We point out, in this paper, the issue of improving the coherence of web archives under limited resources (e.g. bandwidth, storage space, etc.). Coherence measures how much a collection of archived pages versions reflects the real state (or the snapshot) of a set of related web pages at different points in time. An ideal approach to preserve the coherence of archives is to prevent pages content from changing during the crawl of a complete collection. However, this is practically infeasible because web sites are autonomous and dynamic. We propose two solutions: a priori and a posteriori. As a priori solution, our idea is to crawl sites during the off-peak hours (i.e. the periods of time where very little changes is expected on the pages) based on patterns. A pattern models the behavior of the importance of pages changes during a period of time. As an a posteriori solution, based on the same patterns, we introduce a novel navigation approach that enables users to browse the most coherent page versions at a given query time.