Hydro-Morphological Index of Diversity (HMID) to evaluate hydraulic habitat suitability

The changes in hydraulic habitat suitability after the combined artificial flood release and replenishment of sediments were quantified using the Hydro-Morphological Index of Diversity (HMID)^20,21. The HMID is based on the following hypotheses: the structural diversity of a river reach can be characterized by the hydraulic measures of flow depth and velocity and their statistical parameters, which are in turn a surrogate for habitat suitability for the aquatic and semi-aquatic region of a river reach. The HMID takes into account the spatial distribution of flow depth and flow velocity, like other indexes to evaluate habitat and morphology heterogeneity^32,33:

where CV = coefficient of variation of variable i; μ_h and μ_v = mean value of flow depth (h) and velocity (v), respectively; and σ_h and σ_v are the corresponding standard deviations. Generally, HMID-values between 1 and 15 are obtained depending on the river morphology. A river reach can be categorized as^20,21:

1 < HMID < 5: Channelized/heavily altered

5 < HMID < 9: Limited variability

HMID > 9: Fully developed spatial dynamics

A 850 m reach was analysed. Measurements of flow depth and flow velocity were made at the exact same locations before and after the flood event, across the river at nine different equally-spaced cross-sections with a distance of about 95 m (Fig. 6). A total of 200 velocity and water depth measurements were taken in the reach, before and after the flood event. The measurement points were equally distributed over the cross sections with a 1 m spacing. Instantaneous flow velocities were measured with a Handheld-ADV (FlowTracker manufactured by SonTek) at 60% of the flow depth. Flow velocities were averaged over a period of 45 s, with a measurement frequency of 1 Hz, which proved to provide a robust mean value. Flow depths were measured with the scale on the vertical bars of the FlowTracker equipment. The location of the sampling points was recorded with a differential GPS. An analysis of statistical convergence revealed that the results obtained with 200 points surveyed in the 850 m in the Sarine river are sufficient. Detailed information concerning sampling and data sufficiency can be found in literature³⁴.

Nine cross-sections (CS) where flow depths and velocities were measured. CS 2–4 correspond to the HMID_{impact zone}, CS 1 and 5–9 to the HMID_{rest zone} (see Table 1). Background © Research unit Ecohydrology, ZHAW.