Erosion and sediment transport mechanisms in drained peatland forest catchments after ditch network maintenance

Leena Stenberg

Erosion and sediment transport mechanisms in drained peatland forest catchments after ditch network maintenance

Julkaisun otsikon käännös: Kunnostusojituksen vaikutus eroosioon ja kiintoaineen kulkeutumiseen metsäojitetuilla soilla

Leena Stenberg

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Suometsiä ojitetaan puuston kasvuolosuhteiden parantamiseksi. Kunnostusojitusta, eli olemassa olevien ojien puhdistusta ja täydennysojien kaivamista, tarvitaan muutaman vuosikymmenen välein. Kunnostusojitus lisää ojien eroosiota ja kiintoaineen kulkeutumista alapuolisiin vesistöihin, minkä vuoksi se on ympäristön kannalta yksi metsätalouden haital- lisimmista toimenpiteistä. Väitöskirjan päätavoitteina on 1) tunnistaa tärkeimmät eroosioon ja kiintoaineen kulkeutumiseen liittyvät mekanismit kunnostusojituksen jälkeen, 2) verrata neulareliefimittausten ja maalaserkeilauksen soveltuvuutta ojien topografian muutosten ja pinnan karkeuden mittaamiseen ja 3) tarkastella tulosten merkitystä käytännön kannalta. Kunnostusojituksen vaikutuksia tutkittiin kahdella koealueella. Santamäensuolla (Karkkilassa) mitattiin lyhytaikaisessa kokeessa veden tihkumisen aiheuttamaa luiskaeroosiota neulareliefillä lisäämällä eroosioriskiä keinotekoisen kastelun avulla. Koivupuron valuma-alueella (Sotkamossa) eroosiomekanismeja tutkittiin kahden kunnostusojitusta seuranneen vuoden ajan. Ojatopografian muutoksia mitattiin neulareliefillä ja maalaserkeilaimella ja tuloksia verrattiin virtaaman ja kiintoainekonsentraation avulla määritettyyn kiintoainekuormaan valuma-alueen sekä sen sisään rajatun alivaluma-alueen purkupisteissä. Kunnostusojituksen vaikutusta kiintoainekuormaan arvioitiin kalibrointikausi-vertailualue-menetelmän avulla. Vertailualueita käytettiin myös yksikkövaluntakäyrän muutosten arviointiin. Tulosten mukaan eroosioon ja kiintoainekuorman kertymiseen vaikuttavat useat vuorovaikutteiset mekanismit ja prosessit. Ojan luiskassa, vesirajan yläpuolella tapahtuvat subaeeriset prosessit, kuten routiminen ja kuivuminen, lisäävät luiskan eroosioherkkyyttä. Luiskaeroosion merkitys kiintoainekuormitukselle oli pohjaeroosiota suurempi. Ohutturpeisella alueella eroosio oli suurimmillaan talven ja kevään aikana. Sen sijaan paksuturpeisella alueella eroosiota tapahtui eniten kesällä. Ojaverkon sisäiset eroosiomäärät olivat selvästi suurempia kuin valuma-alueen kiintoainekuormitus, mikä viittaa ojaverkon pohjalle kasautuneeseen ja mahdollisesti myöhemmin helposti veden mukana kulkeutuvaan kiintoaineeseen. Kasvillisuuden kehittyminen kunnostusojitusta seuraavana kesänä stabiloi ojien reunoja. Valuntapiikit kasvoivat kunnostusojituksen jälkeen, mutta niiden ajoitus ei juuri muuttunut. Kunnostusojitusta enemmän ajoitukseen vaikutti ojitetun pinta-alan osuus valuma-alueesta. Neulareliefin ja maalaserkeilaimen avulla lasketut eroosiomäärät poikkesivat toisistaan, mutta molempien mukaan eroosio ja pinnan karkeus olivat suurempia ohutturpeisella alueella. Eroosiota tapahtui etenkin hienorakeisesta kivennäismaasta, minkä vuoksi olisi tärkeää mahdollisuuksien mukaan välttää kunnostusojitusta ohutturpeisilla alueilla, joilla ojat yltävät eroosioherkkään kivennäismaahan.

Julkaisun otsikon käännös	Kunnostusojituksen vaikutus eroosioon ja kiintoaineen kulkeutumiseen metsäojitetuilla soilla
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Valvoja/neuvonantaja	Koivusalo, Harri, Vastuuprofessori Finér, Leena, Ohjaaja, Ulkoinen henkilö
Kustantaja	Aalto University
Painoksen ISBN	978-952-60-7053-7
Sähköinen ISBN	978-952-60-7054-4
Tila	Julkaistu - 2016
OKM-julkaisutyyppi	G5 Artikkeliväitöskirja

Tutkimusalat

eroosio
kiintoainekuormitus
kunnostusojitus
suometsät

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http://urn.fi/URN:ISBN:978-952-60-7054-4Lisenssi: Määrittelemätön

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@phdthesis{1c0f03221fd44264accea467ff2e8193,

title = "Erosion and sediment transport mechanisms in drained peatland forest catchments after ditch network maintenance",

abstract = "It is common practice to drain peatland forests in order to obtain better conditions for tree growth. Ditch network maintenance (DNM), the cleaning of existing ditches and digging of new supplemental ditches, is needed every few decades. DNM causes some of the most harmful environmental effects of forestry due to the sediment load induced by the increased erosion in the ditch network. The main objectives of this thesis are: 1) to identify the key mechanisms inflicting erosion and sediment load following DNM, 2) to compare pin meter measurements and terrestrial laser scanning (TLS) for roughness assessment and the change detection of peatland forest ditch topography, and 3) to discuss the practical implications of the results. Two experimental areas were included in the thesis. In Santam{\"a}ensuo, bank erosion induced by seepage was studied in a short-term experiment post-DNM by using artificial irrigation and pin meter measurements to quantify the changes (erosion and deposition) in the topography of a cleaned ditch bank. In Koivupuro, erosion mechanisms were monitored for two years following DNM by applying pin meter measurements and TLS for the change detection of ditch topography, as well as simultaneously measuring discharge and suspended sediment (SS) load at the main catchment and sub-catchment outlets. A paired catchment method was applied using a nearby reference catchment to estimate the impact of DNM on the SS load. Reference catchments were also utilized to assess the changes in unit hydrographs caused by DNM.The results indicate that several interacting processes and mechanisms affect erosion and sediment generation from the ditch network. Subaerial processes, such as frost and desiccation, prepare the bank for erosion. Bank erosion had an important role in producing sediment in the network, while the role of bed erosion was more modest. In the area of thin peat layer, erosion was the highest during the winter-spring period, while in the peat ditches, most of the erosion occurred during the summer time. Erosion inside the ditch network was significantly higher than the SS load at the catchment outlet, indicating that there is a vast potential of easily transportable sediments deposited at the ditch bed. Vegetation started to develop in the first summer following DNM stabilizing the banks and reducing sediment transport. The hydrograph analysis showed that DNM increased the discharge peaks and shortened the mean transit times in the catchment. The timing of the peak was more influenced by the proportion of the drained area of the catchment. There were differences in the results, but both methods (pin meter measurements and TLS) indicated that more erosion and higher roughness occurred in the ditch with a thin peat layer, especially in the fine-textured mineral soil part of the ditch bank, which emphasizes the importance of avoiding cleaning areas of thin peat where possible.",

keywords = "ditch network maintenance, erosion, peatland drainage, sediment transport, eroosio, kiintoainekuormitus, kunnostusojitus, suomets{\"a}t, ditch network maintenance, erosion, peatland drainage, sediment transport",

author = "Leena Stenberg",

year = "2016",

language = "English",

isbn = "978-952-60-7053-7",

series = "Aalto University publication series DOCTORAL DISSERTATIONS",

publisher = "Aalto University",

number = "202",

address = "Finland",

school = "Aalto University",

}

TY - BOOK

T1 - Erosion and sediment transport mechanisms in drained peatland forest catchments after ditch network maintenance

AU - Stenberg, Leena

PY - 2016

Y1 - 2016

N2 - It is common practice to drain peatland forests in order to obtain better conditions for tree growth. Ditch network maintenance (DNM), the cleaning of existing ditches and digging of new supplemental ditches, is needed every few decades. DNM causes some of the most harmful environmental effects of forestry due to the sediment load induced by the increased erosion in the ditch network. The main objectives of this thesis are: 1) to identify the key mechanisms inflicting erosion and sediment load following DNM, 2) to compare pin meter measurements and terrestrial laser scanning (TLS) for roughness assessment and the change detection of peatland forest ditch topography, and 3) to discuss the practical implications of the results. Two experimental areas were included in the thesis. In Santamäensuo, bank erosion induced by seepage was studied in a short-term experiment post-DNM by using artificial irrigation and pin meter measurements to quantify the changes (erosion and deposition) in the topography of a cleaned ditch bank. In Koivupuro, erosion mechanisms were monitored for two years following DNM by applying pin meter measurements and TLS for the change detection of ditch topography, as well as simultaneously measuring discharge and suspended sediment (SS) load at the main catchment and sub-catchment outlets. A paired catchment method was applied using a nearby reference catchment to estimate the impact of DNM on the SS load. Reference catchments were also utilized to assess the changes in unit hydrographs caused by DNM.The results indicate that several interacting processes and mechanisms affect erosion and sediment generation from the ditch network. Subaerial processes, such as frost and desiccation, prepare the bank for erosion. Bank erosion had an important role in producing sediment in the network, while the role of bed erosion was more modest. In the area of thin peat layer, erosion was the highest during the winter-spring period, while in the peat ditches, most of the erosion occurred during the summer time. Erosion inside the ditch network was significantly higher than the SS load at the catchment outlet, indicating that there is a vast potential of easily transportable sediments deposited at the ditch bed. Vegetation started to develop in the first summer following DNM stabilizing the banks and reducing sediment transport. The hydrograph analysis showed that DNM increased the discharge peaks and shortened the mean transit times in the catchment. The timing of the peak was more influenced by the proportion of the drained area of the catchment. There were differences in the results, but both methods (pin meter measurements and TLS) indicated that more erosion and higher roughness occurred in the ditch with a thin peat layer, especially in the fine-textured mineral soil part of the ditch bank, which emphasizes the importance of avoiding cleaning areas of thin peat where possible.

AB - It is common practice to drain peatland forests in order to obtain better conditions for tree growth. Ditch network maintenance (DNM), the cleaning of existing ditches and digging of new supplemental ditches, is needed every few decades. DNM causes some of the most harmful environmental effects of forestry due to the sediment load induced by the increased erosion in the ditch network. The main objectives of this thesis are: 1) to identify the key mechanisms inflicting erosion and sediment load following DNM, 2) to compare pin meter measurements and terrestrial laser scanning (TLS) for roughness assessment and the change detection of peatland forest ditch topography, and 3) to discuss the practical implications of the results. Two experimental areas were included in the thesis. In Santamäensuo, bank erosion induced by seepage was studied in a short-term experiment post-DNM by using artificial irrigation and pin meter measurements to quantify the changes (erosion and deposition) in the topography of a cleaned ditch bank. In Koivupuro, erosion mechanisms were monitored for two years following DNM by applying pin meter measurements and TLS for the change detection of ditch topography, as well as simultaneously measuring discharge and suspended sediment (SS) load at the main catchment and sub-catchment outlets. A paired catchment method was applied using a nearby reference catchment to estimate the impact of DNM on the SS load. Reference catchments were also utilized to assess the changes in unit hydrographs caused by DNM.The results indicate that several interacting processes and mechanisms affect erosion and sediment generation from the ditch network. Subaerial processes, such as frost and desiccation, prepare the bank for erosion. Bank erosion had an important role in producing sediment in the network, while the role of bed erosion was more modest. In the area of thin peat layer, erosion was the highest during the winter-spring period, while in the peat ditches, most of the erosion occurred during the summer time. Erosion inside the ditch network was significantly higher than the SS load at the catchment outlet, indicating that there is a vast potential of easily transportable sediments deposited at the ditch bed. Vegetation started to develop in the first summer following DNM stabilizing the banks and reducing sediment transport. The hydrograph analysis showed that DNM increased the discharge peaks and shortened the mean transit times in the catchment. The timing of the peak was more influenced by the proportion of the drained area of the catchment. There were differences in the results, but both methods (pin meter measurements and TLS) indicated that more erosion and higher roughness occurred in the ditch with a thin peat layer, especially in the fine-textured mineral soil part of the ditch bank, which emphasizes the importance of avoiding cleaning areas of thin peat where possible.

KW - ditch network maintenance

KW - erosion

KW - peatland drainage

KW - sediment transport

KW - eroosio

KW - kiintoainekuormitus

KW - kunnostusojitus

KW - suometsät

KW - ditch network maintenance

KW - erosion

KW - peatland drainage

KW - sediment transport

M3 - Doctoral Thesis

SN - 978-952-60-7053-7

T3 - Aalto University publication series DOCTORAL DISSERTATIONS

PB - Aalto University

ER -

Erosion and sediment transport mechanisms in drained peatland forest catchments after ditch network maintenance

Abstrakti

Tutkimusalat

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Pääsy asiakirjaan

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