Flood Assessment and Monitoring using RS and GIS

Flood Assessment and Monitoring using RS and GIS WMO/FAO Training Workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for SADC countries November 14-18, 2005 Tamuka Magadzire USGS/FEWSNET, SADC RRSU

Outline • Introduction: understanding flood processes • Hydrological Cycle • Topography and stream networks • Surface conditions (landcover, soils, antecedence etc) • Analyzing historical and topographic flood risk • Analyzing current/unfolding flood risk • Rainfall analysis • Basin Excess Rainfall Mapping • Hydrological modeling • Flood mapping using RS • Incorporating GIS overlays

Hydrological Cycle Source: Columbia university: http://www.ldeo.columbia.edu/~martins/climate_water/lectures/hcycle.htm

Hydrological Cycle • The hydrological cycle is composed of a number of processes including • Evapotranspiration • Condensation and Cloud formation • Precipitation • Infiltration and Percolation • Runoff and stream flow • Subsurface interflow • Different applications emphasize different components, based on the domain of interest

Hydrological Cycle

Hydrological Cycle • Flooding is as a result of complex interactions between rainfall and surface processes • Generally, the more the rainfall, the greater the likelihood of flooding • The amount of runoff generated plays a significant role in the flooding process

Topography & stream networks • Some common hydrological knowledge • Water flows downhill • Water accumulates downstream, and water falling further from river will take longer to get there • Steeper areas are less likely to be flooded than flat areas • Areas nearer the river network are more likely to be flooded • Areas downstream in flood plains are more flood-prone than areas upstream nearer river source

Topography & stream networks • In addition to this, water interactions in hydrological and flood analysis tend to be confined to river basins • GIS analysis can help us define • the outline of the river basin • the stream networks • the topology of the streams and basins • the topography or terrain related characteristics of a basin that have a bearing on flooding processes • A Digital Elevation Model (DEM) is required

Topography & stream networks • A little more on DEMs • Can be defined as a digital representation of the elevation variations in the earth’s surface • Two models are common: • A raster grid • A Triangulated Irregular Network (TIN) • Many applications for hydrological analysis use raster grids • Sources of DEM include those made from: • SRTM Data • A combination of some or all of Contour, Spot height, River, Lake data *** [Recommended]

Using Arcview’s Terrain Analysis Functions with USGS 1 km DEM Flow Direction Flow Accumulation Flow Length Hill Length Subbasins Downstream Subbasin Slope Source: USGS

Theory behind flow analysis • Flow Direction • Flow Accumulation Source: ArcView Help System

Surface Conditions • Surface conditions affect flooding by affecting the ratio of amount of rainfall to amount of water that infiltrates the soil • Rainfall that does not infiltrate either becomes runoff or standing/ponded water.

Surface Conditions • Surface conditions affecting flooding include: • Soil type (water holding capacity, hydraulic conductivity) • Land cover (imperviousness) • Antecedent moisture conditions • SCS Curve Numbers are one way of quantifying the impact of rainfall on runoff

Surface Conditions Soil Type Land Cover

Historical flood risk • An analysis of historical stream flows can help statistically determine probability of flooding at a point along the river network • This is done using a cumulative distribution analysis of stream flow, to give historical return periods for different stream flows and stage heights • This can be used to infer the severity associated with different flood return periods

Historical flood risk – an example Source: ZINWA

Topographic flood risk • Areas closer to the river, and flatter areas, are more likely to be flooded by a specific rise in river level. • GIS analysis can be used to estimate the area that will be flooded by a given rise in the river level. • The main data input is a high-res DEM • The flood area for different river levels can be calculated to map the flood risk zones for different flood severities

Topographic flood risk – an ArcINFO application • Appropriate GIS software such as ArcINFO can be used to calculate the topographic flood risk • ArcINFO AML example (K. Asante): • Raise the level of the DEM along the stream network • Use the Fill Function to fill the sinks that are generated by raising the stream level. • Subtract the original DEM from the filled DEM to identify the areas affected by stream rise

Topographic flood risk – an ArcINFO application /* Copy the instructions below into an aml (eg makedem.aml) and run from the arc command prompt /* eg *ARC: &run makedem.aml grid setwindow indem indem setcell indem /* strgrid is river grid with 1's in the rivercells and 0's elsewhere fill indem dem # # flowdir flowacc = flowaccumulation ( flowdir ) STRGRID = con ( flowacc >= 100000, 1, 0 ) strlink = streamlink ( ( strgrid / strgrid ) , flowdir ) strline = streamline ( ( strgrid / strgrid ) , flowdir ) strgrid1 = strgrid &do ndepth := 1 &to 15 &by 1 newgrid%ndepth% = dem + (strgrid%ndepth%) fill newgrid%ndepth% dem%ndepth% # # flowdir%ndepth% &s ddd = %ndepth% + 1 tempgrid = dem%ndepth% - dem flowacc%ndepth% = flowaccumulation ( flowdir%ndepth% ) rivall%ndepth% = con(flowacc%ndepth% >= 100000, 1, 0) /* I am assuming a threshold here of 100,000 cells each 30 x 30 m /* This is not automated if the main channel is the source of inundation area sgrid%ddd% = con(tempgrid > 0, (tempgrid + 1), 0) strgrid%ddd% = con((sgrid%ddd% == 0) and (rivall%ndepth% > 0) , (sgrid%ddd% + 1), sgrid%ddd%) /* This ensures that any additional cells along the critical flow path /* are included in strgrid%ddd% before the next computation of flooded area. copy strgrid%ddd% flood%ndepth% kill flowdir%ndepth% all kill flowacc%ndepth% all kill rivall%ndepth% all kill sgrid%ddd% all kill tempgrid all kill strgrid%ndepth% all kill newgrid%ndepth% all &end Source: Kwabena Asante, USGS

Topographic flood risk – an ArcINFO application &do ndepth := 1 &to 15 &by 1 newgrid%ndepth% = dem + (strgrid%ndepth%) fill newgrid%ndepth% dem%ndepth% # # flowdir%ndepth% &s ddd = %ndepth% + 1 tempgrid = dem%ndepth% - dem flowacc%ndepth% = flowaccumulation ( flowdir%ndepth% ) rivall%ndepth% = con(flowacc%ndepth% >= 100000, 1, 0) sgrid%ddd% = con(tempgrid > 0, (tempgrid + 1), 0) strgrid%ddd% = con((sgrid%ddd% == 0) and (rivall%ndepth% > 0) , (sgrid%ddd% + 1), sgrid%ddd%) /* This ensures that any additional cells along the critical flow path /* are included in strgrid%ddd% before the next computation of flooded area. copy strgrid%ddd% flood%ndepth% kill flowdir%ndepth% all kill flowacc%ndepth% all kill rivall%ndepth% all kill sgrid%ddd% all kill tempgrid all kill strgrid%ndepth% all kill newgrid%ndepth% all &end Source: Kwabena Asante, USGS

Topographic flood risk – some results • Such an analysis was done for Chokwe district to determine which settlements would be affected by different river rises. • Similar analysis was done using SRTM DEM for Beitbridge

Example 1 CHOKWE

Topographic flood risk – some results Determinação de Área Inundada Usando DEM e Alturas Previstas Source: USGS & ARA-Sul, Mozambique

MAPA DE INUNDAÇÃO DO DISTRITO DE CHÒKWÉ, E35 Alt. 4-6m Cidade: Macarretane Aldeias: Conhane e Mapapa

MAPA DE INUNDAÇÃO DO DSTRITO DE CHOKWE, E35 Alt 6-8m Cidade: Macarretane Aldeias: Muzumuia, Muianga, Conhane, Mapapa, Chiaquelane, Marranbandjane, Chiguidela, Malhazene, Chalucuane, Zuza e Chiduchine

MAPA DE INUNDAÇÃO DO DISTRITO DE CHÒKWÉ Altura: 8-10m Cidades: Macarretane, Chòkwè e Lionde Aldeias: Muzumuia, Massavasse, Nwachicoloane, Changulene, Muianga, Conhane, Mapapa, Chiaquelane,Marrambandjane ,Chiguidela,Malhazene, Chalucuane, Zuza e Chiduachine

Example 2 BEITBRIDGE

SRTM DEM

Potential flood areas: 1m Source: SADC RRSU

Potential flood areas: 10m Notice the “line” down the centre of the flood map. Answer later Source: SADC RRSU

Potential flood areas: 15m Source: SADC RRSU

DEM Errors “DEM Errors” shown in brown. These are the areas that are filled during the Fill operation Reason for lines in flood areas analysis Source: SADC RRSU

Combining Statistical and Topographic Flood Risk The Beitbridge Study

Flood Risk

Rainfall Analysis • When there is lots and lots of rain, result is often (not always) flooding. • So a first step in analyzing unfolding flood risk is simple rainfall analysis • A subjective analysis that benefits greatly from an enhanced knowledge of the area under analysis, of the recent rainfall history, as of the events upstream • Daily rainfall observations over the last few days, and QPF are useful for heavy storms, while dekadal (10-day) sums are useful for persistent weather • Encourage use of improved rainfall grids incorporating rain-gauge and satellite data

Rainfall Analysis • Every day, NOAA CPC produces Rainfall Estimates for the FEWSNET activity. • These RFE, as well as QPF are put on the USGS FEWSNET website as graphics: • http://earlywarning.usgs.gov/adds • The actual data can also be downloaded: • http://edcwww.cr.usgs.gov/pub/edcuser/fewsips/africa/

Rainfall Analysis

Rainfall Analysis of potential flood situation March 2001

Rainfall Analysis – an example • There was much flooding in 2001, which was analyzed, tracked and reported on in the Regional Flood Watch • The following analysis shows how rainfall estimates, rainfall proxies, and rainfall forecasts were used to track the flood likelihood

ColdCloudDuration DailyRainfallEstimates QuantitativePrecipitationForecasts

Rainfall Analysis of potential flood situation March 2003

Rainfall Analysis – an example • Example of rainfall analysis that was done to support the Regional Flood Watch in anticipation of Cyclone Japhet in March 2003. • The following example illustrates the use of rainfall estimates and the incorporation of antecedent moisture conditions in the analysis

Flood Assessment and Monitoring using RS and GIS