sewers Design Procedures

Sewers Design Procedures

 

Layout the sewer: Draw a line to represent the proposed sewer in each street or alley to be served. Near of on the line; indicate by an arrow the direction in which the wastewater is to flow. Except in special cases, the sewer should slope with the surface of the street. It is usually more economical to plan the system so that the wastewater from any street will flow to the point of disposal by the most direct (and, consequently; the shortest) route. In general, the laterals connect with the mains and these; in turn connect with the trunk sewer, which leads to the point of discharge or to an intercepting sewer.

 

Locate the manholes: Locate a manhole at: (1) Changes in direction;

(2) Changes in slope;

(3) At pipe junctions with the exception of building connections;

(4) At the upper end and ends of all laterals for cleansing and flushing the lines; and

 

(5)       At intervals from 90 to 120 m or less, as required. Give each manhole an identification number.

 

Establishing the limits of the service area: Sketch the limits of the service areas. Search the limits of the service area for each lateral. If a single lateral will be required to accommodate an area larger than can be served by the minimum size of sewer with the minimum slope the area should be subdivided further. Where the streets are laid out assume that the limits are midway between them. If the street layout is not shown on the plan, the limits of the different service areas cannot be determined as closely and the topography may serve as a guide.

 

Determine the area of each service area. Measure the area of each service area by using a scale, and enter the value on the map.

 

1.  Summarize the basic design criteria.

 

a.     Design period (usually saturation period used);

b.     Population density;

c.     Residential wastewater flow (Obtain the peaking factor);

d.     Infiltration allowances;

e.     Inflow allowances

f.     Hydraulic design equation;

g.     Minimum pipe size ;

h.     Minimum velocity; and

i.     Minimum cover.

 

Prepare tabulation form to record the data and steps in the compilations for each section of sewer between Manholes.

 

N.B. If sewer changes direction in a manhole without change of size, a drop of 30 mm should be provided in the manhole. If the sewer changes size, the crowns of the inlet and outlet sewers should be at the same elevation. Branches coming into manholes should have their crowns at the same elevation as that of the large sewer. Drop manholes are used only if the invert of the branch is 0.6 m or more above what its location would be when following the rule just stated.

Minimum slopes of sewers

 

To assure that sewers will carry suspended sediment, two approaches have been used: The minimum (o r self-cleansing) velocity and

 

The minimum boundary shear stress method, also called the'tractive force'

 

Self-cleansing - a full-pipe velocityof at least 0.6 m/s

Minimum slopes of sewers

 

To assure that sewers will carry suspended sediment, two approaches have been used:

 

The minimum (or self-cleansing) velocity and the minimum boundary shear stress method, also called 'tractive force'

 

self-cleansing - a full-pipe velocityof at least 0.6 m/s

 

Design of storm sewers

 

Generally, storm sewers are designed to provide safe passage of vehicles, and to collect, convey and discharge for frequently occurring, low-return-period storms. Storm sewer design involves estimation runoff from an area design of the sewer and other hydraulics structures in the drainage system.

 

Design flow

 

Design flow is the maximum flow that can pass through a specified structure safely. In determining this design flow the possibility of occurrence has be fixed. Once this is fixed the design flow magnitude can be determined.

 

Generally, a design frequency is selected to match the facility's 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.

 

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  Recurrence  Interval(RI).	Thus  the  exceedence	probability  equals  100/RI.

Generally, to design drainage facilities the recurrence interval shown in table 4-1 can be used.

 

Table 4-1 Return Period Based on Type of Structures.

 

 

Drainage Type : Return Period

         

Side Ditch :          10

         

Pipe Culvert :       10

         

Slab/Box Culvert :         25

         

Bridge :       50/100

 

 

The commonly used hydrologic methods used to estimate are the following:

 

• Rational Method - only for drainage areas less than 50 hectares (0.5 kilometer²);

 

• SCS and other Unit Hydrograph Methods - for drainage areas greater than 50hectares;

 

•    Suitable Computer Programs - such as HYDRAIN's HYDRO, HEC 1, and TR-20 will be used to facilitate tedious hydrologic calculations.

 

Rational Method

 

Runoff from an area can be determined by the Rational Method. The method gives a reasonable estimate up to a maximum area of 50 ha (0.5 Km².

 

The rational method makes the following assumptions:

 

•             Precipitation is uniform over the entire basin.

 

•             Precipitation does not vary with time or space.

 

•             Storm duration is equal to the time of concentration.

 

•                  A design storm of a specified frequency produces a design flood of the same frequency.

 

•             The basin area increases roughly in proportion to increases in length.

 

•             The time of concentration is relatively short and independent of storm intensity.

 

•   The runoff coefficient does not vary with storm intensity or antecedent soil moisture.

 

•             Runoff is dominated by overland flow.

 

•             Basin storage effects are negligible.

 

Thus, the peak runoff is calculated according to the following formula:

 

Q = CiA/360

 

Where,

Q = runoff [m³/s]

 

C = runoff coefficient which can be given for a land use or surface type i = design rainfall intensity [mm/hr] A

 

= area [ha]

 

The sewer design procedure is as follows

 

Establish the layout of the storm sewer

 

Estimate the design runoff by the Rational Method Determine the sewer size by the Manning formula

 

Q= 1/n . R^2/3 . S^1/2

 

Check for velocity; if not in the range change the sewer diameter Determine sewer invert elevations

 

 

 

Example A storm sewer is proposed to drain a 12 hectares drainage area shown in the figure below. With given data in the table below determine the design discharge needed to convey 5-year peak discharge.

 

Site    : Area (ha) C       Inlet time (min)

                            

A       : 4              0.8                        10

                            

B       : 8              0.5                        30

 

 

Solution

 

Upstream Area (Manhole 1): A = 4 ha