–1–
1 Impac s o Plan Roo s on Deb is-Flow Bed E osion in
2 Labo a o y Expe imen s
3 Anna J. an den B oek1, Dagma T. Mennes1, Maa en G. Kleinhans1,
4 Lonneke Roelo s1, Jana Eichel1, Daniel D aebing1, Tjalling de Haas1
5 1Depa men o Physical Geog aphy, U ech Uni e si y, U ech , 3584 CB, The Ne he lands
6
Co esponding au ho : Anna J. an den B oek, a.j. andenb [email protected]
–2–
7 Abs ac
8 Deb is lows o en inc ease in size due o bed e osion and en ainmen , enhancing
9 hei haza dous po en ial. Howe e , he e ec s o plan oo ing on deb is- low e osion
10 on ubiqui ous ege a ed slopes emain unknown, which hinde s deb is- low haza d as-
11 sessmen . He e, we in es iga ed he e ec s o oo s on deb is- low bed e osion using scaled
12 expe imen s in a 5 m long, 0.3 m wide labo a o y lume wi h an e odible bed. Roo s o
13 as -g owing So ghum bicolo (Sudan g ass) seedlings we e used as p oxies o ee oo s
14 o quan i y he e ec o a ying oo ing cha ac e is ics on e osion. Ou esul s indica e
15 ha e osion dec eases non-linea ly wi h inc easing Roo Leng h Densi y (RLD) and Roo
16 A ea Ra io (RAR). Inc eases in ei he pa ame e enhance oo –soil con ac , he eby im-
17 p o ing soil s abili y and educing e osion. Among he wo, RLD, and hus he combined
18 e ec o oo leng h and oo densi y, appea s mos in luen ial, as RAR does no cap-
19 u e he h ee-dimensional s uc u e o he oo sys em. Ou expe imen al esul s sug-
20 ges ha inc easing oo -soil con ac a he deb is- low bed educes e osion, dec easing
21 o e en p e en ing deb is- low olume g ow h. These indings imply ha al e a ions in
22 ege a ion cha ac e is ics, such as hose esul ing om o es i es o e o es a ion, a -
23 ec deb is- low e osion and open up possibili ies o biogeomo phic scale expe imen s
24 o slope p ocesses.
25 Keywo ds: deb is low, e osion, ege a ion oo s, Roo Leng h Densi y, Roo A ea Ra-
26
io,
haza d
mi iga ion
27 Highligh s
28
•
Expe imen s show ha oo ing enhances soil s abili y and educes deb is- low e o-
29 sion.
30
•
Roo Leng h Densi y has a s ong non-linea e ec in educing bed e osion.
31
•
Seedlings o e po en ial o s udying oo e ec s on deb is lows in expe imen s.
32 G aphical Abs ac
–3–
33 1 In oduc ion
34 Deb is lows a e powe ul and des uc i e mass mo emen s ha annually cos hou-
35 sands o li es and billions o eu os wo ldwide (P akash e al., 2024). The numbe o ca-
36 sual ies has been di ec ly linked o deb is- low olume (Dowling & San i, 2014), which
37 may g ow se e al o de s o magni ude by bed and bank e osion du ing anspo (e.g.
38 I e son e al., 2011; Jakob & Hung , 2005). The e o e, unde s anding deb is- low e o-
39 sion is impo an o e ec i e haza d assessmen and mi iga ion. P e ious labo a o y,
40 modeling, and ieldwo k s udies ha e shown ha deb is- low bed e osion depends on com-
41 plex in e ac ions be ween deb is- low o ces and bed s eng h. Many low p ope ies ha e
42 been co ela ed o bed e osion a es including olume (De Haas e al., 2022; Chen e
43 al., 2005), dep h (Sc
h
u
¨
c
h
e al., 2011; McDougall & Hung , 2005), eloci y (I e son, 2012;
44 McDougall & Hung , 2005), and basal, shea and impac o ces (Roelo s e al., 2022;
45 Li e al., 2020; Jakob & Hung , 2005), whe e an inc ease in ei he o hese ac o s gen-
46 e ally enhances e osion. The esis ance o he bed agains e osion, o bed shea s eng h,
47 has been ound o depend s ongly on he po e-p essu e o he bed a he bed- low in-
48 e ace. An inc ease in po e-p essu e educes in e g anula ic ion and he e o e weak-
49 ens he bed shea s eng h (Zheng e al., 2023; Roelo s e al., 2023; Zheng e al., 2021;
50 McCoy e al., 2012; I e son, 2012). Vege a ion oo s a e known o play a c ucial ole in
51 imp o ing bed, bank, and soil s eng h in bo h lu ial en i onmen s (Pollen, 2007; Pollen
52 & Simon, 2005) and soil s abili y p ocesses in moun ainous en i onmen s (D aebing e
53 al., 2023; Roe ing e al., 2003; Schmid e al., 2001). By s eng hening he soil ma ix,
54 hey help esis e osion, educe sedimen anspo , and mi iga e he isk o slope ail-
55 u es. Roo s esis ension o ces and dissipa e shea s esses h ough he soil using hei
56 ensile s eng h (Ghes em e al., 2014), inc ease shea s eng h by ancho ing he soil (Poi ie
57 e al., 2018; Kaspa & Singe , 2011), and bind soil pa icles h ough adhesi e sec e ions
58 and chemical eac ions wi h clay (Galloway e al., 2020; Poi ie e al., 2018).
59 While p e ious s udies documen ed deb is lows in e ac ion wi h plan s in many
60 moun ain egions (Figu e 1; Boo h e al., 2020; Bollschweile e al., 2008; Wil o d e al.,
61 2005; Bunn & Mon gome y, 2004; Johnson e al., 2000), we ha e su p isingly li le
62 quan i a i e unde s anding o how oo s a ec deb is- low e osion. Field obse a ions
63 indica e ha oo s can educe deb is- low e osion (Cui e al., 2024; Wil o d e al., 2005).
64 Mon hs o yea s a e wild i es, whe e he abo e-g ound biomass is des oyed, e osion
65 by deb is lows peaks due o weakened soil s eng h as a esul o oo decay (Thomas
66 e al., 2021; de G a , 2018; Meye e al., 2001). In indus ial o es s, deb is lows o en
67 scou channels down o bed ock, whe eas in old-g ow h o es s wi h well-de eloped oo
68 s uc u es, such scou ing is a e (Schmid e al., 2001; Bunn & Mon gome y, 2004). On
69 deb is- low ans, oo webs a e obse ed o o e esis ance o deb is- low e osion (Figu e
70 1b and d; Wil o d e al., 2005). Combined wi h he well-documen ed ole o plan oo s
71 in soil s abili y (e.g. Vannoppen e al., 2015; Ghes em e al., 2014; De Bae s e al., 2007),
72 hese indings sugges ha oo s may play a i al ole in educing e osion caused by de-
73 b is lows (Figu e 2). Despi e he signi icance, ou unde s anding o he e ec o oo s
74 on deb is- low e osion emains limi ed, mainly because o a lack o quan i a i e da a. This
75 lack esul s om he in equen and des uc i e cha ac e o deb is lows, making i ha d
76 o measu e low p ope ies, oo cha ac e is ics, bed s eng h, and in e ac ions be ween
77 deb is low and bed in he ield (I e son, 1997). As da a on deb is- low o ces and o oo
78 cha ac e is ics a e limi ed, hei combined in luence on e osion dynamics emains e en
79 mo e poo ly cons ained.
80 To imp o e ou insigh in o his poo ly unde s ood p ocess, we aim o (1) un a el
81 he impac s o plan oo s on deb is- low e osion and (2) es he applicabili y o as -
82 g owing seedlings o s udy deb is low—plan in e ac ions in scaled labo a o y expe -
83 imen s. He e, we p esen no el expe imen s in a small-scale deb is- low lume wi h an
84 e odible sedimen bed, in which we g ow li e seedlings o o m beds wi h a wide ange
85 o oo ai s. Labo a o y expe imen s enable sys ema ic and quan i a i e explo a ion
–4–
Figu e 1. Examples o in e ac ions be ween deb is lows and ege a ion. (a) Pos -wild i e
deb is low in James own, Colo ado, No embe 2013 (Whi e, 2013), (b) The oo ne wo k in a
o es s and s eng hens he soil, enhancing i s esis ance o he de elopmen o new channels
on a deb is- low an; B i ish Columbia, Canada (Wil o d e al., 2005), (c) Deb is low unning
h ough a o es ed an; Keze gully, China (Cui e al., 2024), (d) A deb is low gully o med a e
a o es i e, exposing oo s p o uding om he gully banks; Glenwood Canyon, Colo ado, USA
(wi h cou esy o F ancis Renge s), (e) A scou hole o med by a deb is low a e a o es i e;
Glenwood Canyon, Colo ado, USA (wi h cou esy o F ancis Renge s) ( ) Exposed oo s a e
se e e deb is- low e osion o a deb is- low an; B i ish Columbia, Canada (Wil o d e al., 2005).
–5–
Figu e 2. Ske ch o (a) he p ima y o ces d i ing deb is low e osion in he absence o eg-
e a ion, and (b) he addi ional mechanical e ec s o ege a ion oo s on deb is- low e osion
educ ion by inc eased soil cohesion and shea s ess dis ibu ion h ough ensile s eng h.
86 o he e ec s o plan oo s on deb is- low e osion unde con olled condi ions. In expe -
87 imen s, ini ial and bounda y condi ions can be p ecisely con olled, allowing o he iso-
88 la ion o indi idual a iables. Deb is lows ha e been success ully scaled in p e ious ex-
89 pe imen s, which, o example, inc eased ou unde s anding o he e ec s o bed and deb is-
90 low composi ion on e osion (Roelo s e al., 2023; Zheng e al., 2021), he seismic ib a-
91 ions and no mal- o ce luc ua ions o deb is lows (De Haas e al., 2021), and sedimen
92 apping by ege a ion (He e al., 2023). Howe e , scan deb is- low expe imen s ha e
93 been conduc ed wi h plan oo s. Fas -g owing seedlings ha e al eady been success ully
94 used in o he geomo phic expe imen s o s udy a ious ea h su ace p ocesses (e.g. Lokho s
95 e al., 2019; De Bae s e al., 2007), bu hei po en ial emains la gely unexplo ed in deb is-
96 low esea ch. Such species o e se e al ad an ages, including he de elopmen o na -
97 u al oo -soil con ac s, a ela i ely sho cul i a ion pe iod, and he abili y o al e oo
98 ai s sys ema ically. We conduc ed con olled labo a o y expe imen s o quan i y he
99 e ec o oo s on deb is- low e osion. In his pape we do no es e ec s o abo e-g ound
100 biomass, which we emo ed, bu ocus on sys ema ically a ied oo ai s ac oss expe -
101 imen s o iden i y key ac o s go e ning e osion educ ion. The da a gene a ed could p o-
102 ide insigh s in o he e ec s o wild i es and de o es a ion on deb is- low e osion, pa -
103 icula ly when abo e-g ound biomass is los and emaining oo s play a key ole (Thomas
104 e al., 2021; de G a , 2018; Meye e al., 2001). A be e unde s anding o hese p ocesses
105 would also con ibu e o mo e accu a e deb is- low models and os e he de elopmen
106 o s a egies o educe deb is- low e osion and mi iga e i s haza ds.
107 2
Me hods
108 To s udy he e ec o oo s on deb is- low e osion, we conduc ed 38 expe imen s
109 in a lume wi h an e odible bed pene a ed by na u al oo s. Each expe imen ’s seed-
110 ing densi ies and ege a ion ages di e ed o a y oo ai s while keeping he bed and
111 deb is- low composi ion cons an . Fo each densi y, an expe imen was conduc ed wi h
112 5-day-old and 8-day-old ege a ion o a y oo leng h and diame e (see Table S1 o
113 condi ions o each expe imen ). The ini ial se o expe imen s was mean o de e mine
114 he bes expe imen al se up and o e i y he applicabili y o he me hod. The e o e, each
115 expe imen al se up was epea ed once. The ou comes o hese expe imen s a e no in-
116 cluded in he esul s. Once consis ency and applicabili y we e con i med, subsequen ex-
117 pe imen s we e conduc ed only once.
–6–
Figu e 3. Schema ic o e iew o he deb is low lume, wi h images showing he lume and
he g own plan s o 5 and 8 days old. All leng hs a e in cen ime e s.
118 2.1
Flume se up
119 The expe imen s we e conduc ed in a deb is- low lume consis ing o a 5.4 m long
120 and 0.3 m wide channel inclined a 34◦, a mixing ank wi h a o ced-ac ion mixe and
121 cus om-made elease ga e (se -up is simila o Roelo s e al. (2023) and De Haas e al.
122 (2021); Figu e 3). Fi e lase senso s cap u ed low hyd og aphs and eloci ies h ough
123 ime-dis ance measu emen s. Fo ce luc ua ions a he bed we e measu ed using a Geospace
124 GS-20DX geophone o ce pla e (0.14 m wide, 0.06 m long), ins alled a 2.90 m and 2.98
125 m downs eam o he elease ga e, eco ding seismic mo emen ( e ical and ho izon al)
126 wi h a 10–1000 Hz esponse. The shea s ess exe ed by he deb is low on he bed was
127 calcula ed as:
128 τ = ρgH sin(S) (1)
129 whe e τ is he shea s ess (Pa), ρ is he densi y o he low (kg/m3), g is he g a i a-
130 ional accele a ion (m/s2), H is he low dep h (m), and S is he channel bed slope. In
131 he lowe 2.5 m o he channel, a ege a ed e odible bed o 0.07 m hick was placed in
132 a dep ession in he channel loo , o which he age and numbe o plan ed seeds we e a -
133 ied o each expe imen . Bed ele a ion o he e odible bed was quan i ied be o e and a -
134 e each expe imen a sub-mm esolu ion wi h a Vialux z-Snappe h ee-dimensional
135 scanne . F om he measu emen s, he e osion olumes o he e odible bed we e quan-
136 i ied by ob aining he di e ence in bed ele a ion be o e and a e each expe imen . The
137 e osion a e was calcula ed by di iding he e osion olume by 1.5 seconds, as mos e o-
138 sion occu s wi hin his ime ange unde he in luence o he boulde - ich deb is low on
139 (Roelo s e al., 2023; De Haas e al., 2022; Zheng e al., 2021). A 46 cm downs eam
140 o he s a o he e odible bed, po e wa e p essu es we e measu ed a 3 and 4 cm be-
141 low he su ace (cus om-made wi h Kelle P essu e PR-23SY, wi h an accu acy o ±0.25
142 %FS). Using a needle, wa e was inse ed in o he bed su ounding he po e-p essu e mea-
143 su emen ins umen s o ensu e sa u a ed condi ions.
–7–
144 Fo all expe imen s, deb is- low and bed composi ions we e kep cons an (Table
145 S2). Deb is lows consis ed o 75% sand, 5% clay (kaolin), 20% g a el, and a wa e ac-
146 ion o 20% by weigh (Figu e 4a and b). The e odible bed was loosely packed and con-
147 sis ed o 96% sand and 4% clay (Figu e 4c and d). The bed had a wa e ac ion o 13%
148 by weigh (Table S2).
Figu e 4. G ain size dis ibu ions used in he expe imen s. (a) G ain size dis ibu ion o he
deb is low and e odible bed used o he expe imen s, (b) he cumula i e g ain size dis ibu ion
o he deb is low and he e odible bed.
149 2.2 Desc ibing low cha ac e is ics
150 To compa e he scaled deb is lows wi h na u al ones, he Bagnold, Sa age, and
151 ic ion numbe s we e calcula ed o each expe imen . These numbe s desc ibe he e-
152 la ionship be ween he o ces ha esis mo ion in deb is lows, comp ising collisional,
153 ic ional, and iscous o ces (I e son, 1997). The Bagnold numbe compa es collisional
154 and iscous o ces (I e son, 1997):
sρsδ2γ
155
N
b
=
(2)
µ
156 whe e Nb is he Bagnold numbe (-), s is he olume ic solids ac ion, ρs is he den-
157 si y o he low (kg/m3), δ is he mean g ain size (m),
is he olume ic ac ion o
158 ines, γ is he low shea a e (1/s):
159
u
γ
=
(3)
H
160 whe e u is he deb is low on al eloci y (m/s), and H is he maximum low dep h (m).
161 The in e s i ial luid iscosi y µ is es ima ed as (De Haas e al., 2015; I e son, 1997):
162 = 1 + 2.5 + 10.05 2
w
+
0
.
00273
16
.
6
(4)
163 whe e µw is he dynamic iscosi y o pu e wa e (0.001002 Pa s). Fo Nb
>
200, he col-
164 lisional o ces domina e he iscous o ces (I e son, 1997). The Sa age numbe compa es
165 collisional and ic ional o ces:
ρsδ2γ2
166
N
s
= (
ρ
(5)
− ρ )gH an ϕ′
µ
µ
s
–8–
s
167 whe e
ρ
is he luid densi y (kg/m3), and
ϕ
′
is he angle o in e nal ic ion, es ima ed
168 a 42˚ (De Haas e al., 2015). The collisional o ces domina e he iscous o ces when
169 Ns
>
0.1 (I e son, 1997). Finally, ic ional and iscous o ces a e compa ed wi h he
170 ic ion numbe :
171
N
=
s(ρs − ρ )gH an ϕ′
(1 − )γµ (6)
172 Fo N > 2000, ic ional o ces domina e iscous o ces (I e son, 1997).
173 2.3 Plan g ow h and quan i ica ion o oo cha ac e is ics
174 To simula e na u al oo -soil con ac as p esen in he ield, li e seedlings o So ghum
175 bicolo we e g own in he e odible bed. S. bicolo is a as -g owing seedling wi h a high
176 ge mina ion a e and s aigh ap oo s pene a ing deep in o he bed. Al hough he oo
177 a chi ec u e o S. bicolo is less complex han hose o ma u e ees, i p o ides a p ac-
178 ical and scalable p oxy o s udying oo –soil in e ac ions. I s oo s exhibi simila me-
179 chanical e ec s in ein o cing soil and esis ing e osion a small expe imen al scales (Lokho s
180 e al., 2019), enabling us o app oxima e he s abilizing e ec o oo s in a con olled se -
181 ing. The ap oo sys ems p esen in S. bicolo allow unambiguous da a collec ion, p o-
182 cessing, and in e p e a ion. The seed densi y was based on Lokho s e al. (2019), who
183 showed ha a densi y o 2 seeds/cm2 is a ypical densi y wi h measu able e ec s in small-
184 scale geomo phic expe imen s o channel bank collapse and e osion by low. Ou seed
185 densi y anged om 0.4 o 6 seeds/cm2. The a e age ge mina ion a e was 70%, g ad-
186 ually declining o app oxima ely 50% h oughou he expe imen s, likely due o he de-
187 c ease in ligh caused by sho e dayligh . This esul ed in oo densi ies anging om
188 0.3 o 4 oo s/cm2, assuming one oo pe seedling. Each sowing densi y was es ed wice:
189 once wi h plan s g own o 5 days and once wi h plan s g own o 8 days, which allowed
190 o a ia ion in oo leng h and diame e . Be o e he s a o each expe imen , he abo e-
191 g ound biomass was cu o wi hou dis u bing he soil o isola e he e ec s o he oo s
192 on deb is- low e osion.
193 Be o e e e y expe imen , we measu ed oo leng h, oo diame e , oo densi y, oo
194 leng h densi y (RLD), oo a ea a io (RAR), and oo ensile s eng h (RTS), consid-
195 e ing ha hese oo ai s ha e been iden i ied as mos impo an o s abilizing soils
196 and con olling e osion by o e land low (Vannoppen e al., 2015; Ghes em e al., 2014;
197 Bische i e al., 2009; De Bae s e al., 2008; Ali & Osman, 2008; De Bae s e al., 2007;
198 Pollen, 2007). Roo leng hs and diame e s we e a e aged o e en oo s and measu ed
199 using an elec onic calipe wi h an accu acy o 0.01 mm. Roo densi y was de e mined
200 by coun ing he numbe o plan s pe uni o su ace a ea. As we used ap oo s in ou
201 expe imen s, he numbe o plan s co esponds o he numbe o oo s. RLD is used o
202 quan i y oo dis ibu ion in soil, and was calcula ed as he oo leng h pe uni olume
203 o soil (Figu e 5a; Vannoppen e al., 2015; Ghes em e al., 2014; De Bae s e al., 2007):
204
RLD
=
Roo Leng h
soil Volume
(7)
205 whe e he RLD is he Roo Leng h Densi y (cm/cm3), he sum o he oo leng hs e e s
206 o he leng h o he o al oo sys em (cm) wi hin he soil olume sampled (cm3), which
207 ansla es o he olume o he e odible bed. Fo ou expe imen s, he measu ed oo
208 leng hs we e scaled up based on he o al numbe o coun ed oo s. The RAR is de ined
209 as he ac ion o a plane o soil occupied by oo s pe uni a ea (Figu e 5b; Bie man &
210 Mon gome y, 2014; De Bae s e al., 2008; S okes e al., 2009):
–9–
211
RAR
d
=
I
A
(8)
A
212 whe e RARd is he Roo A ea Ra io (-), A is he oo a ea pe oo (m2), and A he
213 a ea measu ed (m2). The measu ed a ea is he wid h imes he dep h o he soil in he
214 lume. The oo a ea is calcula ed by:
215
π 2
A
= 4
d
(9)
216 whe e d is he oo diame e (m). A di e en unc ion o calcula e RAR uses he RLD
217 (De Bae s e al., 2006):
218
RAR
RLD
=
RLD
×
A
(10)
Figu e 5. Illus a ion o he calcula ions o he concep s (a) RLD and (b) RAR, whe e W is
he wid h, H is he heigh , and L is he leng h o he soil sample.
219 The appa en cohesion wi h oo s (hence o h ’ oo cohesion’) was calcula ed o
220 assess he addi ional shea s eng h o he soil p o ided by oo s. Roo cohesion quan-
221 i ies he e ec o oo ai s on deb is- low e osion and allows o a compa ison o he
222 shea s ess exe ed by deb is lows. Roo cohesion was calcula ed using he Wu and Wal-
223 d on model (Wald on, 1977; Alam e al., 2018; Wu, 1984), wi h bo h RARd and
RAR
RLD
224 as inpu . As ou oo s g ow pe pendicula o he soil and ba ely change posi ion du -
225 ing shea ing (Schmid e al., 2001), oo cohesion (C ) is calcula ed as:
226
C
=
RTS
×
RAR
(11)
227 The WWM model assumes ha all oo s b eak simul aneously, which we assume alid
228 o as - lowing deb is lows. The RTS p o ides a measu e o he oo s’ esis ance o b eak-
229 ing as a esul o shea and is calcula ed as:
230
RTS
=
F
max
A
(12)
231 whe e Fmax is he maximum o ce ha is exe ed on a oo be o e b eaking (N), and A
232 he oo a ea (mm2) (De Bae s e al., 2008). Fo 26 oo s, he diame e was measu ed
–16–
388 o d e al., 2005; Bunn & Mon gome y, 2004; Schmid e al., 2001). E osion was obse ed
389 be ween pa ches o oo s, whe e oo densi y was lowe . This pa e n ag ees wi h ield
390 obse a ions, as oo ne wo ks sp ead ou wa d om ees, leading o lowe oo cohe-
391 sion and consequen ly mo e e osion (De Bae s e al., 2007; Roe ing e al., 2003).
392 Table 4.3 p esen s he shea s esses and oo ai s measu ed du ing he expe -
393 imen s, alongside alues epo ed om he ield. This compa ison p o ides insigh in o
394 he ela i e magni udes o o ces go e ning deb is- low e osion. RTS in na u al en i on-
395 men s is o en some o de o magni ude g ea e han he shea s esses exe ed by de-
396 b is lows, al hough he ange is la ge. While RTS also exceeds shea s esses in ou es s,
397 he di e ence in magni ude is less p onounced. Addi ionally, oo cohesion in ou expe -
398 imen s is conside ably lowe han in ield condi ions. This disc epancy is expec ed, as
399 ou se up ea u es ap oo s dis ibu ed o e a la ge a ea, whe eas na u al condi ions a e
400 o en calcula ed pe indi idual ee wi h a la ge oo sys em. To compensa e, we c e-
401 a ed high oo densi ies. Howe e , oo cohesion emains on he lowe end, possibly due
402 o he expe imen al oo simplici y. These de ia ions sugges ha in ou expe imen s,
403 he e osion may be highe han expec ed in na u al se ings. None heless, his disc ep-
404 ancy does no unde mine he alue o ou expe imen s, as ou p ima y goal is no o ec e-
405 a e eal-wo ld condi ions, bu o isola e and s udy he in e ac ions be ween deb is lows
406 and plan oo s. Despi e he scaling di e ences, he combina ion o scaled deb is low
407 expe imen s and scaled oo s enables a sys ema ic s udy o deb is low- oo in e ac ions.
408 Ou indings o e insigh in o how oo s ein o ce he soil and a ec bed e osion, wi h
409 undamen al mechanisms and mo phological ea u es ha a e consis en be ween he lab-
410 o a o y and he ield.
411 4.4 Small-scale, big impac : lab e sus ield obse a ions
412 Ou esea ch shows ha he e is high po en ial o using scaled li e plan s in deb is-
413 low expe imen s. This enables us o s udy he e ec s o plan s on deb is lows wi h a
414 deg ee o con ol ha is no a ainable in he ield, and enables sys ema ic explo a ion
415 o changes in bounda y condi ions and ma e ials. Bo h modelling (e.g. Liu e al., 2021)
416 and ield esea ch (e.g. Tang e al., 2018; Michelini e al., 2017; Johnson e al., 2000)
417 ha e been done on plan —deb is low in e ac ions, bu hese s udies emain p ima ily
418 quali a i e due o he lack o quan i a i e in o ma ion ega ding deb is- low o ces and
419 plan ai s. Ou expe imen al s udy, o he i s ime, con ibu es quan i a i e knowl-
420 edge because we can con ol he oo condi ions.
421 Ou pionee ing es s o e many op ions o u u e expansion and imp o emen . Ou
422 expe imen s we e conduc ed using S. bicolo seedlings as a p oxy o ee oo s. T ee oo
423 sys ems a e ypically cha ac e ized by ex ensi e and hie a chical b anching s uc u es,
424 wi h oo s a ying in diame e , o ien a ion, and leng h, and o en in e connec ing o o m
425 complex ne wo ks (Ghes em e al., 2014; Bische i e al., 2009). Mo eo e , na u al en-
426 i onmen s hos di e se ee species o a ying ages, u he inc easing he e ogenei y in
427 oo a chi ec u e and mechanical beha io (Pohl e al., 2011; S okes e al., 2009). Whe e
428 he simple oo sys ems in ou se up likely esul in an unde es ima ion o e osion e-
429 sis ance, u u e expe imen s could employ plan species wi h mo e complex, mul i-scale
430 oo sys ems. Imp o ing measu emen o po e-p essu e and wa e con en could enhance
431 ou unde s anding o how oo s a ec loading condi ions and po e wa e ans e be ween
432 deb is low and bed, which is c ucial o deb is- low e osion (Roelo s e al., 2023; Zheng
433 e al., 2023). Roo s may educe e osion by inc easing pe meabili y and enhancing d ained
434 condi ions (Liu e al., 2021; Kaspa & Singe , 2011; De Bae s e al., 2011; Reubens e
435 al., 2007). Unde hese d ained condi ions, wa e and ai can di use h ough he soil po es
436 wi hou inc easing po e-p essu e, which helps main ain in e g anula ic ion. Howe e ,
437 inc eased pe meabili y could also acili a e he ans e o mois u e and po e-p essu e
438 om he deb is low o he bed, he eby inc easing po e-p essu e in he bed and po en-
439 ially enhancing e osion (Zheng e al., 2023). Fu he mo e, ou expe imen s lacked abo e-
–17–
Table 1. Deb is low shea s esses measu ed du ing he expe imen s and
du ing deb is low e en s in he ield, along wi h oo ai s measu ed in he
expe imen s and he ield. The oo cohesion om ou expe imen s was cal-
cula ed using
RAR
RLD
. The oo ai s gi en by Ve gani e al. (2012) a e
measu ed in he Swiss Alps, he ai s gi en by Bische i e al. (2009) in he
I alian Alps.
Shea
s ess
RTS
Roo cohesion
KPa
N/mm
2
KPa
Ou expe imen s
0.47–0.59
0.85–1.8
1.9 × 10
−4
–4.3 × 10
−3
. . . . . . . . . . . . . . . . . . . . . . . .
Illg aben
a,
b
0.37–5.1
Roßbichelbach
(modeled) c
4
–15
Mon eci o, USA
d
1.6
. . . . . . . . . . . . . . . . . . . . . . . .
Ace pseudopla anus e
3.68–31.48
Cas anea sa i a e,
0.81–45 8.1–19.6
Fagus
syl a ica
e,
3.38–75.22 14.4–86
F axinus excelsio e
3.89–24.91
La ix decidua e,
1.03–110 17.4–38.3
Os ya ca pini olia e,
1.8–30.17 5.4–30
Picea abies e,
2.84–95 13.8–35.4
a
de Haas e al. (2022)
b
Be ge e al. (2011)
c
Die ich & K au bla e (2019)
d Kos ynick e al. (2025)
e
Ve gani e al. (2012)
Bische i e al. (2009)
440 g ound biomass, which could educe deb is low ene gy and size h ough obs uc ions
441 (Liu e al., 2021; Michelini e al., 2017). Howe e , abo e-g ound biomass simul aneously
442 inc eases he chances o oo pullou , dec easing he soil s eng h and inc easing e osion
443 (Chen e al., 2024; S okes e al., 2009). Fu he esea ch could imp o e ou unde -
444 s anding o he combined e ec o abo e and below-g ound biomass on deb is- low in-
445 duced e osion.
446 Ou indings sugges ha p e en ing cu ings o e en plan ing ees on deb is- low
447 ans and ca chmen s can se e as e ec i e s a egies o mi iga ing deb is- low haza ds,
448 as an inc ease in RLD and RAR dec eases e osion and he e o e diminishes he olume
449 and impac o a deb is low. Fu he expe imen s, modeling ha ep esen s he oo ing
450 densi y and s uc u e, and addi ional ield da a will be essen ial o deepening ou un-
451 de s anding and he p edic abili y o hese p ocesses.
452 5
Conclusions
453 We expe imen ally in es iga ed he e ec s o plan oo s on deb is- low e osion o
454 gain a be e unde s anding o he in e ac ion be ween deb is lows and ees. By com-
455 bining measu emen s o deb is- low cha ac e is ics, e osion, and oo ai s, we we e able
456 o quan i y he educ ion o deb is- low induced e osion by oo s compa ed o oo less
457 beds. While ou se up simpli ies na u al oo s uc u es and condi ions and isola es e -
–18–
458 ec s o below-g ound biomass om e ec s o abo e-g ound biomass, i enables a con-
459 olled s udy o undamen al e osion mechanisms.
460 Ou expe imen s showed a clea non-linea ela ionship be ween RLD and deb is-
461 low bed e osion, wi h minimal di e ence be ween 5-day-old and 8-day-old oo s. RAR
462 and oo cohesion ollowed he same non-linea end when calcula ed om RLD, in-
463 dica ing ha he combined e ec o oo leng h and oo densi y is an impo an ac-
464 o o educing deb is- low bed e osion. These esul s imply a po en ial applica ion: eg-
465 e a ing deb is- low-p one slopes could educe e osion and low olume g ow h, he eby
466 lessening hei haza dous impac . The esul s also o e insigh needed o haza d assess-
467 men : a e wild i es, oo decay may weaken oo cohesion, inc easing suscep ibili y o
468 e osion and enhancing deb is- low olume and haza ds.
469 Ou esul s demons a e ha small-scale deb is- low expe imen s wi h li e seedlings
470 can p o ide aluable insigh s in o he e ec s o plan s on deb is- low e osion ha a e chal-
471 lenging o in es iga e in he ield. Fu he expe imen al explo a ion can p o ide a sci-
472 en i ic basis o he mi iga ion o deb is- low haza ds using na u e-based solu ions and
473 gi e insigh s in o he e ec s o changing ege a ion due o wild i es and de o es a ion on
474 deb is- low haza ds. Ou indings poin o a new pa hway owa ds inco po a ing oo cha -
475 ac e is ics in o e osion models, and show he impo ance o ege a ion in deb is- low-p one
476 a eas.
477 Open Resea ch
478 Expe imen al da a, including p e- and pos - low DEMs, low measu emen s, and
479 he oo ai measu emen s a e a ailable ia Yoda (online eposi o y o U ech Uni-
480 e si y) unde his link: h ps://public.yoda.uu.nl/geo/UU01/NYY5H3.h ml. DOI:
481 10.24416/UU01-NYY5H3.
482 Acknowledgemen s
483 This wo k was suppo ed by he Du ch Resea ch Council (NWO) (g an VI.Vidi.233.008
484 o TdH). The au ho s g a e ully acknowledge he help o A jan an Eijk, Bas an Dam,
485 Ma cel an Maa se een, and Henk Ma kies in designing and cons uc ing he lume and
486 measu emen se -ups, as well as hei assis ance du ing he expe imen s. We would also
487 like o hank Cai lin Amels en Jelle Pos huma o helping ou wi h he p e-expe imen s.
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manusc ip submi ed o Enginee ing Geology
–1–
765 Supplemen a y ma e ial
Table S1. O e iew o he changed bounda y condi ions o each expe imen .
Exp
nmb
Age
oo s
Sowed
den-
si y
Roo
den-
si y
RLD
RAR
d
C
RAR
RLD
C
wi h
RLD
days seeds/cm3 oo s/cm3 cm/cm3
−
×10−5
MPa
×10−2
−
×10−4
MPa
×10−3
1
-
-
-
-
-
-
-
-
2
-
-
-
-
-
-
-
-
3a
5
0.4
0.28
0.10
0.647
0.79
1.55
0.189
3b
8
0.4
0.26
0.16
0.721
0.771
3.16
0.338
4a
5
0.5
0.41
0.13
1.31
1.12
3.03
0.259
4b
8
0.5
0.33
0.21
0.981
0.981
4.30
0.430
5a
5
0.76
0.60
0.20
1.92
1.82
4.47
0.422
5b
8
0.76
0.46
0.28
1.26
1.33
5.40
0.569
6a
5
1
0.83
0.48
2.08
2.14
8.48
0.874
6b
8
1
0.75
0.59
1.61
1.85
8.91
1.02
7a
5
1.4
1.1
0.36
3.91
3.37
9.28
0.799
7b
8
1.4
0.87
0.65
1.15
2.10
5.99
1.09
8a
5
2
1.3
0.53
4.03
3.93
11.4
1.11
8b
8
2
1.5
1.21
3.57
4.11
20.6
2.37
9a
5
3
2.3
0.88
6.99
6.80
18.9
1.83
9b
8
3
2.0
1.46
3.61
5.25
18.4
2.67
10a
5
4
2.1
0.42
7.46
6.64
10.3
0.913
10b
8
4
2.0
1.27
4.98
5.65
22.4
2.55
11a
5
6
3.6
0.98
12.9
11.3
24.9
2.19
11b
8
6
3.3
2.14
8.44
9.47
38.4
4.31
Table S2. Key cha ac e is ics o he expe imen al se ings o he lume expe imen s, including
he a ied oo densi ies and he deb is low composi ion and cha ac e is ics.
Deb is low componen s
Uni
Values
Clay (kaolin)
d y weigh ac ion
0.05
kg
2.4
Sand
d y weigh ac ion
0.75
kg
36
G a el
d y weigh ac ion
0.2
kg
9.6
Wa e
d y weigh ac ion
0.2
kg
12
Flume se ings
Uni
Value
Flume angle deg ees
34
◦
Bed componen s
Uni
Values
Clay (kaolin)
d y weigh ac ion
0.04
Sand
d y weigh ac ion
0.96
Wa e
d y weigh ac ion
0.13
Numbe o seeds
Uni
Tes ed ange
Seed densi y
seeds/cm
2 0.4 – 6
manusc ip submi ed o Enginee ing Geology
–2–
Table S3. Resul s o he eg ession analysis o he co ela ion be ween he ne bed change
and he a iables calcula ed om he measu ed ai s. The loga i hmic eg ession is desc ibed as
bedchange = a log( a iable) + b
plan age
Cons an a
Cons an b
R
2
p- alue
Roo densi y
5-
day
2079
-
3717
0.878
1.96 × 10−4
8-
day
2376
-
1838
0.948
2.29 × 10−
6
Roo leng h densi y
5-
day
2254
-
1300
0.936
1.97 × 10−5
8-
day
2278
-
998.3
0.963
2.89 × 10−6
Roo a ea a io (RAR
d
)
5-
day
1774
14720
0.817
8.29 × 10−4
8-
day
2299
22710
0.814
8.62 × 10−
4
Roo a ea a io (RAR
RLD
)
5-
day
2022
10860
0.917
4.97 × 10−5
8-
day
2298
13790
0.868
2.59 × 10−
4
Roo cohesion calcula ed wi h (RARd)
5-
day
1938
-
16510
0.8531
3.75 × 10−4
8-
day
2368
23030
0.916
5.18 × 10−
5
Roo cohesion calcula ed wi h (RAR
RLD
)
5-
day
2169
12020
0.9339
1.59 × 10−5
8-
day 2305 13420 0.9452 1
.
15
×
10
−
5
