GUTTERS and INLETS - الصفحات...

GUTTERS and INLETSGutter : A gutter is a triangle open channel along the curb stone of streets which carry the storm water along streets to inlets.

Inlets : Inlets are structures constructed in the street along the curb stone or across the street to allow water to inter from the gutter to the underground storm collection system.

: Gutter capacityThe gutter capacity of carrying storm water is calculated using the modified Manning’s equation:

38

21

YSnzKQ =

Q = Gutter flowZ = Reciprocal of the cross slope of the guttern = Roughness coefficientS = Gutter slope (longitude )Y = water depth at the curbK = constant = 0.38 (for SI units)

W=ZYY

Gutter

Cur

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Example 1

What’s the maximum discharge which can be carried by a gutter in a street that has the following characteristics? : * Street longitude slope = 1%* n = 0.018* Cross slope = 4%* Curb height = 15cm* Street width = 10m, (3.5 should be clear)

Solution

The gutter flow will be limited either by the spread (3.5 clear width ) or the curb height.

38

21

38.0 YSnzQ =

38

21

)13.0(*)01.0(018.02538.0= = 0.23 m3/s

Z=1/0.04= 25

W= 10-3.5/2= 3.25 = ZY

Y= 3.25/25= 0.13 m

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STREET INLETS: are classified according to LOCATION into two types:• Inlets in sumps• Inlets on grade

Inlets are also classified according to their design into three types:• Curb inlets• Grating inlets• Combined (Grating + Curb)

A. CURB INLETS IN SUMPSCurb inlets in sumps are located at low points in the street where water which is

not removed by the inlet will accumulate rather than pass by.The capacity of curb inlets in sumps is given by the following formula:

LKyQ 23

= m3/s

K = constant, 1.66 (for SI units)y = water depth at the curb, mL = length of the curb opening, m

The value of Q is reduced by 10% to allow for clogging

Curb Stone

Curb inletElevation

LyQ 23

9.0*66.1= LyQ 23

494.1=



Grating inlets with their manholes



Curb Inlets

Grating inlets



Combined: Grating + Curb



B. GRATE INLETS IN SUMPSThe capacity of GRATE inlets in sumps is given by the following formula:

21

KAyQ =K = constant, 2.96 (for SI units)y = water depth at the curb, mA = Open area of grate, m2

The value of Q is reduced by 25% to allow for clogging

m3/s

grate

Plan

Curb Stone

SlopeSlope

Inlets in sumps should be design to drain the entire flow which will reach them since no flow will pass by to other inlets.

21

75.0*96.2 AyQ = 21

2.2 AyQ =



C. INLETS ON GRADEInlets on grade are either with depression or without depression• They maybe curb inlets, grating inlets or combined• They are usually designed to permits (5 to 25%) of the upstream flow to

pass. This results in a less expensive design than would result if the street were dewatered completely.

C.1 CURB INLET ON GRADE

The capacity of THE CURB inlets on grade is given by the following formula

}){( 25

25

aYaYK

LaQa −+=

Qa = gutter flow

Y= depth at curb above normal gutter grade

a= depression of gutter at inlet below normal gutter grade

K= constant depend on units SI (0.39)

ay

ElevationLa = length of the curb opening for 100% interception

To allow for clogging multiply by 0.75 :

}){(75.0*39.025

25

aYaYLa

Qa −+=}){(293.0

25

25

aYaYLa

Qa −+=



Ratio of intercepted flow to total flow for inlets on grade



C.2 GRATING INLET ON GRADEThe capacity of Grating inlets on grade is the same as that for grating in sump. If the grating on grade is in depression the following equation should be modified.

21

22.2 AyQ = Without depression

21

)(22.2 aYAQ += With depression

The factor 2.22 is the result of multiplying the (K ) value of 2.95 by 0.75 to allow for clogging.



Example 2 (for curb inlet on grade)

A gutter with Z=20, n= 0.015 and a slope of 1% carrying a flow of 0.25 m3/s, curb depression (a=60 mm). Find the required curb inlet length to intercept the entire flow and the length to intercept 75 % of the flow only.

Solution

The flow in the gutter is given by the formula:

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)01.0(015.02038.025.0 Y=

38

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YSnzKQ =

Y= 0.137 m

})06.0()137.006.0{(137.039.0

25

25

−+=LaQa = 0.0465 m3/s.m

The inlet length for complete interception La = 0.25/0.0465 = 5.38 m Assume that only 75% of the upstream flow will be intercepted, what the length of curb inlet will be needed.

Q/Qa = 0.75, a/y= 0.06/0.137=0.44 from the figure of Ratio of intercepted flow to total flow for inlets on grade L/La =0.56

L = 0.56 x 5.38 = 3.0 m



Spacing between street inlets

§ The maximum spacing between inlets (L max )depends on the capacity of the street gutter as illustrated in the equation:

38

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YSnzKQ =

So maximum capacity occurs when:

• (Y ) is just smaller than the height of curb stone so the water will not flood over the shoulders of the street

• The storm water does not spread more than the minimum clear distance of the street.

•Thus the maximum distance between inlets is that distance at which the street gutter reach it’s maximum capacity.

Ymax

Y

Inlet

Curb stone

= Q gutter



§ The spacing is also governed by the rain water (QS) falling per unit length (Ls) of the street under consideration. Qs has the units of liters per second per unit length ofthe street ( L/(S.m).

§ Thus Lmax can be calculated from the following equation:

L max = Qgutter / Qs

LsA1A2

C1C2

§ To calculate (QS) follow this procedure: 1. Take a specific length of the street (Ls).2. Identify the catchment areas on the right and the left of the street A1, A2.3. Assign run of coefficients C1and C2 for the two areas A1 and A2.4. Find the time of concentration (Tc)1 and (Tc)2 for each of the two areas

using the Kirpich formula mentioned previously.5. Find the rain fall intensity i1 and i2 for each of the two areas.6. Calculate Q = CiA for both areas ( for the left and the right sides).7. Calculate Qs = (CiA)/Ls8. Note that Qs on the left is not necessarily equal to that on the right. In

this case use the largest of the two to find Lmax.



Example 3

Calculate the value of Qs for a street knowing the following:

For the right hand catchment:A1 = 40000 m2

C1= 0.40I1 = 60 mm/hLs = 200 m

For the left hand catchment:A2 = 30000 m2

C2= 0.60I2 = 70 mm/hLs = 200 m

Solution:For the Right hand catchment:

QR = C1i1A1 = 0.40*(60/1000)*40000 = 960 m3/h

Qs = QR/Ls = 960/200 = 4.8 m3/h = 1.33 L/s.mFor the Right hand catchment:

QL = C2i2A2 = 0.60*(70/1000)*30000 = 1260 m3/h

Qs = QL/Ls = 1260/200 = 6.3 m3/h = 1.75 L/s.m

So take Qs = 1.75 L/s.m to calculate Lmax for this street



Example 4

It is estimated for the catchments area in the figure that the storm water run-off is Qs= 5L/s.m on each side. The street has the following characteristics:• Longitudinal slope S = 3%• Cross slope 3%= 1/z= 33.33• N= 0.015• Curb stone height = 25 cm• Width of the street= 10m• The minimum required clear distance =4 mFind the maximum spacing between inlets.

SolutionYmax = 3x3/100= 0.09 m

Qmax of gutter

Lmax = Qmax (gutter)/ Qs = 0.238/0.005 = 47.6 m

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)09.0()03.0(015.0

33.3338.0=Q = 0.238 m3/s

3%

3 4 3

Ymax



D max = (Qgutter – Q Baypass)/ Qs

Spacing between street inlets when some of the flow is bypassed

As discussed previously, sometimes we allow some of the water in the gutter to bypass which means that the water flow is not completely intercepted. In this case the spacing between inlet other than the first inlet will be shorter than the spacing of the first inlet. The modified equation in this case is:

Dmax = the distance between inlets other than the first

Example 5:

Solution:

For example 4, find the spacing of inlets other than the first (Dmax) if the inlets are designed to intercept 75% of the maximum gutter flow.

QBypass = 0.25 Qgutter = 0.25*0.238 = 0.06 m3/s

Dmax = (0.238 – 0.06)/( 5 *10 -3) = 35.6 m

So the first inlet will be placed at 47 m and the other inlets at a distance of 35.6 m.

Note that (Qgutter – Q Baypass) is the intercepted flow by the inlet (Q intercepted).



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