Parker et al. (2007) derived regime equations from natural British and US-American Rivers. The regime equations allow estmating the width, depth and slope of single-thread rivers by knowing the bankfull discharge and median grain size of the bed surface only. These equations may be used to estimate the width of a river needed after restoration of the riverbanks.

Consider that:

• The equations were derived from and hence are applicable for alluvial, single-thread gravel bed rivers.
• The equations were derived from baseline data, i.e. data from rivers before implementation of measures. These rivers possess a more or less equilibrium sediment budget.
• In addition to the bankful width, a river needs additional space for lateral dynamics (river bank erosion, channel migration, creation of oxbows, etc.).
• The river morphology is different when it gets in contact with inerodible bank protection structures, which cause increased depth. Provide enough width to the river to minimize contact with channel constraints.

References:

Parker, G., Wilcock, P. R., Paola, C., Dietrich, W. E., & Pitlick, J. (2007). Physical basis for quasi‐universal relations describing bankfull hydraulic geometry of single‐thread gravel bed rivers. J Geophys Res, 112, F04005, doi:10.1029/2006JF000549.