Chapter 6

URBAN IRIGATION DESIGN

What Is Stormwater?

Stormwater is water from rain and melted snow that flows across the ground into the storm sewers found at low points and along the sides of streets. The stormwater flows through a system of pipes and roadside ditches that make up the storm sewer system. The water eventually finds its way to lakes, rivers, streams, wetlands, and the ocean.

Standard design

Urban Storm water Management Manual For Malaysia (Manual Saliran Mesra Alam Malaysia , MSMA)

The Urban Storm water Management Manual for Malaysia (MSMA) is a guideline published by the Department of Irrigation and Drainage Malaysia (DID) in 2000 is to manage the storm water instead of draining it away as fast as possible to more environmentally approach by utilizes detention and retention infiltration and purification process.

These objective seem to be achieved when :

When towns grow, natural, vegetated areas are replaced with impervious surfaces like roads, parking lots and buildings. These hard surfaces do not allow ground infiltration of rain and snow melt, and increase stormwater runoff which leads to flash flooding, water pollution and erosion. Stormwater best management practices (BMPs) combine quality engineering, wise planning, acceptable maintenance practices, and public education to achieve cleaner water in our streams, lakes, oceans and drinking water reservoirs.

•The development and implementation of structural and non structural measures to reconcile the conveyance and storage function of storm water systems within the space and and related needs for expanding urban populations.

•Development and implementation of a range of measures or Best management practice (BMPs) to improve the quality of urban storm water runoff to its discharge to receiving water.

For example

Non – structural

Gross pollutant trap

Swale Wet Pond Dry Pond

Kelebihan MSMA

•Mengecilkan /mengurangkan aliran ke dalam sungai – kurangkan kerosakan kepada persekitaran sungai.•Mesra Alam- menggunakan alur berumput.•Lebih selamat- mesra pengguna kerana tiada penggunaan longkang besar yang terbuka.•Mengelakkan pengumpulan sampah sarap – dengan menggunakan perangkap sampah.•maghasilkan lebih kawasan hijau,•Kolan takungan kering boleh digabungkan dengan kemudahan lain sperti taman reakreasi.•Penapisan air larian permukaan.•menggalakkan penyusupan.

Design rainfall intensity

(Rainfall of course the driving force behind all storm water studies and design.)

Rainfall intensity

Involves ARI = (average reoccurrence interval) IDF = Rainfall intensity (depth/duration) , duration, Frequency IDF value formula: ( 30< t < 1000min)

ln(RIt) = a +b ln (t) + c (ln (t))2 + d (ln (t))3

RIt= the average rainfall intensity (mm/hr) for ARI and duration t

R= average return interval, years

t = duration , minutesa,b,and c are fitting constants depends to ARI

IDF value formula

(short duration 5 to 30 minutes )

Pd = P30- Fd (P60 – P30)

which I = Pd/d, d= duration

Runoff estimation

Rational Method

Is one of the most frequently used urban hydrology methods in Malaysia to computing storm water flows from rainfall. ( small catchments up to 80 hectares)The formula is

Qy = C . yIt . A 360

Qy = y year ARI peak flow C= dimensionless runoff coefficient

yIt = y year ARI average rainfall Intensity over time of concentration , Tc (mm/hr)

A= drainage area (ha)

where

Time of concentration , Tc =is often considered to be travel to inlet plus the time of travel in the storm water conveyance systems.

To = overland flow time (minutes)

Td= flow time in channel, curbed gutter or pipe

Overland flow time can occurred either on grass or paved surface

Tc = To + Tch

To = overland sheet flow travel time , minutes

L= overland sheet flow path length, m

n= Manning’s roughness value for the surface

S= slope of overland surface

Channel Flow Time, Tch

The time taken for storm water to travel along the open channel ( using Manning’s equation)

To = 107. n. L 1/3

S 1/2

n= mannings roughness coef

R= hydraulic radius, m

S =friction slope (1/m)

L= length of reach,m

Tch = travel time channel , minutes

Tch = n . L R2/3 S1/2

60

Example 1 a) Determine the time of concentration for post development

generated from a minor drainage of medium density residential area of 0.2577 ha.

Given :Area = 0.2577haOverland flow, Lch = 148mSlope of open drain, S = 1/300mManning’s coef,n = 0.015Min velocity, V = 0.6m/sCatchment area average slop, So = 0.5%

450mmDrain type = 225mm dia HRGW + 450mm wideDrain wide = 450mmDrain depth = 450mm

Solution Step 1 : Determine ARI for the development for medium density , from table 4.1 minor system ARI = 5 yearsStep 2 :The overland flow time, ToTo = 107. n. L 1/3

S 1/2

= 107 * 0.015 * 101/3 = 4.89 minutes 0.51/2

Area, A = 0.45 * 0.45 =0.2025m2Wettwed = 0.45 * 2(0.45) =1.35mHydraulic radious, R = 0.15The channel flow time, tch

Tch = n . L R2/3 S1/2

60= 0.015 * 148 * 0.15 2/3 * 1/300 1/2

60 = 2.27So, Tc = To + Tch= 4.89 + 2.27 = 7.16m

b) Calculate rainfall intensities for 10 years design ARI in Alor Setar .

Given :Area catchment = 0.26haConcentration time, Tc = 15 minutes

Solution :

Given concentration time , Tc = 15 minutes Determine the value of I(intensity) and P (precipitation) for ARI 10 years( for t = 30 & 60 minutes )ln(RIt) = a +b ln (t) + c (ln (t))2 + d (ln (t))3

ln(10I30) = a +b ln (t) + c (ln (t))2 + d (ln (t))3

from table 13.A1 for Alor Setar a= 5.6422, b= 0.1575, c=-0.1329 & d= 0.0056ln(10I30) = 5.6422 +0.1575 ln (30) - 0.1329 (ln (30))2 +

0.0056 (ln (30))3

= 5.6422 + 0.5357 – 0.1329(11.5681) + 0.0056 (39.3455)

= 4.860610I30 = 129.1mm/j

P30 = 64.6mm ( I = Pd/d )

ln(10I60) = 4.443510I60 = 85.1 mm/j

P60 =?

Pd = P30- Fd (P60 – P30)

= 64.6 – 0.72(85.1-64.6) REFER TABLE 13.3= 49.84mm

10I15 = 49.84/(15/60) ( I = Pd/d )

=199.36mm/j

Example 2

Determine peak dischage on peak housing area in Kuala lumpur , Please use a parameter below as assumption in calculation stated peak discahrge :

Area housing = 20 hectarArea density housing = highDrainage type = minor drainagelength of overland flow = 100mLong drain channel = 300mSlope average = 0.2%

Solution

Determine ARI for the development for high density , from table 4.1 minor system ARI = 10 years and major system design ARI = 100 years

Step 1: Determine Tc

Tc = To + TchTo = 11 minutes from chart 4.1 ( flow =100m, slope =0.2% & paved surface, n = 0.015)Tch = L/V ( L = 300m & assume V = 1.0m/s)so Tch = 300/1.0 = 300 second = 5 minutes Value of Tc= 11 minutes + 5 minutes = 16 minutesFor Tc = 16 minutes

Determine P(precipitation) and I(intensity) for ARI 10 years ( t= 30minutea & 60 minutes) from table 13.A1 for Kuala Lumpur a= 4.9696, b= 0.6796 , c = -0.2584 & d = 0.0147please continue ..the answer is 10P16 = 50.214mm10I16 = 188mm/hr

Step 2 : Peak discharge, Q = Qy = C . yIt . A 360

= refer to table 6F using category 3 –housing , C = 0.88Q10 = 0.88 * 188 * 20 360

= 9.191m3/S