It is interesting… International companies come into UAE & KSA and apply ‘well-established’ hydrological practices, built using standardized, comprehensive numerical modelling systems. Meanwhile, Egyptian scientists have been publishing on ‘different’ ways to do hydrology in semi-arid settings. I know which ones seem to be more relevant!!
Arid zone hydrology is not temperate hydrology with less water.
Baseflow is often effectively zero, but flooding is often substantial, fast and furious, actively carrying volumes of sediment that can amplify flood impacts. Recognizing characteristics of arid zone streams which can modify the effectiveness of conventional practice is an important pathway towards better flood risk management. Some of the considerations, and approaches to identify or mitigate arid stream risks are presented.
Wadis as Roads and Borrow Pits: wadi channels form narrow trails of nearly flat silty, sandy or gravelly bed between rugged, rocky hinterland. While they are often a cost-effective route for a roadway, or a convenient borrow-pit for building materials, their structure is a direct consequence of episodes of sediment mobilisation and redistribution. Long periods of stability can be drastically disrupted during a single flood event.
The capacity to deposit tens of thousands of tons of sediment in an extreme flood event is shown by the long tongue of sand deposited along the wadi channel.
Where roads need to be built inside wadis, some key considerations include:
Building to one side of the wadi
Acknowledging stream pressures from wadi constrictions or planform (meanders)
Understanding stream pressures and sediment loads from hinterland sub-channels
Having an awareness of sediment accumulation or mobility
Building unpaved or low-cost roads (e.g. cement-stabilized or geosynthetic)
Being prepared to clean up after a big event!
Water Storage: Construction of detention basins provides a promise of reduced flooding combined with improved access to precious water resources. However, detention can substantially increase the local capacity for sediment trapping.
Under extreme conditions, sediment trapping can focus water pressures, leading to conditions that can outflank or exceed hydraulic control structures. Drastic outcomes of outflanking during a flood event are shown below.
Hydraulic design for detention systems needs to consider their effect on local sediment trapping and integrate design processes that consider performance across difference levels of infilling. This possibly requires consideration of inconvenient scenarios, as sediment rarely deposits where you would like it to!
Culverts and bridges: for roads and rail are a standard element of civil design, with some awesome software available to streamline the design process. However, conventional practice often swerves away from several aspects, important in semi-arid settings:
Culverts are often much smaller in cross-section, steeper and smoother than the streambed upstream and downstream. This causes hydraulic focusing, with flow speeds much greater than within the streambed. The accelerated flow creates strong opportunity for scour, and channelization, providing an opportunity for sediment resuspension and reducing stream stability.
Pier scour at bridges remains an active research topic, which is often poorly resolved in design assessment. Although structural stability may be resolved through conservative embedment, pier scour effects on bed stability and sediment resuspension are rarely predicted accurately.
Interactions between downstream sediment transfer and culverts or bridges often lead to restricted flow capacity. The resulting stream afflux can cause backwater flooding and flow acceleration.
In general, stream stability at culverts and bridges can be improved by expanding stream cross-section through the structure. This can be done introducing additional design criteria, to limit the recurrence of flow conditions initiating streambed mobility.
How Can We Do Better?
Unsurprisingly, the two main avenues for improvement lie in better understanding of water, or better understanding of land...
Understanding Flows
The vast majority of semi-arid catchments are ungauged, often with poor, patchy or non-coverage by rain gauges. Catchment areas are highly inhomogeneous, with variable terrain effecting infiltration and sub-catchment storage characteristics. Flood events are rare, limiting the event set available to characterize recurrence.
Advances in geospatial and pluviometric detection technologies have provided pathways for advancing the understanding of flooding in semi-arid settings:
Geographic-scale understanding of weather systems provides improved capacity to integrate rainfall event records across a region, using sub-regional attributes. This reduces the problem of limited flood events developed from point-source data sets, or sub-division of regional information.
Use of radar or satellite derived rainfall estimates provides a 2D representation, which supports understanding of non-uniform or localized rainfall events - previously applied mainly to extremely large-scale catchment hydrology.
Satellite altimetry and ground classification techniques can support a fine-grained understanding of stream structure, as well as spatially variable infiltration and flow characteristics. The application of terrain 'ruggedness' to water resource assessment by Egyptian scientists also highlights significance of sub-catchment storage in semi-arid settings, as complex local basins require infilling before stream flow establishes fully.
Incorporation of higher resolution rainfall distribution and catchment representation provides a key opportunity for improved resolution of complex flood risks. Examples include flash-flooding developed due to a rainfall sequence running 'down' the catchment, or transient flood water elevation developed through flow instability on steep stream segments.
Understanding Sediments
The significance of sediment processes provides an important third dimension to hydrologists' two favorite tools of flood management - conveyance and detention. Both can be substantially impeded by sedimentation, leading to increased, usually unpredicted flood risk.
The technique of relating sediment transport to stream power provides a valuable first-order estimate of behavior. However, braided into this concept, stream morphology can have significant local influence on sedimentation or where sediment transport leads to flood risk. Features where stream power fails to describe these risks include stream knick points, or locations subject to channel avulsion.
Detailed geomorphic assessment is often beyond practical project scopes. Consequently, a simplified approach can be evaluation of the relative presence of features which distort the relationship between stream power and sediment mobilization. A non-exhaustive list of these features includes:
Low Permeability Areas (e.g. bare rock)
Shear Slopes / Slip Faces
Knick Points
Boulder Runs
Anabranching
Stream Funnels
Critical Steepness
Relative occurrence of these features can be used to characterize the 'geomorphic risk' that sedimentation or local flow controls can modify the evaluation of flood risk. Areas of high geomorphic risk can then be applied to hydrologic and hydraulic modelling outcomes, such as by sensitivity factoring, or targeted evaluation of critical features, e.g. through CFD modelling.
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