SLAD BROOK PILOT: Hydrological Modelling and Flood Alleviation Solutions

SLAD BROOK PILOT: Hydrological Modelling and Flood Alleviation Solutions

SLAD BROOK PILOT: Hydrological Modelling and Flood Alleviation Solutions Ilaria Pretto Water21 (Department of Vision21 Ltd) 30 St Georges Place Cheltenham GL50 3JZ 01242 224321 Slad Brook Pilot August/2008 1 Introduction ......................................................................................... 7 1.1 Aims of this Report ................................................................................ 7 1.2 Background ........................................................................................... 8 1.3 Methodology ....................................................................................... 10 1.4 Locations ............................................................................................. 12 1.4.1.1 Figure: The Five valleys [Google Earth] ........................................................... 12 1.4.2 Ruscombe brook ........................................................................................... 13 1.4.2.1 Figure: Ruscombe brook‟s aerial view [Google Earth] ...................................... 13 1.4.3 Slad brook ..................................................................................................... 15 1.4.3.1 Figure: Slad brook‟s aerial view [Google Earth]................................................ 15 1.4.3.2 Figure: Hill-shade and elevation map (upstream part) ...................................... 15 1.5 Geology ............................................................................................... 17 1.5.1 General introduction ..................................................................................... 17 1.5.1.1 Figure: UK geological map ............................................................................. 18 1.5.2 Slad brook ..................................................................................................... 19 1.5.2.1 Figure: Slad brook‟s geological map ................................................................ 19 1.6 Historical data: the milling industry usage ......................................... 21 1.6.1.1 Figure: Cloth Mills in Gloucestershire 1750-1820 .............................................. 21 1.6.1.2 Figure: Cloth Mills in Gloucestershire 1840[15] ................................................ 22 1.6.1.3 Figure: Cloth Mills in Gloucestershire 1849[15] ................................................ 22 1.6.1.4 Figure: Cloth Mills in Gloucestershire 1867[15] ................................................ 23 1.6.1.5 Figure: Geological map and ancient milling usage in the upstream part ............ 24 1.7 Local looding: historical analysis ......................................................... 25 1.7.1.1 Table: http:// www.environment- agency.gov.uk/commondata/acrobat/draft_cfmp4_1684747.pdf ......................................... 27 2 Topographic analysis ......................................................................... 28 2.1 Introduction ........................................................................................ 28 2.1.1.1 Figure: Slope map ......................................................................................... 28 2.1.1.2 Figure: Aspect map ....................................................................................... 29 2.1.1.3 Figure: Elevation Map .................................................................................... 29 2.2 Survey ................................................................................................. 30 2.2.1.1 Figure: Ordnance Survey 1:10000 Map ........................................................... 30 2.2.1.2 Figure: Localization of the cross sections (1) ................................................... 31 2.2.1.3 Figure: Localization of the cross sections (2) ................................................... 32 2.2.1.4 Figure: Localization of the cross sections (3) ................................................... 32 2.2.1.5 Table: cross-sections ..................................................................................... 40 3 Hydrological analysis ......................................................................... 41 ______________________________________________________________________ - 2 - Slad Brook Pilot August/2008 The hydrology is the main aspect to be considered as in this study we need to calculate the volume of the water to be dispersed upstream. The discharge which exceeds the channel capacity is termed flood discharge. Flood does much of the work of shaping river channels and valleys through erosion and deposition................................................................................. 41 3.1 Whole Catchment ................................................................................ 41 3.1.1.1 Figure: FEH-Whole cathment .......................................................................... 41 3.1.1.2 Table: FEH parameters and descriptors ........................................................... 42 3.1.2 Key location springs ...................................................................................... 43 3.1.2.1 Table: FEH parameters for the main springs .................................................... 43 3.1.3 Ebley station-donor catchment ..................................................................... 43 3.1.3.1 Table: Ebley station parameters ..................................................................... 43 3.1.4 Flow value (discharge) estimation ............................................................... 44 3.1.4.1 Table: Qmed for the different corss-sections ..................................................... 44 3.1.5 Pooing Group selection ................................................................................. 45 3.1.5.1 Figure: Location of the Donor Catchments ...................................................... 46 3.1.5.2 Figure: Graphs of the Indicators ..................................................................... 47 3.1.5.3 Figure: L-moments ........................................................................................ 48 3.1.5.4 Figure: Generalised Logistic Curve .................................................................. 48 3.1.6 Flood Frequency curve .................................................................................. 49 3.1.6.1 Table: Peak Flow for the Flood Frequency curve .............................................. 49 3.1.6.2 Figure: Flood Frequency Curve ....................................................................... 49 3.1.7 Flood Growth Curve ...................................................................................... 50 3.1.7.1 Table: Peak Flow for the Flood Growth curve .................................................. 50 3.1.7.2 Figure: Flood Growth Curve ........................................................................... 50 3.1.8 Hydrograph ................................................................................................... 51 3.1.8.1 Table: Hydrograph parameter ........................................................................ 53 3.1.8.2 Figure: Hydrograph ....................................................................................... 53 From the hydrograph above, the direct runoff is easily recognisable. The direct runoff is the runoff caused by and directly following a rainfall; it forms the major part of the flood hydrograph and excludes base flow. The base flow is that part of the stream discharge that is not attributable to direct runoff from precipitation or melting snow; it is usually sustained by groundwater. The amount of base flow a stream receives is closely linked to the permeability of rock or soil in the watershed. Base flow is important because impervious surfaces created by development will inhibit water from infiltrating into the ground as it did prior to development. Over time this will draw down the groundwater elevation, which in turn affects spring activity, which feeds the river. In short the river's base flow will decrease over time if springs are not replenished by infiltration, which is why base flow is a key indicator to monitor. ............................................................... 53 3.2 First spring .......................................................................................... 54 3.2.1 Catchment descriptors .................................................................................. 54 3.2.1.1 Figure: FEH-Slad spring ................................................................................. 54 3.2.1.2 Table: FEH parameters .................................................................................. 54 3.2.2 Pooling group selection ................................................................................ 55 3.2.2.1 Figure: Location of the Donor Catchments ...................................................... 55 3.2.2.2 Figure: Generalised Logistic Curve .................................................................. 55 ______________________________________________________________________ - 3 - Slad Brook Pilot August/2008 3.2.3 Flood Frequency Curve ................................................................................. 56 3.2.3.1 Table: Peak Flow for the Flood Frequency curve .............................................. 56 3.2.3.2 Figure: Flood Frequency Curve ....................................................................... 56 3.2.4 Flood Growth Curve ...................................................................................... 57 3.2.4.1

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