What is Checkwind?
Checkwind (https://www.revolutio.com.au/software/checkwind/) is a web-based facility for performing the wind profile calculations required by the Building Codes in Australia, Europe, The US and Canada among others. It claims to be a site-specific response to the Code requirements for producing the wind profiles at a specified site.
I obtained a two week trial version of the Checkwind App in order to explore the facilities used by the program and supplied to the user by Checkwind, and to determine the congruence of the results to the code for which they were derived.
What does it do?
In addition to the calculation of the profile as per code, the app automates specification of three key input parameters required by the calculation, namely the basic wind speed for available MRI for the location, the exposure category, and the topographic parameters of shape and slope.
For the US codes the app retrieves the wind data from the ASCE 7 Hazard Tool available on the web. For the Canadian CSA it obtains the NBCC table data. For a number of other countries which publish their wind speed maps on-line the app obtains this information as well.
To derive the exposure category the app detects several different landscape categories which it classifies as open, water, urban on Google Earth satellite images of the area surrounding the site and determines the percentage of the area in each sector at the location of interest. Using this data the sector is classified as C or B or D for the US codes and as roughness length for the Canadian CSA S37 code.
For topography determination the app uses digital elevation data to evaluate transects of the terrain in order to calculate the isolated topographic feature’s crest and lowest point on the transect and then establish the distance at the half-height of the feature. The app seems to use some heuristics to establish the shape of the topographic feature as well as the height and length measurements.
The input required from the user is the type, shape and height of the structure and the location of the structure.
How well does it function?
The first observation I would make is that the Checkwind App is not providing a “Site-Specific” assessment of the situation as prescribed in the codes. It is using a mapped wind which is based on a limited number of observing sites and where the mapping process of necessity glosses over the local variations in weather systems and orography, and over-rides the variability of the wind speed statistics which is location dependent. The suggested approach in the codes for a site-specific assessment is to use local observations and perform the statistics over a long period of record to derive the return period winds. If you want to get the full benefit of site specific assessment you would need to obtain directional exposure and the corresponding directional extreme wind speed.
The app does provide the calculation which the specific code requires using automation of the input parameter determination. The author seems to acknowledge that this automated derivation is not foolproof as he suggests that the app must be used by a knowledgeable professional.
The following provides a summary of the main findings. For the full report click on the link below.
The terrain categorization is adequate for the ASCE 7 and TIA 222 codes, since these require only 3 categories and are loosely defined. For the CSA S37 code which requires specification of the roughness length for the general area surrounding the site terminating at the base of the hill as well as the roughness on the slope of the topographic feature, Checkwind does allow for several zones for each sector, although the zones don’t seem to be tied to the distance from the crest to the bottom of the hill, with many of the sectors having only a single sector defined.
The topography determination obtains the elevation profile along sectors of 15 deg interval. It then seems to calculate the minima and maxima of elevation along the transect in order to establish the height and shape of the feature. This can be a complex procedure due to the highly irregular variations on the slopes of a hill or ridge.
For an example of a case where the hill parameters of height, shape, and off-slope distance are mischaracterized please see the full Checkwind report linked above. In this case the procedure located an escarpment 4 km downwind and used its parameters but set the Lint (off-crest distance) to -4km, which is seven times the half-height distance L. The program did not detect that this is more than 2 time L and hence calculated the speed up factor for the escarpment using a shape parameter of Ridge. This produced the largest speed up for the site.
The above error could only be detected by looking at the report which contains the topography calculation parameters and the results Qh.