Infos and contacts :

Aurore.degre@ulg.ac.be & francois.colard@ulg.ac.beUniversity of Liege

Gembloux Agro-Bio Tech Hydrology and Hydraulic Eng.

References :

-Demarcin P., Degré A., Smoos A., Dautrebande S. (2009). Projet ERRUISSOL. Cartographie numérique des zones àrisque de ruissellement et d’érosion des sols en Région wallonne. Rapport final de convention DGO3-FUSAGx. Unitéd’hydrologie et hydraulique agricole. Faculté universitaire des Sciences agronomiques de Gembloux. 55 p+annexes.- Sohier C., Degré A., Dautrebande S. (2009). From root zone modelling to regional forecasting of nitrate concentration in recharge flows – The case of the Walloon Region (Belgium). Journal of Hydrology, Volume 369, Issues 3-4, 15 May 2009, Pages 350-359.-Soil Conservation Service (1972). National engineering handbook: Hydrology, Section 4. USDA-SCS, Washington, DC.-Wischmeier WH., Smith DD. (1978). Predicting Rainfall Erosion Losses. A guide to conservation planning. USDA, Agricultural Handbook n°537, Washington DC, 58 p.

Micro-catchments classification: a tool for mud flow m itigation

Hydrologic similarities between catchments can be of use in the context of overland flood and mud

flow hazards management. In Wallonia, upstream micro-catchments were mapped for each runoff concentration axis. The outlets of these catchments are considered at the point where water reaches the permanent river stream. No less than 145 547 micro catchments were mapped in Wallonia (16900 km²). Their area varies between 1 and 1233 ha (mean area: 7.8 ha). A data base was built up to synthesize their major characteristics like e.g. area, shape and mean slope. No clear classification appeared.

At this stage, it seems obvious that the major components of these catchments had to be handled

simultaneously from a hydrological point of view in order to produce a clear classification.

New ArcToolBox « SCS GISER »

Input data

Hydrological modelling’s results

Overland and mud flow risks maps

http://cartographie.wallonie.be/NewPortailCarto/Section : Espace Professionnels � Données en ligne � Agriculture et ruralité� ERRUISSOL

Micro-catchments delimitation

By use of the Arc Gis Spatial Analyst hydrology toolbox, the DTM produces a flow direction map and a flow accumulation map. The concentration axis that are upward the permanent streams are identified. We retained 1ha as minimum contributive area.

The points were a concentration axis crosses a permanent stream is considered as a catchment outlet.

By use of the watershed tool (Arc Gis Spatial Analyst), all the micro-cathments related to these outlets are drawn.

The main hydrologic significantdata are available in Wallonia: the digital soil map (1/20 000), the DTM (1/10 000), digital land use (1/10 000) and spatial statisticsfor rain (IDF curves).

The results of this Arc Tool box consist in a full project hydrograph.

This modelling application gives :

- Peak flow value,

- Flood water volume,

- Sediment yield

in response to a project rainfall computed for each micro-catchment.

A hydrologic model was developed using the python programming language in the ArcGis 9.3 environment. This model is based on the NRCS – USDA method. Rain abstraction is calculated on the basis of a CN (curve number) which integrates the soil type, the land use and the slope. Water transfer is computed using a convolution based on triangular unit hydrogrammes. This calculation includes the concentration time which integrates the catchment morphology (shape and slope) and the land use (roughness). The MUSLE equation is also computed in order to handle the sediment problem.

The aforementioned variables can be considered as integration factors of the whole hydrological context of the micro-catchments. It allows us to build advice on overland flow and mud flow mitigation. This approach can also be of help to compare ungauged micro-catchments between each other and possibly transpose the soil conservation practices from a site to another.

The CN and MUSLE semi-empirical methods give very interresting results at a larger scale in Wallonia. They still have to demonstrate their efficiency at the micro-catchment scale, but as shown below, they can be of use in natural risks caracterisation.

Hyetograph et hydrograph for a 100 years return per iod rainfall and 1360 min in three antecedent moisture condition

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Catchment area 89,4 haFlood water volume Antecedent moisturecondition Dry : 482 m³ Normal : 13 379 m³ Wet : 38 192 m³

Funded

This study was conducted as part of the project GISER (Gestion Intégrée Sol Erosion Ruissellement)

Funded by Service Public de Wallonie Direction Générale Opérationnelle Agriculture, Ressources Naturelles et Environnement

Département de la Ruralité et des Cours d'Eau Direction du développement Rural

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Observed discharge

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River discharge

Ex : Ry de Soumoy catchmentat Soumoy (15 sq. km.)

Sohier C. & Al. (2009).

Dr. Degré A. (ULg – GxABT) ; Ir. Mokadem AI. (SPW – DGARNE) ; Ir. Cordonnier H. (SPW – DGARNE) ; Ir. Colard F. (ULg – GxABT)

DTM

Land useSlope

Hydologic infiltration group