Flood Water Retention by Riverine and Terrestrial Forests

This paper addresses flood retention by floodplain forests and the reduction of runoff by forested watersheds as a function of their management. The results from completed projects and the scheme of an ongoing project are presented. In 2001 reforestation measures were conducted to increase the retention potential of the floodplain of the lower River Blies in Germany. Due to the short project duration of about two years, which included planning and realisation of the measures, reforestation had to be restricted to sites that were initially owned by the project members. Thus only 2% of the floodplain could be included. This resulted in a simulated flood peak reduction of less than 1% at extreme flood events. Despite the relatively small mitigation effect in this particular case the project led to valuable findings. This refers for instance to the utilisation of floodplain afforestation as an ecological and flood control compensation measure, which might substitute lost retention volumes due to leeves or other structures in the floodplain. Another flood control project showed by simulation, that there is a potential for flood peak attenuation of up to 15% by reforestation of the total flood plain. Since 2003 investigations have been conducted into the runoff reduction effects of sustainable forestation and of decentralized retention facilities in woodlands. In two forested watersheds of 4.4 and 8.4 km² flood mitigation effects of such measures have been monitored and simulated.