A grassed waterway and earthen dams to control muddy floods from a cultivated catchment of the Belgian loess belt
Introduction
Muddy floods consist of water flowing from agricultural fields carrying large quantities of soil as suspended sediment or bedload (Boardman et al., 2006). They are therefore considered as a fluvial process rather than a mass movement one. Even though they are frequent and widespread in the European loess belt, they are mainly reported from central Belgium (Verstraeten and Poesen, 1999, Evrard et al., 2007a), northern France (Souchère et al., 2003) and southern England (Boardman et al., 2003). Muddy floods cause numerous off-site impacts, such as flooding of property, sedimentation and eutrophication in watercourses.
About 90% of muddy floods observed in the Belgian loess belt are generated on cultivated hillslopes (10–30 ha) and in dry zero-order valleys (30–300 ha; Evrard et al., 2007a). Numerous studies carried out in cultivated catchments of the European loess belt showed that most sediments produced during the Holocene have been stored in the dry valley bottom near the catchment outlet and have not been delivered to downstream rivers (e.g. Bork et al., 1998, Lang et al., 2003, Rommens et al., 2005, de Moor and Verstraeten, in press). Rommens et al. (2006) also estimated the Holocene alluvial sediment storage in a small (52 km2) river catchment of the Belgian loess belt. They showed that sediment supply towards the alluvial plain has increased dramatically since Medieval times compared to the rest of the Holocene period and occurred at a mean rate of 1.3 t ha− 1 yr− 1. Since 50% of sediment eroded from hillslopes was stored in colluvial deposits, mainly located in dry zero-order valley bottoms, muddy floods caused by severe erosion on agricultural land are the mostly likely process transporting sediments from the dry valleys to the alluvial plains. During heavy rainfall in late spring and summer, ephemeral gullies form in these dry valleys. These shallow (∼ 0.1 m) but wide (∼ 3 m) gullies act as an important conveyor of sediment and may aggravate the off-site damage produced by muddy floods (Nachtergaele and Poesen, 2002, Verstraeten et al., 2006).
The huge costs associated with this damage, which appears to have occurred more frequently during the last decade, justifies the urgent installation of mitigation measures (Evrard et al., 2007a). Two types of measures can be carried out to control muddy floods. On the one hand, alternative farming practices implemented at the field scale, such as sowing of cover crops during the intercropping period, reduced tillage or double sowing in zones of concentrated flow, limit runoff generation and erosion production (Gyssels et al., 2002, Leys et al., 2007). However, the implementation of these practices directly depends on the farmer's willingness. Except for sowing of cover crops (e.g. in Belgium; Bielders et al., 2003), the adoption of such practices remains rather limited in Europe (Holland, 2004). It will probably still take several years or even decades before reduced tillage and double sowing are applied generally. On the other hand, ‘curative’ measures aim to reinfiltrate or buffer runoff once it is formed, as well as to trap sediments and pollutants. Typically, grass buffer strips, grassed waterways (GWW) and detention ponds (retaining runoff for a certain time behind a small dam) serve this purpose (Fiener and Auerswald, 2005). Such curative measures are most effective when they are implemented in the framework of integrated catchment management. Hence, a local water board should be responsible for deciding in consultation with farmers where to install these measures within the catchment and for ensuring their maintenance.
From 2001 onwards, municipalities in the Belgian Flemish region are eligible for subsidies to draw up an erosion mitigation scheme (Verstraeten et al., 2003). Several small-scale measures such as dams and GWW are being installed in the field but there is a need to evaluate their efficiency before generalising their installation in problem areas. Furthermore, since muddy floods are generated on large surfaces (10–300 ha; Evrard et al., 2007a), the effect of control measures should be investigated at similar scales. However, previous research has focused on the effect of grass buffer strips and has mostly been carried out on experimental plots (typically 500 m2, see e.g. Van Dijk et al., 1996, Patty et al., 1997, Le Bissonnais et al., 2004). With respect to the effect of GWW in the European context, it has only been assessed at the micro-catchment scale (max. 8 ha; Fiener and Auerswald, 2005, Fiener et al., 2005). Large quantities of concentrated runoff leading to muddy floods cannot be generated on such small surfaces and a specific study is hence needed at the scale of the larger catchments, which are the source areas of muddy floods.
This paper evaluates the effectiveness of a GWW and earthen dams installed in a cultivated 300 ha-catchment in the Belgian loess belt in mitigating muddy floods in the downstream village. The cost-efficiency of the control measures is also discussed.
Section snippets
General context
The Belgian loess belt (∼ 9000 km2) is a plateau with a mean altitude of 115 m gently sloping to the North (Fig. 1a). Annual mean temperature varies between 9–10 °C, while annual precipitation ranges from 700–900 mm (Hufty, 2001). Soils are mainly loess-derived Haplic Luvisols (World Reference Base, 1998). Arable land covers 65% of the total surface (Statistics Belgium, 2006). During the last three decades, the area covered by summer crops (sugar beet — Beta vulgaris L., maize — Zea Mays L.,
Results
Between 2002–2007, 77% of events with ≥ 15 mm precipitation occurred between May and September. Similarly, 70% of runoff events occurred during this period (Fig. 3).
Effectiveness of the grassed waterway and earthen dams
The propagation of the peak discharge was drastically slowed down within the section with the GWW and the earthen dams. However, there was no important reinfiltration in GWW for moderate and extreme storms. This is due to a high soil compaction (bulk density of 1.59 g cm− 3 in the GWW compared to a mean of 1.43 g cm− 3 for cropland in the Belgian loess belt according to Goidts and van Wesemael, 2007). This confirms the results of rainfall simulations carried out in the Belgian loess belt showing
Conclusions
A 12 ha-grassed waterway and three earthen dams were installed in a 300 ha-cultivated catchment in central Belgium, in order to prevent muddy floods in the downstream village. These measures served their purpose by preventing muddy floods in the village, even during extreme events (with a maximum return period of 150 years). Peak discharge per unit area was reduced by a mean of 69% between both extremities of the GWW. Furthermore, runoff was buffered during 5–12 h, due to the combined effect of
Acknowledgements
The authors thank Marco Bravin for his technical assistance in installing the catchment loggers and measurement station and Elisabeth Frot for her help with data collection in 2002. The installation of the measurement devices was financially supported by the Land & Soil Protection Division of the Flemish Ministry of Environment, Nature and Energy. Local farmers (Jean Lejeune, Jean Boonen and Roland Meys) are also gratefully thanked for implementing the erosion control measures.
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