Retention and stability of driftwood in a wood restored lowland stream. Michael Seidel Michael Mutz

Transcription

1 Retention and stability of driftwood in a wood restored lowland stream Michael Seidel Michael Mutz

2 Introduction & objectives Wood is increasingly used in river restoration, but: Composition of woody debris deviates from natural Wood installations are designed to avoid retention Small wood is valuable to many aspects of stream ecology Do large wood structures affect the retention & stability of smaller driftwood even though they were designed to avoid formation of jams?

3 Study area Ruhlander Schwarzwasser Stream Sand bed stream ~ 60 % covered by riparian forest Slope 0.8 Mean Q 0.6 m³/s Width 5.5 m Depth 0.5 m 400 m restored in wood structures blockage ratio % unrestored control 400 m downstream logs crown ~ 30% blockage Blockage > 50 50% % root wads blockage > 70 %

4 Methods a. Mapping & tagging of wood pieces Restored reach: mapping in 3 rd & 4 th year after installation Control reach: mapping in 4 th year tagging of wood pieces in 3 rd year (359 pieces; ~ 63%) not retrieved pieces assumed to be exported Parameters: length, diameter, buoyant or sinking state, position in relation to flow, proportion submerged and proportion buried in sediment min 0.5 m min 0.01 m low disturbance in 3rd year!

5 Methods a. Mapping & tagging of wood pieces b. Tracing restored driftwood reach: mapping with removal in 3 rd & 4 th year after installation Pieces of driftwood tagging successively of wood pieces released in 3 rd year (359 pieces; ~ 63%) control Summer reach: base-flow mapping (n = 100) in 4 th & year winter high-flow (n = 81) parameters: length, diameter, buoyant or sinking state, position in relation to flow, Individually traced until retention or end of the reach proportion submerged and proportion buried in sediment Retained pieces were removed 400 m 5 m 1.38 m

6 Methods a. Mapping & tagging of wood pieces b. Tracing restored driftwood reach: mapping with removal in 3 rd & 4 th year after installation tagging of wood pieces in 3 rd year (359 pieces; ~ 63%) c. Tracing pieces of driftwood without successively removal released control reach: mapping in 4 th year summer Release base-flow between base-flow (n = 100) & winter mean high-flow discharge (n (n = = 81) 75) parameters: length, diameter, buoyant or sinking state, position in relation to flow, individually traced until retention or end of the reach proportion Mapping submerged position for and 8 proportion weeks (~ buried 1 to in 3-week sedimentintervals) retained pieces were removed 400 m? 5 m 1.38 m

7 Methods a. Mapping & tagging of wood pieces b. Tracing restored driftwood reach: mapping with removal in 3 rd & 4 th year after installation tagging of wood pieces in 3 rd year (359 pieces; ~ 63%) c. Tracing pieces of driftwood without successively removal released control reach: mapping in 4 th year summer base-flow (n = 100) winter high-flow 81) d. Time release until between sinking base-flow state & mean discharge (n = 75) parameters: length, diameter, buoyant or sinking state, position in relation to flow, individually traced until retention or end of the reach Branches proportion mapping ofsubmerged position alder & birch for and 8 proportion weeks (2-week buried in sediment intervals) retained pieces were removed diameter: 2.1 (1.8 cm 2.6 cm) length: 50 cm Kept in a tub for 230 days Determination of density within 2 to 3 weeks (by volume & weight)?

8 Methods a. Mapping & tagging of wood pieces b. Tracing restored driftwood reach: mapping with removal in 3 rd & 4 th year after installation tagging of wood pieces in 3 rd year (359 pieces; ~ 63%) c. Tracing pieces of driftwood without successively removal released control reach: mapping in 4 th year summer base-flow (n = 100) winter high-flow 81) d. Time release until between sinking base-flow state & mean discharge (n = 75) parameters: length, diameter, buoyant or sinking state, position in relation to flow, branches individually traced until retention or end of the reach proportion mapping submerged position for and 8 proportion weeks (2-week buried in sediment intervals) e. Sampling of alder of macroinvertebrates (swimming n=7; submerged n=6) & birch (swimming n=3) diameter: retained 1.8 pieces cm were 2.6 cmremoved length: 50 cm - 1 st to 2 nd year after restoration in summer, autumn & spring (pooled) kept - Substrates: in a tub for driftwood 230 days & sand determination m² each of density within 14 to 21 days (by volume & weight) source: Karsten Grabow

9 Results a.) Wood standing stock [per 100 m²] 1. Stream bed & obstacles besides installed wood structures Control: 0.22 m³ 9.7 m² 65 pieces Restored: 0.20 m³ 10.0 m² 77 pieces 350% + 2. Installation of wood structures 0.80 m³ 17.1 m² 78 pieces + 3. Wood associated to installed wood structures 0.89 m³ 22.1 m² 116 pieces 300% 100% 50% 300% 11% 29% 71% 49% 0% volume surface number 1%

10 Results a.) Mobility between 3 rd & 4 th year +310% 10% 23% -67% 1.9 m 1.53 m (Mann-Whitney U; p > 0.001) Highly dynamic: 90% mobile (n=520) & 70% exported (n = 385) from 3 rd to 4 th year But: import (n=2351) >> export (n=385) Stability was related to: not related to: length buoyancy position % submerged or buried

11 Results a.) Wood standing stock Highest retention: emerged > submerged wood structures wood structures (Mann-Whitney U; p = 0.001) position in main current

12 Results a.) Wood standing stock Highest retention: emerged > submerged wood structures wood structures (Mann-Whitney U; p = 0.001) position in main current

13 Results a.) Wood standing stock Highest retention: emerged > submerged wood structures wood structures (Mann-Whitney U; p = 0.001) position in main current No channel-spanning jams Minor to no influence on the cross section blockage ratio

14 n=104 n=57 out of 81 n=99 n=33 out of 81 Results b.) Retention Retention was related to discharge 100% 80% 60% restored 40% 20% control 0% restored control transport distance [m]

15 retained driftwood [%] Results c.) Storage after retention Period of storage: restored reach > control reach after ~ 8 weeks: 23% ~1% n = 17 out of 75 n = 1 out of restored control days after release

16 density [kg/dm³] Results d.) Time until sinking state Sinking after: birch 60 d alder 135 d No difference if floating or submerged 1,1 1,0 0,9 0,8 0,7 0,6 0,5 0,4 birch - floating (n=3) alder - submerged (n=7) alder - floating (n=6) days after insertion

17 Results - e.) Macroinvertebrates 80 Total macroinvertebrate taxa: abundance & number 2.5 times higher on wood abundance [ind/m2] number of taxa EPT taxa: abundance & number 10 & 7 times higher on wood abundance [ind/m2] number of taxa

18 Conclusions Question: Do large wood structures affect the retention & stability of smaller driftwood even though they were designed to avoid formation of jams? Wood installations increased retention & stability of driftwood in reach Composition of installed wood structures became more natural Wood installations in the main current & emerging from the water surface at high flow were most effective for retention Risk potential of jamming was low Ecological benefits predominated Continuous replacement of wood from upstream important Purposeful retention of driftwood should be considered as important objective in stream restoration with wood and stream management

19 Discussion

20 Annex

21 retention - structures

22 n=104 n=57 out of 81 n=99 n=33 out of 81 Results b.) Retention Retention was related to discharge 100% 80% 60% restored 40% 20% control 0% restored control transport distance [m] Number of contacts: restored > control 3.1 / 100 m 1.8 / 100 m 40% more (Mann-Whitney U test; p > 0.01)

23 Discussion Improved habitat quantity for macroinvertebrates by retention of small driftwood Small pieces of wood are as effective as large wood (Lester et al. 2009) Abundance & number confirmed wood as a high quality substrate compared to sand (e.g. Smock et al. 1989, Johnson et al. 2003)

24 Discussion Mobility of wood was high In accordance with findings on the mobility of large pieces of wood in a low gradient headwater stream (e.g. Dixon & Sear 2014) Probably related to a major flood between pre-survey & tagging of wood For maintaining the wood standing stock replacement from upstream was of great importance Uncertainties: Low retrieval of tagged wood (36.5%): mapped at low-flow, each wooden piece removed; share of missed pieces assumed to be low Control downstream of restored reach: high dynamics of wood no significant influence regarding the amount assumed

25 Discussion Retention & formation of jams Highest retention: wood structures emerging from water surface & positioned in the main current Wood transport at high flow (e.g. Bilby 1984) & parallel to flow in the main current (Braudrick & Grant 2001) Changes in the hydraulics of the restored reach assumed to be low Only partial jams after 4 years (according to Manners et al. (2007)) Porosity of accumulations estimated as high Cross-section blockage ratio not significantly increased