Under the authority of the Ministry of Transport, Public Works and Water Management (Eastern Netherlands division), the Institute for Inland Water Management and Waste Water Treatment RIZA executed a monitoring program on secondary channels in the Gamerensche Waard. During the period 1996-2002 a broad and complete program was executed with three main objectives: 1) evaluation of the desired effects, 2) assessment of the undesirable side-effects (risks) and 3) increase of the knowledge about secondary channels.
In the period 1995-1999 three secondary channels were excavated in the Gamerensche Waard along the river Waal (the main side branch of the river Rhine). Regarding the dimensions, these channels are unique for Dutch rivers. These channels are digged out partly from former agricultural grassland and partly they ecxists of connected former sand and clay exctraction pits. The three secondary channels vary with regard to location (inside and outside the summer embankment), length (0.5-2 km), width, depth (0-20 m), discharge (1-3%) and the like. Just one of the channels is flowing permanently, the other two contain flowing water in respectively 4 and 11 months a year.
The monitoring period (1996-2002) was characterized by relatively high river discharges. Therefore the secondary channels were connected to the stream flow more frequently than planned. The total discharge of the three secondary channels is about 2% of the average river discharge. The maximum flow ve-locity in the secondary channels appeared at narrows; the strongest turbulences just downstream of those narrows. With low river discharges, a large influence of navigation on the flow velocity was visible in the channels; this led sometimes to a turnover of the flow direction. The discharge capacity of the Waal with high floods was not lowered significantly by the sedimentation in the secondary channels. The vegetation development led to only a minimal decrease of the discharge capacity of the river.
The predicted sedimentation in the main channel parallel to the Gamerensche Waard did indeed occur, but it appeared difficult to prove because of the inter-ference with slowly passing sand waves/dunes on the bottom of the Waal. The sedimentation resulting from the construction of the secondary channels was of the same order as the measurement uncertainty and the autonomous developments in the riverbed. Roughly no large morphological changes were measured in the secondary channels of the Gamerensche Waard. The erosion- and sedimentation rates in the first years after construction were larger than in the later years. Locally some clear (bank) erosion and sedimentation were recognized. It is expected that the former sand extraction pit will be filled up to the level of the rest of the channel around the year 2050 (net sedimentation rate is about 0.05 to 0.11 m/year).
Soil quality and ecological risks
The bed soil in the secondary channels consists mainly of sand (in the former sand extraction a mixture of silt and sand). The soil quality of the secondary channels has improved during the monitoring period (sedimentation of clean sand). Although the concentrations of toxic substances in the bottom are so high that negative ecological effects can be expected, the determined risks are not so high that these can block the ecological recovery seriously. These risks are gradually decreasing because the sediment becomes more and more sandy and this means also cleaner. There are no possibilities for (a cost-effective) extraction of the sediment due to the heterogeneous composition. Distribution of the sediment to elsewhere in the river system is possible due to the tolerable quality of the sediment.
The abundance of trees and bushes in the Gamerensche Waard is still limited because of the imperfect germination and establishment conditions: on the islands because of the dense grass cover and on the banks of the channels because of the large water level fluctuations. With regard to the floristic com-position, hardly any target species, Red-list species or protected species were found in or near the secondary channels. In spite of this, the floristic quality of the muddy banks of the channels is (very) good, but not exceptional in com-parison with other nature developments projects along the river Waal. In the biggest secondary channel in the Gamerensche Waard some small fields with aquatic vegetation were found in 2002 (Myriophyllum spicatum and Potamogeton pectinatus).
Ecology, macro invertebrats
In the secondary channels of the Gamerensche Waard about 75% of the recently recorded species of the river Rhine occurs. The species diversity of the secondary channels is much higher than in the groyne fields of the main channel. In the slow flowing parts of the secondary channels significantly less exotic species occur than in the main river bed. From the 46 (macroinvertebrate) target species, only 3 species were discovered in the secondary channels of the Gamerensche Waard. The absence of other target species can largely be attributed to the lacking of some specific habitats e.g. gravel, woody debris and aquatic vegetation. Those chironomid species that are characteristic for stable sandy flats recolonised the area quite fast. The sediment type, the water depth, the flow velocity, the morphodynamics, the organic matter content and the soil chemistry together determine the spe-cies composition of the macroinvertebrate community. All these factors are mutually strong dependent on each other; evident relations are difficult to prove. The highest species diversity occurs on silt or vegetation in shallow, slow flowing water with a limited erosion or sedimentation. These factors seem to be of a larger influence on the macroinvertebrate composition than the degree of soil pollution.
In the secondary channels of the Gamerensche Waard various reophilic fish species (preference for flowing water) were found, including five target species (Barbus barbus, Leuciscus cephalus, Chondrostoma nasus, Leuciscus idus and Lampetra fluviatilis). For these reophilic fish the secondary channels function mainly as a growing up area during their earliest stages of life. Later on they migrate to the main channel. The most reophilic fish enters the secondary channels passively by means of larval drift.
Problems for navigation were not detected in spite of the slight sedimentation in the main channel, which is a result of the construction of the secondary channels. The sedimentation as a result of the secondary channels interferes with other large-scale ’autonomous’ processes. Consequently one should al-ways bear in mind the temporal fluctuations in bed level. Inland navigation was not hindered by (cross) currents at the in- or outlet of the channels. The increase of the river discharge capacity resulting from the construction of secondary channels was annulled for a small part (± 15%) by the sedimentation in the bed of the channels and by the bush encroachment elsewhere in the Gamerensche Waard. At this moment dredging or digging the secondary chan-nels in order to maintain the flowing character of the channels is not needed. At one location local bank erosion required intervention to prevent safety problems. Because of the large variation of water types, the secondary channels in the Gamerensche Waard offer suitable habitats for a wide range of species. How-ever, concerning the species composition according to the intended nature target type, it is clear that the secondary channels are not yet complete. The secondary channels in the Gamerensche Waard remained largely at their original location, although locally distinct (bank) erosion appeared. Regular visual inspection of the banks of secondary channels is and will be necessary in future. A slight improvement of the soil quality occurred, because the new sediments in the secondary channels consist mainly of (clean) sand. The sand (transport) turned out to be a much more outstanding factor than the silt (transport). A deep extraction pit as part of a secondary channel functions as a sediment catch indeed, but it does not seem to be necessary at all to prevent filling up with sand.