Aquatic vegetation, hydraulics and sediment transport have complex
interactions that are not yet well understood. These interactions are
important for sediment conveyance, sediment sequestration, phasing of
sediment delivery from runoff events, and management of ecosystem health
in lowland streams. To address this knowledge gap detailed field
measurements of sediment transport through natural flexible aquatic
vegetation are required to supplement and validate laboratory results.
This paper contributes a field study of suspended sediment transport
through aquatic vegetation and includes mechanical removal of aquatic
vegetation with a weed cutting boat. It also provides methods to
quantify vegetation cover through remote sensing with Unmanned Aerial
Vehicles (UAVs) and estimate biomass from ground truth sampling.
Suspended sediment concentrations were highly dependent on aquatic
vegetation abundance, and the distance upstream that had been cleared of
aquatic vegetation. When the study reach was fully vegetated (i.e. cover
>80%), the maximum recorded SSC was 14.6
g/m3 (during a fresh with discharge of 2.47
m3/s), during weed cutting operations SSC was 76.8
g/m3 at 0.84 m3/s (weedcutting boat
0.5-1 km upstream from study reach), however following weed cutting
operations (4.6 km cleared upstream), SSC was 139.0
g/m3 at a discharge of 1.52 m3/s.
The data indicates that fine sediment was being sequestered by aquatic
vegetation and likely remobilised after vegetation removal.
Investigation of suspended sediment spatial dynamics illustrated changes
in particle size distribution due to preferential settling of coarse
particles within aquatic vegetation. Hydraulic resistance in the study
reach (parameterized by Manning’s n) dropped by over 70% following
vegetation cutting. Prior to cutting hydraulic resistance was discharge
dependent, while post cutting hydraulic resistance was approximately
invariant of discharge. Aerial surveying captured interesting changes in
aquatic vegetation cover, where some very dense regions of aquatic
vegetation were naturally removed leaving behind unvegetated riverbed
and fine sediment.