Clark Atlanta University Department of Physics

INTRODUCTION TO SURACE HYDROLOGY NEPRIS SESSION ON 16 MAY 2017 FIELD TRIP TO HENDERSON WATERFALL

Surface hydrology is a science that deals with streams, rivers, lakes, other small water bodies, as well as water in the lower atmosphere (solid, liquid, and gas), water in the ground, and even glacial flow. Related disciplines include geomorphology (the shape of landforms), hydrogeology (groundwater geology), boundary layer and micro meteorology (lower-atmosphere meteorology), speleology (science of caves), and glaciology (science of glaciers). Stream flow will be studied on the field trip to Henderson Waterfall. The term stream will be more or less interchangeable with river in this discussion. A stream is classified into three zones: head, transport, and deposition. The source or beginning of a stream is called the head or headwater region. The head is typically the highest elevation above sea level in a stream and is the zone where sediments are generated. Sediments are particles (or grains) of mineral or rock removed from a land surface by erosion. The process of erosion is described below. The zone of sediment transport is the middle part of a stream where sediments are being deposited to the stream bed (bottom) at the same rate as they are being removed by erosion. The zone of sediment deposition is at the mouth of the stream, and is the place where sediments are deposited to the stream bed. The stream mouth is where the stream ends, typically at a larger stream, a river, a lake, or the world ocean. Important attributes of stream flow include stream velocity, discharge, capacity, competence, and erosional and depositional characteristics. Stream discharge (D) is the quantity of water carried by a stream per unit time (such as an hour). Discharge is influenced by channel width (w), channel depth (d), and stream velocity (v). Mathematically this relationship takes on this expression: Discharge = D = channel width  channel depth  stream velocity = w  d  v From the units associated with each term in the equation, it is obvious that discharge is measured in units of volume per unit time. Examples would include feet3/second, meters3/minute, feet3/hour, or even meters3/year. Additional factors influence stream discharge: Channel cross-sectional area = A = channel width  channel depth Channel roughness = R Channel gradient = G Observed discharge behavior along a stream can be related to these factors through proportionality relationships, where the symbol  means "proportional to." D  A D  1/R D  v D  1/G The first type of proportionality (e.g., D  A) is called a direct proportion because discharge increases as cross-sectional area increases. The second type of proportionality (e.g., D  1/R) is called an inverse proportion because discharge decreases as channel roughness increases. The following discussion explains why these relationships are true. The headwater of a stream is often in the mountains or another elevated location. Near the headwater a stream channel is typically smaller than downstream, which means that the cross-sectional area is smaller. A stream with a smaller cross- sectional area generally does not transport as much water per unit time as does a larger stream. Since the quantity of water transported per unit time is the definition of stream discharge, a smaller stream is generally not as capable of delivering as great a discharge under normal (non-flood) flow conditions as a larger stream is. Channel size increases in humid regions from stream head to stream mouth to accommodate an increasing number of tributaries. Tributaries are smaller streams that feed larger streams or rivers. Tributaries add water volume to the larger stream and generally increase its cross-sectional area. Larger channels provide less resistance to flow, and therefore streams and rivers deliver greater discharges downstream than upstream. Since headwaters are often in mountainous regions, their elevation above sea level is greater than in the other zones of a stream. Mechanical weathering (the breakdown of a rock or mineral by fracturing) produces rock and mineral fragments of various size ranges most rapidly in the mountains. The smallest particles are called clay, and their largest dimension is less than 1/256th of a millimeter (mm). Particle size ranges grade upward from clay into silt (1/256 mm to 1/16 mm), sand (1/16 mm to 2 mm), granule (2 mm to 4 mm) pebble (4 mm to 16 mm), cobble (16 mm to 256 mm), and boulder ( >256 mm, or about 10 inches). The faster that rainwater, wind, or glacial ice can transport particles downhill, the larger the size of the particle that can be carried in this erosional flow. Erosion is the combined effect of weathering and transport. Since the largest rock fragments are produced by weathering in mountainous areas and transported by erosional flow to stream channels, the channel beds are typically rougher in the headwater region. Just as small channels offer greater resistance to flow than do large channels, rough beds offer greater resistance to flow than do smooth beds. Both factors cause the flow velocity to be lower near the stream head. Channel roughness decreases downstream because smaller flow velocities Copyright © 2017 by Randal L. N. Mandock 1 16 May 2017 upstream allow deposition of the larger sediments (e.g., boulders, cobbles, pebbles, gravel) in the headwater region, while the smaller sediments (sand, silt, clay) continue to be transported downstream. Deposition of smaller sediments downstream allows for a smoother channel near the mouth than near the headwaters. This explains the observed decrease in channel roughness from river head to mouth. Smoother channels offer less resistance to flow and deliver greater discharges downstream than upstream. As channel size increases towards the river mouth, a smaller fraction of water is in contact with the channel walls and channel bed downstream than upstream near the headwaters. This smaller fraction of water contact and the smoother channel bed combine to produce a greater flow velocity downstream. The increase in velocity downstream and the increase in channel size result in an increase in river discharge from head to mouth. The typical longitudinal profile (a side-view of the stream from head to mouth) of a river or stream shows a steeper topographic gradient in the headwater region than downstream near the mouth. Even though stream gradient decreases from head to mouth (which would be expected to cause a decrease in flow velocity), the increase in channel size and decrease in channel roughness more than compensate for the decrease in gradient in their impact on river discharge. Thus the combined effects of larger channel size, decreased bed roughness, and higher flow velocity overcome the decrease in gradient and allow a larger discharge at the mouth than at the head. The material carried in a stream is called the stream's load. There are three types of stream load: suspended, bed, and dissolved. The suspended load consists of particle sizes small enough to be suspended in the stream flow and transported downstream without significant contact with the channel walls. These particle sizes are typically clay, silt, and sand. The bed load consists of particle sizes too large to be suspended in the flow at a given flow velocity. These particle sizes are typically granule, pebble, cobble, and boulder. Granules, pebbles, and the smaller cobbles are generally known as gravel. The bed load is transported along the stream bed by rolling, sliding, and bouncing (also known as saltation). The dissolved load is composed of ions from dissolved solids. Whereas simple straining can remove the suspended and bed loads from a sample of stream water taken from near the stream bed, the dissolved load can only be extracted by evaporation or distillation. Stream capacity is the maximum load of solid particles a stream can transport per unit time (Tarbuck and Lutgens, 2015). The relationship of capacity to discharge is a direct proportion: capacity  discharge. Competence measures the maximum particle size a stream can carry. For particles between about 0.5 mm to 50 mm, stream competence is found to be approximately proportional to v2 (competence  v2). This means that when the stream velocity doubles, the maximum particle size the stream is able to transport quadruples (22 = 4). When the stream velocity quadruples, its competence increases by a factor of 42, or 16. This explains why a small stream is capable of moving boulders in its bed load during flood conditions. Stream competence can also be related to particle mass. If stream competence is compared to particle mass, the proportionality relationship is: competence  v6. A base level of a river or stream is a level at which deposition of sediments can sometimes occur. A useful definition is that the base level is the lowest elevation a stream can erode its bed to (Tarbuck and Lutgens, 2015). There are two types of base level: ultimate and temporary or local. The ultimate base level of a stream is usually the world ocean (i.e., sea level), but in some cases it is an inland sea (e.g., the Dead Sea in Israel and the Salton Sea in California). Temporary base levels are local and can change location with time. They are found where a smaller stream or river empties into a larger one, where a stream enters a lake, where a stream pools and deposits sediments on a resistant rock layer (often at the top of a waterfall), and where a stream encounters a dam. Stream valleys are found in the shape of a "V" in transverse cross section. Transverse means across the stream perpendicular to the flow. A bend in a stream is called a meander. Rivers and streams high above sea level are capable of converting their potential energy of elevation into kinetic energy (i.e., energy of motion). These streams rapidly erode their beds by the cutting action of the bed load downward towards sea level. The Grand Canyon is an example of this type of erosion, and its meanders are called incised meanders because they are being cut downward by the Colorado River. A cut bank is a location in a stream where the valley wall is being eroded by the stream. Cut banks are produced on the outside of a bend (meander) in a stream where flow velocity and possible turbulence are greatest. A point bar is a location in a stream where sediments are deposited. Point bars form on the inside of a meander where flow velocity is least and larger particles drop out of suspension to be deposited on the stream bed.

Reference Earth Science, 2015, 14th ed., Prentice Hall, by Edward J. Tarbuck and Frederick K. Lutgens, pp. 131-169.