SWIP Manual Part 1

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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

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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 2 1.4 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 10 2.2 Computation of 80% Dependable Rainfall 10 2.3 Crop Coefficient and Crop Rooting Depth 11

2.4 Percolation Loss 11

2.5 Soil Water Holding Capacity 14

2.6 Actual Evapotranspiration 14

2.7 Change in Storage 14

2.8 Initial Storage 14

2.9 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

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LIST OF TABLES

TABLE NO. PAGE NO.

Regression Coefficients of the Rainfall Intensity-Duration

Frequency 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 Condition 7

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 Baseflow

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LIST OF FIGURES

FIGURE NO. TITLE PAGE NO.

1 Rearrangement of Rainfall Increments 5

1 Water Management Scheme and Crop Depending Variables

for Field Water Balance for Irrigated Wetland Rice. 12 2 Crop Depending Variables for Field Water balance

of Irrigated Corn. 12

4 Crop Depending Variables for Field Water balance

of Irrigated Mungo. 13

of Irrigated Tomato. 13

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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 inflow hydrograph.

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, and Snyder’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 gradient

a = 0.38

Ct = coefficient with values (Linsley’s): 1.2 for mountatins drainage area 0.72 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

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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 where

i = 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 Depth Seq. No. D, Hr I, min/hr. P, mm 1 D1 = ∆D 1 P1 2 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

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________________________________________________________________________ Seq. No. 1. ∆P1 = P1 ∆P14 2. ∆P2 = P2 - P1 ∆P13 3. ∆P3 = P3 – P2 ∆P12 4. ∆P4 = P4 – P3 ∆P10 5. ∆P5 = P5 – P4 ∆P9 6. ∆P6 = P6 – P5 ∆P7 7. ∆P7 = P7 – P6 ∆P6 8. ∆P8 = P8 – P7 ∆P5 9. ∆P9 = P9 – P8 ∆P3 10. ∆P10 = P10 – P9 ∆P2 11. ∆P11 = P11 – P10 ∆P1 12. ∆P12 = P12 – P11 ∆P4 13. ∆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 inches W = 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 AMC III).

- 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, Applicable values are given in Table 6.

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TABLE 1 Regression Coefficients if the Rainfall intensity, f (mm/hr) – Duration, t (hr) Frequency, T Curve for Different Locations: General Equation: i = aT C

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(t+b)d

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

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Note: _{If b -} _{Ø the} resulting rainfall intensity- duration-frequency curves are straight lines (plotted on log, log chart). 4

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5

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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)

Land Use or Cover Farming Hydrologic SOIL GROUP

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 Good 30 60 70 80 pastures Rotation pastures Straightro w Poor 65 75 85 90 Good 60 70 80 85 Contoured Poor 65 75 80 85 Good 55 70 80 85 Crop Straightro w 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)

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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

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 98 85 70 97 80 65 95 75 60 90 70 50 90 65 45 85 60 40 80 55 35 75 50 30 70 45 25 65 40 20 60 35 20 55 30 15 50 25 10 45 7

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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 0.04

1.6 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 qp

Where: 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 U 1 ∆D

A

-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 U 1 ∆D

A -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.)

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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 0.018 1.1 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 = U1 E1 Q2 = U1 E2 + U2 E1 Q3 = U1 E3 + U2 E2 + U3 E1 Q4 = U1 E4 + U2 E3 + U3 E2 + U4 E1 etc. \ 9

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2. FIELD WATER BALANCE COMPUTATION

Establish the best cropping pattern and cropping calendar with the following objectives:

a) minimum irrigation requirements; b) maximum annual production; c) optimum growing

conditions for the given crop and growing stages: d) grow paddy rice during wet season when water

abundant and irrigation is minimal.

Fill the column for the rainfall (rain) with 80% dependability computed using the two parameter

log-normal distribution and the average potential evapotranspiration (PET). To compute for 80% dependable for a

given site the following procedures are to be considered:

a. Collect day rainfall data, defined as the sums of daily rainfall over each defined

10-day period and arrange them as follows: Y e a r

Month Decade 1 2 3 . . . N Mean

Std. Dev. Jan. 1 - - - . . . - - -2 - - - . . . - - -3 - - - . . . - - -Feb. 4 - - - . . . - - -5 - - - . . . - - -6 - - - . . . - - -Dec. 34 - - - . . . - - -35 - - - . . . - - -36 - - - . . . - -

-b. Compute the mean of 10 – day rainfall for all decades

K = 1, 2, 3 . . . 36

N

XK = 1

Σ X

_Ki

N i =1

Where XK = mean of 10 – day rainfall in decade K

X

_Ki = 10 – day rainfall data in decade K and year 1

N

= number of recorded observation in decade K in years.

c. Compute the standard deviation of 10 – day rainfall for decades

K = 1, 2, 3 . . . 36 __

SK = 1

Σ

(XKi - X )2

N-1 i=1

Where SK = standard deviation of 10 – day rainfall in decade K

d. Compute the coefficient of variation of 10 – day rainfall for all decades K = 1, 2, 3, ….36

10

e. Compute the standard normal deviation corresponding to

an axceedance probability, p of 80 %, tp, for p = 80% tp = -0.831

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

Where: B = Ln ( 1 + Z2 ₎

KK = frequency factor in decade K

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

_

RK = XK + SK KK

Where: RK = 10 – day rainfall at 80% dependability

h. Tabulate the results as follows:

Month Decade XK SK ZK KK RK 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 establishment of 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.

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TABLE 8: Percolation For Different Soil Types

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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 Field

Collecting 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 13 Maximum water 200 80 80 80 80 80 80 80 80 80 80 10 0 depth in paddy, mm Minimum water 10 20 20 20 20 20 20 20 20 20 20 0 0 depth, mm Optimum water 100 65 65 50 50 50 50 50 45 45 45 0 0 depth, mm

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

Rainfall

Collection Crop 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 0.7 5 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

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Rainfall Collection

& Crop in the Field

Land Preparation (80 Days)

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

Land Preparation (80 Days)

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

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

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& Crop in the Field Land Preparation (100 Days)

LP 1 2 3 4 5 6 7 8 9 10 LP 1 2 3 4 5 6 7 8 9 10 Crop Coefficient 0.4 0 0.7 0 0.7 0 0.9 5 0.9 5 0.9 5 0.7 5 0.7 5 0.7 5 0.5 5 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 to AET = PET x KC

2.7 Change in storage (ΔSTOR) is equal to

STOR = 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 + ΔSTOR If STORi > allowable max storage

Then DRAINAGE = STORi – allowable max storage STORi = allowable max storage

IRRIGATION = Ø. Ø

If STORi < allowable minimum storage

Then IRRIGATION = Optimum Storage – STORi STORi = Optimum Storage

Drainage = Ø. Ø ELSE

IRRIGATION = Ø. Ø DRAINAGE = Ø. Ø

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Note: For upland crops, allowable min. soil moisture storage is usually assumed to 50% of soil water

holding capacities in the root zone, that is 0.54 (WHC) (ROODEP). Do not irrigate during the last two decade of a given period.

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2.10 Use an irrigation efficiency if 51% for paddy rice (lowland) and 54% for upland crops to the estimated net crop irrigation to get an estimate of system irrigation requirements.

TABLE 9 Soil Water Holding Capacities of Different Soil Textural Classes:

Soil Texture

Total Available Moisture Pv =Pw As Volume% Sandy 8 (6-10) Sandy Loam 12 (9-15) Loam 17 (14-20) Clay Loam 19 (16-22) Silty Clay 21 (18-23) Clay 23 (20-25) 15

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

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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) DQj = direct runoff in decade j (mm) BFj = baseflow 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)

DQj = direct runoff in decade j (mm) BFj = baseflow in decade j (mm)

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TABLE 10 Regional Run – off Coefficient and % Monthly Baseflow Distribution:

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Month Run - off Coefficient, RC Jan. 0.25 Feb. 0.05 Mar. 0.03 Apr. 0.03 May 0.17 June 0.37 July 0.64 Aug. 0.67 Sept. 0.75 Oct. 0.75 Nov. 0.61 Dec. 0.25 Region 2

Month %Baseflow Run - off Coefficient, RC

Jan. 8.76 0.17 Feb. 7.91 0.17 Mar. 7.22 0.08 Apr. 7.05 0.08 May 6.7 0 June 6.42 0.17 July 7.39 0.2 Aug. 8.18 0.34 Sept. 9.37 0.4 Oct. 10.43 0.41 Nov. 10.84 0.44 Dec. 9.72 0.37

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

a = 286.021 b = -9.72 x 10-1 R = 0.74

17 Region 3

Month Run - off Coefficient, RC

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Feb. 0.08 Mar. 0 Apr. 0 May 0.24 June 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

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 0.15 Sept. 8.31 0.15 Oct. 8.53 0.35 Nov. 8.79 0.39 Dec. 9.07 0.47

a = 2, 057.31 b = 18.28 R = 0.87 18

Region 6

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a = 1, 043.65 b = 8.221 R = 0.695

Region 7

Jan. 8.23 0.26 Feb. 8.07 0.15 Mar. 8.09 0.1 Apr. 8.22 0 May 8.23 0.09 June 8.35 0.15 July 8.47 0.3 Aug. 8.66 0.3 Sept. 8.57 0.3 Oct. 8.45 0.3 Nov. 8.37 0.3 Dec. 8.29 0.26

a = 1, 055.85 b = 11.80 R = 0.766

19 Region 8

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Jan. 9.1 0.38 Feb. 8.8 0.28 Mar. 8.6 0.25 Apr. 8.3 0 May 8.1 0.14 June 7.9 0.22 July 7.7 0.3 Aug. 7.6 0.34 Sept. 7.7 0.34 Oct. 7.9 0.51 Nov. 8.4 0.7 Dec. 9 0.7

a = 12.52 b = 14.051 R = 0.872

Region 9

Jan. 8.53 0.3 Feb. 8.33 0.22 Mar. 8.16 0.08 Apr. 7.94 0 May 8 0 June 8.13 0.07 July 8.19 0.14 Aug. 8.32 0.14 Sept. 8.42 0.14 Oct. 8.53 0.24 Nov. 8.66 0.24 Dec. 8.76 0.3

a = 1, 164.37 b = 30.36 R = 0.999

20 Region 10

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a = 2, 119.90 b = 6.09 R = 0.562

Region 11

Jan. 8.42 0.17 Feb. 8.38 0 Mar. 8.35 0 Apr. 8.31 0 May 8.3 0.12 June 8.25 0.12 July 8.27 0.29 Aug. 8.3 0.29 Sept. 8.32 0.26 Oct. 8.34 0.26 Nov. 8.37 0.23 Dec. 8.39 0.22

a = 152.608 b = 7.53 R = 0.751

21 Region 12

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Jan. 8.13 0.21 Feb. 7.99 0.12 Mar. 8.03 0 Apr. 8.13 0.13 May 8.24 0.25 June 8.39 0.35 July 8.54 0.44 Aug. 8.69 0.45 Sept. 8.66 0.45 Oct. 8.53 0.45 Nov. 8.4 0.21 Dec. 8.26 0.21

a = 1, 751.61 b = -4.018 R = 0.915

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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 VALLEY

Cagayan, 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 inslands of 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

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26 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 PERIOD

Rice:

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 - June

Cadios 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 - December

Lettuce August - January Carrot October - December

Mustard August - January Potato(Irish) October - December

Pechay October - December Talinum May - June

October - December

Fruit: Kutchai October - December

Ampalaya May - July Arrowroot May - June

October - January Tapilan May - June

Cucumber May - June September - October

September - December Beets October - January

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September - February Endive September - October

Melon October - January Snap Bean October - December

28

PLANTING CALENDAR PLANTING CALENDAR FOR TYPE 2 CLIMATE

NO DRY SEASON with a very PRONOUNCED MAXIMUM RAINFALL from November to January. The areas covered are Catanduanes, Sorsogon, Eastern part of Albay, the Eastern and Northern parts of Camarines Norte and Camarines Sur, a great portion of the Eastern part of Quezon, the Eastern part of Leyte and a large portion of Eastern Mindanao.

Rice: Fruit:

Lowland October - December Ampalaya June - August

Palagad May - July November - Febraury

Upand June - August Condol January - March

September - November Cucumber March - April

Eggplant January - April

Corn: August - September

Dry season March - May Melon(ordinary) March - June

Rainy season January - February Muskmelon March - June

August - September Okra Whole year

Patola March - September

Peanut: Squash Whole year

Dry season Janury - Febraury Tomato January - April

August - September August - September

Rainy season May - June Upo November - March

Watermelon January - March

Beans: Root:

Batao Febraury - April Camote Year Round

Cowpea or Kibal January March Carrot March - April

May - July Cassava Year Round

November - December Gabi Year Round

Cadios Febraury - March Ginger Year Round

Bountiful Bean January - May Raddish November - December

Mungo Febraury - June March - May

Patani(climbing) January - May Ubi Year Round

Seguidillas Febraury - April

Sitao May - June Others:

Soybean January - March Irish Potato February - March

Tapilan January - March Endive December - March

August October Onion December - March

Garlic November - December

Vegetables: Sinkamas October - November

Leafy: Sweet Pepper February - March

Cabbage January - March August - September

Celery January - March Chayote February - March

Kutchai March - July Arrowroot June - September

Lettuce March - June Beet January - March

Pechay January - March Peas February - March

Cauliflower January - March Jute January - March

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Spinach January - March November - December

29

Seasons are not pronounced, relatively DRY from November to April and WET during the rest of the year. This type of climate covers the Western part of Cagayan(Luzon), Isabela, Nueva Vizcaya, the Eastern portion of the Mountain Province, Southern Quezon, the Bondoc Peninsula, Masbate, Romblon, Northeast Panay, Eastern Negros, Central and Southern Cebu, part of Northern Mindanao, and most of Eastern Palawan.

Rice:

Lowland June - August Mustard May - July

Palagad November - January October - December

Upand April - June Pechay May - June

October - December

Corn: Spinach May - June

Dry season October - December October - December

Rainy season April - June

Third Crop December - February Fruit:

Ampalaya May - June

Peanut: November - December

Dry season September October Chayote May - June

Rainy season April - June November - January

Third Crop December - January Cucumber May - June

October - January

Beans: Eggplant May - June

Batao May - June November - January

Bountiful Bean May - June Melon(ordinary) May - June

November - January October - January

Cowpea or Kibal May - June Muskmelon November - January

November - December Okra May - July

Kadios May - June October - December

October - November Patola May - July

Mungo December - January October - January

September - October Squash May - June

Patani May - June October - December

(climbing) November - December Sweet Pepper May - June

Seguidillas May - June October - December

Sitao May - June Tomato October - January

November - January Upo April - May

Soybean May - June October - January

October - December Condol June - July

Tapilan May - June November - January

November - December Watermelon October - January

Peas April June

November - January Root:

Vegetables: Sweet Potato April - June

Leafy: November - January

Cabbage April - June Carrot October - December

October - December Gabi May - July

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Celery May - July Garlic October December

October - December Ginger October December

Lettuce April - May November - December

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RAINFALL more or less evenly distributed throughout the year.

The areas covered by Type 4 climate are Batanes Province, Northeastern Luzon, Western Camarines Norte and Camarines Sur, Albay, Eastern Mindoro, Marinduque, Western Leyte, Northern Negros, and most of Central, Eastern, and Southern Mindanao.

Rice:

Lowland May - July Lettuce May - June

August - October January - February

Palagad November - January Mustard June - July

Upand April - June September - January

Corn: Pechay May - July

Dry season September - November November - January

Rainy season April - June Spinach April - May

Third Crop November - February Fruit:

Ampalaya May - June

Peanut: September - January

Dry season September - November Chayote May - June

Rainy season May - June November - December

Third Crop November - February May - June

Beans: October - December

Batao May - June Cucumber June - July

Bountiful Bean May - June October - December

October - December Eggplant June - July

Cowpea or Kibal May - June November - January

October - December Melon November - January

Kadios May - July Muskmelon November - January

Mungo May - June Okra June - July

November - January September - October

Patani May - June January - February

November - January Patola May - June

Seguidillas May - June December - January

Sitao May - June Squash May - June

October - January November - January

Soybean May - June Sweet Pepper May - June

November - January September - January

Tapilan May - June Tomato May - June

November - December October - January

Peas June - July Upo May - June

December - January October - January

Vegetables: Watermelon April - May

Leafy: November - January

Cabbage June - September Root:

October - January Camote May - June

Cauliflower April - July September - November

September - January Carrot May - June

Celery June - July November - January

January - Febraury Gabi June - September

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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 1

2.1.2 Active Storage 3

2.1.3 Flood Surcharge 3

2.1.4 Freeboard 6

2.1.5 Outline of Dam Height Computation 7

2.2 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 9

2.4 Embankment Slopes 11

2.5 Seepage Through Earth Embankment 13

2.5.1 Seepage Line 13

2.5.2 Position of Seepage Line 13

2.5.3 Quantity of Seepage 13

2.5.4 Filter Design 21

2.6 Embankment Slope Protection 22

2.6.1 Upstream Slope 22

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Section Title Page

3.0 SPILLWAY 24

3.1 General 24

3.2 Spillway Type and Alignment 24

3.3 Spillway Hydraulics 24 3.3.1 Control Section 25 3.3.2 Discharge Channel 25 3.3.3 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 44

4.3.1 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 51

5.1 General 51

5.2 Canal Layout and Profile 51

5.3 Canal Hydraulics 51

5.3.1 Slide Slopes 51

5.3.2 Permissible Velocity 52

5.3.3 Applicable Formula for Sizing of Canal 52

5.3.4 Freeboard 53

5.4 Design of Canal Structures 53

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LIST OF TABLES

Table No. Title Page

1 Outline of Dam Height and Dam Crest 8

2 Embankment Slopes for Homogeneous Dams 14

3 Embankment Slopes for Zoned Dams 15

4 Permissible Velocities for Non-Cohesive Soils 27

5 Permissible Velocities for Grassed Channel 28

6 Outline of USBR Basin Computations Format 46

7 Cantilever Retaining Wall Parameters 41

8 Discharge Pipe Computations Format 46

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LIST OF FIGURES

Figure No. Title Page

1 Reservoir Storage Allocations 2

2 Reservoir Operation Studies Format and Flow Chart 4

3 Flood Routing Format and Flow Chart 5

4 Modified Homogeneous Dam Sections 10

5 Size of Impervious Core of Zoned Dam 12

6 Slope Stability Chart No. 1 16

9a Elements of Seepage Line 19

9b Diagrams for Determining ∆a and a 20

10 Flow Profile Along Spillway 30

11 Unsubmerged Deflector Bucket 32

12 Type IV USBR Basin 33

13 Type III USBR Basin 34

14 Type II USBR Basin 35

15 Hydraulic Jump Nomograph

(Stilling basin Depth Vs Hydraulic Head

for Various Channel Losses) 39

16 Typical Chute and Stilling Basin Section 42

17 Typical Outlet Works System 45

18 Impact Type Energy Dissipator 49

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ENGINNERING DESIGN

1.0 GENERAL

For the Water Impounding Component of the Rainfed Project, the earth embankment dam type (homogeneous or zoned type) is considered to be more cost effective over concrete or other types of dam. The dam embankment volumes, consisting of natural earth materials, are relatively small and are available at or in the vicinity of the project site. These materials are soil and rock in their many varied forms.

Included in this section are the procedures, criteria and assumptions used in the design of a small earth dam and its appurtenances.

Also included in the later part of this section are the procedures, criteria and assumptions in the design of irrigation works consisting of canals and canal structures as well as access roads to complete the coverage on the physical structural component of the project.

In the procedures and assumptions that follow, it is assumed that dam location, necessary site investigations as well as prerequisites studies on geology, hydrology, etc., have already been undertaken.

2.0 DAM

2.1 Determination of dam Height

In general, the height of the dam is determined on the basis of the following vertical space requirements in the reservoir.

a. Dead or Inactive Storage Space b. Active Storage Space

c. Flood Surcharge d. Freeboard e. Settlement

Space allocations of each of the above items are illustrated in Figure 1. 2.1.1 Dead or Inactive Storage

The number of years for sediment to fill up the dead storage space plus about 20% of the live storage is termed as the expected “economic life” of the project. This time magnitude is an agency policy decision.

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Unless amended later, sediment volume shall be computed on the basis of 25 years of

accumulation in the reservoir. This volume shall be allocated in the dead storage space as shown in Figure 1.

2.1.2 Active Storage

The active storage is allocated primarily for irrigation purposes. This space is determined from reservoir operation studies.

Reservoir operation study basically “water accounting”. No clear-cut formula is involved but the basic principle is to optimize reservoir to meet water requirement.

The study involves trial runs for different hectareage of service area until maximum area is attained with minimum reservoir spill or shortage.

Among the data and assumptions needed to undertake the reservoir operation study are the following:

a. Reservoir inflow

b. Reservoir evaporation loss c. Water requirements

d. Reservoir area-capacity-elevation curves.

e. Reservoir elevation at the end of the operation must be equal to the starting elevation. Items a, b and c are obtained from the results of Hydrologic Studies. Item d is derived from a reservoir topographic map.

Shown in Figure 2 are the typical format and detailed flow chart for reservoir operation studies. 2.1.3 Flood Surcharge

Flood surcharge space is allocated for the design flood.

Maximum surcharge height is the difference between maximum and normal water surface. It is dependent on three factors namely;

a. Spillway size opening.

b. Reservoir capacity-elevation relationship. c. Magnitude and shape of the inflow hydrograph. Flood surcharge height is estimated by flood routing.

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4

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There are a number of methods for flood routing but the basic formula is:

I = O + S --- 1 Where; I = inflow volume

O = outflow volume S = change in storage

A simple and expedient method of flood routing is by arithmetic trial and error. Shown in Figure 3 are format and detailed flowchart for such method.

In this all other methods of flood routing, it is assumed that all outlets are fully closed and all discharges are allowed to pass only over the spillway. Moreover, water surface in the reservoir is at normal level at the start of the flood.

The data required to undertake flood routing computations are the following: a. Hydrograph of inflow design flood.

b. Reservoir capacity-elevation curve.

c. Spillway rating curve or equation given by the following formula for a broad-crested weir:

Q = CLH3/2_{--- 2} Where: Q = discharge over the spillway

C = weir coefficient; 1.704 metric H = surcharge height

L = spillway width 2.1.4 Freeboard

Freeboard space is provided against wave splash along the upstream face of the dam, which may coincide with occurrence of the design flood as well as embankment settlement. It is estimated by the following formula:

For vertical wall

Fb1 = ---3

Fb2 = 2% to 5 % of dam height --- 4

Fb = Fb1 + Fb2 ---5

Where: F b1= freeboard due to wave run-up, m

F = reservoir effective fetch, km V = wind velocity, km/hr

Fb2 = freeboard due to embankment settlement, m

Fb = total freeboard, m