Sustainable Drainage Systems

Question 1

List some sewage pollutants

Answer 1

1. suspended solids (organic material, inert solids)
2. dissolved substances (ammonia, hormones)
3. aesthetic pollution (rags)
4. bacteria, pathogens
5. riverbank erosion

Question 2

Name two traditional engineering solutions to reducing sewer pollution

Answer 2

1. storage tanks (hard engineering)
2. separate sewer systems

Question 3

Name the 4 SuDS pillars/quadrants

Answer 3

Quality, Quantity, Amenity and Biodiversity

Question 4

What are the SuDS design criteria (CIRIA C697)?

Answer 4

1. store or safely pass the runoff from storms without putting public or property at risk.
2. reduce pre-development risk of flooding.
3. prevent downstream bank erosion.
4. reduce urban runoff pollutants and improve quality.
5. provide amenity and ecological benifits.

Question 5

List some SuDS misconceptions

Answer 5

-SuDS are soakaways
-SuDS cannot be used in clay soils
-SuDS are ponds only
-SuDS require a lot of space
-SuDS dont include pipes

Question 6

What is the Greenfield runoff rate?

Answer 6

The target rate of discharge to receiving water for the site via the provision of storage and flow constraints. Good storage and attenuation can keep this low. (typically 5 l/s/ha)

Question 7

What elements of design should be considered when designing a SuDS?

Answer 7

-rainfall
-runoff, rational method
-conveyance
-Inlets, outlets and flow controls
-storage
-Infiltration

Question 8

What is the minimum infiltration coefficient to warrant the design of an infiltration-based SuDS?

Answer 8

4x10^-4 m/h
(or, 1x10^-7 m/s)

Question 9

What is unsustainable about conventional underground combined sewerage systems?

Answer 9

• treats rainwater as a waste product, not a resource
• WTP’s require energy to run
• concrete has high CO2 footprint
• system capacity needs to increase as ubanisation

Question 10

What is the difference between amenity and biodiversity?

Answer 10

A - “better places for people”
B - “better places for nature”

Question 11

What are the positives and negatives of infiltration SuDS devices?

Answer 11

(+) potentially infinite capacity. Also contribute to groundwater recharge.
(-) may be affected by clogging, may cause groundwater pollution if the surface runoff isn’t treated appropriately.

Question 12

What are the positives and negatives of storage SuDS devices?

Answer 12

(+) may be applied anywhere, even where the natural ground conditions are impermeable.
(-) storage capacity will be finite, so there may be runoff in extreme events or if the storage hasn’t emptied prior to the rainfall event.

Question 13

What rainfall return periods should be used to design SuDS?

Answer 13

For site flooding, return periods of 10-30 yrs are typically adopted.
Smaller events (2 yr) are significant for river hydro-morphological protection.

Question 14

What rainfall durations should be used to design SuDS?

Answer 14

The SuDS Manual recommends use of the 1 in 100 year 6-hour event for checking volume control.

Question 15

How many mm of rainfall should be managed using interception?

Answer 15

5 mm

Question 16

List three different SuDS devices that can contribute to initial losses

Answer 16

Rainwater harvesting (RWH). Green roof. Permeable pavement.

Question 17

What two checks are applied in the design of infiltration-based SuDS?

Answer 17

Is the storage volume big enough? Depth of stored water during a design event.
Drain down time. Needs to half drain within 24 hours

Question 18

List four common runoff treatment mechanisms

Answer 18

Sedimentation – sediments settle out of water.
Filtration – trapping within soil/aggregate.
Adsorption – pollutants attach to the surface of soil.
Biodegradation – microbial communities degrade organic pollutants.

Question 19

How do ponds and swales improve runoff quality?

Answer 19

slow velocities encourage sedimentation and adsorption. Plants encourage filtration and nutrient uptake.

Question 20

Why is it better to use a treatment train rather than a single SuDS device?

Answer 20

Redundancy – if one device fails, another may compensate.
Different devices may be better at dealing with different pollutants.

Question 21

What are the main hydrological processes that contribute to retention (or initial losses) in a green roof?

Answer 21

Evapotranspiration in the days before a rainfall event. There will also be some interception by vegetation.

Question 22

List three alternative approaches that have been used to model detention (delay) in a green roof.

Answer 22

Unit hydrograph
time-area diagram
reservoir routing