Improve water retention

Improve water retention

Improve water retention01. Water flow quantity improvement

General description

Water retention and runoff can be altered by different factors such as changes in land cover, soil structure and compacting, stormwater management, loss of floodplains and wetlands. Loss of water retention combined with accelerated runoff typically increases the frequency and magnitude of flood peaks and reduces the availability of water to streams during low flow (base flow) periods (Stream Habitat Restoration Guidelines, WDFW). The alternatives for improve water retention may be applied in combination with other restoration measures (e.g. restoring side channels or former meander is firstly done for restore channel morphology and lateral connectivity but also increases infiltration area) at different spatial scale and location (riparian zone, nearby land, etc). Some techniques for improve water retention are:

• Changes in land use and cover
• Improve stormwater management
• Reduce and limit the amount of impervious surfaces in the watershed

• Change land use practices and zoning regulations to limit the allowable percent of impervious surface in the watershed

• Decommission roads

• Use pervious pavement alternatives where feasible

• Minimize the extent and degree of soil compaction
• Restore stream connectivity to floodplains (see Remove bank fixation, Allow/increase lateral channel migration or river mobility, Remeander water courses, Shallow water courses, Widen water courses, Lower river banks or floodplains to enlarge inundation and flooding, Remove hard engineering structures that impede lateral connectivity, Set back embankments, levees or dikes )
• Revegetate denuded areas within the watershed. Mean annual flow is increased as a result of greater runoff due to clearing and urbanization (Peterson& Kwak 1999)
• Protect, restore, and create wetlands and other infiltration areas (see Construct semi-natural/articificial wetlands or aquatic habitats, Improve backwaters, Reconnect backwaters and wetlands, Restore wetlands, Retain floodwater )

Changes in land use and cover

Forest and vegetation cover have long been recognized as major factors influencing runoff, infiltration and evapotranspiration from shallow water tables. Watershed treatment involving the establishment of tree, bush and other plant cover is widely used as a way of reducing runoff and increasing infiltration. This is frequently assumed to increase recharge and is advocated as a core part of packages to address groundwater overdraft. However, the effect of surface vegetation on groundwater levels is not automatic. It depends on the balance between improvements in infiltration caused by increased vegetation and relative changes induced in evapotranspiration. In some cases, removal of forest cover has caused water levels to rise significantly with major environmental consequences, e.g. in much of New South Wales, Australia. (Groundwater Management: The Search for Practical Approaches. FAO, 2003)

Effects of forest manipulation on water yield

• Carefully executed light, selective harvesting will have little if any effect on streamflow, which increases with the amount of timber removed.

• The data set for the humid tropics supports the general finding of Bosch and Hewlett (1982) that removal of natural forest cover may result in a considerable initial increase in water yield (up to 800 mm per year); possibly more in highrainfall regions, depending mainly on the amount of rain received after treatment.

• Depending on rainfall patterns, there is a rather irregular decline in streamflow gain, with time, associated with the establishment of the new cover. No data have been published regarding the number of years needed for a return to pre-cut streamflow totals in the case of natural regrowth, but it may take more than eight years.

• Water yield after maturation of the new vegetation may: remain above original streamflow totals in the case of conversion to annual cropping, grassland or tea plantations; return to original levels (Pinus plantation after full canopy closure); or remain below previous values (reforestation of grassland with Pinus or Eucalyptus). Coppicing of Eucalyptus after ten years caused even stronger reductions for two years.

Source: Extracted from Bruijnzeel, 1990.

Expected effect of measure on (including literature citations):

• HYMO (general and specified per HYMO element)

Replacement of conventional pipe stormwater systems by bio-filtration stormwater management leads to a decrease on urban runoff, lower peak discharges, delay on stormwater discharge, and shorter duration (Lloyd et al., 2002).

• physico � chemical parameters

Concentrations of Total Suspended Solids (TSS), Total Phosphorus (TP) and Total Nitrogen (TN) discharged from the bio-filtration system are typically lower than pollutant concentrations discharged from the piped system (Lloyd et al., 2002).

• Biota (general and specified per Biological quality elements)

BQE	Macroinvertebrates	Fish	Macrophytes	Phytoplankton
Effect	+	+	+	o

Pressures that can be addressed by this measure

• Hydrological regime modification
• Groundwater abstractions
• Surface water abstraction

Case studies where this measure has been applied

• Renaturierung Untere Havel
• Vääräjoki - Niskakoski
• Kuivajoki - Hirvaskoski
• Spree - Restoration and remeandering of the Müggelspree - downstream Mönchwinkel
• Regge Velderberg
• Millingerwaard - Floodplain rehabilitation
• Vreugderijkerwaard - Side channel
• Narew river restoration project
• Uilenkamp - Meander reconnection
• Warta Middle River Valley
• Enns - Aich
• Buiten Ooij - Sluice operation
• Polder Ingelheim – Restoring former floodplains (INTERREG Sustainable Development of Floodplains)
• Hondsbroeksche Pleij – Restoring former floodplains (INTERREG Sustainable Development of Floodplains)
• Bemmelse Waard – Restoring former floodplains (INTERREG Sustainable Development of Floodplains)
• Rhine - Polder Altenheim
• Drava - River Widening Amlach/St. Peter
• Drava - River Widening Obergottesfeld
• Drava - River Widening Rosenheim
• Rhine - Nebenrinne Bislich-Vahnum (LIFE08 NAT/D/000007)
• Meuse - Overdiepse Polder
• Rhine - Ontpoldering Noordwaard

Useful references

Lloyd S.D., T.H.F. Wong and C. J. Chesterfield (2002) Water sensitive urban design- a stormwater management perspective. [1]

Other relevant information

Improving Stormwater Detention Basins for Better Stormwater Management (PEC's Stormwater Management Facility Retrofit Program. This fact sheet highlights design concepts for stormwater best management practices (BMPs) in urban areas.[2]