Catchment Hydrology: Water Cycle, Rivers & Floods in a Changing Environment (Lecture) - Don Thompson

Scientific concepts, discoveries, and nature phenomena

Flood hazard in a changing climate (21st century focus)

• Hill slope hydrological cycle / catchment hydrology: how precipitation becomes runoff and reaches rivers.

• Climate-driven changes to precipitation

  • Amount, intensity, and frequency of rainfall are changing.
  • Seasonality shifts: likely more winter rainfall and less summer rainfall.

• Runoff generation pathways

  • Infiltration/percolation into soil leading to subsurface flow and then river discharge.
  • Hortonian overland flow: rainfall collects on the surface and runs off when infiltration is exceeded.
  • Saturated overland flow: when soil becomes saturated from below, water ponds, and then flows overland.

• Flood hydrograph concept

  • River discharge increases after rainfall with a lag time due to travel from slopes to the channel.
  • Bankfull discharge marks the transition to overbank flooding and floodplain inundation.
  • Floodplain storage is a key mechanism: flooding occurs when water exceeds bankfull, spreads onto floodplains, and is stored there.

Expected climate/sea-level context for flooding in the UK/England

• Warming trends

  • Historical warming referenced: ~0.8°C increase between 1961–1990.
  • Projected warming (as stated in the talk): ~7.4°C hotter summers and ~4.4°C warmer winters by 2050.

• Extreme events

  • Increased likelihood of extreme heat and extreme rainfall (probabilistic statements given).

• Sea-level rise

  • ~16 cm sea-level rise since the early 20th century (as stated).
  • Potential >1 m rise by 2100 under some scenarios (as stated).

• UK flood risk statistics

  • Homes and businesses at risk are cited (figures referenced in the talk).

Flood-rich period and storm behavior in northwestern Europe/UK

• Observed flood emergencies (examples from the 2000s–2020s)

  • Widespread flooding in 2000 (Oct–Nov).
  • Severe flooding in 2007 (June–July) affecting Yorkshire and the Midlands.
  • Flooding in 2012, 2013–14, 2015–16, 2019–20, and 2020 storm events.

• Storm mechanism described

  • Winter 2013–14: extremely intense storms linked to a strong Atlantic jet stream.
  • Attributed to contrast between cold North America and warm tropics.

• Hydraulic/geophysical flood impacts

  • Floods differ not only in inundation, but also in erosion, infrastructure/mechanical damage, and impacts from high-energy, high-velocity floods from upland catchments.
  • Examples include bridge damage and bank erosion.

Human land use and catchment “connectivity” (controls runoff to rivers)

• Land-use change over time (pre- vs post-WWII England)

  • Pre-WWII landscape: smaller fields, hedgerows, more tree cover, well-vegetated riverbanks, natural meandering rivers, soils tilled with less intensive machinery → higher infiltration capacity.
  • Post-WWII landscape: larger fields, hedgerow removal, year-round cropping, intensive cultivation, heavy machinery → soil compaction, drainage, and reduced infiltration.

• Mechanisms increasing flood generation and speed

  • Compacted soils (vehicle traffic, livestock trampling/“poaching”) → less infiltration → more surface ponding and runoff.
  • Wheelings/runoff pathways from farm operations during planting → faster runoff to channels.
  • Removal of hedgerows and riparian vegetation → increased hydrological connectivity between fields and rivers.
  • River engineering (straightening, artificial banks) → increases conveyance and reduces natural attenuation.

• Agricultural adaptation concept

  • Encourage farming practices that reduce runoff and reduce connectivity (the talk references working with farmers and changing how they perceive/adapt).

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Flood management approaches (methodology/strategy outlined)

Traditional “hard” flood defenses

• Channel resection / enlarging urban river channels

  • Increase channel capacity to keep water out of towns.
  • Trade-off: can damage ecology and river function.

• Levees / raised banks

  • Protect floodplains so land can be used for cultivation/settlement.
  • Risk noted: if levees fail, flooding can become more severe.

Hard engineering examples mentioned

• Flood diversion channels on the River Thames
  • Additional connected channel sections to divert floodwaters away from at-risk areas (e.g., Windsor/Maidenhead; plans referenced for further downstream areas).

Problem with hard defenses: “moving” the hazard downstream

• Levees/local protection can increase downstream flood probability
  • If one area is protected and flooding is prevented there, more water may be delivered downstream, raising risk for other communities.
  • Floodplains are described as natural storage zones; preventing flooding shifts where storage occurs.

More flexible/less ecologically invasive defenses

• Portable levees / demountable flood walls
  • Protect specific urban areas but can be removed after the event.

Flood abatement / nature-based catchment management

A systems approach: intervene before runoff reaches rivers.

• Land/watershed measures

  • Reforest/woodland management in catchments to absorb/slow runoff.
  • Floodplain woodlands and soil management to increase infiltration/storage.
  • Wetlands and catchment sediment traps to reduce sedimentation/silting and store water/materials.
  • Floodplain restoration to increase natural storage capacity.
  • Upland storage structures that hold runoff before it contributes to downstream flood hazard.

• “Slowing the flow” interventions

  • Leaky dams / woody debris structures in headwaters
    • Pond water, delay runoff release, and spread out flood timing.
  • Peatland restoration in uplands
    • Restore vegetated peat surfaces to enhance absorption/slow runoff.
  • Small gully blocks (in eroded peat/land)
    • Retain water locally and delay peak flows.

Evidence and uncertainty (as stated)

• Results are not conclusive for extreme floods, but there is evidence of reduced intensity for smaller/frequent events.

Sustainable Urban Drainage Systems (SuDS)

Apply “slowing the flow” in cities via:

• Permeable roads
• Green/vegetated roofs
• Roughened roofs/roads
• Local sink-aways with vegetation/trees to store and slow runoff before it reaches rivers

“Living with floods” / resilience

Shift from only preventing flooding to also managing impacts and recovery:

• Resilience measures at household level, e.g., flood gates and flood doors.
• Improved recovery and acceptance of flooding as part of a changing climate.

River restoration (ecology + flood function)

• Restore connections between river channels and floodplains
  • Remove/modify engineering that turns rivers into “boxes.”
  • Aim: allow more natural flooding in safer areas to improve ecological balance and support flood management.

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Researchers or sources featured (listed at end of talk)

• Environment Agency (England) (referenced as the producer of a national strategy and as the flood-policy body)
• Don Thompson (speaker; author of the lecture content)

(No individual scientists/researchers are explicitly named in the provided subtitles.)