RATIONAL METHOD OF ESTIMATING RUNOFF

 RATIONAL METHOD OF ESTIMATING RUNOFF

Rational Method: The rational method is used around the world for peak flow estimation of small rural drainage basins and is the most widely used method for urban drainage design.  

The rational method equation is given below:

Q = kCiA

where: Q - peak flow (m3/s).

k - conversion factor equal to 0.00278 (metric). (=1/360)

C - dimensionless runoff coefficient.

i - rainfall intensity (mm/hr).

A - catchment area (ha).

The Catchment Area, A

The catchment area, A, is determined from a map which includes the drainage area of interest. 

The boundaries of the drainage area using a contour map. 

Once the boundaries are known, the area can be determined using the map scale. 

Since the area must be in acres for use in the Rational Method equation, a useful conversion factor is 43,560 ft2/acre.

The Runoff Coefficient, C

The runoff coefficient is the fraction of rainfall striking the drainage area that becomes runoff from that drainage area. It is an empirically determined constant, dependent on the nature of the drainage area surface. 

An impervious surface like a concrete parking lot will have a runoff coefficient of nearly one. 

A very tight clay soil will also have a relatively high runoff coefficient.

Sandy soil would have more infiltration and a lower runoff coefficient. 

In addition to the nature of the surface and the soil, the slope of the drainage area has an effect on the runoff coefficient. 

A steeper slope leads to a higher runoff coefficient. 

Tables showing the values for runoff coefficient for a variety of types of drainage areas in handbooks, textbooks and on the internet. 

Sample values for ready reference is shown below

The Design Rainfall Intensity, i

The design rainfall intensity is the intensity of a constant intensity design storm with the specified design return period and duration equal to the time of concentration of the drainage area.

Once the design return period and duration are determined, the design rainfall intensity can be determined from an appropriate intensity-duration-frequency graph or equation for the location of the drainage area. 

INFILTRATION & RUNOFF DEFINITIONS

 INFILTRATION & RUNOFF

Two important aspects of Hydrological Cycle are

1. Infiltration

2. Runoff

INFILTRATION

All precipitation will not become surface runoff.

Some quantity of precipitation infiltrates into the ground.

The infiltration plays a significant role on the relationship between rainfall and runoff.

If the soil is wet, less water infiltrates and more surface runoff is generated.

Similarly, if the soil is dry, more water (usually) is able to soak into the ground.

RUNOFF

The water left after infiltration flows overground as runoff

Volume of runoff depends on:

 Rainfall intensity and duration

 Type of surface (pervious or impervious)

 Area of catchment

Usually

(a) Low intensity rainfall - Mostly Infiltrates

(b) High intensity rainfall - Infiltrates and become surface runoff

Runoff depends on the catchment characteristics and can be related as

(a)Thick vegetation = low volume, slow runoff

(b) Paved area = high volume, fast runoff 

Introduction to Hydrology

 INTRODUCTION TO HYDROLOGY

Hydrological cycle Comprises of

1. Precipitation

2. Evaporation

3. Transpiration

Precipitation & Evaporation are the two most important elements of the water cycle for hydrologists.

The water cycle is

1. A closed system

2. No water being created or lost, just moved around.

3. Its a Finite resource – but renewable if quality controlled.

Components of a Hydrologic Cycle is as shown in the figure

A Typical flowchart of a Hydrologic Cycle is as shown in the figure below

Precipitation: All forms of moisture being released from the atmosphere.

Rainfall and evaporation are measured by depth (usually millimetres)

One cubic metre (m3) = 1000 litres

One millimetre depth over one hectare = 10 m3 

10cm depth over one hectare = 1000 m3 = 1 Megalitre (ML)

Rainfall and evaporation are also by rate (mm/hour,  mm/day,  mm/year)

SHEAR KEYS IN RETAINING WALL DESIGN

 

FUNCTION OF SHEAR KEYS IN THE DESIGN OF RETAINING WALLS

In determining the external stability of retaining walls, failure modes like bearing failure, sliding and overturning are normally considered in design. In considering the criterion of sliding, the sliding resistance of retaining walls is derived from the base friction between the wall base and the foundation soils. To increase the sliding resistance of retaining walls, other than providing a large self-weight or a large retained soil mass, shear keys are to be installed at the wall base. The principle of shear keys is as follows: The main purpose of installation of shear keys is to increase the extra passive resistance developed by the height of shear keys. However, active pressure developed by shear keys also increases simultaneously. The success of shear keys lies in the fact that the increase of passive pressure exceeds the increase in active pressure, resulting in a net improvement of sliding resistance. On the other hand, friction between the wall base and the foundation soils is normally about a fraction of the angle of internal resistance (i.e. about 0.8φ ) where φ is the angle of internal friction of foundation soil. When a shear key is installed at the base of the retaining wall, the failure surface is changed from the wall base/soil horizontal plane to a plane within foundation soil. Therefore, the friction angle mobilized in this case is φ instead of 0.8φ in the previous case and the sliding resistance can be enhanced.

Why 150mm Cubes Are Used For Testing Concrete Strength

 

IN CONCRETE COMPRESSION TEST, NORMALLY 150MMX150MMX150MM CONCRETE CUBE SAMPLES IS USED FOR TESTING. WHY ISN’T 100MMX100MMX100MM CONCRETE CUBE SAMPLES USED IN THE TEST INSTEAD OF 150MMX150MMX150MM CONCRETE CUBE SAMPLES?

Basically, the force supplied by a concrete compression machine is a definite value. For normal concrete strength application, say below 50MPa, the stress produced by a 150mmx150mmx150mm cube is sufficient for the machine to crush the concrete sample. However, if the designed concrete strength is 100MPa, under the same force (about 2,000kN) supplied by the machine, the stress under a 150mmx150mmx150mm cube is not sufficient to crush the concrete cube. Therefore, 100mmx100mmx100mm concrete cubes are used instead to increase the applied stress to crush the concrete cubes. For normal concrete strength, the cube size of 150mmx150mmx150mm is already sufficient for the crushing strength of the machine.

Problems in Pumping Concrete

 MAJOR PROBLEMS IN USING PUMPING FOR CONCRETING WORKS 

In pumping operation, the force exerted by pumps must overcome the friction between concrete and the pumping pipes, the weight of concrete and the pressure head when placing concrete above the pumps. In fact, as only water is pumpable, it is the water in the concrete that transfers the pressure. The main problems associated with pumping are the effect of segregation and bleeding. To rectify these adverse effects, the proportion of cement is increased to enhance the cohesion in order to reduce segregation and bleeding. On the other hand, a proper selection of aggregate grading helps to 

ExpansioN Joint - Typical Components

 Typical Components of an Expansion Joint

In a typical expansion joint, it normally contains the following components: 

joint sealant, 

joint filler, 

dowel bar, 

PVC dowel sleeve, 

bond breaker tape and cradle bent. 

Joint sealant: it seals the joint width and prevents water and dirt from entering the joint and causing dowel bar corrosion and unexpected joint stress resulting from restrained movement. 

Joint filler: it is compressible so that the joint can expand freely without constraint. Someone may doubt that even without its presence, the joint can still expand freely. In fact, its presence is necessary because it serves the purpose of space occupation such that even if dirt and rubbish are intruded in the joint, there is no space left for their accommodation. 

Dowel bar: This is a major component of the joint. It serves to guide the direction of movement of concrete expansion. Therefore, incorrect direction of placement of dowel bar will induce stresses in the joint during thermal expansion. On the other hand, it links the two adjacent structures by transferring loads across the joints. 

PVC dowel sleeve: It serves to facilitate the movement of dowel bar. On one side of the joint, the dowel bar is encased in concrete. On the other side, however, the PVC dowel sleeve is bonded directly to concrete so that movement of dowel bar can take place. One may notice that the detailing of normal expansion joints in Highways Standard Drawing is in such a way that part of PVC dowel sleeve is also extended to the other part of the joint where the dowel bar is directly adhered to concrete. In this case, it appears that this arrangement prevents the movement of joint. If this is the case, why should designers purposely put up such arrangement? In fact, the rationale behind this is to avoid water from getting into contact with dowel bar in case the joint sealant fails. As PVC is a flexible material, it only minutely hinders the movement of joint only under this design. 

Bond breaker tape: As the majority of joint sealant is applied in liquid form during construction, the bond breaker tape helps to prevent flowing of sealant liquid inside the joint . 

Freeboard in Dams

 Freeboard

Free Board is the vertical distance between the top of the dam and the still water level. Freeboard is computed from the following two considerations:

Wave height considerations

It is equal to wind set up plus 1 1/3 times the wave height above FRL or above MWL (corresponding to design flood) whichever gives higher dam top level. A minimum freeboard of 1m above MWL corresponding to design flood shall be available. If design flood is not equal to PMF then the top of dam should be at least equal to MWL corresponding to PMF. At least 1m high solid parapet is to be provided, not withstanding the above requirements.

Wind velocity generally assumed as below in absence of meteorological data:

For FRL condition - 120 km/hr

For MWL condition - 80 km/hr

T. Saville’s method as given in IS:6512-1984 is used for calculating the wave height/freeboard.

Operation considerations

IS:11223 specifies the following:

The freeboard as specified in IS: 6512 shall be available at FRL and MWL corresponding to all bays operative condition. For gated spillways a contingency of 10% of gates (min. one gate) being inoperative is considered as an emergency. A reduced freeboard may be acceptable under the emergency condition. The dam shall not be allowed to overtop in any case. 

Weight of 1 cubic METRE concrete grade m20

Volume of dry Concrete is usually 1.54 to 1.57 times Volume of wet concrete. During concreting when the wet concrete is placed, it hardens after initial setting time of 30 mins For 1 cu.m.(assumed) of Concrete work M-20 ratio Sum = 1+1.5+3=5.5 Shrinkage or safety Factor =1.57 So Total volume of wet concrete required is :- 1.57 Cubic Metre Volume of aggregate = (3/5.5) x 1.57 = 0.856 Cubic Metre Volume of sand Require = (1.5/5.5) x 1.57 = 0.471 Cubic Metre Volume of cement = (1/5.5) x 1.57 = 0.285 Cubic Metre = 0.285 x1440 = 411 kg For 1 Cubic Metre of M20 (1:1.5:3) Agregate = 0.856 Cubic Metre Sand = 0.472 Cubic Metre Cement = 8.22 bags.

Design Of Core Wall - 2

Design of Core Wall Section

 

Design of Verical Drop