GEOL 108Lg 1st Edition Lecture 15Outline of Last Lecture- Hydrologic Cycle, Concept of Residence Time- Climate and Rain - Dams and Rivers/Water UsageOutline of Current Lecture- Finish River Morphology- Hydrographs- Floods and Flood Frequency- Erosion and Siltation- Groundwater and Groundwater Flow- Overdrafts- ContaminationCurrent LectureWater and Energy IICross-sectional area = channel depth x channel width. If you have taken more than one measurement of channel depth you can calculate the mean. Alternatvely you can plot the width and depth readings on graph paper, then count the area of the stream. In contrast to braided rivers, meandering rivers typically only contain one channel that winds its way across the floodplain. As it flows, it deposits sediment on banks that lie on the insides of curves (point bar deposits), and erode the banks on the outside of curves.Rivers meander (twist and turn) when they are traveling on top of a relatvely flat surface. The reason they do this rather than flow in a straight line is that a straight line is n o t the most eficient path for water to flow. The friction against the surrounding earth causes the river tomove this way.Meandering patterns move downstream. The leading edge is deep, and the trailing edge is shallow. If the land rises, the river cuts deeper and no longer meanders, technically. Instead, itexhibits an entrenched meander. In other words, it is fxed into its previously meandering pattern. The Grand Canyon is a dramatc example.Surface RunofSurface runof is water, from rain, snowmelt, or other sources, that fows over the land surface, and is a major component of the water cycle.Ratng Curve (river height vs. discharge).1. Determined by measuring velocity profle in stream at various discharges2. Q = v*W*D3. Non-linearity results from shape of channel (area and velocity both increase with height of water). Factors influencing hydrographsRainfall - intensity and duratonSize of basinBasin character determines fraction that infltrates or runs of. (topography, impermeable area, soil type, antecedant rain, vegetation)Ratonal Equaton: Q = CIA where C = constant (depends on basin character), I = rain intensity (m/s), A = basin areaEfect of humans on basins:a) Change vegetaton (trees to farms)b) Increase impermeable area (build cities)c) Build damsChanges (a)&(b) decrease lag time and increase area, (c) = opposite. Many urbanized watersheds are now flooded more frequently. Activities upstream have downstream impacts.Flood ForecastngHow can we estmate magnitude of large, infrequent events?a) Install stream gauges, measure hydrograph for a number of years.b) Identfy the highest discharge for each year and list its rank in descending order.c) Calculate recurrence interval for the highest peak flow for each year. RI is the average length of time between flows that exceed this magnitude: RI = (N + 1)/ Mwhere N = years of record and M = rank for this flow. Note that: Frequency = 1/(RI) = annual probability of this flowFactors Contributing to Floodinga) heavy rain b) snow melt c)damfailured) modification of flood plains and levees e) coastal floods from storm surgeErosion and SiltationSiltation is the polluton of water by fne partculate te rrestrial c l astc material,with a partcle size dominated by si l t or cl a y . It refers both to the increasedconcentration of suspended sediments, and to the increased accumulation(temporary or permanent) of fne sediments on bottoms where they areundesirable. Siltation is most often caused by s o il e r o s i o n or sediment spill.SaltationIn geology, saltation is a specifc type of partcle transport usually caused by fluids suchas wind or water. Saltation happens when loose material are removed from a bed, then carried by the fluid before being transported back to the surfaceEx. pebble transport by rivers, sand drift over desert surfaces, soil blowing over fields, snow drift over smooth surfaces.At low fluid velocites, loose material rolls downstream, staying in contact with the surface. This is called creep or reptation. Here the forces exerted by the fluid on the partcle are only enough to roll the partcle around the point of contact with the surface.Deposition of SedimentThis will occur where velocity decreases, because the amount of sediment a parcel of water can carry depends on the water velocity.Groundwater OverviewKey Points - about 50% of drinking water is from groundwater. We want to know about:1. Flow path taken by water2. Budgets for water3. Overdrafts4. ContaminatonKey Terms to know•water table•unsaturated zone (vadose zone)•saturated zone (phreatc zone)More Terminology- permeability = measure of ability to transmit water- porosity = fractons of pore volume- aquifer = unit capable of storing or transmitting useful quanttes of water- aquitard = unit with low flow- confned aquifer = unit bounded above by an aquitard- unconfned aquifer = unit connected with overlying atmosphere- recharge = entry of water into an aquifer- Good aquifers: sands and gravels, jointed units, sometimes Limestones and basalts.- Poor aquifers = fne grained unitsPrinciples of Groundwater Flow•Piezometer = pipe in ground to measure head.•head = height relatve to a level surface) to which water would rise in a piezometer. Water table topography defnes head at different locations in an unconfned aquifer.•Equipotental Surface = surface of constant head. Note that thewater table is NOT an equipotental surface.•Flow in groundwater is perpendicular to equipotential lines.Overdraft: Occurs if more water is withdrawn than is recharged. Results in falling water table.LA GROUNDWATER BASINSGroundwater Contaminants Common Contaminants (selected)1.Solvents2. Hydrocarbons (petroleum products)3. Salt (seawater intrusion, road salt, soil salinization)4. Arsenic (natural)5. Perchlorate6. Agricultural chemicals and waste7. Bacteria (sewage, septc systems)8. Landfll and mine tailings leachate (acidic)Parts of San Gabriel and San Fernando Valley aquifersare superfund cleanup sites (mostly