A method for routine characterisation of shelterbelts

Abstract

This paper describes a working method of assessing the shelter quality of a wind-break, hedge or shelterbelt by a single field measurement, using portable equipment, deployed by an operative with minimal training on any day when the wind is suitable. The automated measurement yields a single characterisation parameter used to access an “expert system” database of information quantifying the benefit to plants or animals of various species. Because shelter depends not just on wind-break porosity but also on other local terrain features affecting the shear and turbulence in the upstream wind, the characterisation measurement is defined to include not only the mean wind speed but also the turbulence. Thus, the assessment of a farm hedge views the shelter benefit, not just of the hedge in isolation, but of the hedge as found within its own local surroundings. The routine measurement requires the simultaneous deployment of two anemometers connected to a pre-programmed data logger. One anemometer is sited upstream of the shelter barrier and one downstream. Both are at a distance h from the nearest shelterbelt face and at a height of 0.4 h, where h is the nominal height of the shelterbelt. The method is established by a combination of field and wind tunnel measurements. In the field, the shelter parameter has been measured for a hawthorn hedge and the flow downstream has been explored extensively. In the wind tunnel, a wind-break model has been constructed to reproduce the same shelter-parameter value, and the downstream flow compared extensively with the full-scale observations. Thus validated, the range of wind tunnel study has been extended to include other wind-break models and wind flow simulations giving a range of shelter parameters. The research was sponsored by, and the exploitation rights are the property of, Woodland Improvement and Conservation, of Huntley, Gloucestershire.