Sources of flooding on floodplains of the tidal Thames HRPP 325 David Ramsbottom, Owen Tarrant & Alan Cooper Reproduced from a paper published in: Proceedings of the 41st Defra Flood and Coastal Management Conference 4 to 6 July 2006 University of York, UK Sources of flooding on floodplains of the tidal Thames Proceedings of the 41st Defra Flood and Coastal Management Conference SOURCES OF FLOODING ON FLOODPLAINS OF THE TIDAL THAMES David Ramsbottom1, Owen Tarrant2 & Alan Cooper1 1 HR Wallingford 2 Environment Agency Key Words: Flooding, tidal, fluvial Abstract The Thames Estuary 2100 (TE2100) project requires an assessment of flood risk for present day and future scenarios for the tidal Thames floodplains, to provide a basis for planning flood risk management over the next 100 years and beyond. To do this it is first necessary to understand the causes of flooding. The main potential sources of flooding include the surge tides on the tidal Thames, and fluvial flooding from the Thames, tributaries, land drainage systems and urban drainage systems. Flooding is affected by the operation of moveable gates including the Thames Barrier and barriers on other tributaries. The paper describes the sources of flooding, the ways in which they interact and the effects that they have on flood levels. The flood management system on the tidal Thames is described, and the impacts of failure of elements of the system are considered. Extreme sea levels and fluvial flows are expected to increase in the future, and the main implications for flood management on the Thames are summarised. 1. Introduction The floodplain of the tidal Thames covers an Tidal flooding has always been a threat to area of about 350 km2, and extends from London. Walls and other defences have been Teddington in West London to the sea near constructed along the estuary and raised over Southend. The floodplain covers highly the years, usually in response to particular developed areas on both banks of the Thames flood events. Following the 1953 floods, a through London including Kew, comprehensive tidal flood defence system Westminster, Canary Wharf, and large areas was constructed including the Thames of residential and commercial development. Barrier on the Thames, extensive fixed tidal East of the Thames Barrier at Woolwich the defences and a range of other barriers and pattern of development is less intense but control structures. there are major urban developments including Thamesmead and Canvey Island, The standard of protection provided for tidal and extensive commercial and industrial flooding is estimated to be 1 in 1000 years in areas. the year 2030, although the standard will gradually reduce as sea levels rise. The There are over 500,000 properties in the defences have a finite life, and therefore a floodplain and well over a million residents. constant programme of maintenance and In addition, many thousands of commuters refurbishment is needed to ensure that the work in the floodplain and the transport defences fulfil their function. infrastructure includes over 70 underground stations. 2006 1 HRPP 325 Sources of flooding on floodplains of the tidal Thames Proceedings of the 41st Defra Flood and Coastal Management Conference In addition to flooding from overtopping of this can be exacerbated by tide locking of the defences, there is a risk of breaching of watercourses during high tide periods, defences. Hence there is a constant need to causing water to back up in the drainage inspect the defences, identify weaknesses and systems. carry out necessary repairs. The paper considers flooding from each of Whilst tidal flooding is the main potential these sources both individually and in cause of catastrophic flooding on the combination, based on work carried out in Thames, other causes include flooding from Work Elements EP3, EP4 and EP10 of tributaries, local drainage and urban TE2100 (HR Wallingford 2006a, b and c). drainage. Whilst these are far less serious Groundwater flooding is not covered as this than potential tidal flooding, the frequency of is not reported to be a serious problem on these local floods can be much higher. The floodplains of the tidal Thames, although this main reason for this is that standards of could change if there are significant rises in protection are much lower, typically 30 to future water levels in the Thames. 100 years on tributaries where water levels are dominated by fluvial flows. The standard 2. Sources of flooding of protection on the Thames in West London for fluvial flooding is also much less than 2.1 Surge tides 1000 years. Predicted extreme surge tide water levels at Southend are shown on Table 2.1. Flooding from tributaries and drainage systems generally occurs when the systems overflow due to high fluvial flows. However Table 2.1 Extreme tide levels at Southend (Year 2000, Source JBA 2003) Return Period (years): 5 20 50 100 200 500 1000 Tide level (m AOD): 3.94 4.22 4.39 4.57 4.70 4.87 5.03 Typical increases in tide levels along the Lower Thames Marshes, and about 4m AOD estuary from Southend are as follows: in central London. • Southend to Thames Barrier at There is uncertainty in the predicted extreme Woolwich: 0.tides9m at Southend even without consideration • Woolwich to Westminster: 0.of3m climate (if the Barrierchange. is Inopen) addition, considerable variation is possible in the increase in level Thus the 1000-year level at Woolwich is between Southend and London. An about 6m AOD. The 1000-year level at important factor is the steepness of the Westminster would be about 6.3m AOD if incoming tide and variations of ± 0.4m are the Barrier did not close, over a metre above possible (HR Wallingford 2006a). Figure 2.1 local defence levels. However the Thames shows the variation in level along the estuary Barrier is operated to prevent overtopping of for different tide shapes. An event in the upriver defences. These levels contrast December 2005 produced levels that were with ground levels of close to 0m AOD in the over 0.3m higher than expected for this reason. 2006 2 HRPP 325 Sources of flooding on floodplains of the tidal Thames Proceedings of the 41st Defra Flood and Coastal Management Conference 7.500 Teddington Tide01 Tide02 Thames Barrier Tide03 Tide04 7.000 Tide05 Tide06 Tide07 Tide08 6.500 TideHR Southend 6.000 Water Level AOD) (m 5.500 'Average' tide shape Tide shape, December 2005 event 5.000 4.500 0 20000 40000 60000 80000 100000 Chainage (m) Figure 2.1 Variations in water levels on the tidal Thames for different tide shapes Not only is the sea level expected to rise as a future surge tide levels at Southend. The rate result of climate change, but the magnitude of increase of extreme levels for all scenarios of extreme surge tides is expected to worsen. except the Defra scenario is significantly Table 2.2 shows a range of predictions of greater than that of Mean Sea Level. Table 2.2 Predicted future extreme tide levels at Southend (Source: HR Wallingford 2005) Return Period 2005 Name of climate change scenario (years) Defra (2003) UKCIP02 High Plus High Plus Plus Medium High 2050 2100 2050 2100 2050 2100 2050 2100 Mean Sea Level 0.10 0.40 0.70 0.28 0.55 0.74 1.70 1.38 3.30 rise (m): 100 4.57 4.87 5.17 5.03 5.73 5.48 6.84 6.12 8.44 200 4.70 5.00 5.30 5.21 5.96 5.65 7.07 6.29 8.67 500 4.87 5.17 5.47 5.43 6.27 5.87 7.37 6.51 8.97 1000 5.03 5.33 5.63 5.63 6.54 6.07 7.63 6.71 9.23 10000 5.51 5.81 6.11 6.26 7.37 6.68 8.44 7.32 10.04 2.2 Fluvial flows on the Thames between Teddington and Brentford is from fluvial flows. The effect on water levels is Predicted extreme fluvial flows on the much less noticeable further downstream Thames at Teddington are shown on Table where levels are dominated by tidal flows. 2.3. The main potential cause of flooding 2006 3 HRPP 325 Sources of flooding on floodplains of the tidal Thames Proceedings of the 41st Defra Flood and Coastal Management Conference Table 2.3 Predicted future flood flows at Teddington (Source: HR Wallingford 2005) Return Period 2005 Name of climate change scenario (years) Defra (2003) UKCIP02 High Plus High Plus Plus Medium High 2050 2100 2050 2100 2050 2100 2050 2100 100 777 932 932 839 925 901 1088 1088 1554 200 869 1043 1043 939 1034 1008 1217 1217 1738 500 1007 1208 1208 1088 1198 1168 1410 1410 2014 1000 1126 1351 1351 1216 1340 1306 1577 1577 2252 10000 1562 1874 1874 1687 1859 1812 2187 2187 3124 2.3 Tributary fluvial flows Tributaries of the tidal Thames are shown in from tributaries have little impact on flood Figure 3.1. Catchment areas and peak flood levels in the tidal Thames, they contribute to flows are shown in Table 2.4 (HR flooding on the tidally influenced reaches of Wallingford 2006b). Whilst the fluvial flows the tributaries. Table 2.4 Characteristics of Tidal Thames tributaries River/Tributary Catchment URBEXT Estimated 100- Degree of area year peak flood urbanisation (km2) flow (m3/s) River Thames to 9,958.7 0.043 777 Slightly Teddington River Crane 86.5 0.350 35 Very heavily River Brent 168.4 0.411 61 Very heavily River Lee 1,412.3 0.120 159 Moderately River Roding 345.6 0.094 80 Moderately Beam River 62.9 0.289 21 Very heavily River Ingrebourne 62.3 0.132 25 Heavily Mar Dyke 103.6 0.049 47 Slightly Mucking Creek 15.2 0.125 18 Heavily Canvey Island Creeks 38.6 Small - Small Beverley Brook 56.4 0.322 22 Very heavily River Wandle 192.5 0.277 32 Very heavily River Ravensbourne 177.9 0.312 47 Very heavily River Darent 253.0 0.053 33 Moderately River Cray 130.2 0.187 37 Heavily 2.4 Fluvial flows from smaller ground where water drains into the watercourses and drainage systems floodplain.