Bedload Trapping in Open Check Dam Basins - Measurements of Flow Velocities and Depositions Patterns Guillaume Piton, S

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Bedload trapping in open check dam basins - measurements of flow velocities and depositions patterns Guillaume Piton, S. Mejean, C. Carbonari, J. Le Guern, H. Bellot, A. Recking

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Guillaume Piton, S. Mejean, C. Carbonari, J. Le Guern, H. Bellot, et al.. Bedload trapping in open check dam basins - measurements of flow velocities and depositions patterns. 13th Congress INTERPRAEVENT 2016, May 2016, Lucerne, Switzerland. pp.808-817. hal-01390582

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Bedload trapping in open check dam basins - measurements of ﬂow velocities and depositions patterns

Guillaume Piton, Eng.1; Ségolène Mejean, Eng.2; Costanza Carbonari, Msc.3; Jules le Guern, Msc.2; Hervé Bellot, Msc.2; Alain Rrcking, PhD2

ABSTRACT In steep slope streams, torrential-hazards mainly result from abrupt and massive sediment deposits. Open check dams are regularly used in natural hazard mitigation to trap sediment and driftwood. A good comprehension of the phenomena that occur in these structures is needed to optimize their design. In this paper, we present new results from small scale experiments addressing (i) a validation of water stage - discharge formula proposed in the literature for slit and slot dams; (ii) recommendations in the use of formula dedicated to deposition-thickness-estimation; (iii) geomorphic and hydraulics descriptions that seek to help ﬁeld practitioners and numerical modelers to better understand what can be observed in labs and in the ﬁeld and what kind of phenomena should be modeled. A special attention has been paid to highlight the implication of our results in the use of formula and in structure design and maintenance.

KEYWORDS slot and slit dams; sediment trap; photogrammetry; Large Scale PIV; small scale model

INTRODUCTION In steep slope streams and especially on their fan part, torrential–hazards mainly result from abrupt and massive sediment deposits. To curtail such phenomenon, soil conservation measures as well as torrent control works have been undertaken for decades. Since the 1950s, open check dams complement other structural and non-structural measures in watershed- scale mitigation plans [Armanini et al., 1991; VanDine, 1996]. Hundreds of these structures have thus been built for about 60 years. Their design evolved with the improving comprehension of torrential hydraulics and sediment transport processes; however numerous open check dams have a general tendency to trap most of the sediments supplied by the head- waters and to weakly self-clean. Secondary effects such as channel incision downstream of the structures often occur after their creations. Sediment starvation trends tend to propagate to the main valley rivers and to disrupt past geomorphic and ecologic equilibriums. To minimize useless dredging operations and to promote sediment continuity, while main- taining the mitigation effect of open check dams, a better selectivity of sediment trapping must be sought in open check dams [Armanini et al., 1991; SedAlp, 2015]. To approach

1 IRSTEA Saint-Martin-d’Hères, FRANCE, [email protected] 2 Irstea, centre de Grenoble, UR ETGR, St Martin d’Hères, France and Univ. Grenoble Alpes, F-38041 Grenoble, France 3 Univ. Firenzi, Italy and Irstea, centre de Grenoble, UR ETGR, St Martin d’Hères, France and Univ. Grenoble Alpes, F-38041 Grenoble, France

IP_2016_FP079 808 | INTERPRAEVENT 2016 – Conference Proceedings optimal structures that would trap sediments during dangerous floods and flush them partially during small floods, we must improve the scientific knowledge on hydraulic and deposition processes that occur in sediment traps during floods. Four trapping processes (TP) eventually act in sediment trap basins (Fig. 1 a): a decrease in transport capacity due to a milder energy slope in the basin (TP1); a decrease in transport efficiency due to flow spreading in a basin wider than the upstream channel (TP2); a drop in the shear stresses in the calm water area upstream of the dam (hydraulic control - TP3); and mechanical blockages against the dam openings (mechanical control - TP4).

Figure 1: a) Trapping Processes (TP) resulting in sediment deposition in sediment trap basins: (TP1) slope decrease; (TP2) width increase; (TP3) delta-type hydraulic control; and (TP4) direct mechanical blockage and; Longitudinal and transverse view of ﬂows through: b) a slot dam and c) a slit dam.

The mechanical blockage process [Fig. 1 (TP4)] is relatively well understood [Piton and Recking, 2015a, 2015b]. On the contrary, case studies testing the existing criteria on (TPs 1-3) remain scarce. This paper addresses three scientific issues, two deal with (TP3): (i) How accurate are the current hydraulic approaches describing the water stage–discharge Eqs.? and, (ii) Do the approaches proposed for deposit thickness estimation give satisfying results? The last issue concerns (TPs 1-2): (iii) what are the flow conditions and the resulting geomorphic patterns that occur during sediment trap basin filling in a relatively wide and mild basin? We gather new elements on these questions, based on an experimental approach using a Froude scale model. The first runs were performed in pure water hydraulics to validate water stage-discharge Eqs.. Sediment was added in the subsequent runs to look at deposition processes.

MATERIAL AND METHODS The small-scale sediment trap was built in a 6-m-long, 1.2-m-wide, 0.4-m-deep, 10%-steep tilting ﬂume. The water was recirculated and measured by a ﬂowmeter with a maximum discharge of 4 l/s. The sediment feeder was composed of a hopper, associated with a conveyor belt, with a maximum solid discharge capacity of 200 to 300 g/s depending on the grain size distribution of the sediment mixture. Two sediments mixtures were used, hereafter refer to as

INTERPRAEVENT 2016 – Conference Proceedings | 809 GSD1 & GSD2, consisting in natural poorly sorted sediments with diameter from 0.2 to

20 mm. The median grain size D50 of GSD1 and GSD2 are 3.8 and 2.4 mm, respectively; and the mean arithmetic diameters are of 6.4 and 4.9 mm, respectively.

High quality pictures of the flume were taken with two CANON 100D cameras fixed on a trolley to the ceiling of the laboratory. Digital elevation model (DEM) of the deposits were reconstituted, with a 1-mm accuracy [Le Guern, 2014], using the Agisfost Photoscan software. Before each DEM measurement, a high speed camera Phototron FASTCAM took videos of the flow at 125 frames/s. The Fudaa software was used to obtain surface flow velocity fields by large scale Particle Image Velocimetry (PIV) [Carbonari, 2015]. A point gauge fixed on three graduated perpendicular rails allowed to measure flow surface and bed altitudes with a 2-mm-accuracy.

RESULTS Open check dam pure water hydraulics The hydraulic control of the deposit [Fig. 1 (TP3)] is a characteristic delta dynamic, i.e. is controlled by the water level of the tranquil water area formed by the open check dam backwater effect. A large number of open check dam are designed as slit or slot types Table 1 (Eq. 1-5)(Fig. 1 b & c). Table 1 gathers the existing water stage–dischargel Eqs.ee describing la the Table ﬂow TE1-. conditions that may occur in these dams: free surface or pressure ﬂow (Fig. 1 b & c).

Table 1: Water stage – discharge formula for slot ant slit dams Flow type Source Equation Eqs. number

2 3 (1) Free surface Zollinger [1983] Q = ⁄3 μw0√2g H

3 (2)* Free surface Armanini and Larcher [2001] √ 2� Q = w0 g( ⁄3)

3 (3)* Free surface D’Agostino [2013] √ 2� Q = w0 g( ⁄3 ∗ 1.2) (4) Pressure flow Torricelli [1644] Q = μw0h0√2g(H − ℎ0⁄2)

2 3 3 (5) Pressure flow Zollinger [1983] Q = ⁄3 μw0 [√2gH − √2g(H − h0) ]

3 Note: see Fig. 1 for parameter definition, with Q the discharge [m /s], w0 the width of the opening [m], h0 the height of the opening [m], µ the contraction coefficients [-], g the gravitational acceleration [m/s²] and H, the hydraulic head [m] with H = h + V²/2g with h the water depth [m] and V the flow velocity in the upstream section [m/s] approximated by Q/Wh, with W the basin width [m]. * Eqs. 2 and 3 are equivalent to Eq. 1 with values of µ = 0.577 and µ = 0.439, respectively.

Eq. 2 and 3 are based on theoretical considerations. Zollinger [1983] proposed using a value of 0.65 for µ but did not provide the calibration data. We propose to use our measurements to discriminate which equation and which µ-value are the most relevant in typical torrential ﬂows.

810 | INTERPRAEVENT 2016 – Conference Proceedings Height dam-conﬁgurations were tested, without sediment transport, to answer this question:

3 slits w0 = [6;10;14cm] and 5 slots w0xh0 = 10x6cm, 10x4cm, 10x2cm, 14x6cm, 14x4cm, 14x2cm. The dam was made of a 7.5mm-thick PVC plate. The basin immediately upstream of the dam was 20cm-wide and its ﬂoor was covered with pebbles 14-18mm in diameter. 64 measurements with Q [0.5;3.8 l/s] and h [16;140 mm] were taken at different stable states. The results are synthetized in Fig. 2 and more details on the experimental set-up, the data and an error analysis can be found in Mejean [2015].

Figure 2: Water stage–discharge relationship analyses: calibration of µ, contraction coefficient for a) free surface flows in slits and slots using Eq. 1 and b) pressure flows in slot using Eq. 5 and; c) comparison between experimental data and computed water-stage discharge. Note: Eqs. 1, 4 and 5 were used taking into account the inertia term V²/2g in H the hydraulic head term of the formula

The linear best fit (Fig. 2 a) confirms the µ-value of 0.65 in free surface flow. Eq. 1 is thus recommended, rather than Eqs. 2 and 3, for equivalent thin-dam-configurations, i.e. for h/e [0.05;0.47], with e the dam thickness in the flow direction (Fig. 1 b). The results still need to be confirmed for thicker dam configurations were the Eq. 2 may be more adapted. For pressure flows in slots (Fig. 2 b), the linear best fit shows a µ-value of 0.68, close to the 0.65 value also retained by Zollinger [1983]. Eqs. 4 and 5 are almost indistinguishable in our results (Fig.2 c) if the hydraulic head, in the Torricelli [1644] formula, is corrected by half the slot height as written in Eq. 4.

For high H/h0 ratio, Eqs. 4 and 5 underestimate slightly h, and conversely they overestimate h for low H/h0 ratio (Fig. 2 c). It results from the use of a constant value of µ whereas the

INTERPRAEVENT 2016 – Conference Proceedings | 811 contraction effects increase with H/h0. Complementary experiments should be performed to

calibrate a variation of µ with the contraction, i.e. varying with H/h0 and W/w0, this in torrential context, i.e. with steep slope, rough beds and sediment transport. In addition our results demonstrate that taking into account the inertia term V²/2g in H the hydraulic head is important: Using the sole h term in the Eqs. would result in an underestimation of the structure discharge capacity and consequently in overestimation of the deposition and trapping performance (see later), which is not conservative in hazard mitigation.

Delta thickness estimation During a trap ﬁlling, when sediments reach the open check dam backwater area, where ﬂow velocities are low, they deposit as a delta. ∆Z, the delta thickness at the front (Fig. 1 a), directly controls the trapped sediment volume, e.g., a lower ∆Z means a lower trapped Formula 6 volume. ∆Z estimation is thus a key step in the trapl design. r Armanini o oloa and Larcher Formula [2001] and 6o Jordan et al. [2003] proposed formulas to estimate ∆Z that can be rearranged as follow [Mejean, 2015]:

[Armanini and Larcher, 2001] Formula 7 l r o oloa Formula 7o W−w W−w ∆Z(T) = ℎ(�) ( 0)⁄[1 + ( 0)] (6) w0 w0 Equation 6

[Jordan et al., 2003] 1 T 92 ∆Z[Jordan(T) = et al.,∫ 2003]ℎ( �)dt EQUATION 7(7) T 0

93 EquationWith 7 W, the basin-width upstream of the open check dam; w0, the slit-width, T the duration since the

94 beginning of the flood and h(T) the flow-depth upstream of the structure at time T (Fig. 1 b & c), With W, the basin-width upstream of the open check dam; w0, the slit-width, T the duration 95 sincevarying the beginningin time during of the floodflood and and computed h(T) the usingflow-depth Eq. 1 and upstream µ=0.65. of the structure at time T (Fig. 1 b & c), varying in time during the flood and computed using Eq. 1 and µ=0.65. 96 It is worth stressing that Eq. 6 has been calibrated in laterally confined flows, i.e. the flow covered all 97 It isthe worth deposit stressing due to that the Eq. flume 6 has relatively been calibrated narrow width in laterally (0.4 m). confined On the contrary, flows, i.e. Eq. the 7 has flow been 98 coveredcalibrated all the in laterallydeposit unconfineddue to the flow flumes, i.e. relatively the deposit narrow showed width a more (0.4 classical m). On delta the contrary,pattern with Eq. 99 7 hasfew been mobile calibrated active channel in laterallys narrower unconfined than the flows, total basin i.e. the width. deposit In addition, showed the a more authors classical of Eq. 6 delta pattern with few mobile active channels narrower than the total basin width. In addition, 100 assumed that the delta thickness is always equilibrated with the flow constraints and thus only the authors of Eq. 6 assumed that the delta thickness is always equilibrated with the flow 101 depends on the instantaneous water depth h(T). On the contrary, Eq. 7 takes into account all the constraints and thus only depends on the instantaneous water depth h(T). On the contrary, 102 Eq.water 7 takes depth into evolution account during all the the water flood depththrough evolution the integral. during the flood through the integral.

103 To testTo testthese these Eqs., Eqs. five, f iveflood flood experiments experiments were were performed performed in inthe the aforementioned aforementioned flume, flume, underwith Qs 104 constantconstant solid solid concentration, concentration, C = , varyingvarying from from 1 1to to 5%, 5%, with with Qs, theQs, solid the discharge.solid discharge. Triangular Qs+Q Triangular hydrographs were used with water discharge reaching 2.75 l/s at the peak for all 105 hydrographs were used with water discharge reaching 2.75 l/s at the peak for all runs. The cumulated 106 sediment supply was the same in the all runs (500 kg). The hydrograph duration was thus inversely 812 | INTERPRAEVENT 2016 – Conference Proceedings 107 proportional to the concentration (see Mejean [2015] for more details).

108 In our experimental conditions, the flows were laterally unconfined, i.e. sediment entering the open 109 check dam-backwater-area was transported in an active channel narrower than the basin width and 110 flowing between deposits terraces. Several photogrammetric measurements were taken on each 111 experiment at different times on the hydrographs, thus for various instantaneous discharges and 112 water depths. Taking it into account and, as could be expected from Eqs. 6 and 7, ∆Z evolved 113 between measurements. Width-averaged-deposit-longitudinal profiles were extracted from the 114 DEMs [Mejean, 2015]. The delta-thickness was identified at the break in the slope and measured 115 from the bottom of the open check dam (Fig. 1 a). Fig. 3 shows the comparison between measured 116 and calculated values of ∆Z, using Eq. 6 and 7.

117 FIGURE 3

118 One can notice in Fig. 3 that, all other parameters being geometrically fixed, Eqs. 6 and 7 119 underestimate ∆Z when using the pure water hydraulics described by the unique Eq. 1 to estimate 120 h(t). To estimate the total water depth of sediment-laden flows in slits and slots, Piton and Recking 121 [2015a] recommend to take into account an additional head loss related to the sediment transport,

runs. The cumulated sediment supply was the same in the all runs (500 kg). The hydrograph duration was thus inversely proportional to the concentration (see Mejean [2015] for more details).

In our experimental conditions, the flows were laterally unconfined, i.e. sediment entering the open check dam-backwater-area was transported in an active channel narrower than the basin width and flowing between deposit terraces. Several photogrammetric measurements were taken on each experiment at different times on the hydrographs, thus for various instantaneous discharges and water depths. Taking it into account and, as could be expected from Eqs. 6 and 7, ∆Z evolved between measurements. Width-averaged-deposit-longitudinal profiles were extracted from the DEMs [Mejean, 2015]. The delta-thickness was identified at the break in the slope and measured from the bottom of the open check dam (Fig. 1 a). Fig. 3 shows the comparison between measured and calculated values of ∆Z, using Eq. 6 and 7.

Figure 3: Comparisons between measured values of delta thickness, ∆Z, and predicted values by a) Eq. 6 and b) Eq. 7, in pure water hydraulics and with ∆Hsed. Taking into account ∆Hsed in the h estimation is necessary to achieve a satisfying estimation of ∆Z.

One can notice in Fig. 3 that, all other parameters being geometrically ﬁxed, Eqs. 6 and 7 underestimate ∆Z when using the pure water hydraulics described by the unique Formula 8 Eq. 1 to estimate h(t). To estimate the total water depthl of sediment-laden r o oloa ﬂows in Formulaslits and 8o slots, Piton and Recking [2015a] recommend to take into account an additional head loss

related to the sediment transport, Δsed = 1.5DMAX,, with DMAX the maximum transported sediment diameter [m]. Using this adaptation to the clear water hydraulics, the water depth is computed using:

ℎ(�) = ℎ�� (�) + 1.5DMAX (8) Equation 8

INTERPRAEVENT 2016 – Conference Proceedings | 813 Taking into account this additional head loss, Eqs. 6 and 7 give a rather good approximation of ∆Z, though some scattering remains (Fig. 3 a & b, grey dots). This scattering is probably due to ∆Z-measurement uncertainties (the transversal profiles at the delta fronts were not flat, thus their representative height, here taken as the median altitude on the profile, are subject to interpretation) combined to a natural variability of the phenomenon, e.g., high deposition or incision preceding the measurement. In addition, the hypothe- sis that the delta-front-shape is always equilibrated with the flow constrains rapidly fall in defect during the hydrograph recession, as supposed initially by Armanini and Larcher [2001], explaining the low ∆Zs computed in Fig. 3 a. Eq. 6 is thus not adapted to compute ∆Z in wide basins during the hydrograph recession. Eqs. 7 give thus a good approximation of ∆Z if the water depth is correctly computed, i.e. taking into account all additional head losses related to torrential hydraulics (strong sediment transport, driftwood accumulation).

GEOMORPHIC DESCRIPTION OF A SEDIMENT TRAP FILLING When entering the basin, flows and sediment pass from a steep-laterally-confined to a milder-laterally-unconfined situation. In this situation, Zollinger [1983] observed both mono-channelized and braided fan-shape deposits. The transition from confined to unconfined flows raises complex issues in field observations and in numerical modeling [Piton and Recking, 2015a]; e.g. does channelized and braided patterns come from different flood-types? or how to compute the deposition slope of an unconfined massive bed-load supply?

13-experiments, 5 with a slit-dam (analyzed before) and 8 with a basin without open-check- dam (same concentration and hydrographs feedings, two sediment mixtures [Mejean, 2015]), were performed to observed deposition-processes and hydraulics’ conditions of bed-load trapping. Cycles of channelized and braided-like-patterns were systematically observed (e.g. Fig. 4). These patterns are simply different phases of a basin-filling, which is not a deposits-continuous-progression but rather jerky-sediment-propagations occurring after reconstitu- tion of sufficient sediment stocks in the inlet-vicinity. Grain size sorting and deposit armoring play key roles in these cycles: braided patterns were observed to be steep and paved while channelized pattern to be milder, with a bed smoothed by the finer subsurface materials released during the channelization.

There is thus not a unique value of deposit-slope but rather a range of slope in which a dynamic-equilibrium fluctuates. A method to estimate the slope range should be developed in further analysis. In addition, laterally-confined complementary experiments demonstrated that the deposition-slope increases in unconfined configurations (see Mejean [2015]). As a method to estimate deposition slope is still lacking, it is generally recommended in new sediment trap design to measure deposition slopes in the field, for example upstream of existing check dams, to estimate the deposition slope in the future trap. Our results demonstrate that deposition slopes measured above check dams, i.e. in quite confined configura-

814 | INTERPRAEVENT 2016 – Conference Proceedings tions, must be considered as minimum values of the possible bedload deposit slopes in sediment trap basins. One must note that it is not the case for mud ﬂows which may deposit with very gentle slopes [Piton and Recking, 2015a].

Partial self-cleaning was also observed systematically: (i) during the hydrograph recession in slit-dam experiments and conversely (ii) during the peak-flows in dam-less experiments. Slit dams prevent peak-flow-releases due to the delta-like dynamics [Fig. 1 (TP3)], which is maximum at the peak flows. The volumes that were stored in the delta-front were subsequently re-eroded and partially flushed during the flow-recession, leaving terraces’-like patterns. The observation of a clear-incised-channel in the downstream part of the deposits is thus an evidence of partial self-cleaning. Adding a slit-grill or driftwood would have created a jam on the slit and prevented this self-cleaning phenomenon [Piton and Recking, 2015a, 2015b].

Figure 4: Photos of flows, DEM representation of deposit thickness and surface velocity fields of a) braided like-patterns during the initial filling of the inlet-vicinity; b) armor breaking leading to a channelization transferring sediment to the basin central part; c) new braided like patterns followed by d) another channelization, whose channel connect the inlet to the outlet, leading to a partial self-cleaning. Flow from the top to the bottom of the pictures.

HYDRAULICS ON MASSIVE DEPOSITS PIV and DEM measurements were analyzed to deduce the slope, Froude and Shields numbers on massive-bed-load deposits (Fig. 5). Before each DEM and LSPIV measurements, the water depth, hgauge, has been measured using the point gauge. At the same coordinates, the

INTERPRAEVENT 2016 – Conference Proceedings | 815

152 There is thus not a unique value of deposit-slope but rather a range of slope in which a dynamic-

153 equilibrium fluctuates. A method to estimate the slope range should be developed in further 154 analysis. In addition, laterally-confined complementary experiments demonstrated that the 152 There is thus not a unique value of deposit-slope but rather a range of slope in which a dynamic- 155 deposition-slope increases in unconfined configurations (see Mejean [2015]). As a method to 152 There is thus not a unique value of deposit-slope but rather a range of slope in which a dynamic153 equilibrium- fluctuates. A method to estimate the slope range should be developed in further 156 estimate deposition slope is still lacking, it is generally recommended in new sediment trap design to 153 equilibrium fluctuate s. A method to estimate the slope range should be developed in154 further analysis. In addition, laterally-confined complementary experiments demonstrated that the 157 measure deposition slopes in the field, for example upstream of existing check dams, to estimate the 154 analysis. In addition, laterally-confined complementary experiments demonstrated that155 thedeposition -slope increases in unconfined configurations (see Mejean [2015]). As a method to 152 There is thus not a unique value of deposit-slope but rather a range of slope in which a dynamic- 158155 depositiondeposition slope-slope in increase the futures in trap unconfined. Our results configuration demonstrates (see thatMejean deposition [2015]). slopeAs a s method measured156 toestimate above deposition slope is still lacking, it is generally recommended in new sediment trap design to 153 equilibrium fluctuates.. A A method method to to estimate estimate the the slope slope ra range should be developed inin further further 159156 checkestimate dams deposition, i.e. in quiteslope is confined still lacking, configurations, it is generally recommendedmust be considered in new sediment as minimum trap design157 value tomeasures of the deposition slopes in the field, for example upstream of existing check dams, to estimate the 154 analysis. InIn addition, addition, l laterally-confined complementary experiments demonstrated that the 157 measure deposition slopes in the field, for example upstream of existing check dams, to estimate the 160 possible bedload155 depositiondeposit -slopeslope s increase in sediments inin unconfined unconfined trap basins. configuration configuration One musts (see noteMejean that [2015] it is). Asnot158 a the method depositioncase to for slope in the future trap. Our results demonstrate that deposition slopes measured above 161158 muddeposition flows which156 slope estimatemayin the deposit future deposition trapwith .slope Ourvery isresults gentlestill lacking, demonstrate slopes itit isis generallygenerally[Piton that and recommended recommendeddeposition Recking, slope in2015a]in newnews measured sedimentsediment. 159 trap trapab ove designdesigncheck to dams, i.e. in quite confined configurations, must be considered as minimum values of the 159 check dams157, i.e. measurein quite deposition confined slopes configurations, in the field, formust example be considered upstream of asexisting minimum check dams, value160 sto of estimate thepossible the bedload deposit slopes in sediment trap basins. One must note that it is not the case for 162160 Partialpossible self bedload-158cleaning deposition deposit was also slope slope observeds inin in thethe sediment futurefuture systematically: traptrap trap.. OurOur basins. resultsresults (i) One demonstratedemonstrate during must thenote thatthat hydrograph that ddeposition it is not slope recession thes measured 161case informud slit above- damflows which may deposit with very gentle slopes [Piton and Recking, 2015a]. 163161 experimentsmud flows159 which and check may conver deposit damssely,, i.e. i.e. with(ii) inin during quite quitevery gentle confined confined the slopes peak configurations, configurations,- flows[Piton and in must da Recking,m - beless considered 2015a] experiments.. as minimum Slit dams values of prevent the 160 possible bedload deposit slopes in sediment trap basins. One must note that it is not162 the casePartial for self-cleaning was also observed systematically: (i) during the hydrograph recession in slit-dam 164 peak-flow-releases due to the delta-like dynamics [Fig. 1 (TP3)], which is maximum at the peak flows. 162 Partial self161-cleaning mud was flows also which observed may deposit systematically: with very gentle (i) during slopes the[Piton hydrograph and Recking, recession 2015a].. in163 slit- damexperiments and conversely (ii) during the peak-flows in dam-less experiments. Slit dams prevent 165 The volumes that were stored in the delta-front were subsequently re-eroded and partially flushed 163 experiments and conversely (ii) during the peak-flows in dam-less experiments. Slit dams164 prevent peak -flow-releases due to the delta-like dynamics [Fig. 1 (TP3)], which is maximum at the peak flows. 162 Partial self-cleaning was also observed systematically: (i) during the hydrograph recession in slit-dam 166164 duringpeak -theflow flow-releases-recession due to, thebuilding delta- liketerraces’ dynamics-like [Fig. patterns. 1 (TP3)], The which observation is maximum of at a theclear peak-165incised flows. The- channel volumes that were stored in the delta-front were subsequently re-eroded and partially flushed 163 experiments and conversely (ii) during the peak-flows in dam-lessless experiments. experiments. Slit dams prevent 167165 in Thethe volumesdownstream that were part storedof the in deposits the delta is-front thus werean evidence subsequently of partial re-eroded self- andcleaning partially. Adding flushed a slit-grill 164 peak-flow-releases due to the delta-likelike dynamics [Fig. 1 (TP3)], which is maximum at166 the peakduring flows. the flow-recession, building terraces’-like patterns. The observation of a clear-incised-channel 168166 or duringdriftwood the165 flow would -reThecession volumes have, buildingcreated that were terraces’ a stored jam -oninlike the the patterns. delta slit-front and The were preventedobservation subsequently this of are selfclear-eroded-cleaning-incised and partially167-channel phenomenon flushedin the downstream part of the deposits is thus an evidence of partial self-cleaning. Adding a slit-grill 169167 [Pitonin the and downstream Recking,166 during 2015a,part the of flow the2015b]- redepositscession. ,, isbuilding thus anterraces’ evidence-likelike patterns.of partial The self observation-cleaning. ofAdding a clear a-168incisedincised slit -grill-channelor driftwood would have created a jam on the slit and prevented this self-cleaning phenomenon 168 or driftwood167 wouldinin thethe ha downstreamdownstreamve created partparta jam ofof the on deposits the slit is and thus prevented an evidence this of partial self-cleaning self-cleaning phenomenon.. Adding169 a [Pitonslit -grill and Recking, 2015a, 2015b]. 170169 FIGURE[Piton 4and 168 Recking, or driftwood2015a, 2015b] would. have created a jam on the slit and prevented this self-cleaning phenomenon 169 [Piton and Recking, 2015a, 2015b].. 170 FIGURE 4 171170 HydraulicsFIGURE 4 on massive deposits 170 FIGURE 4 171 Hydraulics on massive deposits 171 Hydraulics on massive deposits 172 PIV and DEM171 measurementsHydraulics on massive were deposits analyzed to deduce the slope, Froude and Shields numbers on 172 PIV and DEM measurements were analyzed to deduce the slope, Froude and Shields numbers on 173 massive-bed-load deposits (Fig. 5). Before each DEM and LSPIV measurements, the water depth, 172 PIV and DEM172 measurementsPIV and DEM measurements were analyzed were to deanalyzed duce the to de slope,duce theFroude slope, and Froude Shields and Shieldsnumbers173 numbers onmassive on -bed-load deposits (Fig. 5). Before each DEM and LSPIV measurements, the water depth,

174173 hgaugemassive, has- bedbeen173- load measuredmassive deposits-bed using-(Fig.loadload 5) depositsthe. Before point (Fig. eachgauge. 5).. Before DEM At and eachthe LSPIVsame DEM and measurements,coordinates, LSPIV measurements, the the topographical wate the174r depthwate r h, depth gaugeprofile, ,,ha s, been measured using the point gauge. At the same coordinates, the topographical profile,

175174 transverhgauge, hases 174 tobeen the measuredh flowgauge,, ha directions beenusing measured the, was point alsousing gauge. measuredthe pointAt the gauge. same. The At coordinates, surfacethe same velocitycoordinates, the topographical was the interpolated topographical175 profile transverprofile, on a,, 1se- to the flow direction, was also measured. The surface velocity was interpolated on a 1- topographical profile, transverse to the flow direction, was also measured. The surface 175 transverse175 to thetransver flow directionse to the , flowwas direction also measured,, was also. Themeasured surface.. TheThe velocity surfacesurface was velocityvelocity interpolated was interpolatedinterpolated on a 1 onon- aa 11- 176 mm-transversal step on the profiles (N values/profile)velocity. T hewas mean interpolated value of onthe a interpolated 1-mm-transversal176 mm surface-transversal step on step the on profiles the profiles (N values/profile). (N values/profile) The. The mean value of the interpolated surface 176 mm-transversal step on the profiles (N values/profile).. The mean value of the interpolated surface 176 mm-transversal step on the profiles (N values/profile). The mean value of the interpolated surface mean value of the interpolated surface177 velocities,velocities, = , gives aa roughrough estimation estimation of = , the profile mean velocity. The 177 velocities, = , gives a rough estimation of = , the profile mean velocity. The 177 velocities, = ,, gives a rough estimation of = ,, the the profile profile mean mean velocity. velocity. The 177 velocities, = , gives a rough estimationof , the of = , profile the profile mean mean velocity. velocity. The The deposition slope S has been measured on the DEM 178 deposition slope S has been measured on the DEM along a longitudinal profile passing by the 178 deposition 178slope depositionS has been slope measured S has been onmeasured the DEM on the along DEM along a longitudinal a longitudinal profile profile passing by by the the along a longitudinal profile passing by the transversal profile. Rough estimation of, Fr and 178 deposition slope S has been measured on the DEM along a longitudinal profile passing179 by thetransversal profile. Rough estimation of Fr and , the Froude and the Shield stress for the D84 , 179 transversal profile. Rough estimation of Fr and ,, the Froude and the Shield stress for the D84 ,, 84 179 transversal profile. Rough estimation of Fr and , thewith Froude the Froude and the and Shield the Shield stress stress for the for D the , D84,respectively, were computed using: 179 transversal profile. Rough estimation of Fr and , the Froude and the Shield stress for the D84 , 180 respectively, were computed using: and , with ∆ the submerged 180 respectively, were computed using: andand ,, with with with ∆∆ thethe submerged submerged sediment density taken as 1.65. All 180 respectively, were computed using: and , with ∆ the submerged 180 respectively, were computed using: dimensionless and number, with analyzed ∆ the submergedhereafter are representative of local values of flow features in 181 sediment density taken as 1.65. All dimensionless number analyzed hereafter are representative181 sediment of density taken as 1.65. All dimensionless number analyzed hereafter are representative of active channels. They are not averaged on the basin width. 181181 sedimentsediment density 182density locallocal takentaken valuesvalues as 1.65.ofof1.65. flowflow All featuresAllfeatures dimensionless dimensionless inin activeactive channels.channels. number number TheyThey analyzed areareanalyzed notnot hereafter averagedaveraged hereafter ononare thethe representative basinbasinare representativewidth.width.182 oflocal values of of flow features in active channels. They are not averaged on the basin width. 182182 locallocal values values of of flow flow featuresfeatures in in active active channels. channels. They They are arenot notaveraged averaged on the on basin the width.basin width.

Figure 5: Rough estimation of main ﬂow dimensionless numbers: a) deposition slope vs inlet solid concentration; b) Froude number vs inlet solid concentration and; c) Froude number vs Shield number

The absolute values illustrated in Fig. 5 are rough estimations because of the high uncertain-

ties on hgauge , however some general trends of interactions between geomorphology and hydraulics can be observed: (i) the deposition slope strongly fluctuates for a given solid concentration (Fig. 5 a) highlighting the geomorphic cycles' magnitude and that armoring process lead to various equilibrium slope despite a constant solid concentration at the inlet. (ii) The Froude number also strongly fluctuates (Fig. 5 b) which is likely to be mainly related to the varying velocities related to varying bed roughness within the geomorphic and armoring cycles. (iii) Interestingly an unintuitive inverse correlation seems to appear between the Froude number and the Shields number (Fig. 5 c): Low Shields numbers were computed on the milder slopes observed during chenalisations but where the smooth bed allowed high velocities and Froude numbers. Conversely, steep paved braided fan-patterns showed high slopes (and thus Shield numbers) and low velocities (and thus Froude numbers) due to their rough and paved beds. Local measurements of sediment diameter should be done to compute more accurate value of Shields numbers and to confirm this inverse correlation. These preliminary results need to be more deeply analyzed to specify the autogenic fluctuating hydraulics of massive bed-load deposits related to grain size sorting and to define which friction law and transport formula are the most relevant to compute such phenomena.

816 | INTERPRAEVENT 2016 – Conference Proceedings CONCLUSIONS The new elements listed in this paper will help designers to more accurately design and numerically model the structures, speciﬁcally slot and slit dams, so to better adapt them to each site and their natural-hazard-speciﬁcities and mitigation-objectives. They also highlight the varying nature of sediment transport of poorly sorted mixtures and the necessity to push further the research on this subject, which, so far, is not enough understood to provide accurate design methods to practitioners.

ACKNOWLEDGMENTS This study was funded by Irstea, the INTEREG-ALCOTRA European RISBA project, and the ALPINE SPACE European SEDALP project. The authors would like to thank Matjaž Mikoš for his editorial work and two reviewers who help us to improve this paper.

REFERENCES - Armanini et al., 1991. From the check dam to the development of functional check dams. Fluvial Hydraulics of Mountain Regions 37, 331–344. - Armanini & Larcher, 2001. Rational criterion for designing opening of slit-check dam. J. of Hydr. Eng. 127, 94–104. - Carbonari, C., 2015 Experimental observations on the functioning of sediment trap basins: LSPIV measurements of low submersion ﬂows, Msc. Thesis. - D’Agostino, V., 2013. Filtering-retention check dam design in mountain torrents, in: Check Dams, Morphological Adjustments Erosion Control Torrential Streams., pp. 185–210. - Jordan et al., 2003. Modélisation physique d’un piège à graviers, le cas du Baltschiederbach. Wasser Energie Luft 95, 283–290. - Le Guern, J., 2014. Modélisation physique des plages de dépôt : analyse de la dynamique de remplissage, Msc. Thesis. - Mejean, S., 2015. Caractérisation des conditions hydrauliques du piégeage de la charge sédimentaire grossière des torrents, Msc. Thesis. - Piton & Recking, 2015a. Design of sediment traps with open check dams I: hydraulic and deposition processes. J. of Hydr. Eng. In press. - Piton, G., Recking, A., 2015b. Design of sediment traps with open check dams II: woody debris. J. of Hydr. Eng. 142(2). - SedAlp, 2015. Work Package 6 Final report - Interactions with structures. Alpine Space European project - Torricelli, E., 1644. Opera geometrica, Firenze. - VanDine, D.F., 1996. Debris Flow Control Structures for Forest Engineering. Res. Br., B.C. Min. For., Victoria, BC. - Zollinger, F., 1983. Die Vorgänge in einem Geschiebeablagerungsplatz (ihre Morphologie und die Möglichkeiten einer Steuerung), ETH Zurich PhD Thesis.

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