Aminal’s Water division, part of the Flemish Government, is responsible for the management of certain river catchments in the Flemish speaking part of Belgium. The division has developed a number of models of its river systems.
Together with Wallingford Software, Aminal has recently developed a new methodology to create flood frequency maps. The new methodology is designed to estimate the precise flooding risks for specific key points of the river system.
The benefits of Flood Frequency Mapping
Flood forecasting is usually based on frequency maps - maps that show the probability of flooding, measured as the return period in years of a particular severity of flood event.
In former and some current flood estimating methodologies, the frequency results at one or a few locations, usually rain gauge locations, are used to develop flood frequency maps for the whole river system. These methods are based on the assumption that the results at one location- for example flow- can be propagated throughout the modeled network.
In many cases this assumption is incorrect. A rain event may be critical at one place but not at another. For example a local severe summer storm may be felt in the upstream sections of the river basin but may well be unnoticeable in the downstream sections of that river. Inundations along small rivers are often the result of a short and intensive shower of rain, whereas inundations along bigger rivers, which can transport higher discharges in a shorter time, are often the result of long-term high precipitation values.
The Flood Frequency Mapping (FFM) methodology looks at the precise flooding risks at every node in the network, so the different levels and effects of storms along the course of the modeled network can be taken into account. FFM permits a more thorough investigation of the frequency of inundation at specific locations, and of the effectiveness of any flood mitigation measures that may be considered.
The Yser River
The Yser basin, in the Flemish speaking part of Belgium, is bordered by France and the North Sea. The basin has an area of 425 sq miles (1101 km²) with 270 sq miles (700 km²) in Belgium. The source is in Lederzeele, France, and it discharges into the North Sea via a sluice complex in which river discharge is not possible at high tide. The total length of the river is 50 miles (78 km) with 28 miles (45 km) in Belgium. The Yser is a typical river with a fluvial regime: the flows on the border with France can fluctuate from less than 500 galls/sec (2 m³/s) in dry periods to 26000 galls/sec (100 m³/s) in severe circumstances.
In the last two decades the Yser has experienced some severe flooding events. In July 2005, record levels were experienced at a number of places. For the future management of the river it is therefore important to assess the risk of inundation in each specific area.
Building the model
The model of the Yser described in this paper was built using InfoWorks RS. It covers almost 125 miles (200 km) of the river and 3 tributaries. The hydrodynamic model consists of 2300 nodes, 102 storage areas and 55 hydrological boundaries.
To calibrate the hydrologic and hydrodynamic model, a network of almost 20 real time flow and/or stage meters was used. The hydrologic response to the river was modeled with the conceptual PDM-model, which is a part of InfoWorks RS. The input for the PDM model came from several rain and evaporation gauges in and outside the basin. These measurements were taken on a 15 minutes or hourly basis, and are available from 1898 onwards. The staggering of the rainfall was measured through the Thiessen polygon method. The stages and flows in the rivers and valleys were modeled with InfoWorks RS.
Indispensable in creating representative floodmaps is a digital elevation model (DEM). In this case study a very detailed DEM with a resolution of approximately 5x5 yards (5x5 m) was used.
Summary of the FFM methodology
The FFM methodology consists of the following steps:
· Selection of the rain events to be modeled;
· Running the selected events through the model;
· Statistical analysis of the results;
· Producing the frequency mapping.
InfoWorks RS automates each of these steps, making it possible to obtain flood frequency results quickly.
Selection of the rain events to be modeled
Ideally, flood frequencies would be estimated by running 100 years of historical rain data through the model, but such a calculation could easily take several months. To overcome this, the FFM methodology involves selecting a number of the most critical events- perhaps 50 or 100 events- along the course of the river over the time span, thereby reducing the computation load, but still using the model to examine the most severe average river conditions.
The selection is based on reviewing the partial duration series or peak over threshold (POT) of the historical river flow and stage data. Events are selected for input into the model if their magnitude exceeds a certain threshold. This selection is easily done inside InfoWorks RS by using the POT analysis tool and the Episode Collection tool; the POT tool enables the selection of the independent storms above a certain threshold, and the Episode Collection tool merges all the independent storms that are obtained in the different POT analysis.
Running the selected events through the model
The next step of the methodology is to run each of these selected rain events through the model. InfoWorks RS handles this as a single task, using a ‘Multirun’ tool that executes all the events out of the episode collection.
On completion of the runs and the production of results for every event, the frequency analysis on every node in the network can be undertaken.
Statistical analysis of the results
The results for every node, such as river sections and storage areas, can be examined for every one of the many simulation runs in the Multirun. In InfoWorks RS this is done by the Frequency Analysis tool.
The methodology involves producing frequency (probability) curves of the maximum stages (water depths) or flows, for every node. The continuous frequency curve is fitted onto the results points curve by InfoWorks. The curve used is either a Pareto or a Weibull distribution.
Flood Frequency Mapping
For each return period the frequency results can immediately be represented. The results can be reproduced in the form of flood frequency maps, tables and figures.
It is important to note that, using 50 year data, the frequency results are accurate only up to return period of approximately 50 years. Extrapolation of the distribution curve past the most severe storm can give an indication of higher return periods, but this remains uncertain. In other studies by Aminal’s division Water, 45 years of data were used, giving reliable return periods up to more than 100 years.
The full paper
This is a brief summary of the methodology used in the Yser study by Aminal. For a fuller explanation of the methodology, and the detail of how it was applied to the Yser project, click below on the pdf file of the full article by Sven Verbeke, Aminal, from which this summary was made.