Building standard 3.3 - flooding and groundwater guidance review: research

Appendix D The Highland Council Guidance

Matters to Consider

• is the development site at risk of flooding from any source?
• would development of the site lead to increased flood risk elsewhere?
• would safe access and egress to and from the development be possible during flood events?
• is the development likely to prevent safe access to and maintenance of bodies of water and/or flood defence measures?
• is the development design employing SuDS?
• how is natural water emanating from the site being dealt with and managed, surface or ground water that will not be part of the storm water treatment?
• how is extraneous water being dealt with, uphill surface or ground water?
• what is to be done during construction phases to control water contamination and limit flow rates?
• will future users of the development be faced with difficulty obtaining insurance or mortgage finance?

FRA Content

Background Information

• Study area description.
• Outline of development proposal.
• Geo-referenced location and site plans showing all bodies of water or sources of flood risk which may have an influence on the site.
• Plan of site illustrating pre and post development ordnance datum levels to a recognised scale.
• A plan and description of any structures that may influence local hydraulics. This will include bridges and pipes/ducts crossing the watercourses together with culverts, screens, embankments or walls, overgrown or collapsing channels and their likelihood of choking with debris.
• Good use of photographs illustrating important features such as culverts etc.
• Catchment description.
• If appropriate, information on current flood alleviation measures including the level of protection and condition.
• Identification on the ownership of any water related structures and assessment of their condition.
• Information of historic flood events, photographs, levels, trends in the vicinity of the development.
• Clear drawings, plans and maps to a recognised scale relevant to the site.
• Information on consultations undertaken with others.

Methodologies

Description of the type or source of any present flooding risk.

Methodologies

• In the event that hydrological and/or hydraulic modelling is required it is important to ensure that the appropriate method has been chosen and explained in the FRA, justifying how the chosen model will accurately reflect the complexity of the hydrological processes.
• Appropriate model calibration and verification should be carried out where possible. Un-calibrated models should be accompanied by appropriate sensitivity analysis.
• For fluvial assessments all technical records and data sets derived from the Flood Estimation Handbook (FEH) or other (if appropriate).
• For coastal assessments all technical records and data sets derived by following the industry accepted methodology.
• All modelling should be completed using recognised industry software to determine design water levels and a sensitivity analysis undertaken to determine the sensitivity of design water levels with regards to the key model parameters (e.g. design flow, boundary conditions and roughness).
• It is considered best practise to include an additional allowance for climate change, preferably utilising latest estimates from UKCIP. In general a 20% increase in peak flows should be adopted to assess the potential long term impacts of climate change on future flood risk.

Results

• Comment and recommendations on application of appropriate design to suit flood risk.
• To ensure a complete FRA, the reporting of any modelling study is compulsory and should address important issues to an appropriate level of detail.
• If the FRA is underpinned by hydraulic modelling the results should be summarised in a tabular format, including results for appropriate sensitivity analysis.
• If applicable pre- and post development flood levels should be compared for a range of return periods up to 1 in 200 years.
• If the proposal is likely to result in a loss of flood plain storage then a quantitative assessment should be made of the anticipated loss on a level-for-level and volume- for-volume basis. Proposals to compensate for such loss should also be included.
• Should site flood levels be influenced by downstream hydraulic features such as culverts, an assessment should me made of the potential impact various blockage scenarios may have on expected flood levels.
• An appropriate freeboard, to mitigate uncertainty in relation to flood estimation, climate change and exacerbating factors such as wave action, should be suggested in line with industry standard principles and available guidance.
• An assessment of the impact of the design flood on access to and egress from the development will be required.

Conclusions

• The conclusions should include a summary of the findings detailing any

Conclusions

• recommendations that have been made.
• The report should also indicate how all flood risks have been identified and appropriately mitigated or managed. The plans of the development should clearly take cognisance of these conclusions.
• A statement outlining how, in the author(s) opinion, the development proposal complies with current flood legislation and policy should be provided.
• A summary of residual risk after any proposed flood mitigation measures have been suggested and recommendations for further study/risk reduction.

Drainage Statement

• A description of the existing drainage characteristics of the site including general/ surrounding topography, the existence of any public sewers in the area.
• A concept drawing of the proposed development including proposed/ anticipated drainage arrangements; identification of likely permeable and impermeable (hardstanding) areas (including a summary of area sizes)

Full DIA

• A description of the existing drainage characteristics of the site including general/ surrounding topography, the existence of any public sewers in the area.
• A concept drawing of the proposed development including proposed/ anticipated drainage arrangements; identification of likely permeable and impermeable (hardstanding) areas (including a summary of area sizes)
• An examination of the current and historical drainage rights
• An outline drawing of how the drainage design provides SuDS techniques in accordance with recognised design manuals
• Waste water drainage proposals
• If connection to a public sewer is proposed then agreement letter from Scottish Water should be provided
• If no public sewer is available in the settlement then evidence of compliance with SEPA's Policy and Supporting Guidance on the Provision of Waste Water Drainage in Settlements (available from SEPA planning )
• Subsoil porosity tests at the location of any intended infiltration device and the proximity of the seasonally highest water table
• The soil classification of the site
• If any discharge is to be made to a body of water then the intended rate of discharge, the point of discharge and the outfall structure proposals should be provided
• Calculations comparing pre- and post development runoff rates for a range of storm events (e.g. 1:2, 1:30, 1:100 and 1:200 return periods) if applicable. An additional allowance for the potential impacts of climate change

Full DIA

• Demonstration that post development peak run-off volumes do not exceed that for pre-development for the range of return periods i.e. that the pre-development drainage situation is improved or at least mimicked17
• Demonstration that the drainage solution selected will ensure that properties on and off the proposed site are not at risk of flooding from the appropriate rainfall return period
• Details of the accountable body responsible for vesting and maintenance for individual aspects of the drainage proposals and confirmation in writing that these bodies will vest/adopt the system
• Waste water drainage proposals and confirmation in writing that they will vest in Scottish Water (if applicable)
• Confirmation of measures to be employed to manage surface water runoff during the construction phase
• Topographical survey of the site and adjoining land if available