Inflows into urban drainage systems

Introduction• Sewer water consists of wastewater or rainwater or a mix

of both.• The components that make up the wastewater flow from

a community depend on the type of collection system used and may include the following:

Domestic (also called sanitary) wastewater Industrial wastewater Infiltration/Inflow (I/I)• The wastewater flow is also known as dry weather flow;

wet weather flow refers to the flow under rainfall conditions.

Cont..• Accurate estimation of sewage discharge is necessary for hydraulic

design of the sewers.• Far lower estimation than reality will soon lead to inadequate sewer

size after commissioning of the scheme or the sewers may not remain adequate for the entire design period.

• Similarly, very high discharge estimated will lead to larger sewer size affecting economy of the sewerage scheme lower discharge in the sewer may not meet the criteria of self

cleansing velocity and leading to deposition in sewers.• Actual measurement of the discharge is not possible if the sewers do

not exist.• Since sewers are designed to serve for some more future years,

engineering skills have to be used to accurately estimate the sewage discharge.

Domestic wastewater• In many networks, the domestic component of wastewater is

the most important.• Most domestic wastewater is the water left over from

consumptive uses. • The volume of domestic wastewater discharged in sanitary

sewer represent, in general 60 to 80% of the total drinking water volume distributed.

Losses• A part of the supplied drinking water is lost during the

consumption process and does not reach the sewer system.• This refers for instance to water used for watering gardens,

fire-fighting, street cleaning, filling up swimming pools etc • Leakage losses that occur in the distribution network also

play a large part in explaining the difference between drinking water consumption and wastewater production.

• These losses can make up 10 to 50% of drinking-water production.

For example in Addis Ababa the losses drinking water supplied is more than 40%.

Industrial wastewater• The amount of industrial drinking-water consumed varies per

type of company.• The quantity and quality of wastewater evacuated through the

sewer by the main industries must be carefully analyzed and measured.

• If flow rates are impossible to measure average values values like the one shown in the table can be used.

• In general, water used as cooling agent is evacuated in the storm sewer system.

• Sometimes certain industries (soft drink and breweries) do not drain their wastewater into the sewerage system. This must be taken into account.

Industrial wastewater

Daily volume and pollutant load of wastewater produced from various

commercial sources

Infiltration/Inflow • This is additional quantity due to groundwater seepage into

sewers through faulty joints or cracks formed in the pipes. • The quantity of the water depends upon the height of the water

table above the sewer invert level. • If water table is well below the sewer invert level, the infiltration

can occur only after rain when water is moving down through soil.

• The quantity of the water entering sewers depends upon the permeability of the ground soil and it is very difficult to estimate.

• Therefore, in sewer system design a leak flow rate of 0.2 m3 /(km/h) sewer per hour is often taken into account

• For sewers below the groundwater table (in the United States up to 3 m3 /(km/h)).

Net quantity of sewage• The net quantity of sewage production can be

estimated by considering the addition and subtraction

*unaccounted private water supplies - People using water supply from private wells, tube wells, etc certain industries utilize their own source of water.

• Generally, 60 to 80% of accounted water supplied is considered as quantity of sewage produced.

Variation in Domestic wastewater flow rate

• Wastewater is not produced evenly throughout the day; at night there is barely any wastewater flow.

• The largest amount of wastewater is produced during about 10 hours of the day.

• Wastewater production in most systems shows a peak in the morning and evening.

Peak Factors • Usually to determine minimum and maximum flowrates, peak

factors are used.• Peak factors vary according to regions and characteristics of the

users.• When a sewer system is already in place, these peak factors are

determined by measuring the flowrate. • When there is no existing sewer system we rely on peak factors

suggested by government agencies or specialists in the field.

Cont..• In France the following peak factor is applied:

• In the United States various formulas are used: For Desmoines, for example, the following formula applies:

Peak Factors

Example

Cont..• For estimating design discharge following relation can

also be considered: Maximum daily flow = 2 X the annual average daily

flow. Maximum hourly flow = 1.5 times the maximum daily

flow . = Three times the annual average

daily flow

Design Period • Urban drainage systems have an extended life-span and

are typically designed for conditions 25–50 years into the future.

• The design period depends upon the following: Ease and difficulty in expansion, Amount and availability of investment, Anticipated rate of population growth, including shifts in

communities, industries and commercial investments, Hydraulic constraints of the systems designed, and Life of the material and equipment.

Cont..• Following design period can be considered for

different components of sewage scheme. 1. Laterals less than 15 cm diameter : Full development 2. Trunk or main sewers : 40 to 50 years 3. Treatment Units : 15 to 20 years 4. Pumping plant : 5 to 10 years

Fundamental Hydrology (Revision)

• In calculating the rate of runoff in a stream resulting from a rainfall event, we must first determine the size of the area over which the rain falls.

• For every stream, a well defined area of land intercepts the rainfall and transports it to the stream. The area of land is called the catchment area, watershed, or drainage basin.

• The imaginary line that outlines the boundary of the drainage basin is called the basin divide and is determined by the topography of the land.

• Delineating the basin divide is done on a contour map of the land surrounding the stream and is the first step in computing runoff.

• Reading Assignment 1. Read about the major principles involved in delineating a

drainage basin?

Time Of Concentration • When the size of the drainage basin has been determined, the

next step in finding Q is to compute the time of concentration.• Time of concentration is the amount of time needed for runoff

to flow from the most hydraulically remote point in the drainage basin to the point of analysis.

• The path or route taken by the most remote drop is called the hydraulic path.

• The time is determined by adding all the individual flow times for the different types of flow as the drop makes its way toward the point of analysis. Therefore,

tc = t1+ t2+ t3 +…………..+tn * where t1 , . . . , tn represent the travel times for overland flow,

shallow concentrated flow, stream flow, and any other type of flow encountered.

• Overland flow ( sheet flow)• Shallow concentrated flow• Stream flow

Overland flow (sheet flow)• is usually the first type of flow as the drop starts from the remotest

point.• It is characterized by sheet flow down a relatively featureless slope

similar to the manner in which water flows across pavement.• This is the slowest of all types of flow and is computed by either a

nomograph or empirical formula.• Typically, overland flow cannot travel more than 100 feet before

consolidating into a more concentrated flow.• One way to estimate the overland flow time is to use Figure 5-2

to estimate overland flow velocity and divide the velocity into the overland travel distance. (ERA manual)

• For design conditions that do not involve complex drainage conditions, Figure 5-3 can be used to estimate inlet time.

Shallow concentrated flow• occurs when the natural indentations of terrain cause the

runoff to form into small rivulets.• Since the rivulets are more concentrated, the flow efficiency

is increased, and therefore the velocity is also increased.• Time for shallow concentrated flow is determined by

empirical nomograph, such as that shown in Figure

Stream flow (open channel flow)

• is usually the last (and the fastest) flow to occur along the hydraulic path.

• Time for stream flow can be computed by using Manning’s equation.

Rainfall • Rainfall occurs in haphazard patterns, making it very difficult

to quantify for design purposes.• Rain data measured at an individual rain gauge is most

commonly expressed either as depth in mm or intensity in mm/h.

• Most commonly the size of a storm is described by the number of mm of rainfall together with the duration of the rainfall.

• For example, a rainfall event of 5 mm over 12 hours is in one category, and a rainfall of 5.0 mm over 24 hours is in another. Although both events produced the same rainfall, one is more intense, and intensity of rainfall is very important in computing runoff.

Rainfall Frequency• Probability of occurrence is described by the term return period,

which is the average number of years between two rainfall events that equal or exceed a given number of inches over a given duration.

• If you examine the Figure, you will see that only one storm of a 24-hour duration reached the 6.5-inch level in 100 years of data.

So a 6.5-inch, 24-hour storm is said to be a 100-year storm. Also,you will see that in 100 years, two storms equaled or

exceeded 5.0 inches. So a 5.0-inch, 24-hour storm is called a 50-year storm.

What is a 25-year storm in this example?

Bar graph of highest 24-hour rainfall amounts in 100 years of records for a given location.

Rainfall intensity duration frequency Curve

• The relationship between rainfall intensity and duration for various return periods for a given location is shown on intensity-duration-frequency (I-D-F) curves.

• The intensity-duration relationship is central to the Rational Method for determining peak runoff,

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IDF curve

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Example • Determine i and P for a 20-min duration storm with 5-yr return period in Chicago

From the IDF curve for Chicago,

i = 3.5 in/hr for Td = 20 min and T = 5yr

P = i x Td = 3.5 x 20/60 = 1.17 in

Runoff Calculation• Runoff estimation for hydraulic sizing of stormwater runoff facilities such as pipe systems, storm inlets and culverts, small open channels, and Detention facilities

Runoff Calculation• Peak discharge: sometimes called peak flow, is the maximum

rate of flow of water passing a given point during or after a rainfall event.

• There are different methods used for peak runoff : Rational Method Modified Rational Method NRCS Method

Rational Method• Developed in 1800s in England as the first dimensionally

correct equation.• simple conceptual method• estimates peak runoff rate for a selected frequency• appropriate for urban and rural catchments less than 50 hectares

in which natural or man-made storage is minor• Rainfall intensity at a duration equal to the time of

concentration (TC) is used to calculate the peak flow• relates the peak discharge (q, m3/sec) to the drainage area (A,

ha), the rainfall intensity (i, mm/hr), and the runoff coefficient (C)

Modified Rational Method• For higher intensity storms infiltration and other losses have

a proportionally smaller effect on runoff • The runoff coefficient are usual applicable for 10 year or less

recurrence intervals • Rational method is modified to account for reduction of

infiltration and other loses during high intensity storms

* The product of Cf times C shall not exceed 1.0.

The basic steps for applying the Rational Method • Step 1: Apply I-D-F Data Develop or obtain a set of intensity-duration-frequency (IDF)

curves for the locale in which the drainage basin resides. Assume that the storm duration is equal to the time of concentration and determine the corresponding intensity for the recurrence interval of interest. Note that the assumption that the storm duration and time of concentration are equal is conservative in that it represents the highest intensity for which the entire drainage area can contribute.

• Step 2: Compute Watershed Area The basin area A can be estimated using topographic maps,

computer tools such as CAD or GIS software, or by field reconnaissance.

The basic steps for applying the Rational Method • Step 3: Choose C Coefficients The runoff coefficient C may be estimated using Table 2-5 if

the land use is homogeneous in the basin, or a composite C value may be estimated if the land use is heterogeneous.

• Step 4: Solve Peak Flow Finally, the peak runoff rate from the basin can be computed using

the equation Q = CiA.

EXAMPLE PROBLEM - RATIONAL METHOD• Estimate the maximum rate of runoff at the inlet to a

culvert on a road near Debre Markos. Determine the maximum rate of runoff for a 10-year and check a 25-year return period.

Site DataArea of the drainage = 35 hectareDesign storm frequency = 10 years Length of overland flow = 45 m Average overland slope = 2.0% Length of main basin channel = 700 mSlope of channel = 0.018 m/m = 1.8 % n = 0.090Hydraulic radius = 0.6 m

Land Use and Soil DataFrom 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%

Solution