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The problemMethodologies – the pros and consExample projectsHydraulic modellingFurther examplesConclusions & recommendations
Agenda
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Why do we find hydrology in urban areas much harder to predict than for rural catchments?
Limited gauged urban catchments so lack of observed dataResponse to rainfall very different – water levels rise quicker, higher peaks and higher volumesDue to 2 physical differences:o Impermeability (urban ~ 60 – 90%)o Structures & the artificial drainage systems – reacts very
different to rural catchments• Depends on storage capacity size and condition• Can significantly alter the shape of the unit hydrograph
Hydrology is both a science and an art – judgement and balance are key
The problems
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Methods (1)
Only calibrated on 7 catchments –do not use it!
Treats moderately urban as essentially rural. Hydrograph produced.
Revitalised Flood Hydrograph Method (ReFH)
No good for heavily urbanised catchments due to drainage network affects. Only peak flow provided. Hard to get a good pooling group due to the lack of small, urban gauged catchments.
Urban adjustment applied to QMED and growth curve. Calibrated using ~ 34 catchments. Recommended FEH method.
FEH Statistical (preferred FEH method)
Limited calibration. Tends to overestimate flow. Uncertainty in unit hydrograph
Treats urbanisation more directly than statistical. Hydrograph produced.
FEH Rainfall Runoff
ConsProsMethod
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Methods (2)
Fixed FSR growth curve. Peak flow only. Based on FSR maps which are out of date and large scale, particularly URBAN rather than URBEXT
Different equations for urban and rural catchments. Developed for small catchments.
IoH Report 124
Can only do for select return periods. Peak flow only. Based on FSR maps which are out of date and large scale.
Utilising Wallingford Procedure. Quick to calculate
Modified Rational Method
ConsProsMethod
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Example 1 - Weymouth
Investigating the impact of development on a number of small catchments in Weymoutho FEH techniques – hydrological differento Analysis of geology – hydrological differento Modified Rational & IoH 124 method – very
similar results due to large scale of FSR maps• Soil same value for all of Weymouth• Rainfall same across whole area• Only difference was size of catchment
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Example 1 – Weymouth (cont)
FSR Average rainfall FSR Soil map
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Example 2 – Middle Beck
Intensely urbanised, particularly in lower reachesFEH catchment boundary not accurately defined
Very urbanCulverted watercourseGIS and LIDAR used
Full range of methods undertaken Produced massively varying resultsNo observed data for comparisonReview further during calibrationMore gauging needed!
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Example 3 – Goulds Brook
• Small permeable catchment• Donor sites searched – limited number available• Pooling group – difficult to have enough stations whilst
maintaining goodness-of-fit• Pooling re-visited – used just 5 very similar catchments i.e.
not 5T• Much shallower growth curve from original pooling group i.e.
smaller group 200yr ~ initial group 5yr!• Comparisons with observed suggest first results more likelyHighlights range of different results you can obtain – judgement
needed!
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Modelling
When modelling urban catchments it is particularly important to consider all sources of flooding:Surface water runoff & highway floodingo High % impermeableo More overland flow paths e.g. roadso Water can re-join the river in unexpected placeso Be careful when calibrating fluvial events – recorded
incident may have been surface water
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Modelling (2)
When modelling urban catchments it is particularly important to consider all sources of flooding (cont):Capacity of drainage networko Very different reaction to rainfall – can take water from a
system but also discharges into the systemo Easily blockedo Generally only 30yr design standard – reacts different for
5yr & 100yr events
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Example 4 - Swanage
ISIS-TUFLOW model to determine ABDInitial hydrology completed, model results reviewedVery limited flooding for 100 year event without defences –so why were two big storage reservoirs built???Compared to historic incidentsGood example of using model to help determine appropriate hydrology- cyclic
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Example 5 – Haydon Wick Brook
Heavily urbanised catchment north of SwindonInitial hydrology undertaken – statistical best, with RR hydroAlthough two inflows purely due to surface water drainageDrainage network significantly affects the flow regimeNow also using Thames Infoworksmodel outputsUndertaking testing of inflows in model
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Example 6 – Beverley Brook, London
ISIS-TUFLOW model to improve flood map & determine ABDUsed FEH Rainfall Runoff MethodCalibrated parameters based on observed eventsUnit hydrograph tailed off before the observed –underestimation of volumesAttenuation was underestimated – Tp adjustedHighlighted heavy impact of drainage networkMore gaugingsrequired
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Conclusions
Expect variationsThink carefully about the nature of the catchment prior to undertaking any assessmentBe aware of the limitations of each method usedConsider a hybrid approach e.g. peak flow from one method but unit hydrograph from another (ReFH) Always use as much observed data where possible, provided it is relevant and correctCurrently lacking best practice / a comparison of methods –this is needed!Inputting flows into the model – judge sensibility based on extentsCannot just grind the wheel – we need to think!
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Urban hydrology
Good hydrology is the key to a successful modelling project
But…
+ =
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Better result & confidence
