ERA manual for design of storm sewer

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Chapter 5

Drainage Design Manual - 2002 Hydrology

5.1 DEFINITIONS AND SYMBOLSHydrology is generally defined as a science dealing with the interrelationship between

water on and under the earth and in the atmosphere. For the purpose of this manual,

hydrology will deal with estimating flood magnitudes as the result of precipitation. In the

design of highway drainage structures, floods are usually considered in terms of peak

runoff or discharge in cubic meters per second (m3/s) and hydrographs as discharge per

time. For structures that are designed to control volume of runoff, like detention storage

facilities, or where flood routing through culverts is used, then the entire discharge

hydrograph will be of interest.

The following are concepts that are important in a hydrologic analysis. These concepts

will be used throughout the remainder of this chapter in dealing with different aspects of

hydrologic studies:

The soil moisture conditions of the catchment area at the beginning

of a storm. These conditions affect the volume of runoff generated

by a particular storm event. Notably they affect the peak discharge

only in the lower range of flood magnitudes approx. below the 15-

year event threshold. As floods become more rare, antecedent

moisture has a rapidly decreasing influence on runoff.

The natural depressions within a catchment area that store runoff.

Generally after the depression storage is filled, runoff will begin.

The number of times a flood of a given magnitude can be expectedto occur on average over a long period of time. Frequency analysis

is the estimation of peak discharges for various recurrence intervals.

Another way to express frequency is with probability. Probability

analysis seeks to define the flood flow with a probability of being

equaled or exceeded in any year.

A composite of the physical characteristics that influence the flow

of water across the earth's surface, whether natural or channelized. It

affects both the time response of a catchment area and drainage

channel, as well as the channel storage characteristics.

A graph of the time distribution of runoff from a catchment area.

A graph of the time distribution of rainfall over a catchment area.

A complex process of allowing runoff to penetrate the ground

surface and flow through the upper soil surface. The infiltration

curve is a graph of the time distribution at which this occurs.


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Hydrology Drainage Design Manual - 2002

The storage of rainfall on foliage and other intercepting surfaces

during a rainfall event is called interception storage.

The time from the centroid of the excess rainfall to the peak of thehydrograph.

Sometimes called peak flow. The maximum rate of flow of waterpassing a given point during or after a rainfall event.

The water available to runoff after interception, depression storage

and infiltration have been satisfied.

The elevation of the water surface above some elevation datum.

The time it takes a drop of water falling on the most remote point

hydraulically in the catchment area to travel through the catchment

area to the outlet.

The direct runoff hydrograph resulting from a rainfall event that has

a specific temporal and spatial distribution and which lasts for a unit

duration of time. The ordinates of the unit hydrograph are such that

the volume of direct runoff represented by the area under the

hydrograph is equal to one millimeter of runoff from the catchment

area.

To provide consistency within this chapter, as well as throughout this manual, thefollowing symbols will be used. These symbols were selected because of their wide use

in hydrologic publications.

Symbol Definition Units

A Catchment area hectares, sq.km.

BDF Basin development factor %

C Runoff coefficient -

Cf Frequency factor -

CN SCS-runoff curve number -

Ct, Cp Physiographic coefficients -d Time interval hours

DH Difference in elevation m

I Runoff intensity mm/hr

IA Percentage of impervious area %

Ia Initial abstraction from total rainfall mm

K Frequency factor for a particular return period and skew -

L Lag hours

l Length of mainstream to furthest divide m

Lca Length along main channel to a point opposite the

catchment area centroid km

M Rank of a flood within a long record -


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n Manning roughness coefficient -

N Number of years of flood record years

P Accumulated rainfall mm

Q Rate of runoff m3/s

q Storm runoff during a time interval mm

R Hydraulic radius m

RC Regression constant -RQ Equivalent rural peak runoff rate m

3/s

S or Y Ground slope m/m, m/km or %

S Potential maximum retention storage mm

SCS Soil Conservation Service -

SL Main channel slope m/m

SL Standard deviation of logarithms of peak annual floods -

ST Basin storage factor %

TB Time base of unit hydrograph hours

tcor Tc Time of concentration min or hours

TL Lag time hours

Tr Snyders duration of excess rainfall hoursUQ Urban peak runoff rate m

3/s

V Velocity m/s

X Logarithm of the annual peak -

5.2 HYDROLOGIC DESIGN PRINCIPLES from Standards defines the general principles for

hydrological and drainage design in accordance with this manual. The hydrological data

available for Ethiopia is generally limited so the procedures that can be applied are

consequently imprecise. No specific standards or definitive criteria for hydrological

analysis are suitable for recommendation at this time. For standard procedures to be

adopted confidently, storm water runoff coefficients and procedures shall be calibrated.

Since hydrologic considerations can influence the selection of a highway corridor and the

alternate routes within the corridor, studies and investigations shall be undertaken at the

Planning Stage (see ). Also, special studies and investigations maybe required at sensitive locations. The magnitude and complexity of these studies shall be

commensurate with the importance and magnitude of the project and problems

encountered. Typical data to be included in such surveys or studies are: topographic

maps; aerial photographs; streamflow records; historical highwater elevations; flooddischarges; and locations of hydraulic features such as reservoirs, water projects, and

designated or regulatory floodplain areas.

A hydrologic analysis is prerequisite to identifying flood hazard areas and determining

those locations where construction and maintenance will be unusually expensive or

hazardous. Since many levels of government plan, design, and construct highway and

water resource projects that might affect each other, interagency coordination is desirable

and often necessary. In addition, agencies can share data and experiences within project

areas to assist in the completion of accurate hydrologic analysis.


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All proposed structures are sized using the specified design frequency as provided in

Table 2-1 of . The following text

further develops the choices presented in that table.

A design frequency shall be selected to match the facilitys cost, amount of traffic,

potential flood hazard to property, expected level of service, political considerations, and

budgetary constraints, considering the magnitude and risk associated with damages from

larger flood events. With long highway routes having no practical detour, where many

sites are subject to independent flood events, it may be necessary to increase the design

frequency at each site to avoid frequent route interruptions from floods. In selecting a

design frequency, potential upstream land use that could reasonably occur over the

anticipated life of the drainage facility shall be considered.

Hydrologic analysis should include the determination of several design flood frequencies

for use in the hydraulic design. These frequencies are used to size different drainage

structures to allow for an optimum design, that considers both risk of damage andconstruction cost. Consideration shall be given to what frequency flood was used to

design other structures along a highway corridor.

Since it is not economically feasible to design a structure for the maximum runoff a

catchment area is capable of producing, a design frequency must be established. The

frequency with which a given flood can be expected to occur is the reciprocal of the

probability or chance that the flood will be equaled or exceeded in a given year. If a

flood has a 20 percent chance of being equaled or exceeded each year, over a long period

of time, the flood will be equaled or exceeded on an average of once every five years.

This is called the (RI). Thus the exceedence

probability equals 100/RI.

The designer should note that the 5-year flood is not one that will necessarily be equaled

or exceeded every five years. There is a 20 percent chance that the flood will be equaled

or exceeded in any year; therefore, the 5-year flood could conceivably occur in several

consecutive years. The same reasoning applies to floods with other return periods.

Cross Drainage:A drainage facility shall be designed to accommodate a discharge with a

given return period(s) for the following circumstances. The design shall be such that the

backwater (the headwater) caused by the structure for the design storm does not:

increase the flood hazard significantly for property; overtop the highway; or exceed a certain depth on the highway embankment.Based on these design criteria, a design involving roadway overtopping of short duration

for floods larger than the design event is acceptable practice. Usually, if overtopping is

allowed, the structure may be designed to accommodate a flood of some lower frequency

without overtopping.

Storm Drains:A storm drain shall be designed to accommodate a discharge with a given

return period(s) for the following circumstances. The design shall be such that the storm

runoff does not:


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increase the flood hazard significantly for property; encroach on to the street or highway so as to cause a significant traffic hazard; or limit traffic, emerging vehicle, or pedestrian movement to an unreasonable extent.Based on these design criteria, a design involving a street or road inundation of short

duration for floods larger than the design event is an acceptable practice.

After sizing a drainage facility using a flood (and the hydrograph) corresponding to the

design frequency, it is necessary to review this proposed facility with a base discharge.

This is done to insure that there are no unexpected flood hazards inherent in the proposed

facility(ies). The review (check) flood shall be at least the 100-year event, or as provided

in , Table 2-1. In some cases, a flood event larger than the specified review

flood might be used for analysis to ensure the safety of the drainage structure and

downstream development.

Certain hydrologic procedures use rainfall and rainfall frequency as the basic input rather

than flood frequency. It is commonly assumed that the 10-year rainfall will produce the10-year flood. Depending on antecedent soil moisture conditions, and other hydrologic

parameters, there may not be a direct relationship between rainfall and flood frequency.

5.3 HYDROLOGY

The analysis of the peak rate of runoff, volume of runoff, and time distribution of flow is

fundamental to the design of drainage structures. Errors in the estimates will result in a

structure that is either undersized and causes more drainage problems or oversized and

costs more than necessary. On the other hand, it must be realized that any hydrologic

analysis is only an approximation. The relationship between the amount of precipitationon a drainage basin and the amount of runoff from the basin is complex, and too little

data are available on the factors influencing the rural and urban rainfall-runoff

relationship to expect exact solutions.

In the hydrologic analysis for a drainage structure, it must be recognized that there are

many variable factors that affect floods. Some of the factors that need to be recognized

and considered on an individual site by site basis are:

rainfall amount and storm distribution; catchment area size, shape and orientation; ground cover; type of soil; slopes of terrain and stream(s); antecedent moisture condition; storage potential (overbank, ponds, wetlands, reservoirs, channel, etc.); and catchment area development potential.

The type and source of information available for hydrologic analysis will vary from site

to site and it is the responsibility of the designer to determine what information is


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available and applicable to a particular analysis. A comprehensive list of data sources is

included in of this manual.

5.4 HYDROLOGIC ANALYSIS PROCEDURE FLOWCHARTThe hydrologic analysis procedure flowchart Figure 5-1 shows the steps needed for the

hydrologic analysis and the designs that will use the hydrologic estimates.


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5.5 RAINFALL CURVESRainfall data have been collected from many Ministry of Water Resources meteorology

stations in the country (see Table 5-2). They have been subjected to statistical techniques

to develop the information needed from hydrologic analyses. The results indicate that the

country can be divided into several hydrological regions which display similar rainfall

patterns, as indicated on the map in Figure 5-8 at the end of this chapter. Using thestatistical analyses, rainfall intensity-duration curves have been developed for commonly

used design frequencies. Figures 5-9 through 5-12 at the end of this chapter show the

curves presently available.

Meteorological

Region

Station Years

of

Record

Meteorological

Region

Station Years

of

Record

A1 Axum 18 B Bedele 19

Mekele 35 Gore 45Maychew 24 Nekempte 27

A2 Gondar 40 Jima 45

Debre Tabor 22 Arba Minch 11

Bahir Dar 35 Sodo 28

Debre Markos 44 Awasa 26

Fitche 25 C Kombolcha 46

Addis Ababa 33 Woldiya 23

A3 Nazareth 40 Sirinka 17

Kulumsa 31 D1 Gode 29*

Robe/Bale 19 Kebri Dihar 38

A4 Metehara 28 D2 Kibre Mengist 24

Dire Dawa 46 Negele 45

Mieso 35 Moyale 18

* max 24 hour rainfall not given Yabelo 34

Years of record through 1997

The rainfall data available is too sparse to develop highly accurate intensity-duration-

frequency curves. The 24-hour rainfall depth records were generally adequate to projectthe frequency of 24-hour rainfall depths. Based on the monthly rainfall depths and

patterns, the country was divided into regions and sub-regions.

The 24-hour depth frequency data for each sub-region was analyzed for each station in

the sub-region. Regression analysis was used to develop depth-frequency curves for each

sub-region. The curves were selected so that approximately 75% of the data lay BELOW

the selected design curve. This means that the total 24-hour rainfall estimated for a

particular region will be greater on average than the rainfall depth that might actually

occur, and that these rainfall curves are reasonably conservative estimates of rainfall

depths. It is recommended that hydraulic engineers using these curves do not also take


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conservative estimates of the other independent variables used in runoff estimation as

this will likely lead to excessively conservative and uneconomical designs.

The amount of data available for shorter duration storms was too sparse for the

development of intensity-duration-frequency curves, and was insufficient to do a

frequency distribution plot for each rainfall period. In order to develop intensity-duration-

frequency curves for each rainfall region, the ratios of the short duration data availablewere compared to the 24-hour data. Based on this comparison and making similar

comparisons for published rainfall data from other regions, principally the United States,

it seemed that reasonable estimations of rainfall depths occurring in shorter periods could

be expressed as a fraction of the 24-hour rainfall depth.

Many recommendations for depth-duration-frequency curves in the technical literature

suggest a "broken-leg" approach such that the depth duration frequency equation for

shorter duration rainfalls, less than one hour, is different from that derived for longer

duration rainfalls. Because of the scarcity of data this approach was not taken and one

curve was developed. The amount of rainfall data obtained for peak rainfall intensities of

periods shorter than one-half hour was too limited to be useful. The curves presented aresatisfactory for rainfall durations of one-half hour or more. Intensities for periods shorter

than 15 minutes appear to be overestimated by the curves presented.

It is recommended in this manual for the design of most drainage structures that the

minimum time of concentration be taken as fifteen minutes. The design of gutters and

inlets may be based on shorter rainfall durations, but this isnt serious conservatism. The

overall drainage system - drainage conduit - will usually be designed for a storm duration

of nearly 15 minutes or more, thus the most expensive part of the drainage system will

not be unnecessarily over-designed.

Relevant background information collected during the course of the preparation of thismanual is presented in

5.6 HYDROLOGIC PROCEDURE

Streamflow measurements for determining a flood frequency relationship at a site are

usually unavailable. In such cases, it is an accepted practice to estimate peak runoff rates

and hydrographs using statistical or empirical methods. In general, results from using

several methods shall be compared, not averaged. Standard practice is to use the

discharge that best reflects local project conditions with the reasons documented. Use isoutlined with each hydrologic procedure given below.

The methods presented in this manual were selected to be consistent with the methods

available in the computer program HYDRAIN Integrated Drainage Design Computer

System and HEC 1. The hydrologic model within the HYDRAIN system is called

HYDRO and this model allows the user to select among several hydrologic procedures.

The possibilities generally applicable to available data for Ethiopia include:


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storm cells are relatively small with extreme intensity variations thus making the

Rational Method inappropriate for catchment areas greater than 50 hectares.

(2) The frequency of peak discharges is the same as that of the rainfall intensity for thegiven time of concentration.

Frequencies of peak discharges depend on rainfall frequencies, antecedent moisture

conditions in the catchment area, and the response characteristics of the drainage system.

For small and largely impervious areas, rainfall frequency is the dominant factor. For

larger drainage basins, the response characteristics control. For catchment areas with few

impervious surfaces (little urban development), antecedent moisture conditions usually

govern, especially for rainfall events with a return period of 10 years or less.

(3) The fraction of rainfall that becomes runoff (C) is independent of rainfall intensityor volume.

This assumption is only reasonable for impervious areas, such as streets, rooftops, and

parking lots. For pervious areas, the fraction of runoff does vary with rainfall intensity

and the accumulated volume of rainfall. Thus, the application of the Rational Method

requires the selection of a coefficient that is appropriate for the storm, soil, and land use

conditions. Many guidelines and tables have been established, but seldom, if ever, have

they been supported with empirical evidence.

(4) The peak rate of runoff is sufficient information for the design.

Modern drainage practice includes detention of urban storm runoff to reduce the peak

rate of runoff downstream. Using only the peak rate of runoff, the Rational Method

severely limits the evaluation of design alternatives available in urban and in some

instances, rural drainage design.

The rational formula estimates the peak rate of runoff at any location in a catchment area

as a function of the catchment area, runoff coefficient, and mean rainfall intensity for a

duration equal to the time of concentration (the time required for water to flow from the

most remote point of the basin to the location being analyzed). The rational formula is

expressed as:

Q = 0.00278 CIA (5.1)

where:Q = maximum rate of runoff, m

3/s

C = runoff coefficient representing a ratio of runoff to rainfall (see

Tables 5-3 through 5-5)

I = average rainfall intensity for a duration equal to the time of

concentration, for a selected return period, mm/hr

A = catchment area tributary to the design location, ha

The coefficients given in Tables 5-3 through 5-5 are applicable for storms of 5-yr to 10-

yr frequencies. Less frequent, higher intensity storms will require modification of the


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coefficient because infiltration and other losses have a proportionally smaller effect on

runoff (11). The adjustment of the Rational Method for use with major storms can be

made by multiplying the right side of the rational formula by a frequency factor C f. The

rational formula now becomes:

Q = 0.00278 CCf IA (5.2)

Cfvalues are listed below. The product of Cftimes C shall not exceed 1.0.

Soil Type

Terrain TypeA B C D

Flat, 15% 0.18-0.22 0.24-0.30 0.30-0.40 0.38-0.48

Description of Area Runoff Coefficients

Business: Downtown areas 0.70-0.95

Neighborhood areas 0.50-0.70Residential: Single-family areas 0.30-0.50

Multi units, detached 0.40-0.60

Multi units, attached 0.60-0.75

Suburban 0.25-0.40

Residential (0.5 hectare lots or more) 0.30-0.45

Apartment dwelling areas 0.50-0.70

Industrial: Light areas 0.50-0.80

Heavy areas 0.60-0.90

Parks, cemeteries 0.10-0.25

Playgrounds 0.20-0.40

Railroad yard areas 0.20-0.40Unimproved areas 0.10-0.30

Source: Hydrology, Federal Highway Administration, HEC No. 19, 1984

Surface Runoff Coefficients

Street : Asphalt 0.70-0.95

Concrete 0.80-0.95

Drives and walks 0.75-0.85

Roofs 0.75-0.95

Source: Hydrology, Federal Highway Administration, HEC No. 19, 1984


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Recurrence Interval (years) Cf

5 1.0

10 1.0

25 1.1

50 1.2

100 1.25

The results of using the Rational Formula to estimate peak discharges is very sensitive to

the parameters that are used. The designer must use good engineering judgment in

estimating values that are used in the method.

Time of Concentration

The time of concentration is the time required for water to flow from the hydraulically

most remote point of the catchment area to the point under investigation. Use of the

Rational Method requires the time of concentration (tc) for each design point within the

catchment area. The duration of rainfall is then set equal to the time of concentration and

is used to estimate the design average rainfall intensity (I). For a specific drainage basin,

the time of concentration consists of an inlet time plus the time of flow in a closed

conduit or open channel to the design point. Inlet time is the time required for runoff to

flow over the surface to the nearest inlet and is primarily a function of the length of

overland flow, the slope of the drainage basin, and surface cover. Pipe or open channel

flow time can be estimated from the hydraulic properties of the conduit or channel. An

alternative way to estimate the overland flow time is to use Figure 5-2 to estimateoverland flow velocity and divide the velocity into the overland travel distance.

For design conditions that do not involve complex drainage conditions, Figure 5-3 can be

used to estimate inlet time. For each catchment area, the distance is determined from the

inlet to the most remote point in the tributary area. From a topographic map, the average

slope is determined for the same distance. The runoff coefficient (C) is determined by the

procedure described in a subsequent section of this chapter.

To obtain the total time of concentration, the pipe or open channel flow time must be

calculated and added to the inlet time. After first determining the average flow velocity in

the pipe or channel, the travel time is obtained by dividing velocity into the pipe orchannel length. Mannings Equation can be used to determine velocity (see

).

Common Errors

Three common errors should be avoided when calculating tc. First, application of

simplified general equations such as Kirpich for determining tccan result in too short of a

time of concentration particularly when the average basin slope varies significantly from

the mean channel slope as in steep mountainous areas. Neglecting the overland flow time

can also dramatically shorten the time of concentration thus increasing the design peak


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runoff. Computing tc for two reaches of main channel, from the low point to the 0.7

point, then from there to the end of the channel, has been found to give better results.

Second, in some cases runoff from a portion of the catchment area that is highly

impervious may result in a greater peak discharge than would occur if the entire area

were considered. In these cases, adjustments can be made to the catchment area by

disregarding those areas where flow time is too slow to add to the peak discharge.

Sometimes it is necessary to estimate several different times of concentration to

determine the design flow that is critical for a particular application.


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Third, when designing a drainage system, the overland flow path is not necessarily

perpendicular to the contours shown on available mapping. Especially in urban areas, the

land will be graded and swales will intercept the natural contour and conduct the water to

the streets, which reduces the time of concentration. Care shall be exercised in selecting

overland flow paths in excess of 100 meters in urban areas and 200 meters in rural areas.

Rainfall IntensityThe rainfall intensity (I) is the average rainfall rate in mm/hr for a duration equal to the

time of concentration for a selected return period. Once a particular return period has

been selected for design and a time of concentration calculated for the catchment area,

the rainfall intensity can be determined from Rainfall-Intensity-Duration curves. Rainfall-

Intensity-Duration curves for use in Ethiopia are given in Figures 5-9 through 5-12 at the

end of this chapter.

Runoff Coefficient

The runoff coefficient (C) is the variable of the Rational Method least susceptible to

precise determination and requires judgment and understanding on the part of thedesigner. A typical coefficient represents the integrated effects of many drainage basin

parameters. The following discussion considers the effects of soil groups, land use, and

average land slope.

Three methods for determining the runoff coefficient are presented based on soil groups

and land slope (Table 5-3), land use (Table 5-4), and a composite coefficient for complex

catchment areas (Table 5-5).

Table 5-3 gives the recommended runoff coefficient (C) for pervious surfaces by selected

hydrologic soil groupings and slope ranges. From this table the C values for non-urban

areas such as forest land, agricultural land, and open space can be determined.

Hydrological Soil Groups for Ethiopia

Soil properties influence the relationship between runoff and rainfall since soils have

differing rates of infiltration. Permeability and infiltration are the principal data required

to classify soils into Hydrologic Soils Groups (HSG). Based on infiltration rates, the Soil

Conservation Service (SCS) has divided soils into four hydrologic soil groups as follows:

Group A: Sand, loamy sand or sandy loam. Soils having a low runoff potential due to

high infiltration rates. These soils primarily consist of deep, well-drained sands and

gravels.

Group B: Silt loam, or loam. Soils having a moderately low runoff potential due to

moderate infiltration rates. These soils primarily consist of moderately deep to deep,

moderately well to well drained soils with moderately fine to moderately coarse textures.

Group C: Sandy clay loam. Soils having a moderately high runoff potential due to slow

infiltration rates. These soils primarily consist of soils in which a layer exists near the

surface that impedes the downward movement of water or soils with moderately fine to

fine texture.

Group D: Clay loam, silty clay loam, sandy clay, silty clay or clay. Soils having a high

runoff potential due to very slow infiltration rates. These soils primarily consist of clayswith high swelling potential, soils with permanently-high water tables, soils with a


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claypan or clay layer at or near the surface, and shallow soils over nearly impervious

parent material.

Data from direct field measurements on soil permeability and infiltration rates for

Ethiopian soils are very limited. Data is generally available only for soil types located

near major irrigation projects and agricultural research stations. The hydrological soils

groups presented in Table 5-7 are based on limited field measurements and from profilemorphology and physical characteristics, and are subject to further review and

refinement.

Soil Types Hydrologic Soil Group

Ao Orthic Acrisols B

Bc Chromic Cambisols B

Bd Dystric Cambisols B

Be Eutric Cambisols B

Bh Humic Cambisols C

Bk Calcic Cambisols BBv Vertic Cambisols B

Ck Calcic Chernozems B

E Rendzinas D

Hh Haplic Phaeozems C

Hl Luvic Phaeozems C

I Lithosols D

Jc Calcaric Fluvisols B

Je Eutric Fluvisols B

Lc Chromic Luvisols B

Lo Orthic Luvisols B

Lv Vertic Luvisols CNd Dystric Nitosols B

Ne Eutric Nitosols B

Od Dystric Histosols D

Oe Eutric Histosols D

Qc Cambric Arenosols A

Rc Calcaric Regosols A

Re Eutric Regosols A

Th Humic Andosols B

Tm Mollic Andosols B

Tv Vitric Andosols B

Vc Chromic Vertisols D

Vp Pellic Vertisols D

Xh Haplic Xerosols B

Xk Caloic Xerosols B

Xl Luvic Xerosols C

Yy Gypsic Yermosols B

Zg Gleyic Solonchaks D

Zo Orthic Solonchaks B

Source: Ministry of Agriculture


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As the slope of the drainage basin increases, the selected runoff coefficient C should also

increase. This is caused by the fact that as the slope of the catchment area increases, the

velocity of overland and channel flow will increase allowing less opportunity for water to

infiltrate the ground surface. Thus, more of the rainfall will become runoff from the

catchment area.

It is often desirable to develop a composite runoff coefficient based on the percentage ofdifferent types of surface in the catchment area. Composites can be made with Tables 5-3

and 5-4. At a more detailed level composites can be made with Table 5-3 and the

coefficients with respect to surface type given in Table 5-5. The composite procedure can

be applied to an entire catchment area or to typical "sample blocks as a guide to

selection of reasonable values of the coefficient for an entire area.

5.8 EXAMPLE PROBLEM -RATIONAL METHODThe following is an example problem which illustrates the application of the Rational

Method to estimate peak discharges.

Estimate the maximum rate of runoff at the inlet to a culvert on a road near Debre

Markos.

Site Data

From a topographic map and field survey, the area of the drainage basin upstream from

the point in question is found to be 35 hectares. The Rational Method is selected as per

subsection 5.2 as the area in question is less than 50 hectares.

The initial estimate is that the structure required is a small culvert. The road has a

functional classification of a Link Road, with a design standard of DS3 ( ), indicating, as per Table 2-1, a designstorm frequency of 10 years. Determine the maximum rate of runoff for a 10-year and

check a 25-year return period. The following data were measured:

Length of overland flow = 45 m Average overland slope = 2.0%

Length of main basin channel = 700 m

Slope of channel = 0.018 m/m = 1.8 %

Estimated Mannings n Roughness coefficient (n) of channel:

See : Channels not maintained, dense brush, n = 0.090

Hydraulic radius = A/P, or, as per Glossary, can be approximated by average depth, =

0.6m

Land Use and Soil Data

From existing land use maps, land use for the drainage basin was estimated to be:

Residential (multi-units, attached) 40%

Undeveloped (2.0% slope),with good vegetative cover 60%

For the undeveloped area the soil group was determined from field analysis to be:

Ao Orthic Acrisols Hydrologic Soils Group B 100%

The land use for the overland flow area at the head of the basin was estimated to be:

Undeveloped, (Soil Group B, 2.5% slope) 100%


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Overland Flow

The runoff coefficient (C) for the overland flow area from Table 5-3 is 0.12-0.17, use

0.14.


From Figure 5-3 with an overland flow length of 45 m, slope of 2.0 % and a C of 0.14,

the inlet time is 17 min. Channel flow velocity is determined from Mannings formula

(see , Formula 6.6):

V = (1/n)R2/3

S1/2

Using n = 0.090, R = 0.6 m and S = 0.018m/m, V = 1/1 m/s. Therefore,

Flow Time = (700 m)/(1.1 m/s)(60 s/min) = 10.61 min

and tc= 17 + 10.61 = 28.61 min - say 29 min

Rainfall Intensity

From Figure 5-8 Debre Markos is in Region A2. From Figure 5-10 for Region A2 with a

duration equal to 29 minutes,

I10 (10-yr return period) = 67 mm/hr

I25 (25-yr return period) = 80 mm/hr

Runoff Coefficient

A weighted runoff coefficient (C) for the total catchment area is determined in the

following table by using the values from Tables 5-3 and 5-4.

(1) (2) (3)

Percent Weighted Runoff

Total Runoff Coefficient

Land Use Land Area Coefficient (1) x (2)

Residential (multi-units, attached) .40 .68 .27

Undeveloped (Soil Group B) .60 .14 .08

Total Weighted Runoff Coefficient .35

Peak Runoff

From the rational equation (5-1):

Q10 = 0.00278CIA = 0.00278 x 0.35 x 67 mm/h x 35 ha = 2.28 m3/s

Q25 = CfCIA = 1.1 x 0.00278 x 0.35 x 100 mm/h x 35 ha = 3.75 m3/s

These are the estimates of peak runoff for a 10-year and 100-year design storm for the

given basin. The culvert, channel, and erosion protection design would proceed with

these values.


20/44

Chapter 5


5.9 SCSUNIT HYDROGRAPH

Techniques developed by the U. S. Soil Conservation Service (12) for calculating rates of

runoff require the same basic data as the Rational Method: catchment area, a runoff

factor, time of concentration, and rainfall. The SCS approach, however, is more

sophisticated in that it considers also the time distribution of the rainfall, the initial

rainfall losses to interception and depression storage, and an infiltration rate that

decreases during the course of a storm. With the SCS method, the direct runoff can be

calculated for any storm, either real or fabricated, by subtracting infiltration and other

losses from the rainfall to obtain the precipitation excess (14).

A catchment area is determined from topographic maps and field surveys. For large

catchment areas it might be necessary to divide the area into sub-catchment areas to

account for major land use changes, obtain analysis results at different points within the

catchment area, or locate stormwater drainage structures and assess their effects on the

flood flows. A field inspection of existing or proposed drainage systems shall be made to

determine if the natural drainage divides have been altered. These alterations could make

significant changes in the size and slope of the subcatchment areas.

The SCS method is based on a 24-hour storm event which has a Type II time distribution.

The Type II storm distribution is a typical" time distribution which the SCS has prepared

from rainfall records. It is applicable for interior rather than the coastal regions and

should be appropriate for Ethiopia. The Type II rainfall distribution will usually give ahigher runoff than a Type I distribution. Figure 5-4 shows this distribution. To use this

distribution it is necessary for the user to obtain 1) the 24-hour rainfall value (from

Figure 5-13) for the frequency of the design storm desired, and then 2) multiply this

value by 24 to obtain the total 24-hour storm volume in millimeters.

A relationship between accumulated rainfall and accumulated runoff was derived by SCS

from experimental plots for numerous hydrologic and vegetative cover conditions. Data

for land-treatment measures, such as contouring and terracing, from experimental

catchment areas were included. The equation was developed mainly for small catchment

areas for which daily rainfall and catchment area data are ordinarily available. It was

developed from recorded storm data that included total amount of rainfall in a calendar

day but not its distribution with respect to time. The SCS runoff equation is therefore a

method of estimating direct runoff from 24-hour or 1-day storm rainfall. The equation is:

Q = (P- Ia)2/ (P - Ia) + S (5.3)

Where: Q = accumulated direct runoff, mm

P = accumulated rainfall (potential maximum runoff), mm

Ia = initial abstraction including surface storage, interception, and

infiltration prior to runoff, mm (see Table 5-15)

S = potential maximum retention, mm


21/44


22/44

Chapter 5


The relationship between Iaand S was developed from experimental catchment area data.

It removes the necessity for estimating Iafor common usage. The empirical relationship

used in the SCS runoff equation is:

Ia= 0.2S (5.4)

Substituting 0.2S for Iain equation 5.3, the SCS rainfall-runoff equation becomes:

Q = (P - 0.2S)2/ (P + 0.8S) (5.5)

S is related to the soil and cover conditions of the catchment area through the CN. CN

has a range of 0 to 100, and S is related to CN by:

S = 1000/CN 10 (5.6)

Figure 5-5 shows a graphical solution of this equation which enables the precipitation

excess from a storm to be obtained if the total rainfall and catchment area curve number

are known. For example, 180 mm of rainfall with a Curve Number of 85 would result in

direct runoff of would result in 135 mm of runoff.

Runoff is rainfall excess or effective rainfall - the amount by which rainfall exceeds the

capability of the land to infiltrate or otherwise retain the rainwater. The principal physical

catchment area characteristics affecting the relationship between rainfall and runoff are

land use, land treatment, soil types, and land slope.

is the catchment area cover, and it includes both agricultural and

nonagricultural uses. Items such as type of vegetation, water surfaces, roads, roofs, etc.

are all part of the land use. applies mainly to agricultural land use, andit includes mechanical practices such as contouring or terracing and management

practices such as rotation of crops.

The SCS uses a combination of soil conditions and land-use (ground cover) to assign a

runoff factor to an area. These runoff factors, called runoff curve numbers (CN), indicate

the runoff potential of an area. The higher the CN, the higher is the runoff potential.

influence the relationship between rainfall and runoff by affecting the

rate of infiltration. The SCS has divided soils into four hydrologic soil groups based oninfiltration rates (Groups A, B, C, and D). These groups were previously described for

the Rational Formula (see Section 5.7, Table 5-7).

Consideration shall be given to the effects of urbanization on the natural hydrologic soil

group. If heavy equipment can be expected to compact the soil during construction or if

grading will mix the surface and subsurface soils, appropriate changes shall be made in

the soil group selected. Also runoff curve numbers vary with the antecedent soil moisture

conditions, defined as the amount of rainfall occurring in a selected period preceding a

given storm. In general, the greater the antecedent rainfall, the more direct runoff there is

from a given storm. A five-day period is used as the minimum for estimating antecedent

moisture conditions.


23/44

Chapter 5



24/44

Chapter 5


Antecedent soil moisture conditions also vary during a storm; heavy rain falling on a dry

soil can change the soil moisture condition from dry to average to wet during the storm

period.

The following pages give a series of tables related to runoff factors. The first tables(Tables 5-8 through 5-11) give curve numbers for various land uses. These tables are

based on an average antecedent moisture condition, i.e., soils that are neither very wet

nor very dry when the design storm begins. Curve numbers shall be selected only after a

field inspection of the catchment area and a review of cover type and soil maps. Table 5-

12 gives conversion factors to convert average curve numbers to wet and dry curve

numbers. Table 5-13 gives the antecedent conditions for the three classifications.

Care shall be taken in the selection of curve numbers (CNs). Use a representative

average curve number, CN, for the catchment area. Selection of overly conservative

CNs will result in the estimation of excessively high runoff and consequently

excessively costly drainage structures. Selection of conservatively high values for allrunoff variables results in compounding the runoff estimation. It is better to use average

values and design for a longer storm frequency. Often the runoff computed using

conservative CN's for a ten year storm will greatly exceed the computed runoff for

average CN's for a 25 or even 50 year storm. The hydrologic designer could consider

doing both in making the most appropriate selection of design discharge.

For antecedent moisture conditions (AMC) in Ethiopia, use dry for Region D1, wet for

Region B1, and average AMC for all other regions. The portion of Region A2 in the

vicinity of Bahir Dar should also be treated as wet. When wet AMC is used, it is unlikely

that the vegetation density will also be poor to sparse.


25/44

Chapter 5


Cover description Curve numbers for hydrologic soil groups

Cover type and Hydrologic condition Average %impervious

area2

A B C D

Open space (lawns, parks, cemeteries, etc.)3

Poor condition (grass cover 75%)

Impervious areas:

Paved parking lots, roofs, driveways, etc.

(excluding right-of-way)

Streets and roads:Paved; curbs and storm drains (excluding

right-of-way)

Paved; open ditches (including right-of-way)

Gravel (including right-of-way)

Dirt (including right-of-way)

Desert urban areas:

Natural desert cover

Urban districts:

Commercial and business

Industrial

85

72

68

49

39

98

98

83

76

72

63

89

81

79

69

61

98

98

89

85

82

77

92

88

86

79

74

98

98

92

89

87

85

94

91

89

84

80

98

98

93

91

89

88

95

93

Residential districts by average plot size:0.05 hectare or less

0.1 hectare

0.135 hectare

0.2 hectare

0.4 hectare

0.8 hectare

65

38

30

25

20

12

77

61

57

54

51

46

85

75

72

70

68

65

90

83

81

80

79

77

92

87

86

85

84

82

Developing urban areas

Newly graded areas (pervious areas only, no vegetation) 77 86 91 94

1Average runoff condition, and Ia = 0.2S

2The average percent impervious area shown was used to develop the composite CNs. Other assumptionsare as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN

of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. If theimpervious area is not connected, the SCS method has an adjustment to reduce the effect.3CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations

of open space cover type.


26/44

Chapter 5


Cover descriptionCurve numbers for

Hydrologic soil group

Cover

Type Treatment

2 Hydrologic

condtion3 A B C DFallow Bare soil

Crop residue

cover (CR)

-

Poor

Good

77

76

74

86

85

83

91

90

88

94

93

90

Row

Crops

Straight row (SR) Poor

Good

72

67

81

78

88

85

91

89

SR + CR Poor

Good

71

64

80

75

87

82

90

85

Contoured (C) Poor

Good

70

65

79

75

84

82

88

86

C + CR PoorGood 6964 7874 8381 8785

Contoured &

terraced (C & T)

Poor

Good

66

62

74

71

80

78

82

81

C&T + CR Poor

Good

65

61

73

70

79

77

81

80

Small grain SR Poor

Good

65

63

76

75

84

83

88

87

SR + CR Poor

Good

64

60

75

72

83

80

86

84

C Poor

Good

63

61

74

73

82

81

85

84C + CR Poor

Good

62

60

73

72

81

80

84

83

C&T Poor

Good

61

59

72

70

79

78

82

81

C&T + CR Poor

Good

60

58

71

69

78

77

81

80

Close-seeded SR

or broadcast

Poor

Good

66

58

77

72

85

81

89

85

Legumes or C

Rotation

Poor

Good

64

55

75

69

83

78

85

83

Meadow C&T Poor

Good

63

51

73

67

80

76

83

801Average runoff condition, and Ia = 0.2S.

2 Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.3 Hydrologic condition is based on a combination of factors that affect infiltration and runoff, including (a)

density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass or closed-

seeded legumes in rotations, (d) percent of residue cover on the land surface (good > 20%), and (e) degree

of roughness.

Poor : Factors impair infiltration and tend to increase runoff.

Good : Factors encourage average and better than average infiltration and tend to decrease

runoff.


27/44

Chapter 5


Cover description Curve numbers for hydrologic soil group

Cover typeHydrologic

conditionA B C D

Pasture, grassland, or range-

continuous forage for grazing2

Meadow-continuous grass,

protected from grazing

Brush-weed-grass mixture

with brush the major element3

Woods-grass combination5

Woods6

Farmsbuildings, lanes,

driveways, and surroundinglots

Poor

Fair

Good

--

Poor

Fair

Good

Poor

Fair

Good

Poor

Fair

Good

--

68

49

39

35

48

35

304

57

43

32

45

36

304

59

79

69

61

59

67

56

48

73

65

58

66

60

55

74

86

79

74

72

77

70

65

82

76

72

77

73

70

82

89

84

80

79

83

77

73

86

82

79

83

79

77

86

1 Average runoff condition, and Ia= 0.2S

2 Poor: < 50% ground cover or heavily grazed with no mulch

Fair: 50 to 75% ground cover and not heavily grazed

Good: > 75% ground cover and lightly or only occasionally grazed3Poor: < 50% ground cover

Fair: 50 to 75% ground cover

Good: > 75% ground cover4Actual curve number is less than 30; use CN = 30 for runoff computations.5CNs shown were computed for areas with 50% grass (pasture) cover. Other combinations of conditions

may be computed from CNs for woods and pasture.6 Poor : Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning.

Fair : Woods grazed but not burned, and some forest litter covers the soil.

Good : Woods protected from grazing, litter and brush adequately cover soil.


28/44

Chapter 5


Cover typeHydrologic

condition2 A

3B C D

Mixture of grass, weeds, and low-

growing brush, with brush the minor

elementMountain brush mixture of small trees

and brush

Small trees with grass understory

Brush with grass understory

Desert shrub brush

Poor

Fair

GoodPoor

Fair

Good

Poor

Fair

Good

Poor

Fair

Good

Poor

FairGood

63

5549

80

71

6266

48

30

75

58

41

67

51

35

77

7268

87

81

7474

57

41

85

73

61

80

63

47

85

8179

93

89

8579

63

48

89

80

71

85

70

55

88

8684

1 Average runoff condition, and Ia = 0.2S2 Poor : < 30 % ground cover (litter, grass, and brush overstory)

Fair : 30 to 70 % ground cover Good: > 70 % ground cover3 Curve numbers for Group A have been developed only for desert shrub

CN For Average

Conditions

Corresponding CNs For

100

95

90

85

80

75

70

65

6055

50

45

40

35

30

25

15

5

Dry

100

87

78

70

63

57

51

45

4035

31

26

22

18

15

12

6

2

Wet

100

98

96

94

91

88

85

82

7874

70

65

60

55

50

43

30

13

Ethiopian Rainfall Region D1 (< 100 mm) Source: Ref. 15Ethiopian Rainfall Region A2 & B1 (mean monthly Peak > 300 mm)


29/44

Chapter 5


Antecedent

Condition

Conditions

Description

Growing Season

Five-Day

Antecedent

Rainfall

Dormant Season

Five-Day

Antecedent

RainfallDry

Average

Wet

An optimum Condition of

catchment area soils, where soils

are dry but not to the wilting point,

and when satisfactory plowing or

cultivation takes place

The average case for annual floods

When a heavy rainfall, or light

rainfall and low temperatures, have

occurred during the five days

previous to a given storm

Less than 36 mm

36 to 53 mm

Over 53 mm

Less than 13 mm

13 to 28 mm

Over 28 mm

Source: Soil Conservation Service

The next step in the SCS Method is to determine the Time of Concentration.

Travel Time

Travel time (Tt) is the time it takes water to travel from one location to another in acatchment area. Tt is a component of time of concentration (Tc), which is the time for

runoff to travel from the hydraulically most distant point of the catchment area to a point

of interest within the catchment area. Tcis computed by summing all the travel times for

consecutive components of the drainage conveyance system.

Following is a discussion of procedures and equations for calculating travel time and

time of concentration.

Travel Time

Water moves through a catchment area as sheet flow, shallow concentrated flow, openchannel flow, or some combination of these. The type that occurs is a function of the

conveyance system and is best determined by field inspection.

Travel time is the ratio of flow length to flow velocity:

Tt= L/(3600V) (5.7)

Where:

Tt = travel time, hr

L = flow length, m

V = average velocity, m/s

3600 = conversion factor from seconds to hours.


30/44

Chapter 5



The time of concentration is the sum of Tt values for the various consecutive flow

segments:

Tc= Tt1+ Tt2+ Ttm (5.8)

Where:

Tc = time of concentration, hr

m = number of flow segments.

Sheet Flow

Sheet flow is flow over plane surfaces. It usually occurs in the headwater of streams.

With sheet flow, the friction value (Manning's n) is an effective roughness coefficient

that includes the effect of raindrop impact; drag over the plane surface; obstacles such as

litter, crop ridges, and rocks; and erosion and transportation of sediment. These n values

are for very shallow flow depths of about 0.03 m or so. Table 5-14 gives Manning's n

values for sheet flow for various surface conditions.

Surface Description

Smooth surfaces (concrete, asphalt, gravel, or bare soil

Fallow (no residue)

Cultivated soils:

Residue cover 20%

Residue cover > 20%Grasses:

Short grass

Dense Grasses

Range (natural)

Woods:2

Light underbrush

Dense underbrush

n1

0.011

0.05

0.06

0.17

0.15

0.24

0.13

0.40

0.801 The n values are a composite of information complied by Engman (1986).2 When selecting n, consider cover to a height of about 0.03 m. This is the only part of the plant cover

that will obstruct sheet flow.

For sheet flow of less than 100 meters, use Manning's kinematic solution (Overton and

Meadows 1976) to compute Tt:

Tt= [0.091 (nL)0.8

/ (P2)0.5

s0.4

] (5.9)

Where:

Tt = travel time, hr

n = Manning's roughness coefficient (Table 5-14)

L = flow length, m

P2 = 2-year, 24-hour rainfall, mm

s = slope of hydraulic grade line (land slope), m/m


31/44

Chapter 5


This simplified form of the Mannings kinematic solution is based on the following:

1. shallow steady uniform flow,

2. constant intensity of rainfall excess (rain available for runoff),

3. rainfall duration of 24 hours, and

4. minor effect of infiltration on travel time.

Another approach is to use the kinematic wave equation. For details on using thisequation consult Ref. 16.

Shallow Concentrated Flow

After a maximum of 100 meters, sheet flow usually becomes shallow concentrated flow.

The average velocity for this flow can be determined from equations 5.10 and 5.11, in

which average velocity is a function of watercourse slope and type of channel.

Unpaved V = 4.9178 (s)0.5

(5.10)

Paved V = 6.1961 (s)0.5

(5.11)

Where:

V = average velocity, m/s

S = slope of hydraulic grade line (watercourse slope), m/m

These two equations are based on the solution of Mannings equation with different

assumptions for n (Mannings roughness coefficient) and r (hydraulic radius, meters).

For unpaved areas, n is 0.05 and r is 0.12; for paved areas, n is 0.025 and r is 0.06.

After determining average velocity, use equation 5.7 to estimate travel time for theshallow concentrated flow segment.

Open Channels

Open channels are assumed to begin where surveyed cross section information has been

obtained, where channels are visible on aerial photographs, or where blue lines

(indicating streams) appear on Ethiopian Mapping Authority (EMA) topographic maps

(1:50,000). Average flow velocity is usually determined for bank-full elevation.

Mannings equation or water surface profile information can be used to estimate average

flow velocity. When the channel section and roughness coefficient (Mannings n) are

available, then the velocity can be computed using the Manning Equation

V = (r2/3

s1/2

)/n (5.12)

Where: V = average velocity, m/s

r = hydraulic radius, m (equal to a/pw)

a = cross sectional flow area, m2

Pw = wetted perimeter, m

s = slope of the hydraulic grade line, m/m

n = Mannings roughness coefficient


32/44

Chapter 5


After average velocity is computed using equation 5.12, T tfor the channel segment can

be estimated using equation 5.7.

Reservoir or Lake

Sometimes it is necessary to compute a Tcfor a catchment area having a relatively large

body of water in the flow path. In such cases, Tcis computed to the upstream end of the

lake or reservoir, and for the body of water the travel time is computed using the

equation:

Vw= (gDm)0.5

(5.13)

Where:

Vw = the wave velocity across the water, m/s

g = 9.81 m/s2

Dm = mean depth of lake or reservoir, m

Generally, Vwwill be high (2.44 9.14 m/s)

One must not overlook the fact that equation 5.13 only provides for estimating travel

time across the lake and for the inflow hydrograph to the lake's outlet. It does not account

for the travel time involved with the passage of the inflow hydrograph through spillway

storage and the reservoir or lake outlet. This time is generally much longer and is added

to the travel time across the lake. The travel time through lake storage and its outlet can

be determined by the storage routing procedures in .

Equation 5.13 can be used for swamps with much open water, but where the vegetation

or debris is relatively thick (less than about 25 percent open water), Manning's equation

is more appropriate.

Limitations

Manning's kinematic solution should not be used for sheet flow longer than 100 m.Equation 5.9 was developed for use with the four standard rainfall intensity-duration

relationships.

In catchment areas with storm drains, carefully identify the appropriate hydraulicflow path to estimate Tc. Storm drains generally handle only a small portion of a large

event. The rest of the peak flow travels by streets, grassed areas, and so on, to the

outlet. Consult a standard hydraulics textbook to determine average velocity in pipes

for either pressure or non-pressure flow. A culvert or bridge can act as a reservoir outlet if there is significant storage behind

it. Detailed storage routing procedures shall be used to determine the outflow through

the culvert.

5.10 EXAMPLE PROBLEM:SCSMETHODThe following is an example problem which illustrates the application of the SCS

Method to estimate peak discharges.

Estimate the maximum rate of runoff at the inlet to a proposed drainage structure located

near Nekempte.


33/44


34/44

Chapter 5


Travel Time

For Segment A-B (see Figure 5-6): Sheet flow, natural range, slope = 0.10 m/m, length =

50m. From Table 5-14, Mannings n = 0.13. The 2-year, 24-Hour rainfall is determined

from Figure 5-13 to be 65mm. From Equation 5.9,

Tt= [0.091 (nL)0.8

/ (P2)0.5

s0.4

] = 0.127 hr

For Segment B-C: Shallow concentrated flow, unpaved, s = 0.04 m/m, length = 500m.

From Equation 5.10:

V = 4.9178 (s)0.5

= 0.984 m/s

From Equation 5.7:

Tt= L/(3600V) = 0.141 hr

For Segment C-D: channel flow, natural stream channel, winding with weeds and pools,

s = 0.01m/m, length = 2000m. From the survey, bottom width = 2m, sideslopes = 1V:1H,

25-year storm depth = 1m.

See Table 6-1 : Mannings n = 0.050.

A = Cross-sectional flow area = (2 x 1) + 2[1/2(1)] = 3m2

Pw= wetted perimeter = 2m + 2 R = Hydraulic radius = A/Pw= 3/4.828 = 0.621m

From Equation 5.12

V = (r

2/3

s

1/2

)/n = 1.46 m/s. From Equation 5.7:Tt= L/(3600V) = 0.381 hr

Total Time of Concentration = 0.127 + 0.141 + 0.381 = 0.649 hr

Peak Runoff

From the above data and calculations, catchment area = 100ha, Runoff Curve Number =

84, Time of Concentration = 0.649 hr, 25-year, 24-hour Storm, P = 118mm, Run-Off Q25

= 81mm.

Determine Initial Abstraction from Table 5-15:


35/44


36/44

Chapter 5



37/44

Chapter 5


NOTE: RAINFALL DATA USED IN THE PREPARATION OF THIS FIGURE HAVE BEEN COLLECTED FROM MANY MINISTRY OF

WATER RESOURCES METEOROLOGY STATIONS (SEE TABLE 5-2). IN THE COURSE OF THE PREPARATION OF THIS MANUAL,

THEY HAVE BEEN SUBJECTED TO STATISTICAL TECHNIQUES. THE RESULTS INDICATE THAT THE COUNTRY CAN BE DIVIDED

INTO THE ABOVE HYDROLOGICAL REGIONS DISPLAYING SIMILAR RAINFALL PATTERNS. THE INFORMATION IS SUBJECT TO

REVIEW, AND FUTURE DATA MAY INDICATE THE NEED FOR A FURTHER REFINEMENT IN BOTH VALUES AND REGIONAL

BOUNDARIES.


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42/44

Chapter 5


duration 5 10 20 30 60 120 1440

Chicago 14 22 34 42 56 62 100Conc Pipe

Manual 19 29 36 50 60 80 100

Curve 1: 13.75ln(x) Curve 2: 14.5*LN(x)-5.45

5 1440 5 1440 10 1440

22 100 18 100 26 100

duration 5 10 20 30 60 120 240 360 720 1440

Curve 1 22 32 41 47 56 66 75 81 90 100

Curve 2 18 28 38 44 54 64 74 80 90 100

spline depth 20 30 40 50 60 80 90 100 110 120

% 16 25 33 44 50 67 75 84 92 100

spline mm/hr 240 180 120 100 60 40

% 24 hour 200 150 100 0,83 50 33

Chi % 24 hour 168 132 102 84 56 31

curve 2 214 167 114 88 54 32

y = 15,441Ln(x) - 11,23

y = 15,135Ln(x) - 3,7868

y = 13.75Ln(x)

0

20

40

60

80

100

120

1 10 100 1000 10000

Chicago from VT Chow Conc Pipe Dsgn Manual

Srie3 SPLINEBEST CURVE Logarithmique (Chicago from VT Chow)

Logarithmique (Conc Pipe Dsgn Manual) Design

Logarithmique (SPLINE) Logarithmique (BEST CURVE)

2 5 10 25 50 100

Bahir Dar 74 106 131 163 187 211

Region D 67 89 105 127 144 161

Reg. B & C 65 84 98 118 132 147

A1, A4 60 79 93 113 127 142

A2, A3 52 67 79 95 107 118

duration 2 5 10 25 50 100 duration 2 5 10 25 50 100

5 134,9 193,2 237,3 295,6 339,6 383,7 5 117,3 152,3 178,7 213,7 240,1 266,6

10 113,8 162,9 200,1 249,2 286,4 323,6 10 98,9 128,4 150,7 180,2 202,5 224,8

20 80,1 114,6 140,8 175,4 201,5 227,7 20 69,6 90,3 106,0 126,8 142,5 158,2

30 62,4 89,4 109,7 136,7 157,1 177,5 30 54,3 70,4 82,7 98,8 111,1 123,3

60 38,9 55,7 68,4 85,3 98,0 110,7 60 33,8 43,9 51,6 61,6 69,3 76,9

90 29,0 41,5 50,9 63,4 72,9 82,4 90 25,2 32,7 38,4 45,9 51,5 57,2120 23,3 33,4 41,0 51,1 58,7 66,3 120 20,3 26,3 30,9 36,9 41,5 46,1

duration 2 5 10 25 50 100 duration 2 5 10 25 50 100

5 121,1 161,0 191,3 231,2 261,4 291,7 5 108,2 143,2 169,6 204,6 231,0 257,5

10 102,1 135,8 161,3 195,0 220,5 246,0 10 91,3 120,7 143,0 172,5 194,8 217,1

20 71,8 95,5 113,5 137,2 155,1 173,1 20 64,2 85,0 100,6 121,4 137,1 152,8

30 56,0 74,5 88,5 106,9 120,9 134,9 30 50,0 66,2 78,5 94,6 106,9 119,1

60 34,9 46,5 55,2 66,7 75,4 84,1 60 31,2 41,3 48,9 59,0 66,6 74,3

90 26,0 34,6 41,1 49,6 56,1 62,6 90 23,2 30,7 36,4 43,9 49,6 55,3

120 20,9 27,8 33,1 40,0 45,2 50,4 120 18,7 24,7 29,3 35,4 39,9 44,5

duration 2 5 10 25 50 100

5 94,1 122,4 143,8 172,1 193,5 214,9

10 79,3 103,2 121,3 145,1 163,2 181,3

20 55,8 72,6 85,3 102,1 114,8 127,5

30 43,5 56,6 66,5 79,6 89,5 99,4

60 27,1 35,3 41,5 49,6 55,8 62,090 20,2 26,3 30,9 36,9 41,5 46,1

120 16,3 21,2 24,9 29,7 33,4 37,1

region

24 HOUR DEPTH

frequency

IDF-BAHIR DAR, mm/hr IDF-region B & C, mm/hr

frequency

IDF-region A1& A4 mm/hr

frequency

IDF-Region A2& A3, mm/hr

frequency

frequency

IDF-Region D, mm/hr

frequency


43/44

Chapter 5



44/44

Chapter 5


1. Highway Drainage Guidelines, Volume 11, Guidelines for Hydrology, Task Forceon Hydrology and Hydraulics, AASHTO Highway Subcommittee on Design.

2. Federal Highway Administration. 1990. HYDRAIN Documentation.3. Gebeyehu, Admasu, , Hydraulics Laboratory,

Royal Institute of Technology, Stockholm, Sweden, 1989.

4. Newton, D. W., and Herin, Janet C. 1982. Assessment of Commonly UsedMethods of Estimating Flood Frequency. Transportation Research Board.

National Academy of Sciences, Record Number 896.

5. Potter, W. D. Upper and Lower Frequency Curves for Peak Rates of Runoff.Transactions, American Geophysical Union, Vol. 39, No. 1, February 1958, pp.

100-105.

6. Sauer, V. B., Thomas, W. O., Stricker, V. A., and Wilson, K. V. 1983. FloodCharacteristics of Urban Catchment areas in the United States -- Techniques for

Estimating Magnitude and Frequency of Urban Floods. U. S. Geological Survey

Water-Supply Paper 2204.

7. Wahl, Kenneth L. 1983. Determining Stream Flow Characteristics Based onChannel Cross Section Properties. Transportation Research Board. National

Academy of Sciences, Record Number 922.

8. Overton, D. E. and M. E. Meadows. 1976. Storm Water Modeling. AcademicPress. New York, N.Y. pp. 58-88.

9. U. S. Department of Transportation, Federal Highway Administration. 1984.Hydrology. Hydraulic Engineering Circular No. 19.

10. Water Resources Council Bulletin 17B. 1981. Guidelines for determining floodflow frequency.

11. Wright-McLaughlin 1969.12. Soil Conservation Service (SCS) Technical Release No. 55 (2ndEdition).13.

Applied Hydrology, V. T. Chow et al.14. SCS National Engineering Handbook, Section 4.

15. USDA Soil Conservation Service TP-149 (SCS-TP-149), A Method forEstimating Volume and Rate of Runoff in Small Watersheds, revised April

1973.

16. Regan, R. M., A Nomograph Based on Kinematic Wave Theory for DeterminingTime of Concentration for Overland Flow, Report No. 44, Civil Engineering

Department, University of Maryland at College Park, 1971.

ERA manual for design of storm sewer

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