Catchment sediment load limits to achieve estuary sedimentation targets

Figures & data

Figure 1  Location maps. A, Pāuatahanui Inlet, showing sub-estuaries. (Base map: MapToaster Topo, software by Integrated Mapping, © MetaMedia Ltd.) B, Catchment, showing sub-catchments. (Base map: TUMONZ, Vision Software for Management & Technology Systems Ltd.) C, Pāuatahanui Inlet, North Island of New Zealand. (Base map: TUMONZ, Vision Software for Management & Technology Systems Ltd.)

Figure 2  Analysis of sediment load limits for the case of two sources of sediment depositing in one sink. Sediment load limits chosen from the region denoted by ‘Valid sediment load limits’ will fail neither target and will be achievable by mitigation.

Figure 3  Analysis of sediment load limits for the case of three sources of sediment depositing in one sink. Sediment load limits chosen from the region denoted by ‘Valid sediment load limits’ will fail no target and will be achievable by mitigation.

Figure 4  Analysis of sediment load limits for the case of two sources of sediment depositing in two sinks. Sediment load limits chosen from the region denoted by ‘Valid sediment load limits’ will fail no target and will be achievable by mitigation.

Table 1  The constituent-particle fate matrix (F _c,e,p ), which was specified.

	Flood-tide delta	Central subtidal	Kakaho intertidal	Horokiri intertidal	Pāuatahanui intertidal	Browns Bay intertidal	Browns Bay subtidal	Ocean
Coarse
Kakaho	0	0.10	0.90	0.00	0.00	0.00	0.00	0
Horokiri	0	0.10	0.15	0.50	0.25	0.00	0.00	0
Ration Point	0	0.10	0.15	0.25	0.50	0.00	0.00	0
Pāuatahanui	0	0.10	0.05	0.l5	0.60	0.10	0.00	0
Duck Creek	0	0.10	0.00	0.00	0.70	0.10	0.10	0
Browns Bay	0	0.00	0.00	0.00	0.00	0.80	0.20	0
Fine
Kakaho	0	0.64	0.08	0.00	0.00	0.00	0.08	0.20
Horokiri	0	0.70	0.00	0.02	0.00	0.00	0.08	0.20
Ration Point	0	0.60	0.00	0.00	0.04	0.00	0.16	0.20
Pāuatahanui	0	0.56	0.00	0.00	0.04	0.04	0.16	0.20
Duck Creek	0	0.54	0.00	0.00	0.02	0.08	0.16	0.20
Browns Bay	0	0.00	0.00	0.00	0.00	0.00	0.80	0.20
Note: This represents annual-average sediment dispersal patterns. Sub-catchments (c) are shown along the left and sub-estuaries (e) along the top.

Table 2  Catchment Landuse for Environmental Sustainability (CLUES) predictions of the annual-average total (sum of all grain sizes) sediment run-off at the base of each sub-catchment under the present-day land use, and partitioning of the total sediment run-off between the two sediment size fractions.

Sub-catchment	Annual-average total sediment run-off (kg)	Fraction coarse	Fraction fine
Kakaho	2.32	0.2	0.8
Horokiri	6.87	0.2	0.8
Ration Point	0.44	0.2	0.8
Pāuatahanui	5.52	0.3	0.7
Duck Creek	1.35	0.35	0.65
Browns Bay	0.051	0.4	0.6
Note: CLUES predictions by Wriggle Coastal Management, Nelson (unpublished data).

Table 3  Sub-estuary total area and area over which catchment-sourced sediment was allowed to deposit in each sub-estuary.

Sub-estuary	Area (m^2)	Fraction of area deposition allowed
Flood-tide delta	1,171,437	0
Central subtidal	1,746,993	0.7
Kakaho intertidal	482,226	0.5
Horokiri intertidal	317,345	0.75
Pāuatahanui intertidal	520,382	0.75
Browns Bay intertidal	104,575	0. 5
Browns Bay subtidal	264,808	0.7
Ocean	–	–

Table 4  Predicted and observed total (sum of all particle sizes) and constituent–particle (total multiplied by bed-sediment composition) annual-average sedimentation rates (mm/year).

	Flood-tide delta	Central subtidal	Kakaho intertidal	Horokiri intertidal	Pāuatahanui intertidal	Browns Bay intertidal	Browns Bay subtidal	Ocean
Coarse
Predicted	–	0.3	2.5	3.4	3.7	3.7	0.2	–
Observed	–	0.5	7.3	7.5	7.3	7.1	0.8	–
Ratio	–	0.52	0.34	0.45	0.50	0.51	0.30	–
Fine
Predicted	–	5.4	0.5	0.3	0.4	3.6	6.4	–
Observed	–	10.2	0.6	0.6	0.6	7.1	14.4	–
Ratio	–	0.53	0.81	0.48	0.62	0.50	0.44	–
Total
Predicted	–	5.7	3.0	3.7	4.0	7.2	6.6	–
Observed	–	10.7	7.9	8.1	7.9	14.2	15.2	–
Ratio	–	0.53	0.38	0.45	0.51	0.51	0.44	–
Note: ‘Ratio’ is the ratio of predicted to observed.

Table 5  The total-sediment fate matrix (F _c,e ), calculated from the constituent-particle fate matrix and sub-catchment sediment run-off (Equation 22).

	Flood-tide delta	Central subtidal	Kakaho intertidal	Horokiri intertidal	Pāuatahanui intertidal	Browns Bay intertidal	Browns Bay subtidal	Ocean
Kakaho	0	0.532	0.244	0	0	0	0.064	0.16
Horokiri	0	0.5832	0.03	0.1128	0.05	0	0.064	0.16
Ration Point	0	0.5	0.03	0.05	0.132	0	0.128	0.16
Pāuatahanui	0	0.422	0.015	0.045	0.208	0.058	0.112	0.14
Duck Creek	0	0.3886	0	0	0.2554	0.087	0.139	0.13
Browns Bay	0	0	0	0	0	0.32	0.56	0.12
Note: This represents annual-average sediment dispersal patterns. Sub-catchments (c) are shown along the left and sub-estuaries (e) along the top.

Table 6  The total-sediment source matrix (O _c,e ), calculated from the fate matrix and sub-catchment sediment run-off (Equation 23).

	Flood-tide delta	Central subtidal	Kakaho intertidal	Horokiri intertidal	Pāuatahanui intertidal	Browns Bay intertidal	Browns Bay subtidal	Ocean
Kakaho	–	0.15	0.65	0.00	0.00	0.00	0.10	0.15
Horokiri	–	0.48	0.24	0.74	0.18	0.00	0.30	0.44
Ration Point	–	0.03	0.02	0.02	0.03	0.00	0.04	0.03
Pāuatahanui	–	0.28	0.10	0.24	0.61	0.71	0.42	0.31
Duck Creek	–	0.06	0.00	0.00	0.18	0.26	0.13	0.07
Browns Bay	–	0.00	0.00	0.00	0.00	0.04	0.02	0.00
Note: This shows the annual-average origin of sediment that deposits in each sub-estuary. Sub-catchments (c) are shown along the left and sub-estuaries (e) along the top.

Figure 5  Analysis of capacity sacrifice for the case of two sources of sediment depositing in two sinks.

Figure 6  Equation (8) expanded and rearranged to apply to Pāuatahanui Inlet, plotted for a range of values of Φ for each sub-estuary, where S _e,TARGET=ΦS _e,PRESENT. Sediment run-off is normalised by present-day sediment run-off. The figure guides the discovery of sediment load limits for Pāuatahanui Inlet. Refer to the text for details.

Figure 7  Example case in which the target is a 30% reduction in sedimentation in all sub-estuaries. A,Valid (will not fail any sedimentation targets) and invalid (will fail at least one sedimentation target) sediment load limits. B, Average capacity sacrifice  as a function of sediment run-off from the rural and developing sub-catchments.

Figure 8  Example case in which the targets are 10% reduction in sedimentation on the Kakaho intertidal flats; 50% reduction in sedimentation on the Pāuatahanui intertidal flats and in Browns Bay subtidal; and 30% reduction in sedimentation in the three remaining sub-estuaries. A, Valid (will not fail any sedimentation targets) and invalid (will fail at least one sedimentation target) sediment load limits. B, Average capacity sacrifice as a function of sediment run-off from the rural and developing sub-catchments.

Figure 9  Capacity sacrifice Δ _e for an individual target of 30% reduction in sedimentation rate in the Browns Bay intertidal flats. Δ _e is plotted for invalid and valid sediment load limits, where Δ _e <1 signifies that the individual target sedimentation rate will fail to be achieved. Also shown is  for the case where the goal is to achieve simultaneous targets of 10% reduction in sedimentation on the Kakaho intertidal flats; 50% reduction in sedimentation on the Pāuatahanui intertidal flats and in Browns Bay subtidal; and 30% reduction in sedimentation in the three remaining sub-estuaries.

Table

A e	Area over which deposition occurs in sink (sub-estuary) e
c	Source (sub-catchment) number
C	Number of sources (sub-catchments)
D e	Mass of sediment deposited in sink (sub-estuary) e
e	Sink (sub-estuary) number
E	Number of sinks (sub-estuaries)
F c,e	Fraction of sediment mass from source (sub-catchment) c that deposits in sink (sub-estuary) e
F c,e,p	Fraction of sediment mass of size fraction p from source (sub-catchment) c that deposits in sink (sub-estuary) e
L c	Mass load of sediment (sediment run-off) from source (sub-catchment) c
L c,e	Mass load of sediment (sediment run-off) originating from source (sub-catchment) c and depositing in sink (sub-estuary) number e
L c,LIMIT	Sediment load limit for source (sub-catchment) c
L c,MAX	Maximum mass load of sediment (sediment run-off) from source (sub-catchment) c that will not fail a particular S e,TARGET
L c,MIN_ACHIEVABLE Minimum achievable (by mitigation) mass load of sediment (sediment run-off) from source (sub-catchment) c
O c,e	Fraction of sediment mass that deposits in sink (sub-estuary) e that originates from source (sub-catchment) c
p	Sediment size fraction
P	Number of sediment size fractions
S e	Sedimentation rate (total of all sediment size fractions) in sink (sub-estuary) e
S e,p	Sedimentation rate for constituent size fraction p in sink (sub-estuary) e
S e,TARGET	Sedimentation rate target in sink (sub-estuary) e
S e,PRESENT	Present-day sedimentation rate in sink (sub-estuary) e
Γ	Period of time
Δ e	Capacity sacrifice (or target overachievement) in sink (sub-estuary) e
	Average capacity sacrifice (or target overachievement) over all sinks (sub-estuaries)
Λ	Number of sources (sub-catchments) for which the intention is to set sediment load limits or to offset mitigation
Φ	Fraction (between 0 and 1), where S e,TARGET=ΦS e,PRESENT
Ψ	Factor by which source sediment load is anticipated to increase
Ω	Number of sources (sub-catchments) for which there is no intention is to set sediment load limits or to offset mitigation
α	Number of source (sub-catchment) to be developed
η	Number of source (sub-catchment)
ρ	Density of deposited sediment
i	Index (step number, in the limit-setting process)
λ	Index (number of source)
ω	Index (number of source)