Results and discussion
Storm generated surface runoff resulted in 18 ponding events at the
Hauraki site and 19 ponding events at the Awahou site during this
12-month study. Ponding events occurred most often during the winter
months compared to the other seasons (Fig. 2) . Water samples
were analysed for 13 of these ponding events at the Hauraki site, and 14
events at the Awahou site, since not all ponding events generated flow
rates high enough to trigger auto-samplers. Discharge samples were
collected during 10 events at the Hauraki site, and 13 events at the
Awahou site, since not all events generated discharge flows to be
sampled due to leakage and soil infiltration.
Concentrations
The annual SS inflow concentration was 17 g m-3 at the
Hauraki site, and 96 g m-3 at the Awahou site.
Concentrations peaked in the winter at the Hauraki site, while there was
no clear temporal trend for inflow concentrations at the Awahou site
(Fig. 2) . Event inflow concentrations did not tend to
correspond to event runoff magnitudes and varied widely between events.
The annual MFP SS discharge concentration was 28% lower than inflows at
the Hauraki site, and 29% lower at the Awahou site. These results
suggest that DBs effectively facilitated sedimentation during ponding,
and attenuated deposited sediments in the ponding area. Discharge
concentrations were lower than inflows during 7 of the 10 events
analysed at the Hauraki site, and 10 of the 13 events analysed at the
Awahou site (Fig. 2 ). Inconsistencies in concentration
treatment efficiencies were observed between and within event types at
both sites (Table 2 ).
The wide range of concentration treatment efficiencies observed in this
study were influenced by multiple factors. Treading damage, deposited
animal excreta and previously deposited sediments in the ponding area
could have contributed to SS discharged from the DB that were not
accurately accounted for by the pro rata correction of the contributing
catchment area and affected the concentration treatment efficiency
results.
Variations in particle sizes delivered to the DBs, which were not
measured in this study, could have also been a contributing factor to
the varying concentration treatment efficiencies observed between events
and the sites in this present study. Large particles that have greater
densities settle more readily than smaller sized particles with lower
densities, which would be less likely to settle and more likely to be
remobilised and discharged from the DBs (R. W. McDowell et al., 2003). A
greater number of larger particles, which require more energy to
mobilise and transport, could have been delivered to the DBs during
higher magnitude runoff events, particularly Overflow Events, and could
be partially responsible for SS concentrations decreasing during all
Overflow events in this study, while this was not the case for all
Non-Overflow Events (Table 2 ).
During Overflow Events at both sites, the SS concentrations between the
portions of inflow contributing to ponded water going over the top of
the upstand riser and emergency spillway (i.e. overflow discharge) and
the overflow discharge, did not decrease to the same extent as the
concentration decreased between the between overflow discharge and the
following release discharges (Table 3) . These results
are somewhat surprising since we would expect the decanting of the
uppermost layer of water performed by the upstand riser (Fig.
1 ) and emergency spillway would be highly effective at preventing SS
discharge. The data suggests however, that longer pond residence times
experienced by the release discharge compared to the overflow discharge
(an average of 14 hours between overflow discharge and the following
release discharge at both sites) allowed for greater sedimentation to
occur. Longer retention times have been found to increase sediment
removal efficiencies in a study of sedimentation ponds (Brown et al.,
1981).
The data suggests ponding runoff for longer than the currently suggested
3 days could achieve greater concentration treatment efficiencies,
however, this could risk damaging pasture productivity. Removing the
upstand riser/outlet valve/discharge pipe installation (Fig.
1 ), and allowing all ponded water to infiltrate the soil, would avoid
discharging the bottommost portion of ponded water where SS are likely
to concentrate and/or be stirred up by turbulence when unplugging the
outlet valve to drain the pond. Also, placing the outlet valve 10 cm
above ground level would enable a small portion of the ponded water left
after draining the pond to infiltrate the soil. This later revision
would prevent the discharge of a lower portion of ponded runoff, and
would decrease the area potentially affected by prolonged inundation,
compared to avoiding the release procedure entirely. Lastly, approaches
to achieve greater SS concentration treatment efficiencies could include
the use of flocculants.
Yields and loads
The key finding of this study was that impeding stormflow with DBs
effectively attenuated 789 kg and 1280 kg of SS at the Hauraki and
Awahou sites, respectively. The 51% and 60% decrease in annual SS
loads discharged from the DB catchments was a result of the DBs’ ability
to facilitate sedimentation, which often decreased event SS
concentrations, and soil infiltration, which decreased the volume of
runoff discharged from the DB. These results suggest DBs should be
effective at reducing P losses from pastures in the Lake Rotorua
catchment, due to the high proportion of sediment bound P delivered to
the lake (Hamill, 2018).
Annual SS inflow yields were 28 kg ha-1 at the Hauraki
site, and 109 kg ha-1 at the Awahou site, although
runoff inflow yields were greater at the Hauraki site than the Awahou
site. The annual SS inflow yields at both sites in this study were much
lower than the estimated annual SS yields entering streams in the same
area of the Lake Rotorua catchment from May 2010 to May 2012 (479-741 kg
ha-1 y-1) (Abell, Hamilton, &
Rutherford, 2013). Factors affecting the catchments’ hydrological
responses to precipitation, including antecedent soil conditions and
localised differences in storm rainfall intensity and duration, and
differences between the catchment sizes, geomorphologies, and land use
and management factors, affected runoff generation and erosion
(Dougherty, Fleming, Cox, & Chittleborough, 2004), and likely accounted
for the SS inflow yield differences between study sites in this present
study and the results reported by Abell et al. (2013).
At both sites during this present study, runoff and SS inflow yields
were lowest in the spring and increased during each subsequent season,
peaking during the winter period (Fig. 3 ). This was not
surprising, as the contributing catchment is grazed by dairy cattle soil
treading damage and erosion is likely to increase when soils are wet (R.
W. McDowell et al., 2003) . Additionally, greater SS yields tended to
correspond with greater runoff yields, particularly during the high
runoff magnitude Overflow Events (Fig. 4 ). These results are
consistent with other studies that found greater runoff magnitudes tend
to mobilise and transport greater quantities of sediments and nutrients
from pastures in New Zealand (Cooke, 1988; Smith & Monaghan, 2003) and
the Lake Rotorua catchment, specifically (Abell et al., 2013; Dare,
2018).
The results of this study suggest the DBs at both sites were able to
consistently decrease SS loads discharged from the DB catchments, even
during rare, high runoff magnitude events. The greater inflow magnitudes
during Overflow Events at the Hauraki site contributed to a greater
portion of runoff undergoing overflow discharge, and consequently, the
difference in the portion of inflow undergoing soil infiltration and SS
yield treatment efficiencies between the sites during these high
magnitude events (Fig. 5) .
The results from the high magnitude Overflow Events emphasize the
importance of being effective during these high magnitude events since
they were responsible for 61% and 66% of the annual SS inflow loads at
the Hauraki and Awahou sites, respectively, and 39% and 59% of annual
SS yields attenuated. This finding is important to note since large
storm events have been found to be responsible for the majority of SS
loading to streams in the Lake Rotorua catchment (Abell et al., 2013).
Although DBs effectively attenuated SS loads during Overflow Events,
these large magnitude events still generated 84% of the annual SS
discharge yields at the Hauraki site, and 77% at the Awahou site. These
results are likely related to the majority of annual runoff discharges
also occurring during Overflow Events at both sites (Table 4) .
These results suggest that reducing runoff discharges from DBs by
facilitating soil infiltration played a key role in effectively
decreasing SS loads, which highlights the importance of optimising DB
designs to maximise soil infiltration of ponded runoff and avoiding
excess overflow discharge.
The ability of DBs to consistently decrease SS loads, particularly
during high magnitude runoff events, is also significant because some
land management strategies may be overwhelmed by extreme hydrologic
conditions (Kleinman et al., 2006; R. W. McDowell & Sharpley, 2002;
McKergow et al., 2007). Sediments deposited as a blanket across the
relatively wide DB ponding area was observed during this present study,
and likely contributed to the consistency in DB performance during this
study (Table 1 ) (McKergow et al., 2007). Importantly, it is
likely sediments deposited in the DB ponding area will be attenuated for
longer periods of time than other mitigation strategies, such as buffer
strips and treatment wetlands, that have more concentrated sediment
deposition areas and are susceptible to flushing during high magnitude
events (McKergow et al., 2007). The ability of the DB to impede the
stormflow of each runoff event reduced the kinetic energy of water,
which enables the transfer and/or remobilisation sediments, and
particularly the ‘first-flush’ of the initial runoff, could have had a
major influence on the DBs’ ability to decrease SS loads transported in
surface runoff during each event in this study (Bieroza et al., 2019).
The role of soil infiltration in annual SS yield treatment efficiencies
is important to note. The concurrent study by (Levine et al., In review)
found lower infiltration rates in the ponding area than outside the
ponding area (Table 1 ). The decreased soil infiltration rates
in the ponding area could be due to a combination of factors including
treading damage in the lower lying and wetter areas (R. W. McDowell et
al., 2003), the large volumes of water moving through the soil causing
stress and deterioration in soil structure, and sediments deposited in
the ponding area clogging soil pores and/or forming a less permeable
surface soil layer (Hendrickson, 1934; Reddi, Ming, Hajra, & Lee, 2000;
Rice, 1974). Therefore, the data suggests infiltration rates, and
consequently SS yield treatment efficiencies, may be highest in newly
constructed DBs, and may decrease over time. Also, soil infiltration
rates, and consequently SS yield treatment efficiencies, could decline
more rapidly if DBs are located in areas where erosion is more intense
and a greater amount of sediments are deposited in the ponding area
compared to this present study, if deposited sediments are responsible
for infiltration rate declines. Causes of declining soil infiltration
rates in the ponding areas, as well as methods of mitigating soil
infiltration rate declines, such as aerating the pond area soils or
employing subsoil amendments, should be investigated.
During this study, event discharge concentrations were lower than
inflows more often than higher, and SS yield treatment efficiencies were
greater than runoff yield treatment efficiencies (Fig. 5 ).
These results indicate that sedimentation facilitated by impeding
stormflows with DBs caused lower SS discharge concentrations. Therefore,
DBs would likely be able to decrease SS discharge yields in areas where
soil infiltration rates and pond storage to catchment area ratios are
lower than those in this present study. Other factors influencing the
proportion of runoff infiltrating the soil and sediment sizes delivered
to the DBs would affect yield treatment efficiencies.
Lastly, revising the DB design to remove the upstand riser/outlet
valve/discharge pipe installation would prevent SS leak and release
discharges, which could be remobilised and delivered to downstream
surface waters in subsequent runoff events. We calculated that applying
this revised DB design would have prevented an additional 147 kg of SS
from being discharged from the Hauraki site, and an addition 216 kg at
the Awahou site, increases of 16% and 14% of the annual load
attenuated at each site, respectively. The costs and benefits of
revising the DB design should be considered since the increased
inundation period could damage pasture productivity.