1.
GENERAL
This section covers the stormwater drainage works for the section of the main road located between stations 0+000 and 3+500. The objective of the proposed stormwater system is the collection and disposal of surface run-off water generated over various roads and open lands within the project area. This will include the main road and a short length from the branches.
2.
STORMWATER DR DRAINAGE WO WORKS
2.1
DESIGN CRITERA
1) The following following criter criteria ia were adopted adopted for the the concept design design of the project project.. These criteria were selected in accordance with the British Standards (BS) and codes of practice. 2.2
DATA CO COLLECTION
Information required for hydrological analysis and design was collected. The collected data include the following:
1. Topog Topogra raphi phicc Cont Contou ourr Maps Maps:: a 100 m cont contour our inte interv rval al topo topogr grap aphi hicc maps maps were were obtained and were used for the delineation of the watershed.
2) Rainfall data: Rainfall data was collected from rainfall gauging stations.
3) Survey: Additional survey should be carried out for the e xisting culvert status.
2.3
HYDROLOGIC ANALYSIS AND DESIGN
Hydrologic analysis is the most important step prior to the hydraulic design of a highway draina drainage ge struct structure ure.. It includ includes es the estima estimatio tion n of the catchme catchment nt physi physical cal parame parameter ters, s, calculations of the time of concentration, establishment of intensity - duration - frequency curves, and calculation of runoff.
2.3.1 2.3.1 EST ESTIMA IMATIO TION N OF CATC CATCHME HMENT NT PHYS PHYSICAL ICAL PARAME PARAMETER TERS S
Physical parameters of the drainage area are very significant for the hydrologic analysis. Boundaries of catchments are delineated from the 100m topographic maps which was not very significant. Main streams are also traced as well as the maximum and minimum elevations along them. The area of each catchment as well as the difference in elevation wit within hin the catc catchm hmen entt are are used used for for com comput puting runo runoff ff quan quanttitie itiess. The The runo runoff ff coefficient/curve number for every catchment is generally estimated from the ground cover, the topography and the shape of the area. These are discussed in the following sections. 2.3.2
RUNOFF QUANTITY
2.3.2.1
RUNOFF FORMULA
Several methods, each with its own assumptions and constraints, may be used to estimate watershed watershed runoff. Two methods are used used in the preliminary preliminary analysis analysis for estimating estimating runoff from the drainage areas crossed by the project. The application of each method depends on the availability and type of rainfall data, flow records, and the catchment size. Methods considered in this analysis are:
•
The Rational Method for areas < 50 ha.
•
The SCS Unit Hydrograph Method for areas
≥
50 ha.
a- The Rational Method This method is based on the assumption that a steady uniform rainfall rate in time and space will produce maximum runoff when all parts of the watershed are contributing to outf outflo low. w. This This cond condit itio ion n is met met when when the the stor storm m dura durati tion on exce exceed edss the the time time of concentration. It is used to calculate surface runoff discharges generated from a design storm with a specific return period and a duration time equal to the time of concentration
of the catchment areas. The method relates rainfall to runoff using the following formula:
Q
=
C I
A
360
where Q
=
Maximum rate of runoff, m3/s
A
=
Catchment area, hectares.
I
=
The rainfall intensity in millimeters per hour, for the period of maximum
rainfall of a given frequency of occurrence and for a duration corresponding to the time of concentration. C
=
Runoff coefficient
The The run-o run-off ff coef coeffi fici cien entt is the the rati ratio o of runo runoff ff to rate of rain rainfa fall ll..
Taki Taking ng in
consideration the type of the Project and its location. b - The SCS Unit Hydrograph Method The Unite United d Stat States es Soil Soil Cons Conser erva vati tion on Serv Servic icee (SCS (SCS - now now the the Natu Natura rall Reso Resour urce ce Conservation Service) method estimates runoff using in addition to rainfall, catchment chara charact cter eris isti tics cs such such as ante antece cede dent nt soil soil mois moistu ture re condi conditi tion ons, s, type typess of soil soil,, init initia iall abstraction of rainfall, slope, length of the longest channel, and surface treatment and land cover. These characteristics are reflected reflected by a Curve Number (CN) (CN) value.
This number typically ranges from 25 (for low runoff depressions) to 98 (for paved imperv imperviou iouss areas) areas).. An initia initiall abstra abstracti ction on factor factor (Ia) (Ia) can be specif specified ied.. The SCS-CN SCS-CN method typically uses an initial abstraction of 0.2S, where S is a maximum soil storage depth (in inches) and is calculated from the equation below (other values may be used).
S =
1000 CN
where:
−
10 CN = Curve Number S = Maximum storage depth
Soils are classified, into four hydrologic groups: A, B, C, and D based on their runoff potential. Soil A has a low runoff potential, it has a high infiltration infiltration rate and high rate of wate waterr trans transmi miss ssio ion. n. aggregated aggregated silt. silt.
This This grou group p cove covers rs soil soilss such such as deep deep sand sand,, deep deep loes loess, s, and and
Soil B has moderate moderate infiltrat infiltration ion and water transmiss transmission ion rates. rates. This
group includes includes shallo shallow w loess and and sandy loam. loam.
Soil C has has slow infil infiltrati tration on and water
transmission rates even if thoroughly thoroughly wetted. This group includes layered soils with high fine textures such as clay loam, shallow sandy loam, soils low in organic contents, and soils of high clay contents. Finally, soil D has a very high runoff potential due to low infiltration and water transmission transmission rates. This group includes most of clay soils and soils of high swelling swelling potentials. potentials. Table 1 below shows typical typical CN values for different different land land use/cover/soil complexes.
Table : Areas
Typi ypical Runo Runofff Coe Coefficients Values (C) (C) For For Rur Rural
Watershed Characteristics
A Relief
0.40 Steep terr terrai ain n
B Soil Infiltration 0.20
C Vegetal Cover 0.20
rugged No No effe effect ctiv ivee soil soil No No
effe effect ctiv ivee
D Surface Storage
0.20
plan plantt
Negligible
:Ave :Avera rage ge cover; cover; either either rock rock cove cover; r; bare bare or very very depression thin
mantle
sparse soil cover
shal shallo low; w;
:
surface
few
and
drai draina nage ge
ways ways
slopes greater than
or
30%
negligible
steep and small, no ponds
infiltration
or marshes 30%
0.30
capacity 0.15
Hilly with average
Slow to take ake up Poor Poor to fair air; clea clean n
slopes of 10 to
water;
30%
other other soil soil of low low poor poor natu natura rall cover cover;; ponds of marshes.
0.15
clay;
infiltration
or cult cultiv ivat ated ed crop cropss
0.15
or of small drainage ways, no
less than 10% of area
capa capaci city ty such such as under good cover
Low; well defined system
heavy gumbo 0.20
0.10
Rolling
with
average slopes of 5
0.10
Normal,
0.10
deepFair Fair to good good,, abou aboutt
loam
50% of area in good
to 10%
Nor Norm mal; al;
cons consiider derabl able
surface depression storage;
grass grass land land woodlan woodland d typi typical cal of prai prairi riee land lands, s, or equivalent cover
lakes ponds, and marshes
0.05
less than 20% of area 0.0
0.10
0.05
Relatively flat land
High, deep sand or Good Good
average slopes 0 to
other
soil
5%
takes
up
that
to
about 50% of area in
water good
readily and rapidly
exce excell llen ent; t; High, High, surfac surfacee depres depressio sion n
grass
land;
stor torage age
high high;;
drai draina nage ge
system
not
sharply
woodland
or define defined, d, large large flood flood plain plain
equivalent
storag storage; e; large large number number of
ponds and marshes Note: Runoff coeffici coefficient ent is equal to the sum of coefficient coefficientss from the appropriate appropriate block in rows rows A, B, C, and and D. Thes Thesee runo runoff ff coef coeffi fici cien ents ts shal shalll be propor proportio tioned ned to the
percentage of area covered. It shall be noted that all units units of runoff nalysi nalysiss in the SCS method are in inches (unless (unless otherwise stated), conversion to metric units is possible at the end of analysis.
The SCS-CN method calculates the volume of runoff given the input rainfall depth and the CN value. The relation is given by
Q=
( P - 0.2S ) 2 P + 0.8S
where:
Q
= the accu accumu mullated ated dept depth h of runo runoff ff (inche nches) s);;
P
= the accumulated depth of storm rainfall (inches); and
S
= the the val value ue of S is a func functi tion on of the the CN CN valu valuee as give given n ear earli lier er..
Accordi According ng to existi existing ng condit condition ions, s, hydrol hydrologi ogicc soil soil groups groups C and D were were chosen chosen to represent the hydrological condition of the soil in general. The Curve Number (CN) associated with normal (average) Antecedent Moisture Conditions (AMC II) for desert soils with with poor vegetation vegetation cover ranges ranges from 80 to 85 but for low lands it is 77. The project area is considered to be in a low land.
The shape of the SCS flood hydrograph is standard and depends on the watershed area and the lag time of the basin. The lag time is about 0.6 times the time of concentration. The peak flow for one unit of rainfall excess is given by
Q peak =
2.08A
T
R
where Q peak
= the peak discharge in (m3/s);
A
= the drainage area in (km2); and
TR
= the time of rise of the flood hydrograph which equals the lag time plus onehalf of the storm duration in (hours).
2.3.2.2 Rainfall Rainfall Intensity Intensity
The draina drainage ge design design is based based on the rainfa rainfallll-int intens ensity ity durati duration on relati relations onship hip (IDF) (IDF) extracted from Warry Gaging Station. 2.3.2.3 Recurrence Recurrence Interval Interval
The recurrence recurrence interval was select select according according to Qatar Drainage Drainage manual. Generally, Generally, the following intervals are adopted. Road Body Culverts Underpasses
10 years 50 years 50 years.
2.3.2.4 Time of of Concentration Concentration
The time of concentration is the longest time, without unreasonable delay, required for a drop of water to flow from the upper limit of a drainage area to the point of collection or concentratio concentration. n. This time depends depends on the size and the shape of the catchment area, its hydraulic characteristics, and upon the hydraulic characteristics of the drainage system.
For cross drainage works (culverts); the time of concentration may be estimated using Kirpich or the Kinematic wave equation. equation. For areas less than 50 ha, Kirpich is used; while the Kinematic wave is used for larger areas.
Kirpich’s equation is given by: 1
Tc
=
52
x
L1.155 H 0.385
where: Tc =
Time of concentration, min.
L =
Hori Horizo zont ntal ally ly pro proje ject cted ed len lengt gth h of dra drain inag agee basi basin n alon along g the the main main wat water er cou cours rse, e,
(m) H =
Diff Differ eren ence ce in elevat elevatio ion n betw betwee een n the farth farthes estt point point on the the draina drainage ge area area and the
point of collection, (m)
The Kinematic equation is defined as follows:
L0.6 × n 0.6 T `c = 0.93 × 0.4 0.3 i × S Tc =
Time of concentration, min.
L = Length of Overland flow (ft) n = Manning Overland Roughness i = Rainfall Intensity (in/hr) For road drainage works in urban areas, the time of concentration for each area served by stormdrains may be divided into two parts:
to t p
= =
Time of entry. Time of flow in the conduit.
The time of flow in the storm sewer may be evaluated from the design velocities and the length of the reach considered. The time of entry may be determined by the following formula developed by the Federal Aviation Administration (FAA). The minimum time of entry is taken 6 minutes. T c = 1.8 (1.1-C) L0.5 /S 1/3 Where, tc = Time of concentration, minutes C = Ration Rational al method method runoff runoff coeffic coefficien ientt L = Leng Length th of overl overlan and d flow flow,, ft S = Surf urface ace slop slope, e, %.
2.4
SIZING OF CH CHANNELS
The Manning formula is used for the design of the collection network – pipes or channels. The equation states: Q = (1/n) x A x (R2/3 ) x (S 1/2 ) Where, Q = The discharge in cu.m/sec. n = The The rou rough ghne ness ss coef coeffi fici cien entt of of the the chan channe nell or or pip pipe. e. A = The area of flow in sq.m. R = The The hydr hydrau auli licc radiu radiuss in m. and is is the rati ratio o of the flo flow w secti section on over over the the wett wetted ed perimeter. S = The The slo slope pe of chan channe nell or or pi pipel peline ine in in m/ m/m. Values of n adopted for concrete channels is 0.016 The following criteria has been set for the design work: A minimum free board of 50 cm for the drainage channels is adopted. The free board is based on the peak design flow and is a safety margin for carrying either higher frequency storms or for future increase in surface run-off. 4) 5) 2.7 2.7
STREET INL INLETS (IN (INT TERCEPTORS)
The street inlets shall be installed at sag points road intersections and whenever the spread of water across the road is expected to extend beyond the permissible limit. They shall be connected to the positive drainage collection system with a pipe gradient equal or greater than 1% or to the positive drainage collection system.
