One of the test areas that have been selected is the Cranbrook catchment, located within the London Borough of Redbridge in the Northeast part of Greater London. The catchment is predominantly urban with two off line lakes, a couple of parks, playing fields and a golf course. This area was selected because it has a history of both fluvial and pluvial flooding, with flood events reported since 1926 and recent events in October 2000, February 2009 and January 2012.
A UK case study : the Cranbrook catchment
Figure 1. Location of the Cranbrook catchment in relation to the Roding River catchment and the River Thames
The test area we have selected for this project is the Cranbrook catchment, located within the London Borough of Redbridge (situated in the Northeast part of Greater London).
The drainage area of this catchment is approximately 910 hectares. The main water course is about 5.75 km long, of which 5.69 km are piped or culverted. The catchment is predominantly urban with two off line lakes, a couple of parks, playing fields and a golf course. This area was selected because it has a history of both fluvial and pluvial flooding, with flood events reported since 1926 and recent events in October 2000, February 2009 and January 2012.
The Cran Brook is a tributary of the Roding River, which in turn is a tributary of the River Thames. The Roding River constitutes a boundary condition for the overland and sewer networks of the Cranbrook catchment, given that the water levels in the Roding River (when at high stage) affect the capacity of the drainage system of the Cranbrook catchment. For this reason, attention will be given to both, the Cranbrook catchment as well as the Roding River catchment during the RainGain project.
Figure 2. Monitoring system of the Cranbrook catchment
In order to monitor the area and gather real time data that can be used for calibration, validation and real time operation, a monitoring system has been put into place. The sensors that have been installed are the following:
- 3 tipping bucket rain gauges, with 1-2 min data “sampling”. These rain gauges are installed in the roof tops of 3 high schools of the area.
- 1 pressure sensor for Roding River level monitoring. Real time frequency: 5/10 min.
- 2 sensors for water depth measurement in sewers. Real time frequency: 5/10 min.
- 1 sensor for water depth measurement in an open channel which goes through the biggest park of the area (i.e. Valentine’s Park).
All of the sensors are equipped with data acquisition and real-time access wireless communication units, which allow for measurements to be accessed in real-time via Internet.
Other rainfall data available for the area
Figure 3. Radar coverage in the Cranbrook catchment
In addition to the raingauges installed in the study area, the following rainfall data is available and will be tested and improved during the RainGain project:
- Raingauge data from the London Grid for Learning: over 40 rain gauges in Great London, with real time access to 1 minute resolution data and access to historical data since 06/06/2006.
- Raw radar data : there are two C-band radars whose signal covers the Cranbrook catchment: the Chenies radar and the Turnham radar. Research aiming at improving radar signal for obtaining better temporal and spatial resolution is currently being carried out at Imperial College in collaboration with The Met Office, as part of the RainGain project
- Nimrod data : it corresponds to composite radar data which has been calibrated (or enhanced) using rain gauge observations, satellite images and Numerical Weather Prediction (NWP).
- STEPS (Short Term Ensemble Prediction System) forecast : this is the forecasting system currently used by the Met Office. STEPS is a hybrid method which combines rainfall nowcasting techniques (based upon calibrated radar and rain gauge observations) and Numerical Weather Prediction (NWP) models (based on the solution of a set of equation of fluid dynamics and thermodynamics to estimate the future state of the atmosphere in synoptic scale –≥1000 km grid– or mesoscales –from approx. 5 to several hundred km grid–). These two techniques are combined with the aim of carrying out high-resolution (5/15 min and 1/2 km) rainfall prediction with longer lead time (3/6 hr).
Urban pluvial flood model of the area
Figure 4. Dual drainage model of the Cranbrook catchment
In order to reliably model urban pluvial flooding, it is necessary to realistically represent the urban fabric in its complexity, taking into account the local topography and the interactions between the overland and sewer networks, as well as the boundary conditions that determine the performance of the system. For the Cranbrook catchment physically based models will be developed, which take into account the interaction between the overland (surface) and the sewer network; this is known as the “dual-drainage concept”. These models will be customised in such a way that they can accurately predict flood extents, while keeping computational requirements (specially running-time) within limits which enable real-time applications (mainly real-time forecasting of urban pluvial flooding). The different rainfall inputs mentioned above will be tested by feeding them into the urban pluvial flood model of the study area.