Workshop 16 - 17 September 2004Sandilands, United Kingdom

COMRISK Subproject 8 “Risk Assessment Lincolnshire, Pilot Studies” (SP8)

Location and Issues23km coastline between Mablethorpe and Skegness

Assets located within risk area include densely populated areas and many smaller towns and villages

Existing standard of defence and Tidal Flood extents for present defence

conditions

4.5mODN

Design berm width

1:15 beach slope

Existing and proposed defences at Ingoldmells (not to scale)

Scale of Shortfall

Location and volume of beach recharge between 1999 and 2004

Aim

• To provide an operational tool to assess the value (in terms of risk reduction) of various beach management scenarios.

• Input variables to be: beach profile and available budget.

1. Divide study frontage into section.2. Identify a range of permutations for beach

volume change across each identified zone. 3. Determine mode of defence failure (overtopping

and breach) for the range of beach volumes within each zone.

4. Run flood propagation model for scenarios.5. Develop a user system to enable input

information on beach volumes within each zone, as determined by annual surveys, to access a series of look-up tables identifying associated risk.

Outline Method

1. Divide study frontage into sections

Wave Zones

Coastal Cells

Existing Seawall Structure and

Beach Profile

2. Identify existing defence standard and beach volume change across each identified zone

Apr 2002 - Jan 2004

% Volume Change

11 Steps/ Prom/ Swall 500 -

12 Steps/ RWall 500 -4%

13 Steps/ RWall 200 -2%

14 SApron/ RWalll -5%

15 Armour/ Prom/ Steps/ RWall -1%

16 Steps/ RWall 500 0%

17 Steps/ RWall 500 -8%

18 Steps/ SApron/ RWall -7%

19 Steps/ SApron/ RWall 5%

20 Steps/ Rwall -2%

21 Steps/ Rwall -1%

22 SApron/ RWalll 500 0%

23 Steps/ RWall 4%

24 100 11%

25 Steps/ RWall 500 6%

26 Steps/ RWall 3%

27 SApron/ RWall 6%

28 Armour/ Steps/ RWall 200 9%

29 Armour/ Steps/ RWall 200 2%

30 -1%

31 Steps/ RWall -4%

32 Armour/ Steps/ RWall 500 1%

33 Steps/ RWall -7%

34 -6%

35 Steps/ RWall 500 4%

36 Armour/ Steps/ RWall 500 3%

37 Armour/ Steps/ SApron/ Prom/ Swall 500 0%

38 -1%

39 Steps/ Prom/ SWall 200 -4%

40 Steps/ Prom/ SWall 50 1%

41 0%

42 Steps/ Prom/ RWall 200 -8%

43 -8%

44 Seebee/ PromRWall 200 -4%

45 Dunes -8%

46 -11%

47 Dunes 500 -1%

48 Dunes 5%

49 Dunes 8%

50 Dunes 6%

51 Dunes 9%

52 Dunes 11%

53 -3%

54 -7%

55 BGR/ Beam/ Dunes -8%

56 -15%

57 Dunes -2%

58 -1%

59 Steps/ Prom/ SWall 3%

60 Steps/ RWall 18%

61 Steps/ Dunes 200 10%

62 -2%

63 Steps/ RWall/ Prom 2%

64 16%

65 20%

66 7%

67 Armour/ Prom/ RWall 500 -2%

68 Armour/ SApron/ Prom/ SWall 50 -1%

69 Armour/ SApron/ Prom/ SWall -5%

70 -11%

71 Armour/ SApron/ Steps/ Prom 200 -2%

72 Armour/ Steps/ RWall 200 10%

73 Armour/ Steps/ RWall 0%

74 Steps/ Prom/ SWall -6%

75 SApron/ Seabee/ SApron/ Steps/ RWall 100 -5%

76 -1%

77 Steps/ Prom/ SWall -4%

78 SApron/ Prom/ SWall -5%

79 Steps/ RWall 200 4%

Profile No.

Return period of event causing defence failure

(based on Apr 2002 beach profiles)

Defence

3. Determine mode of defence failure (overtopping and breach) for the range of beach volumes within each zone.

Failure modes• Face Erosion• Toe Erosion• Overturning• Reduction in bearing

capacity• Piping• Extreme Overtopping

Most prevalent structural failure mechanism for the Lincshore seawalls is due to extreme overtopping of the structure

Low beach levels

Overtopping and beach changes

Overtopping = F (storm, water level, structure, beach)

Crest level, Berm width, Toe level, SlopeSediment size

BEACH PARAMETERS

VOLUME

Overtopping = F (storm, water level, structure, beach volume)

Overtopping

property

crest back

Face

Dune

OT1 OT3OT2

OT1 Overtopping Face/Crest Overtopping Face/Crest is the actual overtopping rate over 1st return wall

OT3 Overtopping Back/Property The Overtopping Back/Property is the actual overtopping over the final line of defence onto the property

OT2 Overtopping Crest/Back Overtopping Crest/Back is the (effective) overtopping rate over the splash wall onto dunes. Please note that it might be less than the actual overtopping rate but the effective overtopping determines the erosive force which cause structural damage

Next Steps

• Run flood propagation model for overtopping volumes.

• Mapping of risks/damages as a result of flooding and overtopping.

• Compare results with findings from other studies (RASP, Foresight etc)

Key lessons learnt• Difficult to link overtopping with beach profile:

large number of beach variables.• Large number of modelling runs required to

simulate scenarios in order to generate comprehensive look-up table.

• Flood areas and economic damages are primarily influenced by breach occurrences. Very sensitive to overtopping limit selected.

• Majority of economic damages concentrated in limited number of flood reservoirs.

• Assumes the sediment supply continues as per the existing source.