Agrohydrology Manual

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MANUAL FOR AGROHYDROLOGY

AND

ENGINEERING DESIGN

FOR SMALL WATER IMPOUNDING

PROJECT (SWIP)

Department of Agriculture

BUREAU OF SOILS AND

WATER MANAGEMENT

Diliman, Quezon City

March 1997



TABLE OF CONTENTS

DESCRIPTION PAGE NO.

1. ESTIMATION OF RUN-OFF and DERIVATION OF INFLOW HYDROGRAPH

1

1.1 Establishment of Project Data 1

1.2 Estimation of Basin Lag Time and Time Concentration 1

1.3 Computation for Rainfall Depth 21.4 Rainfall Increments Determination 2

1.5 Rearrangement of Rainfall Pattern 3

1.6 Derivation of Synthetic Unit Hydrograph 8

1.7 Convolution of Equation for Flood Hydrograph 9

2. FIELD WATER BALANCE COMPUTATION 10

2.1 Establishment of Cropping Pattern and Cropping Calendar 102.2 Computation of 80% Dependable Rainfall 10

2.3 Crop Coefficient and Crop Rooting Depth 112.4 Percolation Loss 11

2.5 Soil Water Holding Capacity 14

2.6 Actual Evapotranspiration 14

2.7 Change in Storage 142.8 Initial Storage 142.9 Estimation of Water Storage at End of Decade 14

2.10 Irrigation Efficiency 15

3. ESTIMATION OF 10-DAY RESERVOIR INFLOW 16

3.1 Estimation of 10-Day Inflow for Region I, II, & IV 16

3.2 Estimation of 10-Day Inflow for Other Regions 16

4. ANNEXES

A. Philippine Water Resources Region 24

B. Climate Map of the Philippines 25

C. Monthly Distribution of Potential Evapotranspiration

of Selected Places in the Philippines 27

D. Planting Calendar 28



LIST OF TABLES

TABLE NO. PAGE NO.

1 Regression Coefficients of the Rainfall Intensity-DurationFrequency Curve for Different Locations 4

2 Soil Groups for Estimation of Watershed Index W. 6

3 Antecedent Moisture Condition for Estimation

of Water Index W. 6

4 Values of Watershed Index W. 6

5 Adjustment of Watershed Index W for Antecedent Moisture Condition7

6 Recommended Retention Rate for Hydrologic Soil Groups 8

7 T/Tp versus q/qp for Dimensionless Hydrograph 9

8 Percolation for Different Soil Types 12

9 S W H C of Different Soil Textural Class 15

10 Regional Run-off Coefficient and % Monthly BaseflowDistribution 17



LIST OF FIGURES

FIGURE NO. TITLE PAGE NO.

1 Rearrangement of Rainfall Increments 5

2 Water Management Scheme and Crop Depending Variables

for Field Water Balance for Irrigated Wetland Rice. 12

3 Crop Depending Variables for Field Water balance

of Irrigated Corn. 12

4 Crop Depending Variables for Field Water balanceof Irrigated Mungo. 13

5 Crop Depending Variables for Field Water balance

of Irrigated Tomato. 13

6 Crop Depending Variables for Field Water balanceof Irrigated Peanut. 14



AGROHYDROLOGIC STUDIES AND ANALYSES

There are 3 general computations to be considered in the study. These are as follows:

1. Estimation of Run-off and Derivation of Inflow Hydrograph (25 yrs.)

2. Field Water Balance Computation

3. Reservoir Inflow Computation

1. ESTIMATION OF RUN-OFF AND DERIVATION OF INFLOW

HYDROGRAPH

This would require the following data and inputs to be taken from the project site. These are

topographic map soil and land capability mp or report, land use/vegetation map or report and

rainfall intensities. The following arranged procedures would be helpful in deriving the inflowhydrograph.

1.1 Establishment of the Project Data

a. Drainage Area, A, in sq. km.

b. Mainstream length from outlet to highest ridge, L.c. Mainstream from outlet to point nearest basin centroid, Lc.

d. Total fall (elevation difference) from highest ridge to outlet, H, in meter.

e. Watershed gradient,

f. Soil type of watershed (dominant) to determine the soil group the identified soil

type in the watershed belong to.

g. Watershed cover/land use.

1.2. Estimation of Basin Lag Time, TL and time of Concentration TC using Method, andSnyder’s Method (revised), Time to peak, Tp and peak runoff, qp.a. Compute for unadjusted TL

(TL in hours)

Where: L = mainstream length from outlet to highest ridge, in miles

LC = mainstream length from outlet to the nearest basin centroid.

Y = watershed gradienta = 0.38

Ct = coefficient with values (Linsley’s):

1.2 for mountatins drainage area0.72 for foothill drainage area

0.35 for valley drainage area b. Adjust estimate of TL

Adjusted TL (for ∆ D = 0.4 ≠ )

Adjusted TL = TL + ¼ ( ∆ D - )

1

c. Compute time of concentration, TC, in hours.



TC = TL / 0.70

d. Compute the time to peak, Tp using

Tp = ½ ∆ D + TL (adjusted)

Where: ∆ D = time duration of one inch of excess rainfall (USDA SCS)

suggested values of ∆ D as 0.5 hr. (or 0.40 hr.) where Tc < 3; 1

hr. where 3<Tc<6:1/5 Tc where Tc>6.e. Compute the Peak rate of Runoff, qp, in cms/mm excess rainfall:

qp =

Where:

A = drainage area, sq. km.TL = time lag (adjusted), hr.

qp = cms/mm

1.3 Compute for rainfall Depth P for different duration D, utilizing equation:

P = iD wherei = rainfall intensity computed using Rainfall Intensity Duration, Frequency

Curve for different location in the Philippines (Table 1).Gen. Equation :

D = Duration

The tabulation of rainfall depth Pi versus Duration Di is thus:

Duration, Rainfall Intensity Rainfall DepthSeq. No. D, Hr I, min/hr. P, mm

1 D1 = ∆

D 1 P12 D2 = 2D1 2 P2

4 D3 = 3D1 3 P3

n Dn = 2Dn n Pn

1.4 Obtain rainfall increments ∆ Pi and rearranged them according to three maximization

patterns:

1. Peak ∆ P1 at middle time position, i = n/2

2. Peak ∆ P1 at 1/3 time position, i = n/3

3. Peak ∆ P1 at 2/3 time position, i= 2n/3 + 1

The sequences for peak at the different positions mentioned are shown in figure I.

Considering that the highest qp is usually computed or obtained from the 2/3 time position pattern,

the hydrograph to be derived will utilize this pattern without anymore working the other 2 patterns

for comparison, thus tabulation would only be as follows:2

Rainfall Increments Rearranged Rainfall Increments



APi, mm in 2/3 Position of peak pi

________________________________________________________________________Seq. No.

1. ∆P1 = P1 ∆P14

2. ∆P2 = P2 - P1 ∆P133. ∆P3 = P3 – P2 ∆P12

4. ∆P4 = P4 – P3 ∆P10

5. ∆P5 = P5 – P4 ∆P96. ∆P6 = P6 – P5 ∆P7

7. ∆P7 = P7 – P6 ∆P6

8. ∆P8 = P8 – P7 ∆P59. ∆P9 = P9 – P8 ∆P3

10. ∆P10 = P10 – P9 ∆P2

11. ∆P11 = P11 – P10 ∆P1

12. ∆P12 = P12 – P11 ∆P413. ∆P13 = P13 – P12 ∆P8

14. ∆P14 = P14 – P13 ∆P11

15. ∆P15 = P15 – P14 ∆P15

This rainfall-increment pattern is subjected to estimation of losses in the next step for the determination of

rainfall excess amounts.

1.5 For the rearranged rainfall pattern considered,

-Apply the Soil Conservation Service (SCS) Method to obtain Initial Abstraction, Ia:Ia = 0.2s

Where: Ia = initial abstraction, in inches

s = 1000 – 10

W

= maximum potential difference between rainfall and runoff, in inchesW = watershed index, also called the runoff curve number N or CN

= function of soil group, antecedent moisture condition (AMC), and land use

cover in the watershed

- Refer to Table 2 (Soil Group), Table 3 (Antecedent Moisture Conditions, Table 4 Value of W for

different land uses/covers, assuming AMC II) and Table 5 (Adjustments of W for AMC I and AMCIII).

- The computed initial abstraction Ia is subtracted from the rainfall over the necessary initial number

of time increment until Ia is satisfied.

- After subtracting Ia, a uniform retention rate f is applied in succeeding time increments so that

retention depth subtracted each time from a rainfall increments is at most equal to f AP, Applicablevalues are given in Table 6.

3



TABLE 1 Regression Coefficients if the Rainfall intensity, f (mm/hr) – Duration, t (hr)

Frequency, T Curve for Different Locations: General Equation: i = aTC

(t+b)d

Note:

If b - Ø the resulting rainfall intensity-duration-frequency curves are straight lines (plotted on log, log chart).

4

REGION STATION/LOCATION a b c d R

1 Vigan, Ilocos Sur 47.295 0.20 0.2710 0.577 0.9882

Baguio City 51.414 - 0.2337 0.343 0.9800

Laoag City 60.676 0.30 0.2370 0.554 0.9944

2 Tuguegarao, Cagayan 47.263 0.40 0.2290 0.598 0.9949

Aparri, Cagayan 53.503 0.20 0.2780 0.610 0.9916

3 San Agustin, Arayat,

Pampanga 48.749 0.40 0.2330 0.690 0.9973

Sta. Cruz, Pampanga 41.687 0.85 0.2220 0.611 0.9976

Dagupan, Pangasinan 53.665 0.10 0.1340 0.575 0.9959

Matalava, Lingayen 0.890 0.10 0.2220 0.611 0.9973

Iba, Zamabales 51.960 0.80 0.2020 0.448 0.9951

Cabanatuan City 62.961 0.20 0.1395 0.754 0.9950

Cansinala, Apalit,

Pampanga 36.597 - 0.2280 0.568 0.9962

Gabaldon, Nueva Ecija 43.209 0.10 0.2150 0.487 0.9942

4 Infanta, Quezon 67.327 0.30 0.2010 0.617 0.9867

Calapan, Mondoro Or. 54.846 0.30 0.2460 0.768 0.9969

MIA 46.863 0.10 0.1940 0.609 0.9979

Pot Area, Manila 58.798 0.20 0.1980 0.679 0.9981

Tayabas, Quezon 39.710 - 0.1320 0.461 0.9912

Casiguran, Quezon 77.587 0.70 0.2380 0.717 0.9849

Alabat, Quezon 55.424 0.20 0.2310 0.491 0.9880

Ambalong, Tanauan,

Batangas 41.351 - 0.2310 0.511 0.9620

Angono, Rizal 62.314 0.70 0.1910 0.630 0.9934

5 Daet, Camarines Norte 44.553 - 0.2240 0.570 0.9971

Legaspi, City 55.836 0.20 0.2480 0.591 0.9958

Virac, Catanduanes 49.052 0.20 0.2480 0.591 0.9958

6 Iloilo City 44.390 0.15 0.2040 0.670 0.9970

7 Cebu Airport 59.330 0.40 0.2400 0.812 0.9956

Dumaguete City 100.821 1.00 0.2370 1.057 0.9963

8 Borongan, Eastern

Samar 51.622 0.10 0.1680 0.581 0.9972

UEP, Catarman, Samar 61.889 0.40 0.2300 0.681 0.9905

Catbalogan, Samar 51.105 0.10 0.2020 0.620 0.9948

Tacloban, Leyte 39.661 0.10 0.1660 0.629 0.9968

9 Zamboanga City 48.571 0.30 0.2090 0.803 0.9973

10 Cagayan de Oro 78.621 0.50 0.1950 0.954 0.9992

Surigao City 61.486 0.60 0.2520 0.602 0.9901

Binatuan, Surigao

del Sur 57.433 0.10 0.1340 0.577 0.9932

11 Davao City 81.959 0.50 0.1740 0.945 0.9986



5



TABLE 2: Soil Groups for Estimation of Watershed Index W

Soil Group Description of Soil Characteristics

A Soils having very low runoff potential,

For Example, deep sands with little silt or clay.

B Light soils under/or well structured soils having above

Average infiltration when thoroughly melted.

For example, light sandy loams, silty loams.

C Medium soils and shallow soils having below-average

Infiltration when thoroughly melted. For example,

clay loams.

D Soils having high runoff potential. For example,

heavy soils, particularly days of high swelling

capacity, and very shallow soils underlain by

dense clay horizons.

TABLE 3: Antecedent Moisture Conditions for Estimation of Watershed Index W

Antecedent Moisture Condition Rain in pervious 5 days

(AMC) Dormant Season Growing Season

I less than 0.5 in. lass than 1.4 in.

II 0.5 – 1.1 in. 1.4 to 2.1 in.

III more than 1.1 in. more than 2.1 in.

TABLE 4: Values of Watershed Index W

(Assuming Antecedent Moisture Condition II)

Farming Hydrologic SOIL GROUP

Land Use or Cover Treatment Condition A B C D

Native pasture - Poor 70 80 85 90

or grassland - Fair 50 70 80 85

Good 40 60 75 80

Timbered Areas - Poor 45 65 75 85

Fair 35 60 75 80

Good 25 55 70 75

Improved Permanent

pasturesGood 30 60 70 80

Rotation pastures Straightrow Poor 65 75 85 90

Good 60 70 80 85

Contoured Poor 65 75 80 85Good 55 70 80 85

Crop Straightrow Poor 65 75 85 90

Good 70 80 85 90

Contoured Poor 70 80 85 90

Good 65 75 80 85

Fallow - - 80 85 90 95

6

(Table 4 Con’t)



The meaning of the terms listed under the heading “Hydrologic Condition” are as follow:

a. Native pastures: Pastures in poor condition is sparse, heavily grazed pastures with less than half the

total watershed area under plant cover. Pasture in fair condition is moderately grazed and with between half

and three-quarters of the catchment under plant cover. Pasture in good condition is lightly grazed and with

more than three-quarters of the catchment area under plant cover.

b. Timbered areas: Poor areas are sparsely timbered and heavily grazed with no undergrowth. Fair areas are

moderately grazed, with some undergrowth. Good areas are densely timbered and ungrazed, with

considerable undergrowth.

c. Improved permanent pastures: Densely sown permanent legume pastures subject to careful grazing

management are considered to be in good hydrologic condition.

d. Rotation pastures: Dense, moderately grazed pastures used as part of a well planned, crop-pasture-fallow

rotation are considered to be in good hydrologic condition. Sparse, overgrazed or “opportunity” pastures

are considered to be poor condition.

e. Crops: Good hydrologic condition refers to crops which form a part of a well planned and managed crop-

pasture-follow rotation. Poor hydrologic condition refers to crops managed according to a simple crop-

follow-rotation.

TABLE 5: Adjustment of Watershed index W for Antecedent moisture Condition

Corresponding Value of W for:

AMC = II AMC = I AMC = III

100 100 100

95 87 99

90 80 9885 70 9780 65 95

75 60 90

70 50 9065 45 85

60 40 8055 35 75

50 30 70

45 25 6540 20 60

35 20 5530 15 5025 10 45

7



TABLE 6: Recommended Retention Rate for Hydrologic Soil Group (USBR)

Hydrologic Soil Group Retention Rate, inches/hour

A 0.4

B 0.24

C 0.12

D 0.041.6 Derive the synthetic unit hydrograph, using T/Tp versus q/qp for dimensionless hydrograph

(Table 7)

-interpolate from the values of Table 7 the selected values of discharge ratios q/qp for values of time

ratio equal to

T/Tp = ∆D , 2∆D , 3∆D etc.TP TP TP

Until q/qp is less than 0.001

-Compute the ordinate of Synthetic Unit hydrograph as follows:

Ui = (q/qp) i qpWhere: Ui = ordinate of synthetic unit hydrograph in cms/mm (i= 1, 2, 3 . . . )

q/qp I = interpolated value of q/qp from smooth dimensionless hydrograph.

qp = Computed peak rate of runoff in cms/mm

-Obtain correction factor k for synthetic unit hydrograph

K = 3.6 Σ U1 ∆DA

-Correct to ordinate Ui ( i = 1, 2, 3 . . . )

Uu (Corrected Ui) = original Ui

K

-To check, K should be equal to one when using the same formula:

K = 3.6 Σ U1 ∆DA

-In tabulated form we will have:

Seq. No. Time Dimensionless Hydrograph Unit Hydrograph Cms/mm

i T, hr T/Tp q/qp Ui = (q/qp) i qp Uu = Ui/ki

1 ∆D ∆D/Tp Values (q/qp)1 qp Uu1

2 2∆D 2∆DTp interpolated (q/qp)2 qp Uu2 3 3∆D 3∆D/Tp From (q/qp)3 qp

4 4∆D 4∆D/Tp Table 7 (q/qp)4 qp

n n∆D n∆D/Tp (q/qp)n qp Uu n

Σ Ui Σ Uu

8

(Dimensionless and ideally close to 1:

D in hours; A in sq. km.)



TABLE 7: T/Tp versus q/qp for dimensionless hydrograph

Time Ratio Disch. Ratio Time Ratio Disch. Ratio

T/Tp q/qp T/Tp q/qp

0 0 1.5 0.66

0.1 0.015 1.6 0.56

0.2 0.175 1.8 0.42

0.3 0.16 2 0.32

0.4 0.28 2.2 0.24

0.5 0.43 2.4 0.18

0.6 0.6 2.6 0.13

0.7 0.77 2.8 0.098

0.8 0.89 3 0.075

0.9 0.97 3.5 0.036

1 1 4 0.0181.1 0.98 4.5 0.009

1.2 0.92 5 0.004

1.3 0.84 Infinity 0

1.4 0.75

1.7 To the rearrange pattern of excess rainfall, apply the synthetic unit hydrograph Qi ( i = 1, 2, 3 . . . )

according to the convolution equations:

Q1 = U

1E

1Q2 = U1 E2 + U2 E1

Q3 = U1 E3 + U2 E2 + U3 E1

Q4 = U1 E4 + U2 E3 + U3 E2 + U4 E1

etc.

\

9





e. Compute the standard normal deviation corresponding to an axceedance probability, p of 80 %, tp,

for p = 80% tp = -0.831

f. Compute the frequency factor for all decades K 1, 2, 3. . . . 36

Where: B = Ln ( 1 + Z2 )

K K = frequency factor in decade K

g. Compute the 10 – day rainfall at 80% dependability for all decades

_

R K = XK + SK K K

Where: R K = 10 – day rainfall at 80% dependability

h. Tabulate the results as follows:

Month Decade XK SK ZK K K R K

Jan. 1 - - - - -

2 - - - - -

3 - - - - -

. . . . . .

. . . . . .

. . . . . .

Dec. 34 - - - - -

35 - - - - -

36 - - - - -

Mean 80% dep or 10 – day rainfall

at project site =

Fill- up the crop-coefficients (kc) and crop-rooting depth columns according to the establishmentof cropping calendar and crop growing stages. Refer to Figures 2 to 6. For wetland rice, the crop

coefficient at all stages can be assumed equal to one (1).

Make a reasonable assumption for probable percolation losses (mm/day) or refer to Table 8.

11



TABLE 8: Percolation For Different Soil Types

Clay --------------------------------------------------------------------------------- 1.25 mm / day

Silty Clay --------------------------------------------------------------------------- 1.50 mm / day

Clay Loam-------------------------------------------------------------------------- 1.75 mm / day

Silty Clay Loam-------------------------------------------------------------------- 1.75 mm / day

Sandy Clay Loam------------------------------------------------------------------ 2.0 mm / day

Sandy Loam------------------------------------------------------------------------ 4.0 mm / day

Figure 2: Water Management Scheme & Crop Depending Variables Used In Field Balance

Computation For Irrigated Wetland Rice

Rainfall Land Land Crop in the FieldCollecting Period Soaking Preparation (100 Days)

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13Maximum water

depth in paddy,

mm

200 80 80 80 80 80 80 80 80 80 80 10 0

Minimum water

depth, mm10 20 20 20 20 20 20 20 20 20 20 0 0

Optimum water

depth, mm100 65 65 50 50 50 50 50 45 45 45 0 0

FIGURE 3 Crop Depending Variables For Field Water Balance For Irrigated Corn

Rainfall

CollectionCrop in the Field

& (110 Days)

Land Preparation

LP 1 2 3 4 5 6 7 8 9 10 11

LP 1 2 3 4 5 6 7 8 9 10 11

Crop Coefficient 0.65 0.65 0.75 0.8 0.85 0.9 0.9 0.9 0.9 0.75 0.5

Rooting Depth

(mm) 100 200 300 450 600 700 775 825 875 900 900

12



FIGURE 4 Crop Depending Variables Used in the Field Water Balance For Irrigated Mungo

Rainfall Collection

& Crop in the Field

Land Preparation (80 Days)

LP 1 2 3 4 5 6 7 8

LP 1 2 3 4 5 6 7 8

Crop Coefficient 0.35 0.5 0.7 0.9 0.9 0.85 0.77 0.7

Rooting Depth 80 150 230 300 300 300 300 300

FIGURE 5 Crop Depending Variables Used in the Field Water Balance for Irrigated Tomato

Rainfall Collection

& Crop in the Field


LP 1 2 3 4 5 6 7 8

LP 1 2 3 4 5 6 7 8

Crop Coefficient 0.35 0.5 0.7 0.9 0.9 0.85 0.77 0.7

Rooting Depth (mm) 80 100 300 400 500 600 700 700

13



FIGURE 6 Crop Depending Variables for the Field Water Balance for Irrigated Peanut

Rainfall Collection

& Crop in the Field


LP 1 2 3 4 5 6 7 8 9 10

LP 1 2 3 4 5 6 7 8 9 10

Crop Coefficient 0.40 0.70 0.70 0.95 0.95 0.95 0.75 0.75 0.75 0.55

Rooting Depth(mm) 80 150 200 250 300 350 400 500 600 600

2.5 Make a reasonable assumption of soil water capacities WHC in volume percentage of soils used

for upland crops. (10% - 20%).Refer to Table 9.

2.6 Actual evapotranspiration (AET) is equal toAET = PET x KC

2.7 Change in storage (STOR) is equal toSTOR = RAIN - AET - PERCO - for paddy rice

STOR = RAIN - AET for upland crops.

2.8 Initial Storage (INIT) is estimated using the following formula

INIT = (Raini + Raini – 1) (0.70) for paddy rice

INIT = (Raini + Raini – 1) (0.50) for upland crops

2.9 Estimate the water storage (STOR) at the end of a given decade:

STORi = STORi – 1 + STOR

If STORi > allowable max storageThen DRAINAGE = STORi – allowable max storage

STORi = allowable max storage

IRRIGATION = Ø. Ø

If STORi < allowable minimum storageThen IRRIGATION = Optimum Storage – STORi

STORi = Optimum Storage

Drainage = Ø. ØELSE

IRRIGATION = Ø. Ø

DRAINAGE = Ø. Ø

14





3. ESTIMATION OF 10 – DAY RESERVOIR INFLOW

3.1 For Regions I, III, IV, characterized by distinct wet and dry seasons, 10 – day reservoir inflow are

estimated as follows:

a. DQj = RCj . Pj

Where:

DQj = direct runoff in decade j (mm)

RCj = runoff coefficient in decade j, equal to estimated mean monthly

runoff coefficient

Pj = 80% dependable rainfall

b. 10 – day Baseflow

BFj = F .Qj – 1

Where:

BFj = baseflow in decade j (mm)

F = 10 – day reservoir factor

= 0.002 + 0.026 (D.A.) where DA is drainage area in sq. km. (This

regression equation analysis of several small watersheds <100 km2

In the country).

Qj – 1 = Total runoff (or inflow) in the previous decade (j-j), mm

c. 10 – day Reservoir Inflow

Qj = DQj + BFj

Where:

Qj = reservoir inflow in decade j (mm)



3.2 For the other regions in the country which are predominantly characterized by indistinct, short, or no dry

season with more or less continuous rainfall, 10 – day reservoir inflow are estimated as follows:

a. 10 – day Direct Runoff

DQj = RCj . Pj

Where:DQj = direct runoff in decade j (mm)

RCj = runoff coefficient in decade j, equal to estimated monthly runoff

coefficients

Pj = mean 10 – day rainfall in decade j (mm)

b. Annual Baseflow

BF = a + b . DA

Where:

BF = annual baseflow

a.b. = regression factor for the region where the project is located

(Table 10)

D.A . = Drainage Area, (sq. km.)

c. 10 – day Baseflow

Qj = DQj + BFj

Where: Qj = reservoir inflow in decade j (mm)



16





Region 3

Month Run - off Coefficient, RC

Jan. 0.45

Feb. 0.08

Mar. 0

Apr. 0

May 0.24June 0.34

July 0.58

Aug. 0.7

Sept. 0.75

Oct. 0.7

Nov. 0.4

Dec. 0.5

Region 4

Month Run - off Coefficient, RC

Jan. 0.45

Feb. 0.44

Mar. 0.19

Apr. 0

May 0

June 0.19

July 0.19

Aug. 0.26

Sept. 0.33

Oct. 0.47

Nov. 0.57

Dec. 0.5

Region 5

Month %Baseflow Run - off Coefficient, RC

Jan. 9.17 0.5

Feb. 8.69 0.38

Mar. 8.28 0.3

Apr. 7.91 0.25

May 7.64 0.1

June 7.66 0.08

July 7.86 0.15

Aug. 8.08 0.15Sept. 8.31 0.15

Oct. 8.53 0.35

Nov. 8.79 0.39

Dec. 9.07 0.47

Linear Curve Fit : BF = a + b (D.A)

a = 2, 057.31 b = 18.28 R = 0.87

18



Region 6


Jan. 8.06 0.39

Feb. 8.1 0.19

Mar. 7.96 0.16

Apr. 8.1 0.16May 8.26 0.16June 8.45 0.18

July 8.66 0.44

Aug. 8.73 0.44Sept. 8.6 0.33

Oct. 8.47 0.49

Nov. 8.37 0.39Dec. 8.21 0.39


a = 1, 043.65 b = 8.221 R = 0.695

Region 7


Jan. 8.23 0.26Feb. 8.07 0.15

Mar. 8.09 0.1Apr. 8.22 0

May 8.23 0.09June 8.35 0.15

July 8.47 0.3

Aug. 8.66 0.3Sept. 8.57 0.3

Oct. 8.45 0.3

Nov. 8.37 0.3Dec. 8.29 0.26


a = 1, 055.85 b = 11.80 R = 0.766

19



Region 8


Jan. 9.1 0.38

Feb. 8.8 0.28

Mar. 8.6 0.25

Apr. 8.3 0May 8.1 0.14June 7.9 0.22

July 7.7 0.3

Aug. 7.6 0.34Sept. 7.7 0.34

Oct. 7.9 0.51

Nov. 8.4 0.7Dec. 9 0.7


a = 12.52 b = 14.051 R = 0.872

Region 9


Jan. 8.53 0.3Feb. 8.33 0.22

Mar. 8.16 0.08Apr. 7.94 0

May 8 0June 8.13 0.07

July 8.19 0.14

Aug. 8.32 0.14Sept. 8.42 0.14

Oct. 8.53 0.24

Nov. 8.66 0.24Dec. 8.76 0.3


a = 1, 164.37 b = 30.36 R = 0.999

20



Region 10


Jan. 8.51 0.49

Feb. 8.43 0.4

Mar. 8.36 0.37

Apr. 8.29 0.32May 8.21 0.15June 8.16 0.15

July 8.21 0.15

Aug. 8.27 0.24Sept. 8.3 0.24

Oct. 8.34 0.28

Nov. 8.4 0.25Dec. 8.49 0.52


a = 2, 119.90 b = 6.09 R = 0.562

Region 11


Jan. 8.42 0.17Feb. 8.38 0

Mar. 8.35 0Apr. 8.31 0

May 8.3 0.12June 8.25 0.12

July 8.27 0.29

Aug. 8.3 0.29Sept. 8.32 0.26

Oct. 8.34 0.26

Nov. 8.37 0.23Dec. 8.39 0.22


a = 152.608 b = 7.53 R = 0.751

21







ANNEX A – I PHILIPPINE WATER RESOURCE REGIONS

Water Resources Region No. 1 – ILOCOS

Ilocos Norte, Ilocos Sur, Abra, Benguet, La Union and part of Mt. Province.Predominant Climate : Type I

Water Resources Region No. 2 – CAGAYAN VALLEYCagayan, Isabela, Nueva Viscaya, Quirino and parts of Mt. Province, Kalinga-Apayao, Ifugao and Quezon.

Predominant Climate : Type III

Water Resources Region No. 3 – CETRAL LUZON

Nueva Ecija, Pamapanga, Pangasinan, Tarlac, Bulacan, ZamabaleS, Bataan and portions of Benguet and Aurora

Province.

Predominant Climate : Type I

Water Resources Region No. 4 – SOUTHERN TAGALOG

Rizal, Cavite, Laguna, Batangas, Quezon and Metropolitan Manila in Luzon, and the island provinces of

Marinduque, Mindoro, Romblon, and Palawan.Predominant Climate : Type I

Water Resources Region No. 5 – BICOL

Camarines Norte, Camarines Sur, Albay, Sorsogon in the South-eastern Peninsula of Luzon and the inslandsof Catanduanes and Masbate.

Predominant Climate : Type II and Type III and Type IV

Water Resources Region No. 6 – WESTERN VISAYAS

Negros Occidental, the sub-province of Guimaras, and the island of Panay which consist of the provinces of Aklan,Antique, Capiz and Iloilo.

Predominant Climate : Type I and Type III

Water Resources Region No. 7 – CENTRAL VISAYAS

Cebu, Bohol, Siquijor, Negros Oriental

Predominant Cliamate : Type III

Water Resources Region No. 8 – EASTERN VISAYAS

Samar and Leyte Islands.

Predominant Climate : Type IV

Water Resources Region No. 9 – SOUTHWESTERN MINDANAO

Misamis Occidental, Zamboanga del Sur and Zamboanga del Norte together with Sulu Archipelago.

Predominant Climate : Type III and Type IV

Water Resources Region No. 10 – NORTHERN MINDANAO

Agusan del Norte, Misamis Oriental and part of Agusan del Sur, Bukidnon and Lanao del Norte.

Predominant Climate : Type II

Water Resources Region No. 11 – SOUTHEASTERN MINDANAO

Davao del Sur, Davao Oriental and Surigao del Sur and South Cotabato provinces.

Predominant Climate : Type II and Type IV

Water Resources Region No. 12 – SOUTHERN MINDANAO

Lanao del Norte, Lanao del Sur, Bikidnon, North Cotabato, Maguindanao, Sultan Kudarat and South Cotabato.Predominant Climate : Type III and Type IV

24



25





27



PLANTING CALENDAR

PLANTING CALENDAR FOR TYPE I CLIMATE

TWO PRONOUNCED SEASONS : DRY from November to April

WET during the rest of the year

All the provinces of the western part of the islands of Luzon, Mindoro, Negros,

and Palawan are covered in Type I.

CROP PERIOD CROP PERIODRice:

Lowland June - September Muskmelon November - January

October - December Okra May - June

Palagad January - February October - December

Upand April - June Patola May - June

October - January

Corn: Squash May - June

Dry season Ocrober - January October - December

Rainy season May - June Tomato October - January

Upo October - January

Peanut: Watermelon November - January

Dry season November - January

Rainy season May - June Root:

Camote(Sweet May - June

Beans: Potato) December - February

Batao May - June Gabi May - June

Bountiful Bean May - June Ginger May - June

October - December Raddish October - December

Cowpea May - June Sinkamas October - December

October - November Tugue May - JuneCadios May - June Ubi May - June

Mungo July - September Cassava May - June

November - February October - December

Patani May - June

October - January Others:

Seguidillas May - June Garlic October - December

Sitao May - June Onion October - December November - February Sweet Pepper May - June

Soybean May - June September - December

Condol May - June

Vegetables: October - December

Leafy: Chayote May - June

Cabbage October - December October - December

Cauliflower October - February Spinach October - November

Celery October - February Sweet Peas October - DecemberLettuce August - January Carrot October - December

Mustard August - January Potato(Irish) October - December

Pechay October - December Talinum May - June

October - December

Fruit: Kutchai October - DecemberAmpalaya May - July Arrowroot May - June

October - January Tapilan May - June

Cucumber May - June September - October

September - December Beets October - January

Eggplant May - June Jute May - June

September - February Endive September - October

Melon October - January Snap Bean October - December

28









TABLE OF CONTENTS

Section Title Page

1.0 GENERAL 1

2.0 DAM 1

2.1 Determination of Dam Height 1

2.1.1 Dead or Inactive Storage 12.1.2 Active Storage 3

2.1.3 Flood Surcharge 3

2.1.4 Freeboard 6

2.1.5 Outline of Dam Height Computation 72.2 Dam Crest Width 7

2.3 Selection of Type of Earth Dam 7

2.3.1 Homogeneous/ Modified Homogeneous Type 7

2.3.2 Zoned Embankment Type 92.4 Embankment Slopes 11

2.5 Seepage Through Earth Embankment 132.5.1 Seepage Line 13

2.5.2 Position of Seepage Line 13

2.5.3 Quantity of Seepage 13

2.5.4 Filter Design 212.6 Embankment Slope Protection 222.6.1 Upstream Slope 22

2.6.2 Downstream Slope 23



Section Title Page

3.0 SPILLWAY 24

3.1 General 24

3.2 Spillway Type and Alignment 243.3 Spillway Hydraulics 24

3.3.1 Control Section 25

3.3.2 Discharge Channel 253.3.3 Terminal Section 31

3.4 Structural Requirements 40

4.0 OUTLET WORKS 43

4.1 General 43

4.2 Specific Type and Physical Arrangement 43

4.3 Outlet Works Hydraulics 444.3.1 Section of Design Discharge Head Combination 44

4.3.2 Sizing of Discharge Pipe 44

4.3.3 Sizing of Impact Type Dissipator 48

4.4 Structural Design Considerations 48

5.0 IRRIGATION WORKS 515.1 General 51

5.2 Canal Layout and Profile 51

5.3 Canal Hydraulics 51

5.3.1 Slide Slopes 515.3.2 Permissible Velocity 525.3.3 Applicable Formula for Sizing of Canal 52

5.3.4 Freeboard 53

5.4 Design of Canal Structures 53

Appendix I General Design Criteria for Canal Structures 55





LIST OF FIGURES

Figure No. Title Page

1 Reservoir Storage Allocations 22 Reservoir Operation Studies Format and Flow Chart 4

3 Flood Routing Format and Flow Chart 5

4 Modified Homogeneous Dam Sections 105 Size of Impervious Core of Zoned Dam 12

6 Slope Stability Chart No. 1 16

7 Slope Stability Chart No. 2 17

8 Slope Stability Chart No. 3 189a Elements of Seepage Line 19

9b Diagrams for Determining ∆a and a 20

10 Flow Profile Along Spillway 30

11 Unsubmerged Deflector Bucket 3212 Type IV USBR Basin 33

13 Type III USBR Basin 3414 Type II USBR Basin 35

15 Hydraulic Jump Nomograph

(Stilling basin Depth Vs Hydraulic Head

for Various Channel Losses) 3916 Typical Chute and Stilling Basin Section 4217 Typical Outlet Works System 45

18 Impact Type Energy Dissipator 49

19 Types of Irrigation Canal Layout 54











5





2.1.5 Outline of Dam Height Computation

Shown in Table 1 is an outline form for dam height computations.

2.2 Dam Crest Width

Width of the dam crest is computed on the basis of the following criteria:

a. Minimum width for maintenance purposes = 4.00 m

b. W1 = 5/3 --------------------------------- 6

c. W2 = + 10 --------------------------------- 7

Where: W1 = width of the dam crest, m

H1 = dam height, mW2 = width of dam crest, ft

H2 = dam height, ft

The largest dimension computed from the above items is adapted as the width of thedam crest.

2.3 Selection of Type of Earth Dam

The availability and excavation costs of the materials often dictate the type of the construction of

dam embankment to be considered. An abundant supply of materials of low permeability (e.g. sandy orsilty clay, other clayey material) points to the use of a homogenous earth dam. Sufficient quantities of both pervious and impervious materials indicate the suitability of a zoned dam.

2.3.1 Homogenous/Modified Homogenous Type

A purely homogenous type of dam is composed of a single kind of embankment

material exclusive of slope protection. The basic requirement for this type is that the embankment

material must be sufficiently impervious to provide adequate water barrier and that the slopes must bestable under critical loading condition. Recognizing the basic short coming of purely homogenous

section which is in seepage and pore water control, the usually adopted type of dam is the modified

homogenous type. The modification is carefully placed pervious material which help to control seepageflow and pore pressure development. This permits the use of steep slopes by lowering the phreatic level

within the embankment, the flowing of fine particle with the seepage water is also screened off

preventing piping. The two types of modified homogeneous dam are shown in Figure 4.

If a rockfill toe is provided, as shown in Figure 4a, a filter must be constructed betweenthe embankment proper and the rockfill toe.

7



Table 1

Outline of Computation For Dam Height

& Dam Crest Elevation

1. Creek Bed Elevation = __________________ m

2. Min. Water Surface Elevation = __________________ m

3. Normal W.S. Elevation = __________________ m(From Reservoir Operation Studies)

4. _____ Year Flood Surcharge Height = __________________ m(From Flood Routing)

5. Max. W.S. Elev. 3 + 4 = __________________ m

6. Freeboard due to Wave run-up, F b = __________________ m

7. Preliminary Dam Crest Elev. (5 + 6) = __________________ m

8. Preliminary Dam Height (7 – 1) = __________________ m

9. Embankment Settlement, (2% to 5% of 8) = __________________ m

10. Final Dam Height (8 + 9) = __________________ m

11. Final Dam Crest Elevation (1 + 10) = __________________ m

(Round-off to nearest 0.50 m)

8



To perform its function of lowering the phreatic or seepage line and to provide added stability tothe downstream portion of the dam, the horizontal drainage blanket as shown in Figure 4b, should extend

from the downstream toe deep into the embankment. A chimney drain will help to provide better

collection capability. The blanket should not however extend so far upstream as to shorten much theseepage path to critical extent. It is recommended that the horizontal filter blanket extend, from

downstream toe to a distance not grater than one third of thye base of the dam.

2.3.2 Zoned Embankment Type

This type consists of a central impervious core flanked between zones of more perviousmaterials. The central portion, is called the core while the more pervious flanks, the shell. The shell

enclose and support the core. To prevent migration of material from the core to the shell, a transition

zone is provided. Essentially the transition is filter and therefore designed as such. The upstream shell

affords stability against rapid drawdown and the downstream shell acts as a drain to control seepage.

Three major advantages of using this type of embankment are listed below:

a. Steeper slopes could be adopted with consequent reduction in total volume ofembankment materials.

b. A wide variety of materials could be utilized.c. Utilization of materials excavated structure could be maximized.

d. More stable and affords added stability against earthquake, cracking and settlement.

Figure 5 shows the suggested size of minimum core for the following conditions:

a. Impervious or shallow pervious foundation penetrated by a positive cut-off trench. Use

Minimum Core A.

b. Exposed pervious foundation and covered pervious foundation not penetrated by a

positive cut-off trench. Use Minimum Core B.

9



10











Table 3

Recommended Slopes for Small Zoned

Earth Fill Dams on Stable Foundation

1/ From “ Design of Small Dams “ , U.S. Bureau of Reclamation

2/ Minimum and maximum size of cores are shown in Figure 4.3/ Drawdown rate of 0.15m/day following prolonged storage at high reservoir level.4/ Definitions

CW - Well graded, gravel-sand mixture, little or no fine

GP - Poorly gravel, gravel-sand mixture, little or no fine.SW - Well graded sand, gravely sand., little or no fine

SP - Poorly graded sand, gravely sand, little or no fine.

See Table 2 for definitions of core materials.

15





17





19



20



Therefore, equation 9 would become;

q = k ( dy/dx ) y ------------------------- 12

Expanding equation 12 and transposing terms;

------------------------- 13

Since the discharge passing through any vertical plane is the same;

at x = 0 (Figure 6), dy/dx = 1 and y = yo. Hence,

q = kyo -------------------------- 14

Equation 14 is applicable only to horizontal filters and could also be applied to other cases to getapproximate discharge.

2.5.4 Filter Design

Filter drain is required between the impervious core and outer shell of zoned dams and

on horizontal drainage blanket or toe drains of modified homogeneous dams to prevent migration of small particles and to screen off fine materials that flow with seepage water.

To serve its purpose, the filter must satisfy the following requirements:

a. Graduation must be such that the particles of soil are prevented from enteringthe filter and clogging it.

b. Capacity of the filter must be such that it adequately handles total seepage

flow.

c. Permeability must be great enough to provide easy access of seepage water sothat uplift forces are reduced.

Multi-layer of filters although more effective must be avoided in general since theseare costly. If sufficient quantities of filter material are available at reasonable cost, it would bee more

economical to provide thick layers rather than process material to meet exact requirements for a thin filter

design.

The following limits are recommended to satisfy filter stability criteria to provide

ample increase in permeability between based material and filter.

a. D15 of filter = 5 to 40, provided that the filter does not contain more thanD15 of material 5% of material passing No. 200 sieve.

b. D15 of filter = 5

D85 of material

21



c. The grain size curve of the filter should be roughly parallel to that of the base material.

D15 is the size at which 15 percent of the total; soil particles are smaller. The percentage is

by weight as determined by mechanical analysis.

D85 size is that at which 85 percent of the soil particles are smaller.

If more than one filter layer is required, the same criteria are followed. The finer filter is

considered as base material for selection of the gradation of coarse material.

The design and sizing of the filter drain is done using Darcy’s equation ( see Equation 9 ),

transformed to a more convenient form of application,

( for horizontal drain )

Where; Q = design seepage value equivalent to 5-10 times estimated

embankment seepagek = average permeability of filter material

t = thickness of drain; L = length of Drainw = width of drain (perpendicular to flow)

The design and sizing of other type of drain (e.g., toe drain, chimney drain etc.)

can be done in a similar manner.

2.6 Embankment Slope Protection

2.6.1 Upstream Slope

For a “well protected” reservoir, i.e., a condition where the upper reaches of the basin

is shielded by a high mountain barriers, only plain gravel would be necessary to protect the upstream face

of the dam.

For unprotected reservoirs, the upstream slope of the dam should be protected against

the destructive effect of wave action. Usual types of surface protection include rock, concrete pavement.Concrete pavement should be considered only in extreme cases since it is too expensive.

22





3.0 SPILLWAY

3.1 General

A spillway is the safety valve of an earth dam. Its main function is to release surplus orflood water which cannot be contained within the active storage space of the reservoir. Adequate capacity

of the spillway is of primary importance especially for an earthfill dam which is likely to be destroyed if

overtopped.

In addition to providing sufficient capacity, the spillway must be hydraulically and

structurally adequate. It must be located so that spillway discharge will not have the chance to erode orundermine the downstream toe of the dam. The bounding surfaces at critical sections must be protected

with concrete lining or erosion resistant material to withstand the high scouring velocity created by the

drop from the reservoir to the tailwater level.

3.2 Spillway Type Alignment

It is advisable to concentrate on the side channel or chute type of spillway s much as

geologic conditions will allow. One exception however is the use of a trickle spillway in combinationwith a grassed discharge channel. Further discussion on this latter type of combination will be present5ed

in later sub-sections.

The side channel and chute types have marked advantages over the closed and buried

types, i.e., siphon, culvert and drop inlet such as:

a. Simplicity, facility and ease of construction b. Readily accessible for inspection and emergency repair.

c. Lesser possibility of being clogged with debris. Between the side channel and chute

type, the former is hydraulically less efficient but it is more adaptable were low

surcharge head is required by adopting a long overflow crest. Both types require adischarge channel cut along abutment hillside leading to the same stream below the

dam. For economy, the shortest possible reach to the same or to some other natural

waterway downstream should be selected for alignment of the discharge channel.

In other instances, a saddle on either the left or right side of the proposed dam offers a

good possibility for a chute spillway. The alignment may lead to an adjacent drainage way or to the samestream below the dam. Unless volume excavation is excessive or excavation is too difficult, the possibility

of having the spillway pass through the saddle should be considered in the design.

3.3 Spillway Hydraulics

The side channel and chute spillway have three main components in common namely; the

control section, discharge channel and the terminal section.

24



3.3.1 Control Section

a. Side Channel Spillway – The control section may consist of a concrete ogee weir or

sill. Hydraulic analysis of the control section is treated in detail in “Design of Small

Dams” published by the US Bureau of Reclamation.

b. Chute Spillway - The control may consist of an ogee or harp-crested weir or just a

flat approach which may be lined or not depending on approach velocity or structuralrequirements. For economy, simplicity and ease of construction use of the flat

approach control should be the first priority over the weir type. Hydraulic design of

the former is simple and straight forward.

3.3.2 Discharge Channel

a. Slope – The slope of the discharge channel should approximate the general slope ofthe existing ground to entail lesser excavation. Multiple slopes, however, should be

minimized or avoided. As far as practicable, a single sloped straight channel should

be adopted for hydraulic efficiency structural stability.

b. Channel Shape and Lining – The cross sectional shape of the channel may be

trapezoidal, rectangular, or a combination of both. It may be unlined grassed,riprapped or concrete line depending on channel velocity.

c. Permissible Velocity – Shown in Table 4 are permissible velocities for cohesive

soils which may be used as the basis of design for unlined channels. Permissiblevelocities for different types of grass lining are shown in Table 5. It should be notedhowever that most of the grass types listed in Table 5 are not found locally. These

therefore should be use only as basis of comparison with similar types of grass found

locally.

d. Hydraulics – For unlined, grasslined and riprapped discharge channels,

determination of the flow depth and other elements along the discharge channel shall

be based on the Manning’s Formula:

Manning’s Formula:

-------------------------------- 18

Where: V = velocity, mps

R = hydraulic radius, mS = channel slope

n = channel roughness coefficient

25



Another convenient form of the Manning’s formula is;

--------------------------------------- 19

Where: Q = discharge, m3/ s

A = cross sectional area of flow

For A = bd = md2

----------------------------------------- 20

P = b + 2d m2

+ 1 ----------------------------------------- 21

R = A/P ----------------------------------------- 22

Equation 19 could be expanded and rearranged resulting into a form convenient for trial

and error solution such as,

----------------------- 23

Where: b = channel bed width, md = flow depth, mm = channel side slope

26



Table 4

Maximum Permissible Velocities

and the Corresponding Unit Tractive Force

Clear

Water UnitWater Transporting Colloidal Silts

V Tractive V Unit Tractive

(feet per Force (feet per Force

Material n

second) second)

Fine sand, colloidal 0.020 1.50 0.027 2.50 0.075

Sandy loam, non-colloidal 0.020 1.75 0.037 2.50 0.075

Silt loam, non-colloidal 0.020 2.00 0.048 3.00 0.11

Alluvial silt, non-colloidal 0.020 2.00 0.048 3.50 1.15

Ordinary firm loam 0.020 2.50 0.075 3.50 0.15

Volcanic ash 0.020 2.50 0.075 3.50 0.15

Stiff clay, very colloidal 0.025 3.75 0.26 5.00 0.46

Alluvial silts, colloidal 0.025 3.75 0.26 5.00 0.46Shales & hard pans 0.025 6.00 0.67 6.00 0.67

Fine gravel 0.020 2.50 0.075 5.00 0.32

Graded loam to cobbles 0.030 3.75 0.38 5.00 0.66

when non-colloidal

Graded silts to cobbles 0.030 4.00 0.43 5.50 0.80

when colloidal

Coarse gravel, non- 0.025 4.00 0.30 6.00 0.67

Colloidal

Cobble and shingle 0.035 5.00 0.91 5.50 1.10

27



Table 5

Permissible Velocities

For Grassed Channels

1/ From U.S. Conservation Service. Values apply to average uniform stands of cover.Velocities exceeding 5 fps are to be used only where good covers and proper maintenance can be

obtained.

2/ Not to be used on slopes steeper than 5%3/ Used on mild slopes or as temporary cover until permanent covers are established.

28



For concrete line discharge channels where the flow is supercritical, the Energy, Manning andContinuity Equations shall be used.

Referring to Figure 10.

Z + dc + hvc = d1 + hv1 + hf 1 ---------------------------------------- 24

Dc = critical depth computed by trial and error from the formula,

( bdc + mdc2 )

3 = Q

2 ---------------------------------------- 24a

b + 2md g

hvc = Qo2 ______ ---------------------------------------- 24b

[( bdc + mdc2 )

22g]

hv1 = Qo2 ______ ---------------------------------------- 24c

[( bdc + md12 )

22g]

hf 1 = ( SA + SB ) L ---------------------------------------- 24d

2

SA = ( Qn )2 ( b + 2dc M

2 + 1 )

4/3 ---------------------------- 24e

(bdc + mdc2 )

10/3

SB = ( Qn )2 ( b + 2d1 M

2 + 1 )

4/3 ---------------------------- 24f

(bdc + md12 )

10/3

Equation 24 involves a trial solution in d1 until both sides become equal.

e. Channel Freeboard – Freeboard along the discharge channel shall be computed from

the formula,

Fc = 2.0 + 0.025 V3 d ------------------------------- 25

Where: Fc = freeboard, ft.

V = average velocity of the channel reach, fpsD = average depth of flow within the reach, ft.

29



30



3.3.3 Terminal Section

For unlined and grassed discharge channels, the terminal structure should be as simple as

possible. This may be consist of a concrete sill at downstream end of the channel and a dumped riprap

from the sill to a distance downstream equal to the channel width.

For riprapped and concrete lined channels, the terminal structure may consist of an

unsubmerged deflector bucket as shown in Figure 11 or a hydraulic jump type basin. Examples of thelatter are shown in Figures 12 to 14.

a. Unsubmerged Deflector Bucket – This type of terminal structure works on the principle of a projectile. Flow from the deflector leaves the structure as an upturned

jet and falls some distance downstream from the structure. This is the simplest and

cheapest to construct when the discharge reach consists of hard rock or non-erodible

material.

The path of the jet is described by the basic equation,

Y = x tan A ______x2 ________ -------------------------------- 260.36 cos

2 ( d + hv )

Where: A = exit angle of the bucket lip

d = depth of flow at the bucket. This could

Be determined from Equation 24

hv = velocity head

The horizontal range of the jet is computed by the formula,

X = 1.8 sin2 A ( d + hv ) --------------------------------- 26a

Hydraulic design considerations areas follows:

1. The exit angle must not greater than 30°

2. The bucket radius should be long enough to maintain a smooth and concentric

flow. Minimum bucket radius should not be less than 5d.

31



32



33





35



b. USBR Hydraulic Jump Type Basin – As the name suggests, this type of terminal structure dissipatesthe flow energy by forming a hydraulic jump. There are three types in use by the USBR as

shown in Figure 11, 12 to 14, selection of type is based primarily on the Froude Number, F

in which:

F = -------------------------- 27

Where: v1 = velocity at entrance to the basin

D1 = depth of flow at entrance to the basin

For the determination of jump depth d2, the nomograph shown in Figure 15 would be

useful and convenient. Basin length, L is determined from the L/d2 versus F curve accompanying each

figure in Figure 9 to 11. Other basin dimensions figures.

c. Freeboard – For the USBR basins, the following empirical expression provides ample

freeboard allowance:

F b = 0.1 (v1 + d2) ----------------------------- 28

Where: v1 = velocity of entrance to the basin

d2 = flow depth at basin as determined from Figure 12

d. Outlined of Computation for USBR Basin

Shown in Table 6 is an outline form of USBR Basin computations.

36





Table 6 continued . . .

8. d2 ( see Figure 15 ) = ________ft,__________ m

9. V2 = Qo = ____________________ mps bd2

10. d1 = = ____________________ m

11. V1 = Qo = ____________________ mps

bd1

12. F1 = V1 = ____________________

13. Type of Basin (select from Figure 9 to 11) = ____________________

14. L/d2 (see nomograph for corresponding type = ____________________

of basin in 13)

15. L = ____________________ m

16. Chute Freeboard = 2.0 + 0.025 V = ________ft,__________ m

V = V1 = _____________ fps

2

d = ( dc + d1 ) = __________ ft2

17. Basin Freeboard = 0.1 (V1 + d2) = ____________________ m

38



39







42







45



Table 8

Outline of Discharge Pipe Computations

From the Reservoir Operation Studies, select a critical Demand Qd – Head (h)

combination. A critical combination would be a large under low head.

1. Qd --------------------------------------------- = ______________ m3 /s2. Head, h --------------------------------------------- = ______________ m

3. Preliminary Pipe Diameter, dp = = ______________ m

4. Trail Pipe Dia., dt ------------------------------------- = _______________ m

( Try next larger size than d p )

5. Vt = ____Qd _______ ------------------------------------------------------ = _______________ mps( dt

2 / 4 )

6. Total Minor Losses, hm = a + b + c + d = _______________ m

Consider losses due to trashrack, bend, entrance and gate valve.

a. Trashrack Loss = _K t Vn2

------------------------ = _______________

2g

K t = 1.45 ------------------------- = _______________

an = net trashrack area ----------------------------- = _______________

ag = gross area of rack ----------------------------- = _______________

Vn = velocity through net trashrack -------------- = _______________

46



Table 8 continued . . .

b. Entrance Loss = K e Vt2 ------------------------ = ________________

2gK e --------------------------------- = ________________

( see Table 33, page 472, Design of Small Dams, USBR)

c. Bend Loss = K b corr x Vt2

----------------------- = ________________

2g

K b 90° ------------------------- = ________________( see Fig. 311a, page 475, Design of Small Dams USBR )

Correction Factor for K b 90° -------------- = ________________

( see Fig. 311a, page 475, Design of Small Dams USBR )

K b corr = K b 90° x Correction Factor ------- = _________________

d. Valve Loss = K v V2 ----------------------------- = _________________2g

K v (For gate valve, select, select below)

Fully open = 0.19

¾ open = 1.15½ open = 5.60½ open = 24.00

7. Friction Loss, hf = fL ---------------- = _________________ m

L ( total length of pipe ) ------------------------ = __________________ m

dt --------------------------------------------------- = ________ft, ________ m

f = 185 n2 --------------------------------------- = __________________

( dt )1/3

8. Total Head Loss, ht = hm + hf ------------------ = __________________ m9. Net Head, hn = h – ht ---------------------------- = __________________ m

10. Net velocity, Vn = -------------------- = __________________ mps

11. Qt = Vn dt2 ------------------------------------ = __________________m

3/s

4

12. Qt≥ Qd ; if not, repeat nos. 4 to 1113. Final Pipe Diameter, df ----------------------- = ___________________ m

( After successive trials)

47



Generally, sizing of the discharge pipe involves the solution of Energy of Bernoulli Equation.

Losses incurred along the pipeline are determined and these are subtracted from the total head obtain thenet head. Actual velocity is then computed based from the net head. The cross sectional area of the pipe

multiplied by the actual velocity would yield the actual pipe discharge. This discharge, compared to the

irrigation demand would indicate whether the assumed pipe diameter is adequate to handle the demand.

4.3.3 Sizing of Impact Type Dissipator

This device, as shown in Figure 18, is limited to a maximum capacity of approximately 50

feet per second. It can be used at the terminal of an open chute or a closed conduit structure.

Dissipation is accomplished by the impact of the incoming jet on the vertical baffle and by

eddies formed by the deflected jet after it strikes the baffle. For better performance, the bottom of the

baffle must be at the same level as the invert of the upstream channel or pipe.

Sizing of the device would proceed in sequence as shown in Table 9.

4.4. Structural Design Considerations

For the concrete structural components of the outlet works system described above, only a

minimum of temperature reinforcement is required.

Joints of the steel discharge pipe must be water tight to prevent leakage into the surrounding

embankment. This would require couplings that remain watertight after movement or settlement of the

pipe.

Methods of bedding and backfilling should be such as to insure against unequal settlement

along the pipe length and to the secure the most possible distribution of load on the foundation. Extreme

care should be taken to secure tight contact between the fill and the conduit surface to prevent seepage

and insure lateral restraint on the structure.

48



49



Table 9

Outline of Impact Stilling Basin Computation

1. Hydraulic Head, H = _________________ m

H = NWS Elev. – Elev. C

2. Pipe Diameter, dt = _________________ m

3. Equivalent Square Opening, ds = = _________________ m2

4. Velocity, Vs = = _________________ mps

5. Froude No, F = Vs = _________________

6. Basin Width, W = 2.85 ds F 0.58 = _________________ m

7. Other Basin Dimensions

H = ¾ W = ______________ m

a = ½ W = ______________ m

b= 1/6 W = _____________ m

c= 3/8 W = _____________ mL = 4/3 W = _____________ m

50



5.0 IRRIGATION WORKS

5.1 General

The irrigation works component shall consist of a network of open canal laterals and drainageditches provided with the necessary structures to control the distribution of irrigation water design of

canals shall be done on the basis of the NIA criteria and other applicable criteria which shall be discussed

in the following subsections.

5.2 Canal Layout and Profile

There are three possible types of open irrigation canal layout. These are illustrated in Figure

19.

In laying out canals, the designer should have an overall view of the topographic map anddetermine beforehand where the canals should pass. Routes passing along steep slopes should be avoided

as much as possible for larger canals to minimize or eliminate costly structures.

For feasibility study purposes, ground profiles along canal routes may be taken fromtopographic maps. This is done by taking elevation of contours that cross the canal alignment and plotting

such elevation against distance on suitable cross section paper.

5.3 Canal Hydraulics

5.3.1 Side Slopes

Earth canals for irrigation and drainage purposes re generally trapezoidal in shape with

side slope determined from experience and stability studies of the bank material. Usually, a side slope of

1.5:1 is found adequate for most earth materials. This slope however, be steepend if soil conditions so

warrant. For rock or hardpan materials, a side slope of 1/3:1 or 1/2:1 may be used. Materials that areinitially hard but subsequently would became unstable of their property of being easily pulverized or

disintegrated after exposure to the elements should have flatter side slopes which may be 1:1 and provide

with the concrete lining if necessary.

51



5.3.2 Permissible Velocity

Normally, small canals ( which are expected for the Rainfed Project ) have a minimum

permissible velocity of 0.30 per meter.

The ideal condition is to design the canal for a velocity which will be neither too fast to

cause scouring nor too slow to cause silting. As guide, the following Kennedy Formula would be useful;

Vs = cd0.64 ---------------------------------- 29

Where : Vs = velocity for non-silt and non-scour, sediment

laden water, fps.d = depth of flow, ft.

c = coefficient for various soil conditions as follows:

Soil condition c

Fine, light, sandy 0.84

Coarse, light, sandy 0.92

Sandy, loamy 1.01Coarse, silt, hard 1.09

For clear water

Vs = cd ½ ----------------------------------- 30

5.3.3 Applicable Formula for Sizing of Canal

For simplicity, Manning’s formula shall be used in sizing the canals. This was given in

Equation 18 which is repeated hereunder:

V = _1 R 2/3

S1/2

----------------------------------- 18

( bd + md2)

5/3 = __n_ ----------------------------------- 23

( b + 2d m2 + 1)

2/3 S

For convenient computations.

Values of the Manning’s Roughness Coeffivcient, n are 0.025 to 0.03 for earthined canals

and 0.015 to 0.018 for concrete lined.

52



5.3.4 Freeboard

The canal freeboard generally used by the U.S. Bureau of Reclamation is approximately ¼

the water depth during low flow stage plus 0.30 meter with a maximum of 2.00 meters. The established

freeboard criteria of the NIA is 40% of the water depth with a minimum of 0.30 meter. The USBR criteriaseems excessive for the larger flow depth. It is more reasonable therefore to adopt a freeboard based on

the best ranges of both as follows:

a. For flow depth from 0.18 to 1.99 m:

F b = 0.40 d (minimum = 0.30) ---------------------------------- 31

b.For flow depth = 2.00 m and greater:

F b = 0.25 + 0.30 (minimum = 2.00 m) --------------------------- 32

5.4 Design of Canal Structures

Conveyance structures are designed on hydraulics and structural requirements. Hydraulic

design refers to the proper sizing of the structures and the provision of adequate head

Allowance for flow, while structural design refers to the provision of adequate wall thickness and steelreinforcements to the structure to enable it to sustain the imposed loads. A number of criteria for design of

canal structures have already been formulated by the NIA, and have been the basis of all other designcriteria formulated by other agencies.

Appendix I present hydraulics and structural design criteria formulated by Technosphere or the

irrigation of the Lower Agusan Development Project (LADP). A major portion has been adopted from NIA design criteria. The same criteria can be used in the design of irrigation canal structures for theRainfed Project.

Irrigation canal structures for the Rainfed Water Impounding Component are relatively small

compared to those of major irrigation projects such as those undertaken by the NIA.

Structural requirement are therefore simple. Required thickness for walls are normally 0.10m

with a minimum steel reinforcement as required for temperature reinforcement only.

53



54



APPENDIX I

GENERAL DESIGN CRITERIA FOR CANAL STRUCTURES

1.0 Hydraulics

1.1 Head Losses

All conveyance structures should be designed and checked against all possible head losses.

The more common head losses are due to friction, transitions, bends, trash racks and changes in water

section or velocity.

1.1.1 Friction Loss: For box or barrel typed structures, the friction loss can be calculated

by the formula hy = SL;

Where: S

For pipes, the friction loss may be calculated by the formula:

hf = fL x v2 where Darcy’s “f” can be obtained

D 2g from graph, on page 50 using

n = 0.015 for Reinforced Concrete.

55



1.1.2 Transition Loss: Transition loss can be estimated by using the formula:

Ht = C v22 – v1

2 The value of the coefficient “C” can

2g be obtained from the tabulation below.

Type of Open Transition to Closed Inlet Outlet

Conduit

Stream lined warp to rectangular opening 0.1 0.2

Straight warp to rectangular opening 0.2 0.3

Straight warp with bottom corner fillets 0.3 0.4

to pipe opening

Broken back to rectangular opening 0.3 0.5

Broken back to pipe opening 0.4 0.7

Closed Transition

Square or rectangular to round (maximum 0.1 0.2

angle with centerline = 7 ½ degrees

1.1.3 Bend Loss: Bend loss in closed conduit can be calculated from the formula:

H b = K b v2

2g

The coefficient, K b, can be obtained from the graph on pages 51 to 52 for any value of

“deflection angle”.

1.1.4 Trashrack Loss: Trashrack losses may be estimated as dollows:

Velocity through rack (fps) loss in feet

1.0 0.101.5 0.30

2.0 0.50

More accurate values may be obtained from Diagram on page 35.

Freeboard : In lined and earth canals, the minimum freeboard shall be 20% but notless than 6 inches.

56



1.2 Inlet and Outlet Transitions of Structures

1.2.1 Transitions are generally used at the inlet and outlet of structures and where changes

occur in water section. An accelerating water velocity usually occurs in inlet transitions and a decelerating

velocity in inlet transitions. The most common type of open transitions to closed conduits are thestreamlined warp, straight warp, and broken-back. Because the construction of the warped transition will

require a very thick consistency of the concrete during placing and therefore its consolidation cannot be

attained, the adoption of the “broken-back” transition is preferred.

1.2.2 Inlet transition for minimum hydraulic loss and smooth operation should have a

submerge or seal of 1.5 (hv p – hvc) or 8 cms. minimum measured between the upstream water surface ofthe inlet transition and the opening in the transition headwall.

hv p = velocity at barrel inlet

hvc = velocity at canal just at plane of inlet cut-off wall.

Outlet transition should have no submergence of the opening in the headwall. If

submergence exceeds 1/6 of depth of opening at the outlet, the hydraulic loss should be computed on the

basis of sudden enlargement rather than as an outlet transition.

1.2.3 Open transitions to multiple closed will involve some additional hydraulic loss.Average friction loss should be added for large transitions, but maybe neglected for small transitions. The

slope of the floor on a broken-back outlet transition should be 6:1 or flatter.

1.2.4 Length of Transition

Inlet = 3.5 times depth of the normal water level

Outlet = 4.5 times depth of normal water level

When the velocity inside the barrel exceeds twice the velocity of canal at outlet,

provide scour protection works (riprap on gravel blanket) just after the broken-back transition with a

length equal to 2.5 times the depth of water in the canal.

1.2.5 Cut-off-walls of Transitions

Depth : At inlet C1 = ½ times the depth of normal water level (in multiples of 10 but not

less than 60cm., not greater than, 120cm.)

Minimum concrete thickness of cut-off-walls;

13 to 15 cm. for 60 and 80 cm. In depth

15 to 20 cm. For deeper C.O.W

571.2.6 Minimum freeboards at transition cut-offs for siphon, tunnel and similar structures.



Normal Water Depth at Cut-Off Minimum Freeboard

0 to 4 cm. 15cm.

41 to 60 cm. 23 cm.61 to 150 cm. 30 cm.

151 to 210 cm. 38 cm.

211 to 270 cm. 46 cm.

For small structures such as transitions connecting to 0.46m. Ǿ pipe and smaller, top of

transition walls may be level. For larger structures, the freeboard at the transition head wall should begreater than at the cut-off. The earth bank freeboard should be increased 50 percent (30-cm. Maximum)

adjacent to siphons, wasteways and checks without overflow. The increased freeboard to extend away the

structure at a minimum distance of 15 meters.

1.3 Sizing of Canal Structure (Barrel or Pipe)

1.3.1 Maximum allowable velocity inside barrel

Large Siphons : Vs = 3.00 m/sec.

Small Siphons : Vs = 3 Vc for small canal (but use minimum velocityof 1.20 m/sec. To prevent silting.

Check Structures: Vs = 1.00 m/sec. (with stop planks)

= 1.50 m/sec. (gated)Elevated Flumes: Vs = 2.00 m/sec. But should not approach critical

Velocity at structure irregularities

1.3.2 Submerged Barrel Section

Design barrel for available head by utilizing the actual hydraulic losses due to transitions,

friction, bends and other losses. In case no head is available, 5cms. head may be used only after

examination of the profile.

58



59



60



61



62



63



1.3.3 Prelminary Approximation of Barrel Size

All gate openings shall be predetermined by using formula:

Q = CA (2gh)1/2

and then check by using hydraulic losses;

Where: C = 0.75 or see submerged tubes coefficient appropriate

values on page 54

Q = discharge through opening in cu.m./sec.

A = area of opening in sq.m.

h = available head in meters.

2.0 Structural Design for Canal Structures

2.1 Loadings

Knowing the nature and functions of canal structures, temporary, permanent and unusual loading

conditions exist. The structures are subjected to changing effects such as foundation reactions,temperature stresses, exposure conditions and varying earth hydrostatic loading.

2.1.1 Dead Loads

Commonly used dead load weights for canal structures are as follows:

Loads Weight ( kg/m3)

Water 1,000Dry Earth 1500 to 1600*

Compacted Wet Earth 1800 to 1900*

Compacted Saturated Earth 2100 to 2200*Submerged Earth 1100 to 1200*

Plain Concrete 2,300

Reinforced Concrete 2,400Steel 7,850

* For impervious backfill, adopt lower values and for backfill containing sand and gravel, use higher

values. Compaction is assumed at optimum moisture content. For lesser compaction, use lower values.

64



2.1.2 Lateral Earth Pressure

The horizontal pressure of dry earth is usually about 480 kg/m2 per meter of depth and that

saturated earth is about 1400 kg/m2 per meter of depth. Lateral earth pressure can be calculated using

Rankine’s Formula:

Rankine’s Formula

Where: = unit weight of earth in kg/m3

Ǿ = angle of internal friction

Ө = angle of surcharge

2.1.3 Live Load on Operating Platforms

Operating platforms for radial gates shall be designed for the rated capacity of the hoist actingon either cable, in addition to the weight of the radial gate hoists and equipments.

Operating platforms without stoplogs 500 kg/m2

Operating platform with stoplogs 750 kg/m2

65



2.1.4 Super-imposed Load Pipes

This includes weight of the earth and/or water on the structure. Concentrated wheel loads

transmitted through the earth cover to pipe structures shall be taken as uniform loads converted to

equivalent earth cover for various heights of earth cover over the top of the pipe with MS 18 wheel loads(As per AASHTO) as given below:

Total Equivalent Earth Cover for Wheels Loads

(m)

Height of Earth

Cover

(m)

MS 13.5 Loading MS 18 Loading

0.6*

0.9**

1.2

1.5

1.8

2.12.4

3.60

2.30

2.10

2.25

2.45

2.652.90

4.60

2.80

2.40

2.50

2.65

2.803.05

* Minimum earth cover for standard concrete pipes for thresher crossings

** Minimum earth cover for standard concrete pipes for road crossings.

For earth cover less than 0.60 m., special provisions such as concrete encasement of pipe or slabcovers, etc. are made. Wheel load effect is negligible when the cover is more than 2.4 m.

Wheel load impact factors used for earth covers less than 0.9 meter are as follows:

(a) 10% for earth covers from 0.6 to 0.90m.,

(b) 20% for earth covers from 0.3 to 0.6m and(c) 30%if the earth cover is less than 0.30m.

2.1.5 Hydrostatic Pressure

A fluid pressure of 1000 kg/m2 per meter of depth is caused by water on any structural member

required to retain water all the time, or under some ephemeral conditions of loadings. The horizontal force

due to water pressure can be represented by a triangular load whose resultant is at two-thirds of the

distance from the water surface to the base of the section under consideration.

66



2.1.6 Uplift Pressure

`Uplift pressure, which may cause by the water percolating under or long the sides of hydraulic

structures, reduces the effective weight of the structure. The total uplift pressure acting on the structures is

calculated as follows:

U1 = Woh1

U2 = Woh2

Where: U = total uplift

pressure, (t/m)

U1 = uplift pressure

at left side, (t/m)U2 = uplift pressure

at right side(t/m)

Wo = unit weight of

water (t/m2)L = bottom width of

structure

2.1.7 Forces Due to Water Current

For cross drainage works, effect of flowing water is some significance especially there are angularflows. For design purposes, flow is assumed at 20˚ to direction of pier, computing pressure on the pier by

formula:

P = 78 V2

Where: P = intensity of pressure in kg/m2

V = component of velocity (normal to the pier) in m/sec. and taken as twicethe maximum mean velocity component. Intensity is taken as zero at the

bottom of pier and maximum at the free surface of water.

For in line structures where velocities of flow through canal are low, effect of flowing water is

negligible.

67



2.1.8 Wind Load

Wind loads on small irrigation structures are not included in the structural and stability analysis.

For major bridges and elevated canal structures, wind should be considered.

2.1.9 Seismic Forces

Additional earth and water pressure imparted to small canal structures are not included in thedesign considerations. Temporary stress caused by seismic loads is minor since the earth, water5 and

concrete masses are small.

For major structures like bridges, elevated flumes and long overchutes, earthquake forces shall be

considered in the structural and stability analysis. In the design of such structures, codes published by the

Department of Public Works and Highways shall be followed.

2.2 Loading Combinations

In actual practice, all the forces and loads explained above do not act simultaneously on the

structure. There can be similar loading combinations, imposing varying degrees of stresses in thestructures.

The following guidelines for loading combinations shall be followed;

a. Design shall be based on the most adverse combination of probable load

conditions, but should include only those loads having a reasonable probability ofsimultaneous occurrence.

b. Earthquake forces should not be considered to occur simultaneously with design

flood of maximum wind forces.

c. Permissible stresses may be exceeded up to a maximum limit of 33.33% when

stress due to earthquake or wind are combined with those due to dead and live loads.

2.3 Stability Analysis

For a structure to be stable, the following test criteria shall be satisfied.

a. The structures as a whole should be safe against sliding and overturning.

b. Unit stress in the material of a structure and pressure on the foundation shall not exceed

permissible limits.

68



2.3.1 Safety Against Sliding

Any structure subjected to differential lateral pressure must be able to resist the tendency to slide.

Resistance to sliding is developed by shearing strength along the contract surface of the structure base and

the foundation material itself.

For sliding analysis of small structures, shearing strength developed by the cohesion is omitted but

that developed by mechanical friction at the base and foundation interface shall be considered. The factoof safety against sliding shall be as follows:

Factor of Safety, (FS) = Vertical Forces x tan Ө ≥ 1.5 (under normalHorizontal Force conditions)

1.2 (with earthquake)

Different Values of tanӨ:

Concrete to soil (direct contact), tan Ө = tan (2/3Ǿ)Concrete to rock, tan Ө = 0.6

Concrete to soil(with sand and gravel bedding), tan Ө = 0.6 min.

Where: Ǿ = angle of internal friction of soil.

2.3.2 Safety Against Overturning

A structure is considered safe against overturning if the sum of the stabilizing moments exceeds

the sum of the overturning moments acting on it.

The resultant of all forces acting on the structures should fall within the middle third of the

structure base and if earthquake is considered, it should fall within the middle two-thirds of the structure base. This location of resultant force also provides a more uniform bearing pressure on the foundation.

2.3.3 Safety Against Percolation

To prevent piping of foundation materials from beneath or adjacent standardized canal structures,

sufficient cut-off and structural lengths which shall allow a percolation factor of 2.5 or more should be

provided.

Lane’s weighted-creep method is commonly used for percolation path studies related to canal

structures. Lane’s weighted-creep ratio is equal to the weighted-creep length divided by the effective

head. The former is the sum of: (a) vertical path along the structure (steeper than 45˚), (b) one-third of thehorizontal path distance along the structure (flatter than 45˚), or two times any percolation path distance

that shortcuts

69



through the soil. Effective head is the difference in water surface elevations at the beginning of path and

point of relief.

Lane’s recommended weighted-creep ratios are:

Materials Ratio

Very fine sand or silt 8.5 : 1

Fine sand 7.0 : 1Medium sand 6.0 : 1

Coarse sand 5.0 : 1

Fine gravel 4.0 : 1

Medium gravel sand 3.5 : 1Coarse gravel including cobbles 3.0 : 1

Boulders with some cobbles 2.5 : 1

And gravel

Soft clay 3.0 : 1Medium clay 2.0 : 1

Hard clay 1.8 : 1Very hard clay or hard pan 1.6 : 1

For pipe structures, if the computed weighted- creep ratio is less than that recommended, collarsshould be provided.

70



2.3.4 Safety Against Foundation Failure

a. Maximum vertical pressure of foundation material near the toe shall not exceed the

allowable bearing pressure of soil.

b. Values of allowable bearing pressure in final design calculations shall be as follows.

Normal Bearing

Pressure (kg/cm2)MATERIAL

Normal

Condition

With

Earthquake

N Monoaxial

Strength

10.0 15.0 - 100

6.0 9.0 - 100

hard w/o cracks

or w/ few cracks

and uniformROCK :

hard w/ cracks

soft 3.0 4.5 - 100

GRAVEL : dense Not dense

6.03.0

9.04.5

--

--

SAND : dense

medium

3.0

2.0

4.5

3.0

30 ~ 50

15 ~30*

-

-

Very hardCLAY : hard

Medium

2.01.0

0.5

3.01.5

0.75

15 ~308 ~ 154 ~8

2.0 ~ 4.01.0 ~ 2.0

0.5 ~ 1.0

Where: N = standard penetration value

* = If N is less than 15, soil is not suitable for foundation.

71



2.4 Structural Considerations

2.4.1 Permissible Stresses for Reinforced Concrete

Reinforced concrete design for all canal structures may be done by the working stressmethod based on concrete strength (fc’) of 210 kg/cm

2 and reinforcement with specific yield strength (fy)

of 2800 kg/cm2.

The allowable stresses for design of canal structures are as follows:

Compressive stress in concrete(fc) ----------------------- 85.0 kg/cm

2

Tensile stress in steel

Reinforcement (fs) ----------------------- 1,270.0 kg/cm2

Shear stress (V) ----------------------- 4.0 kg/cm2

The allowable Bond Stress,u for #10 bars or smaller shall be computed with the formula:

Where: D = nominal bar diameter in (mm)

f’c; is in kg/cm2

2.4.2 Modulus of Elasticity

2.4.2.1 The modulus of elasticity, Ec , for concrete may be taken as 15,200 f’c( kg/cm2 ).

F’c (kg/cm2

) Modulus of ElasticityEc (kg/cm

2)

180 2.04 x 105

210 2.20 x 105

240 2.35 x 105

2.4.2.2 For non-prestressed steel reinforcement, the modulus of elasticity,Es may

Be taken as 2.1 x 106 (kg/cm

2).

2.4.2.3 The modular ratio of elasticity (n) shall be computed as :

72



2.5 Detailing of Concrete and Reinforcement

2.5.1 Minimum Wall Thickness

Cantilever concrete wall shall have a minimum thickness of 8 cms. per meter height up to

2.5 m. height, subject to a minimum of 12.5 cms. For walls more than 2.5 m. height, base thickness

shall be 20 cms. plus 6 cms. Per meter height in excess of 2.5 m. No wall with two layers reinforcementshall be less than 17 cm. thick.

2.5.2 Minimum Temperature Reinforcement

The following rules may be used to determine the cross sectional area of temperature or

nominal reinforcement required. The percentage indicated are based upon the cross section of the area

(excluding fillets) of the concrete to be reinforced where the thickness of the section exceeds 38 cm, athickness of 38 cm should be used in determining the temperature reinforcement.

a. Temperature reinforcement shall not be less than 10 mm at 30 cm in

exposed faces for single layer reinforcement nor less than 10 mm Ǿ at 45 cmin the unexposed face.

b. Single layer reinforcement

1. Reinforcement lining 10 cm. and less in thickness with discontinuous wire

fabric reinforcement and weakened planes at 3.70m to 4.60m centers, p = .001 or.10%

2. Slabs and linings not exposed to freezing temperature or direct sun with joints not

exceeding 9m, p = .0025 or .25%3. Slabs and linings exposed to freezing temperature or direct sun with joints not

exceeding 9m, p = .003 or .30%4. Slabs and lining as above with joints exceeding 9m.

Category 2 above p = .0035 or .35%

Category 3 above p = .40%

c. Double Layer Reinforcement

1. Face adjacent to earth with joints not exceeding 9m, p = .001 or .10%

2. Face not adjacent to earth not exposed to freezing temperature or direct sun andwith joints exceeding 9m, p = .0015 or .15%

3. Face not adjacent to earth but exposed to freezing temperature or direct sun and

with joints not exceeding 9m, p = .002 or .20%

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Agrohydrology Manual