Pushing the Boundaries of Flood Modeller 1D

Tel: 01824 702220 Website: www.waterco.co.uk

David Hughes

26th November 2015

The first few slides have been removed as they contained potentially sensitive information

Why use Flood Modeller 1D?

Robust, industry-standard software

High degree of confidence in output

Existing 1D model – cost effective

1D-only – high-level answer required at this stage, not detailed

< (if input is good!)

Existing Model

1D ISIS model of Black Brook and tributaries from previous flood

mapping study in 2013

Based on existing datasets - partially geo-referenced

Partial survey coverage (data supplied with model)

Some sections extended with, or generated entirely from LiDAR

Some (perhaps key!) structures missing

Limited hydrology data (missing hydrographs)

Waterco Model Setup

Stanley Brook sections were extracted

New downstream boundary generated (normal depth unit)

New hydrology study completed – full range of hydrographs for all

watercourses

Site visit carried out – basic measurements of missing structures taken

Spoke to property owners

Representing Debris Dams

Non-standard structure – no “debris dam” 1D unit – not an issue!

Representing key flow paths

1) Flow through dam

2) Flow over dam

Combination of units may be required

Several options available

Representing Debris Dams – Option 1

Spill unit with notch –

Unit represents both the flow through and over the structure

Sensitivity testing on weir coefficients and slot dimensions

Pros

Single unit – simple setup and less nodes

Cons

Single unit – no way to split flow through / over structure

Representing Debris Dams – Option 2

Spill unit coupled with orifice unit –

Spill represents flow over the top of the structure (no notch)

Orifice to represent the “leakiness” of the structure

Sensitivity testing on orifice parameters

Pros

Two units – allows flow paths to be modelled separately

Cons

More units to set up, more parameters to test

Representing Debris Dams – Option 3

Blockage unit coupled with spill –

Blockage to reduce the available conveyance of a section

Spill to represent flow over the top of the structure

Sensitivity testing on blockage value and parameters

Pros

Two units – allows flow paths to be modelled separately

Cons

More units to set up, more parameters to test

Model Results

Results of debris dam simulations:

Very little peak flow reduction in all simulations

Hydrograph is significantly delayed – traffic light system

Potentially part of a successful flood-risk solution

Fluvial Event(AEP)

ScenarioPeak Flow

(m³/s)Simulation Time

(hours)Flow Difference

(m³/s)Time Shift

(hours)

50% Existing 1.141 5.58 - -

50% Option 1 (spill) 1.132 6.62 -0.009 1.034

50% Option 2 (orifice) 1.130 6.62 -0.011 1.034

50% Option 3 (blockage) 1.140 5.75 -0.001 0.167

The model was used to assess the performance of the debris dams in

terms of flood risk reduction downstream

Funding was sourced to construct four debris dams along the lower

section of Stanley Brook

Ecological benefits too

Constructed in June 2015

Project Outcome

An Interesting Point…

A new 1D/2D FMP-TUFLOW model is being used to study all options

Stanley Brook is now modelled within the 2D domain using z-lines to

allow direct rainfall modelling

Debris dams modelled as layered flow constrictions (2d_lfcsh)

Early results show very good agreement with initial 1D-only results!

Goes to show… a 1D solution isn’t always a worse solution.

Conclusions

Debris dams may not “flatten” the flood hydrograph

They can add a significant time delay to a flood hydrograph

They could be part of the overall flood risk solution

Flood Modeller 1D can be used to represent “odd” structures

Know and push the boundaries of modelling!

Any questions?

Tel: 01824 702220 Website: www.waterco.co.uk