Played in Rain water?
Yes, it’s me long years back when I only bothered about this rain water ‘Dance’ than its ‘Drainage’! Strange… but reality, we all enjoyed this era of our life. It is a problem when water is stagnated all over the time and its discharge to compatible manner is a must. No, don’t imagine some awful place of waste water, it’s really an amazing experience of learning specially with me. This will differ from small scale to large scale systems and the type of contamination occurring from subsequent catchment areas. Well on a serious note, this has now become an environmental issue suppress our lifestyle and needs a special attention.
The discharge of storm water/ rain water depends on the type of surface it falls. This drainage system collects rainwater on unpaved areas, roofs, roads and any other areas not likely to be subject to contamination. The system discharges off working area and eventually to the sea/ river/ storm water pond/ low lined areas/ existing network, as untreated/ directly to water treatment units.
Project Engineering Work Processes -
An environmental execution strategy is established for each project. The project design criteria are specified in the project scope summary document. Any additional information or clarification shall be resolved with the project engineer before initiating the underground drainage system design.
A detailed overview of how the drainage system is designed is explained in the video below. Click here to open. Do like and share if you felt useful.
Basic Concepts & Terminologies -
Firstly, we should have our basics right and concepts crystal clear, so let’s have some bookish definitions. And for this you have to glance through this video below. If you want any further more terminologies cleared, please write in comment section.
Input Data & Design Considerations -
To design storm water, below listed input shall be required,
- Rainfall intensity of catchment area (I) mm/hr, or Intensity duration frequency curve (IDF) and shall be provided in Basis document.
- Catchment area (A) m2, shall be calculated from plot plan.
- Fire water demand (Qfire) m3/hr (if required)
- Minimum /Maximum Velocity of flows (m/sec) shall be followed.
- Manning’s coefficient for pipe/trenches (n) shall be followed from standards.
- Minimum pipe/trench size shall be followed from standards.
- HPP/FGL Elevations.
- Surface Runoff Coefficient (C) shall be followed from standards
- Pipe material should be specified.
- Pipe Design capacities (Full/ Partially full)
- Pipe stretch length in consecutive catch basin or manhole or trench section.
Design & Analysis Storm Water Design -
A) Rational Method of Runoff Calculations :
Rational formula is used to calculate the quantity of runoff at a given point in a storm drain system. Using this method, the peak flow can be calculated at any point in the system. This method is used to estimate the peak discharge/surface runoff for all system of network.
For Runoff coefficient values, here are some standardised tabulated practices followed in the design.
The surface runoff is calculated as follow:
Q = C x I x A
where,
Q = Design runoff/surface water runoff, cum/hr
A = Surface area, m2
C = Runoff coefficient (Please click here for standard coefficients)
I = Rainfall intensity, m/hr
To design storm water system, peak flow rate can be calculated for 10 years or even 100 years storm unless a different criteria is required by the state or local authorities.
The general formula used to calculate the rainfall intensity shall be,
I = k / (Tc + b)
where,
k = Constant,
b = Constant,
Tc = Time of concentration. ( TE + TF )
Time of concentration(t) -
The time of concentration is the combined time taken for rain water to runoff from the farthest point of the catchment area to the drainage system and drain through the system to the design point in the drainage system under consideration. The TC consists of two components, time of entry and time of flow.
Time of entry flow shall be calculated,
TE = [3.26 (1.1-C)D1/2] / (S1/3)
where,
TE = Time of entry (in Minutes)
C = Impermeability coefficient
D = Distance of most remote point to point of discharge(m)
S = Ground slope (%)
Time of flow shall be calculated -
TF = L / V
where,
L = length of channel, pipe (m)
V = Flow velocity (m/s)
where, the calculated time of concentration is less than 10 minutes, the “Tc” shall be based on the 10 minutes value until the calculated “Tc” exceed 10 minutes.
B) Drain sizing :
The Manning Formula shall be used to size pipes and open channels when the flow is uniform.
V = (1/n) R2/3 S1/2
Q(cal) = A x V
Q(cal) = A x (1/n) R2/3 S1/2
where,
R = Hydraulic radius = = Area of the cross section of flow divided by the wetted perimeter,
S = Slope of the total head line or may be taken as the slope of the pipe or channel,
= Manning roughness coefficient Depends on pipe material,
V = Mean velocity of flow m/sec,
= 0.6m/s (Minimum velocity Self cleansing)
= 2.4m/s (Maximum velocity no Scouring condition)
Q(cal) = Calculated discharge m3/hr
A = Cross sectional area of the pipe or channel, m2.
Pipe Network shall be generally calculated for 75% depth of flow or as per project / work specific requirement.
Drainage networks have been designed for the maximum flow resulting from the greater of the following combinations:
Rainfall (Qstorm) + Any other known contributing continuous ‘dry weather flow’.
Fire water (Qfire) + Any other known contributing continuous ‘dry weather flow’.
Design of the drainage network shall be based on the fire scenario to be established by HSE/Safety Department.
Safe Design Check: Design storm water or discharge Q(storm) shall be greater than the Q(cal) for safe design.
Underground Drainage Designed by other party firm -
If the underground drainage design is being outsourced then the execution is as shown below –
- Scope of work, specification and design basis shall be shared with third party firm.
- Enquiry package, Bid, and PO shall be placed and finalize the third party for design.
- Drawings(2D) & details shall be reviewed when received from outsourced firm/third party.
- Tie in locations shall be properly checked & verified as per input received from site if any.
- 3D model shall be updated inhouse as per drawing received from third party & clash check shall be done with other underground facilities.
Check, verify & approve third party design calculation and drawings.
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