HW-

notes.docx

Chapter 6, Serrano. Surface Runoff and Streamflow.

Terminology: Overland Flow, Effective Precipitation, Streamflow Measurement, Stage-Discharge Curves, Storm/Event/Runoff Hydrograph, Hyetograph, Gage Station, Watershed outlet, Peak Flow Rate, Time of Concentration, Lag Time, Storm Runoff Volume (SRV), Storm Runoff Hydrograph (SRH), Unit Hydrograph

· Go over Fig 6.1 – Note the different flow contributions and velocities: overland flow (fast), subsurface flow (slow), and deep groundwater flow (very slow)

· Overland Q – important in high intensity storms; also important in deforested watersheds or those with a high % of impermeable cover;

· Subsurface Q – near surface flows, in the root and upper soil horizons; important contribution in low intensity, soaking, storms;

· Groundwater Q – keeps going for weeks/months, often providing the base flows for streams/rivers; these low flows are very important ecologically

· Consider the relative contributions to streamflow from each of these sources before and after significant changes in the watershed cover, e.g. going from forest to urban development.

· Example: Tusayan near Grand Canyon deciding whether to allow large development in forest.

· Currently little flow in forested areas due to high permeability of soils and high % of vegetative cover;

· In town, very little infiltration and have immediate generation of overland flow when rain starts

· Recall: Effective Precipitation is rain that is after infiltration and is available to contribute to runoff/streamflow/overland flow.

· Gage Station – located at stable stream site; all drainage from the watershed above gage passes through the channel at the gage station location.

· Streamflow Measurement – see p 285-289; procedure as follows:

· Establish cross section perpendicular to flow;

· Subdivide cross section line into subsections; determine area of each subsection;

· Measure stream velocity in middle of each subsection at two depths: 0.2 and 0.8 of total depth;

· Average these two velocities

· Multiply each subsection Area by the average velocity measured in that subsection, to get flow, Q, for each subsection;

· Sum all of the subsection Q’s to get total Q at that cross section; note Water Surface Elevation (WSE)

· Create Stage/Discharge Curve p 289-291– this can be done at a gage station or at an established cross section;

· Need multiple discharge measurements at different flows, along with WSEs all done at the same cross section;

· Plot Q vs WSE; this is your Stage/Discharge Curve; with this we can estimate Q by just reading the WSE from a permanent depth measurement staff at the gage station or cross section; see Fig 6.7 p 291

· Continuous Streamflow Measurement p 289-290 – this requires continuous measurement of the WSE at a gage station;

· Continuous measurement is usually accomplished with a float gage (p 290), or an underwater pressure transducer that measures the weight of the water column above the transducer and converts that to WSE

· WSE measurement is then converted to a Q using the Stage/Discharge relationship for that site;

· Requires associated data logger or recorder or means to transmit data to a remote site; See Fig 6.5 p 290

· Streamflow Hydrograph – simply a graph of Streamflow, Q, vs Time

· Annual Hydrograph, see Fig 6.8 p 292 and Fig 6.9 p 293 – note the Baseflow, which is the minimum flow that occurs at all times, as opposed to the spikes, which represent storm related runoff; Fig 6.9 is graph from USGS site; area under the curve represents the total volume of water generated from the watershed over the time period shown; you can also roughly estimate the volume contributed to the stream from groundwater and subsurface flow and that coming from overland flow or runoff;

· Event or Storm or Flood Hydrograph – see Fig 6.10 p 297; this is also Q vs Time, but for a single storm event, so your time axis is on the order of hours or days;

· Read about inflection points, especially the first one which indicates start of Effective Precipitation or runoff; last inflection point indicates break between storm related flow and groundwater contribution; inflection points are often difficult to ascertain due to erratic nature of storms;

· With respect to reading the hydrograph, mainly consider Volume of Storm Hydrograph (above the GW flow level), time when runoff begins and ends, peak flow (Qp), storm-related flows (Qr), and the relationship between the hydrograph and the hyetograph;

· See Figs 6.12 and 6.13 p 300-301; Note the inset Hyetograph above the storm Hydrograph; this is an excellent way to graphically represent the Precipitation information relative to the Runoff/Streamflow information;

· Know these parts of the Hyetograph: a) representation of rainfall intensities (mm/hr) by histogram; b) infiltration estimate, the f line; this is either a declining rate line (first order decay rate, e.g. Horton’s model), or more simply a constant rate line (index method); c) Effective Precipitation estimate, which is the histogram above the f line

· The Effective Precipitation volume can be estimated with these Hyetographs; this volume should be approximately equal to the Storm Runoff Volume, SRV, as shown on the Hydrograph;

· Lag Time p 300 – roughly the time between the peak rainfall intensity (hyetograph) and peak runoff, Qp;

· Time of Concentration – the longest travel time in the watershed; in other words the ToC is the amount of time a drop of Effective Precipitation takes to flow from the farthest point in the watershed to the watershed outlet, e.g. at the gage station;

· Therefore, the Time Base of the Hydrograph is the duration of Effective Precipitation plus the ToC

· The Unit Hydrograph – p 309; by definition the UH is the SRH (Storm Runoff Hydrograph) generated by 1 mm of Effective Precipitation, Pe;

· The depth of Pe linearly predicts Q; this is generally a rough assumption but the UH approach is used for most watershed models and is used in design of storm sewers, reservoirs, spillways, detention basins and more;

· NOTE: the hydrograph for a storm represents a watershed response to a storm that is unique to that watershed; we divide that unique hydrograph by the Effective Precipitation depth, mm, to get m3/sec per mm of Pe.

· A Storm Hydrograph represents the watershed’s signature in how it manages rainfall.

· With the UH, which is developed from any given storm event, we can

· Create a Storm Hydrograph of a storm with a different duration than the original storm;

· Create a Storm Hydrograph of a storm with different precipitation intensities than the original storm;

· There are 3 Basic Types of UH Problems. We will become competent at all three.

· 1. Creation of UH from a given storm and given hydrograph (Problem Type I)p 311

· Pick ‘suitable’ storm. Looking for well shaped hyetograph and hydrograph.

· Note storm duration, ts. NOTE: the UH that is created is a UH specific to this storm duration. Later on we can estimate other duration storms using this UH.

· Looking at Hyetograph histogram, estimate f, infiltration rate. Either constant rate or declining rate model.

· Note peak of Effective Precipitation.

· Determine Effective Precipitation volume from Hyetograph (intensity x time)

· Volume under hydrograph curve minus groundwater flow also approximates Pe depth;

· Divide Effective Precipitation volume by watershed area to get Effective Precip depth, mm: (Pe volume)/WSarea = Ro , or runoff in mm depth over the whole watershed.

· QUH(t) = (Q(t) – QGW)/ Ro, where QUH(t) is the normalized flow of the UH at time t; Q(t) is flow read from the Hydrograph for time t, and QGW is flow due to groundwater at time t, also read from the Hydrograph.

· Basically, creation of a UH for a storm normalizes the runoff hydrograph by dividing by Pe depth spread over the whole watershed area.

· Go Over the two examples on p 313. The first Ex 6.5 derives a UH from Hydrograph data but limited rainfall data. The second Ex 6.6 derives a UH with a bit more rainfall data plus Hydrograph data.

· 2. Create Hydrograph for Storm of Different Duration (UH Problem Type II)

· Lagging Method – new duration is a whole integer multiple of original storm from which UH was derived, e.g. 2x or 4x, etc.; the new storm duration is nts

· Simply plot UH moved over ts units; e.g. if original storm has duration ts of 2 hours, plot the first UH starting at time zero, plot second UH starting at time 2 hours, next one starting at 4 hours and so on until you get to the duration of your storm of interest.

· Add up all the ordinate values at any given time from overlapping hydrographs to produce a composite hydrograph;

· Divide the ordinate values of the composite hydrograph by n

· Plot new ordinate values vs time over new time base to get new UH for storm of duration nts.

· See Ex 6.7 p 316 and Fig 6.18 p 315

· S Hydrograph Method - new duration is not a whole integer multiple of original storm from which UH was derived, for example creating a 5-hour UH from a 2- UH.

· See Ex 6.8 p 318 and Figs 6.19 and 6.20 p 317

· As with the Lagging Method, plot the original UH several times and add the ordinates to produce an S-shaped hydrograph, as in Fig 6.20.

· Again, add up ordinates from multiple UHs to create composite S-hydrograph; plot the composite S-hydrograph starting at time zero

· Plot another composite S-hydrograph but shifted new time duration, nts, over from time zero.

· Plot new UH for new time duration nts by taking difference between ordinates of the two S-hydrographs at a given time and dividing that difference by n. Plot new ordinates vs time.

· This approach works for durations longer or shorter than the original storm duration.

· 3) Prediction of the Storm Runoff Hydrograph (UH Problem Type III)

· THIS is the Streamflow Forecasting Problem; This is the main problem that we will use the UH approach to solve. This is Flood Forecasting Problem, i.e. using the UH to estimate Q’s from high intensity, low frequency storms.

· Given a UH for duration ts time

· Storm hyetograph with rainfall intensities every ts time interval; in other words our rainfall data time units correspond to our UH storm duration

· If not, must convert your UH to a UH of appropriate time duration using UH Problem Type II, above.

· Need Infiltration model or estimate;

· Calculate Ro (Effective Precipitation depth – see Problem Type I) for each rainfall intensity per each time interval ts; for each time interval you will calculate a Roi;

· Multiply UH for each time interval ts by the corresponding Roi estimated for that interval;

· Do this for each interval

· Add ordinate values for each time to create new combined SRH; (NOTE: locate the first ordinate for each UH associated with each time interval with the time when that time interval began);

· Add an estimate of groundwater flow, QGW, to represent baseflow;

· See Ex 6.9 p 320 and 321; See Fig 6.21 p 319