Modelling hydrology and sediment transport in a drained peatland forest - Focus on sediment load generation and control after ditch network maintenance

Kersti Haahti

Modelling hydrology and sediment transport in a drained peatland forest - Focus on sediment load generation and control after ditch network maintenance

Julkaisun otsikon käännös: Hydrologian ja kiintoaineen kulkeutumisen mallintaminen ojitetussa suometsässä – Painopisteinä kiintoainekuorman muodostuminen ja hallinta kunnostusojituksen jälkeen

Kersti Haahti

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Boreaalisen vyöhykkeen soita on ojitettu laajasti viimeisten vuosisatojen aikana. Fennoskandiassa ojitus on keskittynyt suometsien kasvuolosuhteiden parantamiseen ja kuivatuksen ylläpito kunnos-tusojituksin on tärkeä osa suometsätaloutta. Ojitus tunnetaan haitallisena kiintoainekuorman lähteenä, mutta sen muodostumiseen ja etenkin hallintaan liittyvä prosessiymmärrys on puutteellista.Tässä väitöskirjassa kehitetään prosessipohjainen mallikokonaisuus kuvaamaan ojitetun suometsän hydrologiaa ja ojaverkoston kiintoaineprosesseja kunnostusojituksen jälkeisessä tilan-teessa. Mallia sovelletaan väitöskirjan eri vaiheissa 5.2 hehtaarin suometsäalueelle, jossa kunnos-tusojituksen jälkeen seurattiin intensiivisesti alueen hydrologiaa ja kiintoaineprosesseja. Mallikehityksen ensimmäisessä vaiheessa metsämaan hydrologian mallinnukseen yhdistettiin kuvaus virtauksesta ojaverkostossa. Tämä mahdollisti ojaverkoston virtausolosuhteiden ajallisen tarkastelun, mikä on olennaista kiintoaineen kulkeutumisen mallintamisen kannalta. Lisäksi metsämaan hydrologian hajautettu mallintaminen osoitti, että menetelmää voidaan hyödyntää paikalliseen kuivatustilaan vaikuttavien tekijöiden määrittämisessä. Mallin laajentaminen kiinto-aineen kulkeutumisen kuvaamiseen puolestaan osoitti, että mallinnuksen avulla voidaan osoittaa eri prosessien rooli kiintoainekuorman muodostamisessa, sekä täydentää kokeellisesti hankittua tietoa. Kiintoainemalli kuvaa pohjaeroosion, sateen aiheuttaman seinämäeroosion, flokkien laskeu-tumisen ja laskeutuneiden hiukkasten stabiloitumisen ojan pohjalle. Tuloksista havaittiin, että kunnostuksen aikaansaama häiriintynyt kerros ojan pohjalla oli kuvattava mallissa, jotta heti toimenpiteen jälkeen esiintyvät korkeat pitoisuudet saatiin simuloitua. Kevätsulannan aikainen kuorma oli pääasiassa lähtöisin pohjaeroosiosta, kun taas kesäsateiden aikana seinämistä sadepisa-roiden vaikutuksesta irtoavalla turpeella oli merkittävä vaikutus. Lopuksi mallia sovellettiin suo-metsätaloudessa käytettävien vesiensuojelumenetelmien tehon arvioimiseen. Tulokset antoivat viitteitä, että pohjaeroosio voidaan saada melko tehokkaasti kuriin perkauskatkoilla tai padottavilla rakenteilla. Sen sijaan jo liikkeelle lähteneen kiintoaineen pysäyttäminen laskeutusaltailla tai liete-kuopilla osoittautui vähemmän tehokkaaksi. Mallilla pystyttiin myös tarkastelemaan ojaveden-pintaa nostavien rakenteiden vaikutusta saran kuivatustilaan. Tulosten mukaan rakenteiden vaikutus oli vähäinen viitaten siihen, etteivät puuston kasvuolosuhteet heikentyneet. Väitöskirjassa rakennettu mallikokonaisuus muodostaa uudenlaisen lähestymistavan suometsän spatiaalisen kuivatustilan ja kunnostusojituksen jälkeisen kiintoainekuorman muodostumisen ja hallinnan tarkasteluun. Mallin tarpeellisia jatkokehityskohteita olisivat mm. tarkastelun laajen-taminen ohutturpeisille eroosioherkille alueille, sekä mallin hyödyntäminen vaihtoehtoisten hakkuumenetelmien arvioimiseen suometsätaloudessa.

Julkaisun otsikon käännös	Hydrologian ja kiintoaineen kulkeutumisen mallintaminen ojitetussa suometsässä – Painopisteinä kiintoainekuorman muodostuminen ja hallinta kunnostusojituksen jälkeen
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Valvoja/neuvonantaja	Koivusalo, Harri, Vastuuprofessori Kokkonen, Teemu, Ohjaaja Warsta, Lassi, Ohjaaja Younis, Bassam A., Ohjaaja, Ulkoinen henkilö
Kustantaja	Aalto University
Painoksen ISBN	978-952-60-7889-2
Sähköinen ISBN	978-952-60-7890-8
Tila	Julkaistu - 2018
OKM-julkaisutyyppi	G5 Artikkeliväitöskirja

Tutkimusalat

hajautettu prosessipohjainen mallintaminen
hydrologia
kiintoaineen kulkeutuminen
kunnostusojitus
turvemaa
metsätalous
ojaeroosio
vesiensuojelu

Pääsy asiakirjaan

http://urn.fi/URN:ISBN:978-952-60-7890-8Lisenssi: Määrittelemätön

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@phdthesis{b666bf199d934c838f4ab6439ae53ccf,

title = "Modelling hydrology and sediment transport in a drained peatland forest - Focus on sediment load generation and control after ditch network maintenance",

abstract = "Peatland drainage has been common practice in the boreal zone for centuries. In Fennoscandia, drainage has focused on improving forest production, and the maintenance of drainage conditions is an important component of operational forestry. Elevated sediment loads released after drainage have been widely recognized, but associated process understanding is still limited, especially when it comes to controlling the loads. In this thesis, a distributed process-based model is tailored and applied to describe hydrology and sediment transport in a drained peatland forest with aims to assess the generation and the control of sediment loads after ditch network maintenance (DNM). The chain of model applications revolves around a 5.2 ha forestry-drained peatland, where hydrology and sediment processes were intensively monitored after DNM. The first model development step included coupled modeling of soil hydrology and ditch network flow. This enabled the spatiotemporal assessment of flow conditions in the ditch network, which is a necessity for sediment transport modeling. Distributed modeling of catchment hydrology further showed potential to disentangle the importance of spatial factors on local soil moisture conditions, which is relevant as drainage aims to control these conditions. Extending the model to describe sediment transport in the ditch network demonstrated its utility in identifying the role of different processes and in complementing experimental data. The model described bed erosion, rain-induced bank erosion, floc deposition, and consolidation of the bed. Simulation results sug-gested that the loose peat material, produced during excavation, contributed markedly to high concentrations immediately after DNM. Erosion during spring snowmelt was driven by ditch flow whereas during summer rainfalls, bank erosion by raindrop impact was an important process. Finally, the model was applied to investigate scenarios featuring sediment control structures imple-mented in operational peatland forestry. Results suggested that bed erosion can be efficiently prevented with breaks in ditch cleaning and structures ponding water. It was proven less efficient to trap already eroded material with sedimentation ponds and pits. Furthermore, coupled modeling of soil hydrology and ditch processes showed that structures raising ditch water level had a minor effect on water table in the strips between ditches, plausibly not impairing tree growth. The process-based modeling presented in this thesis provided a yet unexplored approach to comprehensively evaluate spatiotemporal hydrological variables, sediment load generation and alternatives for sediment control after DNM in a forestry-drained peatland. Future prospects of such a modelling approach include extending the assessment to sites on shallow peat with a higher erosion risk, and the investigation of forestry management options, e.g., harvesting alternatives, on drained peatlands.",

keywords = "distributed process-based modeling, ditch cleaning, ditch erosion, forestry, hydrology, peatland drainage, sediment transport, water protection, hajautettu prosessipohjainen mallintaminen, hydrologia, kiintoaineen kulkeutuminen, kunnostusojitus, turvemaa, mets{\"a}talous, ojaeroosio, vesiensuojelu, distributed process-based modeling, ditch cleaning, ditch erosion, forestry, hydrology, peatland drainage, sediment transport, water protection",

author = "Kersti Haahti",

year = "2018",

language = "English",

isbn = "978-952-60-7889-2",

series = "Aalto University publication series DOCTORAL DISSERTATIONS",

publisher = "Aalto University",

number = "45",

address = "Finland",

school = "Aalto University",

}

TY - BOOK

T1 - Modelling hydrology and sediment transport in a drained peatland forest - Focus on sediment load generation and control after ditch network maintenance

AU - Haahti, Kersti

PY - 2018

Y1 - 2018

N2 - Peatland drainage has been common practice in the boreal zone for centuries. In Fennoscandia, drainage has focused on improving forest production, and the maintenance of drainage conditions is an important component of operational forestry. Elevated sediment loads released after drainage have been widely recognized, but associated process understanding is still limited, especially when it comes to controlling the loads. In this thesis, a distributed process-based model is tailored and applied to describe hydrology and sediment transport in a drained peatland forest with aims to assess the generation and the control of sediment loads after ditch network maintenance (DNM). The chain of model applications revolves around a 5.2 ha forestry-drained peatland, where hydrology and sediment processes were intensively monitored after DNM. The first model development step included coupled modeling of soil hydrology and ditch network flow. This enabled the spatiotemporal assessment of flow conditions in the ditch network, which is a necessity for sediment transport modeling. Distributed modeling of catchment hydrology further showed potential to disentangle the importance of spatial factors on local soil moisture conditions, which is relevant as drainage aims to control these conditions. Extending the model to describe sediment transport in the ditch network demonstrated its utility in identifying the role of different processes and in complementing experimental data. The model described bed erosion, rain-induced bank erosion, floc deposition, and consolidation of the bed. Simulation results sug-gested that the loose peat material, produced during excavation, contributed markedly to high concentrations immediately after DNM. Erosion during spring snowmelt was driven by ditch flow whereas during summer rainfalls, bank erosion by raindrop impact was an important process. Finally, the model was applied to investigate scenarios featuring sediment control structures imple-mented in operational peatland forestry. Results suggested that bed erosion can be efficiently prevented with breaks in ditch cleaning and structures ponding water. It was proven less efficient to trap already eroded material with sedimentation ponds and pits. Furthermore, coupled modeling of soil hydrology and ditch processes showed that structures raising ditch water level had a minor effect on water table in the strips between ditches, plausibly not impairing tree growth. The process-based modeling presented in this thesis provided a yet unexplored approach to comprehensively evaluate spatiotemporal hydrological variables, sediment load generation and alternatives for sediment control after DNM in a forestry-drained peatland. Future prospects of such a modelling approach include extending the assessment to sites on shallow peat with a higher erosion risk, and the investigation of forestry management options, e.g., harvesting alternatives, on drained peatlands.

AB - Peatland drainage has been common practice in the boreal zone for centuries. In Fennoscandia, drainage has focused on improving forest production, and the maintenance of drainage conditions is an important component of operational forestry. Elevated sediment loads released after drainage have been widely recognized, but associated process understanding is still limited, especially when it comes to controlling the loads. In this thesis, a distributed process-based model is tailored and applied to describe hydrology and sediment transport in a drained peatland forest with aims to assess the generation and the control of sediment loads after ditch network maintenance (DNM). The chain of model applications revolves around a 5.2 ha forestry-drained peatland, where hydrology and sediment processes were intensively monitored after DNM. The first model development step included coupled modeling of soil hydrology and ditch network flow. This enabled the spatiotemporal assessment of flow conditions in the ditch network, which is a necessity for sediment transport modeling. Distributed modeling of catchment hydrology further showed potential to disentangle the importance of spatial factors on local soil moisture conditions, which is relevant as drainage aims to control these conditions. Extending the model to describe sediment transport in the ditch network demonstrated its utility in identifying the role of different processes and in complementing experimental data. The model described bed erosion, rain-induced bank erosion, floc deposition, and consolidation of the bed. Simulation results sug-gested that the loose peat material, produced during excavation, contributed markedly to high concentrations immediately after DNM. Erosion during spring snowmelt was driven by ditch flow whereas during summer rainfalls, bank erosion by raindrop impact was an important process. Finally, the model was applied to investigate scenarios featuring sediment control structures imple-mented in operational peatland forestry. Results suggested that bed erosion can be efficiently prevented with breaks in ditch cleaning and structures ponding water. It was proven less efficient to trap already eroded material with sedimentation ponds and pits. Furthermore, coupled modeling of soil hydrology and ditch processes showed that structures raising ditch water level had a minor effect on water table in the strips between ditches, plausibly not impairing tree growth. The process-based modeling presented in this thesis provided a yet unexplored approach to comprehensively evaluate spatiotemporal hydrological variables, sediment load generation and alternatives for sediment control after DNM in a forestry-drained peatland. Future prospects of such a modelling approach include extending the assessment to sites on shallow peat with a higher erosion risk, and the investigation of forestry management options, e.g., harvesting alternatives, on drained peatlands.

KW - distributed process-based modeling

KW - ditch cleaning

KW - ditch erosion

KW - forestry

KW - hydrology

KW - peatland drainage

KW - sediment transport

KW - water protection

KW - hajautettu prosessipohjainen mallintaminen

KW - hydrologia

KW - kiintoaineen kulkeutuminen

KW - kunnostusojitus

KW - turvemaa

KW - metsätalous

KW - ojaeroosio

KW - vesiensuojelu

KW - distributed process-based modeling

KW - ditch cleaning

KW - ditch erosion

KW - forestry

KW - hydrology

KW - peatland drainage

KW - sediment transport

KW - water protection

M3 - Doctoral Thesis

SN - 978-952-60-7889-2

T3 - Aalto University publication series DOCTORAL DISSERTATIONS

PB - Aalto University

ER -

Modelling hydrology and sediment transport in a drained peatland forest - Focus on sediment load generation and control after ditch network maintenance

Abstrakti

Tutkimusalat

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