subways).
Gradlent or grade The rise or fall per unit horizontal length of a pipe, road, etc. (e.g. 1 in 20). It can also be expressed as the number of degrees from the horizontal (e.g. 2.8P) as more normally as a percentage (e.g. 5%).
Gully A small chamber in the channel line by the side of the road. It is covered with a grating through which surface water drains away. The gully outlet is at a higher level than the invert and water escapes via a trap sa that silt can collect in the chamber. The silt is periodically removed by mechanical or manual methods. Paved shoulder An emergency stopping area along the outer edge of the carrjageway ta allow vehicles to stand clear of the carriageway if broken down, and for other emergency use. Typically 2 . 5 1 wide. On the inner (median) edge of dual carriageways, paved shoulders typically 1.2m wide on the Inner (median) edge of the carriageway can aceommadate some slight driver errar and reduce the risk of damage ta the edge of the road. lnterchanga When applied to roads refers to junctions where at least one of the roads flows through without interruption by means of bridge(s) (is. grade separation). Vehicles change direction using connectar roads (see paragraph 10.10).
Crown The centreline, and hence generally the highest point, of a single carriageway.
Diverging lane The start of a connector road where it gradually leaves the main road.
Junetbn The point where two or more roads join together Were vehicles can change their direction of travel.
Dual Carriageway or Divided Highway. As for carriageway, but in two parallel carriageways separated by a medlan or central resews, each carriageway carrying vehicles in one direction.
Link In design this is used to describe the free flowing length of road between junctions which Intempt that flow.
Earthworks Before constructing the main strueturn1 element of a road (the pavement) it will be necessaiy to modify the existing ground level ta accommodate the grade of the road. This operation is known as earthworks. Where the pavement lies above existing ground Iwel the ground is raised into an embankment using suitable fill. For economy, suitable fill is taken from areas where the pavement is lower than existing ground level (i.e. cuttings). Unsuitable materlal may be found underlying the embankments or in the cuttings and has to be removed off-site. Alternatively, it may be possible to use it in landscaping areas. Unsuitable material includes perishable and organic materials, and topsoil and day of liquid limit exceeding 90 and/or plasticity index exceeding 65.
Merging Lane The end of a connector road where it gradually meets with the main road.
Formation or Grade The surface of the ground in its final shape after completion ofthe earthworks, Construction of the road pavement follows with the laying of any subbase material, or aggregate base where subbase can be omitted. Note that the subgrade layer (see pavement) is part of the subgrade earthworks.
Pavement The structure, prirnarjly stony in nature, placed on the soil in the areas where traffic may travel (i.e, carrjageway and paved shoulders). This normally consists of, in ascending order: n
Subgrade layer A layer for placing on poor quality ground to improve the subgrade (or formation) to sufficient quajity to accept subbase. Special material is not required where the soil is already of good quality, i.e. CSR greater than 8%. Consists of low quality rock or sand with a CBR of more than 25%. Soiis in Oman are generally of smcient quality not to require a special subgrade layer.
a
Subbase Usually the first major component of the pavement and consists of angular gravel or partly crushed gravel or crushed rock with a CBR of more than 30%.
1
I n
Basecourse Crushed rock or partly crushed gravel screened to give a closer grading than subbase, and hence forms a higher strength layer, minimum C8R 80%. Can be strengthened further by the use of a binding agent of either asphalttc cement (bRurninous basecourse) or portland cement (soil cement).
SULTANATE QF OMAN HlGHWAY DESIGN MANUAL - VOLUME 1
4.5
Asphalt Surfacing A blend of crushed rock, sand (natural or crushed), filler {portland cement ox limestone dust) and bitumen to give a strong, flexible surface on which traffic may travel. In Oman practice it consists of a top layer called wearing course with a maximum stone size of 2Omm. Where two surfacing layers are used, the Sawer layer is called bituminous basecourse.
rn
main route (24 hour flows greater than 5000 vehicles) should have a minimum clear running width of 6.5m.
1
lesser traffic mutes (24 hour Rows between 2000 and 5000 vehicles) should have a minimum clear running width of 6.0m.
Note that these figures are clear runnina widths. Where parking is anticipated, a width for parking must be added to give the width between curbs. In addition, care must be taken that emergency setvices can gain access to an area by an alternative route if a vehicle should break down on a narrow pait of the highway.
Right of way (ROW) The area of land dedicated for the public to use for travelling from place to place. It is also used for accommodating public utilities e.g, telephone, etectricity, water.
Site Land which is
The minimum clear width of carriageway to be provided for two way streets should be:
minor roads and streets (24 hour flows between 500 and 2000 vehicles) should have a minimum clear running width of 5.5m. 4.25m between curbs may be provided in areas of very law flow, or aver short lengths, for example in existing local streets being upgraded.
Ramp Common terminology for a steep section uf roadway, usually not part of a public highway. In US terminology is also a slip road (see paragraph 10.10)
Rip-rap Stones for revetment generally from 20 to 70kg weight. prdecting the bed or banks of a wadi from scour by water. Can also be used to prevent slopes of loose material from being dislodged. May be bound together with cement based mortar (mortared rip-rap) te give increased stability.
FEBRUARY 1994
4.6
Lane widening is required on small radius curves as shown in Table 4.3. It should be noted that this addMona1 width will not be sufficient at smaller radii when large vehicles (e.g. I t m rigids, articulated trucks) are required to stay in lane. Where it is essential that trucks do not encroach into adjoining lanes, then templates or computer simulations should be used to determine road widths where radii are less than 50m (References 4.1 ta 4.5). Figure 4.5 gives a suitable layout for commonly met situations, and Figure 4.4 gives turning templates for a range of typical vehicles.
4.7
The introduction of the extra width should be applied uniformly along the transition curve. In the improvement of existing curves the widening should generally be made on the Inside of the curves.
4.8
Mountainous areas offer particular problems as often the road has to turn back on itself and return across the same ground slope at a higher level. Only as a last resort and with the agreement of the Client can this be done by using hairpin bends (see Figure 4.2). Generally on steeply sloping
used or is to be used for construction.
Underpass The opposite of flyover: where one road is depressed beneath another. Viaduct A bridge carrying a road over an obstruction (wadi, road). Generally used for multi-span structures where the length is much greater than the height.
Further definitions can be found in Reference 4.7.
ground to avoid extendve sidelong fill the road formation is created by cutting into the hillside. Hairpin bends have to be designed with the smallest possible radius compatible with the road standards to minimize the extent of excavation and retaining works on the bend. The details shown on Figure 4.2 have successfully been used in the Sultanate and allow two lotonne trucks to pass on the bend. This may be overprovision for many roads within the Sultanate, and where hairpin bends are necessary the vehicle or vehicles to be accommodated must be agreed with the Client. Similar details to those shown on Figure 4.2 can be developed by using swept path templates or programs for the stipulated vehicle (References 4.2 to 4.5).
Cane Width and Cadageway Wldth 4.2
The width af a highway link between junctions must be sulficient for its intended position in the road hierarchy (see Chapter 2). It must achieve acceptable levels of safety and operating efficiency based on the volume of predicted traffic at the end of the design life period (generally 20 years). Reference 4.1 contains a method of defining levels of service which is summarized here in Table 4.1. Table 4.2 shows the lwel of service that should be adopted in the design year for various types of road.
4.3
Where demand is sufficient to justify b u r or more traffic lanes, dual carriageways shauld be provided. Typical design Rows that may be assumed for different types of carriageway are shown in Table 4.4.
4.9
4.4
Typical cross-sections for various categoties of road are shown in Figures 3.2 to 3-5,3.7 and 3.8. The standard lane width used in the Sultanate of Oman is 3.65m on primary and secondary roads and streets, 3.5m on local roads and streets, and 3.25rn on access streets. Lower widths may be used for streets where necessary to overcome particular problems due to restrictions and availability of land. Hcwever, on primary and secondary streets narrow widths should oniy be considered over very short distances ifspecial l ~ c aproblems l exist and significant cost savings can be made. Where the narrowest widths are adopted, design speeds shauld then be kept below GOkmIh, which must also be reinforced by applying a speed limit, except where lengths are extremely short such as at junctions.
As speeds approaching and leaving a hairpin bend will Be low, there is no need to apply transition curves. Care should be taken that the gradient around the inner channel of the bend does not become excessive, and to achieve this the gradient on the centreline will have to be limited to about 4%, the exact figure depending on the width of road provided. Edge lines should b e plotted to check that the inner and outer channels are not too steep and that changes in the gradient do not occur too abruptly.
4.10
One way mads may occasionally be necessary in otder existing urban areas or as service roads. In such cases the minimum widths stated in paragraph 4.5 should still be applied, although higher Rows will usually be permissible. The actual flew that can be achieved will be greatly influenced by the type of access from the road, parking restrictions, etc.
I
SULTANATE OF OMAN HIGHWAY DESIGN MANUAL VOLUME 1
-
Highway Capacity 4.11
4.12
The capacity of a road is the maximum hourly rate at which vehicles can pass a point or uniform section of road. The capacity achieved depends on many factors, including carriageway width, gradients, curves, the percentage of heavy goods veicles and the numbor of junctions. Typical daily capacities are given in Table 4.4.
average daily trafiic, year specified; also called AADT (average annual daily traffic) or AADF (average annual daily flow).
DHV
design hourfy volume (usually the 30th highest hourly volume of the design year)
K
is the ratio of DHV to ADT (Usually 8-18%)
D
directional distribution of traflfc during the design hour. The larger on+way volume expressed as a percentage of the total volume. Normally varies between 50% and 80%. 60% is a typical figure.
T
percentage of trucks, excluding light delivery trucks, within the PHV
4.13
In many cases. particulary in urban situations, consideration of the peak hourly flow will be the critical consideration. Reference 4.8 gives a detailed methodology while Tables 4.5 to 4.8 give typical data applicable to Oman.
4.14
Table 4.6 gives the absolute maximum capacity for highways constmeted to high standards, i,e. 3.65m lanes, adequate shoulders. lateral clearances greater than 1.8m. no substandard stopping or passing sight distances and no trucks. Practical design capacities are less, as shown in the Table. Typical further reductions are:
4.15
25mm high (seeVolume 2, SCD 2.1 to 2.7). Non-mountable (half battered) curbs are normally used where sidewalks are immediately adjacent, and mountable curbs (less than 150rnm high) are used where there are no sidewalks and for most medians and roundabout inner circles. 4.18
On rural roads paved shoulders without curbs should normally be provided. Paved shoulders provide support to the main running carriageway and also enable broken down vehicles to stand off (or partly off) the carriageway. Curbs will not normally be provided at the back of paved s h o u l d e ~ except where concentrations of water may occur and it is necessary to collect and channelise rainwater e.g. ends of bridges, low spats of carriageway, steep gradients on slip roads, embankments. In these areas extruded curbing (protection curb) may b e used, being quick to construct and less hazardous to vehicles at high speeds.
4.19
Lip cuhs (half battered curbs on their back) are used to delineate channelizing islands in rural areas where speeds are 80krnlh and over,
4.20
Curbs placed for traffic channelization at junctions should be painted alternately black and yellow,
4.2-1
Where possible, significant pedestrian movements should be kept apart from vehicular traffic. Often this is not practical in urban areas and a 2.0m wide paved sidewalk should be provided alongside the carriagewy (1.5rn on local and access streets). At local pinch points in difficultsituations thls may be reduced to -1 .Om for very short lengths. These are minimum values and should be increased where large pedestrian flows are expected (see also Chapter 12).
Abbreviations used In discussion of highway capacities include:
ADT (year)
m
2% reduction for 3.5m lanes.
m
1 truck is equivalent to 1.7 passenger vehicles in level terrain, 4 passenger vehicles in rolling terrain, and 8 passenger vehicles in mauntainous terrain.
In urban areas the capacity of a street is generally governed by the capacity of the junctions
Verges 4.22
Beyond the sidewalk (or curb/paved shoulder where no sidewalk is provided) an unpaved verge or shoulder at least 1.Om wide should be provided. f he verge should slope at 5% away from the paved area. This verge provides support to the edge of the sidewalk (or curblpaved shoulder). In urban areas it can be omitted alongside a sidewalk if the sidewalk abuts a building or other substantial boundary feature.
4.23
Where structures occur within 4.5m of the running edge of the carriageway, curbed marginal strips should be provided to reduce the danger of collisions. These should be hard surfaced under bridge decks. Safety barriers may also be required: Figure 13.1, 13.2 and 15,1t o 15.3.
4.24
Where 2-way service roads run parallel to a main road
(see
Chapter 5 ) . Where the traffic and junctions can be strictly controlled, typiml capacities of primay streets are shown in Table 4.7. 4.f 6
Table 4.8 shows the design capacities of 2-lane Sway highways for various running speeds, in different types of terrain and varying percentage of trucks. A 70-80 kmJh average running speed should be used tor most rural roads in level and rotling terrain. A 60-70 km/h average running speed would be applicable for roads approactling urban areas and wherever feasible for roads in mountanous areas. 50-60 km/h should be used for uninterrupted flow in urban areas and for rural roads in mountainous terrain where higher speeds are not feasible.
Edges of Cardageway 4.17
The edge of carriageways on urban roads will normally be curbed with precast concrete curbs. Curbs (together with their insitu concrete suppott) retain the structure of the carriageway and prevent vehicles driving onto and off the carriageway at unauthorized Iceations. Curb faces are normally I00 - 200rnm high, except at pedestrian and vehicular crossing points where the face is reduced to
FEBRUARY I994
Central Reserves (Medians) 4.25
Central reserves segregate opposing flows of traffic to offer higher levels of safety. They would normally be appropriate on multi-lane roads with high traffic movements where speed limits are greater than SQkmlh.
4.26
Central resews also provide useful areas for siting street furniture (traffic signs, lighting columns, gantry legs, etc). Widths must be s a c i e n t to give adequate clearance to such items as shown in Figure 13.2. Items in the central reserve may require protection by safety barriers, particularly larger items such as high mast lighting, gantry legs and celebration arches.
-
4.27
4.28
4.29
The preferred. width of central reserves is shorn in Table 4.9. Requirements for the provision of curbs and safety barriers are shown in Figure 13.2. Where the central reserve is more than l l m wide the likelihood d an errant vehicle crossing the central resenre and colliding with an oncoming vehicle is very small. In this case the safety barrier may be omitted where there are no hazards in the central resenre (lighting columns, etc). It should be replaced with a central fence to prevent vehicles from making unauthorised U-turns. If it is intended in the future to increase the width of the carriageway from 2 lanes in each direction to 3 lanes in each direction, then the additional width for constnrcticn should be allowed in the central resenre.
If pedestrian crossing facilities are provided, a minimum median width of 2.0m is required to permit a pedestrian pushing a pram or wheelchair to wait in safety (see paragraph 14.14).
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SULTANATE OF OMAN HIGHWAY DESIGN MANUAL. - VOLUME 1
FEBRUARY f 994
-
REFERENCES American Association of State Highway and Transpo&tion Officials 'A Policy on Geometric Design of Highways and Streets" AASHTO, 3990. Freight transpod Association
- "Designing for Deliveries'.
FTA (1983)
-
Savoy Computing "TRACK: Libraw of Vehicle Swept Paths". Savoy Computing Ltd., London (1983)
Savoy Computing - "AutoTRACK" (fnteractie program fur determining vehicle swept paths). Savoy Computing Ltd., London (1988)
-
Freight Transpart Association "Freight Fads 2183 Designing for Heavier Lorries". FFA (1983)
-
- , ,\
DOT - TD27/86 "Cross Sections and Headrooms*. DOT (1987)
Crossfalt John S Scott Tne Penguin Dictionary of Civil Engineeringn. Penguin. 4.30
4.31
Except on curves where superelwation is applied (Chapter 5),carriageways should have a crossfall of 2% from the crown on a single carriageway, or from the central reserve uf a dual carriageway, outwards towards the edge of the carriageway. Lack of crossfall gives poor drainage af the surface water and may cause aquaplaning. Excessive crossfall can be a hazard to road users and should be avoided. Sidewalks should also have a crossfall af 2%. This fall should normally be towards the carriageway where the sidewalk is contiguous with the carriageway. The fall should normally be away from the carriageway where the sidewalk is separated from the carriageway. A maximum crossfall of 4% is recommended, but this may be increased to SO% locally at crossovers or dropped curbs. Care should be taken that when the longitudinal fall is combined with namtal crossfall, it does not give falls in excess of 10% in any direction.
Other Aspects 4.32
Clearance requirements ta structures and street furniture are given in Figure 13.2. Light structures such as gantries and celebration arches, which may be severely damaged by being struck by an overheight vehicle, shauld have a minimum headroom of 5,8rn. Heavier structures, unlikely to be significantly damaged by overheight vehides, should have a minimum headroom of 5.5m. When calculating headrooms allowance should be made for the deflection and settlement of the structure. In addition, allowance should be made for future resurfacing of the road (minimum 50mm overlay). On steep grades or crossfalls, and tight sag curves, adequate additional allowances shauld be included in the hadmoms; R&erefle3 4.5 may be used in this regard.
4.33
Sus lay5ys shauld be provided at appropriate places. f he Oman National Transport Company must be consulted for their requirements at an early stage of the project. Typical bus laybys are shown in Figure 4.6.
Highway Aesearch Board (US) "Uighway Capacity Manual, special Report No.209WHRB Washington D c (1985).
SULTANATE OF OMAN HIGHWAY DESIGN MANUAL - VOLUME 1
11.
DRAINAGE AND UTlLITIES
toothiPs, the Ministry of Water Resources should be consulted to determine the adjustment fador to be applied to account for transmission losses. 1t .Q
11.I
The Sultanate of Oman lies on the eastern fringes of one ofthe driest desert areas in the world. The average annual rainfall for most parts of the country is less than 106mrn. However, because of the extreme variabilky of the rainfall pattern, the average annual rainfall can be exceeded in just a single day's rairrfall.
j1.2
This hydrological characteristic has a tendency to produce short duration high intensity starms giving rise to flash floods with consequent damage and disruption to the transport network. The designer should be aware af the potential ferocity of flood waters, and develop a drainage conveyance and protection system which is equally robust in all its elements.
6 1.3
11.4
11.5
11.6
The MWR study programme also encompasses the detailed study of the risk of flooding in urban areas. Althougth this infamation has predominantly been prepared to assist town planners, it is nevertheless of great benefit to the highway designer. Detailed maps showing the predicted extent offlooding in the major urban areas can be obtained from the Ministry of Water Resources, Surface Water Department. The fallowing subsections provide design criteria and procedures for catchment discharge, open channel flow analysis, Irish Crossings, Irish Bridges, culvert design and urban drainage design. The hydraulic design of any drainage structure should be fully documented.
The frequency of flooding to be accammodated at any particular site will depend on the class~cation of the road or street, the national strategic importance of the highway and the economic loss should the drainage facility need to be repfaced. Table I 1.t shows the minimum flood frequencies to be accommodated in particular situaltions without damage to the road or drainage structure, or disruption to traffic.
I 1 .I 0 For lrish Crossings and Bridges, disruption to traffic can be assumed to occur when the water depth across more than half of the carriageway exceeds 150rnrn. 11.11
The prediction of the frequency and magnitude of Rooding has been carried out by the Ministrj of Water Resources, Surtacs Water Department, as part of The Nationwide Flood Study Programme. Flood frequency curves have been developed which predict the flood discharge far a variety of stem return periods frcm a 1 in 5 year event to a 1 in 11Q year storm event. The return period of a storm may also be expressed as the probability of that magnitude of storm occuring in a particular year. In other words, a storm which is predicted to occur only once in every I00 years can be said to have a 1%chance of occuring in any one year. Likewise, a storm with a 1 in 5 year return period has a 20% chance of occuring in any one year. The dood frequency curves are based on rainfall data obtained from the extensive network of gauging stations throughout the country. Rainfall records from the Muscat gauging station have been kept for approximately 100 years. All other stations have been monitored for much shorter periods. Since the gauging stations are generally located in the foothills of mountains the estimation of flood flows elsewhere in the lower reaches d the catchment will need to be adjusted to take aceaunt of transmission losses. Furthermore, far catchment areas of less than 10 km2the flood frequency curves should be used with caution and compared against other traditional empirical methods of estimation such as the Rational Method.
FEBRUARY 1994
For natural catchments less than 10 km* the discharge should be detemined using the Rational Method and compared against that using the Flood Frequency Curves for Oman. The lesser of the two discharge values shall be used for design purposes. Rational Formula:
Where Q = peak discharge at the catchment outletfroad intersection [m31s) C = run-off coefficient. i = rainfall intensity for a specific frequency d storm (mmh), A = Catchment Area contributing to the flow (km2) ,
Typical values for C, the run-off coefficient, am shown in Table 11.2.
Where different surfaces occur within the same catchment area an equivalent 'C" value can be determined by the following equation:
Equivalent C =
fC1 x A l l + (C2 x A21 e.....lCn x An1 (A1 + A2 +....An)
Where C1 is the run-off coefficient for area A l , etc. Rainfall Intensity "i'is obtained from the regional intensity-Duratiorr Freguency relationship (Figure 1I .2,derived from Reference I t .a). The only input required is the return period of the storm (ie frequency) and the time of concentmtion (ie duration). The time of concentration is the time taken for water to flow from the furthest point of the catchment to the site under cansideration. When rainfall from all parts of the catchment is contributing to the run-off* the peak discharge will occur. The time of concentration may be detemined using the widely accepted Kirpich formula:
Catchment OtsctEarge 7
for natural catchment areas greater than 10 km2 the Rood Frequency Curves for Oman shall be used as produced by the Illinistry of Water Resources, S u k c e Water Department. The data is applicable to any geographic area in the Sultanate of Oman and is shown on Figure 11.1.
t 1.8
The only input data needed to obtain the peak discharge for floods of a speck frequency at any site is the drainage area h square kilometres (krn*) of the catchment contributing flowto the selected site. However, where the site under consideration is located in the flood plain, remote from the
Where T, = Time of Concentration (minutes) L = Catchment main stream length (m) S = Mainstream slope (mlm)
!+wing determined the time of concentration (duraticn) and chosen the appropriate starm d u r n period (frequency), the rainfall intensity "i"can be read from the Intensity-Duration-Frequew cuwes.
SULTANATE OF OMAN HIGHWAY DESIGN MANUAL - VOLUME I
Open Channel Flow Analysis 11.I 2
R = S=
An open channel may be a man-made or natural channel (ie wadi) in which water Rows with a free surface. Open channels are the primary method of coping with susface run-off. Open channel hydraufics is of particular importance to highway design because of the interrelationship of channels to all highway drainage structures.
Concrete lined channel Smooth rubble channel Rena mattresses or gabions Rough rip-rap Gravel, cobbles and not many large boulders Cobbles with large botdders
period).
11.15
11.I 6
11.d7
The depth of flow In the channel is an important guideline in choosing the type of drainage structure to be adopted and the optimum elevation of the highway. In the case of bridges, a minimum vertical clearance (freeboard) between top water level and underside of bridge deck must be allowed which will determine the lowest elevation of the bridge. Similarly, at culverts, the height of the opening should be compatible with the expected nominal flow depth and the selected ejevation af the road. However, the elevation of the road may be chosen to allow overtopping (some of the flow goes over the road) or to prevent overtopping (all the flow goes under the road) in the design fload depending on the importance af the route and the vulnerability of the structure to damage. See paragraphs 13.7 to 13.11. The velocity of the channel flow is used to determine the type and extent of protection works necessary to prevent scour from undermining the drainage facility or structure. The magnitude of the velocity can also be used to check the overall integrity af the drainage structure. For exarnpfe, velocities greater than 2.5mls could erode an asphalt road surface at an Irish Crossing. In this case a more durable concrete road surface would be necessary.
Open channel flow is usually cSassMed as: uniform or non-uniform; steady or unsteady; and subcritical, critical, or supercritical. Of these, non-uniform, unsteady, subcritical flaw is the most common type of flow In open channels. Due to the complexity and difficulty invoked in the analysis of non-uniform unsteady flow, most hydraulic computations are made with certain simplified assumptions which allow the application of steady, uniform, or gradually varied flow principles and methods of analysis.
Use of steady Row methods assumes that the discharge at a point does not change with time, and the use bf uniform flow methods assumes that there is no change in velocity, magnitude, ar direction with distance along a stream line. Steady uniform Row is thus characterised by constant velocity and Steady uniform Row is an idealised concept of open channel flow which seldom occurs in natural ebnneis. However, for most pmctical highway applications :he flow is considered steady. and changes in width, depth, or direction are svfficientiy small that flow can be considered uniform. The changes in channel characteristics occur over a long distance such that flow is gradually varied. For these reasons, use of uniform flow theory is usually within acceptable degrees of accuracy.
11. I 9
The Manning equation is used far open channel analysis where uniform flaw exists or can be resonably assumed. This can be expressed as follows:
Where Q = n= A =
discharge (m3/s) Mannings roughness coefficient cross section of ROW area (rn?
0.016 0.020 0.025 0.040 0.030 0.50 0.040 0.70
-
There is no exact method for selecting "n* values in Manning's equation as this coefficient expresses the resistance of flow which consists of many vatiables.The factors affecting Manning's "nuinclude the following:
Surface roughness Vegetation Channel irregularity Channel alignment Scour and sedirnenMion Obstructions Size and shape of channel Flow depth and discharge
11.20 For Row in natural channels the cross section at the point of interest should be determined, together with the general slope of the channel bed. From site observations, where deep channels are concerned, an assessment shouId be made of the probable highest historical fload level. In flatter terrain, top af bank level should be cansidered initially as the limiting flaw height. 11-21 Using Manning's equation the channel discharge is caieuSated for various depths of flow up to the predicted highest water level. The depth versus discharge is then plotted giving the designer a visual display of the stage discharge characteristics. 11.22
The analysis of open channels having irregular sections is best acccmplished by dividing the cross section into subsections of reasonably uniform geometric shape. The discharge from each subsection is computed to determine the total discharge.
11.23
Having plotted the stage-discharge characteristics of the channel the depth af flow can be read off the graph for any discharge. The "catchment" discharge can now be determined as described in paragraphs 11.7 to I d .I l for a range of storm return periods which are then related to channel flow depth. The average velocity of ffow at each depth is determined by dividing the discharge by the area of Row.
flow rate at each section along the channel. 11.18
hydraulic radius ( A M P where WP is the wetted perimeter of flaw area (m)) slopeofchannelbed(m/m)
Typical values of 'n' are:
11.I3 The analysis of the flow h the channel will provide the depth of flow and the velacity of the Row for a range of discharge rates which can then b e related to the frequency of flooding (stom return
11.14
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11.24 Using the output from the channel discharge characteristjcs the sensitivity of a variety of storm conditions at the site can beassessed and the optimum drainage conveyance and protection system chosen fur the selected design storm.
SULTANATE OF OMAN HIGHWAY DESIGN MANUAL - VOLUME 1
i t .53 The quantity of water to be discharged from the highway is determined using the Rational Formula as follows:
The gully spacing G (m)is determined by dividing the allowable maximum gutter Row q by the gutter inflow Q,.
where Q = peak discharge (IJs) C = run-off coefficient i = rainfall intensity (rnrnlh) for a specificstorm frequency A = catchment area contributing to the flow (rnq
Typical values for C, the run-off coefficient, are:
Initially the main gully positions will be located at law points or prior to a road junction or rollavers. From these fixed gully positions the spacing of upstream intermediate gullies can be determined.
6 1.55
1.0for paved areas 0.3 for unpaved areas A rainfall intensity i of 50mmJh (1 in 2 year storm) or 75rnmh (1 in 5 year storm) can be assumed for urban drainage design where gullies are used (a 15 minute event).
q = gutter ffow k r the appropriate l o a d width (11s) K = constant depending on the width of flooding for flood width of 1.5m, K = 135 for flood width of 2.0m, K = 291 far flood width of 2.5m, K = 527 s = longitudinal slope of gutter (rnlm)
For crossfalls other than 2%
A typical gully detail is shown in Figure 11.12. The gully may be canstrueted a s a single inlet ar as a multiple series of inlets depending on the amount af gutter Row to be discharged. The overall net length of gully opening required can be determined from capacity charts for the gully taking into account the longitudinal slope. Reference 11.5 covers a variety of common gully types.
11.56
11.54 The spacing of gulnes will depend on the allowable encroachment of flood water from the curb into the running carriageway for the particular category of road. For urban expressways (primary streeb) the allowable flood width would be the width of the paved shoulder. Elsewhere in urban areas, where no paved shoulder exists, the allowable flooding width is 1.5 metres. The quantity of water flowing along the curb line (i.e. gutter flow) where the crossfall is P A , can be expressed by the followjng equation:
where
FEBRUARY 1994
Knowing the amount of water entering each gully the sizing ofthe pipe network can be determined based on the following parameters: m
desirable minimum flow velocity 1.Om/s or absolute minimum 0.8rnJs
a
minimum pipe diameter 200mm
m
pipe roughness value k = 0.6mrn
I
minimum pipe cnver IOOOmm
8
at changes in pipe diameter the pipes are €a be designed soffit to soffrt
a
distance between manholes should not exceed 50m
11.57 An example of a typical storm water sewer design calculation sheet is shown in Table 17.5, References 11.6 and 11.? may be used to design pipe layauts. Alternatively, there are a wide range of computer programs available. 11.58
In semi-urban areas, where curbs are used but an outfall may be readiiy made to a nearby ditch or wadi, it may be adequate to create offlets as shown in Volume 2, SCD 3.1 and 3.2. Medlan Drainage
where n = Manning's roughness coefficient (= 0.01) A = area of flow in the channel (m2) p = wetted perimeter (m)
The value of q is determined for the valious changes in the longitudinal gradient of the gutter by dividing the road into convenient sub-lengths. The total flow Q from the catchment is divided by the respective length of road to give the flow into the gutter per metre length (Qd.
11.59
Dished medians must be adequately drained by regular crass drains. These should be provided as follows: rn
low points (except Irish Crossings)
a
250m from high points and Irish Crossings
a
250m maximum centres elsewhere
A typical arrangement for a median catchpit is shown in Fgure 11.12.
