1 - Urban Drainage Systems Flashcards
What does a storage gauge do?
A funnel that collects rain as it falls, which is only suitable for long term measurement
What are autographic gauages?
A device that records the amount of rainfall falling in a short time.
What is radar?
Satellite technology - colour coded to show different weather events.
What is rainfall expressed in?
mm, duration is expressed in hours.
What is rainfall intensity?
i = x / D
Rainfall depth / Duration
What is the catchment area?
The area of land which a creek or river recieves its water
In a natural catchement, what are boundaries?
Ridges and hills
In agricultural and urban areas, how are the natural oundaries altered?
Because water flow is controlled via : Channels, gutters, drains, sewers and ditches.
What does a rainfall event that occurs twenty times in 100 years have a return period of?
5 Years
What is the ministry of health rainfall intensity method?
i = a / (D+b)
Where a and b are constants and D is the duration in minutes.
What is the ratio, r map?
The ratio of the M5-50min map to the M5-2day map
What do r values below 0.2 represent?
Drizzle
What do r values above 0.4 indicate?
Intense storms
What 2 types of water require drainage?
Wastewater and foulwater
What does poor stormwater drainage result in?
Inconveniance, damage, flooding and health risks
What does stormwater contain?
Pollutants from the rain and catchment area.
What are drains?
Pipes carrying flow from individual properties.
What are sewers?
Larger pipes carrying flow from groups of properties or larger areas.
What is sewerage?
The whole infrastructure - drains, sewers, manholes and pumping stations.
What is sewage?
Any unwanted wastewater or the content of swers conveying the water-brone wastes of a community.
In the UK, what percent of sewerage system is combined?
70 percent.
Why are combined systems not economically feasible?
They carry very little water during dry conditions - so most of the time carry very little water.
Why is separate systems preferred?
Reduces pollution discharge to watercourses.
What are the advanatages of combined systems?
Cheaper, smaller footprint, house drainage systems less complex.
What are the disadanatages of combined systems?
Pollution in watercourses, flood water is foul, large treatment works necessary.
What are the advantages of separate water systems?
Less pollution, smaller wastewater sewers - higher velocities maintained.
What are the disadvanatages of seperate systems?
More expensive, no treatment of stormwater/
What are the consequences of urbansiation on where water travels?
More runoff, less infriltration and less evapo-transpiration.
Does rainfall reach the river quicker after a storm in a natural or urban storm?
In an urban storm
In hydrology, what is the time of concentration?
The time required for rain falling at the farthest point of the catchment to the measuring point.
What is the time of flow in each sewer pipe ?
tf, the underground flow time.
What is the time of entry, te?
The over-ground flow runoff time
What is the time of concentration the sum of?
The underground flow time and the overground flow time
tc = te + tf
In a stormwater sewer, what is the time of entry?
The longest time it takes for a drop of water when it hits the surface in the catchment to when it enters the sewer pipe.
Why do storms with a longer return period have a lower time of entry?
More intense, produce more water on catchment surface and hence faster overland flow and reduced time of entry.
What is the time of flow, tf?
tf = Length of pipe / velocity in pipe
What does the flow of water equal in asewer?
Q = xA / t = iA
Why is the flow never actually iA? What is it instead?
Real catchments are rarely totally impermeable.
Thus the peak flow Qp = CiA, where c is the runoff coefficient.
What is the equation of Cv?
Percentage runoff / 100
What does 1 hectare equal?
10000m^2 or 0.01km^2
When designing stormwater sewers, what do you set the time of concentration to?
The duration of the storm.
If the duration of rainfall is less than the time of concentration, why is the peak flow low?
Because the entire catchment is not contributing together - contributions form remote parts are contributing after those near parts have ceased.
What is the step by step process of the rational method?
1 - Find area of catchment
2 - Find time of concrentration
3 - Find rainfall intensity
4 - Therefore find the peak flow
In the percentage runoff equation, what is PIMP, UCWI and SOIL?
PIMP is the precentage impermeability
SOIL is an index relating to the water-holding capacity of soil.
UCWI is the urban catchement wetness index.
What does the percentage impermeability represent the degree of?
Urbanisation in the catchment
PIMP = Ai / A x 100
Where Ai is the impervious area.
The diameter of concrete stormwater pipes start at 225mm and go up in increments of what?
75mm
What does the Colebrook-White equation give us?
The velocity in a rough, full pipe.
What is the rougness of a new, concrete pipe?
0.06mm
What have to fill up before water reaches the surface?
Manholes need to fill up.
What is the vertical level of a sewer defined by?
Its invert level (IL). Which is the lowest point inside the pipe.
What is the soffit level in a pipe?
The highest point inside the pipe.
What is the crown level in a pipe?
The highest point outside the pipe.
What is the depth of a pipe?
The lowest point on the outside of the pipe.
Why are access points required in a drainage system?
Access points are required for the connection, testing, inspection and cleaning of the sewer pipes.
In sewers, what are the 6 reasons to have manholes?
Change in directions, head of run, change in gradient, change in diameter, major functions, and every 90m.
In small diameter sewers, why is the flow intermittent?
Only one or two households may discharge into them.
What method is used to design small sewarage schemes?
The discharge unit method
What is the design flow rate, Q, of a foul water system?
kDU * sum of the square root of the discharge units.
What is kDU?
A dimensionless frequency factor which ranges from 0.5 - 1.0, 1 for congested buildings.
Why is a partially full foul water pipe required?
Because the waste generates noxious and explosive gases which need to be dispersed by air.
For geometrical considerations, what does the angle subtended equal, in relation to the depth of flow d, and the diameter, D?
Angle = 2*cos^-1 (1 - 2d/D)
What does sin2theta equal?
2sin theta cos theta
What is the definition of the depth of water in a pipe?
The height of water surface above the pipe invert.
What is the top width in a pipe?
The flow width at the surface.
What is the wetted perimeter?
The length of pipe circumference that is in contact with the water.
What is the hydraulic radius?
The ratio of wetted perimeter to the area.
What is the hydraulic mean depth?
The mean depth of water.
What does a Butler-Pinkerton Chart show?
The hydraulic mean depth as the diameter of the pipe and flow rate increase - gives you estimates for the velocity at different gradients.
Why are sewers required to keep the proportional flow depth, d/D below 0.7?
In order to allow for the ventilation of drains.
What pipe roughness value do you use in foul water sewers?
0.6mm
What is the equation of the Froude Number?
Fr = velocity / root (gravity x mean hydraulic depth)
Why are most foul water sewers designed for subcritical flow?
As if the flow were to go subcritical via a hydrualic jump, this creates turbulence and erosion.
Why are SUDS being increasingly used?
To mitigate the flows and pollution from runoff.
What is the philosophy of SUDS?
To replicate the natural drainage from a site before development and to treat runoff to remove pollutants.
What are the benefits of using SUDS?
Environmental benefits, such as wildlife habitat, improved aesthetics or community resources.
As SUDS reduce peak flows to watercourses or sewers, what does this reduce the risk of?
Downstream flooding
How do SUDS improve water quality?
By removing pollutants.
How do SUDS increase base flows into watercourses?
Through the slow release of water into groundwater systems.
When SUDS are included in the design stage, what are the benefits of this?
Easier, cheaper and a higher quality of design.
What are pervious pavements?
Pervious is a combination of porous surfaces, which allow water to infriltrate across its entire surface, and permeable surfaces - which consist of a material that is imprevious but has patterns of voids over the surface.
What are the pros of using Pervious pavements?
- Reduces the rate and volume of runoff
-Reduces the effects of pollution in the runoff
-Reduced need for deep excavations for drainage
-Costs are comparable to conventional surfacing and drainage systems
-Ponding does not generally occur.
What are the cons of using pervious pavements?
-Frequent sweeping required
-Needs changes in construction techniques.
Why are Green roofs used?
Peak flows and volume of runoff are reduced, so they can take pressure of a SUDS system or conventional drainage.
What are the cons of green roofing?
Damage to waterproof membrane can be critical, and maintenance is higher than conventional roofs.
What are swales?
Broads and shallow channels that are designed to convery or store runoff and remove pollutants
What is a dry swale?
Relies on slowing the flow and allowing it to infriltrate the soil below
What is enhance dry swale?
A filter layer of soil over an underdrain keeps the swale dry most of the time, preventing the stagnant build up of water.
What is a wet swale?
A swale that during storms, forms a boggy wetland.
What are the benefits of swales?
Easy to incorprate, reduce peak flows, filter pollutants, low costs
What are the negatives of Swales?
Vulnerable to large storms, large size required to deal with runoff from less frequent storms.
What do infiltration devices do?
Take runoff, temporarily store it then allow it to percolate into the ground.
Why is there limited use of infiltration devices in the UK?
Because of high water tables and clay soils.
Are clay and silt ground conditions suitable for a soakaway?
No
How far away from foundations and buildings must soakaways be?
A minimum of 5m
What are detention basins?
Engineered or natural basins that are designed to store water after storm events to remove pollutants.
What is a sediment forebay?
Soemthing that removes sediment from any runoff entering the pond.
What are the pros of detention basins?
Can be used in most ground conditions, aestetic
What are the negatives of detention basins?
Land takes limit, safety hazard for children.
