Introduction to Urban Drainage. Scott Arthur. Civil Engineering at the School of the Built Environment. [Class 1 - Into+ 1]

Transcription

1 Introduction to Urban Drainage Scott Arthur Civil Engineering at the School of the Built Environment [Class 1 - Into+ 1]

2 Do we even need a drainage system? [Class 1 - Into+ 2]

3 Greenfield Hydrology When rain falls on to undeveloped land, most of the water will percolate through the soil to the nearest watercourse or groundwater reservoir. A small proportion of the rainfall [~20%] becomes direct runoff and usually slowly [because the surface is rough ] drains into watercourses. This means that the effects of rainfall are spread out over a period of several hours. Even short, heavy storms may have little impact on flow rates in the receiving waters because much of the rain water may be absorbed into the ground. [Class 1 - Into+ 3]

4 Why don t urbanised catchments work like this? [Class 1 - Into+ 4]

5 Developed Catchment Hydrology When a catchment is developed, the proportion of land covered by impervious surfaces will increase, preventing the natural infiltration of rainfall into the ground. Often the remaining open ground cannot accept water as rapidly as it did in its natural state. [Class 1 - Into+ 5]

6 How did we traditionally solve this problem? [Class 1 - Into+ 6]

7 Conventional Drainage Systems System Characteristics Even within developing countries, substantial parts of the population will have no connection to a sewerage system, in the EU this varies from 4% in the UK to 43% in Portugal. Where connections do exist, these will be to three types of system: Combined - A single pipe which carries both SW + WW. Separate - A system where SW + WW is carried by separate pipes. Hybrid A system which is mostly separate, but the WW system will have some SW inputs. [Class 1 - Into+ 7]

8 Combined Systems In France, Germany & the UK ~70% of sewer lengths are combined. During rainfall events SW flows predominate - >90% is not unusual (i.e. 9 Parts Rain + 1 Part Sewage). It is not economical to size sewers to ensure they can accommodate all rainfall events most surcharge at a frequency <1:2 years It is not economical to size WWTW to treat all flows (6 DWF is the norm). [Class 1 - Into+ 8]

9 Typical Combined System Layout [Class 1 - Into+ 9]

10 Combined Systems CSOs The solution to the capacity issue is CSO s (Combined Sewer Overflows). This solution means that flows caused by medium or high rainfall events will be diverted to a local watercourse. During rainfall events a combined system will receive WW + SW. All flows above a certain ( setting ) level will be discharged into the environment ( spill ). Flows below the setting level will be passed for treatment ( continuation flows). CSO s are normally fitted with devices which have the aim of preventing gross solids from leaving the system. [Class 1 - Into+ 10]

11 Basic CSO Performance & Terminology [Class 1 - Into+ 11]

12 Basic CSO Performance & Terminology [Class 1 - Into+ 12]

13 CSO [Class 1 - Into+ 13]

14 CSO [Class 1 - Into+ 14]

15 CSO [Class 1 - Into+ 15]

16 CSO [Class 1 - Into+ 16]

17 CSO - Edinburgh [Class 1 - Into+ 17]

18 CSO [Class 1 - Into+ 18]

19 CSO - Edinburgh [Class 1 - Into+ 19]

20 CSO [Class 1 - Into+ 20]

21 Heavy Rains Pose Bathing Risk [Class 1 - Into+ 21]

22 Thames Sewage Solution Needed [Class 1 - Into+ 22]

23 Tunnel to Resolve Sewage Pollution [Class 1 - Into+ 23]

24 Action Pledged on Thames Sewage [Class 1 - Into+ 24]

25 CSO s in the public [Class domain 1 - Into+ 25]

26 Glasgow s Unsatisfactory Intermittent Discharges (UIDs) Typical CSO spill volumes of approximately 50 million cubic metres per annum to overall Clyde system o Olympic sized swimming pools Combined Sewer Overflows (CSOs) o ~ 550 CSOs in Greater Glasgow study area o Over 200 unsatisfactory CSOs (water quality) o Remaining 300+ have aesthetics problems [Class 1 - Into+ 26]

27 CSOs on the Clyde [Class 1 - Into+ 27]

28 [Class 1 - Into+ 28] Water Quality on the Clyde CSOs have a major negative impact.

29 [Class 1 - Into+ 29]

30 The Sewer Monster s Christmas Tree [Class 1 - Into+ 30]

31 CSOs may cause problems, but don t forget why they are needed. [Class 1 - Into+ 31]

32 [Class 1 - Into+ 32] CSOs may cause problems, but don t forget why they are needed.

33 CSOs may cause problems, but don t forget why they are needed. [Class 1 - Into+ 33]

34 CSOs may cause problems, but don t forget why they are needed. [Class 1 - Into+ 34]

35 CSOs may cause problems, but don t forget why they are [Class needed. 1 - Into+ 35]

36 CSOs may cause problems, but don t [Class 1 - Into+ 36] forget why they are needed.

37 Is there another alternative to CSOs? Can t Civil Engineers Save Us? Is there another alternative to CSOs? [Class 1 - Into+ 37]

38 Stormwater Storage Tank Sheffield 13,175 m 3 Storage Tanks [Class 1 - Into+ 38]

39 Storage Tanks [Class 1 - Into+ 39]

40 Separate Systems Most systems installed in the UK since 1945 will be separate systems. To cut costs the separate WW & SW pipes are often laid side-by-side. Foul systems are designed to convey peak WW loads. Surface water systems are designed to accommodated a given design storm. SW is normally discharged into an adjacent watercourse. [Class 1 - Into+ 40]

41 Typical Separate System Layout [Class 1 - Into+ 41]

42 Surface Water Drain Outlet [Class 1 - Into+ 42]

43 Separate Systems - Drawbacks Although separate systems remove the problems associated with CSO s, they do still have some drawbacks: Expensive. SW can still carry a pollutant load (hydrocarbons, road salts etc). SW will still enter foul sewers via infiltration and/or inflow (due to ignorance and/or malpractice). If only 1% of connections are incorrect, the environmental benefit is lost. [Class 1 - Into+ 43]

44 Surface Water Drainage Systems are not free from pollutants. [Class 1 - Into+ 44]

45 Gully pots, if well maintained, can stop sediments entering the system. [Class 1 - Into+ 45]

46 Receiving Water Impacts In urbanised catchments, direct runoff can increase to more than 80 per cent of the rainfall volume. Furthermore, because paved surfaces may be smoother than natural surfaces, water may travel over them faster and as a result runoff will reach the receiving watercourses more quickly. The flow rates in the receiving waters are therefore much more sensitive to rainfall intensity and volume than those in an undeveloped catchment. [Class 1 - Into+ 46]

47 Receiving Water Impacts (ctd) Volumes and rates of runoff from catchments both increase significantly after development. Peak flow rates can increase by 1000%, which means that streams and rivers have to cope with larger and often sudden runoff flows. It also means that drainage is needed to reduce flood risk within the developed areas. [Class 1 - Into+ 47]

48 Receiving Water Impacts (ctd) In urbanised catchments, direct runoff can increase to more than 80 per cent of the rainfall volume. Furthermore, because paved surfaces may be smoother than natural surfaces, water may travel over them faster and as a result runoff will reach the receiving watercourses more quickly. The flow rates in the receiving waters are therefore much more sensitive to rainfall intensity and volume than those in an undeveloped catchment. [Class 1 - Into+ 48]

49 Receiving Water Impacts (ctd) [Class 1 - Into+ 49]

50 Receiving Water Impacts (ctd) The increased flows that may result from traditional drainage systems can cause further problems; for example: They may cause streams to scour deeper. Eroded sediments may be deposited downstream in slower-moving reaches of the river. Aquatic habitats may be damaged due to changes in the flow profile. Increased flows can also lead to flooding downstream. [Class 1 - Into+ 50]

51 Receiving Water Impacts (ctd) Furthermore, many existing piped drainage systems are operating at full capacity; they may be unable to take additional runoff and will require major work to enlarge their capacity if they are to cater for new developments or climate change! [Class 1 - Into+ 51]

52 Moving Forward Conclusion: Perhaps we should design our drainage system to act as an undeveloped catchment(?). How can this be done? [Class 1 - Into+ 52]

53 Sustainable Drainage Systems (SuDS). Sustainable use of resources is on the development agenda. In particular, there is a need to strike a balance between the management of floods, surface water drainage, costs and the need to conserve natural resources. One factor in achieving this is the widespread use of Sustainable Drainage Systems (SuDS). SuDS are designed to manage surface water runoff in a more sustainable way than traditional drainage systems. [Class 1 - Into+ 53]

54 Sustainable Drainage Systems (SuDS). They provide more natural approaches to runoff management and, when incorporated into developments, help to prevent increases in flood or water pollution risk downstream of the development. They can also provide direct improvements to amenity and biodiversity. [Class 1 - Into+ 54]

55 So how do SuDS change things? 1. Runoff from a developed area should not exceed that prior to development. 2. Runoff should not result in any downgrading of downstream watercourses. 3. Pollution generated by a development should be treated within the development area before discharge. 4. The wider needs of the community are considered in the development of the design. [Class 1 - Into+ 55]

56 SuDS Triangle [Class 1 - Into+ 56]

57 SuDS Triangle [Class 1 - Into+ 57]

58 Sustainable Drainage Systems in Action [Class 1 - Into+ 58]

59 Sustainable Drainage Systems in Action [Class 1 - Into+ 59]

60 Sustainable Drainage Systems in Action [Class 1 - Into+ 60]

61 Sustainable Drainage Systems in Action P&R [Class 1 - Into+ 61]

62 Pre-Development [Class 1 - Into+ 62]

63 Ground Water Pre-Development [Class 1 - Into+ 63]

64 Ground Water Conventional Surface Water Drainage [Class 1 - Into+ 64]

65 Ground Water SuDS Surface Water Drainage [Class 1 - Into+ 65]

66 SuDS Surface Water Drainage [Class 1 - Into+ 66]

67 Thank You [Class 1 - Into+ 67]