Mauritius A Geomorphological Analysis Essay

Geomorphic analysis in the engineering and implementation context is used to quantify channel morphological parameters as they relate to design of a stable system. Geomorphic analysis provides:

  • Quantitative channel and stability assessment tools
  • Foundation for natural channel design
  • Prediction of short- and long term channel change
  • Optimize design for stability and natural channel processes
  • Estimate maintenance requirements

All of these components interact with each other to form the ultimate channel configuration. In urban channels these elements often become “out-of-phase” with each other as the channel adjusts to imposed watershed conditions.

General Channel Stability There are levels of analyzing channel stability and developing solution types. Generally the approach is based on the extents of the affected processes and constraints typically limit the selected solution type. Channel stability can be looked at on a large watershed scale or down to site specific problems.

Approaches to Channel Stability

  • Watershed-scale
    • Upland Stormwater Management Controls
    • (ponds, disconnected impervious cover, impervious cover limits)
  • Reach-based
    • Channel lengths with common hydraulic/morphologic characteristics
  • Site Specific
    • Stabilization of isolated stretch of channel (usually for property protection)

General Channel Adjustment For watershed, stream reach based and site-specific situations, the Stream Restoration Program utilizes the concept that a stream reach equilibrium is dominated by the hydrology, hydraulics and sediment load. A relationship proposed by Lane can be used to qualitatively identify these physical processes dominating the system.

Qw = water discharge S = channel slope Qs = sediment discharge Ds = sediment size

Our experience shows that the most common response to urbanization in degrading reaches can be represented as:

The (+) signs indicate an increase in water discharge (Qw) and coarsening of the channel bed material (Ds); the S- indicates the river slope would decrease through meandering (planform adjustment) and/or downcutting (geometric adjustment) and the relative sediment supply would decrease in an incising reach. This qualitative analysis provides a basis for more quantitative analyses.

Channel Planform Channel planform is evaluated to assess the condition of stream meanders and the tendency of the channel to migrate laterally. Channel planform characteristics are most readily assessed using historical aerial photography and mapping. The most commonly used geomorphic planform variables are:

  • Sinuosity
  • Meander Amplitude and Belt Width
  • Meander Wave Length
  • Meander Arc Length
  • Meander Radius of Curvature
  • Meander Arc Angle
  • Riffle-Pool Spacing
  • Channel Width

Channel Sinuosity and Meander Belt

Sinuosity is the ratio of the length of the centerline of the channel (CL) to the length of a line defining the general trend of the valley or stream reach (VL) and describes the amount of meandering in a stream.

Sinuosity = CL / VL

Channel Planform Characteristics

Some commonly used relationships for planform in natural stable systems are as follows:

  •   10 to 14W
  • Riffle spacing  5 to 7W or  ½ 
  • rc  2 to 3W

In general pools are located in bends, riffles are located near crossings. It should be noted that the relations for wavelength and radius of curvature have been most often been identified in stable natural systems and should be used with discretion in the urban environment. This is because impervious watershed conditions accelerate the erosion process and can cause a shift from the natural condition. However these relationships are used as a starting point for many channel reconstruction projects. They can be used to determine whether a system is “out-of-phase” and provide design targets for stabilization projects. From historical observation and common planform relationships the Stream Restoration staff are able to ascertain the probability of bank retreat in a particular area.

Channel Geometry and Profile Channel geometry refers to the cross sectional and longitudinal parameters that affect the amount of channel conveyance and hydraulic forces on the channel boundary. Some common channel geometry parameters are:

  • Channel Width
  • Flow Area
  • Hydraulic Radius
  • Hydraulic Depth
  • Depth of Flow (maximum depth)
  • Width/Depth Ratio
  • Bank Height
  • Channel Profile

The channel geometric parameters vary throughout a stream reach depending on location these can be averaged to estimate the "reach-average" conditions for certain types of evaluation and analysis.

Relationships that relate channel geometry to hydrology are termed “regime equations” and are based on observations of a large group of streams. These relationships usually take the form of:

  • W = aQb
  • d = cQf
  • V = kQm
  • S = fQz

For width (W), depth (d), velocity (V) and slope (S)

As with planform channel geometric relations are only relevant in stable systems and should be used with discretion in the urban environment. In areas with rapid land-use change such as developing watersheds relationships such as these may be useless for design. However they may be used for comparative purposes. In older or undeveloped watersheds they may prove more functional. In general more detailed analyses are required to determine appropriate stable channel geometry in areas where watershed land use has been altered.

Drainage area is also used as a surrogate to discharge in channel geometry relationships. It has been observed that the trend is and upward shift in the relationship for channel width to drainage area as a result of urbanization.

The channel adjustment process resulting from urbanization can also be expressed with incised channel evolution model proposed by Schumm (1984).

The critical bank height at which mass failure begins is described as hc. when the bank height (h) exceeds hc (Stages II - III) geotechnical failures can be expected.

Observations in Austin indicate that the progression from Stage I to II occurs quite rapidly (10- 30 years) and the widening and restabilization process (Stage IV – V) occurs over a much longer time frame. Most of our urban streams that have been impacted are currently in stages II & III. This identification allows us to utilize other empirical and analytical methods appropriately. The channel evolution model serves to tell us:

  • Where the Channel has Been
  • Where the Channel is in its Evolution
  • Where the Channel is Going

It is important to identify the channel stage of evolution in order to develop appropriate mitigation strategies and reduce future adverse impacts.

Bed Material Characteristics The size, shape, composition and distribution of material in the channel bed are important to the channel stability. These characteristic are used to determine the mobility of the channel bed and subsequently the erosion potential. In general larger sediment sizes (cobbles/boulders) act to stabilize the channel bed, where smaller particles (sand/silt) are more readily erodible. The distribution of particle sizes in bed material mixtures affects the ability of water to mobilize these sediments. The characteristics of the bed material are analyzed through visual observation and gradations developed from sieve analyses or pebble counts.

Bed Material Gradation Curve

A well-sorted sediment mixture consists of grains that are of uniform size and a poorly-sorted sediment contains particles of many sizes. A poorly-sorted sediment may be indicative of a high energy/flashy system. A poorly-sorted stream may also include large particles that armor the channel bed.

The shape of the bed material affects its stability. Angular particles will provide more stability than rounded particles because of the interlocking and friction characteristics.

The chemical composition of the bed material particles affect how it breaks-down, changes size and shape as the material moves downstream. Weaker materials such as shale and limestone degrade faster than quartz-based sediments.

Bank Composition The type of material and stratigraphy in a channel bank affects its erosion potential. Bank stratigraphy is identified and measured in the field. Geotechnical analyses are performed to analyze the strength characteristics of the bank materials. Many channels in Austin are comprised of composite channel banks with bedrock, clay, alluvium and soils.

Composite Channel Bank in Shoal Creek

Composite Channel Bank in Onion Creek

Riparian Vegetation Vegetation acts to provide channel stability as the root systems strengthen the bank material and resist erosive forces. Deep rooted plants and trees give internal strength to the soil mass comprising the channel bank. Shallow rotted plants such as grasses provide more erosion resistance to surficial forces from flowing water. In addition riparian vegetation is an essential component of the aquatic ecosystem.

Roots in the Channel Banks

Geomorphology is the study of the features that make up the earth’s surface and their relationship to the underlying geology. A geomorphological study will provides a conceptual picture of coastal processes and the potential behaviour of the coastal system. This includes taking into account changes in the bedrock composition that could affect the potential rate of future coastal evolution. The results tend to be qualitative, rather than quantitative. This section starts with a description of how a sediment budget may be used to provide a view about future beach levels in front of a coastal structure. The section then moves onto describe useful projects that have has a significant geomorphological component, namely Futurecoast and Eurosion and introduces the concept of the coastal tract as a way of approaching very long term coastal evolution.

An example of the geomorphological approach is given by Honeycutt and Krantz (2003)[1] who illustrated how the local geology affected shoreline change rates along the Delaware coast, using data from high-resolution seismic-reflection profiles, cores and historic shoreline positions. They believe that it may be possible to quantify the effect of large-scale changes in geology on shoreline erosion, but not small-scale ones. Honeycutt and Krantz (2003) provide a scientific basis for modifying calculations of past shoreline change rates to estimate future shoreline change rates.

Many geomorphology studies use a range of tools, including predictive numerical models. As such many geomorphology studies are effectively a composite of the different modelling techniques, as advocated by, for example, Cooper and Pilkey (2004).[2]

Sediment Budget

Sediment budgets are often constructed to assist with coastal management. A sediment budget allows an estimate to be made of the rate of accretion or erosion of sediment within a pre-defined area of the coastal zone (see Rosati 2005 [3], for a recent review). The main steps involved in constructing a sediment budget are:

  • Set appropriate boundaries for the sediment budget and for internal boundaries that separate sub-cells within the overall area to be considered;
  • Identify sources, pathways, stores and sinks of sediment within the budget area;
  • Calculate the rate of erosion from sources and stores and accretion in stores and sinks. These estimates may come from numerical models but are more likely to be derived from data;
  • Calculate the sediment transport rates at the boundaries of the subcells and estimate the uncertainty in each transport rate. The calculations of transport rate may come from data but are more likely to be derived from numerical models; and
  • Integrate the gains and losses within each section to obtain an overall sediment budget.

A good sediment budget will provide a useful indication of whether a beach in front of a coastal structure is likely to be subjected to beach lowering due to loss of sediment from the entire beach. Even if this is not the case and beach volumes have been constant or increasing, a coastal structure may be subject to beach lowering due to local effects.

Futurecoast

Futurecoast (Burgess et al., 2002 [4]) was commissioned by the UK Department for the Envoronment, Food and Rural Affairs (Defra) to improve the understanding of coastal evolution for the open coast of England and Wales. Futurecoast is the obvious starting point for any assessment of future coastline behaviour over decadal timescales for these coastlines. It contains:

  • Shoreline behaviour statements that give an improved understanding of coastal behaviour and qualitative predictions of future coastal evolution at both large and small scales;
  • Assessment of future behaviour for an unconstrained scenario (with no defences or management) and a managed scenario (where present management practices continue indefinitely); and
  • A ‘toolbox’ of supporting information and data including cliff behaviour statements, historical shoreline changes, wave modelling, an uncertainty assessment, morphological measurements including beach width, a coastal geomorphology reference manual and a thematic studies on onshore geology, offshore geology, coastal processes, climate change and estuaries.

Eurosion

Eurosion (European Commission, 2004[5]) was a European study into coastal erosion at a European scale. Its outputs were:

  • A map-based assessment of European coasts exposure to coastal erosion;
  • A review of existing practices and experience of coastal erosion management;
  • Guidelines to incorporate coastal erosion into environmental assessment, spatial planning and hazard prevention; and
  • Policy recommendations to improve coastal erosion management.

Eurosion’s maps can be used to assess the coastal typography, geology and coastal erosion trends of a region. The maps also include the location of engineering works (whether harbours, jetties groynes or breakwaters). There is an additional map for regional exposure to coastal erosion.

Eurosion concluded that a more strategic and proactive approach to coastal erosion is needed for the sustained development of vulnerable coastal zones. It developed the concept of coastal resilience: the inherent ability of the coast to accommodate changes induced by sea level rise, extreme events and occasional human impacts, whilst maintaining the functions fulfilled by the coastal system in the longer term. To promote coastal resilience, Eurosion introduced the concept of favourable sediment status: the situation where the availability of coastal sediments support the objective of promoting coastal resilience in general and of preserving dynamic coastlines in particular. This should be achieved for each coastal sediment cell by designating strategic sediment reservoirs: supplies of sediment of appropriate characteristics that are available for replenishment of the coastal zone, either temporarily (to compensate for losses due to extreme storms) or in the long term (at least 100 years). They can be identified offshore, in the coastal zone (both above and below low water) and in the hinterland.

A coastal sediment cell is a coastal compartment that contains a complete cycle of sedimentation including sources, transport paths, and sinks. The cell boundaries delineate the geographical area within which the budget of sediment is determined, providing the framework for the quantitative analysis of coastal erosion and accretion. Eurosion considered that coastal sediment cells constitute the most appropriate units for achieving the objective of favourable sediment status and hence coastal resilience (European Commission, 2004).

Coastal tract modelling

Coastal evolution over centuries to millennia requires a broader outlook than for shorter time and space scales, in that the shoreline evolution must be linked to the behaviour of the continental shelf and coastal plain (Cowell et al., 2003[6]). The coastal tract is the combination of lower shoreface, upper shoreface and back barrier that comprises the first-order system within a coastal tract cascade, whereby greater and greater levels of detail are needed to model coastal evolution at shorter and shorter time-scales and space-scales (Figure 3). The coastal tract cascade formalises concepts for separating coastal processes and behaviour into a scale hierarchy. An additional concept, that of coastal tract templating was also introduced by Cowell et al. (2003[7]) to provide a protocol for defining a site-specific problem by transforming data into a data model.

The aggregate dynamics of the coastal tract are modelled using behaviour-orientated coastal change models and constrained by sediment mass conservation (Cowell et al., 2003). The rate of coastal advance is governed by the balance between the change in sediment accommodation space caused by sea level rise and sediment availability.

References

  1. ↑ Honeycutt, M.G. and Krantz, D.E., 2003. ‘Influence of the geologic framework on spatial variability in longterm shoreline change, Cape Henlopen to Reheboth Beach, Delaware’, Journal of Coastal Research, Special Issue 38, 147-167.
  2. ↑ Cooper, J.A.G. and Pilkey, O.H., 2004. Alternatives to the mathematical modelling of beaches. Journal of Coastal Research, 20(3) 641 – 644.
  3. ↑ Rosati., J.D., 2005. Concepts in sediment budgets. Journal of Coastal Research, 21(2) 307 – 322.
  4. ↑ Burgess, K.A., Orford, J., Townend, I., Dyer, K. and Balson, P., 2002, ‘FUTURECOAST: the integration of knowledge to assess future coastal evolution at a national scale’, In Proceedings of the 28th International Conference, Coastal Engineering 2002. McKee Smith (Ed), World Scientific, pp 3221 – 3233.
  5. ↑ European Commission, 2004, ’Living with coastal erosion in Europe – Sediment and space for sustainability’, Luxembourg office for official publications of the European Commission. 40 pp ISBN 92-894-7496-3.
  6. ↑Cowell, P.J., Stive, M.J.F., Niederoda, A.W., de Vriend, H.J., Swift, D.J.P., Kaminsky, G.M. and Capobianco, M., 2003. The coastal-tract (part 1): a conceptual approach to aggregated modelling of low-order coastal change. Journal of Coastal Research, 19(4): 812 – 827.
  7. ↑Cowell, P.J., Stive, M.J.F., Niederoda, A.W., Swift, D.J.P., de Vriend, H.J., Buijsman, M.C., Nicholls, R.J., Roy, P.S., Kaminsky, G.M., Cleveringa, J., Reed, C.W. and de Boer, P.L., 2003. The coastal-tract (part 2): Applications of aggregated modelling of lower-order coastal change. Journal of Coastal Research, 19(4): 828 – 848.

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