I'm rereading Norman, and he does talk about the wave model; his primary division of Chinese dialects is into Northern, Southern, and a transitional Central zone, which includes Wu, and has received multiple waves of Northern influence. Interestingly his discussion (Chapter 8) is based on 12 cities as major data points, including Kunming, but none in Sichuan. The Yunnan dialect area originated with a Northern Chinese population colonizing a previously non-Chinese-speaking areas, bypassing influence from southern Chinese dialects, and Kunming is still noted for its intelligibility with Beijing. On the other hand, Sichuan has more conservative, less Mandarinized dialects in southwestern Sichuan. Norman has almost no mention of Sichuan; it appears only once in the index. Taking this into account, the situation in Sichuan looks similar to those in Hunan and Jiangxi, but has been glossed over in favor of a contiguous Southwestern Mandarin area because of the obscurity of southwestern Sichuan.