Presentation on theme: "1 Soils & Hydrology II 9.Soil Water 10.Precipitation and Evaporation 11.Infiltration, Streamflow, and Groundwater 12.Hydrologic Statistics and Hydraulics."— Presentation transcript:
1 Soils & Hydrology II 9.Soil Water 10.Precipitation and Evaporation 11.Infiltration, Streamflow, and Groundwater 12.Hydrologic Statistics and Hydraulics 13.Erosion and Sedimentation 14.Soils for Environmental Quality and Waste Disposal 15.Issues in Water Quality
2 What is the significance of understanding streamflow? Why are we concerned with how it relates to Landscapes? Streamflow is important because it is related to: –Construction of houses, bridges, spillways, and culverts –Surface Runoff over landscapes, including flooding –The associated processes of Erosion, Transport, and Deposition. –Drinking and Irrigation water supplies, especially during droughts –Recreational activities, such as boating and fishing –Navigation of commercial shipping and transport
3 Hydrograph: –Plots precipitation and runoff over time. –Runoff can be discharge, flow, or stage
16 Hydrograph Behavior: Also related to channel patterns
17 Measurement Units cfs: cubic feet per second gpm: gallons per minute mgd: million gallons per day AF/day: Acre-Feet per day cumec: cubic meters per second Lps: liters per second Lpm: liters per minute 1 cfs –2 AF/day –450 gpm –28.3 Lps 1 m 3 /s = 35.28 cfs 1 mgd 1.5 cfs 1 gpm = 3.785 Lpm
18 WEIR: Used to provide accurate flow measurements
20 Weir Types Circular opening: Q = c r 2 h 1/2 Rectangular: Q = c W h 3/2 Triangular: Q = c h 5/2 where Q is flow, cfs c are weir coefficients h is stage, ft r is the pipe diameter, ft W is the weir width, ft
21 Rectangular Weir V-Notch (Triangular) Weir Coweeta Hydrologic Station
22 Field Velocity Measurements Flow Equation: Q = v A where Q is the discharge, cfs v is the water velocity, ft/s A is the flow cross-sectional area, ft 2
25 Manning's Equation When flow velocity measurements are not available v = (1.49/n) R 2/3 S 1/2 where –v is the water velocity, ft/s –n is the Manning's hydraulic roughness factor –R = A / P is the hydraulic radius, ft A is the channel cross-sectional area, ft 2 P is the channel wetted perimeter, ft –S is the water energy slope, ft/ft
26 Hydrologic Statistics: –Trying to understand and predict streamflow Peak Streamflow Prediction: –Our effort to predict catastrophic floods Recurrence Intervals: –Used to assign probability to floods 100-yr flood: –A flood with a 1 chance in 100 years, or a flood with a probability of 1% in a year.
27 Return Period T r = 1 / P –T r is the average recurrence interval, years –P is exceedence probability, 1/years Recurrence Interval Formulas: –T r = (N+1) / m –Gringarten Formula: T r = (N+1-2a) / (m-a) where –N is number of years of record, –a = 0.44 is a statistical coefficient –m is rank of flow (m=1 is biggest)
28 River Stage: The elevation of the water surface Flood Stage The elevation when the river overtops the natural channel banks.
34 Peak Flows in Ungaged Streams Q n = a A x P n where –A is the drainage area, and –P n is the n-year precipitation depth –Q n is the n-year flood flow Q 2 = 182 A 0.622 Q 10 = 411 A 0.613 Q 25 = 552 A 0.610 Q 100 = 794 A 0.605
36 Curve Number Method Most common method used in the U.S. for predicting stormflow peaks, volumes, and hydrographs. Useful for designing ditches, culverts, detention ponds, and water quality treatment facilities.
37 P = Precipitation, usually rainfall –Heavy precipitation causes more runoff than light precipitation S = Storage Capacity –Soils with high storage produce less runoff than soils with little storage. F = Current Storage –Dry soils produce less runoff than wet soils
38 r = Runoff Ratio => how much of the rain runs off? –r = Q / P r = 0 means that little runs off r = 1 means that everything runs off –r = Q / P = F / S r = 0 means that the bucket is empty r = 1 means that the bucket is full F = P - Qorr = Q / P = (P - Q) / S –the soil fills up as it rains Solving for Q yields: –Q = P 2 / (P + S)
39 S is maximum available soil moisture –S = (1000 / CN) - 10 –CN = 100 means S = 0 inches –CN = 50 means S = 10 inches F is actual soil moisture content –F / S = 1 means that F = S, the soil is full –F / S = 0 means that F = 0, the soil is empty Land Use CN S, inches Wooded areas25 - 83 2 - 30 Cropland 62 - 71 4 - 14 Landscaped areas 72 - 92 0.8 - 4 Roads 92 - 98 0.2 - 0.8
40 Curve Number Procedure First we subtract the initial abstraction, I a, from the observed precipitation, P –Adjusted Rainfall: P a = P - I a –No runoff is produced until rainfall exceeds the initial abstraction. –I a accounts for interception and the water needed to wet the organic layer and the soil surface. –The initial abstraction is usually taken to be equal to 20% of the maximum soil moisture storage, S, => I a = S / 5 The runoff depth, Q, is calculated from the adjusted rainfall, P a, and the maximum soil moisture storage, S, using: –Q = P a 2 / (P a + S) –or use the graph and the curve number We get the maximum soil moisture storage, S, from the Curve Number, CN: –S = 1000 / CN - 10 –CN = 1000 / (S + 10) We get the Curve Number from a Table.
41 Example A typical curve number for forest lands is CN = 70, so the maximum soil storage is: S = 1000 / 70 - 10 = 4.29" A typical curve number for a landscaped lawn is 86, and so –S = 1000 / 86 - 10 = 1.63” A curve number for a paved road is 98, so S = 0.20” Why isn’t the storage equal to zero for a paved surface? –The roughness, cracks, and puddles on a paved surface allow for a small amount of storage. –The Curve Number method predicts that I a = S / 5 = 0.04 inches of rain must fall before a paved surface produces runoff. For a CN = 66, how much rain must fall before any runoff occurs? –Determine the maximum potential storage, S = 1000 / 66 - 10 = 5.15" –Determine the initial abstraction, I a = S / 5 = 5.15” / 5 = 1.03" –It must rain 1.03 inches before runoff begins. If it rains 3 inches, what is the total runoff volume? –Determine the effective rainfall, P a = P - I a = 3" - 1.03" = 1.97" –Determine the total runoff volume, Q = 1.97 2 / (1.97 + 5.15) = 0.545"
45 Unit Hydrograph Example A unit hydrograph has been developed for a watershed –The peak flow rate is 67 L/s for 1 mm of runoff and an area of 100 ha –What is the peak flow rate for this same watershed if a storm produces 3 mm of runoff? The unit hydrograph method assumes that the hydrograph can be scaled linearly by the amount of runoff and by the basin area. In this case, the watershed area does not change, but the amount of runoff is three times greater than the unit runoff. Therefore, the peak flow rate for this storm is three times greater than it is for the unit runoff hydrograph, or 3 x 67 L/s = 201 L/s. What would be the peak flow rate for a nearby 50-ha watershed for a 5-mm storm? –Peak Flow: Q p = Q o (A / A o ) (R / R o ) where Q p is the peak flow rate and Q o is for a reference watershed, A is the area of watershed and A p is the area of reference watershed. –Q = (67 L/s) (50 ha / 100 ha) (5 mm / 1 mm) = 168 L/s –In this case, the peak runoff rate was scaled by both the watershed area and the runoff amount.
47 Streamflow and Land Management –BMPs improve soil and water quality Most of our attention is placed on preventing pollution, decreasing stormwater, and improving low flows. –Forestry Forest streams have less stormflow and total flow, but more baseflow Forest litter (O-Horizon) increases infiltration Forest canopies intercept more precipitation (higher Leaf-Area Indices, LAI) Forest have higher evapotranspiration rates Forest soils dry faster, have higher total storage
48 Forest Management –Harvesting High-lead yarding on steep slopes reduces soil compaction Soft tires reduces soil compaction Water is filtered using vegetated stream buffers (SMZs) Water temperatures also affected by buffers –Roads Road runoff can be dispersed onto planar and convex slopes Broad-based dips can prevent road erosion –Site Preparation Burning a site increases soil erosion and reduces infiltration Leaving mulch on soils increases infiltration Piling mulch concentrates nutrients into local "hot spots" Distributing mulch returns nutrients to soils Some herbicides cause nitrate increase in streams
49 Forestry Compliance in Georgia with Water Quality Protection Standards (1991-2004)
50 Agricultural Land Management –Overland flow is a main concern in agriculture increases soil erosion, nutrients, and fecal coliform increases herbicides, pesticides, rodenticides, fungicides –Plowing exposes the soil surface to rainfall (and wind) forces mulching + no-till reduces runoff and increases infiltration terracing and contour plowing also helps –Pastures (livestock grazing) increases soil compaction reduces vegetative plant cover increases bank erosion rotate cattle between pastures and fence streams Urban Land Management –Urban lands have more impervious surfaces More runoff, less infiltration, recharge, and baseflow Very high peak discharges, pollutant loads Less soil storage, channels are straightened and piped, no floodplains –Baseflows are generally lower, except for irrigation water (lawns & septic)
51 Benefits of Riparian Buffers Bank Stability: –The roots of streambank trees help hold the banks together. –When streambank trees are removed, streambanks often collapse, initiating a cycle of sedimentation and erosion in the channel. –A buffer needs to be at least 15 feet wide to maintain bank stability. Pollutant Filtration: –As dispersed overland sheet flow enters a forested streamside buffer, it encounters organic matter and hydraulic roughness created by the leaf litter, twigs, sticks, and plant roots. –The organic matter adsorbs some chemicals, and the hydraulic roughness slows down the flow. –The drop in flow velocity allows clay and silt particles to settle out, along with other chemicals adsorbed to the particles. –Depending on the gradient and length of adjacent slopes, a buffer needs to be 30-60 feet wide to provide adequate filtration.
52 Denitrification: –Shallow groundwater moving through the root zones of floodplains is subject to significant denitrificiation. –Removal of floodplain vegetation reduces floodplain denitrification Shade: –Along small and mid-size streams, riparian trees provide significant shade over the channel, thus reducing the amount of solar radiation reaching the channel so summer stream temperatures are lower and potential dissolved oxygen levels are higher. –Buffers need to be at least 30 feet wide to provide good shade and microclimate control, but benefits increase up to 100 feet. Organic Debris Recruitment: –River ecosystems are founded upon the leaves, conifer needles, and twigs that fall into the channel. –An important function of riparian trees is providing coarse organic matter to the stream system. –Buffers only need to encompass half the crown diameter of full-grown trees to provide this function.
53 Large Woody Debris Recruitment: –Large woody debris plays many important ecological functions in stream channels. –It helps scour pools, a favored habitat for many fish. –It creates substrate for macroinvertebrate and algae growth, and it forms cover for fish. –It also traps and sorts sediment, creating more habitat complexity. –Woody debris comes from broken limbs and fallen trees. –The width of a riparian buffer should be equal to half a mature tree height to provide good woody debris recruitment. Wildlife Habitat: –Many organisms, most prominently certain species of amphibians and birds use both aquatic and terrestrial habitat in close proximity. –Maintaining a healthy forested riparian corridor creates important wildlife habitat. –The habitat benefits of riparian buffers increase out to 300 feet.