Describe how quantification within the drainage basin can help to lessen the impact of hazards occurring within it. (25 marks)

The drainage basin is an area drained by a river and its tributaries, the edge of this area marked by an area of high ground where any precipitation falling will flow into one of the surrounding drainage basins. There may be several types of drainage basin from a small stream such as the one running parallel to Brook Street in Tring or something as large as the Mississippi drainage basin, covering several states in the United States of America. Recognising that the drainage basin can be a very large area indeed, it is important to recognise that there are hazards involved with it. By quantifying (or measuring) within the drainage basin it may be possible to predict the likelihood of hazards. Hazards within the drainage basin may range from simple flood or drought to more obscure Weils Disease in stagnant lakes or ponds.

It may be useful to think of the drainage basin as an open system with inputs, processes and outputs (see overleaf). Within this system every variable can be quantified. Perhaps most important is the only input to the system - precipitation. While the measurement of precipitation within the drainage basin is relatively easy, readings almost always differ as a result of their location in the drainage basin. This is easily overcome by taking an average of measurements from each rain gauge in the drainage basin. Commonly, a rain gauge is merely a cylinder in which rain water is collected and then a measurement in millimetres is taken. An average of these figures can then be taken. However, while knowing the amount of precipitation falling within the drainage basin, this figure can in no way predict hazards as it stands on its own.

Precipitation is intercepted, be it by the ground or by objects that lie above the ground, such as Vegetation or by Urban objects. Should gutters, drains, roofs and so on intercept rainfall, then due to the impermeable nature of the features, water runs overland into gutters and finally into river channels. When vegetation intercepts rainfall it takes one of two courses: Stemflow - i.e. flowing down trunks, etc. - or Throughfall - falling off leaves. Ultimately when water falls in the vicinity of vegetation it is most likely that some will be taken up by plant roots. When the plant photosynthesises, it transpires, letting moisture out of the plant through stomata. The collective term for the evaporation and transpiration of water in Evapotranspiration. The rate of Evapotranspiration is hard to quantify due to the affects of temperature fluctuations and pressure changes. For instance, most water will be evaporated or transpire in summer when temperatures are hot and pressure high.

Precipitation that is not taken up by plants or taken straight to the river channel by man made means will run off in natural channels or be infiltrated by soil. Infiltration is the movement of water into soil and the rate at which this occurs relies upon several factors. The infiltration capacity can be affected by rock or soil type. For example, if a rock is permeable (such as limestone) then infiltration will be quick and easily stored in cracks or pore spaces. If, on the other hand, rock is impermeable (clay for example) then infiltration will be less rapid and will lead to more overland flow and the rocks in ability to infiltrate water means that the risk of flooding is heightened. The rock and soil types also have an affect on the grounds field capacity. This is the amount of water that the soil can store once all excess water has run off. Should the ground exceed the field capacity then it becomes saturated and the flooding is incredibly likely. Alternatively if weather has been particularly cold and ground is frozen, or particularly hot and the ground is baked dry, then the infiltration capacity will be reduced and overland flow or flooding is increasingly likely.

It is by taking several quantifiable features of the drainage basin, combining and subsequently interpreting the information they give, that we can most accurately predict hazards. The water balance shows the state of equilibrium in the drainage basin between the inputs and outputs as a graph, expressed as:

where P = precipitation; Q = runoff; and E = Evapotranspiration. The graph below shows the water balance for an average British year.

It is important to notice that for several months in the year Evapotranspiration exceeds precipitation, while during most of the year precipitation, exceeds Evapotranspiration. During the period when precipitation exceeds Evapotranspiration there is a water surplus as more water is being input into the system than in being output. During summer months when Evapotranspiration exceeds precipitation soil moisture is reduced due to plants and humans utilising the water stored in the soil for photosynthesis. However, there is never a water deficit as precipitation levels are so high throughout the year that during this time there is enough moisture in the soil to accommodate the needs of plants and humans. When precipitation again exceeds Evapotranspiration in August there is not a water surplus immediately as the soil must recharge itself, not until soil moisture reaches the field capacity will the drainage basin flood.

In other parts of the world where the balance between precipitation and Evapotranspiration differs, the graph will represent something quite different. For example, in desert regions, Evapotranspiration is most likely to exceed precipitation for most of the year meaning that precipitation is never enough to replenish the soil. Therefore the soil will never reach the field capacity and there will always be a water deficit, or drought. Alternatively, should an precipitation always exceed Evapotranspiration then the soil will be saturated throughout the year and there will be a water surplus and therefore flood.

Infiltration may also be affected by the nature of the precipitation. Prolonged rainfall will saturate ground and infiltration will be replaced by surface run off - increasing the likelihood of flood. Intense storms such as convectional thunderstorms may be more intense that the infiltration capacity of the soil meaning more surface run off and the rising water level in rivers. This is often likely to create flash floods such as that in Biescas in the Spanish Pyranees, 1996. Flash Floods and mudflows were caused by torrential rain two days before the floods falling on ground that had been baked dry by the hot summer weather. The soils infiltration capacity could not cope with the torrential rain, at one point three inches of rain fell in two hours. The affects of the flood were worsened by the dry soil being easily eroded sending a wave of mud down the Gallego valley and over the campsite of Virgin de Las Nieves, killing over 20 people. Such a disaster could have been avoided had the owners of the campsite known that such and area was prone to such flooding at that time in the year. By using data collected from previous years and applying such data to the water balance we can easily predict possible hazards, in particular, flood or drought.

Another way to predict the effect of torrential rainfall is study a storm hydrograph, as shown over leaf. There are many controls on the shape of such a graph, for instance the time in which it takes for the drainage basin to reach peak discharge from the peak rainfall (lag time). Hydrographs show the response of a river to a rain storm in the drainage basin. The size, shape and relief of the drainage basin help us to predict the shape of the river to the storm. If the basin is small then precipitation reaches the main channel quickly and has a shorter lag time. Should the basin be large then the lag time will be considerably longer due to the further distance the water must travel. Similarly if the basin is circular then there will be a short lag time and higher peak flow as all points on the water shed are equidistant from the gauging station. If the basin is elongated then there is a longer lag time and a lower peak flow because the water must travel further giving a longer time for infiltration or Evapotranspiration to take place.

a circular drainage basin                                                         an elongated drainage basin

The relief of the drainage basin will effect the hydrograph. Simply, if the drainage basin has steep sides then water will reach the gauging station quicker than a drainage basin with gentle slopes. This said the type of drainage basin will affect the hydrograph. By this we mean that the morphometry, or form, of the drainage basin will affect the hydrograph and the drainage basin’s ability to drain water. Horton devised ‘The Laws of Drainage Composition’ which established a hierarchy of streams ranked in according to ‘order’ - where all initial unabranched streams are first order, becoming second order at the confluence of two first order streams (see below). Horton attempted to correlate several parameters within the drainage basin by means of quantification. He suggested that: the correlation between stream order and the number of streams was negative, meaning that there is likely to be a high order stream if there are many lower order streams; the correlation between mean stream length and stream order is positive, meaning as stream order increases their length rises as well; and finally, the correlation between mean drainage are and stream order is positive, meaning the larger the basin the greater the likelihood of a high order stream.

The amount of streams in the drainage basin also relies on many factors. The ability of the soil to infiltrate water will either increase the amount of streams (if the infiltration capacity is low) or decrease the amount of streams (if the infiltration capacity is high). Similarly in times of drought the amount of high order streams will decrease due to the lower amount of water and therefore fewer streams all together as some parts of the basin do not need to be drained. In times of flood the amount of streams increases to cope with the excessive amount of water in the basin that requires draining. Like many features of the drainage basin the order of streams is not an accurate guide as to the prediction of hazards, but we can infer from the morphometry of the drainage basin that the variation of stream order can help identify hazards and predict areas of the drainage basin that are prone to flooding or drought.

The regime of the river is the term used to describe the annual variation in discharge. This is a graph similar to the storm hydrograph but it covers a much greater time (the whole year) as opposed to the short time the storm hydrograph covers. The graph clearly shows the relationship between mean annual precipitation and total discharge. Being able to predict the annual rainfall and the discharge of the river helps predict when hazards are likely to occur. For instance when discharge is low it could be assumed that the drainage basin will experience drought conditions and when the discharge is high it can be inferred that the drainage basin will experience flood conditions.

As the river channel is the main way in which water is discharged from the drainage basin the elements involved with it are also important. The velocity of the river is perhaps a feature that is often over looked. Of course some parts of the drainage basin will have areas where the velocity of the water is high causing great hazard to humans. Drowning in the river may only occur in higher order rivers which are commonly deeper, and faster flowing than lower order streams which encourage recreational activities such as boating or swimming, while in lower order streams or pools of water also contain many invisible hazards. Stagnant pools of water, or water that is not flowing are often homes to disease such as Malaria in wells in African countries or Weils Disease in Canals in Britain. While any area of water is hazardous to animals if not treated with due care, the existence of bacteria in water is a problem that can only be avoided by knowing areas prone to such bacteria. Quantifying the velocity of the river, or pool, will guide us when investigating unsafe areas. These problems can be easily overcome by posting signs in hazardous areas.

The channel itself, or more specifically what is in the channel, can also be quantified. The ability to transport water and sediment depends on the shape of the channel. If the channel is deep and narrow, then it will be most efficient at transporting water and therefore draining the basin. If the channel is shallow and wide, the its ability to transport water is poorer meaning that it will be more prone to flooding due to its inefficiency to move water. This is as a result of a greater wetted perimeter or the area in contact with the banks. Friction cause the movement of water and sediment to slow. By taking the cross sectional area and wetted perimeter, the hydraulic radius can be found, this shows how efficient a river is at transporting water and sediment. The hydraulic radius is worked out by:

The higher the figure, the more efficient the river is at transporting material. It is this that can tell us how efficient the river is at transporting water and sediment during different conditions (e.g. Bankfull Discharge or Flood, Normal flow, or drought). Should the river be inefficient at moving water then it may be assumed that during bankfull discharge the river will be unable to cope with excess water and will therefore break it banks.

It is possible that using all the quantifiable features in the drainage basin can help us take steps to avoid hazards. During times of drought the obvious solution may be to use water sparingly. However, major problems arise when the drainage basin is in flood conditions. Flooding causes great damage to arable farmland let alone to houses and urban feature. In the developing world where land comes at a premium, many of the poorer classes are forced to make poor quality housing in the flood plain of high order river. During times of particularly heavy precipitation the flood plain often becomes covered in water, destroying the poor housing. 89% of Bangladesh’s population are subsistence farmer, in rural communities, and have no means of moving to a less disaster prone area. Following floods in 1974 and 1984, the Red Cross estimated that 40000 people drowned due to overpopulation on the flood plain.

There is little that can be done to such a large river to avoid the disastrous nature of flooding. On a smaller scale, 10 kilometres north-west of Windemere, Langdale Beck (in its upper stages - Oxendale) is particularly prone to flooding. The braided, wide channel at Oxendale was particularly problematic in times of flood. While the load, velocity, and mass of the water in the channel is considerable less than in the lower stages, the area is none the less effective at eroding the channel and flooding. To over come the problem of flooding the channel was dredged of excess bedload and deepened, making the hydraulic radius greater. The banks were then fortified with concrete to avoid lateral erosion and the creation of great loads. On a similar, but much larger scale, the River Thames underwent dredging and reinforcement to prevent flooding London. The Thames Barrier prevents any tidal or storm surges causing the river to flood.

In the Santa Clara drainage basin in southern California several features of the drainage basin are quantified to help provide an early warning system for flooding. This ultimately worked in 1980 when over 5 inches of rain fell in 24 hours. As the ground became saturated, water levels rose leaving the flood plain under water. As the water was discharged from the drainage basin river channels were changed, channels became choked, new channels were created, and bridges and roads were destroyed. There was little loss of life due to evacuation of particularly hazardous regions.

In conclusion the drainage basin is a complex object, involving may processes and features that can be quantified. The interpretation of such data and the ability to interpret the information as visual aids (graphs) helps give an idea as to the likelihood of hazards. These hazards can range from the existence of bacteria in stagnant water - Malaria and Weils Disease - to more obvious hazards such as drought or flooding. The affects of flooding are arguably more devastating that those of drought in the UK, but for countries such as Ethiopia drought and the existence of bacteria are far more devastating. Of course, as with any hazard the ramifications of a hazard will be far worse in a less economically developed country.