Load-based Sediment Delivery ratio - SRG

The Load-based Sediment Delivery Ratio model reduces the amount of sediment leaving a functional unit (FU) as a function of the amount of load generated in the FU. Essentially, the ability of a filter (such as a riparian zone) is limited, and if considerable load is applied to a filter, then trapping efficiency will drop to the point where all incoming material is passed straight through.

Scale

This model is applied at the functional unit scale, but can have catchment-wide effects. The model assumes a daily time-step.

Principal developer

Cooperative Research Centre for Catchment Hydrology. The model was originally developed for EMSS (Hairsine 2001).

Version

Source v2.10

Availability/conditions

Load-based Sediment Delivery Ratio is automatically installed with Source.

Flow phase

Some load from a FU can be directed through a filter (eg. riparian zone) to alter the delivery of sediment to the stream. Riparian zones are applied by defining the percentage of different stream orders within each sub-catchment that are to be "treated" (based on the proportion of the total stream length - ie. each sub-catchment could have a different type of riparian zone applied). The amount of trapping that occurs in the riparian zone is given by a sediment delivery ratio (SDR) that varies from 0 to 1, depending on loading (t/km/yr) as shown in Figure 1.

The sediment loading rate threshold (SLRT) is the threshold below which everything is trapped. The Sediment Loading Rate at Sill (SLRS) is the maximum loading rate above which nothing is trapped.

The filter model is constrained by the proportion of the total stream length that is covered by a riparian zone.

This approach has the following assumptions:

• The delivery ratios are for the riparian zone only. Sediment loading rates to the riparian zone already contain the delivery ratios associated with the flow path above the zone;
• The model is daily though most of the concepts come from studies of event and simulation studies;
• The daily time-step implies that only one event is considered per day; and
• Only constituents in quick flow are filtered. Constituents in the slow flow remain unchanged.

Features of this model include minimal parameterisation and compatibility with loading functions and specification of stream network. This model also captures some of the behaviour of different type of buffer models:

• Threshold model (e.g Herron and Hairsine 1996) where overland flow is subject to enhance infiltration in riparian zone so that small events have zero delivery and there is a threshold behaviour;
• Capture of sediment by settling (eg. Dillaha et al. 1989) where sediment is trapped according to sediment size class so that SDR is a function of incoming sediment velocity distribution, incoming sediment concentration and the incoming water discharge rate per unit width of flow; and
• Finite storage model (eg. Prosser & Karssies 2001) where the riparian zone has a finite storage of sediment which when filled results in SDR approaching 1.

Input data

Data for the SLRT and SLRS values are required, for which the model linearly interpolates between these values to determine the output loading rate.

Default values for SLRT and SLRS of 0.1 and 10 t/km/yr respectively are suggested.

The model is constrained by the proportion of the total stream length that is covered by a riparian zone (ie. stream riparian proportion / stream length in metres).

Parameters or settings

Model parameters are summarised in Table 1.

Output data

A time series of sediment load.

Configuration

Only a single filter model can be applied to each FU and sub-catchment combination.

Reference list

Herron, NF & Hairsine PB 1998, ‘A scheme for evaluating the effectiveness of riparian zones in reducing overland flow to streams’, Australian Journal of Soil Research, vol. 36, no. 4, pp. 683-98.

Karssies L, & Prosser, IP 2001, ‘Designing grass filter strips to trap sediment and attached nutrient’, Proceedings of third Australian Stream Management Conference, CRC for Catchment Hydrology, pp. 349-353.

Dillaha, TA, Reneau RB, Motaghimi, S & Lee D 1989, ‘Vegetative filter strips for agricultural nonpoint source pollution control’, Transactions ASAE, vol. 32, pp. 513-519.

Bibliography

Hairsine, P 2001, Notes on the EMSS Riparian zone model for sediment and nutrient delivery, Unpublished Notes, CSIRO Land and Water.