Elina Kasvi

Seamless Mapping of River Channels at High Resolution Using Mobile LiDAR and UAV-Photography

Accurate terrain models are a crucial component of studies of river channel evolution. In this paper we describe a new methodology for creating high-resolution seamless digital terrain models (DTM) of river channels and their floodplains. We combine mobile laser scanning and low-altitude unmanned aerial vehicle (UAV) photography-based methods for creating both a digital bathymetric model of the inundated river channel and a DTM of a point bar of a meandering sub-arctic river. We evaluate mobile laser scanning and UAV-based photogrammetry point clouds against terrestrial laser scanning and combine these data with an optical bathymetric model to create a seamless DTM of two different measurement periods. Using this multi-temporal seamless data, we calculate a DTM of difference that allows a change detection of the meander bend over a one-year period.

Annual bank and point bar morphodynamics of a meandering river determined by high‐accuracy multitemporal laser scanning and flow data

The knowledge has been insufficient concerning the effects of peak flows, and local bend and flow characteristics on annual morphodynamics of consecutive bends in meandering rivers. Therefore, it was determined how flow peak magnitude and duration affect morphodynamics, how the short-term spatial evolution of a given meander bend associates with the neighboring bends, and how local bend and flow characteristics affect morphodynamics. The annual bank and point bar morphodynamics of eight consecutive bends of a subarctic meandering river were analyzed between 2009 and 2012 on the basis of high-accuracy multitemporal data, measured by mobile and terrestrial laser scanning and an Acoustic Doppler Current Profiler. According to the results, multiple years of highly accurate data are crucial for a broader picture of meandering channel evolution. The results showed for the first time in detail that none of the years were similar in terms of point bar and bank morphodynamics. The duration of point bar submergence and maximum water stage was more important for evolution of the meandering channel than the local effects of each bend. The detailed topographical data of the present study confirmed that the higher the flow and water stage peak the more deposition occurred on point bars. More importantly, the independence of the short-term spatial evolution of meander bends from the association with neighboring bends was confirmed. Erosion patterns did not relate particularly to the sinuosity or radius of curvature. A clear relation between velocity and bend curvature, on which some meander migration models rely, was not found.

Data Processing and Quality Evaluation of a Boat-Based Mobile Laser Scanning System

Mobile mapping systems (MMSs) are used for mapping topographic and urban features which are difficult and time consuming to measure with other instruments. The benefits of MMSs include efficient data collection and versatile usability. This paper investigates the data processing steps and quality of a boat-based mobile mapping system (BoMMS) data for generating terrain and vegetation points in a river environment. Our aim in data processing was to filter noise points, detect shorelines as well as points below water surface and conduct ground point classification. Previous studies of BoMMS have investigated elevation accuracies and usability in detection of fluvial erosion and deposition areas. The new findings concerning BoMMS data are that the improved data processing approach allows for identification of multipath reflections and shoreline delineation. We demonstrate the possibility to measure bathymetry data in shallow (0–1 m) and clear water. Furthermore, we evaluate for the first time the accuracy of the BoMMS ground points classification compared to manually classified data. We also demonstrate the spatial variations of the ground point density and assess elevation and vertical accuracies of the BoMMS data.

Modern empirical and modelling study approaches in fluvial geomorphology to elucidate sub-bend-scale meander dynamics

Major developments in theory and modelling techniques have taken place within the past couple of decades in the field of the fluvial geomorphology. In this review, we examine the state-of-the-art empirical and modelling approaches and discuss their potential benefits and shortcomings in deepening understanding of the sub-bend-scale fluvial geomorphology of meander bends. Meandering rivers represent very complex 3D flow and sedimentary processes. We focus on high-resolution techniques which have improved the spatial and temporal resolution of the data and thereby enabled investigation of processes, which have been thus far beyond the capacity of the measurement techniques. This review covers the measurement techniques applied in the field and in laboratory circumstances as well as the close-range remote sensing techniques and computational approaches. We discuss the key research questions in fluvial geomorphology of meander bends and demonstrate how the contemporary approaches have been and could be applied to solve these questions.

The effects of ice cover on flow characteristics in a subarctic meandering river

The effects of ice cover on flow characteristics in meandering rivers are still not completely understood. Here, we quantify the effects of ice cover on flow velocity, the vertical and spatial flow distribution, and helical flow structure. Comparison to open-channel low flow conditions is performed. The Acoustic Doppler Current Profiler (ADCP) is used to measure flow from up to three meander bends, depending on the year, in a small sandy meandering subarctic river (Pulmanki River) during two consecutive ice-covered winters (2014 and 2015). This article is protected by copyright. All rights reserved. Under ice, flow velocities and discharges were predominantly slower than during the preceding autumn open-channel conditions. Velocity distribution was almost opposite to theoretical expectations. Under ice, velocities reduced when entering deeper water downstream of the apex in each meander bend. When entering the next bend, velocities increased again together with the shallower depths. The surface velocities were predominantly greater than bottom/riverbed velocities during open-channel flow. The situation was the opposite in ice-covered conditions, and the maximum velocities occurred in the middle layers of the water columns. High-Velocity Core (HVC) locations varied under ice between consecutive cross-sections. Whereas in ice-free conditions the HVC was located next to the inner bank at the upstream cross-sections, the HVC moved towards the outer bank around the apex and again followed the thalweg in the downstream cross-sections. Two stacked counter-rotating helical flow cells occurred under ice around the apex of symmetric and asymmetric bends: next to the outer bank, top- and bottom-layer flows were towards the opposite direction to the middle layer flow. In the following winter, no clear counter-rotating helical flow cells occurred due to the shallower depths and frictional disturbance by the ice cover. Most probably the flow depth was a limiting factor for the ice-covered helical flow circulation, similarly as the shallow depths hinder secondary flow in open-channel conditions. This article is protected by copyright. All rights reserved.

Tulvariskien hallinta uusilla teknologioilla

Tulvat ovat vahingollisimpia luonnon aiheuttamia tuhoja Euroopassa. Viimeisen kahdenkymmenenviiden vuoden aikana puolentoistasataa tulvaa ovat aiheuttaneet lähes tuhannen ihmishengen menetyksen, puoli miljoonaa ihmistä on joutunut muuttamaan pois kodeistaan, taloudelliset menetykset ovat olleet n. 30 miljardia euroa. Suomessa tulvien aiheuttamat vahingot ovat olleet huomattavasti vähäisempiä, mutta paikalliset vahingot ovat olleet merkittäviä. Kevään 2005 tulvat Ivalossa ja Kittilässä aiheuttivat noin 5 milj. euron vahingot. Rankkasadetulva syksyllä 2007 aiheutti 22 milj. euron vahingot Porissa. Tulvien aiheuttamat rahalliset vahingot yhdessä ihmishenkien menetysten kanssa sekä tulvien lisääntyminen ja esiintymisajankohdan muutokset ovat voimistaneet tulviin kohdistuvaa tutkimusta sekä Suomessa että ulkomailla. Lisääntyneet tulvat ja niihin liittyvät vahingot Euroopassa johtivat EU:n tulvadirektiivin laatimiseen, jossa jäsenvaltioita velvoitetaan kartoittamaan sekä mahdollisten suurtulvien tulva-alueet että tulvista aiheutuvat riskit ja edelleen laatimaan kokonaisvaltaiset hallintasuunnitelmat tulvariskien hallintaan. Tulvadirektiivi antaa viitekehyksen tulvatilanteiden ennakointiin sekä tulvaennustuksiin ja -riskien ennustuksiin Suomessa. Se velvoittaa jäsenvaltion arvioimaan potentiaaliset tulva-alueensa. Suomessa tällaisia alueita on seitsemänkymmentä. Näille alueille on tehty yleispiirteinen tulvavaaraeli tulvan laajuuskartoitus. Tulvavaarakartoituksessa kartoitetaan eri toistuvuusajoilla (esim. tilastollisesti kerran 20 vuodessa tapahtuva tulva) tapahtuvien tulvien laajuudet ja syvyydet. Tulvavaarakartat laaditaan Suomessa vähintään toistuvuusajoille 1/20, 1/50, 1/100, 1/250 ja 1/1000 vuodessa. Tulvavaarakarttojen lisäksi tehdään tulvariskien kartoitus. Tulvariskikartoissa esitetään tulvavaarakarttojen toistuvuuksien mukaisesti esiintyviin tulviin mahdollisesti liittyvät vahingolliset seuraukset mukaan lukien seurauksista mahdollisesti kärsivien asukkaiden määrä, alueella harjoitettavan taloudellisen toiminnan tyyppi ja laitokset, jotka voivat aiheuttaa äkillistä veden tai maaperän pilaantumista tulvatilanteessa ja toisaalta seurauksista mahdollisesti kärsivät suojelualueet. Näiden kartoitusten ja asiantuntija-arvioiden perusteella Maaja metsätalousministeriö nimesi 21 aluetta vuonna 2011, joilla vesistöjen tai meren tulvimisesta aiheutuvat riskit ovat merkittäviä. Näistä alueista 17 sijaitsee sisämaassa vesistöjen varrella ja neljä rannikolla. Merkittäville tulvariskialueille tehdään paraikaa tulvariskien hallinnan suunnitelmia. Tulvariskien hallintasuunnitelmissa on esitettävä tulvariskien hallintatavoitteet ja toimenpiteet niiden saavuttamiseksi. Suunnitelmissa käsitellään kaikkia tulvariskien hallinnan näkökohtia. Niissä keskitytään tulvien ehkäisyyn, suojeluun sekä valmiustoimiin ja otetaan huomioon myös vesistöalueen erityispiirteet. Mahdollisen suurtulvan aiheuttamien vahinkojen on alustavasti arvioitu kohoavan Suomessa jopa 550 milj. euron suuruisiksi.