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Infiltration Performance
Published in K. Ferguson Bruce, Stormwater Infiltration, 2017
One type of flooding and drainage problem in urban watersheds occurs in unobstructed open channels, where stream stage (water level) is a direct function of instantaneous flow rate. Urbanization typically increases peak storm flow rates. Increased peak flow rates lead to higher stream stage and increased overbank flows onto surrounding properties, hence the flooding and drainage problem. The increased peak rates are partly a result of increased storm flow volumes from urban impervious surfaces. Increased volume could cause an increased peak rate by itself, even if timing of flow is not changed. However, urban development also alters the timing of flow by increasing velocity across smooth impervious surfaces on the one hand, and attenuation in detention reservoirs and at drainage obstructions on the other. Watershed geography also affects timing: hydrographs from subwatersheds merge where tributaries join; if even a part of a tributary hydrograph overlaps the peak of the main stream, peak composite flow is increased and flooding and drainage problems worsen.
An experimental study on the overbank sedimentation in an asymmetrical compound channel
Published in Wim Uijttewaal, Mário J. Franca, Daniel Valero, Victor Chavarrias, Clàudia Ylla Arbós, Ralph Schielen, Alessandra Crosato, River Flow 2020, 2020
C. Juez, C. Schaerer, H. Jenny, A.J. Schleiss, M.J. Franca
However, excessive overbank sedimentation not only potentially jeopardizes ecological processes that develop in the floodplains but also can result in an increased flood risk. A high sedimentation rate leads to a reduction of the channel cross section area and consequently to a reduced channel conveyance capacity. Furthermore, the drainage capacity of the floodplain can be decreased, contributing to an increased frequency of inundations (Kiss 2011).
Fundamental Flood Hazard Issues in the Alluvial Fan Environment
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Alluvial fan landforms may be either active or inactive, or a combination of both (French et al., 1993; NRC, 1996; FEMA, 2002). The Badwater alluvial fan in Death Valley shown in Figure 18.1 is a classic example of an active alluvial fan. The key characteristics of active alluvial fans include the following:Location on an alluvial fan landform: Fan-like flood processes may occur on other geomorphic landform types, but active alluvial fan flooding occurs only on alluvial fan landforms (NRC, 1996).Flow path uncertainty: Changes in the path of flooding over time are perhaps the key characteristic of active alluvial fans (French et al., 1993; Fuller, 2012c). Flow path uncertainty may be caused by avulsions or by other changes in flow distribution on the fan surface. This uncertainty must be accounted for in flood hazard assessments.Net depositional environment: Over geologic time scales, active alluvial fans experience more deposition than erosion, resulting in aggradation of the active fan surface. Note that long-term average aggradation rates on active alluvial fans may be low relative to maximum flood depths during a single flood event or even when considered over time scales of less than 100 years, particularly for fans that are not subject to debris flows. However, portions of individual active alluvial fans may experience extreme rates and depths of deposition during a single flood, especially where there is a risk of debris flows. While active fans experience net aggradation over long time periods, localized portions of an active alluvial fan may also experience short-term scour and erosion.Geologically young surfaces where broad flood inundation is possible: Broad areas of active alluvial fans are periodically inundated by flooding. Inundation may be frequent or infrequent, depending on climatic and watershed conditions, but it occurs regularly within the time scale of analysis. Flood inundation may occur within stable or shifting channels in a variety of stream patterns, but a large percentage of flooding occurs as overbank or sheet flooding outside the boundaries of well-defined channels. It is unlikely that the entire active surface would be inundated during a single flood event (French, 1988).
Dynamics and scales of transmission losses in dryland river systems: a meta-analysis
Published in Australasian Journal of Water Resources, 2022
Never Mujere, Mhosisi Masocha, Hodson Makurira, Dominic Mazvimavi
Terminal water storage in pools, subsidiary channels, floodplains and artificial embankments along enlarged channel segments can persist for long durations and become an important transmission loss component. However, as pools become fully saturated and channel fully active during floods, the rate at which flow is transmitted downstream along the channel or river transmission efficiency increases (Knighton and Nanson 1994). This reduces transmission losses as most of the flow moves downstream. Overbank flows ponded in floodplain depressions can reconnect with main channel, albeit at a slower rate than channel flow due to friction with riparian vegetation. Studies show that terminal storage accounted for 11.2% of the transmission losses along the 180 km reach of the Diamantina River (Jarihani et al. 2015).
Controls on the architectural evolution of deep-water channel overbank sediment wave fields: insights from the Hikurangi Channel, offshore New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2022
Daniel E. Tek, Adam D. McArthur, Miquel Poyatos-Moré, Luca Colombera, Charlotte Allen, Marco Patacci, William D. McCaffrey
Flow velocity may be enhanced on channel overbanks with steep outer-levee gradients that slope away from their conduit and hindered on overbanks that slope toward the channel (Figure 21B; Kane et al. 2010; Nakajima and Kneller 2013). In the studied reach of the Hikurangi Channel, the oceanward overbank is horizontal, or slopes toward the channel throughout the studied stratigraphy. This appears to have hindered overbank flow and inhibited the formation of sediment waves on the oceanward overbank (Figure 22). On the landward margin, the outer-levee gradient is interpreted to have shallowed progressively as slope-traversing drainage networks were established, and overbank flow downstream of the apex of expanding bends acted to redistribute sediment in the trench, leading to a largely flat trench-floor and gently-sloping to flat outer-levees (Table 2, Figures 3B and 22).
On the governing equations for horizontal and vertical coupling of one- and two-dimensional open channel flow models
Published in Journal of Hydraulic Research, 2020
Cesar A. Simon, Eddy J. Langendoen, Jorge D. Abad, Alejandro Mendoza
Knight and Demetriou (1983) found values of mean apparent shear stresses on vertical and horizontal interfaces equal to 0.025 Pa and 0.05 Pa, respectively, for a straight compound channel with bed slope equal to , and floodplain to channel width of 4. Prinos and Townsend (1984) found mean apparent shear stresses on vertical interfaces ranging between 0.025 Pa to 0.035 Pa for Manning n values varying between 0.011 to 0.022, respectively, for a straight compound channel with bed slope equal to , and floodplain to channel width of 3.5. Applying the apparent shear stress relationship for a straight channel of Prinos and Townsend (1984) to the meandering channel modelled here, yields an apparent shear stress less than one third of the smallest value calculated by the model. Clearly, a meandering channel presents more complex overbank flow dynamics at the transition between the main channel and floodplain.