Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Harry Williams, Geomorphology 1Sediment ErosionWater flowing through a channel has the ability to transport sediment supplied to it from hillslopes and/or erode its banks and bed to produce sediment to transport. The amount of sediment transported depends mainly on the volume of flow; this is of course related to the size of the drainage basin and will fluctuate according to inputs of precipitation.The volume of streamflow, or discharge, is measured in cubic meters (or feet) per second. A graph showing the fluctuation in discharge through time is a HYDROGRAPH.Harry Williams, Geomorphology 2Hydrograph for Denton Creek near Justin, March-May, 1990.stormsHarry Williams, Geomorphology 3Sediment Sources in Denton Creek Drainage Basin. Denton Creek drains portions of Montague, Wise and Denton counties. Clastic sediment sources in the drainage basin include rills, gullies and channel bank and bed erosion. Sediment eroded from the drainage basin is deposited into Grapevine Lake.Harry Williams, Geomorphology 4Harry Williams, Geomorphology 5The combination of heavy rain (for example during spring thunderstorms) and sandy ground with low permeability causes surface run-off and erosion in the northern portion of the drainage basin. The erosion creates large gullies and smaller rills.Harry Williams, Geomorphology 6Some gullies in northern Wise county form very large branching gully systems.One branch of a large gully systemHarry Williams, Geomorphology 7Decatur Krum Denton ->Harry Williams, Geomorphology 8Krum area – marls, limestonesHarry Williams, Geomorphology 9Gas wells near KrumHarry Williams, Geomorphology 10North east of Decatur, sandstoneHarry Williams, Geomorphology 11Harry Williams, Geomorphology 12Harry Williams, Geomorphology 13Harry Williams, Geomorphology 14Harry Williams, Geomorphology 15Harry Williams, Geomorphology 16Harry Williams, Geomorphology 17Fragments of the underlying limestone bedrock are exposed in the stream bed, indicating bed erosion.Limestone bedrock in the drainage basin produces dark clayey soils. The steep banks of small streams are evidence of bank erosionHarry Williams, Geomorphology 18South of Decatur – limestones – gas wellsHarry Williams, Geomorphology 19Close up showing channel bed and bank erosion; little gullying.Harry Williams, Geomorphology 20A small earthflow caused by recent heavy rain on the bank of the creek. This, and the fallen tree just above the earthflow, is evidence of ongoing bank erosion.Denton Creek at the Denton-Wise county border. The sand in the creek bed is transported downstream when sufficiently high stream flows occur.Harry Williams, Geomorphology 21mountainshillscliffs ridgesSo, the visible sources are rills, gullies and bank/bed erosion - how is the sediment transported? Generally, 3 main sediment transport modes are recognized:1. Solution load - dissolved rock carried in the flow; generally, this is higher where much of the flow is derived from groundwater pathways, which allow water to stay in contact with rock for long periods. It is also higher where local bedrock is prone to chemical weathering - e.g. the limestones in the southern portion of Denton Creek drainage basin (this sediment is in effect “invisible”).2. Suspended load - finer sediment (usually clay and silt) suspended by turbulence in the flow (does not contact bed).3. Bed load - coarser sediment (sand + gravel) that slides, rolls or skips along the stream bed - amount depends on the tractive force exerted by the flow + resisting force of bed material.Harry Williams, Geomorphology 22canyonsvalleysbeachesdeltasThe USGS collects stream flow and suspended sediment data for selected streams, e.g. the Trinity River south of Dallas, March 1977.date Q(cfs) Q(cms) C(mg/L) Load(ton/day) Load(tonnes/day) 01-MAR-77 3180 90 300 2580 2340 02-MAR-77 2920 82.7 200 1580 1430 03-MAR-77 3230 91.5 450 3920 3560 04-MAR-77 7500 212 1150 26200 23800 05-MAR-77 12000 340 1170 36600 33200 06-MAR-77 13600 385 470 17300 15700 07-MAR-77 15100 428 350 14300 13000 08-MAR-77 15900 450 250 10700 9710 09-MAR-77 16500 467 200 8910 8080 10-MAR-77 16800 476 170 7710 6990 11-MAR-77 15500 439 160 6700 6080 12-MAR-77 10500 297 190 5390 4890 13-MAR-77 5850 166 370 5840 5300 14-MAR-77 4070 115 390 4290 3890 15-MAR-77 3430 97.1 360 3330 3020Harry Williams, Geomorphology 23Notice the rising limb of the flow “flushes out” the suspended sediment.Harry Williams, Geomorphology 24Where does the sediment go? In the case of Denton Creek, some of it is deposited in the floodplain surrounding the creek, some is deposited on the bed of the creek and the rest ends up in Grapevine Lake.Harry Williams, Geomorphology 25Denton Creek floodplainHarry Williams, Geomorphology 26Denton Creek floodplain, farther northHarry Williams, Geomorphology 27How do we measure stream discharge (Q)? : the volume of water flowing through the creek can be calculated by multiplying the cross-sectional area of flow (A) by the mean flow velocity (V). The cross-sectional area of flow can be found by simply measuring the depth of the stream at regular intervals (e.g. 1 meter) across the channel. Mean flow velocity can be measured by a flow velocity meter. Usually a number of measurements are collected from different positions across the stream and then averaged to find themean velocity.Harry Williams, Geomorphology 28Example discharge calculation (based on Denton Creek):1 meter3 meters3 meters6 metersCross-sectional area of flow = 9 m2Stream flowMean flow velocity = 0.5 m/s (from velocity meter)Q = 9 x 0.5 = 4.5 m3/sHarry Williams, Geomorphology 29How do we measure suspended sediment load (Ss)?: the amount of sediment suspended in the flow can be calculated by filtering the sediment