User Tools

Site Tools


start:hype_model_description:hype_sediment

Sediment

Introduction

Sediments are simulated as suspended sediments (SS) and nitrogen in algae (AE). The sum of the two substances is an additional output (total suspended sediments, TS). Suspended sediments are not simulated in the soil of HYPE. Suspended sediments are first introduced into the runoff of the soil by soil erosion.

The main states are concentration of SS and AE in all water stores of HYPE, i.e. soil, river, lakes, but also snow, aquifers (in the store and on the move there or away), irrigation water, floodplain water, and water transfer (that is delayed one time step) could hold suspended sediments and algae. In the current model though, concentration of SS and AE is only positive in river and lakes, and flows originating from them. In addition two pools of settled sediment are simulated; a pool of delayed sediment in runoff, and a pool of (temporarily) settled sediment in river. Note: No store of “settled sediment” of the soil is simulated, i.e. HYPE has an unlimited source of soil for erosion.

Adds an ImageCaption tag
Figure 1: Schematic figure of sediment model.

Sources of sediment load

Soil erosion

The main source of sediment in HYPE is from soil erosion. Soil erosion is modelled in several steps. First particles are mobilized from the soil by rain or surface runoff. The result is here called “mobilized sediments”. The mobilized sediments are assumed suspended in an “eroding flow”, which is surface runoff plus macropore flow. Secondly, if there is surface runoff, the mobilized sediments is transported away from the field with the surface runoff and macropore flow (“eroded sediment”). A filtering can be applied to reduce the amount of eroded sediment. If there is no surface runoff, there will be no transport of eroded sediments. For SS, the eroded sediments are taken from an infinite pool. For PP, the eroded particulate phosphorus is taken from the soil pools. Thirdly, the eroded sediments in macropore flow and surface runoff (and tile runoff though that is generally zero) are delayed in a temporary pool. The release of suspended sediments from this pool is determined by the total runoff from the class, and is following the total runoff off the land.

Step 1: Mobilization of particles from soil

HYPE has two alternative models for mobilization; soil erosion model 1 and soil erosion model 2.

Soil erosion model 1 (MMF-based model): The first model is based on Morgan-Morgan-Finney erosion model (Morgan et al., 1984) and calculates particles mobilized by rainfall energy and surface runoff. The kinetic energy in rainfall is calculated as a function of rainfall (rain, mm/ts) and day of the year (dayno). If the precipitation falls as snow, or if it falls on snow-covered ground or if it is smaller than 5 mm/day no mobilization occurs in the model. Some of the raindrop's energy can be absorbed by vegetation. Crop cover is defined as the portion of land that is sheltered from raindrops; for a description of how this is calculated, see Chapter Crop cover and ground cover. The factor common_{cropcover} is the sheltering effect that the main and secondary crops give together. It varies over the year due to crop growth and management. The mobilization by rain (MobilisedRain, g/m2/ts) is also influenced by soil erodibility (soil dependent parameter soilerod (g/J)).

Rainfall_{energy} = rain * ( 8.95+8.44*LOG10 (rain*2* ( 0.257+ 0.09*sin ( 2 pi* ( dayno-70 ) {/}365 ))))

MobilisedRain = Rainfall_{energy}* (1. - common_{cropcover}) * soilerod

When surface runoff occurs, soil particles are eroded and carried away as the soil surface is exposed to shear forces. The mobilization (MobilisedSR, g/m2/day) is calculated from the surface runoff (sflow, mm/day), subbasin average slope, a parameter for soil cohesion (soilcoh (kPa) soil type dependent), and a general parameter (sreroexp). This type of erosion can be mitigated by protective vegetation or vegetation residues that are in contact with the ground. The calculation of this reducing factor (groundcover) is described in Chapter Crop cover and ground cover. The factor common_{groundcover} is the combined effect of the main and secondary crops.

MobilisedSR = {{(sflow*365)^{sreroexp}} * (1-common_{groundcover}) * {1/{0.5*soilcoh}} * sin(slope/100)} / 365

All mobilized particles are not staying mobilized, because sometimes the transport capacity of the particle-bearing water (eflow) will not suffice for the task. If this is the case, a transportfactor reduces the particle amount mobilized:

transportfactor = MIN(1.0,(eflow {/} 4)^{1.3})

Finally mobilized sediment (mobilSed, kg/km2/day) is calculated as the sum of rain and surface runoff caused mobilization as:

mobilSed = 1000 * (MobilisedRain + MobilisedSR) * transportfactor

Soil erosion model 2 (HBV-SED based model): The second model is based on HBV-SED model (Lidén, 1999; Lidén et al., 2001) and calculates particles mobilized by rain (rain). When the ground is frozen or there is snow on the ground, the no particles are mobilized. The mobilization also depends on soil and land characteristics in the form of model parameters and data on subbasin characteristics.

mobilSed = 1000. * ({slope / 5})^{erodslope} * erodluse * erodsoil * {EI/erodindex} * rain^{erodexp}

The parameters erodslope, erodexp and erodindex are general and thus same for the whole model domain. The parameters erodluse and erodsoil are land-use and soil type dependent. Subbasin input are: slope, the subbasins' average slope, and an erosion index, EI. The many parameters give the possibility to simulate erosion as dependent on slope, soil type, land use or subbasin. In addition an adjustment can be made with a monthly model parameter erodmon to help match seasonality of erosion.

modileSed = mobileSed * erodmon(month)

where month is the current month.

Step 2: Transport of eroded sediments off the field

If at one time step there is no surface runoff, there will be no transport of suspended sediment. Still, calculation continues with step 3, and earlier delayed eroded sediment may reach the local stream. If there is surface runoff, a fraction of the mobilised sediments is assumed to go with surface runoff (sflow). If there is surface runoff and tile runoff and macropore flow, a fraction of the mobilized sediments is assumed to travel with the macropore flow (mflow). The respective fractions depend on the respective size of the two flows, but are reduced by filtering of the particles. The filtering depends on landuse, the proximity of agricultural land to water, presence of buffer strips etc. The filtering effect on suspended sediments in surface runoff (srfilt) is parameterized with land use parameters and subbasin input, while the filtering effect on suspended sediments of macropore flow is parameterized with a soil type parameter (macrofilt).

srfilt = otherfilt + alfa * (1. + bufferpart * (bufferfilt - 1.)) + innerfilt * (1. - alfa)

erodedSed = (srfilt*sflow+macrofilt*mflow)*mobilSed/eflow

eflow=sflow+mflow

The total amount of eroded sediment (erodedSed, kg/km2/day) is the sum of contributions from surface flow and macropore flow. For SS, the eroded sediments are taken from an infinite pool. For PP, the eroded particulate phosphorus is calculated from the eroded sediment (see Soil erosion) and then taken from the solid soil pools (humusP and partP). This difference exists because HYPE does not simulate any pool of “soil sediments”.

Step 3: Suspended sediment reaching the local stream

The concentration of suspended sediments of the runoff leaving the soil is calculated based on total runoff (runoff, mm/ts) and a pool of sediment particles (relpool). The pool collects the suspended sediments in the respective runoff pathways, i.e. primarily the eroded sediment from surface runoff and macropore flow calculated in the previous step. The pool constitutes a temporary delay. The release of sediment (release (kg/km2)) from the pool temporary is calculated with two general parameters (pprelmax and pprelexp):

release = MIN(relpool, relpool *(runoff {/} pprelmax)^{pprelexp})

The suspended sediments released are following the total runoff off the land. The release divided by the total runoff gives a suspended sediment concentration, which is used for all runoff pathways. This concentration is set to all runoff from the class, i.e. the sediments are transported by all flow paths off the land.

The sediment may be removed from the temporary pool between the land surface and the local stream if no erosion occurs. A 1st order decay, with parameter eroddecay, is used so that sediment doesn’t remain detached forever.

Rural household outlets and point sources

Suspended sediments may be added to the model as diffuse rural households outlets (private sewers) or point sources. Similar to other substances in HYPE, they are added as flow with concentration of the total suspended sediments and with a fraction determining the suspended part, while nitrogen in algae make up the rest (GeoData.txt PointSourceData.txt). To add only suspended sediments set the fraction to 1. The diffuse source is divided into two parts, where the division is determined by a general parameter (locsoil). One part is added directly to the local river. The other part is added to the water of the bottom soil layers of each class in the subbasin. The distribution between classes is done proportional to the area of each class. Point sources are added in the main river. Point sources are described in the Chapter on Water management (point sources) and rural household diffuse source in the Chapter on Nitrogen and phosphorus in land routines (rural sources).

Section Symbol Parameter/Data File
Soil erosion soilerod, soilcoh, seroexp, erodslope, erodluse, erodsoil, erodindex, erodexp, erodmon, bufferfilt, innerfilt, otherfilt, macrofilt, pprelmax, pprelexp, eroddecaypar.txt
slopeslope_meanGeoData.txt
EIeroindex
alfaclose_w
bufferpartbuffer
Rural household outlets and point sources subid, ps_vol, ps_type, ps_tsconc, ps_ssfrac, fromdate, todatePointSourceData.txt
loc_ts, loc_vol, loc_ssGeoData.txt
locsoilpar.txt
Modules (file) Procedures Sections
npc_soil_processes (npc_soil_proc.f90)particle_processes_for_runoffSoil erosion
calculate_hbvsed_erosion
calculate_erosion
datamodule (data.f90)load_pointsourcedataRural household outlets and point sources
read_pointsourcedata
get_current_pointsources
npc_surfacewater_processes (npc_sw_proc.f90)add_point_sources_to_main_river
add_diffuse_source_to_local_river
npc_soil_processes (npc_soil_proc.f90) local_diffuse_source

Soils

Suspended sediments are not simulated in the soil calculations of HYPE. Suspended sediments are first introduced into the runoff of the soil.

Sediment in rivers and lakes

If the siltation model option is set, the sedimentation of lakes is added to a sediment pool. The sedimentation of rivers is always added to a sediment pool.

Primary production and mineralization

Primary production in lakes and rivers is affecting sediment simulation by being a source of algae organic nitrogen (AE). Mineralisation, as a sink of AE, is the reverse process. Nitrogen in algae is assumed to grow and decline with the same function as production and mineralisation of organic nitrogen (ON). The production and mineralisation processes are modelled together and only one is active at the time. If nitrogen is simulated by HYPE it uses the actual estimated production/mineralization for AE, otherwise the potential production/mineralisation (minprodNpot) is used for AE. The actual production is limited by the availability of inorganic nutrients. If sediments are simulated without simultaneously simulating nitrogen, the mineralisation of algae is limited to available amount, but the production is unlimited.

The potential production/mineralisation is depending on temperature (Tw, T10, T20), long-term total phosphorus concentration (TP), surface area of water body (area) and a parameter (wprodn).

minprodNpot = wprodn * TPfcn * tmpfcn * area

TPfcn = {TP-limsedPP} / {(TP-limsedPP) + hsatTP}

tmpfcn = tmpfcn1*tmpfcn2

tmpfcn1 = T_w / 20.

tmpfcn2 = (T10 - T20) / 5.

Temperatures T10 and T20 are calculated as the average water temperature (Tw) of 10 and 20 days. They determine if production or mineralization dominates at the current time. The water temperature is calculated through weighting the air temperature (T) and yesterday's water temperature (see basic assumptions of rivers and lakes).

The phosphorus function (TPfcn) is dependent on a general half saturation parameter hsatTP, and a limitation parameter limsedPP. If phosphorus is simulated by HYPE the long-term average concentration (TP) is calculated from that, otherwise a lakeregion parameter is used (tpmean).

Sedimentation of lakes

Sedimentation in lakes is a sink for SS and AE and works the same way as for nutrients. Sedimentation (sed, kg/day) is calculated as a function of concentration in lake water (conc) and lake area (area). The settling velocity parameter (sedss, sedae, m/timestep) is general. The concentration used in the equation may be limited (lim) by general parameter (limsedSS) for SS, but not for AE (lim=0).

sed = par_sed * (conc-lim) * area

Sedimentation/Resuspension in rivers

In the river we simulate both sedimentation and resuspension. No particles are removed from the simulation by sedimentation. For the original model, and a variant, they are redistributed over time and can come back through resuspension based on flow. For the other alternative model it is possible to resupend more particles than is previously settled. This double process of sedimentation and resuspension is used for suspended sediments (SS). Particles in the sediments is collected in a pool (sedimentpool, kg).

For the original model the sediment pool will increase as particles from the water volume (waterpool, kg) settle at low water flows, and vice versa. At high water flow the sedimentation is lower and the resuspension higher, and more particles returns to the water. The net effect of these combined processes are given by the variable sedresp. It is governed by the current flow (flow) in relation to the bankful flow (qbank) and a general parameter sedexp. The net effect is determined by the sign of the variable (-1 <sedresp <1), and the variable size give the fraction of the pool that is transfered per day. The process will give either net sedimentation (settl, kg/day) or resuspension (resusp, kg/day).

sedresp=max(-1.,min(1.,{{qbank-flow}/qbank}^{sedexp}-{flow/qbank}^{sedexp}))

settl = {lbrace}{matrix{2}{2}{{sedresp * waterpool}{sedresp > 0}{0}{sedresp < 0}}}

resusp = {lbrace}{matrix{2}{2}{{- sedresp * sedimentpool}{sedresp < 0}{0}{sedresp > 0}}}

where flow is the current river flow (m3/s) and qbank is the flow when river is filled to the brim. The latter flow is calculated as the second largest simulated flow in the last year. It is adjusted with a correction factor of 0.7 before use in the sedresp equation. The first alternative model is the same as the original model but the correction factor can be calibrated. It is given by the general model parameter qbank.

The second alternative model is based on a simplified Bagnold equation (Neitsch et al., 2011). Suspended sediment concentration is compared to a calculated maximum amount of sediment that can be transported in the river to determine if sedimentation or resuspension occurs. The maximum transport depends on the peak velocity across the river (v_peak). This is estimated from flow in the river (Q) using the hydraulic geometry relationship (river velocity) which gives the average river velocity and a peak adjustment factor (vpeak).

v_peak = vpeak * k * Q ^ m

where vpeak,k and m are general parameters. The maximum amount of sediment that can be transported in the reach (cmax_SS, mg/L) is calculated using the peak velocity and two general model parameters (spcon and spexp).

cmax_SS = spcon * {{v_peak} ^ {spexp}} * 1000000.

If the simulated HYPE suspended sediment concentration (conc_SS) is larger than the maximum transport concentration, then sediment is deposited in the temporary pool. Resuspension occur in the opposite case. The net transport (sedresp):

sedresp = {lbrace}{matrix{2}{2}{{(conc_SS - cmax_SS)* riverbox}{conc_SS > cmax_SS}{(conc_SS - cmax_SS)* riverbox * suspch}{conc_SS < cmax_SS}}}

where riverbox is volume of water in attenuation box of the river and conc_SS its concentration of suspended sediments. The effect of channel erodibility and channel vegetation cover is captured by a river channel erodability/vegetation cover modification factor, suspch.

As default the amount of sediment that can be resuspended is limited by the amount of sediment that has been deposited before (i.e. the temporary pool (sedimentpool)). This can be changed by using a parameter addsed which allows users to apply a limit to the amount of sediment that can be resuspended in HYPE in excess of the amount of sediment in the temporary pool. If addsed=0, then the amount of sediment that can be resuspended is limited to the amount of sediment in the temporary pool. If addsed=1, then there is no limit on the amount of sediment that can be resuspended (the full amount from equation above will be resuspended). For values in between the additional resuspension increases linearly:

resuspension = sedimentpool + ({-{sedresp} - sedimentpool})*addsed

Siltation

It is optional to simulate the lake sediment in HYPE. If the siltation model option is not used the settled sediment (Sedimentation of lakes above) is lost from the system. If the siltation option is used then the settling sediment add to a sediment pool. This is true for all substances that sediment in lakes, but only the substances SS and AE are used further. The settled substances may be compressed over time and their density thus affected. They may fill up the lake and affect its water depth and outflow. They may be removed or flushed at regular or irregular intervals.

The density of sediments may be constant in time and space. It is the given by ldseddens, a general parameter, or determined by the subbasin's soil particle fractions (d_ini). The sediment density may also increase with time through compactation (d_comp). This is modelled to depend on the sediment age.

d_ini=c_sand*p_sand+c_silt*p_silt+c_clay*p_clay

where soil fraction (p_sand, p_silt, p_clay) are given per subbasin and coefficients (c_sand, c_silt, c_clay) depend on reservoir operation mode.

d_comp=d_ini+0.434*K_ini*({T/{T-1}}*ln(T)-1)

K_ini=k_sand*p_sand+k_silt*p_silt+k_clay*p_clay

where coefficients k (k_sand, k_silt, k_clay) depend on reservoir operation mode, and T is the sediment age in years. For ages less than one year the initial density (d_ini) is used.

There are four reservoir operation modes (resop_mode) implemented in HYPE (Strand and Pemberton, 1982);

  1. sediment always submerged or nearly submerged,
  2. normally moderate to considerable reservoir drawdown,
  3. reservoir normally empty,
  4. riverbed sediments.

Table of coefficients for sediment density for the different reservoir operation modes (resop_mode).

Coefficient resop_mode 1 resop_mode 2 resop_mode 3 resop_mode 4
c_sand 1550 1550 1550 1550
c_silt 1120 1140 1150 1170
c_clay 416 561 641 961
k_sand 0 0 0 0
k_silt 91 29 0 0
k_clay 256 135 0 0

Flushing

Flushing is used to remove sediment that has accumulated in a reservoir over time. It is a sediment management method that can be regulated in different ways. The flushing modes are depending on capacity, sediment age or day of year. In each case a starting day is found and a rate of flushing calculated. Additionally an ending day may be set or the flushing is ended when another threshold is met.

Flushing can be simulated for outlet lakes where the reservoir operation mode is not set to river sediments (resop_mode == 4). Flushing is not simulated in ilakes. Sediment density is assumed to be constant while sediment flushing is operating. The flushed sediment may be removed from the simulation or transported with the outflow to the downstream subbasin.

Flush mode 1 - Capacity

Start to remove accumulated sediments from reservoir after available reservoir storage capacity drops below a specified threshold (sm_start, a fraction). Flushing stops when available reservoir storage capacity (fraction) exceeds the second threshold which is calculated as the start level plus the capcacity that is to be restored (sm_caprest). The flushing rate (capacity/day) is calculated as the capacity to restore spread over the number of days specified with variable sm_duraday. Note that the actual duration of flushing for each event may vary depending on how much upstream sediment enters the reservoir while flushing is occurring delaying the restoration of the capacity.

Flush mode 2 - Day of year

Start to remove accumulated sediments from reservoir at a specified day of the year (sm_start). The flushing may be every year or specified to occur at another frequence, e.g. every other year (sm_yfreq). Flushing stops after a duration given by sm_duraday. Flushing rate (capacity/day) is calculated as storage capacity to flush, specified with variable sm_caprest (fraction of capacity), over the duration of the flushing. Note that the amount of reservoir storage capacity that is restored for each event may vary depending on how much upstream sediment enters the reservoir while flushing is occurring. Note that all of the sediment may be flushed from the reservoir before the planned end date if the amount of sediment in the reservoir is not greater than or equal to the flushing rate times the number of days of flushing, basically there isn’t enough sediment in the reservoir, so the reservoir runs out of sediment before the end of the flushing period.

Section Symbol Parameter/Data File
Primary production and mineralizationarea, lakeregion GeoData.txt
wprodnpar.txt or LakeData.txt
hsatTP, limsedPP, tpmeanpar.txt
Sedimentation in lakes parsed sedss, sedaepar.txt or LakeData.txt
lim limsedSSpar.txt
Sedimentation and resuspension in rivers sedexp, qbank, vpeak, addsedpar.txt
k,mhygeomk, hygeomm
spcon, spexpsuspconss, suspexpss
suspchsuspch, suspchannelpar.txt or GeoData.txt
Siltation ldseddenspar.txt
p_sand, p_silt, p_claysand, silt, clay GeoData.txt
resop_modesm_resopLakeData.txt
sm_mode, sm_dest
sm_start, sm_duraday, sm_caprest, sm_yfreqsm_start, sm_duraday, sm_caprest, sm_yfreq
Modules (file) Procedures Sections
npc_surfacewater_processes (npc_sw_proc.f90)substance_processes_in_river Sediment in rivers and lakes
substance_processes_in_lake
production_mineralisation Primary production and mineralization
calculate_lake_tpmean
calculate_river_tpmean
lake_sedimentationSedimentation in lakes
river_sedimentation_resuspensionSedimentation/Resuspension in rivers

Diagnostic output variables - sediment load

The load of sediment leaving a subbasin is calculated for suspended sediments (cSSl) and total sediments (cTSl).

Net load of main river and outlet lake can be calculated for suspended sediments (SS) and total sediments (TS). The output variables ID are nlSS and nlTS. Net load of main river and outlet lake is calculated from the inflow and outflow of substance to the main river and outlet lake in a subbasin as one system. The inflow is composed of upstream coming inflow and local inflow to the main river. The outflow is the outflow of the subbasin through the main and secondary branch. The net load is the load of outflow minus the load of inflow. A bit of caution, do not misstake this output as the net load from a catchment, i.e. the load in the main river leaving the catchment.

Included in the net load is the effect of production/mineralisation of algae in the lake (for TS), point sources. Reduction of the net load is the effect of sedimentation, but also abstraction of water for irrigation or other purposes. In the case of zero outflow, or if the incoming load is larger than the load leaving the system, the net load becomes negative. Negative load could thus be caused by e.g. sedimentation, but also temporary holding up the sediment in the system. For subbasins holding a part of a lake, i.e. a lake basins, it is also possible that the water flow between the lake basins is (temporary) opposite of what is defined as downstream in GeoData. In this case the outflow of the subbasin in an inflow, and is withdrawn from the net load. Thus the net load of main river and outlet lake become negative.

References

Lidén, R., 1999. A new approach for estimating suspended sediment yield, HESS, 3(2):285-294.

Lidén, R., J. Harlin, M. Karlsson, and M. Rahmberg, 2001. Hydrological modelling of fine sediments in the Odzi River, Zimbabwe, Water SA, 27(3): 301-314.

R. P. C. Morgan, D. D. V. Morgan, and H. J. Finney, 1984. A predictive model for the assessment of soil erosion risk, Journal of Agricultural Engineering Research, vol. 30, pp. 245–253.

Neitsch, S.L.. J.G. Arnold, J.R. Kiniry and J.R. Williams, 2011. “Soil and Water Assessment Tool Theoretical Documentation Version 2009”, Texas Water Resource Institute Technical Report No. 406, Texas A&M University, Texas.

Strand, R.I. and E.L. Pemberton, 1982. “Reservoir Sedimentation”, Technical Guidelines for Bureau of Reclamation, Denver, Colorado.

start/hype_model_description/hype_sediment.txt · Last modified: 2021/09/16 09:26 by cpers